K901009 ) IVWINV 1 —=— —NOllViIDvI—-4 + Eb = = —y¥aNnni—— ae 7 : 7 = —_——1S3YOI—1V5IIO4 == i | == Seal Ne ————— ‘ Wit iI q DIWMIULP YIAON Ul suoymioig fo UOTNGLAYSIG, Puvd PJUIAANII() ies a YZ S5NDONW2 ey LONYALS WAH LNOS “74 FAA ee 7s Jr Pa ee AA AAA, SILLAPEEEEELL CPPS LOLLEEEL | HANNO MM, SOMOS lisa YO Z \a3s3a- SW snonaiaad ¥301W3H-3NId ANIMAL ECOLOGY O Ofh#TOO TOEO O NIM 1IOHM/18IN aaa aaa ANIMAL PRENTICE-HALL, INC. ENGL E WiOlO De Gl iE SiN Ji ECOLOGY S. CHARLES KENDEICGH Zoology Department University of Illinois ANIMAL ECOLOGY S. Charles Kendeigh ©—1961 by Prentice-HAatt, Inc. Englewood Cliffs, N. J. All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher. Library of Congress Catalog Card No.: 61-12332 Fourth printing. ..... June, 1965 PRINTED IN THE UNITED STATES OF AMERICA 03714-C To my teachers of ecology LYNDS JONES, OBERLIN COLLEGE JOHN E. WEAVER, UNIVERSITY OF NEBRASKA VICTOR E. SHELFORD, UNIVERSITY OF ILLINOIS Preface The science of ecology, born at the beginning of the present century after a gestation period of several hundreds of years, has now matured into an honored and respected scholarly disci- pline and field of research. This book is an effort to summarize the basic concepts and principles of the subject and present the elementary factual in- formation with which a person to be competent in this field should be familiar, especially as these things apply to animals. After a Background section for orientation, local communities and habitats are discussed in some detail. It is my firm belief that one begin- ning the study of ecology should first of all be- come thoroughly acquainted with the places where animals may be found in nature, what kinds of organisms occur in different habitats, the abun- dance and interrelations of organisms in these habitats, the behavior and the life requirements of the principal species, and the structure and succes- sion of communities. The reader well founded in this knowledge is ready to understand the ecologi- cal processes and community dynamics which are presented in the third section. In the fourth and final section, the reader is introduced to the broad field of geographic ecology, which will give him some knowledge of how animals are distributed over the world, and why they occur where they do. Physiological ecology, the study of the manner in which organisms respond and adjust to envi- ronmental factors, is dealt with sparingly. The proper development of this subject takes one ex- tensively into laboratory experimentation, which is best left to the advanced student. Our emphasis is kept on the study of the free-living organism in its natural environment. Although the quantita- tive aspect of ecology is emphasized, I do not be- vi lieve that in an introductory book it is desirable to approach the subject from a statistical point-of- view. Too few readers will have an adequate sta- tistical background, and the introduction to the subject matter of ecology should not be delayed until an adequate background of statistics is ob- tained—as necessary as that is to advanced work in the field. I have not thought it desirable to devote a spe- cial chapter to applied ecology or, more particu- larly, to wildlife management. The fundamental principles of wildlife management are the same as the fundamental principles of ecology, since wildlife biology is the ecology of game species. Throughout the book, however, I have tried to show the relation of basic concepts to problems in wildlife management. The special obligation of the wildlife manager is to make practical use of these principles for the promotion of wildlife pop- ulations. This book is designed for a course given at the junior-senior-graduate level, to students who have at least a year’s background in zoology. I give such a course during the autumn semester. If I were to give it during the spring, | would probably change the order of study of the four sections to I-IV-III-II. Section IV would here substitute for Section II in providing the student with some knowledge of communities before undertaking section III. This would permit field studies late in the spring to be more closely correlated with the discussion of local habitats and communities. During the first semester at the University of Illinois I have a half-day or a full-day trip every Saturday until winter weather sets in, and there are two half-day winter trips. Also included in the field work is one weekend camping trip to study communities not found locally. The stu- dents get to see at first hand a large variety of animals, and to measure population sizes by quan- titative methods that may be crude but are never- theless effective in stream riffles and pools, ponds of different ages, bogs, lakes, grassland, deciduous and coniferous forests, and seral stages as they develop on rock, sand, pond, bog, floodplain, and abandoned strip-mine areas. Some experimenta- tion is also done in the field to analyze the manner in which both aquatic and terrestrial species re- spond to environmental factors. There is a small amount of laboratory work for learning quantita- tive methods of counting plankton, examining dif- ferent kinds of respiratory systems in aquatic organisms, searching and identifying micro- organisms in the soil, experiments in choice of habitats, and map-making. Methods for measur- ing productivity are discussed but actual practice with these methods is left for an advanced class. Enough material is given on plants in this book, it is hoped, to bring out their essential place in the ecosystem and to emphasize the bioecologi- cal point-of-view. I believe it would be possible for an instructor to use this textbook in a course in general ecology by supplementing in lectures the material and concepts presented in the book with additional material on plants. Some care has been taken with taxonomic nomenclature. Common names are used through- out the text as far as possible, with the scientific nomenclature restricted to the index. Authorities followed for most scientific names are the follow- ing. Mammals—North America: Miller and Kel- logg 1955; Eurasia: Ellerman and Morrison-Scott 1951. Birds—A.O.U. Checklist 1957. Reptiles and amphibians—Schmidt 1953. Fish—Bailey 1960. Invertebrates—as given by authors, not standardized. Trees—Dayton et al. 1953. Grasses —Hitchcock 1951; and other plants, Fernald 1950, Rydberg 1954. Common names of mam- mals are mostly from Hall 1957; birds, A.O.U. Checklist 1957; reptiles and amphibians, Conant 1958; and fish, Bailey 1960. Pertinent references to literature are cited in the text in connection with each major topic. These references serve only as an introduction to the very extensive literature in ecology. Finally, I wish to acknowledge the help of many persons in the preparation of this text, par- ticularly, Stanley A. Cain, Edward S. Deevey, Ralph W. Dexter, Paul L. Errington, F. E. J. Fry, Clarence J. Goodnight, F. T. Ide, Bostwick Ketchum, Ernst Mayr, Howard T. Odum, Or- lando Park, Frank Pitelka, W. E. Ricker, Gordon A. Riley, M. D. F. Udvardy, and R. H. Whitta- ker. In addition, several of my colleagues at the University of Illinois read and commented on early drafts of chapters. John Riina, of Prentice- Preface vil Hall, Inc., was very cooperative in having two or three and occasionally four people read various chapters, and two persons read the entire manu- script. The final manuscript was expertly edited by Oren Hunt, whose help was invaluable. Illustrations come from several sources. I] am most grateful to Dr. Victor E. Shelford for sup- plying many drawings originally published in his Animal Communities of Temperate America (1913); to the Illinois Natural History Survey for original illustrations from several of their publications; to the U.S. Forest Service who al- lowed me to select what I wanted from their ex- vill Preface tensive file of photographs; to the Friez Instru- | ment Division; to the University of Wisconsin News Service, to a number of individuals for sup- plying photographs or other illustrative material for which acknowledgement is made in the legends of the figures, and to Colleen Nelson, Katherine Little, and Nan Brown for preparing special drawings. University of Illinois Champaign, Illinois SCI BACKGROUND LOCAL HABITATS, COMMUNITIES, SUCCESSION ECOLOGICAL PROCESSES AND COMMUNITY DYNAMICS Contents NO Scope and History of Ecology, 1 General Nature of Environmental Responses, 6 The Biotic Community, Its Structure and Dynamics, 18 Measurement of Populations, 31 Streams, 42 Lakes, 59 Ponds, Marshes, Swamps, and Bogs, 79 Rock, Sand, and Clay, 9% Grassland, Forests, and Forest-Edges, 120 Dispersal, Migration, and Ecesis, 145 Reactions, Soil Formation, and Chemical Cycles, 163 Cooperation and Disoperation, 174 Food and Feeding Relationships, _ 187 Energy Exchanges, Productivity, and Yield, 200 Reproductivity and Population Structure, 210 x GEOGRAPHIC DISTRIBUTION OF COMMUNITIES Contents Z| 28 Regulation of Population Size, 219 Trruptions, Catastrophes, and Cycles, 234 Niche Segregation, 245 Speciation, 257 Distributional Units, 268 Paleoecology, 280 Temperate Deciduous Forest Biome, 293 Coniferous Forest, Woodland, and Chaparral Biomes, Tundra Biome, 315 Grassland Biome, 324 Desert Biome, 332 Tropical Biomes, 340 Marine Biomes, 351 Bibliography, 373 Index, 404 301 Background: The Scope and History of Ecology The word ecology, derived from the Greek words oikos meaning habitation, and logos meaning discourse or study, implies a study of the habitations of organisms. Ecology was first described as a separate field of knowledge in 1866 by the German zoologist Ernst Haeckel, who invented the word oekologie for “the relation of the animal to its organic as well as its in- organic environment, particularly its friendly or hos- tile relations to those animals or plants with which it comes in contact.” Ecology has been variously defined by other in- vestigators, as “scientific natural history,” “the study of biotic communities,” or “the science of community populations” ; probably the most comprehensive defi- nition is the simple one most often given: a study of animals and plants in their relations to each other and to their environment. OBJECTIVES Ecology is a distinct science because it is a body of knowledge not similarly organized in any other division of biology ; because it uses a special set of techniques and procedures; and because it has a unique point-of-view. The essence of this science is a comprehensive understanding of the import of these phenomena : 1. The local and geographic distribution of organisms ; 2. Regional variations in the abundance of or- ganisms ; 3. Temporal changes in the occurrence, abun- dance, and activities or organisms ; 4. The interrelations between organisms in popu- lations and communities ; 5. The structural adaptations and functional adjustments of organisms to their physical environment ; 6. The behavior of organisms under natural con- ditions ; 7. The evolutionary development of all these in- terrelations ; and 8. The biological productivity of nature and how this may best serve mankind. METHODS To the achievement of these objectives the following methods or points of attack are funda- mental : Observation in detail of how organisms live un- der natural conditions. PROMINENT LEADERS IN THE DEVELOPMENT OF ECOLOGY IN AMERICA F.E. Clements, plant ecologist. ns Aldo Leopold, wildlife manager. 2 Background Concentration of studies not on the rare but on the most abundant and influential organisms in the community. Measurement and evaluation of physical fac- tors in the actual microhabitat occupied by or- ganisms. Correlation of findings of experimental studies of organisms in the laboratory with observations of those organisms in the field. Use of quantitative—not just qualitative—tech- niques in field studies as well as laboratory studies. A study of organisms in the field may bring to light problems which will be most expediently worked out in the laboratory ; but field and laboratory investi- gations must be integrated. The investigator must often study the morphology of dead organisms in the laboratory, and there perform experiments on living animals and plants held under carefully controlled experimental conditions. But unless such studies are perspective to the normal life of an organism, as it is lived in natural conditions, they are not ecology. The use of exact quantitative techniques is, of course, a general characteristic of all science. But special difficulties arise when such techniques are ap- plied to free-living organisms in natural conditions. For example, size of animal populations has, in the past, often been described in such vague terms as “rare,” “common,” or “abundant.” These are subjec- tive terms, based largely on an impression gained by the observer of the apparent conspicuousness of the species. As James Fisher, an English naturalist, wrote in 1939, a species has usually been indicated as “rare’’ when actual numbers expressible in one’s and two’s could be recorded; “common” when the observer began to lose count; and “abundant” when he became bewildered. One of the chief problems of the ecologist is to develop methods by which to meas- ure the absolute size of populations and the produc- tive capacities of different habitats so that the activi- ties of widely varying types of species may be compared. For setting up experiments and organiz- ing and analyzing studies under natural conditions, it is becoming more and more essential that the ecolo- gist become familiar with and employ good statistical procedures (Williams 1954). As a contribution to human knowledge and under- standing, ecology is in the fortunate position of being concerned with the most complicated systems of or- ganization, apart from human societies, with which we have to deal. For this very reason it provides a constant challenge to the imagination as well as to experimental ingenuity. It is more difficult to analyze and isolate the relevant factors in a living community than in a simpler system, but the gain in significant understanding of the material world and in compre- hending the beauty of its organisation is perhaps better in proportion (Macfadyen 1957: 246). RELATION TO OTHER SCIENCES Ecology is one of the three main divisions of biology ; the other two being morphology and phys- iology. The emphasis in morphology is on under- standing the structure of organisms; in physiology, on how they function; and in ecology, on their ad- justments to the environment. These divisions over- lap broadly. To appreciate fully the structure of an organ, one needs to know how it functions, and the way it functions is clearly related to environmental conditions. The morphologist is concerned with prob- lems of anatomy, histology, cytology, embryology, evolution, and genetics; the physiologist, with inter- preting functions in terms of chemistry, physics, and mathematics ; and the ecologist, with distribution, be- havior, populations, and communities. The evolution of adaptation and of species is of mutual interest to the ecologist and to the geneticist ; bioclimatology is a connecting link between ecology and physiology. All areas, in the final analysis, are simply different approaches to an understanding of the meaning of life. SUBDIVISIONS OF ECOLOGY Ecology may be studied with particular ref- erence to animals or to plants, hence animal ecology and plant ecology. Animal ecology, however, cannot be adequately understood except against a consider- able background of plant ecology. When animals and plants are given equal emphasis, the term bioecology is often used. Courses in plant ecology usually dis- miss animals as but one of many factors in the en- vironment. Synecology is the study of communities, and autecology the study of species. In this book we shall survey the fundamentals and basic facts of animal ecology. We will study com- munity ecology, the local distribution of animals in various habitats, the recognition of community units, and succession ; ecological dynamics, the processes of dispersal, ecesis, reaction, coaction, productivity, com- petition, speciation, and regulation of abundance ; and geographic ecology, geographic distribution, palaeo- ecology, and biomes. We will also be interested throughout the text with how species and individ- uals respond and adjust to the physical factors of their environment, but a full study of physiological ecology must be left to another time and place. When special consideration of their ecology is given to one or another taxonomic group, we speak E.A. Birge, limnologist. Henry C. Cowles, plant ecologist (courtesy R.J. Pool). Victor E. Shelford, animal ecologist. The scope and history of ecology 3 of mammalian ecology, avian ecology, insect ecology, parasitology, and so on. When emphasis is placed on habitat, we speak of oceanography, the study of ma- rine ecology; limnology, the study of fresh-water ecology ; terrestrial ecology, and so on. Animal eth- ology is the interpretation of animal behavior under natural conditions; often, detailed life history studies of particular species are amassed. Sociology is really the ecology and ethology of Mankind. Ecological concepts, which may be grouped to- gether as applied ecology, have many practical appli- cations; notably wildlife management, range man- agement, forestry, conservation, insect control, epidemiology, animal husbandry, even agriculture. This preview of ecology indicates the great breadth and unique character of the subject material which justifies the view of ecology as one of the three basic divisions of general biological philosophy. HISTORY That certain species of plants and animals ordinarily occur together and are characteristic of certain habitats has doubtless been common knowl- edge since intelligent man first evolved. This knowl- edge was essential to him for procuring food, avoid- ing enemies, and finding shelter. However, it was not until the fourth century Bc, that Theophrastus, a friend and associate of Aristotle, first described in- terrelations between organisms and between organ- isms and their environment. He has, therefore, been called the first ecologist (Ramaley 1940). The modern concept that plants and animals occur in closely integrated communities began with the studies of August Grisebach, a German botanist, in 1838; K. Mobius, a Danish investigator of oyster banks, in 1877; Stephen A. Forbes, an American, who described the lake community as a microcosm in 1887 ; and J. E. B. Warming, a Danish botanist, who emphasized the unity of plant communities in 1895 (see Kendeigh 1954 for further details and literature citations). C. C. Adams recognized and described many animal communities in his ecological surveys of northern Michigan and of Isle Royale in Lake Su- perior, published in 1906 and 1909. V. E. Shelford presented a classic study of animal communities in temperate America in 1913, and Charles Elton pub- lished an outstanding analysis of community dy- namics in 1927. Although an appreciation of the fact that the whole community is one biotic unit, rather than one unit of plants and another of animals, may be discerned in the writings of some early in- vestigators (eg., J. G. Cooper in 1859), the fact has been brought to modern emphasis in the work of F. E. Clements and V. E. Shelford, especially in their Bio-ecology published in 1939. 4 Background Succession of plant species after burns and in bogs has been known in a general way since about 1685; and European ecologists have studied succes- sion since the late nineteenth century. The present- day interest in succession, however, especially in North America, dates from the plant studies of Henry C. Cowles in 1899 on the sand dunes at the south end of Lake Michigan, and the work of Fred- eric E. Clements, 1916. C. C. Adams and V. E. Shel- ford, in the citations noted were among the first to apply the concept to animals. Geographic ecology, in the modern sense, dates from the generalizations on the world-wide distribu- tion of animals made by the French naturalist, Georges L. L. Buffon (lived 1707-1788), and the explorations of the German botanist, Alexander von Humboldt (lived 1769-1859). There was lively inter- est and many important contributions in this general field during the nineteenth century ; notably, the life- zone concept of C. Hart Merriam (1890-1898) needs special mention. During the present century the con- cept of biotic provinces is identified with L. R. Dice (1943) and the biome concept with F. E. Clements and V. E. Shelford (1939). The broad survey of ecological animal geography made by R. Hesse in 1924 exerted considerable effect and this treatise was later translated into English and revised by W. C. Allee and Karl P. Schmidt (1951). The study of population dynamics, so important in modern ecology, dates back at least to Malthus, who pointed out in 1798 the limitation to population growth exerted by available food. Darwin, in 1859, recognized the importance of competition and preda- tion in developing his theory of evolution. Pearl, 1925, analyzed mathematically the characteristics of population growth, and Lotka, 1925, and Volterra, 1926, developed theoretical mathematical equations to show the manner in which populations of different species interact. These studies led to the classic ex- periments of Gause, 1935, with interacting popula- tions of predators and prey. Nicholson’s publication in 1933 stimulated much thinking concerning the fac- tors that stabilize populations at particular levels. Andrewartha and Birch, 1954, emphasized the im- portance of climate and other factors on determining the size of populations. The measurement and analysis of energy use by organisms for existence and growth is now of very great interest in ecology. Attention to biological pro- ductivity began in the 1930’s in connection with prac- tical pond-fish culturing in Europe and the limnologi- cal studies of Thienemann in Europe and of Birge and Juday at the University of Wisconsin, but the modern crystallization of the subject came with the fresh-water and marine investigations of Lindeman, Hutchinson, and Riley at Yale University (Ivlev 1945) and of Howard and Eugene Odum. An early study of energy relations within terrestrial communi- ties is that of Stanchinsky (1931). Physiological ecology had its historical beginnings in the correlation of biological phenomena with vari- ations in temperature stimulated by Galileo's inven- tion of a hermetically sealed thermometer about 1612 Ap. The French naturalist Reaumur summed _ the mean daily temperatures for April, May, and June in 1734 and again in 1735, and correlated the earlier maturing of fruit and grain during the first year with the greater accumulation of heat. A discovery of parallel significance was of oxygen in 1774 by the English clergyman, Priestley, and the finding by Lavoisier, a Frenchman, in 1777 that it was an essen- tial part of air. Claude Bernard, another French physiologist, enunciated the principle of homeostasis in 1876. This concept originally referred to regula- tory mechanisms which maintained the “internal en- vironment” of the body constant in the face of chang- ing external conditions. Later, the concept came to be applied also to maintenance of community inter- relations. Van’t Hoff, a Dutch scientist, contributed to physiological ecology in 1884 in describing how the speed of chemical reactions increased two- or three-fold with each rise of 10°C. K. G. Semper and Charles B. Davenport clearly established physiologi- cal ecology in bringing together pertinent information in 1881 and 1897-1899 respectively. More recent summaries of knowledge and methods in this general field have been made by V. E. Shelford in Laboratory and Field Ecology (1929) and by Samuel Brody in Bioenergetics and Growth (1945). The development of animal behavior or ethology may be traced back through the natural history of ancient times. More recently the 13 volumes of Thierleben, prepared by A. E. Brehm during the period 1911 to 1918, are noteworthy. H. S. Jennings, 1906, and Jacques Loeb, 1918, made valuable contri- butions to the understanding of the behavior of in- vertebrates. Precise modern techniques and concepts as applied to vertebrates began to take form about 1920 with the development of banding and marking of individual animals by S. Prentiss Baldwin (1919) and the recognition of territories in the nesting of birds by H. E. Howard (1920). The formulation of the concept of releasers as controlling instinctive be- havior by Wallace Craig (1908), K. Lorenz (1935), and N. Tinbergen (1951) has produced a profound effect on present-day thinking. In regard to other divisions of ecology, the crys- tallization of studies in oceanography may be credited to Edward Forbes 1843, Maury 1855, Alexander Agassiz 1888, Petersen and his colleagues 1911, and Murray and Hjort 1912; limnology to Forel 1869, Birge 1893, Juday 1896, Ward and Whipple 1918, Thienemann 1913-1935, and Naumann 1918-1932; and wildlife management to Aldo Leopold 1933. Ecology, then, is of comparatively recent develop- ment as a distinct science, but its roots extend well back into the past. Doubtless the most comprehensive treatment of the subject in all its aspects is Principles of Animal Ecology by Allee, Emerson, Park, Park and Schmidt, published in 1949 (for citations of his- toric interest in this chapter, see this reference). Since ecology is a young science, it should be empha- sized that its concepts and techniques have not be- come standardized and that there is opportunity and stimulus here for many new investigators. The Ecological Society of America was founded in 1915, and in 1960 had a membership of over two thousand. The British Ecological Society, organized in 1913, has a membership of about one thousand. The society in America has given birth to several off- spring during its 45 years of existence: The Wildlife Society, Society of Limnologists and Oceanog- raphers, The Nature Conservancy, and a Section on Animal Behavior and Sociobiology. Several of these organizations have their own journals. The Ecologi- cal Society of America publishes two periodicals: Ecology for short papers and Ecological Monographs for long ones. The British Ecological Society also publishes two: Journal of Ecology for plant papers and Journal of Animal Ecology for animal papers. Oikos began publication in 1949 to represent ecolo- gists in Denmark, Finland, Iceland, Norway, and Sweden. Announcement was made in 1960 of the or- ganization of the International Society for Tropical Ecology to include India and adjacent countries. There will be a Bulletin. The Ecological Society of Australia was organized in 1960. The New Zealand Ecological Society came into existence in 1952 and regularly publishes Proceedings of its annual con- ferences. The Japanese Journal of Ecology, begun in 1954, is the official publication of the Ecological So- ciety of Japan. Most of its articles are in Japanese, but they have summaries in a European language. Finally, many papers of interest to ecologists appear in zoological journals of various sorts that do not carry the word ecology in their titles. The scope and history of ecology 5 The General Nature of Environmental Res ponses Ecology, by definition, deals with the interrela- tions of organisms with each other and with their environment. These interrelations become estab- lished as organisms respond in various ways to con- tacts with one another and with the ever-changing environment. The term environment describes, in an unspecific way, the sum total of physical and biotic conditions influencing the responses of organisms. More spe- cifically, the sum of those portions of the hydrosphere, lithosphere, and atmosphere into which life pene- trates is the biosphere. There are no characteristic or permanent inhabitants of the atmosphere, although the air is traversed by many kinds of animals and plant propagules. Of the hydrosphere, there are two major biocycles, the marine and fresh-water ; of the lithosphere there is one, land (Hesse et al. 1951). A habitat is a specific set of physical conditions (e.g., space, substratum, climate) that surrounds a single species, a group of species, or a large com- munity (Clements and Shelford 1939). The ultimate division of the biosphere is the microhabitat, the most intimately local and immediate set of conditions sur- rounding an organism; the burrow of a rodent, for instance, or a decaying log. Other individuals or species are considered as part of the community to which the organism belongs and not part of its habitat. The term biotope defines a topographic unit charac- terized by both uniform physical conditions and uni- form plant and animal life. In order for organisms to exist they must respond or adjust to the conditions of their environment. The first living organisms probably evolved in the sea and must have possessed very generalized adjust- ments to this relatively uniform and favorable habitat. However, these early organisms had inherent in them the potential for expansion, as they later spread into other and more rigorous habitats, particularly fresh- water and land. As evolution proceeded, organisms became more and more limited in the range of their ability to respond as they became specialized in their adjustments to particular habitats. This led to the great diversification of species that we see at the present time, with each species restricted to its par- ticular microhabitat and place in the community. Organisms respond to differences or changes in their environment in four principal ways: morpho- logical adaptations, physiological adjustments, be- havior patterns, and community relations. Chapters 2 and 3 are a resumé of these responses, the general fundamentals of which must be understood before the subtle relations of an organism to its environment that are the substance of ecology can be appreciated. Probably the most important of distinctions be- tween organisms in a consideration of their morpho- logical responses to the environment is whether they are sessile or motile (Shelford 1914). Most 6 plants are, of course, sessile; most animals, motile. There are, however, some motile plants among uni- cellular forms and male gametes, and there are many sessile or slow-moving animals in aquatic habitats. Sessile organisms respond to variations of the en- vironment primarily by changes in form; motile ani- mals, primarily by changes in behavior. MORPHOLOGICAL ADAPTATIONS Changes in form and structure Consider a sessile organism, the tree. It is essen- tial to the tree that its foliage be exposed to sunlight. As it grows within a forest, it is usually tall and slender, and little branched except at the top, where the cap of foliage reaches into the full sunlight. Grow- ing on the forest’s edge, the tree is shorter, and branching and foliage are dense both at the cap and on that side exposed to full sunlight. The tree which grows solitary in an open place is short, but branch- ing and foliage are dense and uniformly distributed, often starting close to the ground. In similar manner, the variations in form assumed by sessile colonial animals, such as sponges and corals, reflect vicissi- tudes imposed by habitat (Wells 1954). Morphological variations induced by peculiarities of habitat do occur in motile animals: thickening of the shells of clams subjected to strenuous wave ac- tion ; variation in number of vertebrae, scales, and fin rays among fish subjected to different temperatures at critical periods in their growth (Taning 1952) ; changes in the number of facets in the bar-eye of the fly Drosophila as a correlative of temperature varia- tions during a short critical period in larval growth (Krafka 1920) ; the many variations in form and size of internal parasites, depending on crowding and other environmental conditions (Baer 1951) ; pointed tails in certain flatworms crawling over a substratum during growth, contrasted with rounded tails occur- ring in the same species when individuals are experi- mentally prevented from crawling over the sub- stratum (Child 1903). That individuals of the same species are so much alike attests the great extent to which the course and outcome of morphological development is genetically determined. But that there are variations between individuals, and between groups of individuals, of the same species shows that morphological development is also responsive to environmental influences. Modi- fications induced by the environment emerge as the individual develops and are not specifically inherited by the succeeding generation. These modifications are called growth-forms. If the generation following is similar in growth-form to the parent generation, it is a similar morphological response to a similar en- vironment (Schmaulhausen 1949). If the growth- form persists through many generations and appears to be an adaptation, even though not inherited, it is often called an ecad. If and when the growth-form becomes inherited as the result of evolutionary proc- esses, it then becomes an ecotype. Life-form is a gen- eral term referring to the shape or appearance of an organism irrespective of how formed (Daubenmire 1947). The prevalence of particular life-forms among the important organisms helps to separate and char- acterize biotic communities, as we will see repeatedly in later discussions. Life-forms of plants The life-form of a plant is characterized by its vegetative form, its length of life, the arrangement and character of its leaves, whether its stem is her- baceous or woody, its manner of growth, and its means of overwintering. Life-form categories some- times agree with large taxonomic units, such as ferns and mosses, but on the other hand some taxonomic groups contain species exhibiting a variety of life- forms and some life-forms include species only re- motely related taxonomically. There have been many systems proposed for the classification and terminology of the life-forms of FIG. 2-1 Form assumed by the coral Madrepora as it develops in (a) deep water; (b) barrier pools; (c) rough water (from Wood-Jones 1912). The general nature of responses 7 plants, but for the animal ecologist the following simplified classification, based on Pound and Clem- ents (1900), is sufficient for general purposes : 1. Annuals: Passing the winter or dry season in seed or spore form alone, no propagation or accumulation of aerial shoots ; living one year. 2. Biennials: Passing one unfavorable season in the seed or spore form and the next in a vege- tative stage; no accumulation of aerial shoots ; living two years or parts of two years. 3. Herbaceous perennials: Passing each unfa- vorable season in both seed or spore and vege- tative form; no accumulation of aerial shoots; living several to many years. Broad-leaved herbs: mostly terrestrial Sod grasses: a continuous turf Bunch grasses: scattered clumps Succulents: some broad-stemmed cacti Water plants: (1) Submerged: vegetative body entirely underwater. (2) Floating: leaves floating on water sur- face ; water lilies, duckweed. (3) Emerging: leaves extending above water surface; cattails, sedges, rushes. Ferns Mosses Liverworts Lichens Fungi . Algae 4. Woody perennials: Passing the unfavorable season as aerial shoots or masses, often as seeds also ; living many years as a rule. a. Lianas: vines b. Succulents: some tree or barrel cacti c. Bushes: much-branched, low growth, sev- eral stems d. Shrubs: single stem and tree-like but smaller ey brees: (1) Deciduous: shedding leaves during unfavorable season. (2) Evergreen: leaves shed irregularly and tree never completely bare. (a) Needle-leaved: narrow, usually elongated leaves. (b) Broad-leaved: leaves much as on deciduous trees but shed irregu- larly. emo op Fan 6 oe Sn EL im Life-forms of animals Systems to classify the life-forms of animals have been little developed (Remane 1943, 1952). 8 Background The major life-forms of animals more often agree with their taxonomy than do plants, but some life- forms include representatives from several different taxonomic groups. There can be recognized encrust- ing forms such as the fresh-water bryozoa Pluma- tella and some sponges ; coral forms, including grass, leaf, or shrub forms; radiate forms, such as coelente- rates and echinoderms generally ; bivalve forms; snail forms; slug forms; worm forms; crustacean forms ; insect forms; fish, snake, bird, and four-footed forms. Each of these major types may be subdivided; for instance, the four-footed form of mammals (Osburn et al. 1903) : Aquatic (swimming) : seal, whale, walrus Fossorial (burrowing): mole, shrew, pocket gopher Cursorial (running) : deer, antelope, zebra Saltatorial (leaping) : rabbit, kangaroo, jumping mouse Arboreal (climbing) : squirrel, opossum, monkey Aerial (flying) : bat Adaptations Plants and animals of specific life-forms are adaptations to live in particular habitats and to be- have in particular ways (Klaauw 1948). The life- forms listed for mammals are largely adaptations to particular strata (water, subterranean, ground, tree, air) within a community rather than to the habitat as a whole; for instance, the subterranean adapta- tions of mammals living in the Arctic tundra are similar to the subterranean adaptations of mammals in the tropics. In communities lacking one or more strata (for instance, the tree stratum in grassland), animals specifically adapted to the missing strata are also absent. In communities in which all strata are present, a catholic variety of life-forms occurs. In addition to adaptations to stratum and habitat, there occur ecologically significant adaptations for food-getting and metabolism, protection, and repro- duction. The variety of teeth found in mammals and lizards, the variation in shape and size of bills of birds, the different mouth parts of insects, the siphons of clams, the suckers of leeches, the water canal sys- tems of sponges are but a few special anatomical fea- tures especially designed for food-getting. Associated with food-getting is a great diversity in structural adaptations for the digestion of the food, for respira- tion, for circulating food materials and gases through the body, for excreting wastes, for support and move- ment, and for nervous and hormonal regulation. All these internal organs and structures are necessary to the animal for utilizing the energy resources of the environment. All animals are subject to predation or competi- tion and must have means of protecting themselves or offsetting losses in the struggle for existence. Such adaptations take a variety of form such as body armor, concealing coloration, attack weapons, or be- havior patterns of escape. High rates of mortality are offset by high rates of reproduction or, in some lower organisms, by considerable power of regenerating whole organisms from fragmented parts. The manner in which reproduction occurs and the special structures concerned with reproduction vary with each type of animal and often with each indi- vidual species. These adaptations are universal and too numerous even to attempt to classify at this point but are certainly obvious to all. The primary objec- tives in the life of each species are to maintain the existence of the individual and to reproduce its own kind, and all adaptations to live in favorable habitats are designed toward these ends. Natural selection To be heritable, a variation must have been caused either by mutations in the genes or chromo- somes of the individual or by new combination of genes. Mutations are produced at random, and mostly independent of natural environmental condi- tions, although there is some experimental evidence that they may be induced or increased in frequency by cosmic rays, ultraviolet rays, heat, and certain chemicals. There is no reason to believe, however, that environmental factors can ordinarily influence the kind of mutations that occur. Heritable variations in the structure of organisms, and in their physiology and behavior as well, may be favorable, unfavorable, or of neutral value to the ex- istence of the species. Variations that decrease the efficiency of a species in its struggle for existence against competitors and unfavorable environmental conditions usually disappear, but variations that in- crease this efficiency give those individuals that pos- sess them a better chance for survival and for giving birth to similar offspring. Thus, there is natural se- lection of the fitter individuals, and a gradual im- provement in the relations between the species and its environment. It is in this way that adaptations are established. A better understanding of the eco- logical relations between different species and be- tween species and the environment will contribute to a better understanding of the process of evolution. At the same time a thorough understanding of the processes of evolution is necessary to understand how organisms become adapted to live in particular habi- tats (Simpson 1953). A close study of differences between individuals shows that within many species convergent evolution occurs under similar environmental conditions. Many of these variations are genetic and apparently due to natural selection. The best established corre- lations are the following, although even they are sub- ject to frequent exceptions (Mayr 1942, Dobzhansky Keto) BERGMANN'S RULE: geographic races of a species possessing smaller body-size are found in the warmer parts of the range, races of large body-size in the cooler parts. This appears true for cold-blooded as well as warm-blooded animals (Ray 1960). ALLEN’S RULE: tails, ears, bills, and other ex- tremities of animals are relatively shorter in the cooler parts of a species’ range than in the warmer parts. GLOGER’S RULE: in warm-blooded species, black pigments increase in warm and humid habitats, reds and yellow-browns prevail in arid climates, and pig- ments become generally reduced in cold regions. Races of birds in the cooler parts of a species’ range lay more eggs per clutch than races in the warmer parts of the range. Likewise the number of young per litter of mammals averages higher in cooler climates. The stomachs, intestines, and caeca of birds that live on a mixed diet are relatively smaller in the tropical- than in the temperate-zone races of a species. The wings of birds that live in a cold climate or in high mountains are relatively longer than those of close relatives that live in lowlands or in a warm cli- mate. Races of birds in cool climates are more often and more strongly migratory than races in warm climates. Races of mammals in warm climates have less under-fur and shorter contour hairs. Fish of cool waters tend to have a larger number of vertebrae than those living in warm waters. In- crease in salinity tends to induce the same result as low temperature. Fish that inhabit swift waters tend to be larger and more streamlined than inhabitants of sluggish or stagnant waters. Cyprinid fishes, isolated in desert springs, tend to lose their pelvic fins. Land snails reach their greatest size in the area of optimum climate within the range of the species. The relative weight of snail shells is highest in the forms exposed to the highest radiation of the sun or to the greatest aridity. Land snails tend to have smooth, glassy, brown shells in cold climates, and to have white or strongly sculptured shells in hot dry climates. It would appear at first glance that several of these rules have a physiological basis; for instance, The general nature of responses 9 large body size and short appendages give less sur- face area per volume of body and thus minimize heat loss from the body, in cold climates. It is doubtful, however, that the smaller surface area thus attained gives enough reduction of heat loss to be significant in warm-blooded animals and would not apply to cold-blooded ones. Rather the ability of warm- blooded animals to live in cold climates depends more importantly on the insulation of the body surface, its exposure, its vascularization, and its ability to tolerate a cold tissue temperature (Scholander 1955, Irving 1957). The older explanations of selective value of many of these rules are therefore doubtful. PHYSIOLOGICAL ADJUSTMENTS Nature of adjustments Probably the first response of any organism to a change in the environment is physiological. A physiological response must certainly precede any change in form or structure which requires growth. Even a change in behavior must follow a change in some receptor or sense organ followed by nervous function; a fall in air temperature, for instance, brings a drop in the metabolic rate of cold-blooded organisms but a rise in the rate of warm-blooded or- ganisms. Cold may stimulate nerve endings in the skin of birds or mammals and produce shivering and a search for protective cover. Transference from the dark to light may immediately initiate photosynthesis in resting chloroplastids within a plant cell, or a change in turgescence on opposite sides of a sessile zooid may result in a turning movement, an orienta- tion to or away from the light source. Physiological responses are thus internal responses to factors of the environment. Often they are difficult to detect. Types of response Environmental factors influence organisms physiologically in various ways (Fry 1947). These effects may be classified as follows : Lethal: causing death ; for instance, extreme heat or cold, lack of moisture, and so forth. Masking: modifying the effect of some other fac- tor. Low relative humidity increases the rate of evaporation of moisture from body sur- faces so that warm-blooded animals are able to survive at otherwise intolerably high air temperatures. Directive: producing an orienting response in relation to some environmental response so 10 Background that the organism gets itself into favorable conditions. Controlling: influencing the rate at which some process functions, but not entering the reac- tion. Temperature, pressure, and viscosity, for instance, affect secretion, locomotion, and metabolism. Deficient: curtailing an activity because some es- sential ingredient, such as a salt, oxygen, or the like is absent or at unfavorably low con- centration. The same environmental factor may produce dif- ferent effects at different times and under different conditions. Temperature may be lethal, if extreme; masking, as when cold reduces the demand of cold- blooded organisms for food; directive, by inducing a search for more favorable locations; or controlling, as a modifier of the rate of metabolism. Often the distinction between controlling and deficient factors is not made, or they are considered as together con- stituting limiting factors. Threshold and rate Every environmental factor varies through a wider range of intensivity than any single organism could tolerate. Characteristically, there is for each in- dividual organism a lower and an upper limit in the range of an environmental factor between which it functions efficiently. For any one factor, different or- ganisms find optimal conditions for existence at dif- ferent points along the range ; hence their segregation into different habitats. The threshold is the minimum quantity of any factor that produces a perceptible effect on the or- ganism. It may be the lowest temperature at which an animal remains active, the least amount of mois- ture in the soil that permits growth of a plant, the minimum intensity of light at which a photoreceptor is stimulated, and so forth. Above the threshold, the rate of a function increases more or less rapidly as the quantity of heat, moisture, light, or other environ- mental factor is augmented, until a maximum rate is attained. Above the maximum, there is usually a de- cline in the rate of a process either because of some deleterious effect produced, the interference of some other factor, or exhaustion. The curve of decline at high temperatures is usually steeper than the curve of acceleration at low temperatures. Law of toleration For each species there is a range in an environ- mental factor within which the species functions at or near an optimum. There are extremes, both maxi- mum and minimum, towards which the functions of a species are curtailed, then inhibited. In some organ- isms, such as fish, the upper limit of tolerance is reached before activity is reduced to zero. At low temperatures, the lower limit of tolerance may be reached while the animal is still capable, potentially, of considerable activity, and death is the result of other factors. On the other hand, some organisms may survive in an inactive or dormant state under environmental conditions that do not permit activity, only to become functional again when critical factors rise above the threshold. Before the limits of tolera- tion are reached there are zones of increasing physio- logical stress. The species as a whole is limited in its activities more by conditions that produce physiological dis- comforts or stresses than it is by the limits of toler- ation themselves. Death verges on the limits of tol- eration, and the existence of the species would be seriously jeopardized if it were frequently exposed to these extreme conditions. In retreat before condi- tions of stress there is a margin of safety, and the species adjusts its activities so that limits of tolera- tion are avoided. There is variation in hardiness of individuals within a species, so that some hardy indi- viduals find existence possible under conditions that disrupt other individuals. The population level of a species becomes reduced therefore before the limits of its range are actually reached. It is desirable to test by acclimation and breeding experiments whether these differences in physiological adaptiveness be- tween individuals or populations are genetic or phenotypic (Prosser 1955). Species vary in their limits of tolerance to the same factor. The Atlantic salmon, for instance, THRESHOLD Limits of tolerance for some organisms: High Lower Upper , A Soe q \\ Metabolism at maximum \N \\ < “ee S\ \\ activity i Metabolism at rest SA Aestivation of some organisms Activity INTERACTION INTENSITY Low TEMPERATURE High FIG, 2-2 Interaction between environment and cold-blooded organisms: organism activity as a function of environmental temperature (modified from Fry 1947). spends most of its adult life in the sea, but goes an- nually into fresh-water streams to breed. Most other marine fishes are killed quickly when placed in fresh- water, as are fresh-water fish when placed in salt water. The following terms are used to indicate the relative extent to which organisms can tolerate vari- ations in environmental factors. The prefix steno- means that the species, population, or individual has a narrow range of tolerance and the prefix eury- in- dicates that it has a wide range; thus stenohaline FIG. 2-3 Law of toleration in relation to distribution and population level—often a normal curve (modified from Shelford !9!!). Lower limit of tolerance intolerance 4 4 Organisms absent Sones SSS SEAS Zone of Zone of Zone of __|physiological physiological | Zone of stress 1X———RANGE OF OPTIMUM————> stress Upper limit of tolerance g S 8 3 8 G 2 Ss S 8 3 BS ' g wS i) ~ 3 8 € 8 & 3 3 Ne Q GRADIENT The general nature of responses 11 and euryhaline in respect to salinity, stenohydric and euryhydric in respect to water, stenothermal and eurythermal in respect to temperature, stenophagic and euryphagic in respect to food, stenoecious and euryoecious in respect to niche or habitat, and so on. Law of the minimum An organism is seldom, if ever, exposed solely to the effect of a single factor in its environment. On the contrary, an organism is subjected to the simul- taneous action of all factors in its immediate sur- roundings. However, some factors exert more influ- ence than do others, and the attempt to evaluate their relative roles has led to the development of the law of the minimum. The first elaboration of this law was made by the German biochemist, Justus von Liebig, in 1840, who stated : If one of the participating nutritive constituents of the soil or atmosphere be deficient or wanting or lacking in assimilability, either the plant does not grow or its organs develop only imperfectly. The de- ficient or lacking constituent makes those that are present inactive or lessens their activity. If the de- ficient or lacking constituent be added to the soil or if occurring in insoluble form it be made soluble, then the other nutrients become active (Browne 1942). Blackman (1905) developed the more compre- hensive concept of limiting factors when he listed five factors involved in controlling the rate of photosyn- thesis: amount of CO, available, amount of H,O available, intensity of solar radiation, amount of chlorophyll present, and temperature of the chloro- plast. Any one of these factors will control the rate of the process if the factor is present in least favor- able amount, or may actually stop it when insufficient, = 320 = ~_40+ 2 3 f °240 ~25 uy = 20 FE I60 s 15 N © 80 10 a Ze 6 ae a ee O 4 8 12 I6 20 24 28 32 36 40 TEMPERATURE, °C FIG. 2-4 The relation between maximum respiration rate, tem- perature, and oxygen tension (mm Hg as shown by values in the graph) in young goldfish acclimated to each temperature before measurements were taken (Fry 1947). 12 Background even though all other factors occur in abundance. The same principle applies to animal functions. Since the rate of a process may be controlled by too great an amount of a substance, such as heat, as well as by too small an amount, and since the pres- ence or abundance of an organism may be limited by a variety of environmental factors, biotic as well as chemical and physical, and since the limiting effect may be due to two or more interacting factors rather than a single isolated one (Shelford 1952), the law of the minimum may be restated in broad ecological terms, as follows: the functioning of an organism 1s controlled or limited by that essential environmental factor or combination of factors present in the least favorable amount. The factors may not be continu- ously effective but only at some critical period during the year or perhaps only during some critical year in a climatic cycle (Taylor 1934). BEHAVIOR RESPONSES Orientation Behavior responses to changes in environ- mental factors can usually be detected immediately as turning or locomotor activities on the part of the organisms (Fraenkel and Gunn 1940). These move- ments tend to take the organism away from points of danger and into more favorable locations, or to per- form some task essential to existence, or to reproduc- tion. If the movement involves curvature or a turn- ing movement either toward or away from the source of stimulus, the movement is called a tropism. Motile organisms frequently respond by actual locomotion toward or away from the stimulus rather than mere turning, and such guided or directed locomotor move- ments are called taxes. When the movements of the animal are random in direction, and there is no im- mediate orientation to the source of stimulus, but the frequency of turning or speed of the movements is dependent on the intensity of stimulation, such re- sponses are termed kineses. As the result of kineses an animal may arrive by chance in a favorable en- vironment, by which the intensity of the stimulus is reduced or entirely eliminated. To identify the stim- ulus to which the organism is responding, the fol- lowing prefixes are employed: thermo-, tempera- ture; photo-, light; geo-, gravity; hydro-, moisture ; chemo-, chemicals ; thigmo-, contact ; baro-, pressure ; rheo, current; and galvano-, electricity. Jacques Loeb, during the period 1888-1918, vig- orously maintained that all tropisms and taxes of organisms were mechanical, automatic, and explain- able in simple concepts of physics and chemistry. . the overwhelming majority of organisms have a bilaterally symmetrical structure.... Nor- mally the processes inducing locomotion are equal in both halves of the central nervous system, and the tension of the symmetrical muscles being equal, the animal moves in as straight a line as the imperfec- tions of its locomotor apparatus permit. If, however, the velocity of chemical reactions in one side of the body, e.g., m one eye of the insect, is increased, the physiological symmetry of both sides of the brain and as a consequence the equality of tension of the sym- metrical muscles no longer exist. The muscles con- nected with the more strongly illuminated eye are thrown into a stronger tension, and if new impulses for locomotion originate in the central nervous sys- tem, they will no longer produce an equal response in the symmetrical muscles, but a stronger one in the muscles turning the head and body of the animal to the source of light. The animal will thus be com- pelled to change the direction of its motion and to turn to the source of light. . . (Loeb 1918). The idea that all instinctive activities of organ- isms were forced and invariable responses to en- vironmental factors met many objections. H. S. Jen- nings (1906) pointed out that many Protozoa are asymmetrical in body structure and hence could not lend support to the tonus theory. Furthermore, the movements and responses of many organisms to en- vironmental stimuli were not stereotyped, but random in nature; of a trial and error sort. Although much of Loeb’s theory has been disproven experimentally and appears untenable on the basis of observations of animal activities under natural conditions, it crys- tallized the need for objective analysis and interpre- tation of animal behavior, and the avoidance of teleo- logical and anthropomorphic explanations. The study of orienting responses of organisms is of utmost ecological significance since it is largely by means of such responses that organisms find their proper and favorable habitats. Preferendum The behavior responses of animals and their orientation in respect to most environmental factors can be tested experimentally, and results thus ob- tained correlated with the animal’s behavior under natural conditions. There is a variety of procedures and equipment suitable to these purposes (Shelford 1929, Warden, Jenkins, and Warner, I, 1935) and there is distinct value in verifying field observations with experimental analyses. When the number of favorable responses at each unit intensity of an environmental factor is plotted against the entire range of that environmental factor, the usual result is a normal or Gaussian curve. The maximum number of responses normally occurs near the center of the range, with a progressive reduction in number toward each extreme, An extension in each direction from the peak of the responses to in- clude 50, 25, or some smaller percentage of the total responses is called the preferenduwm for that animal or group of animals. Innate behavior Much of the behavior of organisms is deter- mined by heredity and is characteristic of the species in its proper environment. This behavior may be evi- dent at birth or it may not develop until the nervous system, including both the receptor and effector mech- anisms, is fully matured. Such innate behavior is of various degrees of complexity. A reflex is a quick, automatic response of a single organ or organ system to a simple stimulus; for instance, the knee jerk in man. Tropisms, taxes, and kineses may involve a series of reflexes and represent a higher level of in- tegration. An instinct, or inherited behavior pattern, is a complex fixed behavior that is activated, more or less automatically, when the animal is presented with the proper stimulus (Thorpe 1951). The anatomical basis for these various grades of behavior lies in the structure of the nervous system and especially, in higher types of animals, in the in- terarrangement of neurones and synapses with each other and in the neural pathways that become estab- lished. Behavior patterns become elaborated through evolution, are as subject to mutation as any struc- tural part of the body, and are a means whereby ani- mals respond advantageously to the various factors in their normal environment. Stimuli Before an action will take place the nervous mechanism must be released by the reception of a stimulus. Stimuli may be either external or internal to the organism. Protoplasm is sensitive to any kind of stimulation, provided it is intense enough. In higher organisms, however, specialized tissues have become particularly sensitive to one kind of stimulus, and these tissues, or sense organs, are called recep- tors. There are several forms of receptors: photo- receptors, phono-receptors, mechano-receptors, chemo-receptors, thermo-receptors, and stato-recep- tors. Not all types of receptors are present in all organisms, and the structure and effectiveness of those present varies from one kind of animal to an- other. The efficiency of the receptor mechanisms is important, as they largely determine the environ- mental factors to which the animal will respond and the degree of sensitivity involved. Stimuli may be internal, and derive either from The general nature of responses 13 Motivation External factors mood ro Neural mechanism releaser drive Behavior FIG. 2-5 Factors involved in the activation of an instinct. hormones or as kinesthesia involving changes in the tension of muscles and tendons or changes in shape or form of muscle fibers. Motivation is established when there is an accumulation of internal stimuli po- tentials as the result of hormone action, kinesthetics, or changes of metabolism. A combination of motiva- tion with proper external conditions and stimuli sets up a drive, such as the hunger drive, or reproduc- tive drive (Richter 1927). Once a major drive is initiated, satisfaction of it requires a series of events and stimuli at different levels of integration, so that a hierarchy of drives, ac- tions, and stimuli is established. The significance of this hierarchy is that a major activity in the life cycle of an animal does not take place until the organism is in a propcr physiological state, which depends, often in large part, on the environment, and then one action leads to another until consummation is com- pleted. In the male stickleback, for instance, the re- productive drive is not initiated until hormone stimuli are released as the result of gonad enlargement and response to lengthening daily photoperiods. Once the reproductive stimulus is given, the first secondary ala Sa A LEVEL SECOND LEVEL LEVEL THIRD LEVEL LEVEL Reproductive drive Fighting Chasing Biting Threatening Building Digging ~ © Testing of materials Boring Gluing Mating Zigzag dance Leading female to nest Showing entrance Quivering Fertilizing eggs Caring for Fanning a nga Rescuing eggs FIG. 2-6 The hierarchy of drives and actions in the three-spined stickleback (after Tinbergen 1951). 14 Background drive is the establishment of nesting territories by fighting among male fishes. Then the nest is built. Only after this is completed is the male ready to re- ceive the female. Even though an animal may have potential ca- pacities in its sense organs with which to respond to the whole environment, a particular action is trig- gered by stimuli from only a very small part of the environment. This is a fundamental characteristic of innate behavior, and the discovery of these critical sign stimuli or releasers is necessary for an apprecia- tion of the interrelation of animals in a community and how they respond to their environment (Lorenz 1935, Tinbergen 1951). The complete enactment of mating behavior in the stickleback proceeds step by step in an orderly manner, each action a releaser for the next. If any one step is changed, or is interrupted, the behavior subsequent in the sequence does not take place. Re- leasers are of a variety of sorts in different species, but commonly involve particular colors or color patterns, call-notes or songs, shapes, chemicals, or contacts, as well as associated acts, positions, or move- ments on the part of another animal. If these trig- gers are not presented, the behavior does not become expressed even though a specific nervous mechanism is present. The analysis of behavior through obser- vation and experimentation with the objective of un- derstanding how an animal acts under natural con- ditions constitutes the science of ethology, an essential branch of ecology. Ethology differs from psychology in that it is concerned with understanding not only the causality of behavior but also the survival value of behavior patterns under natural conditions, and the evolution of these patterns. Psychology is con- cerned more with analyzing the nervous mechanisms that are involved. Learning All behavior is not, of course, automatic and inherited. Much of it represents the adjustment of fixed patterns to changes in and conditions of the animal’s surroundings (Thorpe 1956). Learning may be defined as the adaptive change in individual be- havior as a result of experience. The simplest form of learning is habituation, that is, learning not to respond to stimuli which tend to be without significance in the life of the organism. Young animals, for instance, have an innate tendency to respond to a wide variety of danger stimuli, such as any sudden movement or noise. However, when such stimuli are presented repeatedly without asso- ciation with further effects, the young animal learns to disregard them. There is some evidence, on the other hand, that instinctive recognition of a special- ized predator of a species shows little or no habitua- tion. Conditioning is a form of learning and consists of the establishment of a connection between a normal reward or punishment and a new stimulus, that is, one that hitherto has had no meaning to the animal. Imprinting is especially well shown in waterfowl and gallinaceous birds. Grey-lag geese reared from the egg in isolation react to their human keepers, or to the first relatively large moving object that they see, as they would the parents by following. This imprinting of the parent companion is confined to a very definite and usually very brief period following shortly on emergence from the egg. Once thoroughly established, the behavior is very stable, if not totally irreversible. Furthermore, this imprinting of a hu- man being as a substitute for its own species will call forth, a year or more later, sexual reactions to man in the mature bird. There is no innate recognition by birds of parent, species, sex, or home locality, but there is evidence that these are learned through as- sociation and contact during the course of develop- ment. Imprinting doubtlessly also occurs in other animals than birds. Imitation is another form of learning. An indi- vidual in a flock or herd may start to feed or run when it observes other individuals feeding or run- ning. A young animal learns much that is traditional of the species by imitating its parents. Vocal imita- tion is conspicuous in the elaborate songs of some birds. Trial and error learning involves trial responses to a variety of stimuli with gradual elimination of all responses and stimuli except the relevant ones. A chick pecks at random at all sorts of objects until it accidentally strikes one which is edible, whereafter the chick has a greater tendency to peck at objects that have a similar appearance. Repetition of the same act usually leads to the formation of a habit. Habits often appear stereotyped but differ from in- stincts in that they have to be learned and are not inherited. Insight learning involves an apprehension of rela- tions and the sudden adoption of an appropriate re- sponse without previous trial and error behavior. The mason wasp of India builds a cluster of clay cells. After depositing an egg in each cell, the female fills it with caterpillars and seals it with a lid. Even- tually the whole cluster is covered with a layer of clay. While a wasp was away hunting for its prey, an experimenter made a large hole in the side of a cell. On its return, the wasp put in a caterpillar which fell out through the hole. A second caterpillar stuck in the hole with a large part hanging out through it. When the cell was completely provi- sioned, the wasp appeared to notice the hole for the first time and carefully examined it. With great and SAECO: — eit ded MALE | FEMALE Zigzag dance<—————Appears leads SE shows nest oo ar eS nest fertilizes<——€ spawns courts FIG. 2-7 Courtship and mating behavior of the three-spined stickleback (after Tinbergen 1951). The general nature of responses 15 prolonged effort she managed to stuff the caterpillar back into place. She then collected a pellet of clay and mended the hole. Such behavior as this involves an apprehension of relations and a sudden adaptive response not preceded by trial and error. Insight learning may be manifested in various ways as through homing ability, detouring around obstacles, tool-using, discrimination of forms and patterns, and so forth. Ecological life histories Developmental life histories trace the origin and growth of structures and functions of an animal from the egg stage until maturity is reached. Such studies are largely embryological in nature. Behavior life histories attempt to analyze the activities of animals in terms of innate and learned behavior, and the neural mechanisms involved. In order to do this, it is often necessary to trace the origin of each activity to the manner in which it first makes its appearance in the young animal. Ecological life histories, on the other hand, are concerned with the activities of a species throughout its life cycle, and in relation to its adjustments to natural conditions. Ecological life histories usually proceed with, first, analysis of the behavior adjustments needed for the survival of the mature animal; then of its reproductive behavior ; and, lastly, of the development of behavior and physio- logical adjustments of the young animal. In general, the proper procedure is... to discover and estab- lish correlations between the behavior of the organism and the conditions in its environment, and then to test the significance of the correlations by appropriate experiments in nature or in the laboratory. The point should be emphasized that you start with nature, that is, with the organism in its environment. Also it should be noted that morphology and physiology of the organism are entirely subsidiary matters, al- though most important to the person interested in knowing how the organism behaves as it does... (Huntsman 1948). The behavior of a species in re- lation to its environment is called its mores (Shelford 1913). The following are important items that should be included in a complete ecological life history of a species : 1. Phylogenetic and geological history. 2. Geographic and habitat distribution with an analysis of adjustments to the physical en- vironment and of biotic interrelations within the community. 3. Variations in population, through time and in space. 4. Changes in seasonal activities and physiologi- cal states: breeding, migration, hibernation. 16 Background STN 10. Useful outlines, methods, and bibliographies for | Food, enemies. Parasites, diseases. Reproductive potential, mortality, rate of pop- ulation turnover. Requirements for reproduction: home range, territory, nest-site, nesting materials, etc. Breeding behavior: mating, nesting, etc. Development of offspring: rate, stages, gen- erations per year, etc. ecological life history studies of different kinds of animals and plants have been published in the scien- tific periodical Ecology since October, 1949. Ecological niche The ecological niche is the particular position in a community and habitat occupied by an animal as the result of its peculiar structural adaptations, its physiological adjustments, and the special behavior patterns that have evolved to make best use of these potentialities. Important factors in the niches occu- pied by white-footed mice and deer mice are de- scribed in Table 2-1. Both mice are equipped with large eyes for nocturnal vision, large external ears for hearing, long vibrissae on the face for aid in run- ning through dark underground burrows, and pro- tective coloration. P. 1. noveboracensis has a longer tail than P. m. bairdii, which appears to be an adap- tation for climbing. It is possible that these two species are segregated into different niches because bairdii is more tolerant of extreme temperatures and low moisture conditions, and hence is more prevalent than noveboracensis in the exposed grassland habitat, but is unable to displace noveboracensis within the forest because of the latter’s tree climbing ability. Every species has its own peculiar niche. No two species can permanently occupy exactly the same niche in the same locality. The living together of many species in the same community is possible only because their various niche requirements are differ- ent. The analysis of the critical factors in these niche requirements is often very difficult but is one of the main objectives of ecology. COMMUNITY INTERRELATIONS The fact that species with similar tolerances and requirements aggregate into similar environ- ments to form communities is a response of special interest. No organism occurs alone. Each must find its place in the community and establish relations with other members of it. The manner in which the response of species to each other is affected is shown in the structure and composition of the community TABLE 2-! Comparison of niches of the white-footed and deer mice, both species of the genus Peromyscus. Factor P. leucopus noveboracensis P. maniculatus bairdii Vegetation, substratum or Space occupied Microclimate Enemies stratum where food found Reproductive site; nesting Surface of ground materials logs, stumps, or tree cavities Diel activity Nocturnal Seasonal activity Deciduous forest; subterranean, terrestrial, arboreal; home range 0.12 hectare Shade, rich humus, moderate moisture, medium temperature Food Seeds, nuts, insects Owls, foxes, weasels, shrews Nests of leaves in burrows, Active throughout year Sparse grassland; subterra- nean and terrestrial only; home range 0.24 hectare Sunlit habitat, low moisture, temperature often extreme Seeds, grass, insects Owls, foxes, weasels, shrews Surface of ground Nests of dried grass in bur- rows, crannies, or clumps of grass Nocturnal Active throughout year and in its internal dynamics, succession, and distri- bution. The analysis of the community responses and interrelations of organisms is a major objective of this book. SUMMARY The environment, or specifically the habi- tat, of an organism consists of the physical conditions that surround it. In order to live in a particular habi- tat, an organism must be morphologically adapted to it. This may be accomplished to a certain extent dur- ing growth, especially in sessile forms, but depends mainly on long evolutionary processes of variation and natural selection. Each organism must also be physiologically adjusted to the various factors of its environment. Species vary in their limits of toler- ance, and those factors in their surroundings that are most immediately unfavorable limit their habitat dis- tribution. In order for an organism to take advan- tage of its morphological and physiological adjust- ments, it must have the proper behavior responses. These inherited and acquired action patterns involve selective orientation in response to environmental stimuli. Occurrence of different species in the same habitat necessitates the establishment of compatible community interrelations. The general nature of responses 17 The Biotic Community— Structure and Dynamics The concept of the biotic community is basic to an understanding of ecology. We will here be con- cerned only with laying a foundation of general prin- ciples. Details will come in later chapters, but for proper orientation we must know something about how the community is organized, how it functions, and how it may be recognized. INTERNAL STRUCTURE AND PROCESSES Community and ecosystem A community, or biocenose, is an aggregate of organisms which form a distinct ecological unit. Such a unit may be defined in terms of flora, of fauna, or both. Community units may be very large, like the continent-wide coniferous forest, or very small, like the community of invertebrates and fungi in a de- caying log. The extent of a community is limited only by the requirement of a more or less uniform species composition. A different community occurs in each different habitat and environmental unit of larger size, and in fact the composition and character of the community is an excellent indicator of the type of environment that is present. Since plant communities and animal communities occur together in the same habitat and have many interrelations, the one can scarcely be con- sidered independently of the other. Together they make up the biotic community, and the biotic com- munity along with its habitat is termed an ecosystem (Tansley 1935). The ecosystem is the best unit for the study of the circulation of matter and flow of energy between organisms and their environment. Communities may be distinguished as major or minor. Major communities are those which, together with their habitats, form more or less complete and self-sustaining units or ecosystems, except for the indispensable input of solar energy. Minor communi- ties, often called societies, are secondary aggregations within a major community and are not, therefore, completely independent units as far as circulation of energy is concerned. When in this book communi- ties are spoken of the reference is to major communi- ties unless otherwise indicated. Dominance When a number of species come together to form communities, each fits into a different niche and plays a different role in the internal dynamics of the community. Dominance is the relative control ex- erted by organisms over the species composition of 18 the community. Species exerting this important con- trol are called dominants. Plants are more frequently dominant in terrestrial communities than are animals. In aquatic communities, animals are relatively more important in this role, although dominance is often not developed. Dominance is most commonly expressed in the reactions of an organism on its habitat (Clements and Shelford 1939). Dominants shoulder the full im- pact of the climate or the environment but modify this effect for other organisms within the community by tempering light, moisture, space, and other con- ditions. Only those other organisms that find these modified physical conditions tolerable can exist within the community. Furthermore, dominants are ordi- narily the most prominent species in the community, make up its greatest mass of living material, and serve as the major source of food, substrate, and shelter for the animals that are present. In a forest community, trees are dominant. They decrease light intensity, increase the relative humidity, intercept precipitation, monopolize most of the moisture and nutrients in the soil, decrease wind velocity, and furnish shelter and food for animals. Grasses play a similar, though less conspicuous, role in prairie com- munities ; sedges, rushes, and cattails in marsh com- munities ; sagebrush in the arid habitat of the Great Basin; mussels and barnacles on a rocky seashore; and so forth. Sometimes dominance is demonstrated in coac- tions, direct effects of organisms on each other. Insome fresh-water ponds, carp and suckers may consume much, perhaps all, of the submerged vegetation. This coaction thus prevents the plant constituents from assuming their usual role in the community, and by so much prevents the occurrence of animal species that depend directly upon the plants. These fish also react upon the habitat by stirring up the bottom, from which they derive organic matter, thereby greatly in- creasing the turbidity of the water. Penetration of light into the water becomes poor, greatly handicap- ping sunfish, bass, and other species which locate food visually (Table 7-3). In primeval days, bison on our western great plains fed on the luxuriant taller grasses more ex- tensively than on the short grasses, with the conse- quence that, over extensive areas, short grass species replaced tall grasses almost entirely. Thus bison were coactant with and dominant over the composi- tion and character of the community (Larson 1940). In a similar manner, when European meadow voles are numerous they reduce the vigor and prevalence of the grass dominants in consequence of their feed- ing and tunnelling in the ground, and angiosperms which are normally absent or scarce appear (Sum- merhayes 1941). Overpopulations of the European rabbit alter the character of the forest by frustrating reproduction of oak, beech, and hornbeam, upon the seedlings of which they feed to the exclusion of other species. When introduced into Australia, the Euro- pean rabbit converted grassy areas into desert-like tracts (Bourliere 1956). Although animals are more common coactors than plants, plants may occasionally exert dominance in this way. As a notable example, chestnut blight (a fungus) virtually eliminated the chestnut tree from the deciduous forest of eastern North America during the first few decades of the twentieth century. This fungus infects the cambium, forms pustules under the bark, and causes the bark to fall off and the leaves to wilt. The blight has eliminated the chestnut from the community, and the consequent opening up of the canopy has allowed the extensive invasion of new species of shrubs, herbs, and animal inhabitants. Trees are the dominants in a forest community, but species in the lower stratum of shrubs modify the habitat still further and even the herbs exert some control over the physical conditions on the surface of the ground. A subdominant species must tolerate the conditions established for it by the dominants; but it in turn is a modifier of the community composi- tion in a secondary manner. Influence By influence is meant effect upon the abun- dance, health, and activities of other organisms in the community but not to the extent of directly excluding species. Influence is conspicuously expressed through coactions, but it may be effected through reactions as well. Insects may partially or wholly defoliate a tree; a pack of wolves may diminish a population of deer over winter ; squirrels may bury acorns and nuts and thereby aid germination of them; parasitic or poisonous plants may lower the vigor or destroy the life of some other plants or animals; animals may burrow into the soil and thereby increase percolation of water and air, a benefit to plants; and all organ- isms, upon death, add organic matter to the habitat. These and other actions influence the community, but unless these influences become extreme they do not absolutely determine whether or not other species will occur in the community. Influence, then, is of essentially the same nature as dominance but is less vigorous in the modifying role that it plays. Evaluating and classifying animals ecologically One of the most important, yet difficult, prob- lems in ecology is evaluation of the roles the different kinds of animals play in community dynamics. The biotic community 19 A basis for classifying species is exclusiveness, fidelity to the community. A species is exclusive when it occurs only in a single area, habitat, or com- munity ; characteristic (selective, preferential) when it is abundant in one area or community but also oc- curs in small numbers elsewhere; and ubiquitous (indifferent) when it is found more or less equably distributed in a wide variety of communities. The terms given in parentheses are synonyms used by plant ecologists (Braun-Blanquet 1932). Exclusive species are often rare and of little importance in the dynamics of a community, but when they are con- spicuous they often make useful indicator species for identifying and recognizing community units. The recognition of characteristic species presents special difficulties, since one must decide how much more abundant a species needs to be in one com- munity to be sure a definite preference over another is indicated. In a distributional study of breeding bird populations in Ontario (Martin 1960), a species was considered characteristic of one type of vegeta- tion if the species was at least three times more abun- dant in it than in any other type of vegetation. This was at population levels of from 1 to 9 pairs per 40 hectares (100 acres). For species reaching popula- tion densities of from 10 to 100 pairs per 40 hectares, preference was considered demonstrated if the species were twice as abundant in one type of vegetation as in any other. For populations greater than 100 pairs per 40 hectares, differences of 50 per cent are prob- ably significant. It seems logical that a stricter test should be applied to small populations, for errors in measuring the size of populations and random popu- lation fluctuations attributable to factors other than choice produce a relatively greater disturbance in the data. An experimental study in measuring foliage insect populations also indicated that populations differing by a ratio of 3:1 could be accepted as sta- tistically significant (Graves 1953). When the bot- tom fauna of two ponds were sampled, true differ- ences could be detected at minimum ratios between their populations of 1.9 (Hayne and Ball 1956). A species, to be termed characteristic, should also be well distributed through a community, this to be in- dicated by its occurrence in at least 50 per cent of all samples taken (Thorson in Hedgpeth 1957). Another criterion for evaluating species is by numbers of individuals present. Other things being equal, a species in time of high population affects other organisms to a much greater extent than it does at times of low population. A species that is perma- nently more abundant than another will consume more food, occupy more nest-sites, and demand more space ; hence its influence will be greater. Predomi- nants are the more numerous constituents of a com- munity, in contrast to members, which are species of lesser importance. The dividing line between these two categories is an arbitrary one. 20 Background The time and duration of occurrence of a species in a community affects the amount of influence it ex- erts. Generally, the longer the yearly period during which a species is active, the more important its role becomes. Species may be classified on a temporal basis into perennials, those which are active in a community throughout the year, year after year; seasonals, which are present or active only part of the year ; and cyclics, which may be very important some years but of negligible importance other years, as evidenced by their wide fluctuations in numbers. Even though present, a species is usually considered inac- tive when it is hibernating or dormant or when it is represented only by eggs, spores, or encysted stages of its life cycle. The effect produced on the community by indi- viduals and species may be modified by the way they form secondary groupings within the community. These minor aggregations of plants and animals are called societies and are of various sorts (Shelford 1932). Layer societies occupy different strata, such as the subterranean, ground, herb, shrub, and tree societies in a forest; local societies are usually parts of layer societies but are more confined in area, as groups of animals occupying an ant hill, a rotting log or stump, or a restricted but distinctive area of ground; and seasonal societies include all the organ- isms at particular times of the year. Other factors that affect the influence of a species in the community are the size of individuals, their metabolism, food habits, and general behavior. A moose consumes more food than a mouse, and a warm-blooded mouse more than a cold-blooded sala- mander of the same size. A carnivore at the top of several food-chains affects the lives of more different species in the community web of life than a herbivore feeding on plants. Burrowing rodents react on the habitat more than do most birds. One factor may cancel another. An individual of a perennial species of carnivorous mammal certainly eats more than an individual cold-blooded herbivorous insect of small size and active only during the warm season. Yet there may be 1000 insects to one mammal, so that in the aggregate a single species of insect may actually produce more disturbance than a single species of mammal. The difficulty of evaluating the relative effects of species is partly alleviated by calculating their respective biomasses and energy requirements. The biomass of a species is the average weight or volume of an average individual, multiplied by the total number of individuals present. The computa- tion of the biomass of each species thus corrects for differences in size and number of individuals between species. Because of differences between species in body moisture and amount of inert substances such as endoskeleton, chitin, shell, and the like, biomass is expressed with greater accuracy in terms of dry weight than wet weight or volume, and is even more accurate if given in terms of carbon or nitrogen con- tent, or calories. Attempts have recently been made to compute more significant biomasses of bird populations, using physiological constants. A biomass composed of few but large individuals has a lower metabolic activity than an equal biomass composed of a large number of small individuals. In one study (Turéek 1956), the importance of different species in the community was evaluated in terms of the total body surface area rather than total weight presented by each species, using the formula N + 10 + 17-87, where N is the num- ber of individuals and ” the average weight of the species. In another study (Salt 1957) the number of individuals was multiplied by the mean weight of the species raised to the 0.7 power (N -1’°-7). One gets the best evaluation of the importance or influence of a species in a community where metabolic activity can be measured directly and expressed in terms of calories per unit of time (Macfadyen 1957, Teal 1957). Productivity A characteristic of communities that has be- come of considerable importance in modern ecological research is productivity. The number of individuals or biomass present in a community at any one time is the standing crop. At the beginning of the year or reproductive season the standing crop is usually small, but as reproduction and growth take place there is an increase in the amount of organic matter making up the biomass of the community. The pro- duction of organic matter per unit of time and area is productivity. Productivity is commonly indicated on a yearly basis, but it is also possible to measure monthly, weekly, or daily production. Small standing crops may have a high productivity and large stand- ing crops a low productivity, hence average bio- mass or standing crop differentials between different communities is not comparative of the productivity of the habitats in which these communities occur. The largest standing crop which a habitat can sup- port without deterioration, or the maximum num- ber of biomass of animals that can survive least favor- able yet tolerable environmental conditions during a stated period of time, is the carrying capacity. Carry- ing capacity is determined not just by the amount of food available, but also by shelter, social tolerance, and other factors (Edwards and Fowle 1955). A variety of methods are being used to measure pro- ductivity of different kinds of organisms and of dif- ferent habitats. It is desirable to indicate productivity as accurately as possible in descriptions or analyses of community dynamics. SUCCESSION Communities are in a more or less continual process of change (Clements 1916). These changes result in part from the reactions and coactions of the organisms themselves and in part from such external forces as changing physiography, changing climate, and organic evolution. The habitat is usually affected as well as the community, and as the habitat changes, new species invade it and become established, and old species disappear. These changes are especially no- ticeable in dominant species, since these species exert a controlling role over the composition and structure of the community as a whole, The replacement of one community by another is succession, and succession continues until a climax or final stage is reached. Succession is a process. The series of steps or communities comprising a successional sequence lead- ing to the climax is the sere. Seres are sometimes classified according to the predominant force that is bringing them about. These forces are biotic, cli- matic, physiographic, and geologic and their resultant seres are commonly called bioseres, cliseres, eoseres, and geoseres. Biotic succession Biotic succession is brought about by forces inherent within the community and in the activities of the plants and animals themselves. The most im- portant of these activities are the organismal reac- tions and coactions that produce modifications in the habitat and interrelations between species. Important reactions involve filling in of ponds with plant and animal remains, the addition of organic nutrients to sterile soil, and the reduction in light intensity by increasing density of plant growth. With progressive improvement of the soil and changing light and mois- ture conditions, a series of new dominants come into the area. When invasion of new species occurs, in- tense competition develops; if the invaders are suc- cessful, the old species disappear as a new community replaces the old one. Contributing factors that may be involved are dif- ferences in growth and dispersal rates, which are different for different species. After a forest fire or logging operation, herbaceous plant growth is im- mediately stimulated ; since herbs grow rapidly, they become dominant within a year or two. Shrubs begin to spread, and tree suckers or seedlings also appear quickly, but because they require a longer time to reach maturity several years may elapse before they gain control of the area. Succession is considerably influenced by the kinds of propagules available in the vicinity. Seeds, spores, and the like are dispersed more or less readily, de- pending on form. Some kinds of animals roam more The biotic community 21 ; 2 : oe ae ™ Sa oh 4 Nae FIG, 3-1 Early stages in the pond sere: open water, floating stage, emergent vegetation, swamp shrubs. Everglades National Park, Florida. widely or spread more readily into new areas than do other kinds. The composition of the community that develops and the rapidity with which it becomes established depends, in large part, on the rates at which different species invade. When the volcanic island of Krakatoa in the East Indies blew up in 1883, all plant and animal life on it and on two adjacent islands was destroyed. The following year, the only living animal reported was a single small spider. In 1886, it was apparent that the pioneer plant stage consisted of a crust of blue-green algae covering the lava. A few mosses were also present, as were many ferns, and a scattering of some 15 species of flowering plants including 4 species of grass. The vegetation stage following the algae con- sisted predominantly of ferns, but the grasses had become dominant over most of the island. By 1906, woodland had appeared, which has since de- veloped into an increasingly luxuriant mixed forest. Dispersal of seeds, spores, and other propagules was effected by wind, sea, animals, and man, in that de- 22 Background scending order of importance. The order in which the propagules reached the islands greatly influenced the succession that occurred; as vegetation developed it reacted on the soil and habitat, bringing about con- ditions amenable to the return of the tropical rain forest (Richards 1952). Very little study of animal life was made until 1908, and then only for three or four days. At that time, 202 species were found on Krakatoa and 29 on a nearby island. There were no earthworms, snakes, or mammals present, but there were many spiders and centipedes, a number of insects, 2 species each of land snails and lizards, and 16 species of birds. A more thorough survey in 1921 revealed 770 species of animals including rats, apparently introduced in 1918, and bats, first noticed in 1920. In 1933, 1100 species were found, including 3 species of earth- worms, but true forest mammals had not yet appeared and many families of forest birds were unrepre- sented. As with plants, the invasion by animals de- pended on wind, sea, and man and other animals, in that descending order of importance. Survival and establishment of the animal species was correlated with the stage of vegetation that was reached. It is of interest, however, that scavenger species appeared first, then omnivores, herbivores, and finally preda- tors and parasites. Succession of animals depended in large part on the speed with which they reached the island and on their finding proper food and shelter (Dammerman 1948). Bioseres may be broadly grouped as priseres and subseres, depending on whether they develop on pri- mary or secondary bare areas. A primary bare area is a sterile habitat, such as rock, sand, clay, or water. A secondary bare area is a denudation resulting from temporary flooding, fire, logging, cultivation, over- grazing, or other phenomenon that does not produce an extreme disturbance of the soil or substratum. Since in the latter the habitat has already sup- ported community life, and since the soil or sub- stratum is already in an advanced stage of develop- ment, the resulting subsere progresses rapidly and the early pioneer stages of the prisere do not usually oc- cur. A subsere will develop following the destruction of any stage in a prisere, and the species composition of the stages in the subsere will be influenced by the particular priseral community that was destroyed. The early stages or communities that make up the prisere depend largely on the type of bare area on which the prisere originates. As succession proceeds, however, later stages in the various seres in any area having a relatively uniform and humid climate come to be more and more alike. The successional devel- opment from widely diversified communities in ini- tially different habitats to closely similar or identical climax communities in habitats that have also become much alike is called convergence. , Bioseres may occur on a small scale in microhabi- tats as well as in major ones. When hay infusions, prepared in the laboratory, are seeded with repre- sentative protozoans, the order of appearance of maximum or peak populations in the various species is bacteria and monads, Colpoda, hypotrichates, Paramecium, Vorticella, and Amoeba. Disappear- ance of species is in the same order, except that Amoeba precede Paramecium and Vorticella. Algae may come in at the final stage, so that a more or less balanced community is established. The succession of species appears a result of the higher reproductive rate of earlier species, and to the fact that the excreta of at least some forms, especially the hypotrichates and Paramecium, are toxic to them (Woodruff 1912, 1913, Eddy 1928). Another common microsere occurs in the death and decay of trees (Graham 1925, Ingles 1931, Savely 1939). The sequence of animal species pres- ent as decay progresses depends on the species of tree, the community in which the tree occurs, the climate, and the geographic locality. The following stages have been recognized: 1) tree dying, but still with leaves and sap; 2) tree recently dead, bark beginning to loosen, termites and other insects boring into wood ; 3) wood well seasoned, bark very loose or off, s/<\ >| Q Birch 35% @ Hemlock 27% zs @ Pine 26% 3 “TA Q Birch 35% Pine 41% 8 \ QOok 8% %, Maple 3% \) Ne oe A Birch 47% — % Jockpine CDSprce 6% =D Fir ~ at ens warmer i ‘ es This type of forest is today in Upper Penn. Mich., N.Wisc., North N.J., and S. Maine Wormer Cold and moist as in N. Quebec today YN Af J WN at Aa wood borers still predominant ; 4) wood softened and permeated with fungus ; fungus beetles, elaterids, and passalids common; 5) wood largely disintegrated and crumbly, snails and millipedes, occur. Wilson (1959), working in New Guinea rain forests, sub- divides stages 2-5 in a different manner, each of which he names after characteristic insects found: 2) scolytid, 3) cucujid, 4) zorapteran, 5) passalid, and 6) staphylinid. Each stage also has a significantly different aggregation of resident ants. Eventually the decaying log becomes a part of the forest floor, and the animal species then present are those in general occurrence. Climatic succession With changes in climate, environmental condi- tions often surpass the limits of tolerance of estab- lished plants and animals. The result is the replace- ment of the existent community by another. A most interesting clisere is the one that has oc- curred since the northward retreat of the continental glacier of Pleistocene time (Sears 1948, Deevey 1949, Table 21-1). Stages in this clisere may be detected FIG. 3-2 The types of trees which have lived and died during the past few thousand years in Quebec. Instrument taking bor- ings of lake bottom for pollen samples is operated from a boat. Symbols representing different pollen grains and the percentage which each species constitutes in the total are shown at the left. The type of climate indicated by the prevailing vegetation at the time is shown at the right (Wilson 1952). The biotic community 23 and even the relative duration of each stage measured from an analysis of the number and persistence of different kinds of pollen grains at various depths in peat bogs. To make such studies, a core of peat is obtained from the deepest part of the bog by means of a special hand auger. The lowest portion of the core is the oldest ; the most recently formed is at the top. Samples of the core at various depths are suit- ably prepared, examined under a microscope, and the pollen grains identified and counted. The predomi- nant kind of pollen at any level of the core represents the probable prevailing species of plants in the vicin- ity at the corresponding period of time, although the proportionality between all kinds of pollen and abun- dance of the various species may not be exact (Davis and Goodlett 1960). Thus, during the last 20,000 years, the clisere in eastern North America is repre- sented in simplified form by the following climaxes and climates, reading downward to present time: spruce, fir cold, moist pine cool, dry hemlock, oak, beech warm, moist oak, hickory warm, dry beech, oak, hemlock cool, moist The climate is conjectured from the relation of similar flora to climate at the present time. The com- plete clisere occurs only in regions near the southern limit of the reach of the glacier. The later stages have not developed in more northern localities where the glacier has been gone for a shorter time and where the climate has not warmed up sufficiently. Climatic succession actually occurs at all levels in the biosere, as the seral stages leading to one climatic climax is replaced by the corresponding stages leading to an- other climax (Table 7-6). Physiographic succession Changes in the earth’s surface bring a change of communities. The sea alternately inundated and retreated from the Atlantic coastal plain during the Pleistocene as ice, in which large amounts of water were tied up, alternately melted and formed. In earlier times, the sea inundated much of the conti- nental interior, and on its recession the eosere of plant and animal communities which developed cov- ered vast areas. Mountain-building brings the replacement of low- land communities with new ones that invade at higher elevations. As mountains erode, the eosere progresses in the opposite direction, until base-level or pene- planation is attained. The development that the eosere will undergo with continued erosion is some- times locally apparent in the difference between up- 24 Background land and floodplain forests. Stages in the erosion cycle, as it occurs in a stream, may be discerned by examination of habitat and animal life progressively from headwaters to mouth. Geologic succession The evolution of new forms of life and dis- persal of them through the world entails replacement of pre-existing forms and gradual change in the com- position and character of communities. The first or- ganisms to appear on earth were unicellular forms confined to the sea (Table 3-1). During the Cam- brian and Ordovician periods, the marine animal life differentiated rapidly into a rich variety of inverte- brate types and the anlage of vertebrates. The Si- lurian and Devonian periods are noteworthy for the invasion of fresh water and land by both animals and plants. Fishes became predominant both in fresh water and the sea. During the remainder of the Paleozoic era, a luxuriant flora evolved, especially in swampy areas. Modern conifers, such as spruce, fir, juniper, tamarack, cypress, and yew made their ap- pearance. Amphibians became the predominant ad- vanced animal types, and a diversified invertebrate fauna occurred in all habitats. In the Mesozoic era, the existing land flora of giant rushes, tree-ferns, and cycads gave way to for- ests of hardwoods which then spread over the world. Conifers persisted. The earliest woody angiosperms probably originated in the Jurassic and included sas- safras and poplar. The forests soon contained elms, oaks, maples, and magnolias. Herbaceous angio- sperms, such as grasses and sedges, appeared towards the end of the era but did not become important in North America until the drying up of the interior of the continent in the middle Cenozoic. Although the Mesozoic is predominantly the age of the giant rep- tiles that lived on the land and in the water and flew through the air, less conspicuous types such as the toothed birds, archaic mammals, and insects were de- veloping rapidly. At the beginning of the Cenozoic era, ancient types of animals, including the great reptiles and many types of invertebrates, became extinct and mam- als rose to predominance. It is significant that the rich development of mammals, birds, and insects came after the worldwide establishment of the angiosperms with their rich nutrient seeds, fruits, and grasses. Pleistocene glaciation brought a major change in the habitat of these animals, and some large mammals disappeared. The last stage of the geosere, the Re- cent epoch, brought the dominance of man. Only future ages will determine whether this stage is cli- max, or whether new and different types of animal and plant life will someday evolve to replace man and TABLE 3-1 Major steps in the geosere of the earth, especially as it applies to North America (modified from Dunbar Cosmic ora: 3-5 billion years ago; tidal disruption of an ancestral sun and origin of earth. Azoic era: formation of a stable cold exterior shell to the earth; origin of oceanic de- pressions and continental platforms; first formation of water and a thin atmosphere. Archeozoic era: first plant life - bacteria, marine algae, (elevation of Laurentian Up- lands and peneplation of continents). Proterozoic era: marine algae abundant; animal life chiefly sponges and segmented marine worms, (peneplanation of continents). Paleozoic era: 500 million years ago. Cambrian period: marine invertebrates only. Ordovician period: marine invertebrates continue predominance; rise of armored fishes. Silurian period: first invasion of land by plants; rise of air-breathing scorpions and millipedes and of fresh-water fishes. Devonian period: first forests and extensive land floras; diversification of fresh- water fishes, rise of labyrinthodont amphibians, and increase in land fauna, especially spiders, mites, and wingless insects. Mississippian period: increase of amphibians. Pennsylvanian period: luxuriant swamp floras cosmopolitan in distribution, mostly of spore-bearing types; fresh-water clams and amphibians abundant and on land, giant insects, spiders, centipedes, snails, and first reptiles. Permian period: (elevation of Appalachian and Ouachita Mountains); decline of ancient flora and rise of conifers; modern insects and advanced types of amphibians and reptiles appear. Mesozoic era: 200 million years ago. Triassic period: (desert climates); plants mostly rushes, ferns, cycads, conifers; stegocephalian amphibians and dinosaurs numerous, archaic mammals appear. Jurassic period: reptiles evolve higher and more diversified forms, first toothed birds and frogs appear. Lower Cretaceous period: woody Angiosperms spreading over world. Upper Cretaceous period: (great inland seas, warm climate world-wide); modern genera of deciduous hardwood trees predominant, sedges and grasses appear- ing; clams and snails common in fresh-water, culmination of dinosaurs, toothed birds, archaic mammals (elevation of Rocky Mountains). Cenozoic era: 60-70 million years ago. Paleocene and Eocene epochs: (inland seas recede, Appalachian region pene- plained (Schooley) but later again uplifted, climate warm and humid) hardwood forests predominant, palms abundant; modern mammals and birds replace archaic forms. Oligocene epoch: 40 million years ago (continent peneplained); turtles, alligators, croco- dile at maximum. Miocene epoch: 29 million years ago (western mountains becoming elevated, climate turning drier and colder); grasses disperse over open plains; insects reach full de- velopment and mammal fauna expands - Pliocene epoch: 12 million years ago (continued elevation of western mountains, espe- cially Sierra Nevadas; lower Great Basin becomes arid); grasslands become exten- sive and desert vegetation develops in southwest; mammals at maximum and man- ape changing into man. Pleistocene epoch: 1 million years ago (continental glaciation); great mammals disappear. Psychozoic era: 25-30 thousand years ago (glaciers recede). Recent epoch: man becomes predominant, rise of civilization. Note: The epochs, Paleocene to Pliocene inclusive, are often grouped and designated the Tertiary Period, and the Pleistocene and Recent epochs the Quaternary Period. 1949). to continue the succession into the indefinite future. As higher types evolved in each taxonomic group, primitive forms mostly died out. The first primitive wingless insects appeared in the Devonian; in the Pennsylvanian there were giant forms of primitive dragonflies, cockroaches, and grasshoppers. One cockroach had a wingspread of nearly 12 cm. By the Permian, these giant forms disappeared and were re- placed by many modern orders. During early Meso- zoic, most modern families of insects were estab- lished, and by the Upper Cretaceous many modern genera are recognizable. Of the two great groups of warm-blooded ani- mals, the earliest mammals had originated by the Triassic. Modern orders did not become well differ- entiated until very late Cretaceous or early Paleocene, modern families by the Oligocene, and modern genera by the Pliocene (Simpson 1953). 25 The biotic community Toothed birds appeared in the Jurassic and dif- ferentiation of types went on rapidly as modern orders were already represented in the Lower Cre- taceous, modern families in the Eocene, modern genera in the Miocene, and modern species in the Pleistocene. The great group of songbirds evolved and dispersed through the world rather late, perhaps in the Miocene; since the Pleistocene epoch, evolu- tion of birds has been largely limited to subspecia- tion. An important aspect of the geosere is the dispersal of new types of animals and plants, for the impor- tance of a new form depends on the extent of its dis- tribution and the size of its population. A successful species saturates the available niches at its center of origin and thence spreads outward in all directions. Dispersal continues until an impassable barrier is reached. Tracing the origin of taxonomic groups, their phylogenetic relations with other taxonomic groups, and their dispersal is the subject matter of zoogeography. This knowledge is desirable for the ecological interpretation of communities, for it helps to explain their species composition and the geologi- cal history of the community itself. The mechanics of evolution at the species level is called speciation, and is as much a problem of ecol- ogy as it is of zoogeography and genetics. Speciation in animals is initiated only when one population be- comes isolated from another similar population so that interbreeding does not occur. This permits vari- ation and natural selection to proceed independently in the two groups and to become fixed in the germ- plasm. Isolation is usually effected by geographic barriers and generally involves occupancy of new niches and development of new coactions with other members of the community. The history of the past ecological relations of species and of whole communi- ties is the subject matter of paleoecology. The dy- namic forces involved in these processes of speciation are among the determinants of the geosere. The climax We have described four types of succession as if each were entirely independent of the other three. This is not the case. All types of succession are going on simultaneously, although the relative importance of any one varies from one habitat to another. Biotic succession is most conspicuous, since it proceeds most rapidly and appears to reach a final, permanent stage in just a few decades or centuries. The climax stage of the biosere is undoubtedly more nearly stabilized, self-maintaining, and in steady state in its particular habitat than are the seral stages, yet it also is sub- ject to gradual change over long periods of time. The climax, as well as the seral stages, changes with cli- mate, physiographic forces, and evolutionary proc- 26 Background esses. However, the clisere usually requires a few thousands of years before changes in the community structure or composition become evident. The prog- ress of an eosere is even slower; geosere, slowest of all. The climax is defined as the last stage in the biosere; no absolute stability or final permanency should be construed, since it is simultaneously a stage in the clisere, eosere, and geosere. The climax may be recognized by the fact that in a uniform climatic area all seres tend to converge into it, and by its steady state in respect to structure, species composition, and productivity. In the climax community, all species, including the dominant spe- cies, are continually able successfully to reproduce and there is no evidence that new and different species are invading. In seral communities, on the other hand, the developing new growth, particularly evident in the dominants, contains many individuals of invading species which will eventually take over and replace species already present. RECOGNITION OF COMMUNITIES The community as an organic entity Although the major community or ecosystem is the generally accepted unit of analysis in syneco- logical studies, there is a difference of opinion as to whether the community constitutes a discrete organic entity. Two different points-of-view are incorporated in the organismic and individualistic concepts which are usually associated with the names of F. E. Clem- ents (1916) and H. A. Gleason (1926), respectively, and more recently Phillips (1934-1935), Tischler (1951), and Emerson (1952) on one side and Bodenheimer (1938), Whittaker (1951, 1952, 1956, 1957) and Curtis (Brown and Curtis 1952) on the other. Ramensky (1926) stated the individualistic concept independently in Russia as early as 1924. The organismic concept considers the community to be a supraorganism, a complex organism, or a so- cial organism. As such, it is the highest stage in the organization of living matter ; namely cell, tissue, or- gan, organ system, organism, species population, community. There is emergent evolution, so to speak, at each higher stage in this hierarchy; the whole is more than merely the sum of its parts. Tissues have properties, characteristics, and functions over and above those of the individual cells involved; the organ, the organ system, or whole organism func- tions in a way not to be predicted from a knowledge of the parts of which each consists. The species pop- ulation has inherent characteristics of density, rate of natality, rate of mortality, and age distribution, while the total community has such unique functions as dominance, cooperation, trophic balance, competition, and succession which are beyond the characteristics of the individual organisms of which the community is composed. The community behaves as a unit in its competitional and successional relations with other communities, in its local and geographic distribution, in its seasonal activities and response to climate, and in its evolution, Although the community varies in its taxonomic composition and structure in different en- vironmental situations, this variation is proportion- ally no greater than occurs in different cells of the same type or between individuals belonging to the same species. The individualistic concept places emphasis on the species, rather than the community, as the essential unit for analysis of interrelations, activities, distribu- tion, and evolution. Each species responds independ- ently to the integrated influence of the various factors of the physical environment and biotic coactions. The environment may be conceived as a pattern of gradients with the intensity of the various factors changing gradually in space from one extreme to the other. The gradient may be a short one, as from the subterranean to the tree stratum in a forest or from the open water of a pond to a nearby swamp or cli- max forest, or it may be longer, as from the bottom to the top of a mountain or even from the tropical to the arctic zones of a continent. The population density of each species is distributed in a form re- sembling a normal curve when plotted along the gradient of a given factor, and the curves of many species in relation to various environmental gradients overlap in a heterogeneous, and apparently almost random, manner. There is seldom agreement be- tween the limits of the distribution curves of any two species; species are not in general bound together into groups of associates which must occur together. Furthermore, the vegetation and its associated animal life very often form a continuum of gradually chang- ing composition and complexity from one extreme of the environmental gradient to the other. A vegeta- tional continuum has no sharp boundaries between individual communities of different types, and the ecologist must choose the manner in which he dis- tinguishes these units so as best to suit his interests and objectives. These two points-of-view are not necessarily in- compatible. There is no doubt that each species is distributed according to its own physiology, its own complex interrelations with other species, and its own tolerances, and that no two species are exactly alike in their responses to the environment. On the other hand, one can be convinced that the community and its habitat, collectively the ecosystem, is a functional system and that every species of necessity occurs in and as a part of such a system so that its distribution is importantly modified by these interactions and community relations. The community is usually recognized and identi- fied by its most important organisms, the dominants and predominants (Shelford 1932). Subdominants and member species, however, are not usually de- pendent on the dominant species directly ; rather, on the environmental conditions that the dominants establish. Different species of dominants in adjacent or related communities may react on the environment in a manner so nearly the same that subordinate spe- cies find suitable conditions for existence in each, although they are usually more characteristic of one than the other. The niche that a species occupies is a finite unit of distribution that can be measured in an absolute and objective manner. The community-stand is an actual aggregation of organisms occurring in a par- ticular locality. In a sense, it is a collection of niches occupied by a particular set of species, but it is some- thing that one can see and study in the field. Because of the great variation in composition and character of community-stands in different habitats and parts of the world, they need to be evaluated and classified in some logical manner for reference purposes. Com- munity-types are abstract groupings of individual community-stands which resemble one another and consequently must be defined rather arbitrarily. Dif- ferent systems of community-types have been pro- posed, each designed to emphasize a particular point- 50 40 30 % STEMS IN STAND SAMPLE POPULATION 1520 25 30 35 40 45 50 55 60 ELEVATION IN FEET, 100's FIG. 3-3 Continuum of tree (a,b,c,d) and foliage insect (e,f, gh,i,j,k) species in an elevation gradient in the Smoky Mountains, Tennessee. (a) Tsuga canadensis; (b) Halesia carolina; (c) Acer spicatum;(d) Fagus grandifolia; (e) Graphocephala coccinea; (f) Caecilius sp.; (g) Agalliopsis novella; (h) Polypsocus corruptus; (i) Anaspis rufa; (j) Cicadella flavoscuta; (k) Oncopsis sp. (after Whittaker 1952). The biotic community 27 of-view. We are, in this book, using the biome system, the various parts and concepts of which will unfold as we proceed. We will consider the com- munity as being at least analogous to an organism in being a functional unit of interacting parts and hay- ing some degree of structural uniformity. Although community-types are certainly not highly discrete and absolute units, recognition and naming of them is one way of indicating positions in the continua along en- vironmental gradients that are occupied by particular aggregations of plant and animal species. Physiognomy The gross structure of a community or its phys- iognomy is an important basis for its recognition. In terrestrial communities, physiognomy is determined by the life forms of the dominant plant species and their spacing. The life forms that prevail in a given area depend on the climate and sometimes the sub- strate or other special features of the habitat and give character to the landscape. The distribution of ani- mal communities is closely correlated with the struc- ture of the vegetation, hence these vegetation-types ~ need to be recognized and defined: Partridge berry Trillium DA wage ) Bittersweet Hemlock A a | Desert: hot, arid habitats with scattered scrubby or thorny vegetation or, in extreme cases, none. Steppe, plains: semi-arid grassland covered with short grasses. Prairie: semi-humid grassland covered with mid- and tall grasses. Chaparral: semi-arid areas covered with bushes and shrubs, usually broad-leaved evergreen. Savanna: grassland with scattered groves of trees or shrubs. Woodland: open stand of small deciduous or evergreen trees with undergrowth of grass- land or desert vegetation. Forest-edge: mixture of trees, shrubs, and open country, ordinarily occurring as a narrow belt on the margin of forests. Forest: closed stand of trees forming a continu- ous canopy over most of the area. Deciduous forest: broad leaves fall during cold or dry seasons. Broad-leaved evergreen forest: no regular sea- son of leaf fall, leaves often sclerophyllous, warm climates. Rain forest: tall luxuriant forests, often with sev- eral strata of trees, foliage retained through- Dogwood Pipsissewa Clintonia ys Moss AI) FIG. 3-4 A mixed deciduous-coniferous plant community (after Dansereau 1951). Above, a semi-realistic diagram of the community; below, symbolic structure of the community depicting life-form, size, function, leaf type, and texture. 28 Background out the year, climate continuously warm and wet. Needle-leaved evergreen or coniferous forest: forests of pines, spruces, firs, larches, hem- locks, and the like. Forest-tundra: stunted open growth of conifer- ous forests in cold climates. Tundra: extensive flat or gently rolling treeless areas occurring in cold climates. Alpine tundra: treeless areas at higher elevations of mountains. Rog: wet areas in northern climates containing sphagnum, heath plants, coniferous trees. Swamp: wet areas covered with trees or shrubs. Marsh: wet areas containing sedges, rushes, cat- tails, and the like. Inasmuch as animals choose niches in response primarily to the physical structure of the vegetation regardless of its taxonomic composition, it is helpful in describing biotic communities to show the vegeta- tion structure in as much detail as possible. This may be done by semi-realistic diagrams or by a sys- tem of symbols (Dansereau 1951). The 50 per cent rule If the primary basis for community recognition is based on the life-form of the dominants, which on land is expressed in the physiognomy of the vegeta- tion and in some aquatic habitats on the life form of the predominant animals, then the secondary break- down of community units must be on the basis of tax- onomic units. Here, the species unit is most useful, as the species is the smallest taxon having objective reality and precise interrelations with its environ- ment. Two aggregations of species occurring naturally in different areas or in the same area at different times are to be considered as distinct communities when at least 50 per cent of the predominant species of each aggregation are if not exclusive at least char- acteristic to the aggregation. This we may call the 50 per cent rule. The recognition of communities should not be influenced by the presence of rare species, for such are near the boundary of their habi- tat or geographic range. It is important to have quanti- tative information on the size of the populations to evaluate the importance of each species before com- munity classification is attempted (Sparck 1935). The distinctiveness of communities must work in both directions; that is to say, 50 per cent or more of the important species of each aggregation must be different from the other aggregation. This means that the two aggregations are more different than they are alike. If the species composition does not exhibit the 50 per cent distinction, the two aggrega- tions are considered as belonging to the same com- munity. If the difference approaches but does not equal 50 per cent, it is often worthwhile to designate the two aggregations as facies if they are seral, or faciations if they are climax, of the same community. It is preferable to use this criterion for differentiating communities, in the light of present ecological knowl- edge, rather than use more involved statistical cri- teria (Bray 1956). The 50 per cent rule has been earlier applied for separating zoogeographic regions (Mayr 1944). Naming communities Since communities are distinguished by differ- ences in life form and taxonomic composition of the dominant or predominant organisms, these charac- teristics are usually used also in naming the com- munity. Where the habitat is well defined but vege- tation is largely or wholly lacking, as in many aquatic communities, habitat may be used in the terminology. Since names are largely a matter of convenience, they should be short and be derived from some easily recognized feature of the community or habitat. Very often the generic names of two, sometimes three, conspicuous dominants are used to name plant com- munities ; two or three predominant characteristic or exclusive animal species, together with the prevail- ing type of vegetation or habitat, are employed to name animal communities. In case of some large communities, geographic names are more convenient. Large geographic units, differentiated on the basis of difference in the climax type of vegetation, are called biomes. They are specifically named by the characteristic form of vegetation present; tundra biome, or grassland biome, for instance. Secondary communities within the biome can be distinguished as climax or seral, respectively, by the suffixes -iation and -ies. An association is a climax plant community identified by the combination of dominant species present; an associes is an equiva- lent seral plant community. Thus we may speak, for instances, of the Fagus-Acer association, which is a climax deciduous forest community, and of the Ca- lamogrostis-Andropogon associes, which is a grass stage in a sand sere (Clements and Shelford 1939). Animal communities on land are related to differ- ent life-forms of plants or types of vegetation, but only seldom to plant communities distinguished by the taxonomic composition of the plant dominants. Thus animal communities must be analyzed and named independently of plant communities. A bi- ociation is a climax animal or biotic community identified by the distinctiveness of the predominant animal species ; a biocies is the seral equivalent. The The biotic community 29 North American deciduous forest biociation is to be contrasted, for instance, with the pond-marsh biocies. When two plant or animal communities merge, either by intermingling of species in the same habitat or by juxtaposition of different communities in the same region, the resultant transitional state is called an ecotone. Ecotones occur between consecutive com- munities in seral development on an area as well as between adjacent existing local or geographic com- munities. SUMMARY A community is an aggregation of organ- isms in a distinctive combination of species. The community and the habitat in which it occurs con- stitute an ecosystem. Inherent within the community are forces of dominance which control the species composition, and of influence which affects the abun- dance, health, and activities of organisms. Dominance is exerted primarily through reactions of organisms on the habitat, influence primarily by coactions of organisms on one another. The relative importance of each of the various species within the community is evaluated on the bases of exclusiveness, abundance, time of activity, secondary groupings, and influence. Reproduction and growth brings a production of or- ganic matter; the rate at which formation of it takes place is called productivity. 30. ~—- Background Communities are constantly changing, the result of reactions and coactions of the organisms, and cli- matic, physiographic, and evolutionary processes. This change is one of succession, an orderly replace- ment of one community by another until a climax, especially evident in bioseres, is reached. The community may be considered as a highly in- tegrated self-contained organic unit or as merely an aggregation of independent species whose preferanda coincide in the same habitat. These are extreme points-of-view ; an intermediate one is adopted in this book. The gross structure of the community is the pri- mary basis for distinguishing and recognizing it. On land, this structure is characterized by type of vege- tation ; in water, by the life-form of the predominant organisms, which are usually animals. Communities are then subdivided according to their taxonomic composition. An aggregation of species is given com- munity status if at least 50 per cent of the predomi- nant species are exclusive to or characteristic of it. Animal communities are named for the type of vege- tation, life-form of predominating species, or habitat, depending on which is the most conspicuous feature ; and secondarily for the predominant two or three ex- clusive or characteristic species that it contains or for the geographic area in which it occurs. Biomes are major geographic community units. Biociations are secondary climax communities distinguished by the distinctiveness of their predominant animal spe- cies. Biocies are the seral equivalents of biociations. Background: Measurement of Populations The analysis of ecological communities must in- clude a measurement of animal populations that the role played by each species may be properly evalu- ated. The ecologist should also be able to determine quantitatively the abundance of species at different times and different places. It is not sufficient in eco- logical research to indicate that a species is abundant, common, or rare; abundance must be expressed in such objective terms as lend themselves to statistical manipulations. In spite of their fundamental impor- tance, available methods for measuring population size are only moderately satisfactory and are in need of vast improvement (Balogh 1958, Davis in Mosby 1960). Indices of abundance are sometimes used; for instance the number of individuals or songs observed per hour, per day, or per trip; per cent (frequency ) of samples in which the species was recorded ; num- ber of nests, dens, tracks, or fecal pellets per unit area; amount of food or bait consumed per unit of time, and so forth. Under certain conditions of uni- form habitat and weather, random distribution of individuals, and uniform conspicuousness of the ani- mals, indices are useful for demonstrating differences in population size within a single species as functions of time or space, but they are seldom accurate enough to allow comparisons between different species. There have been various attempts to correlate rela- tive indices with absolute abundance (Hendrickson 1939, Bennett et al. 1940, Cahalane 1941, Baum- gartner 1938, Emlen et al. 1949, Eberhardt and Van Etten 1956), but the results have been usually un- satisfactory (Clapham 1936, Dice 1952). In most types of ecological research, the aim should be to de- termine absolute abundance or the actual number or biomass of a species in an area of known size. The difficulty in doing so is no greater than in correcting relative indices for all the variables that are involved. STRIP CENSUSES This method is one of counting all indi- viduals of birds and larger mammals seen on each side of a line of travel over a measured distance. Sometimes the count is made only of animals ob- served within a definite distance from the line of travel. In other cases, the effective width, and hence the area, over which the animals are being censused is computed as twice the average distance at which each species is first observed. This makes possible a quick survey of large areas in any kind of terrain, but is subject to inaccuracies of individuals omitted, especially as the distance from the trail increases (Hayne 1949a) ; differences in conspicuousness of different species or individuals exhibiting atypical or unusual behavior ; and variations in visibility as one 3] passes from one type of terrain or vegetation to an- other. A variation of this method, often employed for counting larger animals such as deer, is to increase the width of the census strip by using a line of many observers that progresses uniformly over an area of previously fixed dimension. The animals are counted as they are driven back through the line or out be- tween other observers stationed along the boundary (Rasmussen and Doman 1943). Helicopters may be effectively used for counting large animals in open country (Aldous 1956) ; faster flying aircraft are less successful (Gilbert and Grieb 1957). SAMPLE PLOTS Since it is seldom possible to count all the individuals present in a large area, it becomes neces- sary to take sample counts over small areas where accurate counting of individuals is practical. The problem then arises as to the number, size, shape, and distribution of plots required to give reliable infor- mation on species composition and the mean density for all the organisms involved. Much work on this problem has been done by plant ecologists, and their techniques should also be of use to animal ecologists. Plot distribution and shape Sample plots may be distributed either sys- tematically or at random. Systematic arrangement of plots of uniform size spaced at equal intervals along straight lines is often preferred because of its easy application. However, if the distribution of organ- isms over the area shows a uniform pattern of vari- ation, systematic sampling may indicate densities either too high or too low. Furthermore, systematic sampling does not permit the assessment of error, since statistical theory requires that the location of each sampling unit be independently determined, whereas in systematic sampling, the position of all plots is determined by the location of the first one. The completely random location of sample plots over an area may be somewhat more difficult to apply in the field, but the data obtained are just as precise and have the advantage that the error of sampling can be calculated (Bourdeau 1953). In order to get randomly located sample plots, a map of the entire area is subdivided into numbered plots of the proper size. The plots to be used are then selected by using tables of random numbers. If the same number comes up twice, the duplication should be discarded (Dice 1952). Other plans of sampling, such as stratified random sampling, may sometimes be preferable. 32 Background Where a habitat is perfectly uniform, the shape of a sample plot is not of great importance, although square plots are commonly used. Where a habitat is obviously not uniform a rectangular plot oriented with its long axis across any observed contour-, soil-, or vegetation banding will furnish less variable data than plots that are shorter and wider (Bormann 1953). Circular plots, which possess a_ smaller periphery than any other shape, are useful where the influx and exit of animals must be minimized. Size The size of plot sufficient to include an ade- quate sampling of the species composition of a par- ticular local community varies with species involved and density of populations. Larger plots must be used for larger organisms, richer fauna, for situations in which one or a few species are so markedly pre- dominant that minor species are scattered, and where population levels generally are low. Since the number of species included will vary with the size of the area covered in sampling, some standardization is desir- able for comparing the species composition of differ- ent communities. A standard size for sampling plots may be deter- mined empirically (Vestal 1949). If the numbers of species found on plots of different sizes are plotted against the logarithms of the plot sizes, a sigmoid so- called species-area curve is formed. The characteris- tics of this curve are that an increase in the size of small sampling plots includes, at first, a considerably larger number of species, but later a size of plot is reached, varying with the kind of organisms being counted, beyond which there is little to be gained by increasing the area sampled. Two arbitrarily chosen points on the upper part of this curve, where it is concave toward the scale of plot size, have ecological significance. One of these points represents a plot fifty times the size of the other, containing twice the number of species of the other. The larger plot is close to the upper asymptote of the curve and repre- sents a fair-sized sample plot for practically all pur- poses. The smaller area, located near the point of inflection and containing half the number of species, is the smallest representative area that is sufficient to identify the community, but hardly usable for any other purpose. A third point may be identified, mid- way between these two points on the curve, as the minimum area large enough to include all important species and about half of the minor ones. It clearly defines the community and the approximate ranking of species in points of number and biomass. The area this intermediate point represents is five times the smallest representative area and one-tenth the fair- sized area. 35 30 25 o Wi & 20 a o w S 5 | a SooN ul C Smallest representative area 5 2 10 The sizes of ‘Smallest representative,” Minimum,’ and ‘'Fair-sized" areas may be de- termined from a sigmoid curve—of which Fig. 4-lb is an example—by the following technique. A rectangular sheet of tracing paper is placed over the graph, the bottom edge of the paper coincident with the horizontal axis of the graph. Using the graduations on the log- arithmic scale as a quide, place a mark on the bottom edge of the paper by one of the graduations. To the right of that mark, place another by that scale graduation which is 50 times the value of the first (on Fig. 4-Ib, if the left mark is at 2, the right mark should be at 100). The interval between the marks represents a 50-fold increase in area. From the right mark, draw a vertical line several inches long. Now place the sheet over the graph in such a way that the bottom edge is parallel to the horizontal 5 fo) SS SE ee See (0) 20 40 60 80 100 120 140 AREA IN MILACRES (Units of 4m? each) FIG. 4-la, b Species-area curves plotted (above) on arithmetic and (below) on logarithmic bases, to illustrate the method of determining sampling area sizes adequate for analysis of com- munity species composition. 35 30 a ea Fair-sized area 25 o = re a. 20 oa w ° «x 15 Ww ao = > 2 | 10 100 1000 AREA IN MILACRES (Units of 4m* each) 0 Lt axis of the graph, and the left mark lies on the curve—the vertical line from the right mark should be long enough so that it will continuously intersect the curve. Keeping the bottom edge of the paper parallel to the horizontal axis of the graph, move the paper in such a way that the left mark traces along the curve. Move the sheet until the vertical line intersects the curve ata point which is twice the value of the point at which the left mark is resting pn the curve, as both values are read off the vertical scale (on Fig. 4-1b, when the left mark rests on the curve at a point opposite 15 on the vertical scale, and the bottom edge of the tracing paper is hori- zontal to the horizontal axis of the graph, the vertical line intersects the curve at a point opposite 30 on the vertical scale). The point established by the left mark is the value of the "Smallest representative area,’ and the point established by the perpendicular line intersect- ing the curve is the value of the ''Fair-sized area," as both values are read in area units off the horizontal scale. Find, by measuring, the point on the curve which lies midway between the two points just found—this is the “Minimum area,” and its value is read in area units off the hori- zontal scale (after Vestal 1949). 10000 Measurement of populations 33 In ecological sampling, fair-sized areas should be used wherever possible, but minimum-sized areas are sometimes acceptable. For evaluation of this and other procedures, see Goodall (1952). When the number of species encountered on several randomly distributed sample plots is known, it is possible to estimate statistically the actual number of species present in the whole area (Evans, Clark, and Brand 1955). The size of the plot should also be adequate to in- clude an accurate representation of the population densities of the various species present. Much of the difficulty of accurately determining population densi- ties results from populations being non-randomly dis- tributed in the space they could occupy (Cole 1946). To be randomly distributed, populations must have been scattered by chance rather than coercion, re- gardless of the proximity or distance one from an- other. This seldom occurs either with plants or ani- mals. Plants reproduce by rhizomes, stolons, or suck- ers or by seeds concentrated near the parent plants. Animals usually lay eggs or drop young in local areas or nests, so offspring are at least temporarily con- centrated. Many animals congregate socially or form colonies, concentrate on some local food supply, or are grouped closely together in certain microhabi- tats because of less favorable environmental condi- tions elsewhere. Even the attraction of male to fe- male for reproductive purposes is a variation from random dispersal. Whenever the occurrence of one or more organisms in an area increases the likeli- hood that other organisms will occur nearby, this is spoken of as contagious distribution. Species may also exhibit negatively contagious distributions when they are spaced more regularly than would be ex- pected by chance, as for instance flocking or colonial birds where each individual keeps just beyond the pecking reach of its neighbor. When small-sized plots are used, contagious dis- tribution shows itself in an excessive number of plots containing no individual and of plots containing a large number of individuals with a corresponding deficit of plots with intermediate numbers of indi- viduals. This represents a deviation from the typical Poisson distribution which is expected with random distribution (Snedecor 1956). In a Poisson series, the mean number of individuals per quadrat should equal the variance according to the formula ae 1 ¥(n—1) The letter + is the number in each quadrat, ¥ is the mean number in all quadrats, and n is the number of quadrats. If the value obtained is significantly greater than unity, then contagious distribution is indicated, if the value is less than unity, then nega- tively contagious distribution is indicated. For a 34 Background reasonably large number of sample quadrats, say 20 or more, a deviation from unity would be considered significant if it were greater than 2\/2n/(n — 1)? (Andrewartha and Birch 1954). With contagious distribution of individuals, the aggregates themselves are often randomly distrib- uted, in which case quadrats may be increased in size until they give a random distribution of aggre- gates rather than of individuals. The total popula- tion would then be computed by multiplying the num- ber of aggregates per unit area by the average number of individuals per aggregate. When aggre- gation occurs but is not easily observed, then other procedures must be employed (Cole 1946a, Goodall 1OSZ Ne Number The number of sample plots needed depends upon the precision desired for the statistical char- acteristics to be estimated. The degree of precision required will vary with the trustworthiness of the data and the objectives of the study. In most sta- tistical investigations, a range of 20 to 40 replications is ample (Snedecor 1956: p. 104). Too few replica- tions may fail to detect important differences, but too many are unrewardingly wasteful of time and energy. Any differences noted between population densities of different species on the same area, or of the same species on different areas or at different times, should be significant at least at the 5 per cent level of sta- tistical probability. Where the number of samples is small, the differences must be relatively large to insure this level of confidence. With ecological studies in the field, there are often practical difficulties involved in obtaining a sufficient number of accurate measure- ments to permit reliance on minor differences in pop- ulation size. It is best, therefore, to be conservative in evaluating the importance of differences in popula- tion densities. Special care must be used in evaluat- ing the densities of rare species, as such densities are unlikely to be reliable if based on counts of less than 20 or 30 individuals (Preston 1948). CAPTURE-RECAPTURE METHOD Some general methods of calculating popu- lation densities need to be considered. C. G. J. Peter- sen, of the Danish Biological Station, working with fish in 1896; F. C. Lincoln, of the US Fish and Wild- life Service in 1930, trying to estimate the number of ducks on the North American continent; and Jackson (1933), working with insects, all independ- ently derived a formula for determining the popula- tion size of various species of animals, much used in recent years (Ricker 1948). The method depends first on capturing a fair sample of individuals in a unit area, marking them in some distinctive manner (Ecol. 37, 1956: 665-689), releasing them for redif- fusion over the area, then after a short interval, re- trapping the area. The ratio of marked individuals recaptured to the total number marked should theo- retically be the same as the total marked and un- marked animals captured during the second trapping is to the total population or: ; ; total number marked Total population = x total captured Other formulas make use of accumulating totals of marked and unmarked individuals during successive periods of trapping (DeLury 1958). The greater the percentage of the population marked and subsequently recaptured, the greater is the accuracy of the calculations. However, there are several possible, uncontrollable sources of error: un- equal mortality of marked compared with unmarked individuals ; dispersal of individuals out of the area, influx of animals from outside; increase by reason of reproduction, marked animals not becoming randomly distributed among the unmarked; marked animals being recaptured with greater or less ease than un- marked ones; marks being lost or not reported, and so forth. Some of these possible errors can be cor- rected statistically, and a considerable body of litera- ture has accumulated describing means of so doing (see especially Biometrika since 1951). CAPTURE PER UNIT-EFFORT In a closed or stabilized population, when the same time, traps, and effort are employed to cap- ture or count individuals in the same area at different times and there is no loss or increment in the original population, and weather and other conditions remain the same, the number of new individuals cap- tured or discovered with each subsequent effort be- comes less and less, and should eventually reach zero. When the number of new individuals captured per unit of effort is plotted against the cumulative num- ber of animals captured, a straight line results. A line thus derived from a few catches may be extended to zero, and the total population of animals in the area determined (DeLury 1947, Zippin 1958). A variation of this method is to use the increasing per- centage of marked animals in the total number cap- tured at successive intervals of time, as the increase in these percentages follows a definite trend that would eventually include the total population (Hayne 1949). marked individuals recaptured APPLICATION TO ANIMAL GROUPS Mammals The more conspicuous diurnal mammals are commonly censused by cruising or drives, but noc- turnal forms, especially mice and shrews, usually have to be trapped (but see Emlen e¢ al. 1957). Snap or kill traps are commonly used. When set in a variety of microhabitats they quickly gather specimens to show the species composition of the community. An early attempt at estimating abundance was expressed in terms of the number of individuals caught per trap per night. At the same site, more animals are usu- ally caught during the first night than during later nights ; 10 traps set for 10 nights will not capture as many small mammals as 100 traps set for only one night, although 100 trap-nights are involved in both instances. When the trapping procedure is standard- ized as to location in community, number of traps used, interval between traps, length of trap lines, number of nights trapping, and so on as has been done in the North American Census of Small Mam- mals (Calhoun 1956), it is possible to follow changes in relative abundance from year to year. It is not possible to relate such data to the absolute number per unit area unless the home range of each species in each locality is known (Stickel 1948). The next advance in censusing technique was to confine the location of kill traps to a small area, usu- ally an acre (0.4 hectare). Enough traps, a hundred or more, are included to saturate the area to the end 100 oO {e) @ (e) — ° 70 fo) (e) 7 S {e) T 7 CATCH PER UNIT EFFORT, Ciy Ww oa (e) [e) T a oe ) 200 400 600 800 1000 TOTAL CATCH TO DATE, AW FIG. 4-2 Total population (K = 1170) calculated by extension of a straight line through data on successive catches per unit effort, C(t), plotted against the accumulating total catch, K(t) (from DeLury 1947). Measurement of populations 35 7/8 FIG. 4-3. Individual territories of birds representing two compet- ing species, wood pewee (stippled) and least flycatcher, along a forest trail. Note that territories of individuals of the same species do not overlap, and that territories of the two species are largely but not entirely exclusive. The outline of each territory is based on observations made from the numbered points. The dates of the several data-collection trips are shown for two territories only (Kendeigh 1956). 36 Background of capturing all individuals present during a trapping period of three nights (Bole 1939). Influx and de- parture of animals, however, disturbs the accuracy of the measurement. Influx is usually more of a prob- lem than escape from the area, as the trap bait and re- moval of captured individuals encourages invasion (Stickel 1946). Since all animals whose home ranges approach or overlap the boundary of the trapping area are likely to be caught, a correction for this error may be made by considering the census area to in- clude a surrounding belt equal to one-half of the home range of each species concerned (Dice 1952). In order to reduce the boundary of contact with the out- side area to a minimum, square or circular areas are used, rather than rectangular or irregular-shaped areas. Censuses taken in this manner and live-trap censuses sometimes give comparable results (R. M. Wetzel 1949, Buckner 1957), but in neither case can one usually be certain that he has captured all the inhabitants of the area (Fowle and Edwards 1954). Live trapping, marking, and release of individuals is a more trustworthy means of censusing small mam- mals, but is more laborious and time-consuming (Blair 1941, Stickel 1946). Traps are usually dis- tributed grid fashion at intervals of 15 to 20 meters, over several acres or hectares. Trapping is continued for a week, or until very few or no unmarked ani- mals are captured. Marking is commonly toe clip- ping, ear notching, tattooing, or tags (Taber 1956). Since the animals are immediately freed, the popula- tion equilibrium is not greatly disturbed, and influx of extraneous individuals is negligible. The method has the further advantage of allowing the determina- tion of home ranges. Individual animals differ, how- ever, in the readiness with which they will enter traps (Geis 1955), and this will affect the accurate deter- mination of home ranges. The type of bait used varies with the species be- ing trapped. Seasonal fluctuations in numbers of ani- mals trapped may sometimes be due to variability in the acceptance of bait (Fitch 1954). For mice and shrews, a paste made of peanut butter, oatmeal flakes, and raisins is commonly used. The most recent and promising development of technique for determining home ranges is the label- ing of individuals with radioactive material, then fol- lowing the movements of the freed animals by use of geiger counters (Godfrey 1954, Pendleton 1956, L. S. Miller 1957, Harrison 1958). This procedure has also been used with amphibians (Karlstrom 1957). Birds Airplanes have come into common use for cen- susing large concentrations of waterfowl. Aerial pho- tographs are made and enlarged, and individual birds pin-pointed. Roadside counts, calling-male transects, indices derived from population structure, kill rec- ords, and a variety of other procedures are used to inventory upland game species (Hickey 1955). For determining populations of smaller species during the nesting season, the spot-map method is commonly used and censuses thus obtained are prob- ably reliable within plus or minus 10 per cent, if they are carefully made (Kendeigh 1944). A sample plot of uniform vegetation of at least 10 hectares (25 acres) is marked out in a grid with numbered stakes or tree tags at intervals of not over 50 meters, or the stakes may be placed along a trail. At least five, preferably more, daily counts of singing males, fe- males, and nests are made at suitable intervals throughout the nesting season. Each time a bird is observed it is marked on a map of the plot. At the end of the season all the spots at which a species was observed are placed together on one map. Since in- dividual birds are observed most frequently in the vicinity of their nests and within their territories, the spots fall naturally into groups so that each group indicates the presence of a breeding pair or at least a territorial male. Counting the number of groups of spots for each species gives the total population for the area. For the large predators, gallinaceous birds, or wide ranging species, census plots of much larger size are necessary than for the smaller song birds, so that procedures must be adjusted to the conditions of the habitat and the species involved. For detailed studies of small populations, the birds should be banded and color-marked for individual recognition (Hickey 1943). Foliage arthropods In order to determine the insect and spider composition in the herb, shrub, and tree strata of a forest, use of a variety of collecting methods is de- sirable; net sweepings, light traps, bait traps, ad- hesive snares, and the like (Hoffmann et al. 1949, Morris 1960). Some of these methods may be made semiquantitative to show relative abundance, but there is considerable difficulty in converting the data obtained into absolute abundance. The use of the sweep net can be standardized to give useful and comparable estimates of population densities (Carpenter 1936). A series of 48 strokes of the net through the upper level of the herb stratum synchronized with one’s pace so that successive strokes do not hit the same plants gives approxi- mately the same number of individuals as one would find on the herbs covering one square meter if all could be captured. The net should have a diameter of 33 cm (13 in.), the strokes should be about one meter long (Shelford 195la), and comparative sam- ples should be taken at approximately the same time of day (Adams 1941). A similar number of strokes through the shrub foliage may be used, but the con- version to number of individuals per square meter depends on the extent and uniformity of the shrubs that cover the ground. Inaccuracies involved in sweep net sampling are the result of variations in the activity of the insects and spiders produced by changes in temperature, wind, and humidity; varia- tions in position of the insects on the plants and hence exposure to capture; insects taking flight in advance of the collector ; variations in the height of the herbs; and variations in the length and rapidity of the strokes (DeLong 1932, Hughes 1955). Differences between sexes and species in behavior and life his- tory will also cause variations in the sampling effec- tiveness, Tests on the reliability of population estimates of single species based on the sweep-net method, made by comparing the results of two different workers in the same woods at the same time,showed an agree- ment within 100 per cent in only 36 per cent of com- parisons between single weekly collections, but in 74 per cent of comparisons between averages of weekly collections taken over the entire summer (Graves 1953). This would indicate that variations in popu- lation estimates obtained by sweep-net samples are not significant unless a good series of data is ob- tained, and only then when differences between aver- ages amount to more than 100 to 200 per cent; i.e., when the larger population is at least 2 or 3 times the size of the smaller. Actually, variations in popu- lation size of the same species of insect or spider at different times or in different communities may amount to several hundred per cent, and hence the sweep net method is useful for quantitative studies. Sampling of arthropods in the tree canopy is more difficult. In the absence of wind, small trees can be jarred or shaken so that released animals fall on a cloth spread beneath. With proper equipment, trees may be fumigated with such poisonous sprays as DDT so that the dead insects fall onto cloths spread below. To put the data on a comparative basis, the volume of the space occupied by the foliage may be measured or estimated, and the number of individuals per cubic meter calculated. In deciduous forests of eastern North America the tree canopy is commonly about 10 m thick. A useful standard for comparison with the numbers per square meter of ground, herbs, and shrubs is the number per 10 m*. With taller trees, samples of the foliage for visual counting of the immature stages of arthropods pres- ent may be collected with the aid of aluminum pole pruners and extension ladders, or from trestles or platforms. Foliage samples, especially of coniferous species, should consist of entire branches or longi- tudinal halves, since arthropods may vary in abun- Measurement of populations 37 dance from the newer apical growth to the older basal foliage. If the width of the branch at mid-length is measured, then the length of the foliated part times the width gives the foliage surface. The total foliage surface of representative trees is determined from felled individuals, and the total foliage surface per unit area may be computed from the known density of trees. If the arthropods vary in abundance at dif- ferent levels in the tree, representative sampling must be taken at each level. Considerable variation in ani- mal density also occurs from tree to tree so that sam- pling must be well distributed over the area under investigation (Morris 1960). Soil animals One must resort to a variety of methods to census the different kinds of animals in the soil be- cause of great differences in their size, physical char- acteristics, and behavior (Fenton 1947, Van der Drift 1950, Kevan 1955). The megafauwna consists of the larger millipedes, centipedes, snails, amphibians, rep- tiles, and small mammals. Mammals must usually be trapped. For the other forms mentioned, if there are a half-dozen workers available, a plot 10 meters on a side (100 m?) may be marked out and the ob- servers, forming a line at one side, may gradually work over the plot, turning over all the leaves and sticks. This gives a good count but must be repeated in various parts of the community. FIG. 4-4 Tullgren modification of a Berlese funnel for quantitative sampling of soil animals. 38 Background For quantitative sampling the fauna of fallen logs and decaying stumps, it is convenient to mark out an area 50 meters on a side (0.25 hectare) and then measure the length of all logs and the height of stumps. A medium-sized log and stump are com- pletely torn apart and all animals counted. The total population for the whole area may then be calcu- lated. The macrofauna, consisting of the larger insects and spiders, the smaller millipedes, centipedes, and snails, and the earthworms may be censused by means of a steel ring, 7.5 cm wide, having a sharpened edge, and covering 0.1 m*, which is pressed into the ground until it is flush with the surface. The litter and top 10-12 cm of the soil, which contain most of the ground animals, may then be sorted by hand, either in the field or in the laboratory. Samples brought in from the field should be transported in paper or plastic sacks or tight containers and sorted as soon as possible before the predatory animals in the sam- ple have consumed prey species. Lumbricid earthworms commonly penetrate well below the topmost few centimeters of the soil. Hand- sorting of considerable amounts of soil is both la- borious and time consuming, but gives the most de- pendable results. In one study, small sample plots were thoroughly soaked with a potassium permanga- nate solution, and a later check by hand sorting indi- cated that 80 per cent of the adult and 100 per cent of the immature worms were forced to the surface (Evans and Guild 1947). Attempts at earthworm censusing with other chemicals have been less satis- factory (Svendson 1955). Driving worms to the sur- face by a discharge of alternating current from a probe thrust into the ground has also been tried (Kevan 1955). Enchytraeid or pot-worms belong to the meso- fauna as do the smaller arthropods, such as spring- tails, symphylans, pauropods, proturans, mites, and various insect larvae. Enchytraeids may be extracted quickly and efficiently by putting a layer of sand on top of a soil sample in a special container, and ap- plying heat and water from below. This forces the animals to accumulate in the sand, from which they may be easily separated (Kevan 1955). A common method of extracting small arthropods from litter and soil is by means of the Tullgren modi- fication of the Berlese funnel. Where possible, the soil sample should be kept intact as a block and in- verted into the funnel, bottom side up with an electric light bulb placed above the sample. Forced to retreat from the light and heat and the gradual drying of the soil from the top to the bottom, the animals move down the funnel and fall into the bottle of alcohol below. A week or ten days is usually required to ob- tain all possible animals from the sample. The pro- cedure is subject to a number of faults, however, and must be modified for different soil types and tax- onomic groups to produce the best results (Mac- fadyen 1953). With the flotation method, the litter or soil is placed in a large pan, and warm—not hot—water is poured in to thoroughly soak and cover the soil to a depth of 1-2 cm. The warm water stimulates the animals to activity, and they come to the surface where they may be collected with forceps or suction bottle. For greater efficiency, air may be bubbled through the material to break up clumped masses ; chemicals, such as magnesium sulphate, may be added to increase the specific gravity of the water (Kevan 1955). The microfauna consists mostly of microscopic forms such as protozoans, rotifers, nematodes, tardi- grades, and turbellarians. To quickly obtain a non- quantitative sample, a few grams of soil or litter may be placed on a screen in a glass funnel, and water, warmed to about 40°C, poured over it. The fluid collected from the funnel may then be centrifuged. A slower but more effective method for collecting nematodes is to wrap the sample in cheese-cloth or muslin and immerse it in water in a funnel that has a clamped rubber tube fastened to the stem. Let it stand for a few hours, and the nematodes will collect in the stem of this Baerman funnel. They may then be released into a petri dish for examination (Kevan 1955). To obtain a good idea of the protozoans present, culturing is usually necessary. Edible bacteria are added to a non-nutrient agar or silica jelly in a petri dish, small amounts of properly prepared soil dilu- tions inserted at various points, and the culture in- cubated for two weeks. Final examination of the culture is made under the microscope, and the pres- ence or absence of protozoans at the various points determined. Cultures may also be prepared using soil extract or hay infusion (Kevan 1955). Fish In small streams, a representative section of known length may be blocked off at the upper end by stretching a net from one bank to the other. Seining proceeds from the lower end up to the stretched net. There are limits, of course, to the size and depth of streams that can be examined in this manner, and unless care is taken, fish will escape around the ends or underneath the net. Seines can be used efficiently in this manner only when the bottom is free of large stones or other obstacles. It is sometimes possible to draw long nets over measured areas of ponds and the shallow waters of lakes, but the data obtained usually give a relative abundance only. In deeper water, trammel, gill, or Fyke nets may be set, but each type of gear has limi- tations with reference to species, locality, and time of day. Seines, trammel and Fyke nets that catch fish alive are commonly used, however, in applying the Petersen method for obtaining absolute abun- dance. When fish are removed from a body of water by gear of some sort or by sport fishing, the catch per unit-effort may be a basis for estimating total popu- lation. Creel censuses of the catch of fishermen are commonly taken to measure the yield of fish over periods of time. In modern practice, artificial ponds are usually built in such a way that they may be drained and unde- sired species or surplus populations removed. Fish may be counted and measured in these operations, and only those species which are desired for re- plenishing the population restored to the pond. Small bodies of water, or representative areas of larger bodies, can be blocked off with nets and cen- sused by the use of a poison, such as rotenone (pow- dered derris root). Fish are killed and float to the surface of the water where they may be collected and counted. This method cannot, of course, be used where the population is to be left undisturbed. The poison also kills most zooplankton and some, but not all, other kinds of invertebrates (Brown and Ball 1942). A less drastic method, that of shocking, is most effective in small streams. Two electrodes are in- serted into the water and the electric charge tempo- rarily stuns the fish so that they float to the surface where they can be captured. After the desired data are obtained, the fish are returned to the water and recover rapidly (Lagler 1952). Plankton Plankton nets are commonly made of silk bolt- ing cloth; number 20 or 25 is ordinarily the finest mesh used. Tow nets are made with a conical bag attached to a wire frame, to which the tow string is attached by means of cords. Collections may be re- moved by turning the net inside out in a jar of water, or the organisms may be concentrated in a vial screwed in at the tip of the cone. For surface plankton, it suffices to use the plank- ton net as a sieve and pour through it a known quan- tity of water. The tow net may be dragged behind a boat either at the surface or submerged to any depth by means of weights attached to the tow line. Since the depth at which plankton occur varies with the time of day, vertical hauls sampling all depths are preferred. Comparison of plankton popu- lations at different times or in different areas had best be made in terms of unit surface area. The Wis- Measurement of populations 39 FIG. 4-5 A Fyke fish trap (courtesy Illinois State Natural History Survey). consin plankton net is especially designed for this purpose. Closing nets or traps can be made so that they may be lowered to any desired depth, then closed and brought to the surface. This enables the investi- gator to determine at what depths the organisms oc- cur. The Kemmerer sampler is used extensively for bringing up known volumes of water from measured depths for plankton or for chemical analyses. Nan- noplankton, which passes through the finest tow net, needs to be filtered out or centrifuged out for quanti- tative measurement (Ballantine 1953). Net plankton is ordinarily counted with the use of a Sedgwick-Rafter cell that holds exactly one cubic centimeter at a time, and the number present calcu- lated per unit volume or surface area of the pond or lake. The volume of water filtered is equal to the area of the net opening, times the distance pulled, times a correction factor. No plankton net filters out all the organisms from the column of water through which it is dragged. The efficiency of such nets depends on fineness of mesh, rapidity with which it is pulled, and the abundance of organisms present. Fine-mesh nets offer resistance to water flow, which is further in- hibited as the pores become clogged with organisms, 40 Background so that a part of the water column is diverted around the net as it is pulled. A correction coefficient must be determined for each net and for each different rate at which it is pulled. This may be done by comparing the quantity of catch obtained in the tow net with the density obtained through use of plankton-traps or the Kemmerer sampler. Detailed instructions for con- structing different kinds of nets, and statements con- cerning the advantages, disadvantages, and possible errors in the use of different methods are given by Sverdrup et al. (1942) and Welch (1948). Bottom organisms Dip-nets are commonly used for obtaining mac- roscopic bottom organisms and those attached to sub- merged vegetation. In shallow water, bottom or- ganisms may be scooped out from a_ bottomless cylinder covering a known area. We find that four good scoops with a dip-net are necessary to get most of the organisms from a cylinder covering 0.2 m?, so we sometimes consider two scoopsfull with a dip-net as equivalent to 0.1 m? when the cylinder is not used. The Surber swift-water net is standard equipment FIG. 4-6 A fish-shocker in use (courtesy Illinois State Natural History Survey). for sampling rocky stream bottoms. A frame marks out 0.1 m?, and a net downstream catches organisms dislodged as the rocks are removed into a pail, for closer examination (Fig. 5—2a). Dredges of various shapes and sizes may be pulled along the bottom for measured distances to get or- ganisms in deep-water, but quantitative determina- tions obtained in this way give population estimates that are generally too low. The dredge commonly does not dig sufficiently deep into the bottom; often it skips and slides along the surface without picking up all the organisms that are present. Much more reliable are the Ekman bottom sampler, on soft bot- toms, and the heavier Petersen sampler, used also on sand and harder bottoms (Fig. 6-9). For microscopic organisms small core samples are usually collected and brought back to the laboratory for examination. The bottom samples obtained in various ways must ordinarily be washed through sieves to remove the debris, and the animals put into vials or jars for identification and counting. The size of mesh to be used in the sieve depends on one’s objectives (Reish 1959). We find four nesting sieves efficient, with the top sieve having a coarse mesh (2 per inch) and the lower ones of increasing fineness (10, 20, 30 meshes per inch) to capture smaller organisms. Suppliers of limnological and oceanographic apparatus and sup- plies have been listed by Ryther et al. (1959). It should also be noted that for on-site studies of ani- mals under water, increasing use is being made of photography and even television. The bottom may also be explored at first hand using diving equipment. SUMMARY Although determination of relative abun- dance is sometimes useful in projects of limited scope, the measurement of absolute abundance is generally to be preferred. Measurement of absolute abundance requires the counting of individual animals or meas- urement of their biomasses on strip censuses or sam- ple plots. The size, number, shape, and distribution of sample plots and methods of measuring population densities present special problems that must be ad- justed for each habitat and group of organisms con- cerned. The development of improved methods of population sampling is one of the major needs of ecology today. Measurement of populations 41 Streams When rainwater falls on an uneven surface, it col- lects in depressions. As the water overflows them, the current erodes a narrow channel that deepens with each succeeding shower and may eventually drain the depression. There is usually also a lateral meandering of the stream, by which a valley is formed. The site of the headwaters of such streams is impermanent, and continued erosion forces the headwaters and the channel farther and farther back into the upland. The stream is at first a temporary one, dependent for its waterflow on rainfall runoff, but when its channel is cut below the level of the groundwater table the stream becomes permanent, fed by general, continuous seepage. The headwaters of such a stream are therefore its youngest portions physiographically, and the stream is progressively more aged towards its mouth. In hilly or mountainous terrain, water may ac- cumulate in large basins until ponds or lakes are formed. In the Great Basin of North America, such lakes have not found an outlet to the sea, and evapo- ration has left them with a very high salt content. Ordinarily, however, the water level in such a lake will rise until it overflows at the lowest point on the perimeter. Then the waters continue to flow down- ward until they eventually reach the sea. Streams springing from fixed headwaters (melting snowfields and glaciers, springs) carve valleys that are of essen- tially the same age throughout. Streams less than 3m (10 ft) wide are usually called creeks or brooks; rivers are streams 3 m or more wide. A river system in youth is characterized by val- leys that are narrow and steepsided ; the flow of water is usually fast, there are few tributaries, and there are many waterfalls, ponds, and lakes along its course. As the river system matures, its valleys be- come wider, its slopes more gentle, and its tribu- taries more numerous and longer. Many ponds and lakes are drained, and waterfalls are worn down to rapids or riffles. The areas of upland are well dis- sected, and the land is thoroughly drained. In old age, the river system has reached base-level. The upland has been worn down to low ridges between tributary river valleys, and the region as a whole is called a peneplain. There are no lakes, ponds, or rapids, and the flow of water is sluggish (Strahler L9S1)E HABITATS Exclusive of its lakes, the principal habi- tats in a stream are falls, rapids or riffles, sand- bottom pools, and mud-bottom ponds. The character of the bottom depends primarily on the velocity of the water current, which, along with the volume of stream flow, can be readily measured (Robins and a Crawford 1954). Water flowing at the rate of about 50 ecm-sec is considered swift-flowing; velocities greater than 300 cm-sec rarely occur. Fast currents roll or slide pebbles and rocks along the bottom; move sand partly by rolling and partly by buoyant transportation ; and carry fine materials, such as silt and organic matter, in suspension (Twenhofel 1939). In places where the topographic gradient is steep, the stream bottom will be composed largely of cobble and boulders too heavy to move, and smaller pebbles which are trapped by obstructions. This habitat is called a rapids, if extensive and turbulent; riffles, if of a lesser order. When the gradient is less steep and the water cur- rent thus slower, gravel (particle size 2-64 mm) is deposited first, then sand (0.06-2 mm), but the finer materials are carried along. Only when the current becomes negligible does the suspended material settle so that silt (0.004-0.062 mm) or mud-bottom pools or ponds are formed. Clay has a particle size even smaller (Morgans 1956). These mud-bottomed pools are the most fertile parts of the stream because of the presence of organic matter entrained in the silt. The rate at which oxygen diffuses into water from the atmosphere increases as the turbulence of the water increases; rapids therefore have, often, the highest oxygen content of a stream’s waters. Ordinarily, however, oxygen is near saturation in all parts of a flowing, non-polluted stream. In a general way, rif- fles, sand- and mud-bottom pools represent three stages in the aging of a stream, and ecological study of them gives a good idea of what the eosere would be over a long period of time. Trout streams do not normally exceed 24°C max- imum summer temperature; streams with higher summer temperatures are more characteristically oc- cupied by species of Centrarchidae and Esocidae (Ricker 1934). Streams have been classified into a variety of different types, using the most character- istic fish present as a basis (Van Deusen 1954). The salt content of stream waters depends both in quan- tity and in chemical nature on the fertility of the land drained or the rock strata which produce the springs. STREAM BIOCIES When quantitative sampling is made of the invertebrate populations of streams, one finds that there is a sharp distinction of species found in riffles and those found in mud-bottomed pools (Table 5-1). The sand-bottom pool habitat has few characteristic in- digenous invertebrate species, but it is occupied by small numbers of individuals of species otherwise occurring abundantly in the other two habitats. The unstable bottom apparently prevents the development of a characteristic community. The unionid clams are really the only invertebrate group to become es- tablished in this habitat with any degree of success, although they are not exclusive to it. There are, how- ever, several fish species (Table 5-2) that find sandy pools a favorite habitat, although they depend in large part upon riffle organisms for their food. Many fish overwinter in the deeper, more quiescent sand- bottom pools, especially since low water temperature makes them too sluggish to withstand rapid currents. Mud-bottom pools form in backwaters of the main stream, behind natural or artificial dams in the main channel, or where the current is sluggish. Very often, aquatic vegetation fringes the edges of these pools. These quiet pools are essentially young stages in the development of ponds and support many animal spe- cies indigenous to ponds. Such pond animals as aquatic annelids, dragonfly and damselfly naiads, and burrowing mayfly naiads commonly occur also on the muddy margins of streams in which the main channel has a sand, gravel, or rocky bottom. The stream biocies consists most typically, there- fore, of the inhabitants of the riffles and sand-bottom pools found throughout the course of the river. The riffle and pool organisms make up two different facies in this community. Mud-bottom pools and sluggish streams are occupied by the pond-marsh biocies, to be later described. Plants are not abundant in the stream biocies, although the upper surfaces of rocks in a rifles may be completely covered with branched filamentous algae (particularly Cladophora), and a few species of water mosses (Fontinalaceae) may occur. Di- atoms, mostly sessile forms, may be numerous in early Spring and again in the Autumn. Dominance in the true sense, .such as occurs in terrestrial com- munities, does not exist, although the algae and mosses passively provide food and shelter for active forms. The most characteristic and abundant animal forms of the stream biocies are the caddisfly larvae, mayfly naiads, stonefly naiads, fly larvae, crayfish, snails and clams, sponges and bryozoans, and fish, each occupying its own particular niche (Berg 1948). Plankton is mostly absent in swift-running water (Carpenter 1928, Coker 1954), but may be abun- dant in sluggish, pond-like stretches of large rivers. The fishes listed in Table 5-2 are mostly warm-water fishes. In the colder waters of mountain and northern streams, the fish fauna changes. Trout, sculpins, and sticklebacks become the most conspicuous species. Streams that empty into the ocean may have a special fauna of anadromous (‘upstream’) fish, such as sal- mon, shad, striped bass, and some trout, that spend much of their lives in the sea but migrate into fresh- water streams to spawn, and catadromous fish, such as the eel, which migrate “downstream” into the sea to reproduce. There are a few vertebrates other than Streams 43 fish commonly found in streams. Some salamander species occur only in fast mountain streams; other species are more typical of pond-like pools. The belted kingfisher feeds on stream fishes, and nests in adjacent clay banks. In the western mountains, the water ouzel feeds under water on the insect larvae and naiads of the riffles. Muskrats make their bur- rows in the stream banks and feed on vegetation and clams. Mink patrol the streams for the muskrats and fish that serve them as food. The once-abundant otter is now absent from most localities. Beaver dam streams to enlarge the pools in which they build their lodges and find shelter. Beaver feed on the bark and cambium of aspen, willow, and other trees and shrubs occurring on the shores of the stream. ADJUSTMENTS TO CURRENT Probably the characteristic of a stream most critical to the life therein is the current. All organisms that occur in streams must adjust to it to maintain constant position. Torrential floods scour FIG. 5-2 Apparatus for collecting quantitative samples of bottom organisms in streams: above, swift-water net, covers 0.1 m? (Surber 1936); right, sampling cylinder for use in pools, covers 0.2 m’, has sharpened lower edge. > a FIG. 5-| Diagrammatic arrangement of streams of different physiographic age on the south shore of Lake Michigan. Each number shows the location of that pool nearest a headwaters which first contains these fish: (1) creek chub; (2) redbelly dace; (3) blacknose dace; (4) suckers; minnows; (5) grass pickerel; bluntnose minnow; (6) sunfish, bass; (7) northern pike, lake chubsucker, and others (after Shelford 1913). the stream bed, move rocks and sand, cut new chan- nels, and destroy entire populations. Recovery after such catastrophes, however, may take place within a few weeks or months, especially by those species pos- sessing short life cycles (Moffett 1936, Surber 1936). Position is ordinarily maintained by clinging to the substratum, avoidance of the current, or vigorous swimming, and requires a good development of ori- entation behavior. Clinging mechanisms The growth form of fresh-water sponges is af- fected by a number of factors, but in riffles sponges are usually simple encrustations. In quieter water, long, slender, finger-like processes may form. The distribution of species depends both on current and organic content of the water (Jewell 1935). Plumatella is a common bryozoan that forms an encrusting, plant-like, branching colony on the under- side of rocks or fallen trees in swift water. Pectina- tella, on the other hand, forms a gelatinous spherical ball, and is more commonly found in ponds or slow- flowing portions of streams. Turbellarians, such as Planaria, and swift-water snails, such as Goniobasis and Pleurocera, and the limpet Ferrissia, cling to the substratum by means of flat, slimy, adherent body or foot surfaces, and are most common on the protected lower surfaces of rocks. Mayfly naiads have efficient adaptations which enable some of them to tolerate currents up to 300 em-sec (Dodds and Hisaw 1924). The animals cling to the smooth undersurfaces of the rocks, keeping their heads toward the current and their bodies par- allel with it as they move sideways, forward, and back. The head is flattened, and when pressed firmly against the substratum the water current exerts a downward pressure which helps to hold the animal in position. Compared with forms found in quieter 44 Habitats, communities, succession TABLE 5-1! Size and distribution of invertebrate populations in stream habitats of the Vermilion River, Illinois, as determined by class studies through eight years. Common name Caddisfly larva Mayfly naiads Hellgrammite Riffle beetle larva Riffle beetle adult Limpet snail Bryozoan Fresh-water sponge Flatworm Broad-shouldered water strider Stonefly naiad . Snails White midge fly larva Horse fly larva Fingernail clam Crayfish Damselfly naiad Dragonfly naiad Clams (28 species) Crayfish Snail Red midge fly larva Aquatic annelid Burrowing mayfly naiad Water boatmen Alderfly larva Fishfly larva Crawling water beetle Amphipod Predaceous diving beetle Backswimmer Water scorpion Aquatic isopod Whirl-i-gig beetle Springtail Mayfly naiad Snail Snail Total taxa Total individuals Number per square meter Sand bottom Mud bottom Classification Riffles pool pool Trichoptera 1,006 Heptageniidae, Baetidae 248 Corydalis 46 Psephenidae 19 Psephenidae 4 Ferrissia tarda 2 Plumatella + Spongillinae + Planaria + Rhagovelia + Plecoptera 61 1 Goniobasis livescens, Pleurocera acuta 39 2 Tanypus 16 0 5 Tabanidae 8 1 1 Sphaerium 8 + + Orconectes propinquus 6 1 4 Zygoptera 4 2 1 Anisoptera 2 4 7 Unionidae ++ ++ + Orconectes virilis + + + Physa gyrina 7 0 1 Tendipes + 1 8 Chaetopoda + 1 134 Hexagenia + + 139 Corixidae 6 Sialidae 4 Chauliodes 2 Haliplidae 1 Hyalella 1 Dytiscidae 7 Notonectidae Z, Ranatra + Asellidae ay Gyrinidae 2 Podura aquatica + Caenis i Gyraulus parvus + Lymneidae + 24 14 26 1469+ 13+ 320+ most of their time in burrows, dug into the mud. water, they show a larger thorax and legs, a smaller abdomen, absence of hair on the caudal cerci, shorter middle cercus, and smaller gill lamellae. These modi- fications enhance body streamlining and reduce the drag of the water. Furthermore, the legs are articu- lated in a way which allows the current to press them firmly against the substratum. The body itself swings freely in the current. Mayfly naiads that occur in quiet waters do not have these modifications. They commonly spend They come out at night to swim around and search for food. The abdomen of the mud-inhabiting forms is thick, with little taper, sometimes bowed ventrally, and the three terminal cerci are provided with long stiff hairs that overlap and make an excellent oar for swimming. Stonefly naiads are not limited to stony habitats. Some species occur in the masses of leaves that lodge against rocks or along the banks, in the algae grow- 115) Streams TABLE 5-2 Distribution of predominant fish species in stream habitats of central Illinois (after Thompson and Hunt 1930). Gravel and sand Mud bottom bottom Common Name Riffles pools pools Suckermouth minnow Banded darter Bigeye chub Log perch Green-sided darter +++ e+ Stonecat ar Hog sucker + Fantail darter + Steelcolor minnow + Common shiner + +++ Channel catfish + Hornyhead chub + Stoneroller minnow Silverjaw minnow River shiner +++ ++ Reffin shiner Rainbow darter Quillback carpsucker Smallmouth bass White crappie Ge PoP or ae Orangespotted sunfish Longear sunfish Green sunfish Bluntnose minnow White sucker ++ +++ ++ ++ Northern redhorse Shorthead redhorse Creek chub Johnny darter Golden shiner ++ ++ +++ ++ Creek chubsucker Grass pickerel Blackstripe topminnow Pirateperch Freshwater drum ++ et + Gizzard shad Highfin carpsucker Largemouth bass Bigmouth buffalo Carp ++ +++ + Black crappie Black bullhead + Total species 12 23 24 ing on the rocks, on sand bottoms, and in small mud- bottom streams rich in organic matter. The general form of the body is similar to that of swift-water may- fly naiads, although the gills are filamentous and lo- cated at the base of the legs. Caddisfly larvae occur most abundantly in streams with medium to swift currents, but some species oc- cur only in sluggish rivers, in lakes, or in pond vege- tation. Caddisfly larvae are of especial interest be- cause of the cases they construct, in which the pupae also occur later. In some species these cases are portable. They are made of pieces of leaves, twigs, sand grains, or stones which are cemented or tied together with silk that the animals secrete. In stand- ing or sluggish water, the cases are often large and made of buoyant plant material, or they may be made of sand grains, more fragile and slender. In swift water, the cases are stout, cylindrical, tapered pos- teriorly, and are usually smaller and more solidly constructed of sand, small pebbles, or rock fragments (Dodds and Hisaw 1925). The Hydropsychidae, Philopotamidae, and Psychomyiidae are unique in spinning fixed abodes in the form of a finger, a trum- pet, or a tube. The Hydropsychidae erect a net at the front end of the tube to catch particles of food washed down with the current. Some psychomyiid larvae, particularly Phylocentropus, burrow into sand and cement the burrow walls into fairly rigid cases. Some larvae belonging to the Rhyacophilidae are free-living. Found in algal growth, they crawl around seeking food, and are provided with large abdominal hooks as clinging devices to supplement the legs for clinging. However, they form a stone case, or cocoon, for pupation (Ross 1944). The black fly larvae, Simuliidae, are often very abundant in the swift waters of mountain brooks and northern streams. The larvae secrete from their sali- vary glands a delicate silken thread by which they attach to the rocky substratum, and by manipulation of which they can move short distances. At the pos- terior end of the semi-erect body is a circlet of rows of outwardly directed hooks which, when the muscles of the disk are relaxed, move outwards and catch on to a silk web placed there previously by the larva; the anterior end of the body then swings freely in the current. There is a fan-like food-gathering organ on each side of the mouth. Before pupation, the lar- vae spin a sedentary cocoon. The pointed end faces the current and the other end, open, faces down- stream. Out of it, the peculiar gills of the pupa float in the water (Hora 1930, Nielsen 1950). The net-veined midge larvae, Blepharoceridae, are unique in possessing six unpaired suckers on the ventral side, by means of which they fasten to the substratum. The original segmentation of the body is almost obliterated; it has been replaced by a sec- ondary segmentation correspondent with the number of suckers. Adult riffle beetles (Psephenidae, Dryopidae, Elmidae) are small in size and are the only coleopter- ans that live in or near running water. The legs are not fitted for swimming, but rather possess hooked claws for clutching the substratum. The body is cov- ered with silken hairs that hold a thin film of air about it when the beetle is submerged. The larvae 46 Habitats, communities, succession are disc-shaped and pressed close upon the sub- stratum, to which they cling with their legs and backward-directed spines. They are sometimes called water pennies. When ready to pupate the larvae crawl out of the water. Avoiding the current Diminutive body and appendage sizes and as- sumption of a stream-line shape keep the amount of surface exposed to the full impact of the current at a minimum. The conical shape of the limpet Ferris- sia and the flatter cone of water pennies offer little resistance to water flow. Flat bodies, such as are found in many swift-water animals, appear to be not only an adaptation lowering resistance to current but also to escape it by enabling the animals to seek shelter in crevices and underneath stones (Dodds and Hisaw 1924, Nielsen 1950). Most species, even those with specialized means of clinging to the bot- tom, are more abundant on the undersides of rocks in riffles than they are on the uppersides. Some spe- cies, however, such as the free-living caddisfly lar- vae, rotifers, tardigrades, water mites, and proto- zoans, find shelter within the mass of algae that may cover the top of the rocks. The hellgrammite, tabanid fly larvae, and stream crayfishes possess no special structures for withstanding currents and only occur in riffles providing protection or lodgement under- neath and between rocks. Even swift-water fishes, strong swimmers, take maximum advantage of what- ever protection is available. The clams avoid the full force of stream current, and at the same time retain position, by lodging their bodies between stones. In pools, they bury them- selves in an oblique position in the gravel, sand, or mud. Their posterior ends are directed upstream (ac- cording to Dr. Max Matteson), and their siphons usually maintain contact with open water so there can be circulation through the mantle cavity, for gain- ing food and oxygen. Clams occurring in pools one- half to one meter in depth may remain more or less sedentary, but those occurring in shallower waters move around considerably, especially in response to changes in water level and temperature. Swimming Locomotion of swift-water invertebrates is, in the main, restricted to short-distance crawling. May- fly naiads that occur in riffles do not swim, although related species frequenting quiet waters do so, regu- larly. Only the more vigorous fishes can maintain position in swift currents by swimming, and many of them do so only when feeding. At other times they congregate in the pools that occur between riffles. Salmon and trout are well known for their ability to swim against strong currents, an accomplishment of sheer force of powerful, muscular, tails. The sub- family of darters, Etheostominae, which contains a variety of brightly-colored small fish, are especially adapted to live in the rifles. The air bladder of the darters has become very degenerate, even absent, so that the specific gravity of the body is increased. TABLE 5-3 Rheotactic responses of invertebrates from riffles and nools (from Shelford 1914). Velocity of current 4-6 cm/sec 10-12 cm/sec 16-20 cm/sec Response in percentages Posi- Indif- Nega- Inac- Posi- Indif- Nega- Inac- Posi- Indif- Nega- Inac- tive ferent tive tive tive ferent tive tive tive ferent tive tive RIFFLES ANIMALS Crayfish, Orconectes virilis 30 40 28 2 54 8 16 22 78 2 6 14 Snail, Goniobasis livescens 45 PA 28 0 65 22 0 13 76 tf 0 17 Caddisfly larva, Hydropsyche sp. 23 26 16 35 18 9 6 67 26 2 a 68 Damselfly naiad, Argia sp. 719 0 17 4 63 18 4 15 63 4 0 33 Stonefly naiad, Perla sp. 31 24 3 42 65 6 15 14 61 3 3 33 Mayfly naiad, Heptageninae 25 12 14 49 52 3) 0 45 52 3 0 45 Water penny, Psephenus sp. 26 32 36 6 67 26 0 ra 74 15 11 0 Averages 37 23 20 20 55 13 6 26 62 5 3 30 POOL ANIMALS Damselfly naiad, Calopteryx maculata 78 0 22 0 59 8 0 33 63 0 0 37 Snail, Campeloma subsolidum 51 32 6 11 80 0 0 20 10 0 0 90 Burrowing dragonfly naiad, Macromia sp. aly 36 41 6 12 72 10 6 0 0 Oo 100 Clam, Anodontoides ferussacianus 16 66 18 0 17 67 16 0 0 0 0 100 Fingernail clam, Sphaerium sp. 17 66 17 0 16 67 17 0 0 0 0 100 Averages 36 40 21 3 37 43 9 12 15 0 0 85 Streams 47 FIG. 5-3 Mayfly naiads: (a) adult of Hexagenia limbata; (b) naiad of H. limbata from quiet water; (c) naiad of Heptagenia flavescens from swift water (courtesy Illinois Natural History Survey). FIG. 5-4 Stonefly: (a) adult; (b) naiad, Isoperla confusa (courtesy Illinois Natural History Survey). 48 Habitats, communities, succession FIG. 5-5 (a) External features of a caddisfly larva; (b) larva and case from a weedy lake; (c) larva and case from a spring-fed brook (courtesy Illinois Natural History Survey). FIG. 5-6 Immature stages of the b black fly: (a) larva; (b) pupa; (c) pupa case (Shelford 1913 after Lugger); (d) enlarged detail of arrangement of hooks on the posterior end of the larva (after Nielson 1950). FIG. 5-7 Water pennies, larva of the psephenid beetle: (a) dorsal and (b) ventral views (Shelford 1913); and (c) larva of the net-veined midge; showing the central row of six suckers (after Hora 1930). Streams 49 Their fan-shaped pectoral fins are enlarged, and project at right angles from the lower side of the body. When at rest they maintain position by con- tact with the bottom, fins lodged between pebbles or the body partly buried. They never float suspended in the water, as do other fish; when disturbed, they dart swiftiy from one anchorage to another. The Etheostominae are confined to North America east of the Rocky Mountains. Stream fishes are in general quite sensitive to cur- rent, and the discontinuous distribution of a species within the same stream may be closely correlated with gradient (Trautman 1942, Burton and Odum 1945). The smallmouth bass, for instance, is mostly absent in southern streams of gradient less than 40 cm/km (2 ft/mi); is of moderate abundance in gradients up to 135 cm/km (7 ft/mi) ; is very abun- dant in gradients of 135-380 cm/km (7-20 ft/mi) ; and becomes less common again, until it disappears altogether, in gradients above 475 cm/km (25 ft/m1). Perhaps streams with very slow current do not pro- vide suitable gravel nest-sites for spawning, and in streams with very fast currents they are unable to maintain position. Salamanders that live in swift mountain streams generally have short limbs and toes, reduced size of fins, smaller lungs in the adult and shorter gills in the larvae, and relatively few large eggs, which they fasten to the underside of flat rocks (Noble 1931). Orientation behavior Structural adaptations for withstanding or avoiding current are of no avail without appropriate behavior responses to make use of them. The rheo- tactic responses of animals may be tested either in the field or in the laboratory by means of special ap- paratus. When animals from riffles and those from pools are compared (Table 5-3), it is apparent that, at low current velocities, responses of the two groups of animals are nearly the same. The elongate body, notably of stream animals, brings an automatic turn- ing into the current much as wind directs a weather- vane. As the velocity of current is increased, how- ever, there is a marked increase in the percentage of riffle animals that face into or move against the cur- rent, while a very large percentage of pool animals are swept away by the current or are forced to with- draw into their shells. Caddisfly larvae, free of their cases, are not very able to withstand a strong cur- rent, although within their cases they readily main- tain position. Blackfly larvae can tolerate water currents as swift as 180 cm/sec, and studies indicate that their clinging to the substratum is a response to current rather than to any associated factor, such as food or oxygen requirement (Wu 1931). When tested experimentally, 80 per cent of the stream crayfish Orconectes propinquus were able to maintain position in currents of 50 cm/sec, but only about 20 per cent of the pond crayfish O. fodiens were able to do so (Bovbjerg 1952). Fishes generally respond to current by showing nearly 100 per cent positive response, regardless of whether they be taken from streams or ponds. Since the response involves a tendency to swim upstream, other factors must be involved for the fish to main- tain a constant location in the stream ; otherwise they would all move to its headwaters. Some stream fishes, such as the blacknose dace and the common shiner, can be shown experimentally to respond visually to landmarks on stream bank and bottom to maintain their location. Some pool fishes, such as the sunfish and topminnow, likewise respond visually, but much more sluggishly, and irregularly. Darters are entirely unresponsive to visual stimuli, depending on the tactile stimulus of contact with the bottom for maintaining position (Lyon 1905, Clausen 1931). Smell may be important to some fish for orientation. The backswimmer Notonecta (Schulz 1931) and whirl-i-gig beetle Dineutus (Brown and Hatch 1929) have also been shown to use visual ori- entation in running water. RESPONSES TO BOTTOM The segregation of stream animals _be- tween riffles, and sand- and mud-bottom pools may be, in part, a response to type of bottom. With no current flowing, the species listed in Table 5-3 were, in another experiment, given a choice between a hard bottom and a sand bottom. Eighty-five per cent of the riffles animals selected the hard bottom, but only 10 per cent of the pool animals did so. Of the pool animals, all species made 100 per cent response to sand, except the damselfly naiad, Calopteryx macu- lata, which divided equally between the two types of bottom. When the riffles animals were given a choice between loose stones and a bare bottom, nearly all individuals selected the stones, and they distributed themselves among the stones or on top or underneath in the manner one would expect of them under nat- ural conditions (Shelford 1914). Stream crayfish, when given a choice between mud and cinders, ori- ented 88 per cent to the cinders, while the pond cray- fish responded 40 per cent to cinders and 60 per cent to mud (Bovbjerg 1952). Type of bottom is important to invertebrates for support and locomotion. Sand bottoms are note- worthy as unstable and shifting. Insect larvae and naiads find footing very uncertain; planarians, 50 Habitats, communities, succession sponges, and bryozoans find no stable anchorage ; and rock-inhabiting snails and limpets are quickly buried. Clams, however, find a sandy bottom suitable, if it is firmly packed, as they are adapted to burrowing and plowing their way through a loose substratum. They are able also to move through a mud bottom, but where silting is heavy they close their valves to avoid an accumulation of silt within the mantle cavity and on the gills. The anodontas seem to be the most tol- erant of mud bottoms. Some of the mayfly naiads, such as Hexagenia, are adapted to burrowing in mud, and the surface of the bottom in shallow water is often closely dotted with the openings of their burrows (Hunt 1953). These burrows are relatively permanent in compact mud but would quickly collapse in loose sand. The genus Caenis is peculiar in possessing covers at the anterior end of the abdomen; they protect the gills from becoming clogged with silt. Midge fly larvae and aquatic annelids exist in mud bottoms; they would be ground to bits among moving sand parti- cles. The pond crayfish will burrow into mud down to water level as a pond dries up, but stream cray- fish will not do so and consequently suffer high mor- tality (Bovbjerg 1952). The bottom is important to invertebrates and vertebrates alike for placement of eggs. Some caddis- fly eggs are fastened to smooth rock surfaces in long strings by a cement-like substance. The eggs of other species occur in jelly-like masses and may be secured to plant stems or other submerged objects. Jelly-like masses of snail eggs are often quite common on the undersides of rocks in rifles. Some fish, such as the fantail darter (Lake 1936), make nests in small cavi- ties under stones, but other species, for instance the rainbow darter (Reeves 1907), creek chub (Reig- hard 1908), and river chub (Reighard 1943), build nests in gravel bottoms in the upper parts of riffles. Some of the suckers (Reighard 1920) spawn in shal- low water; their eggs scatter downstream, finding lodgment in various riffles. RESPIRATION AND OXYGEN REQUIREMENTS Oxygen is usually ample in streams, often saturating the water in turbulent riffles. The oxygen concentration is sometimes low, however, in sluggish streams and standing pools. The difference in oxygen tension of the two habitats is reflected in the respira- tory adaptations of the organisms that inhabit them. The lamelliform gills of the mayfly naiads in- habiting mud-bottom pools are larger in size than those of species inhabiting streams, are doubled in number on the anterior abdominal segments of some species, and are almost continuously flicked back and FIG. 5-8 Apparatus for studies of rheotaxis. Right, box for use in streams where water enters at upper end, flows through center trough, and out lower end. Controls may be run in side troughs filled with still water. Above, rheotaxis pan in which current is produced artifi- cially with a rod or finger. An organism's response is positive when it turns to confront the current; negative, when it faces down- stream; indifferent, when it orients crossways. forth for better aeration. The gills of naiads living in riffles, or in waters in which the oxygen content is high, may have the surface area of the gills reduced by two-thirds in proportion to body weight, compared with mud-dwelling forms. They are never flicked, since the water movement continually brings oxygen to them (Dodds and Hisaw 1924). Other species do not flick their gills at high oxygen tensions, but will do so when tension is reduced. In some swift water species, there appears to be sufficient oxygen diffusion through the general body surface to make gills ines- sential equipment (Wingfield 1939). Caddisfly larvae have filamentous gills, and there is some evidence that they increase in number as body size increases and oxygen content of the water decreases. It is probable that oxygen also diffuses readily through the thin skin. A constant current of water is maintained through their cases by undula- tions of the abdomen. Stonefly naiads have poorly developed filamentous gills, located on the thorax, or have none at all. As a result, they are more sen- sitive to variation in oxygen supply than are the other forms mentioned. Streams 51 The respiratory equipment of pond-inhabiting animals permits them not only to live in habitats with lower oxygen tensions but also to survive longer at high water temperatures. Often, these animals dis- play relatively low rates of general body metabolism and oxygen requirement. Such relations between riffle and pond animals have been observed for may- fly naiads, caddisfly larvae, isopods, crayfish, and fishes (Allee 1912-13, Wells, 1918, Fox et al. 1935, Clausen 1936, Whitney 1939, Bovbjerg 1952), and to some extent for limpet snails (Berg 1951). RESPONSES TO STREAM SIZE Of the species of clams indigenous to Mich- igan, the 3 commonest are largely limited to creeks, 14 others to medium-sized rivers, and 5 to large rivers (Van der Schalie 1941). In central Illinois, the num- ber of species of fish per collection increased from about 4.5 in streams draining 4 sq km to 15.5 in streams draining 500 sq km of upland. At the same time the number of fish decreased from 9 to 2.5 per sq m of water surface (Thompson and Hunt 1930). Large species of fish can occur only in stream with sufficient volume of water to permit freedom of move- ment; small fish may find orientation difficult in large rivers. The preference of fish for streams of specific size is evident in the tendency for some spe- cies to travel upstream in times of flood and down- stream in times of drought. An increased number of species downstream cor- relates with greater variety of available niches and moderate environmental conditions. In many in- stances the correlation between distribution of species and stream size, or volume, is not direct but depend- FIG. 5-9 Clam tracks in a sandy pool (courtesy R.E. Rundus, 1956). ent on associated changes in temperature, type of bottom, fertility, silting, pollution, and other factors. Headwaters The headwaters of drainage streams present a highly variable habitat. During dry periods, pools shrink and may disappear ; temperature may be very high in summer and the water largely converted to ice in winter; there may be a lack of oxygen, an ex- cess of carbon dioxide, and a high acidity ; fishes and other organisms may become greatly overcrowded. In times of heavy rain, on the other hand, the stream is swollen, there is considerable erosion of materials into the stream, and animals are washed downstream. At all times food is likely to be scarce. Only the hardiest species can exist under these conditions. The creek chub is a remarkably hardy fish; it may be found in large numbers in shrunken pools, stirring up the water with tail action and gap- ing for air at the water surface. Crayfish burrow into the bottom when the pool dries up. Small snails may survive desiccation of habitat by crawling un- der rocks or into crevices, secreting a mucous mem- brane across the aperture of their shells, and remain- ing dormant until water returns. The occurrence of insect larvae and naiads is hazardous, for if the aquatic stages of their life cycles are characteristically prolonged, they perish at times of low water or drought. Temperature and altitude In drainage streams the temperature of the headwaters is variable, but as the water volume in- creases downstream and becomes more constant, the range of temperature variation decreases. The head- waters of spring-fed streams, or of streams arising at high elevations, usually have a progressive increase in temperature downstream. Some species of stonefly and mayfly naiads and caddisfly larvae are absent from the headwaters of Ontario streams because the temperature never gets high enough to permit them to complete their life cycle. More species are present downstream, and the headwaters species tend to emerge earlier and earlier in the summer while the waters are still cold. Still further downstream, the headwaters species disap- pear altogether. Species that are limited to the lower portions of the stream emerge late in the season, when the waters are the warmest. Closely related species are thus segregated to different positions in the stream by temperature tolerances. Headwaters species have generally a northerly distribution over the continent and the downstream species a southerly De Habitats, communities, succession distribution (Ide 1935, Sprules 1947). Linear dis- tribution of fish in streams may be, in part, a result of differences in temperature tolerance. Brook trout, for instance, do best in waters cooler than 19°C in Virginia, while some varieties of introduced rainbow trout prefer waters above 19°C (Burton and Odum 1945). The altitudinal zonation of various species of invertebrates and fish in mountain streams is well defined, and is in large part contingent on differences in temperature (Dodds and Hisaw 1925). Shape and size of individuals In the Tennessee River, riffles snails of the genus Jo show a progressive change in shape from the headwaters on downstream. There is a decrease in shell diameter, a decrease in globosity, and an in- crease in number and length of spines (Adams 1915). However, the riffles snail Pleurocera was found to in- crease in globosity downstream in Michigan (Good- rich 1937). Some pond snails, such as Lymnaea stagnailis and Galba palustris, develop a larger foot and shell aperture when exposed to wave action (Baker 1919). Primitive types of clams, on the other hand, such as Fusconaia, Amblema, Quadrula, Pleur- obema, and others, change progressively downstream from a large, compressed, smooth shell to one that is shorter, more obese, and sculptured with tubercles (Ortman 1920). Some species of clams show no such changes in shape. In some fish of central Asia (Ni- kolski 1933), the body changes downstream from a torpedo-shape to a flatter, longer form. These changes are probably a result of downstream reduction of water current, increase in amount of calcium in the water, and higher temperatures. The formation of spines and tubercles, for instance, would require an abundance of calcium and quiet water. EVOLUTION In all probability species inhabiting quiet waters are ancestral to those occurring in running waters (Dodds and Hisaw 1925, Hora 1930). In- vasion of stream habitats requires mechanisms for contending with the force of current, and orientation behavior for maintaining position. Convergent evolu- tion has occurred in many kinds of animals under the influence of current, as shown by similarities in struc- ture and habits (Shelford 1914a). Inducements to the invasion of swift waters have doubtless been new sources of food, escape from enemies, and avoidance of competition with the abundant life of lakes and ponds. As adaptations to stream habitats evolved, ani- mals have largely lost their ability to occupy quiet waters. They no longer can tolerate the lower oxy- gen tension, silt bottoms, and the absence of current which brings them food and oxygen, and, in some forms, such as the Hydropsychidae, helps build their shelters and nests. LIFE HISTORIES The life-cycle of stream insects is remark- able for the long duration of the immature stage in many species and the brief life of the adult. The naiads of mayflies pass through a number of molts (20-40), and this immature stage may last from six weeks to two years. When ready to emerge, the naiad comes to the water surface or crawls out onto a stone, molts into a subimago, and flies away. Within a few minutes, or a period of one to two days at the longest, the subimago undergoes another molt, unique in in- sects, into the fully mature adult. The adult insect does not eat and lives only a few hours or days; dur- ing this time reproduction takes place. Mating oc- curs in flight, hundreds or thousands of individuals swarming in flight together. The females lay their eggs aimost immediately after mating. In some spe- cies, deposition is made upon the water surface, the eggs sinking to the bottom; in other species the female crawls down into the water and attaches the eggs, as they are laid, to a rock surface. The eggs have a viscid surface or filaments and quickly be- come attached to submerged objects. Embryonic life may last 11 to 23 days, at the end of which time the naiad is fully formed (Needham et al. 1935, Burks 1953, Hunt 1953). The life-cycle of stoneflies is also 1, 2, or possibly 3 years long in different species, of which time all but a brief interval is spent in the water (Frison 1935). Molting into the adult occurs after the naiad crawls out of the water onto a rock or other project- ing object, and there is no subsequent molt in the adult stage. Adult diurnal stoneflies may feed, al- though the adults of nocturnal species apparently do not. It is of great interest that many species emerge, mate, feed, and carry on all essential activities during the coldest months of the year (Frison 1935). At all seasons, the eggs may be dropped into the water while the female is in flight over the water, or as she alights on its surface. The eggs are mucilaginous and may contain surface filaments or hooks. Caddisfly larvae pupate submerged in cases. As the pupa approaches the adult form, it leaves the case; and, after crawling and swimming, emerges either upon the water surface or on some protruding object. Larval life in different species may be as short as 25 to 80 days, but since overwintering occurs in this stage it may be greatly prolonged. The pupation period is ordinarily shorter than the larval period, and the adults, which probably feed, may live from Streams 53 several days to a few weeks. Species living in tem- perate climates have either one or two generations per year. Some females drop their eggs while in flight but others crawl under the water to deposit them. The eggs are laid in masses in either a single-layered, cement-like encrusting form or in a jelly or gelatinous matrix that swells in water. Eggs are sometimes de- posited on objects above water. Usually, 10 to 24 days are required for their hatching (Balduf 1939). The common hellgrammite of North America ap- pears to require three years to complete its life-cycle, of which it spends two years and eleven months as an aquatic larva. When ready to pupate, the larva crawls out of the water and underneath some loose stone or piece of wood. The adults do not eat and live only a few days. The female lays her eggs in masses attached to supports situated near water or to the upper surface of leaves. Upon hatching, the larvae make their way back into the water (Balduf LOSONE In the crayfish Orconectes propinquus copulation occurs in cool climates from July to November. Fur- ther southward, copulation is delayed until Septem- ber, continues until cold winter weather, and is re- newed again during March and April. Eggs are laid beginning in late March or early April and are car- ried around by the female, attached to her pleopods, or swimmerets. The eggs hatch in 4 to 6 weeks, and the young are carried for another week or two before they become free-swimming. The majority of the young become sexually mature at the end of the first growing season in early October (Van Deventer O37) The female adult black fly deposits her eggs in a mass or string on a stone or other object at water level during late afternoon, usually with only the tip of the abdomen submerged. If the eggs become ex- posed to the air they do not hatch; normally, the lar- vae appear in four or five days at medium water temperature of 20°-22°C. The larval stage persists 13 to 17 days, the pupal period a little more than 4 days, and the adult stage a little over a week when the adults feed, or only 5 or 6 days when they do not (Wu 1931). Stream snails attach their eggs in a jelly mass to the sides of stones during late spring and summer, and development leads directly to the adult. Clams of the family Unionidae, however, have a peculiar mode of reproduction. The sexes are separate, and fertilization of the eggs takes place in the supra- branchial chambers of the female. Development takes place through several weeks in these marsupial gills, and each egg grows into a minute glochidium. These larvae are later shed into the water, where further development requires that the glochidia become at- tached to the gills, skin, or fins of fish. The larvae may be parasitic, feeding on nutrients absorbed from the fish; this stage may last from 9 to 24 days. Later, the cyst formed by the fish around the glochidium weakens, and the young animal escapes to take up a free-living existence. Breeding occurs from May to August in Quadrula and Unio, while in some spe- cies (Anodonta, Lampsilis) breeding does not occur until late in the summer and the glochidia are re- tained in the female over winter. The life history of clams is of special significance in showing that dispersal depends, to a large extent, on the movements of the fish to which the clams are attached. There is evidence that some species of clams cling to particular species of fish only, so that distribution of the two forms in the stream is closely correlated. The fingernail clams Sphaeriidae, on the other hand, are hermaphroditic and lack the glo- chidial stage. The fingernail clams are annuals; the larger unionid clams may live 10 to 15 years (Coker et al. 1922, Boycott 1936, Matteson 1948). Some sponges, and perhaps also bryozoans, are perennial, although they may become fragmented as a result of floods or freezing during the winter ; they may die during times of low water. Both kinds of animals have vegetative buds, gemmules in sponges and statoblasts in bryozoans, that become free of the parent body. The buds are adapted to withstand un- favorable drought or winter periods, and to germinate and form new colonies when favorable conditions re- turn. The nesting habits of some stream fishes have already been mentioned. Some of the darters and dace defend their nests, or small territories around their nests, against intruders; other species appear to not do so. Individuals of territorial species do little wandering, and it is possible that a darter may persist through several generations in the same riffles. There is increasing evidence that some larger species of stream and pond fishes have definite home areas, and that the fish population of a small stream with riffle- pool development may be considered as a series of discrete, natural units. This has been demonstrated with tagged individuals for species of bass, sunfish, suckers, and bullheads (Gerking 1953). Homing tendencies, however, are developed to varying de- grees, and some species appear to move around in a quite random manner (Thompson 1933). FOOD COACTIONS The basic food substances for stream ani- mals are detritus, diatoms, and filamentous algae. Detritus consists of dead fragments of plants; par- tially decomposed, finely divided, plant material ; and a certain amount of dead animal matter. Plankton, either plant or animal, is not normally a common source of food, except in outlets from the lakes and 54 Habitats, communities, succession ponds from which they derive and in the sluggish waters near the mouth of the stream. Larger aquatic plants are not characteristic of swift flowing streams ; they occur in sluggish pools. Filamentous algae, how- ever, may be abundant in riffles, and a rich micro- flora of diatoms, with scattered protozoans, may furnish a thin slimy film over the surface of rocks. Animals are adapted to these food resources as filter feeders, microflora eaters, or carnivores (Nielsen 1950). The caddisfly larva Hydropsyche is a fine example of a filter feeder. This species and related forms con- struct silken nets at the entrances of their shelters and strain out food particles brought down by the current. The anterior legs of some caddisfly larvae and mayfly naiads are furnished with brushes of hair- like setae which catch and transfer the detritus to the mouth as the animal faces the current. Black fly larvae have a pair of fans at the anterior end of the body. These fans are of long, curved setae. The larva folds them periodically, and the mandibles comb or brush off the detritus that collects. Clams siphon water through the mantle cavity, and detritus ma- terial and plankton are carried to the mouth through the activity of the cilia of the mantle, gills, and labial palps. Sponges and bryozoans also take detritus into body cavities for feeding purposes. Feeding on the microflora and filamentous algae are planaria, snails, and various insects. Some cad- disfly larvae have mouthparts specially adapted to scrape the thin film of microflora from the surface of rocks. The maxillae of mayfly naiads serve as a comb or brush with which diatoms are swept up into the mouth. Carnivorous species may also be partly herbivo- rous (Table 5-4). Too, there is apt to be seasonal variation in food habits and there are differences of habit between closely related species. Fall and winter stonefly naiads are largely herbivorous, but spring and summer forms comprise genera that are either carnivorous, herbivorous, or omnivorous. Hellgram- mites are largely carnivorous, feeding on immature insects. Crayfish are omnivorous; they appear to prefer dead and decaying material. The smaller fish, including the darters, are largely insectivorous, but also consume some plant material. Suckers, carp, and catfish feed on bottom debris as well as small living animals and plants. Young bass and trout are largely dependent on insects for food, but as they grow larger they turn also to young crayfish and small fish. The population density of fishes is ultimately determined, therefore, by the abundance of invertebrates and, when fishes rely on vision for finding their food, also on the turbidity of the water. The average weight of food in the stomach of fan- tail darters of all sizes, sampled from October to May in New York State, was found to be 0.01354 g (Daiber 1956). If the average biomass of the living food averages 2.83 g/m*® of bottom, then one indi- vidual of this species could get 209 full meals from one square meter if it captured everything that was there. Similarly, mottled sculpins could obtain 130 meals from a square meter. It would be interesting to know what actual percentage of the invertebrate population can be readily captured by fish and how frequently the fish feed, for correlation with the density of the fish population. Fish, however, also depend to a considerable extent, especially in summer, on small terrestrial organisms that fall, or are washed, into the stream. BIOMASS AND PRODUCTIVITY Of the kinds of animals present in one short coastal stream in California, the caddisfly larvae were found to be not the most populous. But when size was considered, they constituted more bulk than any other invertebrate group (Table 5-5). The inverte- brate biomass per unit area of riffles is invariably much greater than in sand-bottom pools, whether biomass be computed in terms of wet weight, dry weight, or volume. However, the abundance of spe- cies within the riffles depends on whether the stones are loose or are fastened to the bottom, and on whether or not they are covered with algae, moss, or other vegetation (Percival and Whitehead 1929). The biomass of mud-bottom pools may sometimes exceed that of the riffles, especially if it contains the burrowing mayfly naiad Hexagenia (Behney 1937, Forbes 1928, Lyman 1943, Needham 1932, O’Connell and Campbell 1953, Pennak and Van Gerpen 1947, Smith and Moyle 1944). In the mud-bottom Silver Springs stream in Florida, the dry weight biomass of plants averaged 809 g/m?, herbivores 37 g/m’, small carnivores 11 g/m?, and large carnivores 1.5 g/m? (Odum 1957a). Insect populations in streams vary with the sea- son (Table 5-6). Peak populations commonly occur during late spring and again in autumn (Daiber 1956, Lyman 1943, Needham 1934, 1938, Stehr and Bran- son 1938). Populations become reduced in summer because of low water ; in winter, because of low tem- perature and ice. Small streams tend to have greater densities of insect populations per unit area than do large streams. In New York State, streams up to width 2 m have biomasses that average 22.2 g/m? wet weight; from 2 to 4 m, 18.0 g/m?; from 4 to 6 m, 10.1 g/m?; and over 6 m, 7.7 g/m? (Needham 1934). In small streams, the distribution of organisms is nearly uni- form from one side to the other, but in large streams there is a decrease in density from the sides toward midstream (Behney 1937). Larger streams actually Streams 55 TABLE 5-4 Food habits of immature stream insects in Yellow- stone National Park, Wyoming (Muttkowski and Smith 1929). Number of Per cent food specimens types consumed Insect examined nimal Plant Detritus Stonefly naiads 80 54 22 24 Mayfly naiads 109 4 30 66 Caddisfly larvae 115 28 54 18 Diptera larvae 20 0 17 23 contain more organisms, however, in spite of lower densities per unit area, because they have a much larger total bottom surface. The reason for this vari- ation in density per unit area is not clear, but it may be that per given population of sexually mature adult insects in the surrounding region, small streams offer less area than large streams, over which the females can spread their egg-laying. The standing crop of fish in Indiana streams varies from 5.2 to 106 g/m? (46-939 lbs/acre) wet weight for minnows, suckers, centrarchids, darters, and bullheads (Gerking 1949) to 2.7-4.2 g/m? (24- 37 lbs/acre) for rock bass (Scott 1949). The fish crop in warm water streams is generally higher than in cool trout streams, a relation that also holds for the biomass of invertebrates (Pennak and Van Gerpen 1947). Fish are usually more abundant in relatively deep streams than in shallower ones. Brook trout and three other species in one stream in New York State averaged 10.9 g/m? (97.5 lbs/acre), a ratio of 1 :2.1 to the invertebrate food supply (Moore et al. 1934). Of a stream, the richness of a fauna and the size of the biomass that develops depend largely on the fertility and chemical composition of the water. Hardwater streams, with an abundance of salts in solution, tend to have a large and more varied fauna than do softwater streams. Calcium salts, in particu- lar, are required by mollusks for building their shells, and by crayfish for the exoskeleton. The salts and organic matter which are basic substances in all aquatic food chains depend directly on the fertility of the soil over which the water drains. Streams draining areas of fertile soil usually have an abun- dance of stream organisms; biomasses of both in- vertebrate organisms and fish in streams occurring in areas of poor soil are low. The productivity of insects in Algonquin Pro- vincial Park, Ontario, was periodically measured during one summer by collecting, in cages a yard square, all insects as they emerged from the water and transformed into adults. The count varied over different kinds of bottom between June 1 and August 31, 1940, as follows: rubble 6603, gravel 1636, sand 1079, mud 2618 individuals per sq m. Various moun- tain streams in different parts of the country have TABLE 5-5 Relation between numbers per m* and biomass of insect groups in a riffles of a California coastal stream during February and March (after Needham 1934). Total Wet weight number of ay Insect individuals Per cent Grams Per cent Caddisfly larvae and pupae 742 22.2 5.66 43.9 Mayfly naiads 1,853 550 3.61 28.0 Fly larvae and pupae 343 10.3 1.02 7.9 Stonefly naiads 260 7.8 1.58 12.2 Miscellaneous 137 4.1 1.02 tie) Totals 3,335 12.89 shown an annual productivity of trout taken by fish- ermen of 2.2 to 3.9 g/m? (20 to 35 lbs/acre) wet weight (Surber 1937). APPLIED ECOLOGY The chief problems in applied ecology of streams are those of erosion and silting, pollution, and maintenance of biotic productivity at the highest possible level. Erosion and silting Stream erosion becomes considerable when up- land vegetation is so reduced that there is little or no retardation of runoff from heavy rains. Dredging and stream straightening for drainage purposes usually eliminates the riffles habitat. The bare, hard clay that often emerges as the new stream bottom supports very little animal life. Continuous erosion throws a heavy load of fine silt into the stream. This is detrimental. It makes the water opaque; reduces or prevents photosyn- thesis in algae, water moss, and other plant life; handicaps those fish and other animals that depend on sight for finding and capturing food; and clogs the filtering mechanism of various invertebrates. Clams are ordinarily closed less than 50 per cent of the time, but in silted waters they may stay closed up to 95 per cent of the time. Clams secrete mucous to keep the mantle cavity cleansed, but when silting is heavy this may not be sufficient and mortality will result (Ellis 1936). Deposition of silt on rock or sand bottom may bring a considerable change in spe- cies composition of animals present. During the last several decades, greatly increased soil erosion in agri- cultural areas has reduced pan and game fishes in our streams, and rough fish, such as carp, have taken their place. Chronically muddy streams may often be cleared by reforesting the watershed, and by practicing mod- 56 Habitats, communities, succession TABLE 5-6 Seasonal variation in invertebrate populations per m’* in a California coastal stream (Needham 1934). Number of Wet weight Predominant Month individuals in grams _ species February 2,862 7.89 Mayfly naiads March—April 2,324 9.76 Mayfly naiads May 18,254 52.94 Blackfly larvae and pupae August 4,524 19.37 Caddisfly larvae and pupae November 6,531 23.03 Mayfly naiads ern erosion control in cultivated areas. With slower runoff, more rainwater soaks into the ground, and the water table is raised. It is also desirable to main- tain vegetation on the immediate stream banks to slow up undercutting. Streambank vegetation is also beneficial for shading the water and keeping it cool enough for such fish as trout. If artificial dams are necessary, they should be small, and located where the drainage begins in the numerous headwaters of the streams. Contour plowing, strip planting, and sod ditches also slow up water movement in hilly areas and should be practiced. Pollution Pollution occurs when foreign substances are introduced into a body of water in amounts sufficient to change its character and chemical composition. This type of pollution is of two forms: industrial wastes, such as those from lead and zinc works, tan- neries, breweries, paper mills, gas plants, mines, atomic energy plants, etc.; and organic sewage. In- dustrial and mine wastes are often acid, and extreme acidity will kill fish and other organisms. Clams are greatly reduced or disappear altogether in acid waters. Industrial wastes contain a great variety of chemical compounds, including salts of the heavy metals, and many of them are very toxic to fish. Young fish and species of small fish appear especially sensitive, and the polluting materials may cause physi- cal or chemical injury to the gills without actually being absorbed into the body (Doudoroff and Katz 1953). The control of radioactive wastes from uranium mills and other atomic energy plants has become an especially serious problem in modern times. No stream can purify itself of these wastes. However, they become diluted downstream, undergo natural decay, settle out in the mud bottom, and are taken up by organisms. Organisms take up elements at equal rates whether they are radioactive or not. Radioactive elements may thus accumulate and be- come concentrated in organisms to an extent many thousands of times greater than their concentration in water. This is of potential harm to man (Tsivo- glou et al. 1957). Fortunately, streams are but little used at the present time for the disposal of radioac- tive wastes. The ecological significance of radioactive wastes and fallout from atomic explosions has been summarized by Odum (1959). The introduction of small quantities of organic wastes may increase the size and productivity of ani- mal populations by adding to the basic nitrogen sup- ply. The limit of the sewage load that a stream can carry without harm is, however, low and soon reached. As fresh organic material oxidizes, carbon dioxide and toxic gases are released into the stream, and there is a drastic reduction in the oxygen content. Fermentation is more rapid in summer than in win- ter, and may begin in wastes before they are dis- charged into the stream. The decomposing organic material continues to be oxidized as it is carried downstream, and when this action is completed the stream is again pure (Coker 1954). There have been many attempts to determine the degree to which a stream is polluted, by means of chemical analyses of the water. There is difficulty, however, in evaluating the extent to which each of the many chemical compounds to be found is harm- ful to the various kinds of organisms. There is con- siderable variation in this respect, even between dif- ferent stages in the life-cycle of the same species. Furthermore, the sewage load may vary from time to time, and infrequent heavy loads may wipe out the animal life in localities where chemical measurements made at other times do not indicate harmful pollu- tion. Various investigators (Richardson 1928, Ellis 1937, Paine and Gaufin 1956, Gaufin and Tarzwell 1956) have attempted to use invertebrate animals as indicators of pollution. The presence of midge fly lar- vae Tendipes riparius, Glyptotendipes, mosquito larva Culex pipiens, rattail maggot, and sludge fly delimit zones of septic pollution. There are relatively few species that can tolerate septic conditions, but those that do may become very abundant. The oligo- chaete worms Tubifex and Limnodrilus, and certain midge fly larvae, such as Tendipes plumosus, indi- cate low oxygen. In general, pond invertebrates are much more tolerant of low oxygen concentration than are those belonging to stream habitats. Species espe- cially tolerant of pollution are those that have adapta- tions for obtaining oxygen at the water surface, such as the dipteran larvae of Culicidae, Syrphidae, and Stratiomyidae, aquatic Coleoptera and Hemiptera, and pulmonate snails. Gill-bearing species generally require clean water of high oxygen content. Among fish, pond species such as carp, bullhead, perch, and crappie are relatively more tolerant than stream species. Streams 5/7 3 ye plantings 8 oe deflectors @* Ke for islands eek) log wing triangle cover Naa i © anchored log at 8 (3 GAY \ |B eNOS oe ACO” EF stone deflector CURES sa anchored log Fish management Basic to fish management in streams is the con- trol of soil erosion and pollution. In clean, clear streams, both invertebrates and fish can attain high populations through normal reproduction. Artificial propagation and release of reared fish into streams to improve fishing is not necessary except where habitats have been depleted of breeding stock or where the fishing pressure is excessive. The artificial raising and releasing of fish of suitable size for quick recapture in sport fishing is expensive but sometimes justified in highly populated areas. In most regions the fish manager is better concerned with improving habitats and letting the fish repopulate them to full carrying capacity on their own accord. Stream fishes suitable for sport and food are pri- marily those inhabiting the pools rather than the rif- fles. The carrying capacity of streams can sometimes be raised by artificially increasing the number of pools without destroying too many of the riffles, the main source of fish food. The interspersion of ponds along the stream also increases its fertility, since they are the sources of plankton, detritus, and dislodged or escaping organisms. The formation of pools may often be done inexpensively by making simple log dams or deflectors. Occasionally, it may be desirable to haul in gravel from elsewhere to make spawning beds and to provide artificial log or brush shelters (Needham 1938, Lagler 1952). There has been a country-wide practice of intro- ducing species of fish into streams where they did not originally occur. The result has been to greatly mix up and modify the fish fauna; original primitive communities no longer prevail. This is unfortunate for ecological research. The U.S. National Park Service is, however, attempting to preserve a certain number of natural stream areas in their original con- dition, prohibitng fishing therein (Kendeigh 1942a). boulder dam FIG. 5-10 Schematic diagram of possible stream modifications affording improved protection and spawning facilities to fish (after Lagler 1952). stumps brush anchored tree boulder retards SUMMARY Streams contain riffles, sand-, and mud- bottom pools. Inhabitants of the riffles and sand- bottom pools constitute a distinct stream biocies. Mud-bottom pools are inhabited by species from the pond-marsh biocies. Animals adjust to the action of water current by clinging mechanisms, avoidance, or vigorous swimming. They are generally positively rheotactic, and several forms maintain orientation to a particular position in the stream by means of visual landmarks. Segregation to different habitats depends largely on differential response to the substratum; that is, preference respectively for rock, sand, or mud. Animals occurring in mud-bottom pools are usually negatively rheotactic, or become helpless in strong current. They also have adaptations tolerant of lower oxygen concentrations in the water. Changes in the size of the stream, occasioned by various physi- cal factors, also affect the responses of animals. Stream animals have apparently evolved from an- cestral types that occupied the quiet waters of lakes and ponds. The life cycles of many stream insects are remarkable for the long duration of immature stages and the brief life of adults. Animals have various adaptations to feed on detritus in the water, on di- atoms, on filamentous algae, or for being carnivorous. Density of individuals, biomass, and productivity of invertebrates are ordinarily less in sand-bottom pools than in either riffles or mud-bottom pools. Clams and game fish, however, inhabit sand-bottom pools. Fish management requires the proper interdigitation of pools and riffles, as well as control of erosion, silting, and pollution. 58 Habitats, communities, succession Local Habitats, Communities, and Succession: Lakes Lakes are large bodies of fresh water, often deep enough to have a pronounced thermal stratification for part of the year. Typically, shores are barren and wave-swept (Muttkowski 1918). Lakes are formed in youthful stages of river system development. Water from upland runoff, groundwater seepage, springs, and melting snow- fields and glaciers collects in basins. As the basins fill to overflowing, erosion of outlets starts ; as it goes on, outlets are deepened and water level of the lake drops. Products of erosion, carried into the basin by wind and water, and the products of animal and plant decay accumulate, making the water shallow. Morphometry aside, the essential distinction be- tween lake and stream habitats is the characteristic of water movement; continuous, rapid flow is the characteristic of the stream, the Jotic habitat. The lake is a lentic habitat; the water is essentially a standing, quiescent body, although at times wind ac- tion stirs surface layer and margins into great turbu- lence. Habitat factors associated with the lentic en- vironment are uniquely modified to it (Welch 1948, Hutchinson 1957). HABITAT Pressure, density, and buoyancy The pressure imposed on a lake-dwelling or- ganism is the weight of the column of water above it plus the weight of the atmosphere. Most lakes have a maximum depth of less than 30 meters; the Great Lakes of North America vary from 64 to 393 meters in depth, Crater Lake in Oregon is the deepest on the continent, 608 meters (Welch 1952). Maximum pressures are much less than in the ocean, and or- ganisms appear to adjust to them readily. The ab- sence of animal life from deep water is ordinarily a consequence of low oxygen supply, or low tempera- ture, rather than pressure. The density of water varies inversely with tem- perature and directly with the concentration of dis- solved substances. Water is most dense at approxi- mately 4°C. Water becomes progressively less dense as it is cooled below +4°C; ice expands markedly (i.e., becomes less dense) the colder it gets. It is because the coldest water is at the surface in winter that ice forms there, rather than at the bot- tom. In summer, the coldest waters of deep lakes are at the bottom. Dissolved salts increase the density of water; the density of most inland water- bodies is much less than that of the ocean. When great evaporation occurs in a lake having no out- let, as in the Great Basin, the lake may come to contain a higher percentage of salts than the ocean. The few species capable of living in these very salty lakes include some algae and Protozoa, the brine shrimp Artemia gracilis, and the immature stages of ay, two brine flies, Ephydra gracilis and E. hians. There are no fish in the Great Salt Lake of Utah (Wood- bury 1936). By the law of Archimedes, the buoyancy of an object is equal to the weight of the water it dis- places. Buoyancy varies with the density of water, and is influenced by the factors that affect density. Viscosity, the measure of the internal friction of water, varies inversely with temperature and also in- fluences buoyancy. An organism will sink unless it keeps station by swimming movements, or unless it has special adap- tations to decrease the specific gravity of the body and take advantage of any turbulence in the water. Such adaptations take several forms: absorption of large amounts of water to form jelly-like tissues ; storage of gas or air bubbles within the body ; forma- tion of lightweight fat deposits within the body, or oil droplets within the cell; increase of surface area in proportion to body mass, which increases frictional resistance (Davis 1955). When an organism so equipped dies, the special mechanisms quickly cease to function, and it sinks to the bottom. If dead or- ganisms did not sink to the bottom, living organisms, with the exception of some bacteria, could not exist in an aquatic habitat. An interesting phenomenon is cyclomorphosis, a seasonal change in body form that develops in many plankton organisms, both plant and animal, including protozoans, cladocerans, and rotifers. In general, the summer generations have higher crests, longer spines, longer beaks, or longer stalks, than do the winter gen- erations. It is believed that the increased surface area provided in the summer forms is induced by the higher water temperatures obtaining then, and may FIG. 6-1 Cyclomorphosis of Daphnia retrocurva in a Connecticut lake (from Brooks 1946). be an adaptation to the decreased buoyancy of the water at this season, but factors other than temper- ature also appear to be involved (Brooks 1946). Light The daily alternation of light and darkness es- tablishes a rhythm in the activities of many aquatic organisms. Light is essential to plant photosynthesis ; some fish require light by which to feed. Many or- ganisms orient to light, and some are sensitive to light of particular wavelengths, notably ultraviolet. Small, soft-bodied, bottom-dwelling organisms are particularly sensitive to light, and it is thought that the evolution of pigmentation, chitinous exoskeletons, shells, cases, and similar structures may have helped certain otherwise photosensitive species to survive in shallow, well-lighted areas (Welch 1952). A common way to measure the relative transpar- ency of water is to lower a Secchi disk, a white plate 20 cm in diameter attached to a cord marked off in linear units, marking the depth at which the disk dis- appears from sight. The disk is lowered a bit farther, then raised until it reappears, and that depth marked. The two depths are averaged. The light intensity at the depth of disappearance of the disk is usually about 5 per cent of that at the surface (Hutchinson 1957). Other more exact procedures employ photographic methods, pyrlimnometers, or photoelectric cells (Shelford 1929). The depth to which light penetrates into water is affected by intensity of the light, angle of ray in- cidence, reflection at the surface, scattering within the water, and absorption. Penetration anywhere is re- duced when the sun is away from the zenith; is less in waters at high latitudes ; and is much less in winter compared with summer. About 10 per cent of the light falling on Lake Mendota, Wisconsin, during the spring and summer ts reflected ; about 15 per cent during the autumn (Juday 1940). In the unusually clear waters of Crystal Lake, Wisconsin, measure- ments with a pyrlimnometer indicated only a small surface reflection light loss, a penetration of 67 per cent of full intensity to a depth of one meter, and 10.5 per cent of full intensity at 10 meters (Birge and Juday 1929). In pure water, red light is ab- sorbed most rapidly, at a rate of 64.5 per cent per meter ; orange, at 23.5 per cent per meter ; yellow, at 3.9 per cent; green, at 1.1 per cent; blue at only 0.52 per cent. Blue penetrates the farthest. Violet is ab- sorbed at 1.63 per cent per meter. Very little ultra- violet penetrates the water, and nearly all the infra- red is absorbed in the first meter (Clarke 1939, Ruttner 1953). Suspended material in water produces turbidity, and reduces light penetration. In western Lake Erie, 60 Habitats, communities, succession the depth to which 1.0 per cent of surface light pene- trates varies from 9.7 m when turbidity is 5 ppm, to 0.8 m when turbidity is 115 ppm (Chandler 1942). Since phytoplankton require light for photosynthesis, abundance varies inversely with turbidity. Light penetration is also affected by the abundance of or- ganisms themselves, both phyto- and zooplankton. An appreciable amount of light passes through ice in the winter. This enables phytoplankton photo- synthesis to continue. In eutrophic lakes many fish may suffocate when snow overlies surface ice, pre- venting photosynthesis and, thus, the generation of oxygen (Greenbank 1945). The apparent color of water bodies may be the result variously of sky reflections, the color of the bottom, suspended materials, or of plants and animals. But apart from these extraneous factors, water often has an intrinsic color deriving from its chemical contents. The blue color of pure water is a result of blue-light scattering by water molecules. Iron gives water a yellow hue. A green color is usually associ- ated with high concentrations of calcium carbonate. Water from bogs or swamps contains humic ma- terials and is often dark brown. Many waters are essentially colorless. In a Wisconsin lake showing practically no color, maximum photosynthesis of algae occurred at one meter depth on bright days: some photosynthesis occurred down as far as 15 meters. In a highly colored lake, maximum photo- synthesis occurred at 0.25 meter, none at 2 meters (Schomer 1934). Photosynthesis releases oxygen into the water; respiration and decomposition absorb it. The upper layer of a lake, where photosynthesis predominates, is called the trophogenic zone. Below this zone there may still be considerable photosynthesis, but oxygen absorption is greater than oxygen release. The deeper portion of a lake is called the tropholytic zone. The two zones are separated by a thin layer where the oxygen gains from photosynthesis during the day- light hours are balanced by the respiratory and de- composition losses during the day and night. This is the compensation depth, to which generally about one per cent of the full sunlight at the water’s surface penetrates. The compensation level in a dark-colored bog may lie less than a meter below the surface; in a deep, clear lake it may be 100 m down. Wind and currents Wind is an important environmental factor of lakes because of water currents it generates. The effect of wind action depends largely on the extent of the exposed water surface, the presence or absence of protecting upland, and the configuration of the lake relative to the prevailing wind direction. Waves may become sizable in large lakes, but the forward motion of a wave does not involve any great mass of water. The rate of movement of surface water is usually less than 5 per cent of the velocity of the wind. The wave form moves on while the water beneath undergoes a more nearly cycloidal mo- tion, except along the shore, where the wave mass progresses forward and breaks as surf. The water washes back off the beach as an undertow, only to be carried forward again by the incoming waves. The problem of maintaining position here is similar to the problem of maintaining position in streams. The depth of wave action in the open lake and along the shore depends largely on the strength of the wind (Ruttner 1953). In summer, surface water is warmed by solar radiation and its density, weight, and viscosity de- crease. In deep lakes the warm water piles up on the exposed shore until, moving down along the bottom, it encounters colder and denser waters, which resist mixing. The warm water is then diverted horizon- tally to the opposite shore. Thus the lake beomes stratified horizontally into an upper epilimnion, where the water circulates and is fairly turbulent, and a lower hypoliminion, which is relatively undisturbed. This difference in circulation in deep lakes is closely correlated with differences in temperature and oxy- gen characteristics; it is of considerable importance in the distribution of the biota. Temperature The thermal conductivity of water is very low; but because of the thorough mixing of the waters in the epilimnion during the summer by wind action, the temperature is nearly uniform down to the thermo- cline. The thermocline is the zone of most rapid temperature decrease, generally involving a drop of { === sheor plane samme Thermocline = .. 3° ‘ Duis oa ote ca : Hypolimnion. - ‘ . FIG, 6-2 Water currents and thermal stratification in a deep lake. Lakes 6] TEMPERATURE, 50 45 IN METERS DEPTH ey ~~ -~ — ayo eS Soe rs TEMPERATURE, a -~ oF 20 30 40 ae — - . Nee ee oe fo) (2) INSEE ~“ (e) DEPTH @ (eo) 10 AG FIG. 6-3 Vertical temperature distribution throughout a year in a dimictic lake of the second order—Convict Lake, California; elevation 2308 m (from Reimers and Combs 1956). at least 1°C per meter of depth (Birge 1904) and occasionally as much as 7°C per meter. The thermo- cline, as here defined, equals the metalimnion of some authorities who have a different conception of the thermocline (Hutchinson 1957). When the thermo- cline forms, early in the season, it is close to the surface. As the season progresses, it sinks lower, in- creasing the volume of the epilimnion and decreasing the volume of the hypolimnion. The temperature of the hypolimnion is fairly uniform, although it de- clines gradually from the lower edge of the thermo- cline to the bottom, where it is seldom below 4°C. During the autumn, the surface water cools and 62 the thermocline sinks. The epilimnion increases in thickness until it includes the entire lake. The waters are then uniform in temperature and density, at all depths. Even slight winds produce complete circula- tion. This is the autumn overturn, which may last for the several weeks, until ice forms. As surface waters cool below 4°C they no longer sink, and ice may form. Less dense than the underly- ing water, it floats. Immediately below the ice, the temperature of the water is very close to 0°C, but in one or two meters of additional depth it usually rises rapidly to 4°C, although in some lakes temperatures below 4°C occur even at considerable depths. Habitats, communities, succession As the ice melts during the spring and the sur- face waters warm up, a spring overturn occurs when the water at all depths is at the same temperature. The time and duration of the spring overturn de- pends on weather conditions; it may last several weeks. It often occurs intermittently, however, cor- responding with changes in weather and water tem- peratures. When a lake has two overturns during the year, it is called a dimictic lake. Such lakes are character- istic of, but not limited to, temperate climates. In warm, oceanic climates and in the tropics, the sur- face waters may not cool sufficiently to permit com- plete circulation, except during the coldest period of winter. Lakes undergoing a single overturn are called warm monomictic. The temperature of the water in the hypolimnion of such a lake is never lower, of course, than the mean air temperature dur- ing the period of the last complete circulation; in warm climates this may be several degrees above 4°C. On the other hand, lakes in polar or alpine regions may never warm above 4°C, and complete circulation occurs only in the middle of the summer. These are cold monomictic lakes (Hutchinson 1957). The three types of lakes were formerly called tem- perate, tropical, and polar, but this terminology is undesirable since their geographical segregation is not precise. Lakes of the first order are those in which the bottom water remains at or near 4°C throughout the year, and while one or two circulation periods are possible, there is often none. In lakes of the second order, the temperature of the bottom water rises above 4°C during the summer, and there are one or two regular circulation periods during the year. Lakes of the third order do not develop thermal strati- fication, and circulation of water is more or less con- tinuous (Whipple 1927). In general, lakes over 90 meters in depth belong to the first order; those be- tween about 8 and 90 meters belong to the second order ; and those less than 8 meters to the third order. The specific heat of water is greater than most other substances ; accordingly, a vast amount of heat must be absorbed to cause a temperature change. Temperature change is, in any event, slow. Much of the energy of solar radiation is lost by reflection from the water surface. The rest of the radiation is ab- sorbed by the water, the solutes, and the suspended material. But much of the diurnal energy increment may be dissipated by re-radiation at night or in cloudy weather, by evaporation, and by convectional cooling. The amount of heat actually retained by a lake to melt its winter ice and warm it from the winter minimum up to the summer maximum is its annual heat budget (Table 6-1). For many dimictic lakes this is between 20,000 and 40,000 g-cal/m? of surface; there is wide variation in different kinds of TABLE 6-! Monthly change in cumulative heat budget and solar radiation in the Bass Islands region (depth 7.5 m) of western Lake Erie, in 1941. A 20.3 cm ice covering formed in mid-Janu- ary, melted in late March. The maximum heat budget, reached on July 30, was 19,575 g-cal/cm’. The heat budget was about 15 per cent of the total solar radiation received during the year (after Chandler 1944). g-cal/cm? g-cal/cm? Solar Solar Heat radia- Heat radia- Month budget tion Month budget tion January 105 3,364 July 18,765 17,291 February 206 5,849 August 18,112 16,375 March 581 10,201 September 16,331 12,737 April 6,405 13,952 October 11,115 6,829 May 12,581 17,156 November 4,350 4,147 June 16,369 15,960 December 1,369 2,599 lakes. The annual heat budget is important in de- termining a lake’s productivity. Oxygen The distribution of oxygen at various depths depends upon the presence or absence of a thermo- cline, the amount of vegetation, and the organic na- ture of the bottom. The amount of oxygen in water is only one-fortieth to one-twentieth of that present in an equal volume of air when the two are at equi- librium, although their partial pressures are the same. Diffusion of oxygen from the air into comparatively sedentary water occurs very slowly; agitation of the water increases the surface area and promotes a faster rate of equilibration. DEPTH IN METERS fe) _—__. = 4S w (e) oO DEPTH IN FEET 60 20} | On li2 aes), Aur Olea ae Siae CUBIC CENTIMETERS PER LITER FIG, 6-4 Changes in the vertical distribution of oxygen through- out a year in a dimictic eutrophic lake—Lake Mendota, Wis- consin (after Birge and Juday 1911). Lakes 63 The amount of oxygen released by plants varies with their abundance and time of day ; photosynthesis can take place in light. Phytoplankton and_ the rooted vegetation restricted to the shore line are im- portant sources of oxygen to the water. With rapid photosynthesis in relatively small volumes of water, the water may be supersaturated with oxygen for short periods of time. The oxygen supply of lakes is reduced in various ways; most notably through the respiration of ani- mals and plants and the decomposition of organic matter. As lake waters warm up during the summer, their capacity to hold oxygen is reduced and oxygen may be released into the atmosphere. The saturation capacity of water at 0°C is 10.2 cc per liter, but at 25°C it is only 5.8 cc per liter. In some lakes, decom- position of organic material at the bottom may deplete the hypolimnion of its oxygen content for several weeks during the summer; perhaps lower than the level minimal to the support of life. This is called the summer stagnation period. During the winter, if the lake is covered with ice and snow, there may be a win- ter stagnation period. The oxygen supply of the deep waters is renewed with the autumn and spring over- turns. Before decomposition can proceed very far there must be calcium in the water. Hence, decom- position is slow in soft or acid waters. At temperatures of 15°—26°C oxygen concentra- tions of less than 2.4 cc/1 (3.5 ppm) are fatal within 24 hours to several species of fish. From 0°-4°C, oxygen concentrations can decline through 48 hours to 1.4 cc/l (2.0 ppm), or even to 0.7 cc/I liter (1.0 ppm), before the same mortality results (Moore 1942). Some planktonic invertebrates can tolerate oxygen concentrations as low as 0.2 cc/l (0.3 ppm) and, for short periods, even 0.1 cc/l (0.1 ppm). Some bottom-dwelling protozoans, annelids, mol- lusks, and insect larvae may survive actual anaerobic conditions for periods of days, even weeks. Organ- isms that tolerate a lack of oxygen do so by creating an oxygen debt; that is, the lactic acid and other breakdown products produced in consequence of muscular activity simply accumulate until conditions permit oxidation of them. In true anaerobes these acid waste products are eliminated from the body; no oxidation debt is established. European workers, principally Thienemann and Naumann, have devised a classification of lake habi- tats into three main categories on the basis of fertil- ity and the amount of oxygen in the hypolimnion during the summer concentration. The oxygen con- centration in the hypolimnion is, of course, a reflec- tion of the fertility of the lake, since it is inversely proportional to the amount of decaying organic mat- ter. Dystrophic lakes contain considerable organic matter but are infertile because the organic matter does not completely decompose and there is release of organic acids. Oligotrophic lakes are usually deep (over 18 meters) with very little shallow water, and little veg- etation around margins. Bottom contours are V- shaped; they are low in fertility, rich in oxygen in the hypolimnion (orthograde distribution), low in COs, and the color of the water varies from blue to green. The volume of the epilimnion is usually less than the volume of the hypolimnion. The fish popu- lation is not large. Characteristic species are lake trout, whitefish, and cisco. The midge fly larva, Tanytarsus, predominates. Plankton is not abundant. The Finger Lakes of New York are of this type. Eutrophic lakes are usually less than 18 meters deep, the bottom contour is U-shaped, water color varies from green to yellow or brownish green, and there are larger areas of shallow waters and more marsh vegetation. Fertility is high, and because of rich bottom humus the oxygen content of the hy- polimnion is greatly reduced during the summer (clinograde distribution). The CO, content is ac- cordingly high. The volume of the epilimnion is usually greater than that of the hypolimnion. Plank- ton is abundant. The midge fly larva Tendipes is very numerous and the culicid larvae Chaoborus is usually present. The bottom fauna is rich, and there is a large fish population in the epilimnion. Charac- teristic fish species are the largemouth bass, perch, sunfish, and pike. These lakes occur in relatively mature river systems; many lakes in Minnesota and Wisconsin are of this type. Dystrophic lakes are bog-like, very rich in mar- ginal vegetation and organic content. Oxygen is likely to be scarce at all depths. The water is usually conspicuously colored, yellow to brown, and may be acidic because of organic acids and incompletely oxi- dized decomposition products. Plankton, bottom or- ganisms, and fish are usually scarce, but blue-green algae are sometimes abundant. Tendipes may pre- dominate among the bottom forms, but at times only Chaoborus is present. Characteristic fish are stickle- backs and mud minnows. Many lakes of northern latitudes are dystrophic in type. All gradations exist between these three types of lakes, and individual lakes are often difficult to classify. Oligotrophy is indicated if the loss of oxy- gen in the hypolimnion during the summer is not over 0.025 mg/cm?/day ; eutrophy, if it is over 0.055; mesotrophy, if it is between the two (Hutchinson 1957). A lake may change from one type to another as succession proceeds (Lindeman 1942). Probably all lakes start as oligotrophic, but as they accumulate vegetation and decaying organic matter, they change into eutrophic lakes; or, if the organic matter does not completely decompose, into dystrophic lakes. 64 Habitats, communities, succession Eutrophic lakes may later develop into ponds and marshes; dystrophic lakes, into bogs. Biotic succes- sion is scarcely discernible, however, in very large or very deep lakes. The Great Lakes, for instance, will endure until erosion lowers their outlets. Carbon dioxide and other gases Carbon dioxide is required by plants for photo- synthesis. Its presence in lake waters tends to vary inversely with oxygen. Carbon dioxide is derived from the atmosphere, the respiration of both animals and plants, decaying organic matter, ground water, and bicarbonate salts. It may occur in either the free state (dissolved COs), half-bound state (HCOs;), or fixed state (CO3). These three states are asso- ciated respectively with pH values 7, 7 to 10, and above 10. Algae and some rooted aquatic vegetation are able to obtain the half-bound CO. from the solu- ble bicarbonate salts, thereby converting them into the less soluble carbonates : Ca(HCOQOs)>. — CaCOs + COs oe H,O Mollusks, a few insects, and some bacteria are also able to precipitate carbonates. Carbonates precipitate as to make conspicuous marl deposits on the bottom of some lakes. When marl formation becomes consid- erable, there is a decrease in lake fertility and a conse- quent decrease in animal life present, including bottom-inhabiting organisms. When there is sufficient free carbon dioxide in the water derived from sources other than carbonates, they are converted back into bicarbonates and marl does not form. The degree of alkalinity of a lake is measured by the amount of carbon dioxide or acid required to convert the excess carbonates into bi- carbonates, yielding neutral water. Soft-water lakes contain not over 5 cc/l fixed carbon dioxide; medium-class lakes contain 5 to 22 cc/l; hard-water lakes may have from 22 to as high as 50 cc/I (Birge and Juday 1911). Marsh gas (methane) evolves from organic mat- ter decomposing at the bottom. It rises in bubbles to the surface of the water. Methane formation may be extensive during the summer stagnation period. Methane does not appear to be particularly toxic to organisms until it is generated in very large amounts. Hydrogen sulphide results from anaerobic de- composition of sulphurous organic matter. It may be conspicuous in sewage-polluted waters. It is inher- ently very poisonous. Nitrogen occurs in water by reason of diffusion from the atmosphere. When present in excessive amount it has been known to form bubbles in the circulatory systems of fish causing death, but this does not commonly occur in natural waters. Ammonia may occur naturally in water, a result of decomposition of organic matter. Ammonia may also be dumped into streams and lakes from indus- trial plants, often in concentrations toxic to fish. Fish are apparently unable to detect the presence of am- monia in water. Dissolved solids Falling rain may contain as much as 30 to 40 ppm of solids, and the runoff dissolves more as it drains over the upland into streams and lakes. Water draining off siliceous or sandy soils may contain 50 to 80 ppm of dissolved minerals; off more fertile calcareous soils, 300 to 660 ppm. Lake waters com- monly vary from about 15 to 350 ppm of dissolved minerals, although in some lakes of the Great Basin, the total dissolved salts exceed 100,000 ppm. The ocean contains only 33,000 to 37,370 ppm. Inorganic salts especially important for plants in- clude ammonium salts, nitrites, and nitrates as sources of nitrogen ; phosphates to supply phosphorus which, with nitrogen and sulphur, are raw materials for protein synthesis; silicates, which furnish silicon to diatoms and sponges; and salts of calcium, mag- nesium, manganese, iron, copper, sodium, and po- tassium for proper development of chlorophyll and growth of plants and, indirectly, of animals. Mol- lusks require calcium salts for shells. Crayfish and other arthropods require calcium for the carapace ; vertebrates, for their skeleton. Absence of these necessary salts in lake waters limits the kinds and abundance of animals that can live there. Phosphorus and nitrogen are the most likely to be deficient. Nutrient salts tend to accumulate in the deeper waters and at the lake bottom, but they are brought to the surface at the autumn and spring overturns. Lakes in prairie regions tend to have more salts than those in deciduous or hardwood forests, which, in turn, have more salts than lakes in coniferous forest areas (Moyle 1956). The total dissolved content of a lake is important in determining its general level of pro- ductivity (Northcote and Larkin 1956). Little is known about the amount of amino acids, fats, and carbohydrates occurring in natural bodies of water and how much of this nutrient material may be directly absorbed by organisms. Dissolved organic matter is derived chiefly from plankton remains, and other dead plants and animals as well as from bot- tom mud and external sources. In Wisconsin lakes, there is about 15 mg/l, of which crude protein con- stitutes 15 per cent, fats or ether extract 1 per cent, and carbohydrates about 83 per cent. Dissolved or- ganic material becomes higher, of course, in dystro- phic lakes and peat bogs (Birge and Juday 1934). Lakes 65 Hydrogen-ion concentration The acidity or alkalinity of water depends on the ratio between the H+ (or hydronium, H; O+) and OH~ ions. The amount of acidity or alkalinity is commonly expressed in terms of potential hydro- gen ions in a pH scale. The values on this scale rep- resent the logarithm of the reciprocal of the normal- ity of free hydrogn ions. When the number of Ht ions is equal to the number of OH ions, the pH value is 7, the value which represents absolute neu- trality. All pH values less than 7 indicate a greater number of H+ ions than OH™ ions, which is to say the closer the pH value approaches 0, the more acid the water. Above pH 7, there is a preponderance of OH— ions; the higher the pH value, up to 14, the more alkaline is the water. The hydrogen-ion concentration of most unpol- luted lakes and streams is normally between pH 6.0 and 9.0, but extreme values of pH 1.7 and pH 12.0 occasionally occur (Hutchinson 1957). In some bodies of water, the pH value fluctuates consider- ably. Hydrogen-ion concentration increases (low pH values) with active decomposition of organic matter. In general, aquatic animals can tolerate great changes in pH, although the range of toleration varies between species. Mollusks are not ordinarily found in acid lakes, but some snails can survive pH as low as 6, and the fingernail clam Pisidium down to pH 5.7. At the lower pH values, the shells of mollusks become thin, fragile, and transparent, but it is be- lieved that the cuticular covering is partially protec- tive and prevents complete dissolution of the calcium carbonate by the acid (Jewell and Brown 1929). In Campeloma snails, the apex of the shell may com- pletely dissolve, exposing the apex of the visceral mass. Most fish can tolerate pH 4.5 to 9.5 provided there is plenty of oxygen (Brown and Jewell 1926, Wiebe 1931), and many invertebrates will tolerate even greater extremes. Fish as individuals become acclimated to certain pH values, and will select those values when given choice in a gradient. Such ac- climation of individuals may have an effect on their choice of natural habitats, although when forced into a habitat with a different hydrogen-ion concentration, they change their acclimation. Although the direct ecological importance of differences in hydrogen-ion concentrations is doubtful, the measurement of pH may serve as an index of other environmental con- ditions, such as the amount of available carbon di- oxide (with which it varies inversely), dissolved oxygen (with which it varies directly), dissolved salts, etc. Sometimes the difference in species of plankton found in bodies of water with permanently different pH values, for example in granite and lime- stone, is very striking (Reed and Klugh 1924). LAKE BIOCIES If we reserve ponds and peat bogs to sep- arate consideration, there remain two major lake communities. They differ in species composition, abundance of organisms, distribution of niches, pro- ductivity, and physical characteristics. Inasmuch as these two communities correspond fairly well to the oligotrophic and eutrophic types of lakes, we may name them simply the oligotrophic and eutrophic lake biocies. Various facies of each community, or inter- mediate types (Deevey 1941) are affected by varia- tions in the abundance of component species and correspond to differences in temperature, depth, fer- tility, and other features of the habitat. The com- munities that occur in dystrophic lakes ; for instance, are an impoverished facies of the eutrophic lake biocies. In spite of taxonomic differences in constitu- ent species, each lake biocies contains organisms be- longing to the same life-forms and with similar mores so they may be discussed together. Depending largely on their morphological adap- tations and behavior, aquatic organisms are, for con- venience, divided into plankton, neuston, nekton, and benthos, although the differences between the groups are not precise. Seston is a collective term that in- cludes all small particulate matter, both living and non-living, that floats or swims in the water. Plank- ton are free-floating or barely motile organisms, either plant (phytoplankton) or animal (zooplank- ton), that are readily transported by water currents. Most plankton are microscopically small, although some forms are visible to the unaided eye. Species that can be caught with a net are called net plankton to distinguish them from the minute varieties that pass through No. 20 silk bolting cloth meshes. The latter include most protozoan, bacterial, and fungal forms, collectively called nannoplankton. Organisms that depend on the surface film for a substratum are called neuston and are more important in the quiet waters of ponds than in lakes. Nekton are larger animals that are capable of locomotion independent of water currents. Aquatic birds that swim and dive are included in this group. Benthos organisms are attached to or dependent on the bottom for support ; there are sessile, creeping, and burrowing forms. PLANKTON Fresh-water plankton (Welch 1952, Pennak 1946, Davis 1955) includes representatives from the photosynthetic algae, Bacillariaceae (diatoms), Myxophyceae (blue-green), and Chlorophyceae (green), and occasional other form such as Wolffia among the higher plants ; the non-photosynthetic bac- 66 Habitats, communities, succession teria and other fungi; and among the zooplankton, all classes of Protozoa except Sporozoa, Rotatoria, Entomostraca (especially Cladocera, Copepoda, and Ostracoda), some immature Diptera, the statoblasts and gemmules of bryozoans and sponges, the rare fresh-water jellyfish, Craspedacusta, and occasional aquatic mites, gastrotrichs, and others. Fresh-water plankton lack many forms common in the plankton of the ocean. On the other hand, the rotifers, aquatic insects, and water-mites are mostly absent from the sea, and the Cladocera are only poorly represented. It is probable that plankton evolved from benthonic forms occurring near the shore (Ruttner 1953), and many species of groups listed above, notably Ostra- coda and Rotatoria, are still largely benthonic in be- havior. The algae in fresh water may vary in numbers from hundreds of thousands to tens of millions of cells per liter ; Protozoa, from thousands to hundreds of thousands of individuals per liter ; and the rotifers and entomostracans, from less than ten to hundreds per liter. Distribution Many species of plankton are nearly world- wide in distribution, particularly those that occur in the larger lakes. Cosmopolitan distribution and the many primitive types of the plankton community in- dicate that its origin is ancient. Some plankton, how- ever, such as species of the genus Pseudodiaptomus, have a very limited distribution. The plankton found in the open water of small to medium-sized lakes is seldom more than one to three species of copepods, two to four species of cladocerans, and three to seven species of rotifers, although the species change from one time of the year to another. It is also unusual to find more than one species of the same genus at the same time. When two do occur, one of them is usually much more abundant than the other. It is commonplace to find that 80 per cent or more of all limnetic copepods present belong to a single species; 78 per cent of all cladocerans to a single species, and 64 per cent of all rotifers to a single species (Pennak 1957). In any one lake the horizontal distribution of the plankton may be irregular because of water currents, inflowing streams, irregularity of shore line, or swarming of a particular species in local areas. The vertical variations in the composition and abundance of species is even more striking. The chlorophyll- bearing algae require light and are most numerous in the upper stratum, although diatoms commonly occur at greater depths (Fritsch 1931). The verti- cal distribution of zooplankton varies widely with the species, but it is strikingly affected by light, food, FIG. 6-5 Common invertebrates found in lakes, (a) Copepod, (b) cladoceran, (c) ostracod, (d) the snail Amnicola limosa, (e) the snail Valvata tricarinata, (f) the ghost larva Chaoborus albipes, (g) the fingernail clam Pisidium. (Modified from various sources, Pennak 1953.) gravity, dissolved gases, particularly oxygen, and thermal stratification. Few zooplankton occur in the hypolimnion of eutrophic lakes during the summer stagnation period, but occur at all depths during the spring and autumn overturns. Diel movements Several species of net zooplankton exhibit pro- nounced vertical migrations, moving upward into surface strata during the night and returning to greater depths during the day. In some instances this daily shifting of position may extend to 60 or more meters, in other instances it may be only a frac- tion of a meter, and some species do not exhibit the phenomenon at all (Langford 1938). A common ex- planation of these movements is that the animals are negatively geotatic by nature, but that during the day this drive is suppressed by a negative phototaxism Lakes 67 and can be expressed only at night (Parker 1902). An alternative explanation is that zooplankton ac- tively orient to a band of optimum light intensity and move up and down at different times to avoid light of too great or too little intensity (Cushing 1951, Hardy and Bainbridge 1954). These diel movements are most widespread among Cladocera and Copepoda, but other species are also involved. One of the most interesting cases is the dipteran larva Chaoborus punctipennis that rests on the lake bottom during the daylight hours but is often teeming in the surface waters at night. It appears that the buoyancy of this larva varies with the size of its two pairs of air-sacs (Damant 1924). There are a few rotifers, Mysis among the Mala- costraca, and Ceratium among the Mastigophora, in which vertical day and night movements have been demonstrated (Pennak 1944). Seasonal distribution The different species of plankton vary in their response to seasonal changes in the physical and chem- ical nature of the water, in number of generations per year, and in time of occurrence. Accordingly there is a marked seasonal variation in total numbers dur- ZOOPLANKTON PER LITER 25) 20 15 10 5 Oo O 25 5O 75 100 125 150 V °. ny && jtou AA E if, rr ; = ee Les Moai jes ott MD. Ncheas: ee ae BN MOY bss = SS al5 WJ [a) 20 295 5000 4000 3000 2000. 1000. 0 O re 40 re = = td 500 60 70 E Entomostraca N Nauplii R Rotifers 80 A Algae 90 1000 2000 3000 4000 5000 6000 ALGAE PER LITER FIG. 6-6 Vertical distribution of net plankton (left) in an oligotrophic lake and (right) in a eutrophic lake, Wisconsin. Note that the horizontal scale is different for the two lakes, and for the algae as compared with the zooplankton. The cross- hatched horizontal belts show the region of the thermocline (from Birge and Juday 1911). 68 Habitats, communities, succession 20 IN METERS 25 DEPTH Epischura lacustris 35 40 fo) 20 40 60 80 a 45L PER CENT Diaptomus minutus IN FEET DEPTH Cyclops species FIG. 6-7 Vertical distribution of three species of copepods in the daytime (stippled) and at night (black) in the oligotrophic Lake Nipissing, Ontario, on a July day, when the thermocline occurred between 12 and 15m (from Langford 1938). ing the year. In larger and deeper lakes, a maxi- mum population usually occurs between April and early June, a minimum in August, a second maxt- mum in late September or October, and the yearly minimum in late winter, February or March. How- ever, not all species follow this schedule; some spe- cies have a maximum in the spring and not in the autumn, or vice versa; and some species reach great- est abundance during the general summer or winter minimum. A species can also exhibit alternate increases and decreases in population at other times; these, as well as fluctuations in total plankton, are called pulses. At times, especially during the summer when the water is warm, an algal form, most commonly a blue- green species, may become so abundant that it dis- colors the water; these irruptions are known as blooms. The death and decay of such masses of vege- tation may so deplete the oxygen supply that great mortality of fish and other animals results. In some cases the algae produce chemicals toxic to animals. The ways in which environmental factors control seasonal and other changes in population are not all clearly understood, but it is significant that the max- ima in total plankton of deep lakes often come at the times of the two annual overturns, times when food and oxygen are abundantly distributed at all depths. But the bimodal curve may also be found in shallow lakes and ponds that do not possess thermoclines. In small lakes, however, there is greater irregularity, and one, two, three, or no maxima may occur at vari- ous times of the year (Pennak 1946). Periods of high rainfall, which means increased drainage of nu- trients into a lake, may be a factor of importance in producing maxima ; seasonal changes in water tem- perature and oxygen tension certainly are important. There appears to be no relation between the pulses of net phytoplankton and zooplankton suggesting ex- clusive dependency of the latter on the former. BENTHOS Divisions The lake bottom can be divided into a littoral zone and a profundal zone. The littoral zone extends from the water’s edge to the limit of rooted aquatic vegetation. It may be subdivided into the eulittoral zone, between high and low water marks at the water’s edge where the beat- Lakes 69 b ccssoeeees Temperature ae Phytoplankton 300 a eee | ee Zooplankton nN ao (oe) Le) fe) (eo) 150 100 oa {e) O ZOOPLANKTON & PHYTOPLANKTON (I,000'’s) PER LITER vy NM WwW ( ¢) {(o) °c TEMPERATURE, | SEP | OCT | NOV | DEC | JAN| FEB | MAR| APR|MAY | JUN | JUL | AUG | FIG. 6-8 Seasonal plankton populations in western Lake Erie through a year (after Chandler 1940). ing of waves is most effective, and the sublittoral zone, which extends from the lower limit of wave action to the lower limit of rooted vegetation. Where such vegetation is absent, the sublittoral zone may be considered the bottom of the epilimnion down to the thermocline. The profundal zone is the entire bottom below the rooted vegetation, or commonly the bottom of the hypolimnion. The boundary lines between the zones are variable and change with the depth of the thermo- cline. The open water of the lake above the bottom is known as the limnetic zone. Littoral zone The bottom of the littoral zone may be rock, cobble, gravel, sand, or mud. The muddy shallows of protected bays may have considerable rooted vege- tation; they are essentially pond habitats. Differen- tiation of species distribution is primarily between the hard bottom and mud bottom habitats ; sand bot- tom habitats are transitional (Table 6-2). Sand bot- toms ordinarily have the lowest population of most species except clams because they are unstable habi- tats at best; indeed, they are often destructive by reason of the action of sand grains grinding on each other (Rawson 1930, Krecker and Lancaster 1933, Lyman 1956). A lake-bottom and a streambed of similar composition will contain many of the same kinds of organisms because of the similarity in the physical conditions of existence. The respective spe- cies compositions, however, are often different. Oneida Lake in New York has an unusually high mollusk population. Baker (1918) recorded 59 spe- cies and varieties. It is interesting that most of them occurred on mud and sand bottoms. The highest populations were 1890 individuals per m? on mud at depths less than two meters, and 1573 individuals per m? on sand. On rocks and gravels there were only 656 individuals per m?. In eutrophic Douglas Lake, Michigan, bottom de- posits in the littoral areas show zonation down to a depth of about 18 m. Beginning at the shoreline, there are belts of barren, wave-washed sand, muddy sand, sandy mud, and deep-water soft black ooze, in that order. The average number of macroscopic ben- thic animals is large, varying in the different types of bottom from 369 to 1178 to 3822 to 1713 per m?, respectively. The abundance of animals is related not only to the nature of the bottom but also to depth and vegetation present. Where vegetation was scarce there were only 162 animals per m?, but with increas- ing density of plants from sparse to common to abun- dant the population of animals rose to 1531, 2525, and 4407 per m?, respectively. Vegetation was most dense at depths of 7 to 14 m in mixtures of sand and mud. Most abundant animal species in decreasing 70 Habitats, communities, succession | A |eseerars. P7772) order were the amphipod Hyalella azteca, the dip- teran larvae Tendipes and Protenthes, the snail Amnicola, tubificid worms, and the sphaerid Pisidiwm (Eggleton 1952). For comparison, the depth distri- bution of animals in an oligotrophic lake is shown in Fig. 6-10. There is also a fauna of microscopic animals inhabiting the bottom. This consists of Protozoa, Hydra, Rhabdocoela (flatworm), Nematoda, Rota- toria, Gastrotricha, Oligochaeta, Cladocera, Cope- poda, Ostracoda, Acarina (mites), and Tardigrada. These organisms are often very numerous in the thin organic ooze-film that covers mud bottoms (Bigelow 1928), but may penetrate underlying deposits to depths of 20 cm. Sand bottoms also support a varied and abundant microfauna (Pennak 1940, Cole 1955). In general, number of microfauna species and indi- viduals varies inversely as the depth of water; only a few species remain active in the profundal zone dur- ing the summer stagnation period (Moore 1939). Bacteria, a source of food, are abundant in the bottom at all depths. Much of the bottom fauna of the littoral zone consists of immature stages of otherwise terrestrial insects. The pulmonate snails and water mites have evolved from terrestrial species. Other aquatic spe- cies, however, have related forms in the sea and this may indicate their evolutionary origin. The funda- mental problem involved in dispersal from the sea FIG. 6-9 Two types of bottom samplers for measurement of benthic populations. When open, each sampler covers a known area. As each is closed, it scoops up the organisms present. Right, Peterson's bottom sampler for hard or sandy bottoms; left, Ekman's bottom sampler for soft bottoms and deep water (from Welch 1948). into fresh water would be that of osmoregulation, and the ability to live in fresh water has doubtless con- stituted a selection factor in the origin of this com- munity. Profundal zone In oligotrophic lakes, species characteristic of the littoral zone are found at much greater depths than they are in eutrophic lakes, in which the oxygen supply during the summer stagnation period is re- duced. The amphipod Pontoporeia occurs only in the deeper oxygenated cold waters of some northern lakes (Adamstone 1924). It is a relic from the glacial period, when it was probably more widely distributed. The profundal benthos of one oligotrophic lake in British Columbia increased from 470 individuals per m? in January to 1270 in August (Ricker 1952). The most common bottom organisms are the an- nelids Tubifex and Limnodrilus, and the insect lar- vae Tendipes, Chaoborus, Protenthes. There may be a few mollusks, Pisidium, and Musculium for in- stance, nematodes, and other forms, including a microscopic fauna (Eggleton 1931). The midge larvae represent a variety of species. Fifty species occur in one small lake in Algonquin Park, Ontario, that has a pH range of 4.6 to 6.6 and thermal stratification in summer with ample oxy- Lakes ye] Number per m? TABLE 6-2 Size and distribu- ore tion of invertebrate Common name Classification gravel sand mud populations on different types of bottom in the Midge fly larva Cricotopus exilis & others 1,000 littoral zone of western Caddisfly larva Hydropsyche 70 Lake Erie (after Shelford and Mayfly naiads Stenonema tripunctatum, Boesel 1942). S. pulchellum, S. inter- pbunctatum 15 Snail Physa sp. 13 Water penny Psephenus contei Ti Sponge colonies Spongillinae 3 Snail Planorbula crissilabris 2 Leech Glossiphonia 2 Snail Amunicola limosa porata 1 Mayfly naiads Baetis, Centroptilum + Clam Elliptio dilatatus sterkii + Damselfly naiad Argia moesta + Midge fly larva Tendipes pallidus + Bryozoan colonies Plumatella 32 + Parnid beetle & larva Stenelmis crenata 17 + Caddisfly larva Trichoptera 2 1 Clam Leptodea fragilis + 1 Flatworm Planaria + + Snail Goniobasis livescens 12 1 2 Midge fly larva Chironomidae 1 2 1 Clam Amblema costata + 0 + Clam Lampsilis siliuoidea rosacea + 2 1 Clam Obovaria subrotunda 1 Clam Lampsilis ventricosa + Mayfly naiad Ephemera + Alderfly larva Sialidae + Midge fly larva Tendipes flavus + Parnid beetle larva Stenelmis bicarinatus + Clam Anodonta subglobosa + Clam Micromya fabilis + Snail Pleurocera acuta 7 + Clam Fusconaia flava parvula 1 + Mayfly naiads Hexagenia occulata, H. rigida + 33 Midge fly larva Tendipes digitatus 1 6 Midge fly larva Procladius culiciformis + 3 Amphipod Gammarus limnaeus + + Water boatman Arctocorixa lineata a + Midge fly larva Tendipes decorus 1 Snail Valvata tricarinaia 1 Leech Herpobdella punctata 1 Amphipod Gammarus fasciatus + Leech Glossiphonia stagnalis cr Crayfish Cambarus argillicola a Mite Lunnesia undulata + Midge fly larva Cricotopus trifasciatus + Clam Protera alata cts Clam Ligumia nasuta + Clam Truncilia donaciformis Fone = a Total taxa 22 23 22 Total individuals Waa rite 17 49 72. Habitats, communities, succession gen in the hypolimnion, Of this number, 33 species are confined to the littoral and sublittoral zones, 7 to the profundal zone, and 10 occur throughout (Miller 1941). The bottom mud of eutrophic lakes commonly consists of a thin, upper, brown, detritus layer of newly deposited organic matter that has drifted down from above; a relatively thick gray layer containing many fecal pellets and much organic matter, as well as diatoms; and a relatively barren bottom layer. In England’s Lake Windemere, 85 per cent of all bot- tom organisms occur 6 meters below the surface, and 100 per cent 12 meters below the surface in the upper layers (Humphries 1936). Maximum populations of insect larvae in eutro- phic lakes are ordinarily reached during the winter. Minimum populations occur during late spring and summer, both in the littoral and profundal zones, because many immature insects have completed their development and emerged, as adults (Eggleton 1931, Ball and Hayne 1952). Although relatively few gen- era make up the bottom fauna, populations may at times be enormous. Chaoborus larvae alone have been recorded in populations of 97,000 individuals per square meter, and Tendipes larvae at 26,000 indi- viduals per square meter (Deevey 1941). Summer stagnation period The low concentrations or complete disappear- ance of oxygen in the hypolimnion of eutrophic lakes for periods of several days or weeks in the summer requires special adjustments by organisms. Some bacteria are truly anaerobic, and perhaps some ani- mals are, too, but most forms simply accumulate an oxygen debt that is repaid when the autumnal over- turn takes place. It is of interest that the annelid worms and those midge fly larvae that tolerate the lowest oxygen concentrations possess hemoglobin in the blood, the pigment which has the greatest capacity and efficiency in transporting oxygen at low tensions. Tubificid worms extrude farther from their tubes and wave their tails more vigorously for a time as the oxygen content becomes reduced. The nightly excursions of Chaoborus larvae into the oxygenated epilimnion is certainly an opportunity for replenish- ment of their oxygen needs. A considerable propor- tion of the larvae migrate out of the profundal zone during the spring, and do not return until autumn or early winter (Wood 1956). The copepods Cyclops bicuspidata, Canthocamptus staphylinoides, and per- haps others, encyst and lie on the bottom during the summer period (Moore 1939), although this action has not definitely been related to any particular en- vironmental factors (Cole 1953). Some midge fly larvae also form cocoons inactive. NEKTON The nekton of lakes consists principally of fish. There is an interesting small shrimp, Mysis relicta, found in the deeper waters of many northern lakes of North America that is often included with the nekton. This species is believed to be a relic of DEPTH SIN) FEET Ogu 50 100 N 350 —c—c—e Pontoporeia amphipod (right scale) 1250 E —e—e—e Fingernail clams fe \ 300/- seveeeeees Annelids = € : Nematodes 10002 3 Ae —-—- Midgeflies = S SoStSse Ostracods 750 = id 200 Snails = o 2 150, tees, 500 6 = lhe | eh all | BES) ene er ey eee ti Se pues “Oitgce. a mos “1950 & 50 = jae ape 0 10 20 30 40 50 60 70 80 90 100 IIO 120 130140 150 160 170 180 190 150200 250 300 350 400 450 500 550 600 \sq0 Sara, 0) eae eed a naan) ie deleeiall aan UE DEPTH IN METERS FIG. 6-10 Variation with depth in abundances of various organisms in oligotrophic Great Slave Lake (Rawson 1953). Lakes Tis a marine fauna that happened to get cut off from the sea in some past geological period, yet was able to survive as the water became fresh. Numbers and species of fish are more concen- trated in the littoral zone of lakes than in the open, deeper waters of the limnetic zone. Limnetic species, however, invade shallow waters for spawning. In deep waters, fish tend to remain close to the bottom, where their food supply is located, unless there is a deficiency in oxygen there. Caged fish, lowered to various depths in a eutrophic lake, did not survive long below the thermocline (Smith 1925). Fish may, however, make short excursions into the hypolim- nion. In a study of fishes in six Wisconsin lakes, Pearse (1934) found that Usually most fishes per unit area occur in muddy, vegetation-filled, shallow ponds, but the characteristic fishes (carp, crappie, sunfish, dog- fish) are not the most desirable for food. Rich eu- trophic lakes produce considerable quantities of desir- able fishes (perch, largemouth bass, white bass, rock bass). Oligotrophic lakes produce littoral game fish of good quality and size (smallmouth bass, wall-eyed pike, pickerel) and ciscoes in deep water. The aver- age catch with gill nets in two oligotrophic lakes was 3.5 per hour; in two eutrophic lakes, 4.2 per hour; and in two shallow lakes or ponds, 5.1 per hour. In the littoral zone, fish species are segregated according to the composition of the bottom, as are the invertebrates. The species living over rock and gravel bottoms in lakes are mostly different from those in- habiting similar bottoms in streams, but the mud bottom forms are nearly the same as in ponds (Shel- ford and Boesel 1942, Nash 1950). Amphibians and reptiles do not commonly occur in lakes except around margins supporting attached aquatic vegetation, and here pond species occur. Such pond mammals as the muskrat, mink, and otter are not typical of lakes as such, although they are fre- quently found in shallow littoral waters. There are a number of bird species, however, that occur most commonly in lakes: American and _ red-breasted mergansers, loons, pelicans, cormorants, terns, gulls, ospreys, bald eagles, and swallows. These species get their living from the lake, but nest on neighboring shores or islands. In addition, there are many pond and marsh birds that occur along vegetated lake margins. FOOD CYCLE The lake is a closely knit ecosystem whose inhabitants are largely independent of the rest of the world but very much dependent on each other for existence. It is almost a microcosm in itself (Forbes 1887), but it depends on the insolation of the sun for energy, rain and snow for water supply, and on minerals dissolved out of the surrounding uplands for the basic nutrient salts essential to the formation and functioning of protoplasm. Basic to this food cycle are the bacteria. A few bacteria occur free-floating in the water. For the most part, however, they are either attached to algae, to other plankton organisms, to submerged objects, or occur on the bottom as part of the benthos (Hen- rici 1939). Their number varies from one place and time to another, as do the numbers of other organ- isms; they are more abundant in eutrophic than oligotrophic lakes. Their action is to transform the dead organic matter into nutrients, especially ni- trates, that the green plants then absorb. The phytoplankton are the next link in the food cycle because of their ability to manufacture carbo- hydrates with the aid of sunlight and to anabolize proteins after absorbing nitrogen and other com- pounds dissolved in the water. Rooted vegetation around the lake margin is important in this respect, although in large lakes the proportion of food sub- stances formed by marginal vegetation is small as compared to the amount manufactured by phyto- plankton. In Wisconsin lakes, the daily production of glucose during clear days in August varies from 14 to 44 kg per hectare (12 to 39 Ib/acre) (Manning and Juday 1941). Zooplankton feed upon phytoplankton, Protozoa, bacteria, detritus, and each other. Some species ap- pear to discriminate in their choice of food, but most species filter out and ingest all particulate matter, within size limits, non-living as well as living, with which they come in contact. The ratio of number of entomostraca to number of phytoplankton cells has been found to vary from 1:1,800 to 1 :63,000. Ratios of rotifers to phytoplankton vary from 1:50 to 1:37,500 (Pennak 1946). The plant cells, how- ever, are much smaller than individual animals. The mean ratio of zooplankton to phytoplankton by vol- ume is commonly about 1:4 (Davis 1958), but in alpine and northern oligotrophic lakes, the ratio may be reversed (Pennak 1955, Rawson 1956). In the nannoplankton, Protozoa depend largely upon bac- teria, although some forms feed also on algae and detritus ; a few species prey chiefly upon other proto- zoans (Picken 1937). When the plankton dies, it settles to the bottom and furnishes food for the benthos. The accumula- tion of dead plankton and other aquatic organisms on the bottom may be extensive enough to form a dis- tinctive brownish layer. The benthic midge fly and other insect larvae, annelids, clams including the sphaeriids, snails, and bottom-dwelling entomostraca feed on this detritus layer, on organic matter held in suspension, and on algal plankton and attached forms. The variety of their food habits is reflected in 74 Habitats, communities, succession certain anatomical adaptations of fish. Fish feeding on bottom matter have soft-lipped sucking mouths ; fish feeding on plankton have numerous slender gill- rakers ; fish feeding on other fish have large mouths and sharp teeth. The adults of some fish, such as the cisco, gizzard shad, paddlefish, and sunfish, consume large quantities of plankton. The gizzard shad also feeds on bottom mud, straining organic particles out of it and grinding them up in a stomach that re- sembles the gizzard of a chicken. Sturgeon, white- fish, buffalo fish, carp, catfish, bullheads, suckers, sunfish, and many others feed largely on bottom annelids, insect larvae, mollusks, and vegetation in shallow waters. As many as 354 midge fly larvae have been found in a single whitefish stomach; 331 were found in a sturgeon stomach (Adamstone and Harkness 1923). Bass, crappies, perch, pike, gar, and lake trout feed principally on other fish. The bottom feeders scoop up the bottom ooze indiscrim- inately ; several forms maintain contact with the bot- tom by means of sensitive barbels hanging from the chin, but plankton-feeders and carnivorous species de- pend largely on sight for seizing individual prey. Young fish of many species live largely on plank- ton, even though as adults they feed on something quite different. A 10-centimeter perch requires 150 mg dry weight of food per day during the summer, the equivalent of about 37,500 Cyclops. The perch would have to consume Cyclops at a rate of 26 per minute throughout the day in order to ingest such a total. A 20-centimeter perch would require 600,000 Cyclops per day, ingested at a rate of 417 per minute, which is doubtless beyond its efficiency of intake. By con- suming only four small fish 0.3 g dry weight each, the perch could obtain the same energy intake (Allen 1935). Most lake-inhabiting birds subsist mainly on fish, diving for their food. Gulls take only dead fish, which they find floating on the surface or washed up on the shore. Swallows skimming over the water surface consume enoromus numbers of emerging adult midge flies and other insects. BIOMASS AND PRODUCTIVITY The dry weight of total organic matter of seston in 529 fresh-water lakes was found to range from 0.23 to 12.0 mg/l with an average of about 1.36 mg/l (Birge and Juday 1934). Of this, living plankton organisms constituted an amount ranging from 20 to 80 per cent. The biomass of green phyto- plankton is usually, but not always, greater than the zooplankton. The biomass of net plankton may be only one-third to one-tenth of the total net and nan- noplankton. Net plankton is generally more abundant in hard water than in soft water, more abundant in SIZE OF FISH IN INCHES 3 4 5 6 A Large zooplankton* zooplankton PERCENTAGE OF FISH PER FOOD TYPE Benthos \ io [Suk ae O SIZE OF FISH IN CENTIMETERS FIG. 6-11 Change in food habits of perch as they age and in- crease in size (after Allen 1935). eutrophic lakes than oligotrophic lakes (Rawson 1953). The dry weight of net plankton during the summer in 18 lakes of western Canada and in 2 lakes of Wisconsin varied from 0.9 to 17.7 mg/1, and averaged 5.0 mg/m? of water surface area (Rawson WEE) The biomass of benthos varies with the nature of the bottom, amount of vegetation, and depth. When computed for the total bottom of 10 Canadian lakes exceeding 11 m in depth, it was found to vary from 0.07 to 2.47 g/m?, and average 0.63 g/m? dry weight, not counting the shells of mollusks (Rawson 1955). The mean of 36 lakes in Connecticut ranging in depth from 1.1 to 11.1 m varied from 1.09 to 34.8 g/m?, and averaged 7.5 g/m? (Deevey 1941). The bottom fauna of various European lakes has been found to range from 0.69 to 5.65 g/m?. When lakes of different depths are analyzed, it is found that the mean biomass of both net plankton and benthos exist in inverse relation with mean lake depth (Table 6-3). This may indicate that the morpho- metric characteristics of a lake affect its carrying capacity, a factor additional to those of dissolved salt content, oxygen content, and temperature. The biomass of plankton is generally greater than the biomass of benthos. In addition to the five Cana- dian lakes listed in Table 6-3, Deevey (1940) found the ratio between plankton and benthos in five other lakes likewise to vary from 3.8:1 to 10.0:1. In one eutrophic lake in Michigan, the standing biomass of lakes 75 TABLE 6-3 Interrelations between depth, biomass of plankton, and biomass of benthos in five Canadian lakes (from Rawson 1955). Average dry Average dry Average weight of weight of Ratio total depth, net plankton, benthos,’ biomass, meters g/m? g/m? plankton /benthos 11 9.05 2.47 PAs) 26 3.65 0.41 4.5 38 3.2 0.45 5.7 69.5 2.6 0.20 {ler 120 0.9 0.07 9.6 1Minus weight of shells in mollusks fish to benthos was in the ratio of 2.7:1 (Ball 1948). The measurement of productivity is difficult, and methods presently in use require several assumptions. If, throughout the year, the plankton population of Lake Mendota, Wisconsin, should replace itself every two weeks, then the annual productivity is 624 grams ash-free, dry, organic matter per square meter of water surface. Of this amount, 585 g would come from phytoplankton and 39 g from zooplankton. The benthos reproduces less rapidly, nekton, still less so. The annual productions of bottom fauna and fish in Lake Mendota is estimated at 4.5 and 0.5 g/m? re- spectively, and the large aquatic vegetation at 51.2 g/m? (Juday 1940). Disregarding the large aquatic plants, the ratio of productivity between plankton and benthos is approximately 139:1; between benthos and nekton, 9:1; and between plankton, benthos, and nekton taken together, 1248 :9:1. The ratio of annual productivity between plankton and benthos is much higher, therefore, than is the ratio of their biomasses or standing crops. No attempt was made in this study to determine the standing crop of fish. FIG. 6-12 (a) Larva, (b) pupa, and (c) adult of a midge fly (from Shelford 1913 after Johannsen). m6 Habitats, communities, succession In a detailed study of the net productivity of the benthos in the Russian Lake Beloie (Borutsky 1939), it was found that the standing crop increased during the year by 125 per cent. Of this total biomass, DD per cent died without being eaten by other organisms or was replaced by the small biomass of new eggs be- ing laid; 14 per cent was consumed by other organ- isms, chiefly fish; and 6 per cent emerged as adults that subsequently left the lake ecosystem. The re- maining 25 per cent constituted the standing crop of the following year. However, this standing crop was only 56 per cent of what it was the year before, so these percentages are not representative of stabilized populations. It was estimated that in Costello Lake, Ontario, the standing population of midge fly larvae was re- placed during the 135 days of summer eight or nine times in the epilimnion, and two or three times in the hypolimnion. Consumption of larvae by fish was small in shallow waters but amounted to 50 per cent of the standing crop in deep water (Miller 1941). LIFE HISTORIES Although most midge fly larvae, Chirono- midae, are aquatic, some forms live in decaying or- ganic matter, under bark, or in the ground. The earliest larval stage is a wiggler, which may be car- ried by the current or may squirm about from place to place. Later, this wiggler larva becomes sluggish and builds a case or tube, open at both ends, by ce- menting particles of sand, debris, or silt about itself with mucous from its salivary glands. Construction is accomplished in about three hours. The larvae extend themselves from these cases for feeding, and in some species may even move the cases to better feeding areas. The larval period is the longest part of the life cycle; it lasts at least two months (Mac- donald 1956). Most of the pupation period, which is probably less than a week, is spent in the larval case, but towards the end the pupa swims to the sur- face of the water. At this time it is preyed upon ex- tensively by fish. The adult imago struggles out of the case and flies off. The adult lifespan is probably short, as there is no evidence that they feed. They may occur in immense swarms in the evening. Eggs are laid in masses of several hundred or in sticky gelatinous strings that float at the surface attached to some object or sink to the bottom. The eggs hatch in a few days, and the cycle is repeated (Cavanaugh and Tilden 1930, Johnson and Munger 1930). Some larvae (e.g. Procladius, Tanypus) are carnivorous rather than herbivorous or saprophagous. They do not build cases, but roam over the bottom. The num- ber of generations varies in different species from two per year, to one per year, or one in two years, and depends in part on the depth and temperature of their habitats (Miller 1941). The tubificid worms do not leave the lake bottom. Many of them occur in tubes or cases, similar to the habit of midge fly larvae. It is evident by the pres- ence of sexually mature adults and the reproductive cocoons that reproduction occurs principally during the periods of autumn and, especially, spring over- turns. There follows a large increase in numbers of small immature worms (Eggleton 1931). It is obviously impossible to include a descrip- tion of the life history of all species. The cisco or lake herring has been selected to illustrate the life- history of a lake fish (Cahn 1927, Fry 1937). The cisco spends much of the year in deep water, feeding very largely on plankton, strained out by specialized gill rakers. Because its food habits require a large volume of water to be passed through its gill- rakers, the fish swim almost constantly, usually in a constant and definite direction, in schools of from twenty to several hundred individuals. During hot summers the cisco may leave the cool, deep, but oxy- gen-poor carbon dioxide-rich waters and ascend into the epilimnion. There they are sometimes killed in large numbers by temperatures higher than they are able to tolerate. The fish spawn in November or De- cember, when the water temperature drops to 4°C. For this purpose the fish move into water only one to three meters deep, or even up into rivers. The males precede the females by two to five days. When the females arrive, several males consort with each. When she is ready to spawn, the female descends to within 20 cm of the bottom and sheds about 15,000 eggs. At the same time, the accompanying males dis- charge sperm, and fertilization is completed. The eggs are viscous and become attached to rocks or bottom debris. No nest is made and no further atten- tion is paid to the eggs. Incubation may last 10 to 12 weeks ; hatching normally occurs in late March. The young fish later return to deep water and reach breed- ing condition in three years. Doubtless the slow rate of development in this species is related to the low temperature of the habitat. After spawning is com- pleted, the adults may remain in shallow water until water temperatures reach 20°C. This temperature is above their preferendum, although they can tolerate temperatures up to at least 25°C. APPLIED ECOLOGY Applied ecology involves the management of lakes and the control of their resources for man’s benefit. Aside from their use in transportation, in industry, and as sources of drinking water, lakes are of importance to man for fishing, swimming, sight- seeing, and boating. For swimming, clean, clear water with a sand bottom is desirable. Sewage and industrial wastes must be diverted or eliminated for reasons of health and the appearance of the water. Algal growth, when excessive, can be controlled with copper sulphate; and rooted vegetation can be re- duced by sodium arsenite treatment. When chemical treatment of water is limited to low concentrations administered with discretion, there is generally no great harm to fish; some invertebrates, such as midge fly larvae, mayfly naiads, and fresh-water shrimp, are adversely affected (Machenthun 1958). Where there is excessive erosion of the surround- ing upland, silting may render the waters of small lakes turbid, decreasing the growth of algae, a basic food substance for lake organisms. The rapid ac- cumulation of silt on lake bottoms covers up bottom organisms, clogs the gills of mollusks, and generally reduces the lake’s productivity. The obvious remedy is the control of erosion at the source. The management of large lakes to the end of in- creasing fish productivity is difficult because of the area and depth of water involved. Where commercial fishing is commonly practiced in large lakes, a care- ful yearly catch record for each species should be maintained. This will suggest regulations such that annual cropping will not exceed annual production. To improve fishing and increase productivity there must be an increase in a lake’s carrying capacity. Carrying capacity depends on maintenance of good chemical and physical characteristics of the water, an abundance of food, plenty of breeding areas, and ex- clusion of exotic predators. The drastic decline in the annual yield of lake trout in the Great Lakes is at- tributed to invasion by the predaceous sea lamprey. Artificial fertilization of lakes presents greater prob- lems than it does for ponds, but may eventually prove practicable (Hasler and Einsele 1948). Pollution is usually a local problem in large lakes. A moderate pollution of organic wastes may in fact fertilize a lake and produce an increase in the plank- ton and bottom organisms which serve as fish food. Excessive pollution, however, must be controlled, as it interferes with the use of the lake for recreation and as a water supply. The smaller the lake the more practicable becomes management of the habitat. The water level may be manipulated, by damming, to increase the area of shallow water available for spawning at certain sea- sons, or lowered at other times to permit growth of marginal vegetation or prevent spawning of unde- sirable species. Artificial shelters or spawning areas may sometimes be created, yielding a significant im- provement (Hubbs and Eschmeyer 1938). In gen- eral, rearing small fish in hatcheries for later release has not proven economically practicable. Any pro- posed introduction of exotic species should be investi- gated with considerable skepticism. Lakes Ae) SUMMARY Important factors in aquatic habitats are pressure, density, light, current, temperature, oxygen, carbon-dioxide and other gases, dissolved solids, and hydrogen-ion concentration. Of special importance in many lakes is the occurrence of a thermocline that divides the water into an epilimnion and a_ hy- polimnion. The hypolimnion retains a low tempera- ture throughout the year, and in some lakes becomes deficient in oxygen in late summer. These differences in temperature and oxygen greatly affect local and seasonal occurrence of organisms. Lakes are classified several ways on the basis of physical characteristics ; biologically, only two distinct communities, the oligo- trophic and eutrophic lake biocies, are distinguish- able. The life-forms of lake organisms are chiefly plank- ton, benthos, and nekton. Zooplankton exhibits diel movements to greater depths in the daytime and gen- eral dispersal, including movements toward the sur- face, at night. Peak populations are commonly reached in late spring, and again in autumn; low points occur in summer and winter. Benthos de- creases in abundance from the littoral to the profundal zone. Profundal animals in eutrophic lakes are ad- justed in various ways to survive the low oxygen late summer stagnation period. Nekton includes aquatic birds as well as fish. The lake is a closely knit ecosystem, largely inde- pendent of the rest of the world except for its solar energy, inflowing water, and mineral salts. The base of food-chains is composed of detritus, bacteria, and phytoplankton, then zooplankton and small benthic organisms, and finally fish and birds. All dead or- ganisms, as well as their excreta during life, decom- pose so that their nutrient substances start the food- cycle over again. The biomass and productivity of the three life-forms usually rank, from high to low: plankton, benthos, and nekton. The life-cycle and behavior of lake organisms are closely adjusted to the various environmental situa- tions available. Control or management of fish pro- duction by man is difficult, except in lakes of small size. 718 Habitats, communities, succession Local Habitats, Communities, and Succession: Ponds, Marshes, Swamps, and Bogs Pond is a popular term for lakes of the third order that are small, shallow, and, when mature, have rooted vegetation over most of the bottom. There is no clear distinction between ponds and lakes of the first and second orders. The littoral zone of eutrophic lakes, for instance, is pond-like in habitat and or- ganisms. Floating and emergent vegetation com- monly occurs around the margin of ponds to form extensive tracts of marsh. The pond habitat may originate as a shallow basin, as a large pool in a stream, as the result of the filling in of a lake, or from a stream dammed by beavers, man, or landslide. Slow-flowing rivers are essentially elongated ponds, and have a similar fauna (Kofoid 1908, Richardson 1928). Because of the slight water movement in ponds, the surface film becomes an important micro- habitat for some species. Pondwater temperature is often uniform at all depths, but during warm sunny weather, ponds well protected from the wind may show considerable stratification, not only in tempera- ture, but also in oxygen content and other character- istics (Wallen 1955). Daily and seasonal variations in temperature may be great because of the small volume of water. Ice forms earlier and lasts longer in ponds than in lakes, freezing shallow ponds to the bottom in severe winters. Light penetrates to all depths, encouraging growth of vegetation except in high turbidity. Young ponds may have rocky, sandy, clay, or mud bottoms; in mature ponds, there is or- dinarily an accumulation of organic matter and silt. The dissolved oxygen content of ponds varies widely from temporary supersaturation when there is excessive photosynthesis of plants to near depletion when decomposition predominates. Oxygen content is often highest in the spring; very low in late sum- mer ; and sometimes low again under the winter ice cover. Oxygen content is usually higher during day- light hours than during the night because of the daytime photosynthetic cycle of plants. Oxygen may become so low at night as to become critical, espe- cially for fish. Decomposition of organic matter evolves carbon dioxide and, at times, considerable methane, hydrogen sulphide, and other gases. In ponds, as in lakes, there is as wide variation in hy- drogen-ion concentration. As ponds mature and ac- cumulate humus, pH value decreases. PLANT SERE The plant hydrosere, or pond sere, typically contains the following stages and characteristic spe- cies : Submerged vegetation: water weed, pondweed, milfoil, hornwort, naiads, buttercup, bladderwort, eel- grass, and the herb-like alga Chara. 79 FIG. 7-1 Pond animals (from Shelford 1913). Red mite Representative animals of the emerging associa- tion. a, the common newt; b, the common pond snail; c, a predaceous diving beetle. An Amphipod A pelagic rotifer ventral (left) and side (right) views (Above) A garter snake feeding on the dead fish left in a dry-season pond. (Right) Representative animals of the submerged vegetation. a, a viviparous snail; b, a green sun- fish above a yellow perch, both juvenile; c, a shrimp; d, a winter body, or statoblast, of the gelatin-secreting polyzoan. 80 Floating vegetation (Fig. 2.8) : water lily, pond lily, pondweed, smartweed, duckweed, and water hyacinth. All except the last two are rooted in the mud, often at depths of two to three meters, and may have rhizomes from which long petioles extend to the leaves floating on the surface. Duckweeds and water hyacinths are unattached floaters, and cover the surface extensively in some localities. Emergent vegetation (marsh) The dominant species are: cattail, reed, bulrush, bur-reed, swamp loosestrife, wild rice, and sawgrass. They invade waters of over a meter depth, but in shallower waters or in secondary succession they are replaced by a sedge meadow composed of sedge, rush, and spike rush. Swamp shrubs: buttonbush, alder, dogwood, swamp rose, and sometimes shrubby willow and cot- tonwood. Swamp forest: red and silver maples, elm, ash, swamp white oak, and pin oak. Succeeding stages depend on the climate of the region. In arid regions the swamp forest may be poorly developed, and grassland or desert vegetation may come in quickly. In the mesic climate of the Eastern states, an oak-hickory associes follows the swamp forest, replaced in turn by a climax of sugar maple-beech or mixed mesophytic forest. The hydro- sere in the broad-leaved evergreen climax area of southeastern North America brings in cypress and a number of other unique species. Vegetative debris and animal remains, together with inwashed silt, fill the basin gradually, reducing the depth of water and allowing vegetation to en- croach on the periphery. As this process continues, the succession is effected. Ultimately, open water en- tirely disappears as the ground stratum is built up above the water table, and climax vegetation replaces all other types. ANIMAL SERE Animals characteristic of marshes and ponds constitute a distinct pond-marsh biocies which extends into sluggish or base-leveled streams and the littoral zone of eutrophic lakes. Most animal spe- cies are not restricted to a single stage or community of the plant sere, but commonly occur in several stages in varying abundance and for various activities. Fish, for instance, feed in open water but spawn in shallow water among the emergent vegetation. Sub- merged, floating, and emergent vegetation represent different levels or strata in a single biotic community, and each stratum has about the same degree of dis- tinctiveness as forest community strata. With the invasion of swamp shrubs and with the ground level well above the water table most of the year, pond and marsh species largely disappear, re- placed by many characteristic new species. This ani- mal community represents the swamp facies of the deciduous forest-edge biocies, which will be discussed later. The swamp forest is often quite open at its outer margin, and forest-edge species remain com- mon. But as this forest develops a closed canopy and drier ground stratum, it is invaded by the swamp facies of the deciduous forest biociation. In the pond sere there is a succession of animal adaptations from aquatic to amphibious to terrestrial. POND-MARSH BIOCIES Neuston The supraneuston, organisms which move on top of the surface film in pursuit of most of their life activities, consists of the water striders Gerridae, Veltidae, Mesoveliidae; the water measurers Hy- drometridae; the whirligig beetles Gyrinidae; the springtails Collembola; some spiders; and occasional other forms. The gyrinids of several species com- monly occur in social groups (Robert 1955). They are remarkable for having each eye divided by the margin of the head so that the upper portion looks into the air and the lower portion into the water. Several of the forms listed have long legs that dis- tribute the weight of the body over a large area of surface film. The portions of the legs or body that contact the surface film are water repellant. The undersurface of the water film supports an infraneuston of Hydra, planarians, ostracods, cla- docerans, snails, and insect eggs, larvae, and pupae (mosquitoes, certain kinds of midge flies, and so forth). For all except the insects, however, the use of the surface film in this manner is usually transi- tory. Some cladocerans, such as Bosmina and Daphnia, occasionally break through the surface film from below, fall over onto their sides, and cannot return. Plankton The species of plankton found in ponds differ somewhat from those in lakes (Klugh 1927), but the transition from lake species to pond species is a grad- ual and progressive one. Protozoa and Rotatoria are usually more abundant in ponds than in lakes. Ponds, marshes, swamps, and bogs 8] Age of pond TABLE 7:uDevelopment (ecesis) Common name Classification lyr 8yr 2lyr 30yr 80yr of invertebrate bottom Red midge fly larva Tendipes 16 7 6 27 25 populations (calculated in Damselfly naiad Zygoptera 2 23 4 22 51 number per m’) in strip-mine Caddisfly larva Trichoptera 1 = = 2 3 ponds of different ages, as Backswimmer Notonectidae + = + 3 determined by studies conducted Whirl-i-gig beetle Gyrinidae + - + in October, through three years. The ponds | and 8 years old had Alderfly Sialis 23 3 2 no rooted vegetation; the ponds Water boatman : Corixidae 13 x 1 ail andi30\vearscold hadla Burrowing mayfly naiad Hexagenia 21 262 5 1 y c A fittls'vegetationiialpratectad Dragonfly naiad Anisoptera 9 4 11 27 Crawling water beetle Haliplidae 5 + + + coves; the pond 80 years old had submerged, floating, and Clam Unionidae 5 is ze a emergent vegetation. Range of White midge fly larva Tanypus 3 1 35 10 pH: 7.1 to 8.5; locality, near Aquatic annelids Tubificidae, Lumbriculidae 4 + 3 2 Danville, Illinois. Ghost larva Chaoborus + + + 17 Springtails Podura aquatica + + + + Crayfish Orconectes propinquus 1 1 Fly larva Diptera 3 + Water spider Arachnida + + 3 Fingernail clam Sphaerium i = 2 Mayfly naiad Caenis = Snail Physa gyrina 14 58 Amphipod Hyallela 5 105 Other mayfly naiads Ephemerida 1 21 Aquatic isopod Asellus communis 1 5 Flatworm Planaria 1 1 Limpet snail Laevapex 1 1 Snail Gyraulus parvus + 17 Snail Helisoma trivolvis + 9 Water scorpion Ranatra + 1 Leech¢ Hirudinea 5 Snail Lymnaeidae 2 Fingernail clam Pisidium 1 Shrimp Palaemonetes 1 Water strider Gerris + Total taxa 5 3 16 27 28 Total individuals 19+ 113+ 284+ 132+ 371+ Although plankton distribution does not vary with depth as much in ponds as in lakes, seasonal fluctua- tions are as extensive and similar in nature. Eddy (1934) lists 15 perennial species of zoo- and phyto- plankton that may be found in ponds throughout the year, 2 seasonal species which reach their peak of abundance between December and April, 4 between February and June, 12 between March and Decem- ber, and 5 between July and September. Benthos Subaquatic animals dwell not only on the bot- tom but also on the stems and leaves of submerged plants. Aquatic plant species have many kinds of in- sects, amphipods, mites, and snails using them for the food, shelter, or reproductive sites denied them 82 in the mud bottom below. Other kinds of insect lar- vae and oligochaetes are more abundant in the mud than on the plants. The undersurface of lily pads often contains many small organisms, including Pro- tozoa, Hydra, flatworms, rotifers, and snails. The biomass of animals varies directly with the biomass of vegetation, and the quantity of invertebrates is especially great on those plants possessing finely dis- sected leaves (Gerking 1957). Very few species found in lake bottoms are not found in ponds, but the pond-marsh biocies contains many species not found in lakes. The variety of species and number of individuals found in the bottom fauna increase with the age of the pond from the time the pond is formed until at- tached vegetation becomes excessive (Tables 7-1, 7-2). Coincident with the development of the bottom Habitats, communities, succession: fauna is an increase in variety and abundance of plankton (Eddy 1934) and fish. The construction of a beaver dam in a small Ontario river changed the riffle habitat into that of a pond and brought a reduc- tion in mayfly naiads, stonefly naiads, and caddisfly larvae within two years. Other stream animals fell from 68.7 to 15.6 per cent of the total population while midge fly larvae increased from 31.3 to 84.4 per cent (Sprules 1940). Shallow ponds develop more rapidly than deep ones, and mud bottom ponds develop more rapidly than sand- (Shelford 1911) or rock bottom ponds (Krecker 1919). The increase in number of species and individuals in ponds depends on an in- crease in the variety of microhabitats, types and amount of food, and vegetation. With the develop- ment of the pond into a marsh there is generally an increase in humus and an increase in bacteria effect- ing its decomposition, carbon dioxide, and marsh gases. Oxygen and pH decrease. Two predominantly terrestrial orders of insects, Coleoptera and Hemiptera, have invaded the pond community but are not found in lakes except those which have pond-like margins. The Coleoptera are represented by three families of diving beetles, Haliplidae, Dytiscidae, and Hydrophilidae, and by the whirligig beetles, Gyrinidae. The haliplids are herbivorous; the dytiscids are predacious ; some hy- drophilids and gyrinids are predators, others are scavengers. The aquatic bugs or Hemiptera are the Corixidae, which feed on the bottom ooze, the No- tonectidae, which prey upon small Entomostraca, the Nepidae, the Belostomatidae, and the Naucoridae, which are all carnivorous; and the Veliidae, Meso- veliidae, Gerridae, and Hydrometridae, which are probably both carnivores and scavengers. Some of these species, as already noted, are usually found on the surface film, but they may occasionally dive and cling to submerged vegetation. The true diving TABLE 7-2 Succession of dragonfly and damselfly naiads in western Lake Erie (after Kennedy 1922). Forest-edge Lake biocies Pond-marsh biocies biocies Lake mar- gin with Lake mar- floating Old marsh gin with and emer- with invading Dragonfly and Open submerged gent vege- Young Mature shrubs and damselfly naiads lake vegetation _ tation pond pond trees Gomphus plagiatus ++ ++ Gomphus vastus ++ rire Neurocordulia yamaskinensis rar ++ Macromia illinoiensis oa are Argia moesta ++ ++ + Enallagma carunculatum mu ++ ++ Enallagma exsulans + ++ ++ Enallagma ebrium + ++ ++ Ischnura verticalis + ++ Pure ++ Tramea lacerata fos rerk Anax junius ++ ++ + Enallagma signatum we Fic Libellula luctuosa ++ ++ Libellula pulchella +4 re + Lestes rectangularis + ++ * Leucorrhinia intacta rae Erythemis simplicicollis ++ Plathemis lydia ++ Nehalennia irene = Pachydiplax longipennis + es Lestes forcipatus a ron Sympetrum obtrusum ++ ++ Sympetrum vicinum ++ rae Sympetrum rubicundulum Bs Hu Enallagma hageni * Lestes uncatus ri Lestes unguiculatus es Ponds, marshes, swamps, and bogs 83 (ht i Ml 0.5mm FIG. 7-2 The beetle Dryops freshly submerged, crawling along a stem, encased by a bubble of air (after Thorpe 1950). forms, especially the beetles and some of the hemi- pterans, have evolved oar-like legs for rapid pro- pulsion. Respiratory adaptations Air-breathing aquatic insects, as well as the pulmonate aquatic snails Lymnea, Helisoma, Gyra- ulus, Physa, Laevapex, have evolved special mech- anisms and behavior for respiration. Most species rise to the surface of the water at intervals to re- plenish their supply of air. Insects are so buoyant that they must cling to the vegetation or some other II 6 4 ee 4 “Se a FIG, 7-3 Dragonfly niches (after Needham 1949): (1) on sand, Macromia; (2) in sand, Gomphus; (3) in muck, Libellula, Neuro- cordulia; (4) on massed Nitella, damselflies; (5) on tips of Websteria, Enallagma Jaurenti; (6) in open tangles of blad- derwort, Erythemis and damselflies; (7) in fallen brown leafage, object to maintain a submerged position. As soon as they let go of the substratum, they float to the surface and must return by swimming. Snails commonly creep to the surface along plant stems or other sub- merged objects, or suddenly emit mucous threads that float them to the surface (Dr. Max Matteson, personal communication). They find their way to the surface, at times of oxygen need, by negatively geo- tactic behavior. After they have obtained a fresh supply of oxygen, they become positively geotactic (Walter 1906). Pulmonate snails probably also ab- sorb some oxygen from the water ; indeed, some spe- cies appear never to come to the surface. The gill- bearing or branchiferous species of snails are seen to be segregated into rather distinct niches when their habitat relations are analyzed in detail (Baker 1919). Diving beetles carry a bubble of air beneath the elytra, and the entire body of Dryopa is enclosed in air. The hemipteran notonectids and corixids carry a bubble over the ventral surface of the body, trapped there by hair-like setae. The spiracles of the tracheal system open into these bubbles. The body surface encompassed by and setae holding the bubble are water-repellant, or hydrofugous. The fresh air- bubble contains 21 per cent oxygen and 78 per cent nitrogen, the same proportion as the atmosphere. The nitrogen dissolves into the water very slowly. The carbon dioxide given off by the insect passes quickly into the water. As the insect uses up the oxygen in the bubble, it shrinks. The oxygen content of the bubble may be reduced to one per cent, or less, before the insect rises to the surface for a fresh supply ; in water containing little or no oxygen, ris- ing may occur every three or four minutes. If the water contains ample, however, oxygen will diffuse into the bubble as rapidly as it is used, and perhaps three times as fast as the nitrogen diffuses out. Un- der these conditions backswimmers, Notonecta, have survived for nearly 7 hours without coming to the 5 6 10 Pachydiplax longipennis; (8) at sides of ditch, Tetragoneuria, Celithemis, Erythrodiplax; (9) on invading roots of woody plants, Argia fumipennis; (10) at water-line rooted green plants, dam- selflies; (11) in rafts of fallen pine needles, aquatic Hemiptera that are enemies of dragonfly naiads. 84 Habitats, communities, succession surface. The bubble is really a physical gill mecha- nism, but functions only as long as the nitrogen present provides an adequate surface for oxygen dif- fusion. The insect’s trip to the surface is as much to get a fresh supply of nitrogen as it is to get a fresh supply of oxygen (Wolvekamp 1955). The air-breathing respiratory mechanisms of other aquatic insects are equally remarkable. In many larvae, Dytiscus, Culicidae, and other Diptera, and in the aquatic Hemiptera, only the terminal ab- dominal spiracles are functional. The tracheal trunks of mosquito larvae and Dytiscus larvae among others, store considerable air so that the animal may remain submerged for long periods. Ranatra and other Nepidae have long respiratory tubes extending from the tip of the abdomen so that they can cling to vege- tation well below the water surface yet respire di- rectly into the air. Dragonfly naiads pump water through the anus, in and out of an enlarged rectum. The walls of the rectum are abundantly supplied with a network of tracheae for interchange of gases directly with the water. In the larvae of midge flies, black flies, and corixid beetles, the general body surface is richly supplied with fine tracheae for exchange of gases di- rectly with the water. The anal papillae of midge flies and mosquito larvae are not respiratory in func- tion, as formerly supposed, rather they serve for osmoregulation. Tracheal gills, plates, or filaments are found on many immature insects, Ephemeridae, Plecoptera, Zygoptera, Trichoptera, Neuroptera, and some Diptera, and effectively increase the area of surface available for oxygen absorption. The larvae of the beetle Donacia and certain Diptera including mosquitoes have a unique ability to puncture the walls of submerged plants and collect air from the intercellular spaces (Miall 1934). Terrestrial invertebrates The terrestrial insects found in marsh vegeta- tion are in the main adult mosquitoes, midges, dragonflies, damselflies, mayflies, and alderflies, whose immature stages live submerged. On bare ground around ponds may be found toad bugs, shore bugs, springtails, tiger beetles, and sometimes ground beetles and pigmy locusts. Spiders become numerous throughout the vegetation, and the snail Succinea ap- pears. In addition to these true marsh and pond spe- cies, invertebrates belonging to the forest-edge biocies may occasionally be found. Fish Fish are often very abundant (Table 7-3). In- cluded among the species that occur are several bot- TABLE 7-3 Differences in species composition and number of individuals of fish present in two similar-sized Wisconsin ponds (Cahn 1929). Pond Pond with without Species carp carp Carp 5,891 0 Shorthead redhorse 66 0 White crappie 17 0 Bigmouth buffalo 1 0 Northern redhorse 14 10 Walleye pike 4 20 Bowfin 7 340 Northern pike 3 380 Rock bass 1 940 Bluegill 2 1,220 Longnose gar 0 30 Pumpkinseed 0 610 Yellow perch 0 680 Black crappie 0 730 Largemouth bass 0 1,120 Total species 10 11 Total individuals 6,006 6,080 tom-feeders—suckers, bullheads, buffalo, and carp; the last, a species introduced from Europe. By feed- ing on the submerged vegetation and stirring up the bottom they may control the habitat and the compo- sition of species present in the community. This con- dition, however, does not last indefinitely. Vegetation encroaches on the margins of ponds, and the fish are gradually eliminated because of the disappearance of suitable breeding sites. The mudminnow, bowfin, and bullhead are usually the last to disappear before the pond becomes a dry marsh (Shelford 1911). Amphibians and reptiles Salamanders and frogs are basically aquatic animals, although they show varying degrees of adap- tation to terrestrial life. Siren and Necturus have permanent external gills and spend all their lives in the water. Most other forms lay their eggs and pass through their earlier development in water, but the adults are air-breathing and wander over the land. Since their skins must be kept moist, they are con- fined to the vicinity of water, to humid climates, or to damp humus. A few species, such as Plethodon cinereus and P. glutinosus, lay their eggs in the cavi- ties of well-rotted logs and seem largely non-depend- ent on standing water. The ability of salamanders and frogs to live temporarily away from water ap- pears positively correlated with thickness, cornifica- tion, and relative impermeability of the skin. The aquatic tadpoles and larvae are scavengers or herbiv- orous in their food habits, the adults feed on in- Ponds, marshes, swamps, and bogs 85 TABLE 7-4 Populations of breeding birds in units of pairs per 40 hectares (100 acres) in marshes of northern Ohio (after Aldrich 1943). Swamp shrubs Swamp Bird species Marsh forest Virginia rail 22 Least bittern 12 Short-billed marsh wren 10 Florida gallinule 5 Sora 3 Mallard 3 Killdeer 1 Long-billed marsh wren 78 3 Red-winged blackbird Swamp sparrow 68 49 4 Song sparrow 8 49 12 Yellow warbler 80 Traill’s flycatcher 80 Eastern kingbird 24 American goldfinch 21 Tree swallow tf Catbird 31 Green heron 7 Yellowthroat 28 1 Robin 3 Red-eyed vireo Black-capped chickadee Northern waterthrush House wren Ovenbird Downy woodpecker Eastern wood pewee Tufted titmouse White-breasted nuthatch Blue jay Rose-breasted grosbeak Crow Scarlet tanager Yellow-shafted flicker Crested flycatcher Cardinal Wood thrush Veery Hairy woodpecker Black billed cuckoo Red-shouldered hawk Eastern bluebird Brown-headed cowbird Prothonotary warbler Total species lal 13 323 526 Be ee Bee ee NYNYNYNDY Web UT ANoOWoo Pwr ow o Total individuals 121 sects, earthworms, or other animal matter that they catch on land (Noble 1931). Reptiles are terrestrial. Desert reptiles never go to water. Painted, geographic, and snapping turtles bask in the sun on the shore or on protruding logs, but quickly plunge into the water to escape danger, to cool off, or to feed. The alligator and musk and 86 soft-shelled turtles spend nearly all their time in water. The soft-shelled turtle is able to utilize dis- solved oxygen in the water and hence has evolved special readaptation to water. Like other turtles, however, they lay their eggs on land, placing them in holes excavated in sand, loose soil, muck, or de- caying stumps or logs. Water snakes give birth to living young that enter the water immediately. Water snakes feed on insects, small fish and amphibians, crayfish, or whatever other animal food they can find. The food of turtles is similar to that of snakes ; some species are also scavengers. The cottonmouth moc- casin is a prominent poisonous snake in southern marshes and swamps. The massasauga rattlesnake occurs in wet areas to the north. Birds Bird populations are high, and the pond-marsh, swamp shrub or forest-edge, and forest communities are especially clearly defined (Table 7-4). There is an abundance of nest-sites and food, but the aquatic and terrestrial species exploit different niches to avoid competition as much as possible. The aquatic species feed in all stages of the plant sere, beginning with the open water, but nest for the most part in the emergent vegetation (Beecher 1942). Herons com- monly feed in shallow water but nest in tree-top col- onies. Grebes, cormorants, and terns feed on fish in the open water ; the herons, egrets, and bitterns get fish in water shallow enough for them to wade in; cranes and coots are omnivorous; ibis, stilts, snipe, and rails probe around in the mud for invertebrates ; avocets sweep their curved bills back and forth through the water, catching aquatic insects ; gallinules eat seeds, roots, and soft parts of succulent plants as well as some invertebrates ; most ducks feed on sub- merged and floating vegetation and attached animal organisms; song birds inhabiting the marsh feed chiefly on insects captured outside the water. Mammals One of the most characteristic mammals of the marsh is the muskrat. A well-developed marsh may contain one of their haycock-shaped lodges on each acre (2.5 per hectare), with perhaps five animals per lodge during the autumn. The diet of the muskrat is largely the leaves and roots of marsh vegetation, al- though they also feed to some extent on crayfish, clams, snails, and sluggish fish. Overpopulations of two or three lodges per acre (5.0 to 7.5 per hectare) may lead to “‘eat-outs’’ or local destruction of the marsh vegetation (Dozier 1953). The mink is probably the most common mam- Habitats, communities, succession malian predator of the marsh; it is an enemy of the muskrat. Foxes, raccoons, and coyotes may invade the marsh when the water level is low. The otter preys on fish and crayfish of the marsh; it has now been exterminated from most of its former range. The beaver makes its own marsh habitat by dam- ming small streams, flooding the surrounding low- land. Here it builds its large lodge and feeds on the bark and twigs of adjacent aspen, willow, and cot- tonwood, and on the roots of aquatic plants. When the supply of aspen and other food is exhausted, the colony disappears, the dam decays, the water level subsides, and marsh vegetation invades. After some years the pond is converted into a beaver meadow. In the southern Atlantic seaboard and Gulf states, the herbivorous rice rat is common to marsh vege- tation. In northeastern Ohio, the meadow mouse at- tains populations averaging 58 per hectare (23/acre) in the marsh and persists in smaller numbers in the swamp shrub and swamp forest (Aldrich 1943). The smoky shrew averages 8 per hectare (3/acre) in the marsh. The cinereous shrew and _ short-tailed shrew are found in marsh vegetation and are common in the swamp-shrub stage (30 and 52 per hectare, 12 per acre and 21 per acre, respectively). All three of the insectivorous shrews are also found in the swamp forest. The white-footed mouse increases in numbers from the marsh through the swamp-shrub into the forest stages (2.5-22-32 per hectare, 1-9-13/acre). Moles, chipmunks, and squirrels also occur in small numbers where the ground is drier. FOOD CHAINS IN PONDS Many animals in ponds depend for food on floating phytoplankton, bacteria, and bottom detritus. Ponds, however, unlike lakes have additional pro- ducers of organic matter in the rooted pondweeds. Attached to the submerged pondweeds is a_peri- phyton composed of bacteria, diatoms, and green and blue-green algae. This periphyton is important as food to many small crustaceans, immature insects, oligochaete worms, and snails (Frohne 1956). Pond- weeds are consumed by insects, ducks, and herbivo- rous mammals. Filamentous algae are also more abun- dant in ponds than in lakes, and is a source of food to various immature insects and frog tadpoles. Creep- ing predators among the pondweeds and algae are leeches, dragonfly and damselfly naiads, and water mites. Small swimming predators are the dytiscid beetles, most of the hemipterans, and the swimming leeches Erpobdella and Macrobdella. At the top of the food chains, feeding on all these small animals, and often on plants as well, are the fish and other vertebrate groups (Lindeman 1941). SEASONAL CHANGES AND TEMPORARY PONDS Seasonal changes are greater in a pond than in a lake, a consequence of the smaller volume of water. Because of decreased rainfall, increased evaporation, and continuous seepage or drainage, ponds often become greatly diminished during the summer, or the open water may entirely disappear. As the volume of water shrinks, water temperature rises, and the oxygen content and pH decline. Ani- mals must either adjust to these conditions of the pond or disappear altogether. Under the winter ice, active pond life is slight because of the low oxygen and pH, but all groups increase in numbers as the temperature rises during the spring. In one small Chara-cattail pond near the south end of Lake Michigan, the snails Amnicola and Helisoma deflectus, the amphipod Hyalella azteca, the isopod Lirceus danielsi, the back swimmer Plea striola, and diving beetles Haliplidae attained maxi- mum populations during April and May, but then declined in numbers through August, increasing again in the autumn. On the other hand, the water strider Gerris, mayfly naiads Ephemeridae, damselfly naiads Agrionidae, dragonfly naiads Libellulidae, and the snails Physa and Helisoma parvus had highest popu- lations during June, July, and August (Petersen 1926). Maximum populations of Protozoa are also attained during the warm period of the year (Wang 1928). Fish sometimes suffer from lack of oxygen in the winter when the ice cover lasts a long time. Dur- ing the summer thermal stratification often forces them out of the deeper stagnant water. When photo- synthesis is curtailed at night or in cloudy weather mortality may become high. Bird nesting is ordinarily completed by the time marshes dry up in late summer. At this time ducks concentrate in the remaining deeper bodies of water, and other marsh species start their southward migra- tion. Periods of drought are times of stress and in- creased mortality for muskrats, since the lack of water interferes with their normal locomotion and predation upon them by invading terrestrial species increases. Intraspecific competition becomes intensi- fied as animals become crowded together in the shrinking habitat. Many individuals undertake over- land journeys to new areas (Errington 1939). Many invertebrates have spores or eggs resistant to the effects of desiccation, enabled thus to pass over the period during which the pond is dried up. These include representatives of the protozoans, sponges, hydras, turbellarians, nematodes, annelids, bryozoans, rotifers, mollusks, crustaceans, and insects (Mozley Ponds, marshes, swamps, and bogs 8/7 1932, Kenk 1949). It is an interesting experiment to collect top soil from a dried-out pool, place it in an aquarium with fresh water, and see what comes out of it (Dexter 1946). When water returns to the pond the hatching of plankton organisms and their growth to reproductive maturity is very rapid. The life cycle of some species appears shorter than could be readily sustained in a normal year; perhaps this is an adaptation to survive extreme years (Table 7-5). During years when the pond does not fill with water at all, eggs and cysts remain in a dormant con- dition; some have hatched in a year of good condi- tions after they had continued dormant for several poor years. Some, but not all, species of crayfish sur- vive dry periods by burrowing down to the water table. Among the more interesting inhabitants of tem- porary ponds are the phyllopods. The fairy shrimp is not found in permanent ponds, except those with a wide, shallow shore that dries out during the sum- mer. Shrimp nauplii develop quickly from the egg, usually in January or February after the ice melts, but sometimes as early as November if the pond be- comes filled with water after an autumn dry period. In the spring, adults mature in three or four weeks, egg-laying takes place forthwith, and the species may be gone by late May. The period when the pond is dry is passed through in the egg stage, and either the drying or freezing of the eggs facilitates their hatching (Weaver 1943). Some toads, Bufo spp., Microhyla olivacea, and spadefoot frogs lay their eggs, after a warm spring rain, in temporary pools rather than permanent ponds. Development is rapid, and metamorphosis of the tadpoles may be completed in a month’s time, before the pool evaporates (Bragg et al. 1950). LIFE HISTORIES On hatching from the egg, copepods first pass through six free-swimming nauplius stages by a series of molts, during which the small compact ani- mal possesses only three pairs of appendages; then through five copepodid stages, when additional ap- pendages are added ; and, finally, into the adult form. Both sexes occur regularly. Ostracod eggs also hatch into nauplii, but these already possess a shell like that of the adult. Several molts are required, however, before maturity. Some species always reproduce sexually; others are par- tially or always parthenogenetic. The reproduction of the cladocerans is of special interest. Most of the time only females are present, and eggs develop parthenogenetically during the sum- mer into more females. The thin-shelled eggs are held in a brood pouch on the dorsal side of the body, and the young are well grown before they are set free. There are no free-swimming larvae. After a number of generations, the number varying with the species, and as the pond begins to dry up in the sum- mer or winter, conditions reach a point where there is a crowding of females, an accumulation of ex- cretory products, and a decrease in available food. Parthenogenetic male as well as female eggs are then produced. The resulting males are usually smaller than the females, but, subsequent eggs are fertilized by them and a thick shell is formed around them. These ephippial eggs are produced in smaller num- bers and are very resistant to drying and freezing. When the pond again becomes filled with water, the ephippial eggs develop into reproducing females to start the cycle over again (Pennak 1953). The life-cycle of the rotifers bears some resem- blance to that of cladocerans. A few species are vi- viparous, but in most forms development of the egg takes place outside the body and is direct into the adult form. Two kinds of females are not distinguish- able by external characters. One kind, amictic, pro- duces large diploid eggs that are never fertilized and only develop parthenogenetically into more females. The other kind of female, mictic, occurs only at critical times of the year and produces smaller haploid eggs. If not fertilized, these small eggs develop into males ; if fertilized, they form the thick-walled winter eggs which, under subsequent favorable conditions, de- velop into females. The males are usually small com- pared with the females; they lack an alimentary tract, and consequently live only two or three days. Females live one to three weeks or longer. The production of males appears to be periodic and is often correlated with a change in type or amount of food, or degree of crowding. Males have never been seen, and may not occur in, some groups (Pennak L953))e The amphipod Hyalella azteca breeds only dur- ing the warmer months of the year. The male carries the female on his back for 1 to 7 days before copula- tion occurs. Oviposition follows copulation by 12 to 24 hours. The incubation period is 21 days, and the female may carry the young another 1 to 3 days in her brood pouch. A period of 24 to 36 days elapses between successive broods, and each brood is larger than the last. The females may live into a second summer and reproduce again. The young on hatch- ing in the spring have all the adult appendages and can reproduce later in the summer (Gaylor 1921). In the spring, aquatic Hemiptera commonly glue their eggs to submerged vegetation. Some species insert their eggs into incisions made in leaves or stems. The young emerge directly into the water and resemble the adults except that they do not acquire wings until after several molts. In the Sialidae of the Megaloptera, eggs are deposited in masses on leaves 88 Habitats, communities, succession TABLE 7-5 Monthly changes in the ostracod fauna of temporary ponds in central Illinois. The ponds dried up in mid-July (after Hoff 1943). March March Ostracod species 9 17-20 Cypricercus reticulatus + + Cypria turnert + Candona simpsoni + Candona fossulensis Candona distincta _Candona indigena Candona biangulata Cypria maculata Cypria opthalmica Cypridopsis vidua Candona suburbana Cypria obesa or bare ground near water; on hatching, the larvae proceed into the water. Here they stay for a full year, after which they leave the water and pupate for several months in a hollow that they scoop out of moist earth. The adult does not live over winter. Dragonflies may fasten their eggs to plants below or above the water surface, puncture leaves or stems for egg insertion, oviposit eggs in the bottom, or may scatter them through the water and over the bottom. The naiads hatch out in about three weeks and are of varied forms and sizes. Dragonfly naiads may be divided into three groups on the basis of their habits: the climbers that crawl through the vegeta- tion; the sprawlers that lie half buried in the mud with legs extended and backs covered with silt; and the burrowers. They all undergo several molts under water, some forms living 11 months in this stage. For their last molt they crawl up the stem of some plant or onto a rock on the shore, molt, and emerge as adults. They live for a few weeks only. Adult dragonflies commonly feed on adult mosquitoes, and the naiads feed to some extent on the mosquito larvae (Needham and Westfall 1955). Aquatic beetles commonly attach their eggs to water plants or bore holes into plant tissues to hold them. Some hydrophilid beetles make floating silk cocoons containing many eggs, anchoring these co- coons to surface plants. Beetle larvae live only a few weeks before they leave the water and pupate in characteristic mud cells that they build for them- selves. Pupation varies from a few weeks to several months, depending on the temperature, before emer- gence of the adult occurs. The adults are the chief survivors of the winter but sometimes eggs or larvae live through it, too (Miall 1934, Balduf 1935, Rice 1954). Mosquitoes reproduce abundantly in marshes, ponds, or even in small pools, tree holes, or other water-holding depressions. Some species of mos- Ponds, marshes, swamps, and bogs April 1 SO a | Ue © April May May June June July 15 4 18 9 22 10 + + + - + + + + + - + + + a + + + + - + + + + + + + + quitoes lay hard-shelled chitinous-covered eggs on the ground which are capable of withstanding freezing, extreme heat, and drought but hatch very quickly after being covered with warm water. In water, eggs may be laid singly or in rafts. The adult female Culex vexans stands at the margin of the pool or on some floating object and deposits as many as 300 eggs. The individual eggs are cigar-shaped and are placed vertically to form a floating raft. They fit to- gether so snugly that the surface film of water does not penetrate between the eggs, and the surface of the raft is dry. The larvae hatch in 12 to 28 hours and hang head down from the surface film. Vibrat- ing vibrissae continually sweep food particles into the larval mouth. The respiratory tube at the pos- terior end of the body penetrates the surface film and also prevents the body from sinking. At other times the larvae may suspend themselves from the surface film, dorsal side uppermost, and feed on floating ma- terials (Renn 1941). After 3 or 4 molts (5 to 8 days), the larva changes into a quite differently- shaped pupa, which hangs from the surface film by two respiratory tubes proceeding from the thorax. The winged adult may emerge in 2 days. Some spe- cies may have seven broods per year. Sexual and other behavior of mosquitoes varies considerably among species (Horsfall 1955). Ordi- narily only the female mosquito bites, this to obtain the blood nourishment necessary for egg-laying. The male feeds only on plant juices. Studies made on marked individuals of Aedes vexans showed that 73 per cent of the individuals confined their activities to within a radius of 5 miles (8 km), but that 19 per cent traveled 5 to 10 miles (8-16 km) and some even to 16 miles (26 km) away from the point of marking (Clarke 1937). Other species, however, ap- pear not to have such wide ranges. The pulmonate snail Physa gyrina lays its eggs in the spring when water temperatures reach 10°- 89 12°C; thereafter the adult population dies. Snails born in the spring may reach sexual maturity by autumn but oviposition is normally delayed until spring because of cold weather. The life span is usually 12-13 months, but may be prolonged if de- velopment is interrupted by aestivation resulting from the drying up of the pond during the summer (DeWitt 1955). Many warm-water pond fish, such as the black bass and sunfish (Breder 1936), spawn in nests or redds prepared in shallow water by removing all debris and vegetation over circular areas of one-half to one meter diameter. There is some preference for gravel and sand bottoms when they are available. The male remains to guard the several thousand eggs during the few days required for their hatching, and the fanning movements of his tail and fins doubtless help to aerate them. He may also guard the young until they can take care of themselves. Both bullhead parents guard the egg masses and keep them con- tinually agitated for aeration; they may even suck the eggs into their mouths and expel them forcibly. The adults keep the young in compact groups by swimming about them. The European carp may spawn promiscuously a half-million or a million eggs during the early spring. The eggs settle in the water and adhere to the roots and stems of vegetation there. The eggs are not guarded, and the young are left to care for themselves. Salamanders commonly hibernate in humus, un- der logs, or in other nooks or crevices on land. They usually emerge during the first warm rains of early spring and proceed to the nearest pond, there to lay their eggs. The males deposit their spermatophores on submerged leaves or twigs from whence the fe- male picks them up for fertilizing the eggs. The eggs are laid in jelly-like masses and require several days to hatch if the temperature is low. The eggs of Ambystoma maculatum (Gilbert 1944) and A. tex- anum (Burger 1950) hatch more successfully and at a faster rate if they contain unicellular green algae within the capsule. These algae apparently create a symbiotic relationship for oxygen and carbon dioxide. Larval salamanders possess gills, but in all but a few forms these are later absorbed and the adult returns to land. A. tigrinum sometimes breeds while still re- taining the larval gills, and never leaving the water. Frogs commonly hibernate in the mud at the bot- tom of ponds, although some forms, including toads, hibernate in the soil on land. In the spring the males go to small bodies of water where their loud choruses attract the females for mating purposes. The jelly- like masses or strings of eggs require only a few days to hatch, but the tadpole stage lasts longer. Meta- morphosis in toads that lay their eggs in temporary ponds takes place rapidly, but in other species, such as the bullfrog, adults do not occur until two years after the eggs are laid (Wright and Wright 1933). Practically all species of birds characteristic of northern latitudes that nest in the marsh are migra- tory, as the freezing of the water and drying of the vegetation eliminate their food supply. Nests are lo- cated in a variety of situations: on floating masses of plant debris built above the water level, typical of grebes, terns, gulls, black-necked stilt, and ducks ; in plant material, placed in tufts of vegetation or formed into platforms, or nests attached to cattails and other emergent plants well above the water level, typical of cranes, gallinules, rails, avocets, snipes, bitterns, ibises, marsh wrens, swamp sparrows, and blackbirds ; in swamp shrubs, typical of flycatchers and yellow warblers; in holes in trees, typical of tree swallow, prothonotary warbler, and wood duck; in the tops of trees in adjacent forests, typical of herons, cormorants, egrets, and wood ibis. The muskrat is one of the most conspicuous and important mammals of both salt and fresh-water marshes as well as river banks. Along rivers, the animal lives in burrows that it excavates well back in the bank. In marshes, it constructs a dome-shaped lodge, as high as one meter, by heaping up freshly- cut marsh vegetation. The lodge is hollow and dry within, the floor is placed well above the water level. The lodge has several underwater entrances and ex- its. In it the animal cares for its young and finds protection from enemies and weather in both winter and summer. In addition to lodges, the muskrat con- structs shelters, where it may feed out of sight of enemies, and breathing holes, called push-ups, through the winter ice. BIOMASS AND PRODUCTIVITY In a pond in Iowa, the average summer population of bottom invertebrates in water 0.5 m deep averaged 3819 individuals, 1334 mg/m?; in water 1.5 m deep, 1540 individuals, 1370 mg/m?. In the shallow water the most important components of the biomass were, in descending order : snails (shells removed), midge fly larvae, annelid worms, and the amphipod Hyalella. In the deeper water the biomass was mostly midge fly larvae (Tebo 1955). Produc- tivity of the midge fly Tanytarsus, one generation per year, averaged 7.5 g/m? in a Michigan lake (An- derson and Hooper 1956). By mooring a floating cage over open water throughout the season in an English pond, a total of 8988 midge flies and other insects per square meter were caught as they emerged from the bottom mud. In shallow water, where the vegetation was thicker, a total of 5979 individuals per square meter were captured, a total which included fewer midge flies and more dragonflies and caddis- flies (Macan and Worthington 1951). Average standing crops of fish in backwaters and oxbows may be almost 500 lbs/acre (57 mg/m?), 90 Habitats, communities, succession in midwestern North American reservoirs almost 400 Ibs/acre (45 mg/m*), in other reservoirs and ponds 200-300 Ibs/acre (23-24 mg/m?*), in warm-water lakes 125-150 Ibs/acre (14-17 mg/m*), and in trout lakes less than 50 Ib/acre (5.7 mg/m?). There is no tendency for the standing crop to decrease with in- crease in size of the body of water (Carlander 1955). Biomass varies with the fertility of the pond and the food supply. Ponds and lakes receiving water that drains over fertile soil will have more basic food sub- stances. than drainage from poor soils brings. The presence of certain species depends also on suitable breeding sites (Shelford 1911). Biomass is further affected by the food habits of the fish species present. In fertile ponds in Alabama containing species feeding largely on phytoplankton, the median biomass of fish was 925 Ibs/acre (105 mg/m*) ; in ponds with fish feeding largely on in- sects, 550 Ibs/acre (62 mg/m?) ; and in ponds with fish feeding largely on other fish, 175 Ib/acre (20 mg/m") (Swingle and Smith 1941). It was esti- mated that about five pounds of food (2.26 kg) are required to produce one pound of fish (0.45 kg). The same ratio has been found characteristic of ponds in Michigan, (Hayne and Ball 1956). Hence, the biomass of animal life decreases with each additional link in the food chain. In two small Michigan ponds where it was pos- sible to tabulate the entire fish population, the benthic production (at least, that portion used as fish food) during one growing season was calculated at about 17 times the standing crop when fish were present. This equalled 811 Ibs/acre (92.0 g/m?). The pro- ductivity of the fish during the same period was 181 Ibs/acre (20.5 g/m*), giving a ratio of 4.5:1 (Hayne and Ball 1956). When the standing crop of fish remains the same year after year, its productivity is indicated by the number or biomass harvested. In northern Wiscon- sin the maximum annual yield of desirable food fishes is about 21 per cent of the mean standing crop; in central Illinois it is about 50 per cent; in southern Louisiana, 118 per cent (Thompson 1941). The productivity of ponds and marshes for verte- brates other than fish has been measured in a few localities. In northwest Iowa, redhead ducks annually produce about 1.4 young per hectare (56/100acre) ; ruddy ducks, 0.6 young (24/100acre) ; (Low 1941, 1945). Nine species of ducks in the Bear River marshes of Utah average 16 young per hectare (640/ 100acre) (Williams and Marshall 1938). In Idaho, nine species of ducks produce over 22 young per hectare (880/100acre) and Canada geese about 0.1 young (Steel et al. 1956, 1957). On a well-developed marsh, it is generally possible to remove two-thirds of the muskrats each year and still reserve sufficient brood stock for a sustained annual crop. This is about 2.5 muskrats per lodge (Dozier 1953). POND AND MARSH MANAGEMENT The maintenance and control, throughout the year, of the water level of ponds and marshes is important for increased productivity. This may often be accomplished by damming the outlet. It is also important to retard the plant succession which, if left alone, will eventually bring about the total disap- pearance of the habitat. This may be done by cutting, burning, use of chemical sprays, flooding, and ditch- ing. Small ponds, called pot holes, with a good mar- gin of marsh vegetation, or a marsh interspersed with numerous small areas of open water, give the highest yield of waterfowl and other birds, and muskrats. The abundance of waterfowl is often proportional to the extent of the pond margin rather than the acre- age of emergent vegetation. In the Louisiana coastal marshes, the highest sustained yield of muskrats (14.5/hectare/yr or 580/100acre/yr) is in areas with Scirpus americanus (O'Neil 1949). Artificial ponds are easily constructed (Ander- son 1950, Musser 1948) and are an asset to farms as a source of food and recreation as well as water for domestic animals. Such ponds are commonly stocked with bluegill and largemouth bass, although other combinations may be used. High rates of re- production bring the fish population up to full carry- ing capacity within one or two years. If the pond were stocked with an herbivorous fish only, such as bluegills, normal reproduction would soon become so excessive that a dense population of stunted fish would be present. Using a prey-predator combina- tion in proper proportions, the predator (largemouth bass, for instance) will consume the excessive off- spring of the prey species, and the average size of the remaining fish will be increased. The develop- ment of aquatic vegetation in these farm ponds is dis- couraged since it allows too many prey individuals to escape the predator. The fertility of poor ponds can be increased by applying fertilizer encouraging the abundant growth of bacteria, plankton, and bot- tom organisms providing fish food (Howell 1941). The control of turbidity is also important. Clear ponds with less than 25 ppm turbidity may have 12.8 times more plankton and 5.5 times more fish than ponds with a turbidity exceeding 100 ppm (Buck 1956). Repeated stocking of ponds with artificially prop- agated fish is undesirable as there is more trouble in controlling overpopulation than underpopulation. The productivity of a pond is determined not by the number of fish introduced but by available food sup- ply. The available food supply is divided between the individuals present. One study showed that 6500 bluegills per acre (16,250 per hectare) averaged 25.5 grams each, 3200 per acre (8000 per hectare) aver- aged 51.0 grams each, and 1300 per acre (3250 per hectare) averaged 104.9 grams each (Swingle and Ponds, marshes, swamps, and bogs 91 Gms Lbs 180 \ =O , \ 2 140 tos 120 | eye) Yearlings ca 0.2 80 IN i \) vi 60k, ye eT 40. wo SY i Wie, 20) ERO 4 kK SR oS US ! 00 1933] 1934 | 1935 | 1936 | 1937 | 1938 | 1939 | ~ FIG. 7-4 Increase in weight of bluegills upon removal from an overpopulated pond to a pond of lower fish populations (from Bennett, Thompson, and Parr 1940). The cross-hatched portion represents the circumstance of overpopulation; a = autumn, b = summer, c = spring. Smith 1942). When the available food supply must be apportioned to a relatively large population, growth of individuals is retarded, but with small populations there is more food available per indi- vidual, and growth is surprisingly rapid (Fig. 7-4). In Europe, fish, chiefly carp, are raised for food in small artificial ponds and are handled in much the same manner as other domestic animals (Snieszko 1941). Problems involved in stocking and maintain- ing suitable fish populations in small artificial ponds in various parts of North America are summarized in the Journal of Wildlife Management (16, 1952, 233-288). A problem involved in the management of ponds and marshes is the control of mosquitoes. Oiling the water surface will kill mosquitoes, but also renders the habitat unsuitable for other organisms. Mos- quito larvae and pupae are good food for such min- nows as Fundulus and Gambusia that regularly feed at the surface. Stocking of these fish species will often keep mosquitoes under control. Elimination of aquatic vegetation in the shallow marginal areas will do away with hiding places and leave the larvae more exposed to fish predators. Because ponds and marshes produce great num- bers of fish, muskrats, and waterfowl, and are a source of recreation for hunting, fishing, boating, and swimming as well, the actual economic value of main- taining such areas is often greater than it would be if they were drained and planted to crops (Bellrose and Rollings 1949). Proper management of them is therefore a challenge to applied ecologists. BOGS Characteristics Bogs or moors typically develop in the hydro- seres of cold northern regions; while marshes and swamps, which are markedly different than bogs (Dansereau and Segadas-Vianna 1952), are charac- teristically southern in their location. Bogs commonly develop into a coniferous forest climax ; swamps suc- ceed to deciduous forest or other southern climax types. Several thousand years ago, when glacial cli- mates gripped the northern states, extensive bogs developed and have persisted as relic communities in spite of the warming of the climate. These bogs are slowly being replaced by pond-marsh species at equivalent seral stages by cliseral succession (Table 7-6). Bogs occurring in the Great Lakes region are ordinarily small in area and have little or no drain- age. There may be oxygen present in the open water of the larger bogs, but it is characteristically in very low concentration, at most, in small bogs or in the marginal zones. Bog water has a distinct brown color; a low nitrogen content; a low temperature beneath the surface; a low pH, at least in the mar- ginal vegetated zones; and a low dissolved salt con- tent. A false bottom is characteristic of bogs. It con- sists of finely divided plant material of a light brown color, held suspended in the water at varying depths below the surface. This false bottom may extend downward several meters before a true solid bottom is reached. The material disperses on slight disturb- ance and may render all the open water turbid. Ordi- narily, the surface waters are quiet and clear. Dead vegetation does not completely decompose; as it ac- cumulates, it becomes compressed to form peat. The plant bog sere In the early stages of development of the bog, organic detritus may accumulate mostly in the deep- est portions (Potzger 1956). As time goes on, how- ever, a definite concentric-circle zonation of vegeta- tion is established around the margin. As_ peat accumulates, each zone encroaches on the next inner ; the inmost shrinks until all open water disappears. The area becomes finally covered with climax forest (Dachnowski 1912). 92 Habitats, communities, succession 5h '< > ie Gn. Clisere Cold climate Stage Bog Floating vegetation Pond and water lilies, or absent Emergent vegetation Sedge mat Leatherleaf, labra- dor tea, bog rosemary Low shrubs or heath Biosere Mountain holly, chokeberry High shrubs Swamp or bog forest Tamarack, black spruce Hemlock, pine, white cedar or spruce, fir Climax forest War nlimale TABLE 7-6 Relation of biotic Pond succession to climatic succession in ponds and bogs. Vertical Pond and water lilies succession from open water to climax forest is taking place in both the pond and the bog, but Marsh: cattail, reeds, . as the climate gets warmer, there bulrushes Absent is simultaneously a horizontal succession from the various stages in the bog sere to equivalent stages in the pond sere. Buttonbush, alders Soft maple, elm, ash Oak, hickory or beech, sugar maple In some bogs (Gates 1942, Dansereau and Se- gadas-Vianna 1952) the first plant stage may be com- posed of floating vegetation (Nuphar, Nymphaea, Potamogeton, Sparganium), but floating vegetation is often absent and the first stage is a sedge-mat com- posed of sedges, cottongrass, and buckbean. The rhizomes of the sedges grow out into the water and become so interlaced that they form a floating mat. At the water edge the mat may be very thin, but towards shore it may become as much as a meter thick. Since the mat floats on open water it jars easily, hence the name quaking bog—one must watch his step that he does not break through. Sphagnum moss is not essential for the formation of a mat, but it invades the mat quickly and helps bind it together. Sphagnum persists into the shrub and bog-forest stages following. Interesting insectivorous species such as the pitcher plant and sundew are common, as are various members of the orchid family. The next plant stage is dominated by Jow shrubs, which encroach on the floating mat. The leatherleaf, bog rosemary, laurels, labrador tea, sweet gale, and cranberries are important species. A high shrub stage commonly follows the low shrubs at such time as the mat becomes thicker or grounded. Common shrub species are holly, willow, chokeberry, alders, and dwarf birch. The first tree of the bog forest to invade the shrubs is commonly the tamarack, but this species is now less common than formerly because of fire, log- ging, and the depredations of the sawfly larvae Lygaeonematus erichsonii. Black spruce may either invade the shrubs directly or follow the tamarack. Later, the northern white-cedar may become domi- nant and persist for a very long time, but the ultimate fate of the bog, upon addition of upland soil or lower- ing of the water table, is to be covered with the climax forest of the region. Animal life In bogs that have a large body of open water, or an inflow of water entraining oxygen, and in which the pH is not extreme, invertebrate life comparable to that found in ponds and marshes occurs. True bogs, however, have little oxygen and a low pH, and many pond species do not appear. Mollusks are character- istically absent ; sphaeriids may persist but their shells become very thin. Bottom organisms in general are poorly represented because of the tenuous physical nature of the substratum. Desmids predominate among the phytoplankton, although dinoflagellates, Chlorophyceae, and Myxo- phyceae are common. Rotifers and a variety of Pro- tozoa are the principal zooplankters (Graaf 1957). The chief fish found in acid waters in Michigan are the brown bullhead, northern pike, bluegill, yellow perch, and mudminnow (Jewell and Brown 1929). The mudminnow may be found in waters almost de- void of oxygen since it is one of the few species that can live indefinitely by gulping air at the surface. Amphibians and reptiles are not characteristic of bogs, although the leopard frog is sometimes numer- ous on the sedge mat of bogs in Minnesota (Marshall and Buell 1955). Marsh birds are few in species and in no bogs do populations approach the magnitudes found in southern marshes. The muskrat and beaver persist into northern Canada. In general, the productivity and economic value of bogs is very low compared with ponds and marshes. Liming experiments, calculated to improve productivity, are being made. Calcium combines with the humic colloids which then flocculate and fall to the bottom. This clears the water, light penetrates deeper, pH is raised, and greater algal, zooplankton, and fish growth is induced (Hasler et al. 1951). Peat is a special bogs product of importance in northern Ponds, marshes, swamps, and bogs 93 FIG. 7-5 Plant sere at Bryant's Bog, Michigan, from open water through a narrow broken mat stage of sedge, a low shrub stage of leatherleaf, a high shrub stage (in middle rear) of holly, to tamarack and black spruce (courtesy R.E. Rundus). Europe. It is cut out in blocks, dried, and used as fuel. It is doubtful if the aquatic fauna of bogs is suf- ficiently distinct or unique to constitute more than a facies of the pond-marsh biocies. It is succeeded, however, by a distinct shrub biocies that differs from the deciduous forest-edge community. The shrub bi- ocies is replaced by coniferous forest biociations. SUMMARY Ponds differ from lakes in that they are generally small and shallow, and, when mature, have rooted vegetation over most of the bottom. Bogs are limited to northern regions, contain a northern type of vegetation, and are generally acid and deficient in oxygen. As the climate of northern regions slowly warms, stages in the bog plant sere are replaced by corresponding stages in the pond plant sere. The pond sere consists of six or more plant stages but only three animal stages : pond-marsh biocies, decidu- ous forest-edge biocies, and deciduous forest biocia- tion. These animal communities correspond with the types of vegetation in the plant sere, but not with the plant communities identified by taxonomic composi- tion of the plant dominants. The animal community in bogs is an impoverished facies of the pond-marsh biocies. The pond-marsh biocies contains plankton, ben- 94 Habitats, communities, succession FIG. 7-6 Profile of a bog plant sere (from Dansereau and Segadas-Vianna 1952). Water Shrubs lillies ai peat False bottom Altered rock B- horizon Water Parent rock A-horizon thos, and nekton, as do lakes; in addition, neuston is present. Species that constitute these life-forms are mostly different from those in lakes. In ponds pul- monate snails replace the gilled snails of lakes, and clams are of lesser importance. Air-breathing adult beetles and bugs, mostly absent from the lake biocies, are often abundant. Adult stages of aquatic insects and terrestrial forms occur in the surrounding marsh. Fish spend most of their lives in the ponds, but go into the marshes to reproduce. Amphibians, reptiles, birds, and mammals are usually numerous. Food-chains in ponds and marshes are based in part on detritus, bacteria, and phytoplankton, true also of lakes; and, in part, on rooted plants, the periphyton that covers them as well as other objects Humus layer BOG FOREST Black spruce Sphagnum peat Woody peat Live sphagnum Mesic mosses in the water, and filamentous algae. Biomass and productivity are usually greater in ponds than in lakes. Ponds, however, may become stagnant during dry periods, especially in late summer, with great ad- verse affect upon their carrying capacity. Pond and marsh management for high economic yield of fish, waterfowl, and muskrats requires control of the water level and control over plant succession ; an incidental problem is mosquito control. The unique adapta- tions and behavioral adjustments of animals to meet the critical periods of summer stagnation and winter freezing, characteristic of ponds, are most interesting. Ponds and marshes are available to all ecologists for the study of the life-cycles and adjustments of animals in the pond-marsh biocies. Ponds, marshes, swamps, and bogs 95 Rock, Sand, and Clay The origin of life was undoubtedly in the sea. Physiological adjustments were necessary before or- ganisms could occupy fresh water. Although some organisms may have become air-breathers and in- vaded land habitats directly from the sea, most evolu- tion of terrestrial forms has doubtless come from fresh water. Relatively few major groups of animals have been successful in this invasion of land, the most notable being oligochaete worms ; gastropod mollusks ; many arthropods, especially the insects and spiders; reptiles ; birds; and mammals. ADJUSTMENTS TO THE TERRESTRIAL HABITAT Living on land presents many problems. Our present concern is to analyze the ways in which animals have met these problems and to trace the suc- cession of communities in the extreme terrestrial habitats of rock, sand, and clay. Gravity In water, organisms counteract gravity by means of various flotation and swimming mech- anisms. Fluid buoyancy permits water-dwellers to attain huge size; consider the whale. A land animal, on the other hand, must support its entire weight. Some terrestrial animals gain a modicum of support by burrowing into the soil; others drag their bodies over the ground surface. But the supportive advan- tage they thus gain is costly in other directions, for they are slow moving and relatively helpless before predators. The animals best adapted to terrestrial life have evolved appendages in the form of legs or wings that not only raise the body above the ground but are also the means of more or less rapid locomotion and adroit movements over the surface or through the air. Terrestrial adaptation has involved the development of a tough body covering to hold fluids and internal or- gans in place; a skeletal framework to give permanent shape to the body and, as a system of levers, to fur- nish means of locomotion; and powerful muscles to lift and move the heavy body. Gravity thus limits the mass of land animals; dinosaurs, mastodons, and elephants approach the maximum practicable size. Moisture In sharp contrast to aquatic forms, terrestrial animals are not constantly enveloped with a continu- ous watery medium, with the limited exceptions of protozoans, nematodes, and other small organisms living in moist soil. 96 Land animals, lacking constant contact with the water medium, are faced with the problems of obtain- ing water and preventing excessive water losses from the body. Water becomes available to animals in varying amounts in the forms of rain, snow, hail, frost, and fog. Whatever the form it arrives in, the significant things are the amount of free, liquid water added to the substratum, accessible to plants and animals, and the humidity of the air. Considerable amounts of moisture are lost to organisms as run-off water flow- ing into streams, by evaporation back into the air, and as water bound in snow and ice. Evaporation of water from the earth’s surface or from the bodies of organisms increases as tempera- ture rises, air movement (wind) accelerates, and the amount of moisture already in the air decreases. When measured as grains per cubic foot or as milli- meters mercury pressure, the actual amount of mois- ture vapor in the air is known as absolute humidity. This measurement is of less ecological importance than is relative hwnidity, the ratio of amount of water vapor actually in it to the quantity required to saturate the air at existing temperature and baro- metric pressure. Relative humidity is easily deter- mined by means of sling or cog psychrometers, and may be continuously recorded with temperature by hygrothermographs. The evaporation rate of water is more closely re- lated to saturation deficit than it is to relative hu- midity. Saturation deficit is a quantity which cannot be directly measured, it must be calculated. It is that additional amount of moisture required to saturate air under prevailing temperature, relative humidity, and barometric pressure conditions, commonly expressed as grains per cubic foot or as millimeters of mercury pressure. The most exact and desirable measurement of water evaporation is the vapor pressure gradient ob- taining between the organism and the surrounding air. The gradient is positive if water molecules leave the organism at a rate faster than the rate at which the organism is absorbing them from the air, and negative if the reverse is true (Table 8-1). The de- termination of gradient magnitude involves the meas- urements of body temperature of the organism and air, permeability of body membranes, and rate of air movement over the body surface (Thornthwaite 1940). Water is obtained by a land animal by various devices. There may be some direct absorption through body surfaces such as occurs in the toad in moist soil and in some beetle larvae in moist air; this device is important in only a few species. Large mammals frequently travel several miles each day to water holes to imbibe drinking water. Many, but not all, birds require drinking water; some species, for FIG. 8-| Apparatus for measur- ing weather factors. (a) rain gauge (courtesy Friez Instrument Division); (b) cog psychrometer; (c) hygrothermograph; (courtesy Friez Instrument Division); (d) Livingston spherical atmometer. instances quail and partridge, get it as morning dew on vegetation. Butterflies may frequently be observed drinking water from small pools. An important source of water is the free water in food, particularly in succulent vegetation and in the blood and body fluids of animals. Desert animals depend almost en- tirely for water on that contained in their food and on metabolic water, liberated when fats and carbo- hydrates, and to a lesser extent, proteins, are oxidized in their bodies. Water is lost from the body through the skin and lungs as insensible moisture and perspiration. Rapid and largely uncontrolled loss of moisture through the skin of amphibians, snails, annelids, and insect larvae is a limiting factor confining these animals to moist Rock, sand, and clay 97 TABLE 8-| Evaporation and condensation on a free water sur- face in relation to temperature, relative humidity, saturation deficit, and vapor pressure gradient (after Thornthwaite 1940). Water Water Factor evaporates condenses Water temperature 16°C 16°C Air temperature 16°C 27°C Relative humidity 70% 10% Vapor pressure of saturated air 133 mm Hg 263 mm Hg Vapor pressure of air 70% saturated 93 mm Hg 184 mm Hg Saturation deficit 40 mm Hg 79 mm Hg Vapor pressure of the water 133 mm Hg 133 mm Hg Vapor pressure gradient +40 mm Hg -51 mm Hg habitats or to activity only in times of high humidity. Adult insects and other arthropods, reptiles, birds, and mammals have evolved body surfaces of chitin and waxes, scales, or cornification of the surface layers of the skin that largely prevent uncontrolled loss of moisture. Moisture loss through the respiratory surfaces in these forms remains considerable, how- ever. Water is also lost with the feces, although in some species much water is reabsorbed by the large intestine before the feces are ejected. The amount of water removed from the body by excretory organs, particularly the kidneys, varies directly with the amount of water intake and inversely with the amount lost through other devices. The kidneys are critical to maintenance of proper concentration of salts in the blood and body fluids. It is important that water in- take balance water loss. Organisms are very sensitive to disturbances in body water balance, and this factor is very significant in determining the type of niche which a species comes to occupy. In order that animals could exist in terrestrial habitats, they had to acquire the ability to carry on reproductive activities in the absence of water. The chief reproductive adaptations involve the following (Pearse 1950) : internal fertilization ; a shell covering the egg to conserve moisture and salts; food pro- vision to the embryo and young, by yolk in the egg cell, placenta in the uterus of the mother, or direct feeding by the adults; reduction in number of young with more efficient parental care; reduction or elimi- nation of free-swimming larval stages; greater seg- regation of species into different niches to avoid interspecific disturbances. Temperature Aquatic animals are not ordinarily subjected to temperatures below freezing and are in a relatively stable temperature environment, but terrestrial spe- cies are exposed to highly variable temperatures that may reach extremes of about —68°C and +55°C. No single species is required to withstand such a wide range of temperatures, however. Optimum and tolerance limits vary from one species to another, inasmuch as each inherits a specific degree of acclimati- zation. No aquatic species has evolved control over its body temperature. Aquatic warm-blooded mammals and birds are derived from terrestrial forms. An abil- ity to maintain a constant body temperature has sur- vival value in terrestrial habitats, however, and consequently physiological mechanisms for homoto- thermism developed independently in birds and mam- mals. All other land organisms are poikilothermal ; that is, they have no physiological mechanism for maintaining a constant body temperature. Some poikilotherms, such as bees, have developed special behavior patterns, that enable them to maintain fairly constant conditions in the hive by cooperative efforts; some lizards, snakes, and turtles are able to exert some control over their body temperatures by moving into and out of sunlit areas. Rates of activity, food consumption, metabolism, growth, and other physiological functions increase, to a certain limit, with rise of body temperature. Homoiotherms maintain a continuous high rate of functioning because of their constant high body tem- peratures, but the rate at which poikilotherms func- tion varies with the temperature of their habitats. Latitudinal distribution of both poikilotherms and homoiotherms is often limited northward and south- ward by the extremes of temperature that they can tolerate. The rate of energy exchange in poikilo- therms is so directly dependent on the amount of heat in the habitat that the total growth and repro- duction of a species may be determined by the extent to which it can accumulate developmental heat units during the year (Shelford 1929: Chap. 7) ; the prin- ciple is similar to the principle of heat budgets in lakes. The closer a region lies towards either Pole, the shorter the growth season is, and distribution may be limited not by extreme low temperatures as such, but by accumulation of heat energy insufficient to per- mit completion of life cycles. The relation of energy balance in homoiotherms to air temperature is even more complicated (Ken- deigh 1949, Seibert 1949). In cold regions an ani- mal may require all the energy its food provides it simply to maintain its own existence, no surplus available to meet the high demands of reproduction. Under such conditions, a species cannot become per- manently established in a region. A warm-blooded animal requires a range of temperature that is com- fortable and in which it can ingest and metabolize food at a rate sufficient to maintain normal body tem- perature, sustain physical existence, and carry on reproductive activities, too. We can thus speak of existence energy and productive energy, concepts es- 98 Habitats, communities, succession sential to understanding the relation of an organism to the temperature of its environment. All organisms outside the tropics must adjust to meet the critical winter season. In those species ac- tive throughout the year, there is an increase in re- sistance to cold, brought about, in part, by dehy- dration of body tissues (Payne 1927), or by increase in density of plumage (Kendeigh 1949) or fur (Sealander 1951). Those species incapable of maintaining activity in situ over winter either migrate to more favorable regions or hibernate, or the adults die. Many invertebrates survive the winter in re- sistant egg or larval stages. Oxygen Dry air at 760 mm Hg pressure contains ap- proximately 21 per cent oxygen, 0.03 per cent carbon dioxide, 78 per cent nitrogen, and traces of other gases. Oxygen is thus much more abundant, con- stant, and available at all times in air than it is in water. Oxygen availability seldom becomes a critical factor for land animals, with the occasional exception of forms that live in the soil or invade high altitudes. Although terrestrial organisms have evolved sim- ple moist chambers, branched tracheal systems, or complicated lungs to replace the gills found in many aquatic forms, the fundamental requirement of moist membranes for the exchange of oxygen and carbon dioxide between body fluids, tissues, and the sur- rounding medium remains the same. The skin still serves this purpose in some terrestrial forms—an- nelids and some amphibians—but in most forms the moist membranes are within the body. Internal place- ment decreases the loss of water through evaporation. The evolution of an ability to take oxygen directly out of the air apparently preceded the actual invasion of land, and may have been induced in the pond and marsh habitat when oxygen dissolved in the water became reduced or absent during summer stagnant periods (Pearse 1950). The evolution of internal air- breathing organs was probably concurrent with the evolution of mechanisms to prevent excessive water loss from the exposed surfaces of the body. Solar radiation Solar radiation takes the form of an endless procession of waves. The length of a light wave from crest to crest, or trough to trough, determines its character in respect to energy and color; the height of the wave determines its intensity. Wave- length is commonly expressed in millimicrons (Imp = 0.000001 mm = 10 Angstrom units, A). All wave- lengths have a velocity of 299,340 kilometers per second. The solar spectrum varies from 51 mp, which is the shortest ultraviolet radiation, to 5300 mp, which is the longest infrared radiation. The spectrum vis- ible to man is between 390 mu and 810 mp. Consid- erable ultraviolet is absorbed by the atmosphere, and the ultraviolet wavelengths reaching the earth’s sur- face are mostly between 292 mp and 390 mu. Color perception by man is as follows: violet, 390— 422 mp; blue, 422-492 mp; green, 492-535 my; yellow, 535- 586 mu; orange, 586-647 mp; and red, 647-810 mu. The longer waves are rich in heat energy ; the shorter, in actinic energy. Solar radiation may be measured with a pyrheli- ometer, by which readings are given in terms of heat energy (g-cal/cm*/sec). Results are not accurate for the shorter wavelengths. Photoelectric cells accu- rately measure intensities in the shorter wavelengths ; readings are given in foot-candles. The Macbeth il- luminometer measures total sunlight in foot-candles by visual comparison of observed intensity with a standardized and calibrated light source set within the instrument; accuracy is limited by sensitivity of the human eye. With any photometric instrument, the measurement of intensity of any portion of the spectrum requires the use of calibrated color filters that screen out everything but the desired wave- lengths. Different wavelengths have different effects on or- ganisms. Green light is reflected by plants; little is used in photosynthesis. Some early experiments on tadpoles, fish, snails, and other forms (Davenport 1908) indicate that there is an increasing growth rate in different wavelengths, in the following order: green, red, white, yellow, blue, violet. The physio- logical basis of this phenomenon is not known. An excess of infrared may produce overheating of the animal. Ultraviolet in large concentrations is harm- ful to most animals, but in lower intensities is bene- ficial to elaboration of vitamin D. Evidence indicates that ultraviolet combined with rainfall is important in controlling numbers of some mammals, forest-edge birds, and insects. In general, terrestrial organisms are exposed to much higher intensities of solar radia- tion than are aquatic organisms and have evolved horny or chitinous body coverings, hair, or feathers, that function in part to protect internal structures from lethal concentrations. Long-range vision has de- veloped only in land animals and is correlated with the high light intensities characteristic of terrestrial habitats. Diurnation Animals may be divided into diurnal (day- time), crepuscular (late evening and early morning), nocturnal (night), and arhythmic (irregular) species. Animals that occupy microhabitats where tempera- ture and light changes are negligible at most tend to Rock, sand, and clay 99 INCHES OF RAINFALL 14 16 145 140 135 130 125 120 5 110 105 100 95 PETTIT UNITS OF ULTRAVIOLET RADIATION 90 85 80 75 70 20 25 30 35 oie | ee ee | 40 45 50 55 CENTIMETERS OF RAINFALL FIG. 8-2 An ultraviolet-hydrogram for February populations of pronghorn antelope in Yellowstone Park for the years indicated in italic numerals. It appears that the number of young produced in any year has been determined in a sensitive period two be arhythmic; cave crayfish, log-inhabiting beetles, moles, shrews, and some ants, for instances. The microfauna of the soil is probably arhythmic. About two-thirds of the mammal species occurring in both temperate deciduous and tropical rain forests are nocturnal. Birds are predominantly diurnal, except that owls are nocturnal and goatsuckers crepuscular. The majority of amphibian and reptile species are nocturnal ; some frogs and lizards are diurnal. Septembers earlier; hence, the ultraviolet data given for each year is for the second preceding September, and rainfall is for that September through the August following, inclusive (modified from Shelford 1954). Among invertebrates, there is considerable vari- ation. The drosophilid flies are crepuscular, having pronounced peaks of activity at dawn and dusk (Taylor and Kalmus 1954). The major period of activity of many nocturnal animals occurs during the first half of the night period, although they often possess a secondary pre-dawn period of activity. With diurnal animals, the major period of activity usually comes during the first portion of the day, al- 100 Habitats, communities, succession though there may be a secondary pre-dusk period of activity (Calhoun 194446). A classification of the diel activities of animals in respect to controlling influences can be made (O. Park in Allee et al. 1949; 558) : I. Periodic activity. Regularly most active for a specific period in the diel cycle. 1. Exogenous type. Activity rhythm directly induced and controlled by periodically re- current environmental conditions. 2. Endogenous type. Activity rhythm more or less independent of obvious factors in the environment; persists even under controlled, apparently uniform environ- mental conditions. It is possible, however, that obscure environmental factors may still be regulative (Brown 1959). a. Habitual activity. An endogenous rhythm that has become established as the result of previous experience of the individual. b. Inherent activity. An rhythm that is inherited. 3. Composite type. A rhythm pattern that is partly endogenous but is accentuated when the animal is exposed to environ- mental conditions periodically recurrent. II. Aperiodic (arhythmic) activity. No con- sistency between individuals of a species in exhibiting an activity pattern relating to a specific time of day or night. endogenous Endogenous diel rhythms have been demonstrated in Coelenterata, Platyhelminthes, Echinodermata, Crustacea, Insecta, Cyclostomata, Pisces, Amphibia, Reptilia, Aves, and Mammalia, although they are not often inherent. Probably the great majority of species have rhythms of the composite type. Periods of rest or sleep alternating with activity seems to be a funda- mental protoplasmic requirement (Park 1940). Carnivorous, herbivorous, and omnivorous noc- turnal animals occur throughout the metazoans. It is of interest that nocturnal animals are less fre- quently gregarious and social than are diurnal forms. Adjustments for night activity involve development of luminescent organs such as fireflies (Lampyridae) possess ; infrared-sensitive vision, suggested for some insects and birds but not proven for owls (Dice 1945) ; increase in visual acuity by modification of eye structures (Walls 1942) ; keenness of smell dis- played by some mammals; and increased sensitivity to sound, remarkably developed in bats. Bats have evolved a radar system, called echolocation, whereby the animals emit ultra high frequency sound waves, which are reflected from objects back to the ears (Griffin 1953). Color vision, well developed in some diurnal insects, fish, reptiles, amphibians, birds, and mammals, is largely lost in those nocturnal species active at such low intensities of light that colors would be indistinguishable anyway. Correlated with loss of color vision is restriction of body coloration to blacks and whites or intermediate shades. Nocturnal animals escape such diurnal predators as reptiles, birds, and hymenopterous insects. There are noc- turnal predators, to be sure, but predation pressure at night appears to be less intense than during the day. There is decreased competition for food and shelter when some species are active by night, others hy day, over the same range. Butterflies are pre- dominantly diurnal; moths, nocturnal. Animals with moist skin, like snails and amphibians, suffer less evaporation of water from their bodies at night, when the relative humidity is higher and the temperature is lower. There is some belief (Clark 1914) that noc- turnal forms are derived from originally diurnal forms; an adjustment, perhaps, to avoid competition from aggressive diurnal species occupying the same niches. Seasonal variations (aspection) In tropical rain forests there is very little sea- sonal variation in the number of species active and size of populations attributable to length of day, tem- perature, and humidity, for these factors are nearly uniform throughout the year. In other parts of the tropics, however, there are definite wet and dry sea- sons, and a considerable change in numbers and ac- tivities of animals correlates with the seasonal varia- tions in vegetation and food supply. In temperate A ee er eack tractener te ecccrsverenaente go° 22 North Pole 20 Equator HOURS LIGHT PER DAY m |JAN | FE3|MaR|APRIMAY|JUN|JUL [AUGSER |OCT|NOV |DEC| FIG. 8-3 Monthly variation in daily photoperiods at cardinal latitudes of the Northern Hemisphere (after Boggs 1931). Rock, sand, and clay (101 TABLE 8-2 Ecological seasons (Macnab 1958). Aspect Sector Characteristics Hiemal Early November to late March. Deciduous trees nearly bare, herbs mostly dead except for winter-greer. species (Beatley 1956); insects, other inverte- brates, some mammals, going into hibernation or dormancy; last migrant birds disappear. Deep winter condition, little animal activity evident except winter resident birds in shel- tered locations and a few mammals. Some buds swell and subterra- nean sprouts begin to appear above ground, earliest migrant birds appear, animals begin- ning to emerge from hiber- nation. Hiemine Hibernine Emerginine Vernal Early April to late May. First appearance of flowers both of herbaceous and tree species; mammals and perma- nent resident birds begin re- productive activities; sala- manders go to ponds and lay their eggs; all insects, snails, and other invertebrates come out of hibernation. Deciduous trees now fully foli- ated, early spring flowers re- placed by species that tolerate shading; bird migration reaches its peak; insects and inverte- brates become abundant in all strata. Prevernine Vernine Aestival Early June to middle August. Reduced number of flowering herbs but vegetative growth at maximum, birds at height of nesting. Deciduous forest becomes hot and dry, many ground plants dry up; birds quiet and entering molt, molluscs aestivate, foli- age insects attain maximum populations. Cisaestine Aestine Autumnal Middle August to early No- vember. Fruits and nuts ripen, autumn flowers come into bloom, birds at height of southward migra- tion, mammals reach maximum populations but invertebrates decreasing. Foliage of deciduous trees changes color and falls, insect and spider populations shift from higher strata to the ground. Serotinine Autumnine 102 and arctic regions, seasonal differences in length of day and temperature become increasingly great the closer the region lies toward a pole. Correlated with seasonal changes in climate are adaptive adjustments of metabolism and energy bal- ances, regulation of breeding time, change in food habits, and migration or hibernation. Birds breed in the spring and early summer, since lengthening daily photoperiods stimulate maturing of the gonads (Burger 1949). Photoperiodism also controls the breeding time of some mammals, fish, and inverte- brates as well as plants. However, in some species, say trout and deer, shortening rather than lengthen- ing photoperiods are stimulating, and such species regularly breed during the autumn. In deciduous forests, seasonal differences in the development of the foliage greatly affect animals. When trees are bare, sunlight penetrates to the forest floor more readily than when foliage is in full devel- opment. Foliage is important because it is protective cover from weather and offers refuge and conceal- ment from predators; to many species it is a direct source of food. Four main ecological seasons, or aspects, may be recognized; each aspect is divisible into secondary periods, or sectors (Table 8-2). These periods are best developed in the temperate deciduous forest but also occur in modified form in other communities as well. The beginning and end of any aspect cannot be set with exactness, since aspects vary from year to year, with latitude and type of community. Substratum The substratum greatly influences the kind of plants and animals that occur in the pioneer stages of succession. Bare rock presents one extreme physi- cal habitat, sand another, and clay yet another. The substratum affects animals indirectly in terms of the kinds of plants it supports and the variety of niches it affords. Differences between early sere stages not- withstanding, later ones tend to be more and more alike so that convergence occurs. In temperate hu- mid regions, where the seres pass through several stages, the climax communities of all seres are very much alike regardless of the type of bare area on which they originated. ROCK SERE Plant communities Stages in the plant sere on bare rock are lichens, mosses, annual herbs and grasses, shrubs, Habitats, communities, succession 103 *SO4EFS PoflUP) UJe}sey Ul! sesas yueld jo sabeys Bulbseauod yfIM SalziunuWOD jeUIUe 4O UOIFe/9IOD 4-8 “Oj4 XBUIT[D 194}0 10 ABI Y-SNID] f DA ADD-SNnIAaN’Y “030 ‘poomsseq Say ‘afayongq ‘jnusza}ng I Japa xoq-yse-wptq ! aroweods -uin3-Ar1aqyoey { aideur adeur-poom -ar1omeohs uOT}eID0IG dijnj-yse-wyq -U0}}09-MOTIIM puynjaa snor4an& -poomuo}j09 jso10j Snonpioeq sould | (Wqnos) sats01q PexTW UIa}svayynog yeaTsoys-ATToO[Go"] Snutd-Snuig | snjndod-snjndog (yj4ou) satToo1q jsa10j} [ve10g Ieped par (sqiay pue) t soot} ‘ausroyyaey ‘sialig MOTTA Ieqpueg SNUAOD-SNUNAT SUIpBAUL 7 Sqn1iys ap sal00I1q a3pa-j}sai04 uosodoapuy sessei3 sasseiZ ainjseg ssevid adpaswuooig -SYSOASDUDIDD IZAO0TO }99MS sqiay ee sesso _ Sqiey Tenuuy (sqiey Tenuuy) sqioy Tenuuy suayor'y SaT001q pue[sseIn seinsetd Spel} pouopurqy aias uleldpooly (seToosse) aaas ARID ai1as YO0Y Ayunuiwo0D [ewjuy aies pues and forest. The species composition of each stage varies with the chemical nature of the rock, the pre- vailing climate, and the locality. In the first stage, various kinds of lichens com- pete for a foothold, but crustose types usually pre- cede foliaceous types. Mosses and such fruticose lichens as Cladonia follow foliaceous lichens; or may initiate the sere, telescoping the earlier lichen stages (Keever, Oosting, and Anderson 1951). FIG. 8-6 (a) a tardigrade, Echiniscus oihounae, occurring in moss, possessing long filaments (Heinis 1910). (b) ant-lion adult (Shelford 1913). FIG. 8-5 Early crustose lichen, foliose lichen, and moss stages on rock, with ferns in a crevice (courtesy R.E. Rundus). Lichens and mosses soak up moisture in wet weather. They derive mineral nutrients from the un- derlying rock. Carbon dioxide secreted from the rhi- zoids forms a weak acid with water and dissolves the binding material of the small rock particles. Rhizoids may penetrate rock for several millimeters. These plants trap windblown dust and obtain nitrogen from organic compounds in it. When the plants die, they become an addition to the accumulation of organic matter. Herbs, grasses, ferns and later stages invade to continue the crumbling of the rock and buildup of soil. Freezing and thawing of water may crack the rock, and in these cracks wind- and water-borne soil lodges and supports plants. Once shrub and tree roots get started in crevices, their growth exerts a powerful force further splitting and crumbling the rock. Animal life Animal life in the pioneer plant stages on rock is scanty. Ants and spiders roam over the bare rock, and insects of various sorts may stop there, tempo- rarily. Spiders may construct webs and nests in rock crannies or amongst the lichens. Some tardigrades find preferred niches in lichens. Mosses offer a some- what more substantial microhabitat, but only those animals that can tolerate great extremes of flooding, dessication, heat, and cold can survive. Such forms are found in the rhizopod protozoans, nematodes, bdelloidid rotifers, tardigrades, copepods, small in- sects, and mites (Heinis 1910). They often have spe- 104 Habitats, communities, succession cial means of attachment to keep them from being blown away by the wind, such as strong claws or cement glands on the feet, long bristle-like threads to entangle among the moss filaments ; stickers or spines covering the eggs. Since wet periods are often too short to permit complete development, all stages must be tolerant of desiccation, at which time activities and growth are largely suspended. Animal life in general and land snails in particu- lar are usually more abundant in vegetation (grass- land and forests) established on calcareous soils derived from limestone than in the vegetation estab- lished on soils derived from sandstone, granitic, or volcanic rock. Calcium carbonate is a mineral essen- tial to the metabolism of most animals and for build- ing such skeletal structures as bones and _ shells. Snails are less numerous in the grass stage than in the later, moister forest communities that develop in the succession. SAND SERE Plant communities Sand is the product of mechanical pulverization of various rocks. It is deposited by wind and water. Where extensive areas of sand occur, strong winds pile the sand into shifting dunes. These dunes have a characteristic shape as the sand grains are blown up a long, rather gentle windward slope and swept over the crest onto a steep lee slope. Moving dunes may engulf whole forests; they eventually move on, leaving the denuded trunks of trees that they have smothered. The dunes continue to move until they reach the shelter of some other dune, get beyond the full force of the wind, or until invading vegetation covers the surface and anchors them down. The most successful sand-binding plants are the grasses Am- mophila, Calamovilfa, and Agropyron, willows, sand cherry, and cottonwoods. Willows and cottonwoods will survive even when almost buried. Each succeeding stage ties the sand down more firmly, but any break in the vegetation occasioned by a blowdown of trees or disturbance by man may invite the wind to start moving the exposed sand, and change the partially anchored dune again into a moving one. Only when the pine stage or the black oak stage is reached does the dune become relatively secure from the wind. The plant sere on the south shore of Lake Michi- gan consists essentially of the following stages (Cowles 1899) : Lower beach: Washed by summer storms and devoid of vegetation. Middle beach: Washed only by severe winter storms; comparatively dry in summer; upper limit marked by driftwood and debris. Scat- tered annual plants present. Calamagrostis-Andropogon associes (upper beach) : This is where the dunes begin to form. In this early developmental stage (associes ) grasses are dominant, particularly Calama- grostis longifolia, Andropogon scoparius, Agropyrum dasystachyum, Ammophila are- naria, Elymus canadensis ; various biennial and perennial herbs make their appearance. The sandbur grass occurs extensively in some areas. Prunus-Cornus associes: The commoner shrubs are sand cherry, chokecherry, red-osier dog- wood, creeping juniper, and the frost grape vine. Shrubs may invade the grass directly but become more common in the following tree stages. Populus-Populus associes: The first tree stage in the southern portion is made up principally of the eastern cottonwood, and in the northern portion, of the balsam poplar. The trees com- monly occur in open stands with grasses and shrubs forming the lower strata. The habitat is essentially forest-edge. The shrub and cotton- wood stages are often missing so that the sere progresses from the grass directly to the pine or black oak stage. Pinus-Pinus associes: Jack pine, red pine, and eastern white pine may invade one after an- other, commonly forming mixed stands. North- ern white-cedar and eastern redcedar also oc- cur; the former, more commonly northward. Succession to this stage is mainly contingent on stabilization of sand in dunes, and more efficient utilization of water resources. For succeeding stages to emerge, soil must develop by deposi- tion of humus. The floor of pine forest is cov- ered with a carpet of needles, although patches of bare sand still occur. As the sere advances, all bare areas become covered with a layer of humus. Quercus velutina consocies: Black oak often forms a nearly homogeneous stand that may persist for a long time. Quercus-Carya associes: Black, white, and, to a lesser extent, red oaks are commonly mixed with shagbark and bitternut hickories and, in moist habitats, American basswood. Fagus-Acer association: When soil humus and moisture become sufficient, American beech and sugar maple invade the sand to form the final climax stage. In other localities, the taxonomic composition of the communities, especially the later stages, differs considerably. The character of the climax varies ac- cording as climate and geography, but perhaps the Rock, sand, and clay 105 FIG. 8-7 Sand sere at Ludington State Park, Michigan. (a) the lower beach (at right center) is washed by ordinary waves; the middle beach (in center) contains driftwood left by heavy storm waves; the upper beach (at left) has a sand dune well anchored by grass and sand grape (light areas), shrubs (dark areas), and cottonwood trees. (b) grass stage, showing blowouts devoid of vegetation; a mixed pine stage is shown in the distant background (courtesy R.E. Rundus). sere is as complete and as complex in the Lake Mich- igan region as it would be anywhere. Habitat The sand dune habitat is characterized by ex- treme fluctuations in physical conditions, generally resembling those of a desert (Chapman et al. 1926). Temperatures, especially at the ground surface, are very high during bright sunny days; relative hu- midity is very low. Evaporation from spherical at- mometers is 2.5-3 times higher than in forest hab- itats at the same time of day. At night the ground surface temperature may be even lower than that of the air since there is little or no surface covering to prevent rapid heat radiation. Correlated with the diurnal changes of tempera- ture, relative humidity, and light, the kinds of animal active on the sand during sunny days are quite dif- ferent from those active on cloudy or rainy days and at night. When the temperature of the sand nears 50°C, all insects leave the surface. Some climb grasses to get off the ground, others enter their bur- rows. Insects flying above the sand can select an optimum temperature from widely different tempera- tures merely by changing their elevation only a few inches. They make hurried landings when entering their ground burrows. The female velvet-ants are usually among the last to retreat into their burrows 106 Habitats, communities, succession in the morning and the first to leave them in the eve- ning. Experiments show that they of all insects in this habitat are the most tolerant of the high temper- atures. Animals living here must either be physio- logically tolerant of extreme heat or possess behavior patterns that enable them to avoid it. Grasshoppers and other Orthoptera There have been detailed studies of a few spe- cial groups of animals occupying the Lake Michigan sand dunes. Three species of wood roach, 2 species of walking-stick, 20 species of short-horned grass- hopper, 13 species of long-horned grasshopper, and 6 species of field cricket occur in various stages of the sand sere in the Chicago area (Strohecker 1937). A breakdown of this list shows that 7 species, all short-horned grasshoppers, occur in the grass and cottonwood stages ; of these, one species is not found in the pine stage, and the other 6 species disappear by the time the black oak stage is reached. Eight new species of orthopterans, including 4 short-horned grasshoppers, enter the sere at the pine stage, but only 5 species persist into the black oak stage. Alto- gether there are 23 species of orthopterans listed for the black oak forest, an increase of 18 new species. There are only 25 species of orthopterans listed for the climax, but this includes 4 species of camel crickets which for the first time can find their proper niches under logs, and a katydid that appears in the trees. The greatest change in species composition within the sere occurs at the black oak stage upon the disappearance of 67 per cent of the species present in the earlier stages and the appearance of 78 per cent of the species as new forms. Of that 78 per cent, 61 per cent persist through all later stages. The change in species composition at this stage can be correlated with the development of a canopy of foliage and the resulting reduction in light intensity and soil temperatures. The community or niche restriction of the short-horned grasshoppers appears to be deter- mined either by soil conditions or by the vegetation (Isely 1938a). Before short-horned grasshoppers lay their eggs in the ground, the female tests the soil with her ovipositors until she finds soil of proper conditions. Experimental studies show that in cer- tain cases soil texture is the critical factor in the choice of the egg-laying site, while in other cases soil structure or degree of compaction is most important. Soil conditions appear particularly important for the sub-family of band-winged grasshoppers; for other groups, vegetation is of greater significance. In an experimental study of how an available choice between foods may affect distribution (Isely 1938a), one-half of 40 species of short-horned grass- o (e) a (2) S [e} Sunny day SUPERSURFACE Surface O SUBSURFACE 30 N 5 20 25 30 35 40 45 50 55 TEMPERATURE, °C FIG. 8-8 Temperature gradient on a sand dune, on a rainy day and on a sunny day (after Chapman et al. 1926). hoppers showed a feeding preference for grasses and one-half for broad-leaved herbs. The latter group in- cluded the spur-throated grasshoppers. Four species were restricted to feeding on a single plant species; 30 species confined themselves to a few plant species only, and usually of a single plant family at that; only 2 species fed on a wide variety of plants. In several instances grasshoppers starved in cages, when there was an abundance of fresh plant materials pres- ent that were palatable for other species, because their own preferred food species were absent. All five species of false katydids studied in Texas (Isely 1941) confined their choice of food to related species of broad-leaved herbs or forbs, refusing grasses; adults showed a marked preference for the flower parts and tender fruit pods. Two species of shield-backed grasshoppers were wholly carnivorous. The flower-feeding false katydids disappeared from the prairie in late spring and early summer as the flowering plants passed their peak, but the insect- feeding grasshoppers persisted to the end of July or until temperatures became too high for their comfort. Ants Ants cannot get established on the beach be- cause of its unstable character and are scarce even in the grass and cottonwood stages because of the shifting character of the dunes (Talbot 1934). Spe- cies found are crater-formers Lasius niger neoniger and Pheidole bicarinata, and two species of Campo- notus that find protection under the occasional log Rock, sand, and clay 107 TABLE 8-3 Number of species of each spider family found in each sand sere plant stage except the oak-hickory (after Lowrie 1948) Middle Grass Beech- Spider family beach stage Cottonwood Pine Black oak maple Web-builders Ariopidae 2 5 3 5 26 20 Micryphantidae 2 2 0 2 8 3 Theridiidae 1 1 2 5 10 13 Dictynidae 1 1 4 3 2 Linyphiidae 2 0 0 5 7 Agelenidae 1 3 8 Ciniflonidae 1 1 Hahniidae 2 1 Mimetidae 1 il Uloboridae 2 Total 5 11 6 17 59 58 Per cent all species in stage 29 35 34 40 35 48 Non-web-builders Lycosidae 9 4 2 1 24 ijl Gnaphosidae 2 1 0 3 11 6 Salticidae 1 7 5 10 29 17 Thomisidae if 4 10 22 14 Clubionidae 1 1 1 14 7 Anyphaenidae 1 1 3 Dysderidae 1 0 Oxyopidae 1 1 Pisauridae 6 4 Total 12 20 12 26 109 63 Per cent all species in stage 71 65 66 60 65 52 Number individuals of all species in herb stratum per 50 sweeps ; 8 6 10 18 24 that occurs. On hot dry days these ants withdraw to several inches below the surface and emerge only in the cool of the evening. In the pine stage, the slight mixture of humus in the sand is decidedly favorable, food is more abun- dant and varied. Of 18 ant species found, 9 live in patches of open sand with no shelter, 6 require sand with some protection above it (logs, bark, needles), and 3 are strictly log-inhabiting forms. Monomorium minimum and Paretrechina parvula are characteristic species. In the black oak community, 29 species occur of which only 6 live in scattered open areas of sand. These 6 species are quickly crowded out when there is development of a complete leaf covering over the ground. Formica pallide-fulva, which was becoming important in the pine community, is the predominant ant in the black oak stage. Its nests are invariably found under pieces of bark or branches lying on rather open ground. 108 As the sere advances into the white and red oaks stage, open areas of sand disappear, humus and mois- ture increase, logs in all stages of decay occur, the whole area becomes shaded, and the daily extremes in temperature and humidity typical of the open dunes are considerably curtailed. Species of ants character- istic of the early stages disappear, and forms that are found in mesic deciduous forests generally predom- inate, although there are only six species found here that do not also occur in the black oak community. Formica truncicola obscuriventris is the most numer- ous species. The number of colonies and variety of species reach maximum in the oak stages. In the climax beech-maple community, the num- ber of soil-dwelling forms is reduced, perhaps be- cause of the thick rich humus, although log-inhabiting forms are numerous. Lasius niger alienus americanus and Aphoenogaster fulva aquia picea are the only ants abundant in the deep woods ; ants are more numerous in the forest-edge than in the forest-interior. Habitats, communities, succession Although different species reach peaks of abun- dance at different points in the habitat gradient pro- ceeding from open sand to dense forest, the nature of the substratum divides the species into two major groups: those that tolerate and reach their greatest abundance in the sandy areas where vegetation is scattered, and those that are limited by sand and re- quire humus in the soil or the microhabitat of decay- ing logs. The transition or ecotone between these two ant communities comes at the pine and black oak stages. Experimental studies of six species in the genus Formica indicate that physiological differences occur, and that some species are able to invade places of low relative humidity that others cannot. Spiders In the sand dunes on the south shore of Lake Michigan and in adjacent areas, 228 species of spiders are to be found (Lowrie 1948). The number of fami- lies represented, the number of species involved, and the abundance of individuals per unit area increase as plant stages in the sere succeed one another (Table 8.3). Probably because of the greater diversification of the vegetation, the availability of logs, the increase in number of strata, and the consequent greater va- riety of niches, spiders, like ants, are represented by a larger number of species in the oak communities than in the earlier stages of the sere or in the climax. It is of significance that up through the black oak stage new species appear in each succeeding stage with very few dropping out. In the beech-maple climax, however, 51 per cent of the spider fauna occurring in preceding stages are no longer found, while 79 per cent of the species are either new with this stage or came in at the black oak stage and remained. Up to the black oak stage the species composition of the spider population shows ecesis, but with the advent of deciduous forest, the change in the fauna composi- tion is sufficiently extensive to indicate succession of distinct communities. There is also a change in the mores of spiders as the sere advances. Small lycosids that hide during the day under driftwood or other debris and run over the sand at night hunting for insect prey washed up by the waves are most characteristic of the beach. The permanent population is small. A burrowing spider, Geolycosa wrightii, is usually common. The burrows in which the spiders stay during the day may be easily spotted on the beach and through the grass and cottonwood stages. Web-building species are at a disadvantage in the early stages of the sere, how- ever, because of the general lack of vegetation to which their webs may be anchored and because of the destructive effect of unchecked wind. With the appearance of grasses, a substratum in which spiders can build webs becomes available. In later stages, the percentage of web-builders increases considerably as stratification progresses and the forest furnishes a scaffold. Other animal life Strong offshore winds often blow insects out over the water where they are forced down onto the surface and washed ashore. Windrows of such in- sects many thousands of individuals representing a wide variety of species, are sometimes to be seen. Dead fish washed up on shore are fed upon by flesh- flies and histerid, dermestid, and rove beetles. The tiger beetles Cicindela hirticollis, and C. cuprascens, a white ground beetle and other carabids, shore bugs, digger-wasps, robber flies, and other insects and spiders come down from higher ground to feed on the scavenger species and those washed up by the waves (Shelford 1913, Park 1930). The tiger beetles, ground beetles, digger-wasps, and sand spiders build their burrows and larval stages far enough back to escape the summer waves. Termites feed on buried wood that is decaying or on the undersides of logs that have drifted ashore. The piping plover and spotted sandpiper place their nests in the middle and upper beaches. At night, the toad, opossum, raccoon, and the deer mouse come down to scavenge whatever is available. The light coloration of many of the insects and spiders that occur on sand is doubt- less an adaptation for concealment (Hart 1907). The kinds of animals occurring in the grass, shrub, and cottonwood communities are similar ex- cept that new species invade with each successive plant stage. The white tiger beetle Cicindela lepida first appears on the upper beach and reaches maxi- mum populations in the cottonwood stage, as do the digger-wasps, robber flies, and sand spiders. Another tiger beetle, Cicindela formosa, occurs in the ecotone between the cottonwood and the pine stages. Snout beetles, spittle bugs, and miscellaneous other insects are occasionally very numerous. Some 592 species and varieties of beetles have been taken from various stages of this sere (Park 1930). Fifty species were found to occur in the cottonwood stage, 23 in the conifer stage, and about 200 in each succeeding forest stage. The occurrence of bees is dependent to a large extent on the variety and abundance of flowers, but the number of species in each plant stage increases up to the black oak and then declines to the climax (Pearson 1933). Vertebrates are not usually numerous on sandy flats or dunes away from the water’s edge. The vesper and lark sparrows occur among the grasses, the prairie warbler and chipping sparrow are found among the shrubs, and the kingbird is conspicuous Rock, sand, and clay 109 FIG. 8-9 Burrows made in sand ae by arthropods. (a) burrows of a digger wasp, Microbembex ee monodonta. (b) a digger wasp, eg . Bembex spinolae, and a cross- section sketch of its burrow 3 (Shelford 1913). (c) the white & Se tiger beetle and its burrow (Shelford 1913). (d) excavated burrow of a sand spider (courtesy R.E. Rundus). The upper portion, oo shown with a stick in it, is Bi ge intact; the lower portion, in the shadow, is broken open. ~~ in the trees. Tracks of the prairie deer mouse are frequently to be seen on the sand. Fowler’s toad and the hognose snake are the only amphibian and reptile that regularly occur. The grass, shrub, and cottonwood stages ordinarily occupy relatively nar- row belts parallel to the lake shore. Extensive sandy areas inland may have a larger variety of species present (Vestal 1913). The pine community in the sere is not so well de- veloped around the south end of Lake Michigan as it is northward. The coniferous forest penetrates southward from the north, and some northern ani- mals move with it. Nesting birds are represented by the slate-colored junco, red-breasted nuthatch, black- throated green warbler, blackburnian warbler, and myrtle warbler, all belonging to the boreal forest biociation. Forest-edge and deciduous forest birds also occur. The red squirrel is a characteristic boreal mammal that occupies this stage, and the white-tailed deer browses on conifer foliage, especially the white cedar. The six-lined racerunner and blue racer snake appear. Among the invertebrates are the bronze tiger beetle C. scutellaris and the ant-lion. With the advent of the black oak and later forest stages, most species requiring open areas or depend- ing on patches of bare sand disappear. Although the bronze tiger beetle remains abundant in the black oak community, it, as well as the other tiger beetles, disappear in the higher plant stages. Only the green tiger beetle C. sexguttata is in the climax, a species that requires bare spots on the forest floor, but not sand. Reptiles are not common in the sand sere around Lake Michigan, but elsewhere around the world liz- ards and snakes are quite characteristic of sandy habitats. They are remarkable in showing a variety of structural and behavioral adaptations specific to locomotion in sand and for protection of their sense organs and body openings from sand (Mosauer 1932). The sidewinder rattlesnake, for instance, has evolved, in addition to the usual undulatory lateral movement of snakes, a rolling sidewise movement that involves spiral contractions of the body and ap- plies vertical rather than lateral pressure to the sand. Sand offers the snake an unstable footing—lateral undulations alone do less to propel the snake forward than to merely push sand aside. Sand provides firm footing only if it is pushed down upon, hence the effective, if singular, action of the sidewinder. MO) Habitats, communities, succession TABLE 8-4 Percentage location of ovipositor holes and larvae in different soils, experimental conditions (from Shelford 1911, 1915). Tiger beetle Holes or Sand and Niche under species Larvae Number Sand humus Humus Clay natural conditions Cincindella Holes 69 40% 50% 1% 3% Wet sandy beaches hirticollis Larvae 50 56 42 2 0 Cincindella Holes 141 34 48 2 16 Adults on sandy tranquebarica Larvae 129 7 75 2 16 ridges covered with vegetation, larvae on sand or clay Cincindella Holes 117 8 22 0 70 Clay soils, oak- sexguttata Larvae 93 15 53 0 32 hickory forests, prefer leaves on ground On level Adults on sand or Cincindella Holes 51 0 2 0 23 clay, larvae en- purpurea Larvae 47 0 2 0 23 tirely on clay limbalis On slope banks 75 74 FIG. 8-10 The sidewinder rattle- snake and the track it makes (Mosauer 1935). Rock, sand, and clay eal Life history of tiger beetles, Cicindelidae The intimate adjustments of a species to its habitat and the manner in which it selects a particular stage in the sere may be illustrated by briefly describ- ing the life-histories of tiger beetles. Adult tiger beetles are bright-colored, alert, swift fliers. They are frequenters of bare ground. Both adults and larvae feed predatorily on ants, sowbugs, centipedes, spiders, beetles, flies, dragonflies, butter- flies, and larvae of various forms. Tiger beetles com- monly dig shallow burrows in the soil for shelter. They reach sexual maturity after several warm days in spring or early summer after they have emerged from hibernation. They copulate on warm, humid days when there is an abundance of food and sun- light. After laying their eggs, they die. The female deposits one egg at a time, and lays up to 50 in all, in small vertical holes, 7-10 mm deep, which she makes with her ovipositor. The female tests soil with her ovipositor until she locates soil of the required characteristics. Hatching occurs in about two weeks. The larvae are elongated, yellowish, and grub- like. Anteriorly directed hooks, spines, and bristles on the dorsal side of the larval body prevent the lar- vae from being pulled out of their burrows by the larger prey on which they feed. At the site of the ovipositor hole the larva excavates a vertical cylin- drical burrow 8-50 cm deep in temperate climates, much deeper in colder northern regions. Most of the time the larva stations itself at the top of its burrow with its mandibles extended, and with its head and prothorax just closing the round opening. It grabs passing prey and carries it off to the bottom of its burrow to devour it; larger prey are eaten at the en- trance. Inedible parts are cast out on the surface of the ground around the burrow entrance. After feed- ing 3-4 weeks, the larva closes the mouth of its bur- row with soil and goes to the bottom to molt. The second larval stage lasts five weeks or longer, after which there is another molt. The last of the larval stages closes the entrance to its burrow in late August or September and goes to the bottom to hibernate over winter (some species hibernate in the second larval stage). The larva comes out of hibernation in late spring and feeds until summer. Then it closes the entrance of its bur- row and constructs a side chamber in which it pu- pates. The adult emerges in late summer and feeds until October. It then digs a hole in which to hiber- nate over winter. Two years are commonly required to complete a generation, although in various species the interval between successive generations may be one to four years, depending in part on regional tem- peratures. The niche requirements or seral stage preferred by different species are rigid and appear determined, in large part, by the character of the type of soil a species finds suitable for deposition of eggs and larval growth. Studies performed under experimental con- ditions demonstrate the nature of these requirements (Table 8-4) but suggest no physiological explanation (Shelford 1908, 1911, 1915; Balduf 1935). CLAY SERE Plant communities Erosion or calculated removal of overlying ma- terial may leave bare areas of clay exposed. In clay above pH 4.5 annual plants, of which smartweed is particularly important, appear within a few weeks to two years; the higher the clay pH, the quicker the appearance of vegetation. Within two to five years thereafter sweet clover invades and develops nearly complete dominance over large areas. Sweet clover is a biennial, and an exotic species unimportant in the sere in some parts of the country (Bramble and Ash- ley 1955). Prior to its introduction, this stage in the sere on bare clay may have consisted of the perennial grasses still found in small scattered patches, or it may not have been well developed. A shrub stage seldom takes dominance over extensive areas, but thickets of raspberries and blackberries, smooth sumac, trumpet creeper, and various other species succeed the sweet clover and grass stage. The first trees begin to invade early in the sere, but they are scattered and slow of growth, and do not attain dominance for 25 to 30 years. The tree stage is com- monly made up of eastern cottonwood, American sycamore, silver maple, and American elm. Willows occur in wet spots. Herb species of the first two plant stages disappear, for the most part, in the shrub stage. The herb stratum now consists largely of wood nettle. Advanced forest stages of oaks, hickories, basswood, and sugar maple will likely invade in the future; as they occur now in adjacent areas. Animal life The number of invertebrate species tends to in- crease as the sere advances, although not always regularly. In a study of a formerly strip-mined area (Smith 1928), 18 species were found to be important in the annual stage, 41 species in sweet clover, 40 species in shrubs, 32 species in the early forest stage, and 67 species in the upland climax. More species would be found in advanced stages because of the greater variety of niches then available. Thus, in the initial bare area there is only the ground stratum; in the annuals and sweet clover stages there are the ground and herb strata; in the shrub stage there are 2 Habitats, communities, succession: ground and shrub strata. The herb stratum is poorly developed or absent altogether. In the forest there are the ground, herb, shrub, and tree strata. Since the early forest is on a floodplain, the ground is fre- quently swept by floods, and the shrub stratum is poorly represented. The climax forest has all strata, richly developed, and possesses the greatest number of animal species. There is an increase in the abun- dance of individuals per square meter with the pro- gression of the sere: annuals, 268; clover, 531; shrubs, 532; early forest, 748; climax, 2445 (David- son 1932). Beetles, spiders, ants, and mites are the most abundant animals in the annuals stage, and along with aphids remain most abundant also in the sweet clover community. Grasshoppers are fewer in number but especially characteristic of the first three stages ; they practically disappear in the forest. Earthworms are absent in the annuals and scarce in the sweet clover, as are the springtails; as the amount of soil humus increases with the development of the sere, both groups become more and more numerous. Snails first appear in the sweet clover stage and increase in im- portance in the forest stages. The first two stages are not sufficiently extensive to support a distinct bird fauna, but they are quickly invaded by scattered shrubs and trees. A forest-edge habitat is thus established and is occupied by forest- edge birds (Brewer 1958). Beginning with the early forest stage, these forest-edge species are replaced by the forest bird community. The composition and stucture of these two communities will be discussed in Chapter 9. The first small mammal (Wetzel 1958) to invade the annuals and sweet clover stages is the prairie deer mouse. It attains populations as high as 22 per hectare (9/acre). It persits until the shrubs and trees have become well established. Its place is taken in advanced stages by the woodland white-footed mouse. The prairie vole prefers the grassy areas and is found under briars and other shrubs. Peak populations are about 18 per hectare (7/acre). The short-tailed shrew invades the sweet clover stage but does not establish a stable population until the shrubs come in; it persists into the climax forest. Wood- chucks commonly occur throughout the early stages of the sere, but mostly disappear in the forest. The cottontail rabbit is common in the early stages, and the fox squirrel invades with the first trees. FLOODPLAIN SERE A stream continuously deepens its chan- nel, thus lowering the water table of the surrounding land. At times of flood, the stream overflows its banks. The flow rate of water declines as the water passes over vegetated areas, and there is a deposition of silt which may sometimes amount to several inches. In a valley, the lowland area between the river and the bluffs on each side is called the floodplain. In the course of time, the river meanders back and forth across the floodplain, cutting new channels and aban- doning old ones, and frequently leaving a sequence of terraces between its present channel and the sur- rounding upland. A study of these terraces com- monly shows a variety of plant communities that constitutes the plant sere. Plant communities Gravel, sand, or silt is deposited on the inner side of river bends. Attached aquatic vegetation may occur in the water. On land, such herbs as smart- weed, cocklebur, ragweed, beggar’s ticks occur. At some bends small sand dunes may occur, displaying their characteristic plants and animals; usually this stage is narrow at most, and may be entirely absent. On sandy islands in the river or on sandy shores, the sandbar willow often forms dense, shrubby thickets. The first tree stage is ordinarily black willow mixed with eastern cottonwood, and sometimes silver maple. On the floodplain of the Canadian River in Okla- homa (Hefley 1937), the sere proceeds next to an edaphic subclimax of either tall grass prairie or elm- oak. The climatic climax on the surrounding upland is mixed prairie. The normal sequence of stages in this region has become considerably modified by the extensive ecesis of the exotic tamarisk tree, intro- duced from Asia. On the Mississippi floodplain in western Tennes- see (Shelford 1954b) the mature cottonwood-willow associes contains an abundance of vines of several species that form such tangled masses as to be al- most impenetrable. The next stage is one in which sugarberry, sweetgum, American elm, and American sycamore predominate; several other species are present in small numbers. This leads to an oak- hickory stage that includes a complex variety of spe- cies, and eventually to the regional climax of western mesophytic forest. Cypress becomes part of the com- position of the early floodplain forest around the edge of small oxbow ponds or other standing water. The schedule for this sere, the time from the start to the beginning of dominance by each successive plant com- munity, has been estimated as follows: sandbar wil- low, 3 years; cottonwood-willow, 35 years; sugar- berry-sweetgum, 82 years; early species of oaks and hickories, 260 years ; intermediate species of oaks and hickories, 350 years ; early climax of oaks and tulip- tree, 440 years; full development of the climax, 600 years. Elsewhere in the eastern United States, the cot- tonwood-willow stage gives way to a narrow zone of sycamore. Two or three species of elm ,white ash, Rock, sand, and clay 113 ’ TABLE 8-5 Distribution of annelid worm species in the Sangamon River floodplain forest of central Illinois (Goff 1952). Original nomenclature revised by W. J. Harman. Annelid species Family Ruderals silver Dennen eee ee Lumbricus terrestris Lumbricidae ++ Allolobophora iowana Lumbricidae + Bimastos tumidus Lumbricidae Octolasium lacteum Lumbricidae Henlea urbanensis Enchytraeidae Henlea moderata Enchytraeidae Diplocardia singularis Megascolecidae Fridericia agilis Enchytraeidae Fridericia sima Enchytraeidae Friderica tenera Enchytraeidae and boxelder follow; then a mixed forest that in- cludes black walnut, butternut, black maple, Ohio buckeye, red mulberry, American basswood, tuliptree, and hackberry ; next an oak-hickory stage ; and finally the beech-sugar maple climax. The herb and shrub strata are usually well developed in mature floodplain forests. Telescoping or skipping of stages is not un- common in this sere, since variation in ground level or in height of terraces is considerable and the tran- sition between heights is often abrupt. The later stages occur only on the very oldest terraces and may be hard to find at all. Animal life In the bare areas, in the herbs, and among the invading trees occur such beetles as Heterocerus pallidus and Bembidion laevigatum that feed on the algae and detritus present on the shore. They make their burrows in sand. Fly larvae, a cocklebur weevil, a cocklebur mirid, and a cocklebur fly also occur. The tiger beetles Cicindela Jurticollis, C. cuprascens, and on slightly higher ground C. punctulata, prey on the ground species and may even dig them out of their burrows. Spiders, ground beetles, and rove beetles invade from higher stages. In the herb stratum and in the shrubby growth of willows, adult midge flies and other flies are sometimes very abun- dant. Tarnished plant bugs, 12-spotted cucumber beetles, and other insects of open area habitats are present, and there is invasion of various species from the forest itself (Hefley 1937, Shelford 1954b). The animal life of the floodplain forest is much the same as that of the deciduous forest in general (Chapter 9) and does not need to be discussed here except for its unique features. Annelid worms make their appearance in the ruderal stage, become very abundant in the moist soils of the elm-ash and mixed floodplain forests, then decrease in numbers in the drier soils of the late seral stages. They occur mostly 114 Willow- Silver Elm- maple- Elm-bur_ shingle Oak-hickory maple elm oak oak upland ++ ++ ++ ++ ++ ++ ++ + ++ ++ ++ + + ++ + ++ + ++ in the first 5 to 10 cms below the surface in moist soil, but up to 30 cms or more in dry soil. During the winter they keep below the frostline, and in very dry weather they roll up in small knots and aestivate. Ten species occur in the floodplain of the Sangamon River in central Illinois, and each species has its par- ticular range of moisture requirements between the river’s edge and the upland forest (Table 8.5). Snails and slugs are moisture-loving animals and occur in large numbers and great variety in flood- plain forests; it is not hard to find 15 to 20 species with a little searching. Mesodon thyroidus is a com- mon snail, and on a floodplain in central Illinois an average population of 6.3 individuals per m? was found during the autumn (Foster 1937). This amounts to a biomass of living flesh (shell excluded) of 15.8 g/m? (141 Ibs/acre). Succinea ovalis on another old Illinois floodplain (Strandine 1941) aver- aged 6.5 individuals per m? in September with a biomass of only 0.878 g/m? (7.84 Ibs/acre). Snail flesh is an important source of food for such small mammals as the short-tailed shrew. Effects of flooding Animals living on floodplains must usually tol- erate flooding of their habitats almost yearly, and in years with heavy precipitation, often several times annually. All land except the surrounding bluffs may be flooded. Leaves are swept up from the forest floor and piled with other debris against shrubs and the bases of trees. Herbs and shrubs may be damaged, sometimes killed. Observations during time of flood (Stickel 1948) showed that emergent brush, the bases of trees, and debris rafts supported masses of insects, spiders, millipedes, snails, and amphibians. Debris rafts were refuges for box turtles and pine-mice as well. Snakes, turtles, and amphibians were also seen swimming or floating in the water. No white-footed mice were Habitats, communities, succession found in the flood, but the size of the population per unit area, determined by live-trapping immediately after the flood, was the same as it was immediately before the flood, and a number of tagged individuals were found surviving. This species readily climbs trees and may well have passed the danger period arboreally. Tagged box turtles were found on the identical home ranges they had occupied before the flood. This flood lasted only a few days. Severe flooding persisting for long periods is known to have virtually exterminated species of small mammals from wooded floodplains in grassland areas (Blair 1939). Larger mammals, such as rabbits, opossums, and foxes, quickly leave flooded areas and may be tem- porarily concentrated around their margins. Squirrels and raccoons easily obtain refuge in the trees, but if the flood persists for some time, they may have trouble finding food. Woodchucks normally spend considerable time in underground burrows and may be trapped there by floodwaters (Yeager and Ander- son 1944). Invertebrates in the soil are also affected by flood- ing. A gradually rising water-table may eventually displace all the air from a soil, and the arthropods are killed. Many earthworms leave their tunnels when these are inundated and are killed. Some spe- cies that regularly exist in areas subjected to frequent flooding, however, are not injured. Crane fly larvae are flood-resisting. In normal flooding, bubbles of air trapped in the soil provide sufficient oxygen for at least the smaller arthropods (Kevan 1956). The eggs of some floodplain mosquitoes are laid just above the water level of pools during late summer or autumn and must be flooded the following spring be- fore they will hatch. SUBSERES Abandoned fields When farmland is abandoned, succession back to natural vegetation and ultimately to the climax is rapid, since the soil is already relatively fertile and does not need a great deal of conditioning. On the Great Plains the subsere proceeds rapidly through stages of annual herbs, several of which may be ex- otics naturalized from other continents ; mixed annual and perennial herbs; a short-lived perennial grass; dense stands of triple-awned grass ; finally, the climax of short grasses. This last stage may be attained in 10 to 20 years. Small mammals and grazing domestic animals retard the succession by feeding on grasses while avoiding the herbs. Sheep have the opposite effect, preferring the herbs. Harvester ants denude the veg- etation in a circle around their mounds and consume a considerable amount of the available seed supply. Ant coactions may be of very great importance when we consider that the population of a mound may aver- age 10,000 individuals and the number of mounds per hectare range from 0 to 10 (0 to 4/acre) in the annuals stage, 7 to 28 (3 to 11/acre) in the mixed annual and perennial herbs, 12 to 52 (5 to 21/acre) in the first grass stage, 40 to 142 (16 to 57/acre) in the triple-awned grass, and 0 to 32 (0 to 13/ acre) in the final stage (Costello 1944). Six plant stages are recognized in the sere that develops in the mixed prairie region of Oklahoma: an initial stage of mixed herbs; three intermediate stages involving different proportions of triple-awned grass; a subclimax; and the climax of Andropogon and Bouteloua grasses. The insect population con- sists of 293 species representing the following orders, ranked in decreasing abundance: Coleoptera, Hemip- tera, Homoptera, Diptera, and Orthoptera. There was a greater variety of species and a greater abun- dance of individuals in the intermediate stages than in either the early or the climax stages, probably be- cause of the greater variety of plant species present in the intermediate stages. Of the 144 species of in- sects in the climax, 58 per cent entered the sere in its initial stage, 15 per cent in the second stage, 12 in the third, 5 in the fourth, 2 in the fifth, and only 8 per cent were limited to the climax itself. The ecesis of the mature animal community was therefore a grad- ual and progressive one. On the other hand, many species that were present in the early stages did not persist into the climax community (Smith 1940). Succession in abandoned fields of the southern Atlantic and Gulf states is of special interest. During the first year, crabgrass and horse-weed, annuals, pre- dominate. During the second year an aster and a rag- weed, and in the third year the perennial broomsedge grass, become dominant. The grass is invaded quickly by loblolly and shortleaf pines which form closed stands in some areas in as little as 10 to 15 years. These pines do not reproduce in their own shade. They mature in 70 to 80 years, and are replaced by the climax oaks, hickories, beech, and sugar maple which take complete dominance by the time the area is 150 to 200 years old (Oosting 1942). The bird succession (Table 8-6) shows a change from grassland to forest-edge to forest species with an increase of both species and number of pairs as the sere progresses. A great diversity of species in- habits the pine forest since several forest-edge spe- cies persist, while pine warbler, brown-headed nut- hatch, solitary vireo, and yellow-throated warbler are particularly characteristic of it. Pine-mice and meadow voles are common in the grassy stages and pine-mice, cotton-mice and golden mice in the for- ested stages. In Michigan, the sere in abandoned fields passes through the following stages: annuals-biennials; Rock, sand, and clay 115 FIG. 8-11 Stages in the strip-mine plant sere in east central Illinois. (a) sweet clover, aster, ragweed, after 7 years. (b) same area 10 years later, trees invading. (c) silver maple-cottonwood-sycamore floodplain forest after about 40 years (Wetzel 1958). perennial grasses; mixed herbaceous perennials; shrubs ; and finally three tree stages, the first reached in 21 to 25 years. Prairie deer mice are at their most abundance during the early stages, meadow voles in the intermediate grassy stages, and the woodland white-footed mouse and short-tailed shrew in the shrub and tree stages. Such game species as ring- necked pheasants, bobwhite, and cottontail rabbits are common on abandoned farmlands but give way to another group of game species, including white-tailed deer, ruffed grouse, and gray squirrels, when the forest stages become established (Beckwith 1954). Pastures Pastures in northern Ohio contain a sod prin- cipally of blue grass. With light grazing, this sod will resist invasion of other species for a long time, but with heavy grazing, resistance is weakened and unpalatable herbs, briars, and hawthorne come in. The latter two species are armed with prickles or thorns discouraging animal browsing. When they be- come dense enough they kill the grass beneath them. Eastern redcedar may establish itself in horse pas- tures, but not in cattle pastures ; cattle browse it but 116 Habitats, communities, succession horses will not. In the middle of protecting thickets of briars, hawthorns, and redcedar such deciduous trees as elm, ash, tuliptree, sycamore, and oak come in. After a few years they grow beyond the reach of animals, shade out the briars, hawthorns, and red cedar, and establish a forest dominance. Where left undisturbed by man, the succession of native vegeta- tion will thus bring about the elimination of domestic animals from the area and replacement with the biotic climax natural for the region. In western areas too dry for deciduous forest, overgrazing reduces the vigor and abundance of the taller climax grasses, and the short grasses that are less easily grazed are favored. Unpalatable herbs, sagebrush, cacti, and mesquite may also replace grasses Over extensive areas. Although native animals such as the bison and pronghorn may have heavily grazed the original prairie in locally arid regions, the result was less drastic than that produced by the heavy concentrations of grazing stock on our farms and ranches at the present time. When the most favored vegetation was reduced, native animals com- monly dispersed into other areas so that the carrying capacity of the land was not critically reduced. Burns Prairie fires, frequently started by lightning or by Indians, were doubtless important in preventing deciduous forest from succeeding grassland in parts of the middlewest. More lately, fires are started by careless campers or travelers. Fires are especially destructive in coniferous forests, as the clinging dry needles encourage crown as well as ground fires to develop. Many thousands of square miles of forests are burned over annually. The extensive pure stands of longleaf pine on the coastal plain of the southeastern states are probably a consequence of ground fires that regularly occurred at intervals of 3 to 10 years before white men came. The terminal bud of the longleaf pine is well pro- tected by a thick covering of green leaves, one of several characteristics that make the species ex- tremely fire resistant (Chapman 1932). Fire de- stroys all seedling hardwood trees as well as other species of conifers. When coniferous forest is destroyed by fire, the first trees to invade are usually quaking aspen, paper birch, and sometimes balsam poplar. These forests cover extensive areas in Canada and southward on the Rocky Mountains. Jack pine in the north and lodgepole pine in the western mountains either come in with the deciduous trees or succeed them. The cones of these two trees take several years to open and shed the seeds held within, and may not do so at all unless heated by forest fires. Aspen and pine are eventually replaced by the climax forest. In many TABLE 8-6 Breeding bird pairs per 40 hectares (100 acres) in sere developing on abandoned fields, Georgia Piedmont region, averaged from two stations in herb-shrub (1, 3 years old), three stations in grass-shrub-tree (15, 20,25 years old), four stations in pine forest (25, 35, 60, 100 years old), and one station in oak-hickory (over 150 years old) (condensed from Johnston and Odum 1956). Grass- Oak- Herb- shrub-tree Pine hickory Bird species grass (forest-edge) forest climax Grasshopper sparrow 20 8 Eastern meadow- lark 8 6 Yellowthroat 11 Yellow-breasted chat tf Prairie warbler 4 Catbird 1 Indigo bunting 1 American goldfinch + Bobwhite + Field sparrow 36 4 Rufous-sided towhee 9 13 Bachman’s sparrow 5 1 Pine warbler 5 43 White-eyed vireo 3 3 Mourning dove + + Cardinal 6 15 23 Summer tanager 2 14 10 Chuck-wills-widow + + + Brown-headed nuthatch 2 Brown thrasher 1 Solitary vireo of Yellow-throated warbler 1 Pileated woodpecker + Hooded warbler 11 11 Carolina wren 10 10 Ruby-throated hummingbird 6 10 Blue-gray gnatcatcher 5 13 Tufted titmouse 5 15 Eastern wood pewee 4 3 Blue jay 4 5 Carolina chickadee 4 5 Crested flycatcher 4 6 Red-eyed vireo 4 43 Yellow-throated vireo 3 7 Wood thrush 2 23 Yellow-shafted flicker 1 3 Hairy woodpecker 1 5 Downy woodpecker 1 5 Yellow-billed cuckoo + 9 Black and white warbler 8 Acadian flycatcher 5 Kentucky warbler 5 Total species 2 18 30 22 Total pairs 28 104 163 224 Rock, sand, and clay V7 12 Plant succession in abandoned FIG. 8 fields in Virginia. (a) annual herbs invading old -sedge invasion of young pine field. (b) broom corn (c) grass. trees. FIG. 8-13 The Tillamook Burn in coniferous forest in Oregon, 1944 (courtesy U.S. Forest Service). Habitats, communities, succession 118 western areas, particularly in the Sierra Nevada, chaparral may occur in dense stands after fires and persist for a long time. Since the burn subsere in coniferous forest com- monly includes the aspen-birch associes, many of the typical animals in this stage are deciduous forest and forest-edge species, although there is a penetration of coniferous forest species as well. The birds and mam- mals are not generally very numerous in the aspen- birch community, but ground invertebrates may be more abundant here than in the poorly decomposed acidic ground duff found in the coniferous climax. ANIMAL COMMUNITIES Although the plant communities that make up the stages of the different land seres and subseres we have described are numerous and varied, the num- ber of distinct animal communities that can be clearly recognized are few. Actually, we can distinguish in eastern North America only the animal communities of grassland, forest-edge, deciduous forest, south- eastern evergreen forest and coniferous forest. Each of these communities varies in the different habitats of rock, sand, and clay, and in the various subseres, but the variations are of minor significance and are best treated as facies of the larger communities. FIG. 8-14 This aspen forest in New Mexico will be replaced by one of spruce as the young trees now forming in undergrowth reach maturity (courtesy U.S. Forest Service). SUMMARY For terrestrial living, animals must actively support themselves against gravity, obtain water, and prevent excessive water losses from the body. They must be equipped to endure a wide range of fluctuat- ing temperatures, to secure oxygen, to endure in- tense solar radiation, adjust to diurnation (day and night) and aspection (seasonal changes), and yet maintain close contact with the substratum. Succession occurs on all primary bare areas, such as rock, sand, clay, and floodplains, and in such secondary bare areas as abandoned fields, pastures, and burns. In humid regions, all seres converge to the same climax community. There are normally more plant than animal stages in any sere. The suc- cession program of animal communities correlates with the succession program of vegetation-types or life-form of the plant dominants, not with plant com- munities identified by the taxonomic composition of the plant dominants. In eastern North America, we can distinguish only the grassland, forest-edge, de- ciduous forest, southeastern evergreen forest, and coniferous forest terrestrial animal communities. Rock, sand, and clay 119 Grassland, Forests, and Forest-ed ges We have seen that succession of animal communi- ties in humid climates passes through three terrestrial stages before attaining climax: grass, shrubs and scattered trees (forest-edge), and forest. In arid cli- mates, the climax may be reached at the first or sec- ond stage. It is important for us to examine each community in more detail, therefore, if we are to gain an understanding of the ecology of animals prevailing locally in different parts of the world. VEGETATION Grassland vegetation differs from forests in that the above-ground vegetation is completely re- newed each year. Grasses may be divided into three categories on the basis of height: tall grasses (1.5-3 meters tall), such as big bluestem and slough grass; mid grasses (0.5—-1.5 meters tall), such as little blue- stem and needle grass; and short grasses (less than 0.5 meter tall), such as buffalo grass and grama grass. The taller grasses grow in wet habitats, the short grasses in arid habitats. Most native grasses are bunch grasses in that they grow in clumps with the areas between the clumps either bare ground or occupied by other species. Broad-leaved herbs occur- ring between the dominant grasses are called forbs. A few species are sod formers in that their growth is continuous over the ground surface. The leafy aerial parts of perennial grasses die in the winter or in dry season, leaving the underground stems or rhizomes to propagate the plant the following year (Weaver and Fitzpatrick 1934). Forests are composed of trees growing sufficiently close together to dominate the entire area of ground surface. In cold climates, forests are needle-leaved evergreen; in intermittently warm, moist climates, they are broad-leaved deciduous ; and in continuously warm, moist climates, they are broad-leaved ever-~ green. In spite of these secondary differences in life- form, the structure and internal dynamics of all forest communities are quite similar. Useful methods for measuring the density of trees per unit area are de- scribed by Cottam and Curtis (1956). Between forests and open country, the trees are often widely spaced and do not completely dominate the area; open-country shrubs and grasses become interspersed. This transition area is usually narrow around the margins of a mature forest, but where succession is occurring, large areas of shrubs con- taining small or scattered trees are essentially forest- edge in character. Likewise, in agricultural areas, hedge and fence rows, or narrow strips of trees and shrubs along streams, are really edges without the adjacent forest. Essentially, forest-edges provide, in close proximity, forest, shrub, and open ground habi- tats which animals take advantage of in a variety of unique ways. | 20 Deciduous trees shed their foliage in the autumn, are bare over winter, and obtain new foliage in the spring. Coniferous trees, on the other hand, retain their foliage throughout the year, although old dried leaves fall a few at a time at all seasons. Differences in the size, shape, and structure of the leaves are im- portant to many animals. The lack of foliage in de- ciduous forests during the winter permits a greater light penetration to the forest floor, more wind circu- lation, and relatively lower temperatures than in coniferous forests. During the summer, deciduous forests generally have higher but more variable tem- peratures and lower relative humidities than do co- niferous spruce and fir forests (Blake 1926, Dirks- Edmunds 1947). Pine forests, however, commonly develop in habitats that are warm and dry. As shade producers, the deciduous and coniferous trees do not vary as groups, but only as individual species (Weaver and Clements 1938) : Deciduous trees Coniferous trees Heavy shade producers Sugar maple Yew Beech Spruce Basswood Hemlock Firs Thujas Medium shade producers Elms Eastern white pine White oak Douglas-fir Northern red oak Ash Black oak Light shade producers Silver maple Ponderosa pine Bur oak Tamarack Birches Lodgepole pine Poplars Willows It is interesting that light shade producers are spe- cies found in the early stages of succession while the heavy shade producers are mostly climax species. There is an important difference between decidu- ous and coniferous forests in the nature of the de- composing dead leaves that fall from the trees. De- composition of broad leaves is rapid and relatively complete to form a rich humus that mixes gradually with the mineral soil beneath. Needle leaves decom- pose slowly and form a somewhat acid humus sharply defined from the underlying mineral soil. Humus formed in humid grasslands is similar to but richer than that of deciduous forest; in arid grasslands it is poorly developed. The nature of the humus and litter affects the number and kinds of animals that occur in the soil. In grassland there are three strata of vegetation : subterranean, composed of roots and other under- ground plant parts as well as bacteria, fungi, and algae, ground, including the surface litter, and herb, the stems and leaves of the grasses and forbs. The forest not only has these strata, but also one of shrubs and one or more of trees. Animals characteristically limit their major activity to one or more of these strata. HABITAT Grassland, forest-edge, and forest-interior compared At the University of Illinois, no significant dif- ference in mean monthly temperatures, calculated bi- hourly day and night, has been found between the interior of a virgin oak-maple forest and an adjacent open grassland. In the forest, however, the daily extremes are not so great; i.e., the maximum mid- afternoon temperature is not as high, nor the mini- mum night temperature so low, as in the grassland. Relative humidity during a summer day in Iowa was found (Aikman and Smelser 1938) to average 20 per cent lower in grassland than in a shrubby forest-edge, and 5 to 8 per cent lower in the forest- edge than in the forest-interior. There is less differ- ence between the three habitats, however, at night. Rate of evaporation, as measured with Livingston atmometers, is inversely correlated with humidity, being greatest in grassland and least in the forest- interior. Daily changes in relative humidity between day and night tend to vary inversely with the tem- perature, except when there is rain. During four years at the University of Illinois woods, rain gauges recorded 88.8 cm (35.5 in.) per year in the adjacent grassland, upon which full pre- cipitation fell, and 70.1 cm (28.0 in.) throughfall (the amount reaching the ground) under the tree canopy of the forest. There was variation of through- fall from spot to spot in the forest, depending on the location of openings in the canopy and drip-points from the leaves and stems. Stem-flow of water down the tree trunks was not measured. Throughfall and stem-flow together make up the net rainfall. In a shortleaf pine plantation in southern Illinois (Bog- gess 1956), the net rainfall over three years averaged 91.2 per cent of the total rainfall. Interception, the amount of rainfall presumably evaporated back into the air, was 100 per cent in very light rainfalls but a less than 5 per cent of rainfalls exceeding 5 cm (Zhine) In a beech-maple forest in northern Ohio, which bordered on an open field, wind velocity at a distance 245 meters (about 800 ft) inside the west margin Grassland, forests, and forest-edges 124 —MONDAY——.-——T UESDAY—..- WE DNE SDAY—.— THUR SDAY —.—— FRIDAY —.-— SATURDAY—.— SUNDAY —~V— XI Mt xi Mt xi Mt xI Mt xr Mt xi Mt xi Mt 100 FIG. 9-1 A weekly chart from a hygrothermograph placed at shrub level in a deciduous forest in central Illinois. was reduced to a minimum of 10 per cent when the trees were in leaf and 25 per cent when not (Williams 1936). With a protective edge of shrubs, the wind velocity would doubtless have been decelerated more quickly. Summer light intensities are much less under foliage than out in the open. Noontime illumination under shrubs in Iowa averaged 26 per cent of full sunlight ; within the forest interior, 6 per cent (Aik- man and Smelser 1938). The forest floor is not uni- formly illuminated because small openings in the canopy admit sun-flecks of varying intensity. In the cottonwood, pine, black oak, and sugar maple stages of the sand sere at the lower end of Lake Michigan, the percentages of the forest floor shaded during the midday hours were 68, 87, 75, and 90 per cent, re- spectively (Park 1931). There may be some change in the quality of light that filters through the forest canopy, as there is of intensity, as some wavelengths are used more than others in photosynthesis; green is transmitted or reflected and not absorbed. Where a stand of trees abruptly confronts an open field, light penetrates laterally under the forest canopy and, the typical edge configuration reversed, the light per- mits shrubs to extend 40 meters or more into the interior. Vertical gradient There is a gradient in microhabitat factors from above the grasses down to the ground. In one study of virgin prairie (Weaver and Flory 1934), light intensity varied from 100 per cent in full sunlight to 25 per cent at one-half the pile depth of the grasses |22 to 5 per cent at the base of the stems, and, of course, zero per cent in the subterranean stratum. The rela- tive humidity above the grass was 20 per cent; in the grass, 31 per cent. The wind velocity above the prairie grasses was 14.5 km/hr (9 mph) ; at the top level of the grasses, 6.0 km/hr (3.7 mph) ; at the soil surface, zero. The rate of water evaporation from white spherical atmometers was 55.3 cc/day above the grasses, 33.3 cc at top surface of the grasses, 15a cc at one-half the pile depth of the grasses, and only 13.4 cc just above the soil surface. The temperature gradient varies with the height of the grass and be- tween day and night. The vertical gradient of temperature in a decidu- ous forest in central Ohio varies with the season and with the height of macroclimatic temperature (Table 9-1). In the summer, the greatest extremes of tem- perature occur in the canopy, but at other levels, both above and below the ground, summer daily mean temperature is more stable than at any other season. Because the canopy largely controls the air tempera- ture beneath it, there is little or no thermal stratifica- tion between it and the ground. Summer soil tem- peratures are always lower at 1.2 meters below the surface than at the surface. For comparison, air tem- peratures in a coniferous forest in Wyoming during July and August averaged 12.3°C at 0.1 meter above the ground and 7.6°C 0.1 meter below the surface litter (Fichter 1939). During the winter, temperatures in deciduous for- ests are lowest near the ground and more uniform at all higher levels than during the summer, since the absence of a canopy permits greater turbulence, hence less stratification, of the air. Soil temperature at 1.2 meters depth is generally higher than surface tem- Habitats, communities, succession HEIGHT, cm ow + (e) (eo) nN {e) 24 IS 20 23 25 FIG. 9-2 Gradient of air temperatures in and above (a) tall grass medium height at night (after Waterhouse |955). perature ; beneath the litter in the central Ohio area temperatures do not usually go below freezing (Christy 1952). A covering of snow gives added pro- tection against freezing of the leaf litter. Another study (Wolfe et al. 1949) revealed differences be- tween temperatures above and below a snow covering 2 to 10 cm deep during a period of two months aver- aged 8.9°C, and on one occasion reached 15.5°C. Relative humidity decreases from the ground stratum upwards. In a young elm-maple forest in Tennessee, the relative humidity from mid-February to mid-August averaged 77.9 per cent at the surface of the leaf litter, 75.2 per cent in the herb stratum 0.5 meter above the ground, 72.5 per cent in the shrub stratum at 0.9 meter above ground, and 67.4 per cent in the trees at 7.6 meters above ground (Adams 1941). In this same forest, the rate of evaporation between May and November in the four strata respectively averaged 29.4, 60.7, 72.8, and 99.2 cc per week. In a spruce-fir forest in Wyoming, the average weekly evaporation at 0.1 meter was 50.5 cc, at 1 meter 75.2 cc, and at 3 meters 103.9 cc (Fichter 1939). In the Tennessee elm-maple forest mentioned a May - September Elevation Minimum Maximum Range Macroclimate 10.0°C 34.4°C 24.4°C +25.0 meters 7.8 SLT 23.9 +18.9 8.9 30.0 21.1 +6.1 8.9 30.0 21.1 +1.5 8.9 28.9 20.0 Surface of leaf litter 9.4 28.3 18.9 Under leaf litter 12.2 2ore 10.0 - 0.15 12.2 18.9 6.7 - 1.2 10.6 16.1 5.5 A C a— C B 26 TALL SHORT IO II 12 GRASS GRASS °C and (b) short grass on a sunny day, and (c) in grass of moment ago (Adams 1941), the average daily mid- summer light intensities measured with a MacBeth illuminometer for ground, herb, shrub, and tree (be- neath the canopy) levels were respectively 52.3, 60.3, 60.4 to.76.2 foot-candles; in early May, before the foliage was fully developed, intensities of 65.8, 78.3, 104.4, 119.1 foot-candles were measured. Under the leaf litter and in the soil, the light intensity was, of course, zero. Above the trees it was doubtless several thousands of foot-candles. Maximum light intensities from the sun occasionally reach 15,000 foot-candles. There are, therefore, three distinct sections in the vertical gradient; below ground surface, between ground and tree canopy, and above the canopy. Ground insects, millipedes and isopods, when placed in experimental gradients, show a_prefer- endum for lower light intensity, higher humidity, and lower temperature than do insects taken from the herb or shrub startum (Table 9-2). In grassland, motile organisms can quickly vary the microhabitat to which they are exposed by changing their vertical position only a few centimeters, and they do shift in position as the gradient varies at different times of day or from day to day. To obtain an equivalent November - December TABLE 9-1 Vertical SNES SE IEG gradient of temperature -19.4°C 18.3°C 37.7°C—Sima beech forest in -17.8 13.3 31.1 central Ohio (Christy -17.2 13.3 30.5 1952). -16.1 13.9 30.0 -21.1 12.2 33.3 - 8.3 8.3 16.6 0 10.0 10.0 2.8 12.8 10.0 4.4 13.3 8.9 Grassland, forests, and forest-edges 123 TABLE 9-2 Results of experiments conducted in the field to establish light orientation of arthropods taken from different strata. Number of Mixed species Light inten- sity gradient Control Inter- (no gradient) experiments Strong mediate Weak Left Middle Right ae eee ——————— Grassland animals from herb stratum Forest animals from herb and shrub strata Grassland animals from ground stratum Forest animals from ground stratum outro change in microhabitat in the forest gradient requires a shift of several meters in vertical position. Experi- ments show, however, that each forest animal species occupies a stratum approximating its preferendum for a particular microhabitat, in response especially to the relative humidity factor (Todd 1949). Slope exposure Microclimatic differences between North- and South-facing slopes are great. South-facing slopes receive a greater amount of solar radiation and are commonly exposed to the prevailing winds. As a consequence, both air and soil temperatures are higher on South-facing slopes than on North-facing slopes ; relative humidity is lower, soil moisture lower, and the rate of evaporation is higher. The differences between the two slopes are most marked close to the ground, increasingly less so at higher levels (Cantlon 19533) The vegetation on the protected North-facing slopes is usually more mesic in type and more luxuri- ant than on the exposed South-facing slopes, and there is a deeper organic leaf litter on the ground. Types of vegetation characteristic of arid habitats penetrate humid climates on South-facing slopes ; mesic vegetation penetrates the relatively arid cli- mates obtaining on North-facing slopes. Southern types of vegetation invade boreal climates on the warm South-facing slopes, and boreal vegetation in- vades southward on North-facing slopes. In moun- tain areas, vegetation characteristic of lower altitudes penetrates higher on South-facing slopes, and vege- tation of the upper altitudes penetrates farthest down- ward on North-facing slopes. Animals are locally distributed in a similar manner, partly as a direct response to the climate, partly to the differences in vegetation. Many other differences in microclimate occur in various parts of the forest and forest-edge (Wolfe et al. 1949), and in grassland. In studying the distribution of animals in relation to climate, it is obviously not sufficient to consider only the macroclimate. Animals respond to the micro- climate of their particular niches, and the relation 47% 34% 19% 40% 31% 29% 21 330 33 30 37 27 49 34 23 42 23 59 634 29 37 between these microclimates and the prevailing macroclimates of the region must be demonstrated. THE GRASSLAND COMMUNITY Invertebrates Snails, earthworms, and myriapods are not nu- merous in grassland because of the dry habitat. In- sects, however, are abundant ; some 1175 species and varieties have been listed for different grassland plant communities in Iowa (Hendrickson 1930). These belong principally to the orders Orthoptera, Hemip- tera, Homoptera, Coleoptera, and Diptera. Ants, bees, and wasps (Hymenoptera) are also numerous. Spiders make up about 7 per cent of the total arthropod population in grassland. In one study made in Nebraska (Muma 1949), 111 species were collected from 128 hectares (320 acres) of mixed high and low prairie containing some shrubs. Less than a dozen species were web-builders; there is a lack of suitable web-building sites in grasslands. The vast majority were wandering cursorial forms. In re- gard to strata in this prairie, 45 species were re- stricted to the soil and litter, 30 to the herbs, 1 to the shrubs. Thirty-five species occurred in two or more strata. The total population for the area was least in the spring and greatest in the autumn. Peak popula- tions in the ground stratum were reached during the winter, however, because of the presence of many hibernating immature forms. Similar seasonal fluctu- ations occur with other invertebrates, although the peak populations of insects are usually attained dur- ing the summer (Shackleford 1939, Fichter 1954). In grazed pastures and in grassy meadows in New York State, invertebrates average 777 individ- uals per square meter (Wolcott 1937). Of this pop- ulation, ants make up 26 per cent, leafhoppers 15 per cent, other insects 34 per cent, spiders 9 per cent, millipedes 9 per cent, sowbugs 2 per cent, snails and slugs 2 per cent, earthworms 2 per cent, and large nematodes 1 per cent. 124 Habitats, communities, succession Some insects show structural adaptations for liv- ing in grassland (Hayes 1927). May beetles in for- ested regions commonly feed at night on the foliage of trees and have well developed wings, but closely related species in grassland areas feed on low grow- ing plants during the day and are flightless. The de- velopment of pilosity and thick integuments in some insects appears to be an adaptation to prevent evapo- ration. Prairie May beetles pupate in the spring rather than autumn, probably in correlation with their change in food habits, and adults appear in mid- summer rather than late spring. An insect microhabitat of special interest is the dung of the larger mammals. Bison formerly oc- curred in frequency one to 10-20 hectares. Inasmuch as the output of each animal is about 25 droppings per day, the number of these microhabitats available was considerable. Some 83 species of arthropods have been collected from cow dung, mainly beetles and flies, but including annelids, nematodes, and proto- zoans. There is a regular succession of insect species breeding and maturing. The microsere is completed in about eight days, the length of time required for the droppings to dry. The first species that arrive are the obligatory breeders on dung. They have the shortest life-histories, and remain for the shortest time. Predatious and parasitoid species prey on the coprophagous ones. The greatest variety of species is present at the middle of the microsere, but the composition of species varies with the season. Spe- cies disappear as the dung disintegrates into the gen- eral surroundings (Mohr 1943, Laurence 1954). A comparable microhabitat and succession occurs in carcasses of dead animals (Chapman and Sankey 1955). Vertebrates Table 9-3 gives a representative sampling of small mammal populations found in grassland, al- though it is to be expected that the species composi- tion and size of populations will vary locally and from year to year. The mores of grassland mammals, which show how they are adjusted in behavior to live in this community, are tabulated in Table 9-4. Birds are not numerous in grassland. In north- western Iowa (Kendeigh 1941b), grasshopper spar- rows, western meadowlarks bobolinks, ring-necked pheasants, marsh hawks, and short-eared owls aver- aged less than one pair per hectare (2.5 acres). Prairie chickens and sharp-tailed grouse formerly occurred where now is to be found only the intro- duced pheasant. The eastern meadowlark predomi- nates over the western meadowlark in the wetter and smaller pastures east of the Mississippi River. Vesper sparrows and horned larks occur in short grasses, but usually not in climax areas with dense tall grasses. Upland plovers, Henslow sparrows, lark buntings, and longspurs are common locally. Some fourteen species of snakes are generally dis- tributed over the prairies (Carpenter 1940). To the east, the blue racer, massasauga, bullsnake, and garter snakes are frequently found. The prairie rattle- snake is increasingly common westward. The lizards Cnemidophorus sexlineatus, Sceloporus undulatus, and Holbrookia maculata, commonly occur in grassy areas at forest-edges. The horned toad is found in arid habitats. The most characteristic amphibian of grassy areas is the toad. All species breed in the ephemeral bodies of water resulting from the rains of spring and sum- mer. One species, Bufo cognatus, will not breed un- less it rains, even though bodies of water are present. During the hot, dry weather of later summer, the toads retreat to burrows in the earth or to other shelter until favorable conditions again return (Bragg and Smith 1943). Grazing food coactions and range management Since the vegetative productivity of grasses is very high, herbivorous animals, especially large mam- mals, are favored in the grassland community (Ren- ner 1938). Unlike trees and shrubs, the terminal bud on grasses lies close to the ground and is not ordi- narily injured by grazing. Meristematic tissue lies at the base of the leaves so that when the terminal por- tion of the leaf is eaten off, the leaf keeps on grow- ing. Actually, lateral branching at the base of the grass stem is stimulated by grazing, and a thicker and more succulent growth with less fiber is pro- duced. Productivity of grass is reduced if the herbage is removed more than two or three times during the growing season. However, total protein production is not diminished, for frequent clipping results in an increased ratio of leaves to stem, and leaves are much richer in protein content. Light to moderate grazing TABLE 9-3 Population of small mammals per hectare (2.5 acres) in mixed prairie of western Kansas (after Wooster 1939). Mammal species Number Prairie meadow-mouse 9.6 13-striped ground squirrel 7.6 Prairie white-footed mouse 6.8 Harvest mouse 2.8 Little shrew 2.4 Short-tailed shrew 1.3 Black-tailed jackrabbit 0.7 Cottontail rabbit 0.1 Total 31.3 Grassland, forests, and forest-edges 125 TABLE 9-4 A tabulation of certain grassland Mammal species q A] 3 5 o q 8 Herds or packs & + Sue 3 & 3g: ro) 5 a ifs) iS) n Fy x Bison Pronghorn antelope Wapiti White-tailed deer Mule deer Cottontail White-tailed jackrabbit Prairie-dog x Prairie white-footed mouse Prairie meadow-mouse x XG Jumping mouse Pocket mouse Harvest mouse Franklin ground squirrel 13-lined ground squirrel eS Pad: oS PaS ba Pos Pah Pas Pocket gopher Richardson ground squirrel Wolf Coyote Badger Bobcat Skunk Weasel Red fox Swift fox Shrew PS ead Patera’ PaSivode ead inns Cursorial XK KK XK XK XK x xX mammal mores (Carpenter 1940). Food Stratum habits Daily Sea- period sonal of activity activity Sess w| | g ey Tate SEUSS | Re algi- &)] gg] &] & 3 Ss Onis 2| 8/8/81 2)/2| 5 2/3/¢é ies) Bu eal sealies a | a| 2 S| fl a/a|/8/3]5 Sern ml OloO|] a] Bla] ajol|z x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x 52 x x x x x x x x x x x x x x x x x x x x x x x x x x can, therefore, be carried with full or nearly full pro- ductivity. Heavy grazing, however, should not be permitted. In addition to reducing herbage produc- tion, heavy grazing may destroy seed stalks prior to the dropping of the seed or so weaken the plants physiologically that seed is not even produced. The growth of underground rhizomes and vegetative re- production is retarded when photosynthetic activity is reduced. The best pastures are those in which graz- ing animals do not consume more than 70 to 80 per cent of the total herbage productivity of the grasses (Stoddart and Smith 1943). As a rule, not more than about 60 per cent of the current forage volume and 25 per cent of the flower stalks should be har- vested by grazing animals. Overgrazing always brings about a reduction in abundance of the more palatable species and an increase in the less desirable ones with the consequent deterioration of the range and the productivity of the community (Weaver and Tomanek 1951, Kucera 1956). The carrying capac- ity of grassland or the largest number of animals 126 that can be supported without deterioration of the range varies with the type of grasses involved, the climate, and the soil (Table 9-5). Although often overlooked, invertebrates consti- tute one of the three important groups of grazing ani- mals. Individually they may not consume appreciable amounts, but in the aggregate they produce a very significant effect. The total biomass of insects in a New York pasture amounted to 3.2 g dry weight per square meter. This is to be compared to 14.5 g for the dry weight of cows per square meter that the pas- ture was supporting. Feeding experiments showed that in one pasture where grazing by the cattle was moderate and the vegetation was ample, the insects ate more of the grasses and clovers than the cows did, but in another pasture which was being over- grazed and in which the vegetation was short, the cattle ate more than the insects did (Wolcott 1937). Grasshoppers and Mormon crickets are sometimes very destructive in the arid west. In one area in Montana, a population of 25 grasshoppers per square Habitats, communities, succession FIG. 9-3 Forest-edge at William Trelease Woods, University of Illinois: prairie grasses and herbs in foreground, briers and shrubs in middle, forest in background. meter destroyed enough forage on three acres during one month to support one cow for a month (Stoddart and Smith 1943). Rodents and rabbits consume very considerable amounts of grasses and other herbs and cause great damage at times of high populations. In a study per- formed in Arizona (Taylor 1930), grazing by Gunnison’s prairie dogs alone consumed 87 per cent of the total grass production and grazing by cattle and rodents combined, 95 per cent. In California, Beechey’s ground squirrels eliminated 35 per cent of the green forage by the end of the season, pocket gophers 25 per cent, and kangaroo rats 16 per cent (Fitch and Bentley 1949). Since these various rodents have food preferences of grass species simi- lar to those of cattle, there is obviously severe com- petition between them, especially in times of drought. When rodents are not overly abundant, they have some beneficial effects in fertilizing, aerating, and mixing the soil. Among big-game mammals, bison and wapiti are largely grass-eaters, especially during the summer season. Food consumption of bison is about equal to that of cattle, but wapiti eat only about half as much per individual. In Yellowstone Park it has been esti- mated that wapiti may utilize 67 per cent of the avail- TABLE 9-5 Carrying capacity of natural grasslands for big game and livestock (from various sources, compiled by Petrides 1956). Number/mi? Biomass/ mi? Location Game or livestock (2.6 km?) Pounds Kg Oregon Antelope (64%), mule deer (36%) 9 1,000 454 Tanganyika, Africa Bush country game 10 3,300 1,498 Montana Bison (50%), mule deer, elk, bighorn 21 14,000 6,356 Arizona Bison 17 17,000 7,718 Western U.S. Cattle, ave. all grassland types 20 20,000 9,080 Western U.S. Cattle, tall grass prairie 28 28,000 12,712 Nairobi Nat. Pk., Africa (1) herbivorous big game 85 28,000 12,712 Nairobi Nat. Pk., Africa (2) herbivorous big game 134 47,700 21,656 Grassland, forests, and forest-edges 12/7 FIG. 9-4 Interior of a temperate deciduous forest of sugar maple, basswood, and American elm in Wisconsin (courtesy U.S. Forest Service). able grass forage, 47 per cent of forbs, and 30 per cent of browse. Browse and forbs are used more than grasses by pronghorn antelope and deer (Stod- dart and Smith 1943). Because of difference in food preferences, competition between the latter big-game species and cattle, although significant, is not as great as is sometimes supposed. Furthermore, deer and wapiti are able to graze steep slopes and other areas which cattle ordinarily do not (Stoddart and Ras- mussen 1945). Competition between deer and wapiti, sheep and goats is more direct, however, because sheep and goats also feed largely on forbs rather than on grass. Overgrazing produces a change both in the kinds and numbers of animals present (Table 9-6). This is correlated with the change from mid-grasses to short grasses to weedy perennials. The short-horned grasshoppers increase in variety of species with this change, but in other orders of insects, the number of species present in overgrazed pastures either remains the same or declines. There is generally an increase in population level of all groups of arthropods, except beetles, with overgrazing until the pasture deterio- rates to such an extent that erosion becomes severe, then there is a decline in abundance of all groups ex- cept the Hymenoptera and Lepidoptera. Meadow voles, cotton rats, and cottontails are less numerous in overgrazed than in undisturbed grassland, but other rodents and lagomorphs increase in abundance. Tall grass in ungrazed pastures hinders the vision of jack rabbits, kangaroo rats, prairie dogs, and ground squirrels. Some rodents are benefited by the larger and more numerous seeds of the annual weedy species, and pocket gophers find more tap- and bulbous-rooted plants in deteriorated range (Bond 1945). Increased populations of insects and rodents are a result, not a cause, of overgrazing. If grazing by larger mammals is eliminated, suc- cession back to thick grassland will occur in spite of the smaller animals, and prairie dogs and ground squirrels may actually be eliminated from the area (Osborn and Allan 1949). In the luxuriant native prairie of early days, there was seldom overgrazing by such large mammals as bison, antelope, and wapiti, although this sometimes occurred in the more arid Great Plains. Insects and rodents occurred in populations that were in equi- librium with their food supply, and overpopulations of the species were held in check partly by the vege- tation itself and partly by predatory birds, mammals, and reptiles. The most important of the larger preda- tors were the hawks and owls, coyotes, foxes, badgers, black-footed ferrets, bullsnakes, and rattle- snakes (Shelford 1942). In California, it has been estimated that these predators eliminate about half of the annual increase of ground squirrels (Fitch 1948). Because coyotes and wolves occasionally took calves and lambs, they were systematically killed by ranchers; many other predators suffered with them. With the elimination of these predators, one of the checks on the rodent population was removed at a time when increased grazing by livestock rendered 128 Habitats, communities, succession ee - 2 " this control even more desirable. Damage done to the range by increased populations of rodents and rabbits has undoubtedly been much greater than the mone- tary value of an occasional killed lamb, calf, or chicken. In the great grasslands of the West, where human populations are low, there would be advan- tage not only in reducing the amount of grazing by livestock to the carrying capacity of the land but in restoring balanced populations of herbivorous and carnivorous species. THE FOREST-EDGE COMMUNITY Grassland animals are usually restrained from penetrating forests in the same way that true forest animals are restrained from penetrating grass- land, although the home ranges of these species may overlap at the forest margin and in shrubby areas. Since shrubs are especially numerous at the forest- FIG. 9-5 Interior of a virgin coniferous forest of Engelmann spruce in Colorado (courtesy U.S. Forest Service). edge and animals have an opportunity to make use of these as well as both grassland and forest, the forest-edge biocies is well developed for some groups of animals. There are probably no soil or small ground animals characteristic of the forest-edge. There are some foliage insects that find their pre- ferred niches here. Many insects of grassland and agricultural crops that overwinter as adults migrate into the forest-edge to hibernate. Since many game species of interest to man reach their greatest abun- dance on the forest-edge, he has become impressed by this so-called edge effect. When total populations of all species are measured, however, the density of birds (Table 9-11) or mammals is not always higher than in the forest. When two forest types come in contact, for instance different deciduous forest types or deciduous and coniferous forests, there is no con- sistent change in the density of animal species (Barick 1950). The forest-edge is the preferred nest- ing site of many birds (Johnston 1947). Slightly Heavily A Normal Properly over- over- Severely overgrazed ‘ABLE 9-6 Relative Order prairie grazed grazed grazed and eroded abundance (per cent of eee SS total specimens collected) Coleoptera 29 27 19 14 11 of various orders of Hemiptera 17 11 22 36 14 arthropods in normal and Homoptera 21 24 22 26 8 overgrazed grasslands in Hymenoptera 9 11 6 30 45 Oklahoma (after Smith Diptera 19 22 23 30 6 1940). Orthoptera 15 16 34 20 15 Lepidoptera 11 13 22 17 38 Arachnida 25 21 25 21 9 Total 19 19 25 24 13 Grassland, forests, and forest-edges [29 TABLE 9-7 Size of animal populations in forest and forest-edges, May to September, exclusive of mesofauna and microfauna of the soil. Deciduous Coniferous Taxonomic forest, forest, Chaparral, group central Ill.1_ Utah? Utah® Number per hectare Shrews, mice chipmunks 62 31 87 Squirrels, cottontails, raccoons, etc. 1 20 + Birds 12 24 25 Snakes, lizards + + + Frogs, toads, salamanders a 0 + Number per square meter Snails, slugs 79 + 1 Spiders 158 16 10 Harvestmen 12 0 0 Pseudoscorpions 10 0 4 Sawflies, wasps, bees, etc. 22 5 20 Ants 141 17 142 Flies 100 20 14 Moths, butter- flies 8 0 + Beetles 165 5 20 Leafhoppers, aphids 82 27 27 True bugs 40 3 10 Thrips 131 0 1 Psocids 1 0 0 Lacewings 1 0 + Crickets, roaches, etc. 9 0 1 Insect larvae 307 4 20 Centipedes 67 5 3 Millipedes 31 1 2 Sowbug 24 0 0 ‘Including and extending data by Shelford 1951 (a, b) 2Hayward 1945 SHayward 1948 THE FOREST COMMUNITY Since the censusing of each group of ani- mals furnishes special problems, there have been no studies of total animal populations in single forest communities. By Table 9-7, however, it appears that the ratio in numbers of individuals per hectare be- tween different animal groups is of the order: 1 bird, 3 mammals; 13,000 snails and slugs, 20,000 centi- pedes, millipedes and sowbugs; 35,000 arachnids; and 225,000 large insects. The mesofauna would number in the tens of millions (Table9-8), and the microfauna in numbers so large as to be scarcely con- ceivable. In general, the number of individuals rep- resenting a species varies inversely with the body size characteristic of the species. There is, however, considerable variation in population levels both geo- graphically and temporally. We must give special consideration to each of these various groups of ani- mals. Soil animals Some animals, geobionts (Table 9-8), spend all their lives in the ground; certain protozoans, flat- worms, nematodes, annelids, tardigrades, snails, milli- pedes, centipedes, some spiders, mites, pseudoscor- pions, true scorpions, many small wingless insects, some beetles and other winged insects, and a few mammals are examples. Other animals, geophils, live in the ground only as eggs, larvae, or pupae, such as do many flies and beetles; in cocoons, as do some moths; or for hibernation, as do many beetles and bugs. Soil animals are most abundant in undisturbed virgin areas. In a longleaf pine forest suffering fre- quent burning, the number of small animals in the humus layer was reduced to one-fifth and the num- ber in the top 5 cm of the mineral soil was reduced to one-eleventh of the number in unburned areas (Heyward and Tissot 1936). Some 250 species of flagellate, amoeboid, and ciliate protozoans have been recorded in the soil (Sandon 1927), but only a few species are limited in distribution exclusively to the soil since they also occur in freshwater habitats. Many species occur in practically worldwide distribution. Flagellates may range from 100,000 to 1,000,000 or more individuals per gram of soil; amoebae, from 50,000 to 500,000 ; and ciliates, from 50 to 1,000 (Waksman 1952). Over 150 species of rotifers are known as ground inhabi- tants, and about one-third of these species have been found only in the soil. They feed on organic material and, to a lesser extent, on nematodes and proto- zoans. Nematodes may occur to the extent of 1,000 to 10,000 individuals per cubic centimeter. Most of these forms belong to the Anguilluliformes and are more or less worldwide in distribution. They com- monly possess mucous glands in the skin, the secre- tions of which aid locomotion. These nematodes are very resistant to desiccation and will quickly become active when moisture is added to soil that has been dried out for years. Tardigrades occur regularly, sometimes abundantly ; they too are very tolerant of desiccation (Kithnelt 1950). Land planarians are not common except in moist tropical regions. Some of these soil animals are detritus-eaters, some bacterial and algal feeders, some partly carnivorous, and some partly parasitic on plant roots. The majority of these small organisms are active only in soil water, present as a thin film lining the 130 Habitats, communities, succession surfaces of the soil particles. Swimming forms are necessarily very small; often, they appear dwarfed compared to the size they have been brought to in cultures. Nematodes are somewhat less restricted in their movements. They can distort the surface of the water film by means of muscular movements, and thereby bridge intervening air spaces to the next soil particle. Amoeboid organisms and hypotrichous cili- ates usually accommodate their shapes to irregulari- ties of the solid surfaces over which they crawl and can become larger in size but still remain in the water film. The variety of micro-habitats in the soil accom- modating the large number of species that occur in- cludes spaces between surface litter, caverns walled off by soil aggregates, root channels, fissures, and pore spaces between individual soil particles. These micro-habitats vary in size, temperature, and moisture conditions (Birch and Clarke 1953). Most of the insects, as well as the myriapod and arachnid groups that belong to geobiontic fauna, are wingless or nearly so (Lawrence 1953) ; many spe- cies are also eyeless. Special respiratory organs are either absent, the animals relying on their general body surface for gas exchange, or are more or less primitive. Most soil animals must, therefore, live in an environment saturated with moisture, and out of direct sunlight. The springtails jump around by means of a special springing apparatus. Millipedes and centipedes, of course, have numerous legs. Many of these animals feed on plant litter and fungus, but the pseudoscorpions, spiders, some of the mites, and centipedes are carnivorous. Most of these species are annuals or have even shorter life-cycles. Favorable soil moisture and food are most important in limiting their numbers; temperature and hydrogen-ion con- centration are secondary factors. Differences in the character of soil, whether sand or clay, does not ap- pear to affect the size of populations greatly; how- ever, the amount of decaying humus present is im- portant. In Denmark the biomass of soil organisms decreases from oak to beech to spruce forests (Table TABLE 9-8 Numbers of soil animals per m’ (mostly mesofauna) in three different communities. Locality North Carolina! England? Trinidad® Community Oak-Pine Disturbed Tropical grassland rain forest* Season of censusing Year round November July-Sept. Depth of sampling 13 cm 30 cm 23 cm Sowbugs 0 0 12 Pseudoscorpions 128 56 } 138 Spiders 92 142 Mites 22,141 164,363 20,022 Millipedes 96 401 Pauropods 44 629 366 Centipedes 34 648 Symphylids 102 3,867 Telson-tails 149 1,363 0 Japygids 135 6,605 42 Springtails 6,932 61,269 354 Termites 11 0 5,394 Thrips 355 1,129 42 Ants 164 141 2,736 Miscellaneous insects 101 23,047 992 Insect larvae 219 * 1,578 1Pearse 1946 Salt et al 1948 ‘Strickland 1945 ‘From 5 samples from 3 forest reserves, total individuals x 6 = number per m? *Larvae classified with adults 9-9), but there is an increase in number of individ- uals in beech and spruce over the oaks, attributable to increased numbers of mites and springtails, which are so small that they do not greatly affect the bio- mass. Springtails also increase in abundance from oak to spruce to beech in the forests of Yugoslavia (Stevanovic 1956). Biomasses of mites and spring- tails in an English grassland area varied from less than 0.1 to 1.4 g/m?; they were generally at peak dur- FIG. 9-6 Soil fauna. (a) campodeid, (b) japygid, (c) proturan, (d) symphylid, (e) springtail, (f) centipede (from Kevan 1955). Grassland, forests, and forest-edges 137 FIG. 9-7 Some inhabitants of the ground stratum in a temperate deciduous forest. (a) camel cricket, (b) yellow-margined millipede, (c) round red millipede, (d) Mesodon pennsylvanicus, (e) Allogona profunda, (f) Anguispira alternata, (g) Anguispira kochi, (h) Haplotrema concava (Shelford 1913). ing the winter months (Macfadyen 1952). In a hem- lock-yellow birch forest in Michigan, mites and springtails were over twice as numerous in winter as in summer (Wallwork 1959). There are two main groups of annelids in the soil, the large red earthworms, Lumbricidae and Mega- scolecidae, and the small, whitish potworms, Enchy- traeidae. In rich, moist, humus soil, the red annelids may reach populations of over one hundred individ- uals per square meter; potworms sometimes occur in hundreds of thousands per square meter. Earth- FIG. 9-8 Wood-eating beetle, Passalus cornutus. Top left, adult; top right, pupa; bottom, larva (Shelford 1913). i2 worms ingest particles of mixed humus and mineral soil, absorb the organic matter out of them, and defecate around the entrances to and along the length of their burrows. Potworms feed more on plant and animal detritus, but may ingest some mineral parti- cles. Potworms may also exert some control over parasitic nematodes of plant roots (Kthnelt 1950, Jacot 1940). Minimum numbers in Wales occur in late winter, maximum numbers in the early summer, and the biomass varies concomitantly from 2.7— 13.2 g/m? (O’Connor 1957). The native North Habitats, communities, succession Grassland, FIG. 9-9 Foliage arthropods of deciduous forests. (a) daddy long-legs, Phalangida; (b) leafhopper, Cicadellidae; (c) lace-wing, Chrysopidae; (d) bald-faced hornet, Vespidae; (e) leaf-legged bug, Coreidae; (f) plant-louse, Aphididae; (g) walking-stick, Phasmidae; (h) katydid, Pseudophyllinae; (i) tree cricket, Oecanthinae; (j) stink-bug, Pentatomidae; (k) field cricket, Gryllinae; (1) spined spider, Argiopidae; (m) stilt bug, Neididae (courtesy Illinois Natural History Survey). forests, and forest-edges iS American annelid fauna has been seriously disturbed by the widespread invasion of introduced Lumbricus terrestris and Allolobophora caliginosa. These spe- cies are found in forested areas, especially along rivers to which they have been carried by fishermen. In some localities, their activities have altered the basic character of the soil to the jeopardy of the en- tire original forest community. The gastropod fauna is rich in moist, humus soil, but becomes scarce when the soil dries out. It is more abundant in deciduous than coniferous forests, be- cause coniferous forest soils tend to be acid. In east- ern North America there are three common genera eS FIG. 9-10 Foliage insects of coniferous forests. (a) sawfly larvae, Neodiprion lectonei, on jack pine; (b) spruce budworm eggs, larva, chrysalis, and moth (courtesy U.S. Forest Service). 4 of slugs, Philomycus, Deroceras, Pallifera, and a va- riety of snails belonging principally to the Poly- gyridae, Zonitidae, Entodontidae, Haplotrematidae, Pupillidae, and Succineidae. Fifty species of snails were collected in 74 hours of searching in the Great Smoky Mountains (Glenn Webb). The haplotremes are carnivorous, feeding on other snails, but other- wise the gastropods feed chiefly on detritus, algae, lichens, and fungus. The assemblage of small animals that dwells un- der stones, rotting logs, and the bark of trees is some- times called cryptozoa. Many species occurring in this microhabitat also commonly occur through the litter and soil generally, especially in moist climates with rich soil humus. In temperate forests, however, some snails, sowbugs, some spiders, lithobiid centi- pedes, julid and polydesmid millipedes, entomobryid springtails, roaches, earwigs, staphylinid, carabid and histerid beetles, and some ants reach maximum popu- lations as cryptozoa (Cole 1946). The cryptozoan habitat is a favorite of salamanders. Many common soil animals are found as well in the special tree-hole forest microhabitat (Park et al. 1950) ; indeed, some species are specifically limited to tree-holes. Decaying logs and stumps are preferred by many species. During the first three years following the cutting of the pine trees in a North Carolina stand, 130 species of insects, myriapods, annelids, mites, and mollusks were found (Savely 1939). Coleoptera was by far the most numerous order of insects. Of all spe- cies, approximately 7 per cent were phloem-feeders, 15 per cent sapwood-feeders, 44 per cent rotten wood- and fungus-feeders, 30 per cent predaceous, and 4 per cent parasitic. The phloem-eaters were most nu- merous during the first year. Their mode of feeding prepared the way for the later entrance of fungi, fungi-eating species, and predaceous forms. In the decay of logs of such nothern trees as pine, spruce, and fir, the character of the food available is important to the succession that occurs (Graham 1925, Ingles 1931). Bark beetles require fresh green tissues of the inner bark and cambium, and hence occupy the tree only for the few weeks that these tissues remain. The long-horned beetles and wood borers require green tissue for their younger stages. As they mature, they are able to digest the solid wood. The horntail larva can digest solid wood as soon as it hatches. The outer bark is most difficult to digest but it does furnish food for some species of Lepidoptera and Diptera. In addition to invertebrates, there are several vertebrates, particularly mice, shrews, moles, am- phibians, and reptiles, that may be mentioned as part of the soil and ground fauna. These animals often have extensive underground runways and feed on the invertebrates in the soil, as well as on each other. Foliage arthropods Foliage insects and spiders are represented by large numbers both of species and individuals (Gra- ham 1952). Spiders, ants, flies, beetles, leafhoppers, bugs, and larvae ordinarily predominate (Table 9-7) ; population depends directly on the amount of green foliage present. The species present depend on the type of vegetation, stratum, season, time of day, lo- cality, and climate. Outbreaks of particular species may occur irregularly or periodically and several hundreds or even thousands of individuals may occur in each tree. Spruce budworms and walkingsticks are sometimes so abundant that their excrement or eggs dropping to the ground sounds like the patter of raindrops. Birds Breeding-bird populations in forest communi- ties vary with the fertility of the forest, but are com- monly between 100 and 400 pairs per 40 hectares (100 acres), which would be equivalent to 5 to 20 indi- vidual birds per hectare (2 to 4/acre) (Table 9-11). In addition, there is often a large non-breeding popu- lation present. On a 16 hectare (40 acre) spruce-fir forest in Maine there were 154 territorial males present prior to June 13. Between June 21 and July 5 a determined effort was made to reduce the popula- tion of songbirds, and a total of 352 males was taken from the area, more than were actually nesting at the start of the operations. Yet on July 11 there were still 40 males present and proclaiming territories. FIG. 9-11 Adult red-shafted flicker and young at nest in an aspen forest, Oregon (courtesy U.S. Forest Service). One hundred twenty-six females and 49 individuals of undetermined sex were also removed (Hensley and Cope 1951). This floating population is more numerous than the number of nest sites available in the community, but it functions as a pool from which individuals may replace any nesting birds that die, or take over any suitable niche that becomes avail- able for one or another reason. This population pres- sure doubtless keeps the community saturated with breeding birds, tends to maintain the nesting popula- tion at a high peak of efficiency, and is a challenge to individuals to exploit new adaptations and to occupy new niches. It is therefore a potent factor in evolu- tion. In a mixed deciduous-coniferous forest in Europe, a bird population of 662 individuals per 40 hectares (100 acres) was estimated to have a biomass of 47 kg (103 lb). In the same area, there were 528 in- dividual mammals of biomass 264 kg (580 Ib) (Tur- cek 1952). The number and biomass of birds is gen- erally less than that of mammals (Hamilton and Cook 1940). Mammals Rodents (mice, chipmunks, squirrels) and in- sectivores (shrews, moles) are the most abundant small mammals of forest communities. Resident sum- Grassland, forests, and forest-edges Ie TABLE 9-9 Number, biomass, and metabolism of ground invertebrates per m’ in forests of Denmark (after Bornebusch 1930). Type of Forest Oak Beech Spruce Number of Stations 1 6 3 Biomass Biomass Biomass Invertebrate group Number _ingrams Number ingrams Number in grams Lumbricid worms 122 61.0 79 15.8 50 PG} Other humus-eating animals 2,675 15.0 9,338 10.6 10,807 7.0 Predacious animals 181 0.8 264 15. 290 185) Totals 2,978 76.8 9,681 27.9 11,147 11.0 mer populations commonly vary from 25 to 100 per hectare (10 to 40 per acre). In rich, moist, undis- turbed forests, populations may sometimes attain temporary levels of up to 500 per hectare (200 per acre). Considerable data on population sizes and biomasses of individual species have been compiled by Mohr (1940, 1947). In forests of eastern North America (Hamilton and Cook 1940 :469), small mammals fall into several categories. The deer mice and the flying squirrels are adept climbers and often have their homes thirty feet or more [10 meters] from the ground in some hollow snag, deserted nest, or abandoned woodpecker hole. Flying squirrels feed among the trees and descend to the ground to forage about old logs and brush piles. They also dig down into the litter from the surface. Chipmunks forage in much the same manner, al- though they climb less frequently. Deer mice occupy several levels, from the trees to the burrows of moles and shrews. The red-backed mice, the lemmung mice, and probably the jumping mice dig fairly permanent tunnels and runways through the soil and the litter. These they use as bases for food-gathering in both the litter and the upper layers of the mineral soil. These runways are often used by the shrews and the deer mice. The short-tailed shrews dig substantial tunnels. The diminutive long-tailed shrews thread their way through the easily parted litter and top-soil, and make intricate temporary labyrinths daily in search of food. The moles remain in their...tunnels during the daylight hours, but often come to the surface at night, no doubt attracted by the countless invertebrates that swarm over the ground with darkness. In winter they remain safe under the snow. In mixed deciduous and coniferous forests in the Sierra Nevadas the total number of rodents varied from 150 per hectare (60/acre) in July-August to 52 per hectare (21/acre) in December; their bio- mass varied from 27 to 4 kg per hectare (24 to 3.3 Ib/acre) (Storer et al. 1944). The home ranges of individuals of these small rodents and insectivores are commonly only a fraction of a hectare (Blair 1953). Of forest species, the larger the mammal, the fewer its numbers; and, usually, the wider an indi- vidual’s range. The home ranges of the weasel, rac- coon, and bear, are more extensive than those of ro- ISG dents and insectivores. In the aggregate, their biomass does not exceed that of the more numerous smaller species. A population density of one deer per 20 hectares, for instance, translates into a deer biomass of about 2.8 kg per hectare (2.5 Ib/acre). Stratification The community is a structure of the subter- ranean, ground, herb, shrub, and tree strata, and spe- cies are separated into different niches in relation to these strata (Elliot 1930). Food, shelter, and micro- climatic differences are the chief limiting factors. Be- cause microclimate in each stratum varies from hour to hour, day to day, and season to season, classification of a species by stratum must be in terms of the stra- tum it is observed to frequent for the major portion of a relatively long period of time. The inhabitants of the five strata divide into two major groupings, or societies. The soil invertebrates and some mammals move freely back and forth between subterranean and ground strata, and may be considered a society dis- tinct from that which occupies the combined herb, shrub, and tree strata. Insects and birds depending on the foliage for food and reproduction sites move more freely among the latter three strata than between these strata and the ground. In terms of the strata which each occupies, however, the societies are not mutually exclusive, for ground animals such as millipedes and snails climb up onto herbs and tree trunks during humid weather, and foliage animals often rest on the ground, and search for food, hibernate, and lay eggs, there. The majority of arthropod species carry on their main activities within a single stratum for their major activities (Table 9-10). The tree stratum spans a greater vertical distance than any other. Within this broad stratum, arthropods often show segregation to particular heights above the ground (Davidson 1930). Ants, spiders, and beetles appear to move more freely between different strata than do other species. In point of macrofauna population densities of in- vertebrates, the ground stratum ranks highest, fol- lowed by subterranean and herb strata, while shrub Habitats, communities, succession and tree strata rank lowest. The largest mammal populations occur in the subterranean and ground strata. In a European oak-hornbeam forest (Turcek 1951) 15 per cent of the bird species nested on the ground, 25 per cent in the herb and shrub strata, 31 per cent in or on the trunks of the trees, and 29 per cent in the tree canopy. The largest number of indi- viduals (32 per cent) occurred in the forest canopy, although the biomass of these birds constituted a smaller percentage of the total (16 per cent) than did the grotind and herb population (67 per cent). In re- spect to feeding, however, the distribution was differ- ent: 52 per cent found their food on the ground, 9 per cent in the herbs and shrubs, 10 per cent on the tree trunks, 23 per cent in the tree foliage, and 6 per cent in the open spaces between the canopy, trees, and shrubs. Seasonal changes Outside of the tropics, the forest community changes drastically with the seasons such that four aspects may be recognized, each of which is divisible into two or three different sectors. The total popu- lation of the soil macrofauna in the temperate decidu- ous forest is highest during the hiemal aspect because of the migration of many foliage insects into this stratum to hibernate. Forest species hibernate in densities that vary randomly throughout the forest, except where there are differences in topography or substratum. Forest tracts adjacent to grass- or farm- land, however, receive an influx of non-forest species that hibernate principally on the forest-edge gen- erally, and along the south edge in particular, where exposure to solar radiation and protection from cold northerly winds produces warmer temperatures (Weese 1924). During the vernal aspect, insects and other in- vertebrates come out of hibernation, and the adults of forms variously frequenting the herb, shrub, and tree foliage return to their characteristic stratum. The population of ground animals remains relatively high throughout the year, however, which can be at- tributed to the reproduction of the geobionts and to the fact that the immature stages of many foliage arthropods, particularly Diptera, Coleoptera, and Lepidoptera, occur in the ground. Various groups of these geobionts and geophils reach their peak num- bers at different times during the year (Pearse 1946). An insect species may show more than one popu- lation peak during the year (Shelford 1951), depend- ing on the number of generations produced and the specific life-span. A species may appear, attain to very large numbers, and disappear, all in a matter LONG=TAILED.- —=. SHREW HREW F< \SHREW DED: BACKED MOUSE = a ar SELES HAIRY- ee MOLE FIG. 9-12 Relation of small mammals to the forest floor (Ham- ilton and Cook 1940). of a few days, or a few weeks at most. Considerable variation also occurs from year to year in the popula- tion fluctuations of individual species and of whole groups, correlative with differences in weather, par- ticularly temperature and moisture (Leopold and Jones 1947). Bird populations in temperate zones reach peak populations with the passage of transients during the seasons of migration. These migration peaks are in- conspicuous or absent in northern coniferous forests, since most birds that arrive stay to nest. In the tropics, birds are most abundant during the winter period of the north temperate zone, since the fauna then contains many migrant species from the north. Mammal populations in temperate regions com- monly reach their maximum numbers in the autumn, at the end of the breeding season. Populations de- cline progressively as winter wanes, the result of mortalities from severe weather, lack of food, and predation. Comparison of animals in different forest types There are more niches and microhabitats avail- able in forest and forest-edge communities than in any other type of terrestrial community. The strati- fication and the diversification of plant forms are re- sponsible for this. Many forest niches are much the TABLE 9-10 Stratal distribution of arthropod species in Missouri (Dowdy 1951). Oak-hickory Red cedar Distribution of species forest forest Number of species 161 96 Confined to one stratum 69% 78% Confined to two strata 19 15 Confined to three strata 11 5 Confined to four strata 1 2 Found in all five strata 0 0 Grassland, forests, and forest-edges rey O 2700270 400270 E 50 80®600 wo © 100 a lJ = 5o 60030 = oS ro) 250 50030 ag = 2300 ro) Ww = 250 600800 ro) tt 200 330 2830 w 600 1200 1400 630 670 11430 ol50 e e eee 2 320 * 80 80 70 2/0 4270 220 /20 F, 100 a 50 8082230 560200 1 | 860,150 zi (eer eee ay 10) O 50 100 150 200 150 100 50 O DISTANCE FROM WEST & EAST EDGES, m FIG. 9-13 Population (number per m7) of hibernating insects in the soil during the winter at different locations in a 22-hectare (55 acre) rectangular deciduous forest tract (Trelease Woods) in central Illinois. The forest is in contact with grassland on the south side and with farmland on the other sides. The italicized numbers are field and crop insects that have invaded the forest to hibernate, the other numbers are true forest species (modified from Kennedy 1958). same, regardless of whether they occur in deciduous or evergreen forest, and regardless of geographic lo- cation (Blake 1926, Dirks-Edmunds 1947). Species occupying these niches differ, but are often taxonomi- cally related ; related or not, they have similar mores. Thus a predominant, shrub-inhabiting, plant-juice sucking leafhopper in an Illinois oak-maple forest 1s Erythoneura obliqua, but in a Maine pine-hemlock forest it is Graphocephala coccinea. The common herbivorous woods mouse in Illinois is Peromyscus leucopus, but in the Douglas fir-hemlock forests of Oregon it is Clethrionomys occidentalis. An insectiv- orous hole-nesting chickadee in Maine is Parus hudsonicus; in Illinois, P. atricapillus; and in Ore- gon, P. rufescens. Coniferous forests have needle leaves; deciduous forests, broad-leaves. Some special, different niche adjustments are thus required which may not permit a species to successfully occupy both kinds of forest. The tube-building moth attaches its eggs to the pine leaf, and, when the larva hatches, it makes a nest of 6-15 needles, bound together with silk-like threads. The larva, and later the pupa, is protected within this tube but can come out and feed on the end of the leaves of which the tube is constructed. Deciduous leaves are clearly incompatible with these behavioral patterns, so nicely adapted to the peculiarities of pine needles. On the other hand, the red-eyed vireo ex- periences difficult feeding in coniferous trees because of the arrangement and close position of the needle leaves on all sides of the twig (Kendeigh 1945). While the vireo can feed in coniferous forests, it is considerably more profitable for the bird to confine it- self to the deciduous forest, for which it is better adapted. Some animal kinds inhabit the soil and litter of both deciduous and coniferous forests, but a kind may be more abundant in one forest type than the other because coniferous and deciduous leaves form two distinct types of humus. Because of differences in foliage character, persistence through the year, cli- mate, and the considerable difference in the taxo- nomic composition of the plants and animals involved, coniferous and deciduous forests are separated as dis- tinct biomes, each with a number of biociations in different parts of the world. Bird populations are not necessarily consistently higher in one type of forest than in the other (Table 9-11) ; rather, population varies with the luxuriancy of the vegetation. Animal populations in both co- niferous and deciduous forests are generally highest in regions where ample rainfall brings rich develop- ment of vegetation as the basic food supply (Odum 1950). Population densities of both birds and mam- mals decrease progressively westward from the Ap- palachian Mountains to the eastern edge of the prairie as the climate becomes progressively drier (Wetzel 1949, Fauver 1950). The variety of snail species de- creases in a similar manner from moist to dry forest types (Shimek 1930). Forest and game management The forest productive capacity of greatest eco- nomic interest is the timber yield. Forests are also of great importance to man for the protection of water- sheds against erosion; for such recreational purposes as hunting, camping, and hiking; and for the inspira- tional values of unspoiled scenery and primitive na- ture. Complete logging, as practiced universally in colonial and even in modern times, destroys the for- est, most of its animal life, and its usefulness for these purposes. Logging on a sustained yield basis converts the forest into a forest-edge community and allows the forest-edge animals to increase in abun- dance, while the animals dependent on dense forest decline. 138 Habitats, communities, succession 160 120 80 40 40 40 30 20 600 500 Ss NTA 300 FIG. 9-14 Monthly variations in total population size of differ- larvae; (c) spiders; (d) Homoptera; (e) snails; (f) centipedes; ent animal groups in a temperate deciduous forest (Trelease (g) invertebrates (macrofauna) in ground; (h) non-forest Woods) of central Illinois. The data for each month are averages species. of 10-14 year records. (a) Diptera larvae; (b) Lepidoptera Grassland, forests, and forest-edges 139 1600 1400 ve) ~ ‘e) fe) fe) fe) INDIVIDUALS PER 40h (IOOA) @ [e) fe) 200 Sete permanent A reside | JAN | FEB | MAR | APR | MAY | JUN | FIG. 9-15 Seasonal fluctuations in the bird populations of a deciduous forest area in Ohio. Transients include robins and Such game animals as the gray squirrel, black and grizzly bears, moose, the fur-bearing marten, and the ruffed grouse and wild turkey belong primarily to the forest proper, although they often feed in the forest- edge, or brushland, and openings scattered through the forest. Populations of these species may be main- tained simply by preserving large tracts of virgin or dense forests. Most game animals of interest to the ordinary sportsman, however, belong to the forest-edge. These species are the cottontail, fox squirrel, deer, bobwhite, pheasant, and dove. Increase in populations of these TABLE 9-11 Average densities of total breeding-bird populations in forests and forest-edges of different types in eastern Tt : “1 JUL | AUG| SEP | OCT | Nov | DEC Type of vegetation «. ae inter visitor the occasional visitors (modified from Williams 1936). species requires interspersing the forest with open areas, development of shrubby forest margins, or cre- ation of artificial cover along fence rows, uncultivated field corners, around ponds, along drainage ditches or streams, on steep slopes subject to erosion, and on waste lands (Trippensee 1948). Intelligent manage- ment may involve control of plant succession to pre- vent its proceeding to a normal closed forest, and harvesting the forest for timber and game. Proce- dures for managing timber on a sustained yield basis are fundamentally the same as for managing popula- tions of game animals on a permanent basis. Soil North America (compiled from Fawver 1950). Spruce-fir (coniferous) forest Mixed coniferous forest Mixed hemlock - deciduous forest Beech-maple-hemlock forest Mixed deciduous forest Deciduous floodplain forest Oak-hickory forest Mixed deciduous and southern pine forest Southern pine forest Broad-leaved evergreen forest Coniferous forest-edge Deciduous forest-edge Number of territorial Number of males per areas Number of 40 hectares censused species (100 acres) 5 30 311 6 33 207 5 28 224 5 31 190 17 26 255 7 28 229 5 24 181 5 19 157 1 23 163 1 23 162 7 30 241 6 27 265 140 Habitats, communities, succession conservation and erosion control can also be readily combined with wildlife management, especially when trees and shrubs selected for planting to regulate soil erosion are species useful to game as cover and food. The farmer can encourage establishment of small game species on his land by practices that do not in- terfere with the raising of crops. The maintenance of brushy fence rows does not increase the number of insect or other crop pests (Dambach 1948), as has sometimes been maintained. A knowledge of the fun- damentals of life-history and ecology is essential to wildlifé management, as wildlife management is ap- plied ecology and involves the management of the total community, not merely game species in it (Leo- pold 1933). PRESERVATION OF NATURAL AREAS It is of utmost importance for the future of ecological studies that adequate samples of virgin primitive areas—forest communities, tundra, grass- land, desert, tropical and rain forest, and all seral as well as all climax types of communities in all parts of the world—be preserved intact. Balanced primi- tive communities are the result of processes at work through eons of time. Primary communities once de- stroyed, there is never assurance that the secondary communities which develop can ever exactly dupli- cate them. This involves not only the replacement of all species in the original fauna, but also their replace- ment in the same relative numbers so that an inte- grated balanced community is fully re-established. The preservation of such natural areas is of historical value to future generations as a record of natural con- ditions over the country in pre-colonial days. Natural areas serve as controls for the agricultural develop- ment of the country, for the evaluation of various farming practices and uses of the land, and to show the potentialities of vegetative development of various parts of the continent. No one can know the potential value for food, medicine, or domestication of any organism that makes up primitive communities. Large primitive areas are preserved in some of the National Parks, National Monuments, and in some of the larger of our state parks (Kendeigh 1951). Natural, wild, and wilderness areas have been set aside in several of the National Forests. Smaller areas of ecological value are being preserved in state, city, and private preserves. Not all community types are represented; more areas need to be set aside in other parts of the country, and constant vigilance must be exercised to keep them undisturbed. These projects are being sponsored by the Nature Con- servancy, the National Parks Association, the Wil- derness Society, and other organizations which de- serve the support of all ecologists. RAZED-WOOD' LOT TIMOTHY SES PASTURES CORN CORN | CORN FIG. 9-16 Sketch of a 100-acre farm before 4nd after improve- ments to encourage small game. (A) windbreak, of some value to game as cover; (B) hardwood planting, perhaps black locust usable later for fenceposts; (C) a portion of the crop (corn) left, near cover, for wildlife; (D) a field or fence border; (E) emergency food, a few shocks of grain placed near cover; (F) cover planting of coniferous trees; (Q) a quail habitat with food and cover (Phelps 1954 in Virginia Wildlife). Grassland, forests, and forest-edges 141 LIFE HISTORIES We here choose four species to show life- history adjustments to the habitat and community : two mammals for the grassland, a bird for the forest- edge, and a millipede for the forest. Voles (Microtus, Pedomys) The meadow vole, M. pennsylvanicus, and prairie vole, P. ochrogaster, are small, dark gray or brownish, compactly built mice with short legs and tail, small eyes, and partly hidden ears. They spend most of their time in an elaborate system of tunnels, partly underground and partly as almost hidden gal- leries in dense grass. The food of these species con- sists mostly of grasses but also includes legumes, composites, fruits, and occasionally insects. Grass stems grow close together. The voles thrust them aside to form paths on the ground surface. These runways are heavily trafficked networks, and feeding is done in them. Runways formed through clover or alfalfa are less permanent. Underground passages lead to nests and chambers where food is stored. Nest cavities are round, lie 7 to 45 cm below the sur- face, and often have two tunnels leading up to the surface. The cavities are lined with dried grass and leaves (Jameson 1947, Martin 1956). The voles may be active at any hour of day or night throughout the year, but the periods of greatest activity come in early morning and evening. The mean monthly home range of an individual is very small, commonly about 364 m? (.09 acre), although males wander somewhat farther than do females. There is no defense of territory. Because of their small home ranges and high rates of reproduction, vole populations in years of abundance may reach 366 or more per hectare (146 per acre). The level of population fluctuates through the years, however, in response to rainfall and competition, and there is some evidence of a four year cycle. Populations regu- larly decline each winter and increase to an annual peak in the autumn. Predators on the two voles are numerous and varied, and include hawks, owls, crows, weasels, foxes, coyotes, badgers, and snakes (Hamil- ton 1937, 1940, Martin 1956). During the peak of a population cycle, breeding may continue throughout the year, but it is ordinarily curtailed during the winter and periods of summer drought. The number of young in a litter increases with the age of the female but commonly varies be- tween three and five. The duration of the oestrus cycle in the female, if such a cycle actually occurs, is not known, but is likely only a few days long. Voles are promiscuous, and the female may accept a male within a few minutes or hours after the birth of her young and be capable of ovulating and conceiving a new litter within five or six days. The gestation period is short (about 21 days) and it is estimated that 8 to 10 Microtus litters may be produced in a favorable year (Hamilton 1941). P. ochrogaster is less prolific. The young are born pink-skinned, hairless, blind, and with ear pinnae closed. They soon attach them- selves to the teats of the mother, who may even drag them along as she forages. They weigh two or three grams at first but grow rapidly, gaining one-half to one gram per day. Meadow voles at sexual maturity weigh 25 to 30 grams; when fully adult, 40 to 50 grams. The backs of the young voles are covered with soft velvety hair on the fourth or fifth day after birth, the incisors erupt on the sixth or seventh day, and the eyes open and the pinnae unfold on the eighth or ninth day. With their eyes open they be- come more active and may take short trips away from the nest to nibble on succulent vegetation. The young voles are weaned at two to three weeks, but may re- main with their mother for several days longer. Males may become sexually mature when five weeks old, and females may mate successfully when only four weeks old. The mortality rate in young mice is high, but the normal life span of adult meadow voles under natural conditions is 10 to 16 months. Prairie voles appear not to live as long (Hamilton 1941, Fitch 1957). Bobwhite (Colinus virginianus) The bobwhite is found over the eastern part of the United States and south through Central America (Stoddard 1931). It occupies open woodlands, shrubby fields, fence rows, and forest-edges border- ing on grassy fields or farmlands. Nests are usually located within 16 meters of roads, paths, or culti- vated fields. This vegetation serves as cover against both weather and enemies. The bird feeds primarily on seeds, occasionally on fruit and herbage, and, dur- ing the summer and autumn, on insects. It also in- gests a considerable quantity of mineral matter in the form of grit. The young chicks feed largely on insects the first three weeks after hatching, but then become gramnivorous like the adults. With ample cover and food, the species may reach maximum pop- ulation of 2.5 per hectare (1 per acre), but popula- tions of one bird to 2-5 hectares are more common. In the South, populations are fairly stable year after year, but in the North they fluctuate widely (Ken- deigh 1944). The birds have a number of call notes for com- munication between individuals of a pair or covey. These notes serve to attract mates, express alarm or 142 Habitats, communities, succession distress, indicate that the individual is lost, for feed- ing, to reassemble the covey, for battle cries, and so on, Pairing of male and female usually begins in April as the winter covey breaks up, and the males give their bob-bob-white calls. During this period, there may be song competition between males, fight- ing, chasing, plumage displays, and bluffing. Compe- tition is intense because there are more males than females. Two to four weeks may elapse before the pair begins to nest, during which time the two birds stay close together. Nesting may start in April in the South; May to August in the North. Nests are placed in good cover where the herb stratum is open enough so the birds can run around over the ground near the nest. A slight hollow is scratched in the ground, and the nest is commonly constructed of grasses, pine needles, mosses, or whatever is immediately available. A grassy arch is made overhead to serve as a roof and to conceal the nest from predators. It is ordinarily located on well drained high ground. One egg daily is laid until the full clutch of 14 or so is attained. An occasional day may be skipped, and clutches laid early in the year are larger than those laid later. The incubation period lasts 23 days, and incubation may be performed either by the male or female; three out of four times it is the latter. During this period the incubating bird usually leaves the nest for a time to feed early in the morning and often again in late afternoon. The incubating bird joins its mate at a distance from the nest and they feed and rest together from one to occasionally nine hours, depending on the weather. The birds do not need surface water for drinking, but get what water they require from their food or from dew. About 86 per cent of eggs hatch, and all of these within about an hour. The young chicks quickly leave the nest and are cared for and brooded against cold, wetness, and too much sun by both parents for another two weeks. By that time, juvenal plumage is replacing the natal down, and the birds will flush and fly a short dis- tance when disturbed. The young birds become simi- lar in plumage to the adults at the end of 15 weeks. There is some feather molt about the head in the spring and a complete molt from August to October each year. The winter covey forms in the autumn, and com- prises one to three pairs of adults, their surviving young, and a few birds that were unmated. As birds die, small coveys unite and maintain an average size of about 14 birds. A covey may confine its activities within a range of 60 to 16 hectares (24 to 6 acres), and the ranges of adjacent coveys may overlap. The birds commonly freeze when approached by enemies, relying on their protective coloration for escape. If too closely approached they burst forth in rapid flight that carries them in all directions for 400-500 meters, FIG. 9-17 Formation of egg-chambers by the millipede Pseudo- polydesmus serratus. Top and bottom, base of two egg-chambers being formed; middle three, egg-chambers filled with eggs and partially capped over (Hanson 1948). whence they then drop down into other cover. When the enemy disappears the covey call reunites them again. Coveys feed together and roost together. They roost on the ground in compact circles with heads pointing outward. Species predaceous on eggs, chicks, and adults in- clude skunks, rats, foxes, weasels, opossums, rac- coons, dogs, snakes, red ants (eggs), cats, shrikes (chicks), Cooper’s and sharp-shinned hawks, and great horned owls. Numbers of parasites and disease organisms potentially dangerous to it are harbored by the species. Heavy rains may be destructive to nests and young birds in the summer, while extreme cold combined with long periods when snow covers the ground may kill adults during the winter. The popu- lation turnover during a year is therefore large. Millipede (Pseudopolydesmus serratus) This species feeds on decaying leaves and other organic material. Adults occur in populations up to 5 per m?, and immature stages may be present up to several hundred per m?. High populations, however, occur only in poorly-drained, moist forests. During periods of low precipitation, individuals migrate and become concentrated in wet depressions. The de- pendency of the species on moisture is further indi- cated by higher reproduction during wet than dry years. Copulation occurs March to December, but there are two principal peaks of egg-laying; one in April, the other, during the first half of July. This results Grassland, forests, and forest-edges 143 in two generations per year, but these are not geneti- cally distinguishable. There are 7 larval or instar stages. At hatching, the larva has only 7 post-cephalic somites and 3 pairs of legs. At each molt, more somites and legs are added until in the adult there are 20 post-cephalic somites and 30-31 pairs of legs. The first and second instars are whitish in color, but later instars develop a reddish-brown pigment. The April generation reaches the morphological adult stage in the autumn but sexual maturity not until the next spring. The July generation overwinters in the 5th to 7th instars, reaches the adult stage the following spring and sexual maturity in June or July. Molting takes place in small chambers similar to those in which the eggs are laid. The egg chamber is unique. It is made of faecal pellets containing ingested soil and organic material. These pellets are placed in a ring of diameter about 6 mm. More and more pellets are piled on until the ring reaches a height of about 3 mm. Some 200 to 400 eggs are then deposited inside the ring after, which the ring is closed at the top to form a capsule. The whole process requires 6 to 12 hours. After breeding is completed, the adults die. SUMMARY Daily fluctuations of temperature, precipi- tation reaching the ground, light intensity, and wind velocity are greater in grassland than in forests, but relative humidity is usually less. A gradient in habi- tat conditions extends from above the vegetation to the ground in both grassland and forest. Segregation of animal species into subterranean, ground, herb, shrub, and tree strata is partly explained by differ- 144 ences in response to this gradient. North-facing slopes are generally cooler, moister, and with lower light intensities than South-facing slopes. The species composition of animals differs between grassland, forest-edge, and forest. Within each com- munity there is a vertical division into a subterranean- ground society and an herb or herb-shrub-tree so- ciety. Animal density and biomass are generally greater in the former. Food, shelter, and microcli- mate are the chief limiting factors. Outside of the tropics, there is considerable seasonal variation in the abundance of animals. Many niches are similar in forests of different types; say, coniferous and deciduous. The species occupying these niches are often different, however, although they may have similar mores. Grasses tolerate considerable grazing, and grass- land productivity may provide a high carrying ca- pacity for large grazing animals. Overgrazing by large populations of insects, rodents, or domestic stock, however, may bring deterioration of the range. Economic utilization of grassland requires proper bal- ancing of grazing pressure against vegetative pro- ductivity throughout the year. Forests are of great interest to man for timber, protection of soil against erosion, and recreation. Game species are usually more varied and abundant in the forest-edge than in the forest-interior. Game management is concerned with controlling the vege- tation and habitat to produce the highest yield of the desired species and to regulate the number taken. It is necessary to know the intimate life-histories of the species concerned before this can be accomplished intelligently. Finally, it is of utmost importance to ecological study that adequate samples of primitive areas be preserved in an undisturbed condition. Ecological processes and dynamics Ecological Processes and Community Dynamics: Dispersal, Migration, and Ecesis In order to understand the geographic distribu- tion local and otherwise of animals, the eventual suc- cession or replacement of one community by another, and the stabilization of different species at different levels of abundance, we need to know several things. We must discover how organisms invade new habitats or geographic areas; how they react on the habitat ; the manner in which they compete with or otherwise affect each other; and the factors that determine the success or failure of organisms in the struggle for existence. The processes of dispersal, migration, com- petition, speciation, reaction, coaction, and ecesis con- stitute the dynamics of the community, the under- standing of which is a prime objective of ecological thought and research. David Starr Jordan (1928) once stated that the general laws governing the distribution of animals can be reduced to three simple propositions. Accord- ing to Jordan, a species of animal will be found in any part of the earth having conditions suitable for its maintenance unless (1) its individuals have been un- able to reach this region because of barriers; or (2), having reached it, the species has been unable to main- tain itself because of inability to adapt to the region or to compete with other forms; or (3), having arrived and survived, it has subsequently so evolved in the process of adaptation as to have become a species distinct from the original type. DISPERSAL DYNAMICS Dispersal is the spread of individuals away from their homesites. Dispersal movements are usu- ally slow, and cover relatively short distances in the life time of an individual. The cumulative result of short dispersions by successive generations, however, may become conspicuous in the course of years, dec- ades, or centuries, especially when it amounts to range expansion of a species into a new habitat or area. Some remarkable instances of range expansion have resulted from the purposeful introduction of the house sparrow into North America in 1852-53 and the Eu- ropean starling in 1890-91 ; and the accidental intro- ductions of the black rat, Norway rat, and numerous insect pests, at various times. In similar fashion the gray squirrel and muskrat have been introduced into Europe (Elton 1958). Once man had helped them to overcome the ocean barrier that previously held them in check, these species spread unusually rapidly be- cause of the optimum environment on the continent. There is reason to believe, however, that under en- tirely natural conditions the rates of dispersal of all species, once a barrier is passed, would be similar, varying only with respective reproductive potentials, characteristic speeds of locomotion, and relative abil- ities to find unoccupied niches, overcome competition, 145 (e) a OCCUPANCY, Sq Mi, MILLIONS P<§ 1.0 3 t S05 ° oi ORI | 1925 | (935 [945 ee 1920 1930 I940 1950 YEARS FIG. 10-1 Dispersal of the European starling in North America between 1918 and 1949 (compiled from Kessel 1953). acclimatize to new climatic conditions, and acquire new behavior patterns. Manner and means of dispersal Animals find suitable habitats and niches in various ways. In a uniform environment, dispersal movements radiate in all directions from the home area. The greater the density of individuals in the home area, the more quickly distant areas are invaded, and the farther away do individuals move. NORTH e 200 EAST 87 records) SOUTH FIG. 10-2 Relation of breeding localities to birthplaces among robins. Numerals on concentric circles are distances from the origin in miles (1 mile = 1.6 kilometers). Data points beyond the 200-mile radius are not placed to scale (Farner 1945). A common method of achieving dispersal is the broadcasting of enormous numbers of eggs, spores, encysted stages, or young so that they scatter into a wide variety of places in a more or less random man- ner. Those that come by chance into suitable environ- ments persist and become established ; those that enter into unfavorable locations are destroyed or never de- velop. Broadcasting is a wasteful procedure; it is especially characteristic of aquatic species. The fresh- water clam annually produces hundreds, perhaps thousands, of eggs. Only two fertilized eggs need ma- ture that the two parents be numerically replaced when they die, and thus the population of the species be maintained at a constant level. Contrastingly, in those forms, such as birds and mammals, that have developed a high degree of parental care, the number of eggs or young annually produced is commonly a half-dozen or less, and the offspring exercise con- siderable discrimination in their choice of suitable habitat. Although the dispersal of broadcast offspring is not under the control of the parents or the young, it is not often truly random. Water and wind currents, and other agents of dispersal may channel eggs and spores in restricted directions. Such dispersal is described as passive conveyance. In streams, all agents of passive conveyance, except some other ani- mals, direct dispersal downstream. Upstream dis- persal must be the result of active locomotion. Eggs of insects, snails, fish, or other aquatic or- ganisms will sink unless they are buoyed up by cur- rents, possess flotation mechanisms, or are attached to some floating object. Logs, masses of vegetation, and other debris are sometimes torn loose from the banks of rivers and float out to sea carrying the smaller animals attached to or trapped on them. It is estimated that over 300 debris rafts of significant size are formed each century and float out to sea from the mouths of the larger tropical rivers (Matthew 1915). The passengers on such rafts may come to be colonists of remote islands or even other continents. The dispersal of fresh-water animals from one river to another is sometimes effected by erosion, when the process permits one river system to rob a branch of another river system (Crosby 1937). Fol- lowing the recession of the continental glacier in North America, the proglacial Great Lakes had outlet down the Illinois and Mississippi Rivers. Later, a new outlet was established over Niagara Falls, and the Lakes became a part of the St. Lawrence River system. Wind acts in a manner somewhat similar to water currents. Some terrestrial spiders have evolved a spe- cial mechanism allowing them to use mild air currents for dispersal. The young climb up on a clod of earth or other object and spin out long threads or flocculent masses from glands in their abdomen. This continues 146 Ecological processes and dynamics until enough buoyancy is created to lift the spider and carry it away, sometimes distances of hundreds of miles. Strong winds will often carry insects and even birds great distances away from their usual courses. Crop pests may be blown North during the summer and cause damage, but may never become permanently established because they are killed by the northern winter cold. Hurricanes are an important means of colonizing islands far at sea with terrestrial species (Elton 1925, Darlington 1938). There are authentic records of rains of fishes and other aquatic species that were sucked up and transported appreciable distances by tornadoes (Gislen 1948). In studies done in England (Freeman 1946), it was estimated that the number of insects drifting through a rectangle 91 m (300 ft) high and 1610 m (1 mi) long amounted to 12,500,000 per hour. The number was highest during May, June, and Septem- ber, at temperatures above 18°C. The aerial popula- tion over the forests and swamps of Louisiana has been measured (Glick 1939), by means of traps placed on the wings of airplanes, and found to average the following number of individuals per 1000 cubic meters of air: Altitude Daytime Night 6 m (20 ft) 10.3 — 61 m (200 ft) Sy? — 152 m (500 ft) — 6.2 305 m (1000 ft) 22, 2.6 610 m (2000 ft) ei 1.1 914 m (3000 ft) 0.6 0.5 1524 m (5000 ft) 0.3 0.4 Diptera were most numerous, followed during the daytime by Coleoptera, Homoptera, Hymenoptera, Araneida, Hemiptera, and others. Spiders and wing- less insects were greatly reduced in numbers at night because of the lack of vertical convection currents. The density of insects in the air depends on the di- urnal and seasonal activity rhythms of the animals to give them exposure to air currents in their terrestrial habitats. Those species tend to be most numerous that have the greatest wing area per unit weight, hence the greatest buoyancy. The insect population in the upper air is not distinct from that flying in the lower layers, but its density decreases with height in a well-defined logarithmic manner up to and over 1500 m (5000 ft) (Johnson 1957, Taylor 1960). Im- proved methods for obtaining aerial densities employ a suction pump to strain known volumes of air per unit of time (Johnson 1951). Other animals serve as vehicles for passive trans- portation. Bits of vegetation, small animal life, and the eggs of worms, entomostracans, rotifers, insects, snails, and fish may cling to the feet of such water- birds as ducks, rails, and herons and be carried many i) fe) DROPLETS PER |Ocm* a ° 0 0 10 20 30 40 50 60 DISTANCE FROM CENTER OF SPLASH, cm FIG. 10-3 Decrease in concentration of water droplets beyond 5 cm from the center of splash (Wolfenbarger 1946, after Faulwetter). miles when the birds migrate. A species of Succinea snail, native only to St. Croix and Puerto Rico, was found alive in the feathers of a bobolink shot in Cuba. Ferrissia snails have been found attached to the wing covers of aquatic beetles and sphaeriid clams clamped on their legs. These insects occasionally fly from one body of water to another. Some dispersal movements are determined by the manner in which the animals respond to environmen- tal factors. Such directed movements, or taxes (p. 12) may induce the dispersal of animals upstream positive to current rather than downstream. NUMBER OF LARVAE RECOVERED O | 2 3 4 DISTANCE OF RECOVERY, CORN ROW FIG. 10-4 Effect of increase in population at source (200, 500, 1000 eggs deposited) on the dispersal of European corn borer larvae (Wolfenbarger 1946, after Neiswander and Savage). Dispersal, migration, and ecesis 147 FIG. 10-5 Response of the hypotrichian Oxytricha fallax to heat. The slide is heated at X. An Oxytricha at station | changes position as indicated by the arrows, repeatedly moving back- wards, turning to the right, then moving forwards. Finally, at stations 13-14, it directs itself away from the heat, and moves in a straight path towards a cooler region (from Jennings 1906). Trial and error is involved to a considerable ex- tent in the dispersal movements of animals. If, ini- tially, an individual progresses into an unfavorable habitat, it may be able to withdraw and proceed in a new direction. This may continue many times until by chance it discovers an area that is favorable. Trial and error movements are manifested in all groups of animals. In higher types, and even in many lower types, the individual may learn by experience and re- duce the number of false trials that it makes. Thus initially random movements may eventually evolve into directed behavior patterns. Barriers Dispersal continues until a barrier that makes further movement difficult or prevents successful col- onization is encountered. Barriers are of different sorts and are classified as physiographic, climatic, and biotic. The difficulty of surmounting or bypassing any type of barrier varies greatly among species. River valleys, for instance, may be barriers to animals frequenting mountains; but to lowland species, river valleys are important dispersal highways. To fresh-water organisms in river and lake habi- tats, intervening land masses are usually effective bar- riers. The headwaters of different river systems may lie only a few miles apart, yet contain quite different species because the continuous water route down to the mouth of one river and back up to the other may be a distance of several hundreds or thousands of miles. A waterfall may be a barrier for all non-flying aquatic species, and even riffles or swift water may prevent upstream dispersal of pond or lake species. The salt water intervening adjacent rivers flowing into the sea is a barrier to most fresh-water forms that might otherwise invade one river from the other. Terrestrial organisms are hemmed in by a great variety of barriers. The oceans constitute the major physiographic barriers since they separate the faunas of the several continents and isolate islands from each other. Lakes are not effective barriers because they can be readily skirted, but wide rivers coursing long distances between banks of dense vegetation, as does the Amazon River through the tropical rain forests of South America, may limit the range of forest mam- mals, butterflies, flightless beetles, land snails, and even birds (Mayr 1942: 228-9). The Grand Canyon of the Colorado River separates the ranges of the Kaibab and Abert squirrels and several other species, even in country that is semi-arid and open (Goldman 1937). Mountains are sometimes considered barriers to lowland species, and valleys barriers to mountain forms. This is only true if the change in climate and vegetation that such barriers produce are unfavorable to the species. Deserts are important climatic barrters since they are hot, and dry. Temperature affects animals di- rectly, since species have definite limits of tolerance, comfort, and efficiency. Precipitation is important be- cause it controls the type of vegetation that occurs in a region, and by so much the animals adapted to that type of vegetation. A low relative humidity may di- rectly limit the dispersal of moist-skinned species. Excessive solar radiation limits some species to forest habitats, excluding them from open country ; without recourse to shade, such animals would experience overheating and critical loss of water from the body. Short photoperiods may limit distribution northward _ 148 Ecological processes and dynamics during the winter, especially if low temperature ob- tains, and may influence southward distribution of northern animals in the summer. The length of the season between spring and autumn killing frosts, or between dates when its limits of temperature tolerance are reached in the spring and autumn, may determine whether or not a species can complete its life cycle at a given latitude. A corollary consideration is whether the accumulation of heat is sufficient to furnish cold- blooded animals and plants sufficient energy for growth and reproduction. Biotic barriers consist of changes in vegetation, food, competitors, and predators. The adaptations and behavior patterns of many animals fit them to niches in specific types of vegetation; should the previously amenable vegetation change, the animals may have great difficulty in adapting to it. Tree squirrels, for instance, are replaced by ground squirrels in prairies and deserts. Most animals have a great adaptibility to food, so food is not so limiting a factor. Some insects, however, such as certain aphids, are narrowly limited to particular species of plants as a source of food. Where their food plant is not present, they can- not exist. Competition between species is also a potent force in controlling distribution. The boundary between the ranges of the house wren and Bewick’s wren in eastern North America is not sharply defined, varying as a function of competition between the two species. Either species can live in the range of the other, but in the North, the house wren usually wins in com- petition for territory and nest-sites. In the South, the Bewick’s wren is more successful (Kendeigh 1934). Predators, such as the great horned owl and the swifter hawks, tend to urge the bobwhite to confine itself to a forest-edge habitat, where it is less vulner- able to attack. Trypanosome parasites carried by the tsetse fly are effective barriers against successful in- troduction of domestic ungulates in certain parts of Africa, and the rabbits introduced into Australia have limited the range and greatly reduced the abundance of several species of native marsupial. Dispersal of young The dispersal of a species is primarily accom- plished in the immature stages. This is obviously true of eggs and spores, but banding and marking studies have shown that among the higher animals—birds and mammals—it is also the young which disperse the species. Once a bird has reached sexual maturity and nested, it has strong tendencies to return to the same area in following years. The distribution of young birds is not random, however, as they tend to return to the general vicinity of their birthplaces rather than uniformly over the range of the species. Thus only 0.5 per cent of 557 adult house wrens recovered a year after handing (Kendeigh 1941b) nested farther than 3.3 km (2 miles) from the site where they had nested the year of banding, but 15 per cent of the 182 birds banded as nestlings were recovered at greater dis- tances, the longest of which were 32 km (20 miles), 56 km (35 miles), 80 km (50 miles) and 1120 km (700 miles). Dispersal distances for young of other species are proportionally comparable (Haartman 1949), Of small mammals, it is characteristic that once an individual has selected a homesite, it rarely leaves it for another (Burt 1940, Blair 1953). It was observed that in the months following the time at which they had been captured, marked, and released, 95 per cent of 133 adult woodland white-footed mice resumed habitation within 183 m (200 yd) of the site of cap- ture. Rate of dispersal If dispersal from birthplace were typically lim- ited to one direction, then a simple mean of the dis- tances to which the young disperse before they breed would give the dispersal rate per generation. It is the case, however, that dispersal proceeds peripherally in all available directions and may extend to surprising distances (Bateman 1950). The area exposed to invasion and the average time required to saturate that area increase proportionally as the square of the linear distance (d?) from the center, since the total area within which the individual could settle is 7d?. Therefore, the equation | Sd? n for computing mean dispersal distance seems correct (Haldane 1948), although other equations have been suggested (Haartman 1949, Burla et al. 1950, Dice and Howard 1951). Consider the data on house wren nestling recovery distances, presented above. Exclud- ing the truly extraordinary distance of 1120 km, and observing that only about 93 per cent of young female wrens nest when they are one year of age, we compute by this equation an annual dispersal rate of approxi- mately 8 km (5 miles) for this species. The mean dis- persal distance of one group of 154 young woodland white-footed mice (Burt 1940), according to the above equation, is about 176 m (192 yd). However, mice born in the spring mature sexually very quickly and breed in late summer or autumn, so the annual dispersal rate must be somewhat greater than this figure indicates. Dispersal, migration, and ecesis 149 Our dispersal rate data so far have described the outward diffusion of a local population through an area already occupied by the species. Once it has sur- mounted a barrier the dispersal rate of a species into an area previously unoccupied by it should be faster. The European starling was introduced into North America about 1890. From a central locus around New York City it spread at an accelerating pace until, in 1940, it had become established over 6,500,000 sq km (2,500,000 sq miles) (Wing 1943), a mean rate of about 130,000 sq km (50,000 sq miles) per year. With amelioration of the climate in Finland during recent years, the lapwing spread northward between 1899 and 1954 at a mean annual rate of 7 km (4.3 miles) (as computed from Fig. 9 in Kalela 1955) ; the roe deer, between 1850 and 1945, at a mean annual rate of about 9.5 km (5.8 miles) (computed from Fig. 1 in Kalela 1948). The Norway rat invaded south- western Georgia and virtually displaced the previously established black rat at a rate of about 430 sq km (167 sq miles) per year (Ecke 1954). By way of contrast, it has taken the fresh-water amphipod Gam- marus pulex the last 6000 years to disperse across 12 river systems from southern England into Scotland (Hynes 1954). Causes of dispersal It is the case that the reproductive rate of any species is so great that if all offspring survived the world would be overrun with that species within rela- tively few generations. Because species produce a sur- plus of young in most years, there is continuous pres- sure on individuals to move into all suitable niches, and to seek out new areas in which to settle. The im- pact of large numbers of individuals struggling for survival is described as population pressure, and is doubtless the most potent force inducing dispersal. It should be recognized, however, that population pres- sure is not uniformly constant year after year. When because of poor breeding conditions or catastrophe there is a reduction in the over-all population of a species, that decimated species may withdraw into its optimum habitat and be less put upon to exploit new or less desirable areas. In years favorable to the pro- duction of large surpluses of young, a species will often be found in less favorable habitats, even regions it would not otherwise occupy at all (Kluyver and Tinbergen 1953). The broadcasting of eggs or off- spring, or the passive conveyance of them to other re- gions, varies directly with the size of the population producing them, and is hence as much an expression of population pressure as the active search for new areas engaged in by individuals under their own loco- motion. Animals cannot disperse successfully, if at all, into 150 Ecological processes and dynamics new areas to the characteristics of which they are not | structurally, functionally, and behaviorally adapted. If an area the characteristics of which have excluded a species changes so that the species is adaptive to it with the equipment it has, that species can success- fully invade. The American robin, song sparrow, chestnut-sided warbler, house wren, and_ prairie horned lark have invaded Georgia only in recent years as the logging of forests, initiation of early seral, grassy, and shrubby stages, and extensive general cul- tivation of the land have produced habitats meeting the requirements of these birds (Odum and Burleigh 1946). If an area the characteristics of which have excluded a species remains unchanged but the species acquires new structural, functional, or behavioral traits by which it can adapt to that area, the species can in- vade the area. If its newly acquired traits let the spe- cies remain adaptive to its former range, its range is expanded by inclusion of the newly-invaded area. If the food supply fails, homesites or vegetation be destroyed, or a pernicious change in climate occur there, animals may be forced to leave an area to which they were well-adapted and disperse, more or less temporarily, into an area to which they are less well-adapted. The snowy owl, for instance, depends heavily on lemmings and mice for food in its usual range, the Arctic tundra. In apparent correlation with the cyclic decline of the lemming population there, the owls invade our northern states. DISPERSAL PATHWAYS Theories of how animal and plant groups have dispersed over the face of the earth are based fundamentally on the hypothesis that the continental land masses were at one time intimately connected and only later drifted apart to their present locations, or the hypothesis that the continents have been perma- nently fixed in their present positions throughout geo- logical time. Continental drift theory This theory postulates that throughout the Pa- leozoic and most of the Mesozoic the presently dis- tributed continents were grouped into two great land masses: a northern Laurasia was separated from a southern Gondwana by the vast sea of Tethys, al- though narrow connections between the two might have occurred for short periods. In the Cretaceous the land masses fragmented, and the fragments sub- | sequently drifted apart. Laurasia is supposed to have | split into North America, Greenland, Europe, and — most of Asia; Gondwana, into South America, Africa, Arabia, Madagascar, India, Australia, and Antarctica (Wegener 1924, DuToit 1937). This theory is sug- gested by the shapes of the continents that could con- ceivably be fitted together; by the characteristics of the Atlantic Ocean basin that makes it appear to have been formed by a rifting apart of land masses ; by the similar geological stratigraphy shown by invertebrate fossils in South Africa and South America; by some similarities of present-day fauna and flora at the same latitudes on different continents; and by the difficulty tropical species have dispersing over arctic land bridges. Perhaps we should keep an open mind towards this theory (Wolfson 1955), but the weight of present-day evidence, both geological and biologi- cal does not strongly support it (Mayr 1952). Stability of continents Although the major land masses and ocean depths have probably remained substantially un- changed since life first originated, the continents have been repeatedly flooded to various extents by inland seas, elevated, and eroded. Continents now separated have been connected by land bridges in times past; one of the best known connected Asia and North America at the site of the Bering strait. A land bridge now connects North and South America at the isth- mus of Panama, and Eurasia and Africa are connected by the isthmus of Suez. Land bridges may exist for only a short time in the geological sense, but they serve as important dispersal routes for those land ani- mals and plants able to cross them. When a land bridge allows free passage of most animals and plants in either or both directions it is called a corridor (Simpson 1940). If the land bridge is narrow, has an unfavorable climate, a lack of suitable niches, or too many competitors, it is called a filter—only a few specially adapted species are able to pass over it. A sweepstakes route of dispersal is one over which only a few species pass, more or less by chance. Because of its general unfavorableness, it is generally only one- way. The island-hopping dispersal of organisms from southeastern Asia through the south Pacific has been accomplished by relatively few species; the more dis- tant the island, the fewer the species that have reached It. Mountain barriers are not permanent; in the course of time they erode away. New mountains may rise again in the same area or elsewhere. With such changes in physiography come changes in climate and vegetation as well. The explanation of present-day distribution of animal groups requires a knowledge of past changes in the geological history of the earth, as well as in present day characteristics of continents, oceans, and climates. A glance at a globe quickly shows that the con- tinental land masses are concentrated mostly in the northern hemisphere. There are three broad land ex- tensions southward below the equator: the Malay Peninsula, East Indies, and Australia; Africa; and South America. The southern hemisphere otherwise consists largely of vast expanses of oceans. On a land route, North America intervenes South America and Eurasia. Eurasia is the largest continent, is central to all the others, and always has had a great diversity of climate and terrain. The size, arrangement, and posi- tions of the continents are of importance to interpreta- tion of the past evolution and dispersal of animals. Considerable evidence (Matthew 1915) indicates that some large groups of animals, notably mammals, first evolved in Eurasia and then spread to other parts of the world. North America was a less important center of origin and dispersal. Periods of aridity and glaciation are known to have occurred in the northern continents during the Permian, at the end of the Tri- assic, at the beginning and end of the Cretaceous, and during the Pleistocene. Intervening warm, moist, uni- form climates prevailed in the early Carboniferous, Jurassic, mid-Cretaceous, and Eocene. During periods of continental emergence, climates on the northern continents changed from moist and warm to more arid and cold. Animals adaptively lim- ited to moist, warm environments were restricted to tropical regions or dispersed outward into the south- ern land extensions. New, more advanced animal types adapted to the new conditions in the north ap- peared. Monotremes entered Australia at an early date. Marsupials probably originated in Eurasia and dispersed into Australia and South America during late Mesozoic or early Tertiary, although there is no fossil evidence for these suppositions (Darlington 1957). An opossum still occurs in North America, but no marsupial is now present in Eurasia. Marsu- pials did not get into Africa, but are present in South America and have adaptively radiated into a variety of forms in Australia. The predominance of marsu- pials in Australia is probably due to their chance suc- cess, at an early date, in surmounting the sweepstakes route of islands from the Asian mainland. They were the only placental forms in Australia until bats and members of the rat family arrived, much later. There have been successive waves of dispersal of higher mammalian forms into Africa and South America, but many recently-evolved mammalian types are largely confined to the northern hemisphere, where most of the primitive forms there have long since become extinct. The origin and dispersal of birds may have followed the same pattern as that of mammals, but their geological history is less easily traced because birds are not easily fossilized. A modification of Matthew’s theory appears neces- sary, at least for the cold-blooded vertebrates. Fresh- water fishes, amphibians, and reptiles are most richly Dispersal, migration, and ecesis lar) “(2961 uojBuljieq 4ayye) soidosy pjioay PIO 94+ ul UIBllo yo sajued e w e1jeagsn ue ‘saipuy 4seqz out jo awos ‘seosebepey;w 4oj $deoxe—pjiom ay} seco (ojng) speoy $9 [essedsiq 9-9) “O]4 fuscitoris x fuscocephalus theileri Autchinsoni Neoculex vagans and allies perexiguus sinensis gelidus anf borraud/ groups _—— ASIA} guiarti moucheti and related groups musarum \ine argenteopunctatum simpson/ \ine ninagongoen- sis \ine pipiens univittatus grahami \ine tams/ \ine mirificus FIG. 10-7 Phylogeny and dispersal of the mosquito genus Culex developed in tropical regions: all are definitely handi- capped in dispersing into temperate or cold regions by the climatic barriers. Fresh-water fishes have evolved a richer north temperate and arctic fauna than am- phibians and reptiles, since their aquatic habitat pro- tects them better against extreme cold than does the terrestrial or semi-terrestrial habitat of the other two groups. Amphibians extend farther north than do reptiles. Evaluating all types of evidence, Darlington (1957) concludes that these three groups of cold- blooded vertebrates, and probably also the warm- blooded groups, reached their greatest taxonomic di- versification during the Cretaceous and early Tertiary, and not in temperate Eurasia but in the Old World tropics, especially in the Orient. Uniform, warm, humid climates and a great variety of available niches, along with many possibilities for geographic isolation, appear to have induced evolution in these groups, stimuli quite different than those suggested by Mat- 9-7 bahamensis < pipiens \ salinarius nigropalpus peccator erraticus restuons apicalis forsalis N. AMERICA-~_|! S. AMERICA| —— Mel/lanoconion and allies (Ross 1953). thew for the warm-blooded mammals. From the Old World tropics, dispersal proceeded into Africa, into Eurasia, across the Bering land bridge into North America, and finally across the Panama land bridge into South America. Subsequent evolutionary radia- tion of new forms occurred in each continent. A northern route between Asia and North Amer- ica by way of the Bering land bridge as a route of dispersal for tropical species presents problems in re- spect to climate. We know, however, that during the Cretaceous and early Tertiary the climate in these northern regions was much warmer than it is now. We may suppose that the bridge shut off the cold Arctic Sea from the Pacific Ocean, and that the south- ern shores of the land bridge were washed by the warm Japanese current. This would have made it possible for warm temperate species to use the bridge, but probably not tropical species, unless we suppose that the tolerance to cold of the ancestral stock of our Dispersal, migration, and ecesis 1538 FIG. 10-8 Paleogeography of the world during the Upper Cretaceous, showing land bridges and epeiric seas (Ross 1951). present tropical fauna and flora was greater than it is at the present time. Another difficulty that southern species would en- counter on the bridge would be the very long days of the summer and the very short ones of the winter. Tropical species are adjusted to fairly equal photo- periods at all seasons of the year. Seasonal differences in photoperiod are due to the inclination of the earth’s axis and there is no positive evidence that this inclina- tion has changed during geological time. Although it appears very likely that the Bering land bridge was an important route of dispersal between Asia and North America, considerably more study is required before we will satisfactorily understand how this was accomplished by various kinds of animals. Centers of origin The tracing of dispersal routes presupposes a starting point where the taxonomic group, whatever its size, first evolved. These starting points are called centers of origin. Various criteria for determining centers of origin have been suggested (Savage 1958), but caution must be exercised in applying them (Cain 1944). Of the many criteria proposed, the following two are especially important, although neither one is infallible : 1. LocATION OF GREATEST DIFFERENTIATION OF THE TYPE OR THE GREATEST VARIETY OF ENDEMIC RACES, SPECIES, AND GENERA, INCLUDING PRIMITIVE FORMS OR FossiLs. The older a group is and the longer it has occurred at a particular location, the more chance it has had to radiate into different habi- tats, become isolated, and evolve into new varieties. However, a shift of climate or a drastic change in physiography may render an original locality unhabit- able and the group moves elsewhere. Also, a group losing its vitality may contract its range into some area other than the one in which it originated. 2. CONTINUITY AND CONVERGENCE OF LINES OF DISPERSAL. Dispersal from a center radiates in all directions in which conditions are favorable and until insurmountable barriers are encountered. Lines of dispersal may be readily distinguished where one or more taxonomic characters can be traced from primi- tive or generalized types through more and more specialized types the greater the distance involved. However, once a species filters through a break in a barrier and invades an extensive, new, and favorable habitat, there may be increased evolution of new types, and a secondary dispersal center formed. This has happened repeatedly in the geological past, so there is often difficulty in distinguishing which center is the original one for a group. 154 Ecological processes and dynamics Island dispersal Dispersal of animals from continents to islands and from one island to another presents special prob- lems. Many of the world’s prominent islands occur on the continental shelf and are separated from the main- land only by shallow seas. At times of land emer- gence, as when glaciers lock up quantities of water as snow and ice enough to lower the level of the seas, these islands become connected by land bridges to the mainland, and dispersal of forms occurs. The British Isles have been thus connected to Europe; Japan, to Korea and Siberia; Sumatra, Java, and Borneo to Malaya; New Guinea and Tasmania, to Australia; and Newfoundland, to Labrador. On the other hand, the Bermudas, Azores, Hawaiian and other small Pacific islands, and possibly New Zealand, the West Indies, and Madagascar, could not have had _ mainland connections and thus have ‘received their present faunas by some means other than overland dispersal (Chapter 20). Islands adjacent to continents, unless very small or long separated, generally have faunas similar to that on the nearby mainland. Oceanic islands are more difficult to colonize, however, and often have unique unbalanced faunas or chance assemblages of species. Larger islands generally have a more varied fauna than do small islands; mammals, amphibians, and fresh-water forms are often absent or scarce. Fly- ing birds, bats, lizards, insects, snails, and small in- vertebrate forms easily transported on rafts or blown in by strong winds are better represented. Since island faunas are small in point of popula- tion compared with the mainland, there is less com- petition between species. A single genus or family may adaptively radiate into new niches to form a vari- ety of species or races, as did the group of finches in the Galapagos Islands observed by Charles Darwin and later studied by Lack (1945), and insects, honey- creepers, and other forms in the Hawaiian Islands (Zimmerman 1948). Because of lack of competitors and predators, primitive animals isolated on islands may survive long after their relatives on the mainland have perished, as has, for instance, the reptilian Sphe- nodon on New Zealand. Confinement of a species to a limited range permits extensive inbreeding so that the population becomes more homozygous in its vari- ous genetic traits, hence much less adaptable to new situations than are larger heterozygous populations. Traits that would be eliminated by predation pressure on mainlands sometimes become established in pop- ulations on islands. All of these conditions render oceanic island species liable to extinction by invasion of mainland forms, and renders them impotent to re- invade the mainland. Origin of North American fauna A dry land bridge connected North America to Asia across the Bering Sea during most of the Ter- tiary and during glacial periods in the Pleistocene (Hopkins 1959). During early Tertiary, Alaska had a temperate climate, but as the climate became pro- gressively colder in late Tertiary, there was increased filtering of animal groups having access to the bridge. At the present time all land connection has, of course, disappeared, although a few forms, particularly birds, are able to hop the narrow straits. Across this bridge came a heavy traffic of Asiatic mammals (Simpson 1947, Savage 1958), birds (Mayr 1946), reptiles, amphibians, and fish (Darlington 1957), modern in- sects (Ross 1951, 1953), and other groups. There was also some dispersal from North America into Asia, but this reverse movement was much less strong than the one from Asia into North America. During the early part of the Tertiary, all major groups of mammals appear to have moved freely across the land corridor so that there was considerable similarity between the two continental faunas. How- ever, from early Eocene to early Oligocene, new orders, families, and subfamilies arose that were dis- tinctive to each continent. Later in the Tertiary, there were fewer groups dispersing across the bridge, and these were largely confined to genera within the higher taxa already established on each continent. There is no evidence for a land bridge between North America and Europe via Greenland and Ice- land since early Tertiary, although it is possible that one existed earlier. A few animal forms may have been able to hop from island to island across the North Atlantic, but the fauna of North America and Europe are not sufficiently similar to suggest any recent close connection of the continents. Some pan- tropical forms may have crossed the Pacific Ocean from island to island to reach the Western Hemi- sphere, but former land bridges across either the Pacific or South Atlantic oceans are highly unlikely. A continuous land connection occurred between North and South American in late Cretaceous-Paleo- cene time. It was probably at that time that ancient types of mammals, birds, reptiles, amphibians, insects, and other groups got into South America from the north and differentiated into distinct families and other taxonomic categories (Dunn 1931). During most of the Tertiary, no land bridge existed between the two continents, although there were scattered is- lands separated by relatively narrow water gaps which some groups, particularly birds, may have been able to use. The land connection now in existence was apparently formed in the late Pliocene or Pleistocene. Previous to late Pliocene there were about 29 families of land mammals confined to South America Dispersal, migration, and ecesis 155 yrs: \y FIG. 10-9 Tertiary water gaps (hatching) between North and South America (Mayr 1946). and 27 confined to North America ; the two continents had not more than one or two families in common. During the Pleistocene, after the land bridge had been in existence for some time, 22 families were repre- sented on both continents ; 7 had dispersed from South to North America, 14 from North to South America, one is of uncertain origin. Some families have be- come extinct. The present faunas of the two conti- nents contain 14 families in common, 15 families found only in South America, and 9 families only in North America. Thus there has been considerable dispersal over the land bridge in both directions, but not a complete exchange or unification of faunas (Simpson 1940). The modern fauna of North America is thus de- rived principally from Eurasia and South America, and by autochthonous development. By autochtho- nous we refer to species evolved from very old in- digent types that may or may not be represented by related forms on other continents. The proportion of any local fauna that is derived from one or the other of these sources varies with each geographic area and with each group of animals (Table 10-1). Northward on the continent and in the western mountains, the Eurasian bird element is strongly represented. The South American element becomes greater southward, especially in the lowlands of California, Mexico, and Central America. Many Eurasian forms have dis- persed through North America into South America, but no modern forms, at least, of South American origin have been able to invade Asia through North America. MIGRATION Migration, like dispersal, involves move- ments and the invasion of new areas. Migration, as here defined, differs from dispersal in that it is a periodic movement back and forth between two areas (but see Urquhart 1958). In contrast, dispersal is a one-way outward movement. Migratory invasions of areas are temporary and repetitive, but invasions re- sulting from dispersal may be permanent. Migration is best known in birds, as an invasion of breeding area alternating with an invasion of win- tering area, annually. Representatives of other groups of animals also migrate, particularly mammals, fish, and insects (Heape 1931). Migration may be clas- sified as annual, diurnal, or metamorphic (Clements and Shelford 1939). Annual and diurnal cycles are correlated with the two most pronounced time cycles in the physical environment. Metamorphic migrations are movements from one habitat to another in differ- ent stages of an animal’s life cycle. Annual migrations Annual migrations may involve a change of lati- tude, or altitude, or be more local in extent. Latitudinal migrations may traverse only a few miles or may traverse almost from pole to pole. In terms of their occurrence in an area bird species are described as permanent residents, species represented in an area throughout the year even though some in- dividuals migrate; summer residents, species present only during the warmer part of the year, which in- cludes a breeding season that may extend from early spring to late autumn ; winter visitors, species present only during the winter or non-breeding period ; tran- sients, species ephemerally present only during mi- TABLE |0-! Analysis of geographical origin of the breeding bird populations of various communities in North America (Mayr 1946, Snyder 1950, Hensley 1954). Eurasian North American South American Unanalyzed Community Locality Species Pairs Species Pairs Species Pairs Species Pairs Western coniferous forest Rocky Mountains 65% 98% 17% 2% 6% + 12% + Boreal forest Maine, Ontario 52 20 30 79 3 + 15 iL Deciduous forest Ohio 28 23 32 52 28 23 12 2 Desert scrub Arizona 27 39 35 53 22 6 ey + Desert scrub California 11 14 78 49 11 37 0 0 156 Ecological processes and dynamics a a ee ee Ke BREEDING RANGE \\ FIG. 10-10 Migration routes, the longest known, of the arctic tern (Lincoln 1950). gration periods, neither breeding nor wintering in the area; and accidentals, species that are rare or irregu- lar in occurrence. The bird population in most com- munities reaches peaks during the vernal and autumnal aspects, as transients arrive and temporarily swell populations. The autumnal peak is usually the greater because adult populations are incremented by the large number of young birds hatched during the breeding season. Latitudinal bird migrations also occur in the southern hemisphere but are less conspicuous because WINTERING RANGE of the small continental land masses there and small populations of birds involved. There are many causes of bird migration, varying in relative importance with different species. Aquatic species must leave northern areas before their food supply is cut off by the freezing of the lakes, ponds, and rivers. Insectivorous species unable to change to other types of food must migrate before insects go into hibernation or disappear. The metabolism and food requirements of many song birds are so high that even Dispersal, migration, and ecesis 157 when food is abundant they cannot eat enough during the short winter day to give them sufficient energy to survive the long, cold winter night. Migration north- ward in the spring escapes the high summer temper- atures of the south and gets the migration into lati- tudes where the days are long (Kendeigh 1934). Whatever its immediate causes, migration presum- ably evolved because survival was more successful among those individuals that departed than among those that remained (Lack 1954a). The timing of migration is not usually regulated, however, by the factors just listed as causes, for most birds migrate days, weeks, even months before the beginning of intolerable conditions in the autumn and after they disappear in the spring. The annual stim- ulus to migration is complex and involves changes in physiological state, energy balance, and hormones (Kendeigh et al. 1960). The chief environmental- factor stimuli are changes in length of day and night and changes in temperature with the progress of the seasons. The regularity of migration by which species arrive at a given point about the same date year after year is probably a response to the regularity of change in day length. The fact that species may arrive a few days early or late of the usual arrival date is doubtless a result of the superimposed effect of variations in tem- perature, to which the birds are also responding. The mechanics of migration, fly-ways, flocking be- havior, migration routes, and so forth, are too intri- cate for detailed analysis here (Lincoln 1950). Much research is now in progress analyzing the factors in- volved ; let it be sufficient merely to add that migra- tory behavior is organized as an instinctive behavior pattern in the bird’s nervous system. If a stimulus is not presented, the behavior will not be expressed. The stimulus arises when the interaction between inter- nal physiological rhythms and environmental cycles reaches a critical stage. Annual latitudinal migrations are not limited to birds. Bison regularly migrated from northern parts of the Great Plains to pass the winter in southerly reaches, traversing a distance of 300 to 600 km (200- 400 miles). Some bats, particularly the hoary and red bats regularly migrate between Canada and the north- ern states. The fur seal breeds in the Pribilof Islands in the Bering Sea during the summer, and migrates southward as much as 4800 km (3000 miles) for the winter. Some insects migrate. The monarch butterfly breeds in the northern states and migrates several thousands of miles to winter as far south as the Central American tropics. A small proportion of in- dividuals successfully make the return migration in the spring. Evidence is accumulating to indicate two- way migratory behavior in other species of butterflies and insects (Uvarov 1928, Fraenkel 1932, Williams 1958). Migratory locusts or grasshopers occur both in the eastern and western hemispheres. Schistocera gregaria inhabits the arid grassland and semideserts of Africa and southern and western Asia (Uvarov 1928) ; Melanoplus mexicanus occurs in the northern Great Plains of North America. In both species, solitary and migratory phases occur which differ in points of size, wing length, and coloration. The mi- gratory phase apparently develops under conditions of higher temperatures and good breeding conditions so that over-populations occur. When migration begins, immense swarms of adult flying individuals move great distances. Migration in the nymphal hopper stage is more limited. Vegetation is ravenously de- voured wherever the swarm stops. Such migrations were extensive in North America between 1876 and 1879 when populations moved from the northern Rocky Mountain area into the states immediately west of the Mississippi River. Eggs were deposited at the terminus of the migration flights and at least some of the succeeding generation exhibited return flights in following years. Altitudinal migrations are movements of no more than a few miles up and down the slopes of mountains. By descending to lower altitudes in the autumn, an organism obtains some of the same benefits secured by those species that undertake latitudinal migrations ; i.e., less snow, higher temperatures, and more food. Birds restricted to alpine habitats in the summer are common winter residents of lowland areas. Some of the larger mammals, such as the mule deer (Russell 1932) and the American wapiti (Altmann 1952) have very regular migration habits in respect to herding, timing of movements, and migration routes. They move to the high alpine meadows soon after the vege- tation renews its growth in the spring, and come back down to the valleys in time to avoid the deep winter snows of the higher slopes. Local migrations do not necessarily involve a change of latitude or altitude and are often quite lim- ited in distance covered. However, in tropical grass- lands and savannas where wet and dry seasons bring great changes in available water, vegetation, and food, there is a great exodus of both mammals and birds during the dry season and an influx during the rainy season. The Atlantic salmon, after reaching sexual ma- turity, may ascend fresh-water streams in subsequent years to spawn and return each time to the sea. Many deep-water fishes spawn annually in shallow waters and then return to deep water again. Turtles come onto the land to lay their eggs; snakes disperse from their winter dens with the advent of warm weather in the spring; tree frogs go to small pools to mate and spawn; and resident birds move onto their breeding territories. Insects perform regular migrations both into hi- 158 Ecological processes and dynamics bernation and out of hibernation. In the autumn, for- est species migrate downward and may be found in peak numbers first in the shrubs, then in the herbs, then in their hibernacula in the ground. Many insects of the forest-edge, meadows, and agricultural crops also hibernate in the forest, usually a few meters in from the South-exposed edge where they derive some heat from the winter sun. These insects usually mi- grate into the forest in the same stratum, herb or shrub, in which they occur during the summer, then downwards into the soil. These flights into the forest occur with declining temperatures and are sometimes spectacularly large numbers of individuals. As they come out of hibernation in the spring the direction of movement the insects take is just the reverse that taken in the autumn; i.e., upward into their proper stratum, then horizontally outward into open country (Weese 1924). Daily migrations Ascent of plankton towards the surface at night and descent to greater depths during the day occur both in the sea and in lakes. The lake-dwelling culicid larva Chaoborus lies on the bottom during the day but becomes pelagic at night. Snails, slugs, and millipedes in the deciduous forest lie quiescent under logs or litter during the day, but come out at night to crawl around on the forest floor or even climb up on the vegetation to a height of perhaps a few meters. Al- though these excursions are restricted in range, they are more or less regular and periodic, and may be thought of as migrations. Metamorphic migrations Aquatic larvae and naiads of several orders of insects eventually change into adults that leave the aquatic habitat and become aerial. The adult stage is often short-lived. The eggs are deposited in water, or the immature stages return again to water to begin the cycle over again. The length of the cycle may be part or the whole of a year, or a longer period. The seventeen-year cicada is a well-known insect whose nymphs spend 17 years underground feeding on juices from the roots of trees. The adults appear above ground in large numbers in late May, mate, lay their eggs on twigs of various trees, and then disappear, all in a few weeks. The eggs hatch in about six weeks, and the nymphs drop to the ground and bury them- selves for another long period of years. The anadromous Pacific salmon ascends fresh- water streams but once to spawn. The breeders die; it is their offspring that return to the ocean to develop for a period of years before they make the migration. The two species of catadromous eels, of the western hemisphere and of Europe, migrate to the open sea in the region northeast of the West Indies in order to reproduce, The immature stage of the eel, not the adults, returns to the two continents. ECESIS Dispersal or migration of individuals into new areas is without great ecological significance un- less those individuals become established and can build up significant populations. The process by which or- ganisms become established in new areas is called ecesis. Ecesis will occur there if a species disperse into a habitat favorable to it, and if the species can then secure its proper niche or become adjusted to a new niche, new competitors, predators, parasites, and disease organisms. Ecesis, to go to completion, re- quires first the establishment of individuals in an area, then the growth of the population that they form, and finally or simultaneously, the maturing of com- munity structure with the invasion of many other species. Ecesis is often of a temporary nature. Temporary ecesis is the rule with migrant species, as the periodic change of location is normal in their life behavior. Ecesis as range expansion is not surely permanent until the species demonstrates that it can survive criti- cal years in weather cycles. For instance, northward dispersal of tropical forests is, after a point, thwarted by frosts that occur only at rare intervals. Insects may continuously expand their ranges for a period of years, only to be forced back hundreds of miles by a severe drought or cold spell. Growth of populations The growth of a species population from a single individual or pair of individuals is governed by the same laws that govern the growth of the in- dividual itself from zygote to adult organism. When either the size of the individual or the cumulative growth of the population is plotted against time, a characteristic sigmoid curve or logistic curve results. This phenomenon was first demonstrated by Verhulst in 1839. This curve has been used to describe the population growth of such diverse organisms as yeast, Paramecium, Drosophila, Tribolium, and man (Pearl 1927, Park 1939), and even the growth of commu- nities in particular habitats when many species are simultaneously present. Under natural conditions, however, growth of animal populations is subject to so many variable factors, including the change from one morphological stage to another in the life cycle of many species, and the change in the physical environ- ment both daily and seasonally, that the curve is often Dispersal, migration, and ecesis 159 Ww fo) fe) = {o) mS {e) INSTANTANEOUS PERCENTAGE GROWTH RATE Instantaneous percentage o ° Asymptote fo)) fo) oO fo) Point of inflection J “cumulative growth rate a) [o} 50 NUMBER OF INDIVIDUALS, 48 SWEEPS a fe) 2) fo} fe) | ! Jee | FEB|MAR| APR|MAY| JUN| JUL | AUG| SEP| FIG. 10-11 Annual ecesis of the invertebrate community in the herb and shrub strata of a deciduous forest. 223 + 266 + 218 + 273 + 272 Fe : = 250; a = loge 250 —3.5 __log e(20/3.5) _ ARI DOPE pa asp eee Ieee oe (425 = 01052) I+e not fully expressed, even though its trend is present inherently. The sigmoid curve shows that a population grows slowly at first, then at an accelerating rate which is at maximum at the point of inflection, after which the population continues to increase but at a decelerating rate, finally becoming stabilized at the upper asymp- tote. Most growth curves are symmetrical, and the point of inflection is one-half the value of the asymp- tote. The lower concave part of the curve is called the accelerating phase of growth and the upper convex part of the curve, the inhibiting phase of growth. If the number of new individuals added during each unit of time, absolute growth rate, is plotted against time midway in each period, a bell-shaped curve is obtained, the peak of this curve coinciding with the point of inflection on the sigmoid curve. However, the number of individuals involved in the absolute growth rate varies with the length of the time unit used, and the time unit of greatest signifi- cance varies from one species to another. Compari- sons of growth rate of different populations are diffi- cult unless instantaneous growth rates are obtained. The imstantaneous growth rate is the rate of growth at a point on a time scale and is usu- ally expressed in terms of increase per individual or unit biomass per unit of time. It cannot be measured, but it can be calculated from the logistic curve by the differential equation (Park 1939, Andrewartha and Birch 1954) dN IK == IN Te a ca where JN is the size of the populations at any time f; dN/dt stands for the instantaneous rate of change (dN) in the size of the populations during an interval in time (dt) and hence may represent the growth rate at any desired time on the growth curve; r is the biotic potential, innate capacity, or the intrinsic rate of in- crease per individual per unit of time in an environ- ment where there are no limiting factors ; and K is the maximum size of the population reached at the asymp- tote. This equation means that the rate of growth equals the potential rate of increase in the size of the population (yN), multiplied by the fraction of the maximum population size (carrying capacity) still re- maining to be filled (K — N)/K. In its integrated form, sv K Tel +e (a—rt) where the constant a is the natural logarithm of (K — N)/N when t is zero. In order to solve the equation for the logistic growth curve, it is necessary to determine the intrinsic growth rate, ry. In an environment without limiting factors, population growth is logarithmic. The factor r, the value of which varies with species, is the ex- ponent that indicates this growth rate. The elephant, for instance, has a very slow growth rate. It has been estimated, however, that if all offspring survived and in turn reproduced, a single pair could give rise to 19,000,000 elephants in 750 years. On the other hand, a single stem mother of the common cabbage aphid gives rise to an average of 41 young, and there may be 12 generations per year between March 31 and August 15. If they all lived, the progeny resulting would number 564,087,257,509,154,652 individuals in only 4.5 months (Herrick 1926). It is of considerable ecological value to determine both the maximum po- tential rate at which a species could increase under ideal conditions and the factors that prevent this in- crease from being realized. The intrinsic rate of increase, r, has been defined as the maximal rate of increase attained at any partic- ular combination of temperature, moisture, quality of food, and so on, when the quantity of food, space, and other animals of the same kind are kept at an optimum and other organisms of different kinds are excluded from the experiment (Andrewartha and Birch 1954 p. 33). Such an ideal environment may be set up under controlled experimental conditions. Actually, it is sometimes approximated under natural conditions during the very early stages of the accelerating phase of growth. Under such conditions the value of 7 may be approximated from the equation Poet log, (Nie/Nu) = roe N Thus if a population is doubled in a period of three 160 ~~ Ecological processes and dynamics weeks, or if the mean length of life of a generation is three weeks, then r = log, 2/3 = 0.2310 per individ- ual per week. A number of factors affect the intrinsic growth of a species: number of young at each reproduction, the number of reproductions in a given period of time, the sex ratio of the species, the age distribution of the population, their age at reaching sexual maturity, and so forth. The value of r has been obtained for com- paratively few species (Table 15-7 ; Edmondson 1946, Odum 1959, Solomon 1953, DeWitt 1954, Oliff 1953, Root 1960). In Fig. 10-11 the logistic curve has been fitted to the annual ecesis of the invertebrate community of the herb and shrub strata of a deciduous forest (raw data from Fig. 9-14). The upper asymptote, K, was ob- tained by averaging the five randomly fluctuating density values for June through October. The value of r was derived from the increase in community size for 28 days in February-March, but different values of r can be substituted in the integrated equation above until a curve is obtained that best fits empirical data. The value of r being known, the equation for in- stantaneous growth rate was solved for different parts of the curve to give the following values: March, 1.08 individuals per day; April, 3.55; May, 2.65; June, 0.59. By plotting intermediate times it appears that the highest growth rate, 3.87, comes about April 24, at the point of inflection of the growth curve, and also at the time of greatest absolute growth. y is an important constant, the potential rate of population growth with ecesis taking place on an area, where ideal conditions prevail. Neither such condi- tions nor, by so much, an actual rate equivalent to r are realized in Nature except that y may be infre- quently approximated in the initial phases of growth. The actual rate is best expressed by the instantaneous growth rate. It is necessary to take equivalent stages in the growth curves for making growth rate com- parisons between different populations. The point of inflection is of considerable significance in this regard ; it represents the same equivalent age of populations whether they respectively attain to the asymptote in a matter of hours, days, months, or year. The instanta- neous percentage growth rate 100 « aN /dt_ N declines progressively with time. In study of the process of population growth there are many advantages to working in the laboratory with populations of a single species held under experi- mental conditions where environmental factors can be closely controlled and varied at will (Park 1941). In such studies, the rate of growth of the flour beetle in experimental cultures has been found to vary with tem- perature, humidity, and light; according as whether fresh flour is added each day; whether competing forms or predators are introduced; and so on. The final population density turns out to be the same re- gardless of the number of beetles originally introduced into the flour, but it varies with the volume of the medium and other factors. The accelerating phase of growth is probably induced by frequent and successful mating contacts between individuals as populations increase in size. The inhibiting phase of growth is a result of decreasing food supply , accumulation of ex- creta which correlates with reduced fecundity of the adults ; lowered rate of metamorphosis of the imma- ture and increased mortality of the larvae, and can- nibalism of the adults on the eggs. Under natural conditions logistic growth curves for populations of single species are clearly evident when a species invades a new area that is favorable; when a species is recovering from a catastrophe or cyclic depression ; and as a species builds up its popu- lation in the spring after the termination of a winter dormancy or migration. The many factors in natural environments that modify rates of population growth and determine the levels at which populations become stabilized at the asymptote will be considered in Chap- ter 16. The total community is an aggregation of many species. When a bare area becomes receptive to prop- agation of life, only a few hardy plant species become established at first. These pioneer species react on the habitat by providing humus, food, shelter, and shade, making conditions in which other, more sensitive plant and animal, species can invade and become estab- lished. With many species present, interactions or co- actions between them bring about the establishment of dominance, influence, and complete community or- ganization. The result is a closed community. A com- munity thus fully organized discourages invasion by new species. It persists for the longer or shorter time until there is further change or development of the habitat permitting succession to a new community. Each invasion and ecesis of species to form new com- munities follows the sigmoid or logistic curve. Thus the process of growth and ecesis is much the same whether it is at the level of the individual cell, organism, species population, or the complex com- munity. Ecesis of plant communities, as recognized by the species of plant dominants, is often more rapid than ecesis of complete animal communities. For example, some six to eight plant stages may be recognized in the floodplain sere, each stage giving way to the next in an orderly succession (Chapter 8). However, ex- cept for a poor representation of forest-edge species along the banks of the river itself, there is only one Dispersal, migration, and ecesis 16] 400 300 200 100 0 | MAY FIG. 10-12 Seasonal growth and decline of a nest colony of common hornets, Vespa crabo, in France (Bodenheimer in Biol. Rey. 1937, after Janet). animal community. This animal community, the de- ciduous forest biociation, begins its ecesis with the establishment of the first trees, and with each ad- vanced stage in the plant sere more and more animal species invade. The establishment of the complete animal community is not attained until the late or final stages of the plant sere are reached. In the sand and pond seres there are three or four distinct animal communities recognizable that definitely succeed one another, but the ecesis of each animal community is not accomplished until there has been a succession of several plant communities. One must not expect, therefore, the complete ecesis of a distinct animal com- munity to correspond with each distinct plant commu- nity but only when there is a change and full develop- ment of a distinct type of vegetation. “| FEB | MAR | APR | MAY FIG. 10-13 Ecesis of nesting bird species in central Illinois dur- ing the prevernal and vernal aspects (compiled from Smith 1930). SUMMARY Dispersal of animals into new areas may be a range expansion of a species if individuals find un- occupied niches, are able to overcome competition, or are able to acclimatize and adapt to the conditions. Dispersal is chiefly by the immature stages that are broadcast randomly in all directions, conveyed pas- sively by wind, current, or other animals, respond to environmental factors by directed movements or taxes, or find their way by trial and error. Dispersal continues at measurable rates until a physiographic, climatic, or biotic barrier is reached. Factors that in- duce dispersal include population pressure, failure of food supplies or loss of favorable homesites, opening up of new areas elsewhere, and pre-adaptation for new and different niches. The origin of taxonomic groups of organisms may be traced to various centers, usually distinguished by the occurrence of the greatest amount of differentia- tion within the group and the convergence of lines of dispersal. According to Matthew, the major verte- brate groups first evolved in temperate Asia; accord- ing to Darlington, that event took place in the Oriental tropics. The present distribution of related forms over the face of the world has been accounted for by the continental drift theory and the likelier idea of dis- persal over land bridges that have periodically come and gone during geological time. The fauna of continental islands is derived mainly from the adjacent mainland over such bridges in the past, but the fauna of the more distant oceanic islands are often unique and unbalanced, and dependent on the accidental dispersal of miscellaneous species. The fauna of North America is derived principally from Eurasia, South America, and by autochthonous de- velopment. Migration, like dispersal, involves movements and invasion of new areas, but differs from dispersal in that the movements are periodic back-and-forth move- ments between two areas. Migrations may be annual or daily, or may be in the form of changes of habitat at different stages in a life cycle. Annual migrations may be latitudinal, altitudinal, or local. Annual migra- tion is best developed in birds but also occurs in mam- mals, fish, and insects. Ecesis is the establishment of organisms in an area into which animals have come by dispersal or migra- tion. This involves the establishment of individuals, the growth of populations, the invasion of more and more species, and finally the development of mature communities. The growth of species populations and of complex communities commonly follows a sigmoid or logistic curve that may be defined in mathematical terms. 162 — Ecological processes and dynamics Ecological Processes and Community Dynamics: Reactions, Soil Formation, and Chemical Cycles The building up of species populations in a new habitat brings various reactions of the organisms with the habitat which, together with coactions between different members of the community, bring about succession of communities. We need now to examine PLANT REACTIONS We have already considered, many of the reactions of plants such as reduction of light and wind intensities, mitigation of temperature extremes, inter- ception of rainfall, and increase in relative humidity. Plants also exert important effects on the formation, structure, and characteristics of the soil or substratum produced by accumulation of dead plant remains : they further the weathering of rock through acid excretion and the mechanical action of roots ; they offer obstruc- tion to wind- and water-borne materials; they help stabilize moving sand and talus slopes and help pre- vent erosion generally ; they variously increase or de- crease the water content of soil; they foster decom- position of raw humus into usable nutrients, and so forth. Water plants form marl. It is by these reac- tions that plants exert dominance in terrestrial com- munities, and establish the physical conditions of the habitat which must be acceptable to all minor plants and animals that dwell there. Succession of plant stages eventually brings the interactions between habitat and community into equilibrium upon the establishment of the climax (Weaver and Clements 1938). SOIL FORMATION Texture, porosity, consistency, arrange- ment of particles, chemical nature, and organic con- tent of soils are determined by three sets of factors: the parental rock material, the biota, and the climate. Differences in topography modify the relative effects of these three factors, and plenty of time is required before their full effects are realized. Parental rock The basic rock from which the mineral portion of a soil is derived determines, to a large extent, not only its chemical composition but also its structure. For instance, soils derived from limestone are highly calcareous and more alkaline than soils derived from sandstone. Clay soils are derived from feldspar ; sandy soils, from quartzite. Clay forms a finely-textured, compact, water-retaining soil. Sand is coarse-textured 163 and porous. Loam is a mixture of sand and clay and makes the best soil. The presence of iron oxides and silicates produces the red and yellow colors of some soils. Humus produces black soils. Soil from swamps or bogs and very rich in organic material is called muck. Residual soils are formed in situ from under- lying bedrock. Soils may, however, be formed in one locality and moved considerable distances. Soils trans- ported and deposited by wind are called loess; by water, alluvium; by glaciers, till. Biota Plants and animals have a highly important role in the formation of soil, both as they affect its struc- ture (Jacot 1936, 1940) and as they aid in the pro- duction of humus. Plants contribute to the mechani- cal and chemical weathering of rock. Plant roots, especially those of trees, can split large rocks. Lichens, mosses, and even bacteria and fungi excrete acids, in the course of metabolism, which dissolve the sub- stances that cement rock granules together. When plant roots die, fungi convert them to dry, soft, spongy material (punk), used as food by saprophytic micro-arthropods. Usually the bark of the root re- mains intact the longest. Hollow tubes are thus formed that permit water and air to penetrate con- siderable depths into the soil. These channels gradu- ally become filled with silt and animal excreta. The addition of plant and animal organic matter to heavy compact soils or clay tends to open the soil, making it more porous. Addition of organic matter to sandy soils binds the particles closer together, making the soil less porous. Earthworms may be divided into deep- and shal- low-working species (Kevan 1955). Deep-working species dig narrow tube-like channels which may reach 2-3 m down through overlying soil to parent rock. Earthworms ingest soil while burrowing, digest and absorb organic matter from it, and egest the residue in a semi-liquid form which is used to cement the walls of the burrow or else is deposited at the surface as castings. Earthworms prefer easily digested suc- culent vegetation and dung for the purpose, but in the autumn may pull the freshly fallen leaves down into their burrows to use as food or nest linings. Ejected petioles may form midden piles around burrow en- trances. In an undisturbed virgin prairie in Texas, earthworm casts made a layer 2-3 mm thick over the entire ground surface and when air-dried weighed about 2400 g/m? (10.7 tons/acre) (Dyksterhuis and Schmutz 1947). Earthworms are not, however, im- portant soil builders in disturbed grassland ; they may be absent altogether in arid regions. In other studies (Evans and Guild 1947), the dry weight of casts brought to the surface annually by earthworms varied from 475 g/m? (2.1 tons/acre) in a moderately hot dry climate, to 24,000 g/m? (107 tons/acre) in the White River valley of the Sudan, during the rainy season. Earthworm casts compared with the sur- rounding soil show higher total nitrogen, organic carbon, exchangeable calcium, exchangeable magne- sium, available phosphorus, exchangeable potassium, organic matter, base capacity, pH, and moisture equivalent (Lunt and Jacobson 1944). Only certain species make these surface castings ; other species void the ingested soil into subterranean spaces. The ant Lasius niger neoniger spends most of its time in its underground burrows and deposits ex- cavated soil upon the ground surface around burrow entrances. In an old field community in Michigan such deposits amounted, at one sampling, to 85.5 g/m? (750 lb/acre). However, entrances are aban- doned and new ones made,so that in the course of a few weeks a much larger quantity of soil is brought up (Talbot 1953). In the semi-arid Great Plains of western North America there is at least one species of ant that ex- cavates extensively underground and builds a conical mound of this excavated material above the surface. A single such mound weighs approximately 77 kg (170 Ib) ; there are as many as 50 such mounds per hectare (20 per acre) in some localities. Plainly, these little excavators move prodigious amounts of soil. The relatively sterile subsoil is gradually mixed with organic material and spread over the surface of the ground, thus increasing the depth of the fertile top-soil. Scarabeid beetles, bees, wasps, and in tropi- cal regions mound-building termites also move con- siderable subsoil to the surface (Thorp 1949). The crayfish Cambarus diogenes often occurs in poorly drained fields; it burrows down to the water table, sometimes a depth of three meters. Excavated material is brought to the surface and built into chim- ney-like affairs which may be 20 cm high and almost that much in diameter. Where crayfish are abundant, as much as 600 to 2000 g/m? (2.7 to 8.9 tons/acre) of soil per year may thus be moved (Thorp 1949). The burrows of prairie dogs and badgers may ex- tend 2 to 3 meters below the surface, and a single mound of excavated dirt weigh from 100 to 10,000 kg. Mounds made by pocket gophers and ground squirrels weigh from 7 to 180 kg each; it is not unusual to find 42 such mounds per hectare (17 per acre). These animals thus move from 7 to 9 kg of subsoil for each square meter of surface (30-40 tons/acre) in a period of several months (Taylor 1935, Thorp 1949). Large terrestrial animals trample the soil into greater compaction and destroy vegetation at sites where numerous individuals foregather ; around water holes in grassland where bison and antelope come to 164 Ecological processes and dynamics drink, for instance, or winter yards of deer and moose, trails on hillsides, wallowing places, and so on, These reactions are usually very local, however. As animals burrow and bring large quantities of loose soil to ground surface exposure, the likelihood of water and wind erosion destruction is greatly in- creased, especially true if the burrowing is done on hillsides where the flow of water is faster and where the animals always tend to deposit the soil on the downslope side of burrow entrances. On the other hand, the very same activities may decrease erosion where a soil is, in consequence, made more porous so that there is less water runoff. Humus In soil, organic matter that is partly or entirely decomposed is called humus. The amount of humus varies from less than one per cent to as much as 20 per cent of the soil; peat soil may be largely organic material, but much of it resists decomposition, and hence is not true humus. Decomposition breaks down complex organic compounds into simpler ones that are washed back into the soil, thus becoming available again as nutrients. On virgin prairie in Texas the ground litter of dead grasses and herbs amounted to over 300 g/m? when measured in April (Dykster- huis and Schmutz 1947). The annual dry weight of leaves that fall to the ground in deciduous and conifer- ous forests varies from year to year, from site to site, and with the density of the trees, but is in the range of 100 to 900 g/m?. In mature climax forests the rate of decomposition of the litter and re-absorption by plants of the nutrients thus yielded keeps pace with the annual accumulation so that an equilibrium is established. In seral stages, decomposition and utiliza- tion do not keep up with the annual accumulation so that the organic content of the forest floor increases with time. Under spruce, sugar maple, and birch in New Hampshire, the organic ground matter equals 3 kg/m?, but in Florida where high temperatures and rainfall favor rapid decomposition and leaching, there may be only 0.4 kg/m? under old growth longleaf pine (Kittredge 1948, Ovington 1954). The thick organic layer on the ground moderates extremes in the daily and seasonal rhythms of soil temperature, retards freezing of the ground in the autumn and thawing in the spring, and retains soil moisture. Because of humus formation (involving oxidation) and the respiration of plant parts and ani- mals underground, soil air contains little oxygen but much carbon dioxide, and it possesses a higher mois- ture content than does the general atmosphere above ground. This is especially marked in warm summer months when these processes go on more rapidly. The decay of organic matter usually makes the top soil somewhat acid (most commonly pH 5 to 7), but in the mineral subsoil, the acids are often neutralized by the basic salts commonly present. The mineral content of leaf fall varies according to the species of tree, but in the northern United States it averages about as follows (Chandler 1941, 1944) ; Hardwood Coniferous Element forests forests calcium 7.3 g/m? 3.0 g/m? nitrogen 1.8 2.6 potassium 1.5 0.7 magnesium 1.0 0.5 phosphorus 0.4 0.2 Silicon, copper, manganese, carbon, and zinc are also present in the leaves of hardwood trees. Carbon is relatively more abundant and nitrogen less abundant in coniferous than in deciduous leaves ; commonly the carbon/nitrogen ratio is 55:20 (Ovington 1954). Both plants and animals are important agents effecting the decomposition of organic matter and the formation of humus. An animal digests and metab- olizes plant foods, the total quantity of which is re- turned to the soil, in part as the excreta of the living animal, the rest as the body of the dead animal. Fully formed humus is, in fact, derived mostly of fecal ma- terial. The larger herbivorous and carnivorous ani- mals pass urine and feces containing simple nitrog- enous compounds and compounds of phosphorus, potassium, and traces of calcium, magnesium, sulfur, and other elements. Humus is but one point in a con- tinuous cycle of decomposition of plant and animal organic matter, absorption of decomposition products by plants, ingestion and metabolization of plant mat- ter by animals, decomposition of plant and animal or- ganic matter, —ad infinitum. The consumption by saprovores and herbivores of living and dead plant matter and the consumption of herbivores by carni- vores, neither add nor subtract from the total nutrient supply of an ecosystem. The chemical elements avail- able in the air, water, and soil of an ecosystem pass, in one compound and another, from one organism to another, and through one stage in the cycle after another, and they continue thus to circulate within the ecosystem unless and until they are physically withdrawn from it. To remove plant and animal crops from an ecosystem is to withdraw nutrients from it, and thus to reduce the fertility of the system. Fertility can then be maintained only if the nutrient supply is kept replenished by artificial fertilization. Kangaroo rats defecate promiscuously throughout their underground burrow systems. The soluble ni- trate content of the soil in the region of one burrow system averaged 221 ppm and in another one 570 ppm, compared with a maximum of 15 ppm in the Reactions, soil formation, and cycles 165 a ANIMAL ANABOLISM: (death) plant proteins——~ animal proteins —~ protoplasm AMMONIFIC ON ee ace death) =~ proteins——> ammonia (NH3) compounds Heterotrophic bacteria Actinomycetes, fungi Animal katabolism, excretions PLANT ANABOLISM: | a nitrates > amino acids——> proteins —~> protoplasm NITROGEN FIXATION: No——> NH3 (bacteria) (atmosphere) / DENITRIFICATION: NITRIFICATION: NO2—— No (bacteria) NH, compounds—> nitrites (NO9)—— nitrates (NO3) Autotrophic bacteria FIG. I 1-1 Steps and processes in the nitrogen cycle. surrounding desert soil generally (Greene and Rey- nard 1932). It is a reasonable estimate that the total bird population in a deciduous forest would deposit 0.1 g dry weight of organic excrement per square meter in a year’s time; the mammal population, per- haps 0.5 g; and the total invertebrate fauna, possibly 2-3 g. The accumulation of excrement under the roosts of birds is sometimes enough to kill the ground vegetation and even the trees (Young 1936). The guano deposits on the coast of and islands off Peru and elsewhere in the world were originally several meters thick, as the result of centuries of occupancy by nesting colonies of marine birds, but have now been largely depleted by man for use as crop fertilizer (Hutchinson 1950). Bat excrement, deposited in caves, was exploited in years past as a source of salt- peter for gunpowder. The conversion of raw organic matter into ma- terials suitable for re-absorption and utilization by plants is a complicated process and depends almost entirely on the reactions of plants and animals (Lutz and Chandler 1946, Waksman 1952). The digestion of animals produces both mechanical and chemical changes in raw humus that can be measured quanti- tatively (Franz and Leitenberger 1948). The non- nitrogenous substances in fresh litter are sugars, starches, pectins, pentosans, celluloses, cutins, tan- nins, lignins, oils, fats, waxes, and resins. Most of these substances are readily broken down in the soil by fungi, actinomycetes, bacteria, and protozoans, but tannins, lignins, waxes, and resins decompose very slowly. The end products of complete decomposition are H.O and COs, but sometimes decomposition is incomplete and organic acids are formed instead. The most important soil organisms concerned in the decomposition of the litter are the bacteria, both aerobic and anaerobic forms. They are commonly divided into two types. Heterotrophic bacteria obtain their energy from the oxidation of the carbohydrates and fatty substances as above described. They use this energy for the synthesis of cell substances and the production of enzymes that break down complex com- pounds in the litter into simpler compounds, including proteins into ammonia compounds. They then use part of the ammonia compounds in synthesizing the amino acids they need in building their own proteins. Autotrophic bacteria, in turn, are of two types : Chem- osynthetic species that obtain their energy from the oxidation of inorganic compounds (hydrogen, sulfur, hydrogen sulfide, iron, ammonia) and photosynthetic species, which include purple and green sulfur bac- teria, possess a form of chlorophyll, and utilize the energy of sunlight. Chemosynthetic bacteria convert ammonia compounds into nitrites and nitrates, part of which they use in their own anabolism, the rest becom- ing available for plants to absorb. Photosynthetic bacteria use the ammonia compounds in their own anabolism but do not render them directly available to plants. Chemosynthetic bacteria are more abundant than photosynthetic bacteria in soil; photosynthetic bacteria are the more abundant in water. Nitrogen cycle In the nitrogen cycle proteins are broken down yielding ammonia (NH3) compounds in the course of the metabolic processes of all animals and by the activities of heterotrophic bacteria, filamentous fungi, and actinomycetes. The process is called ammonifi- cation. Some of the ammonia is oxidized to form 166 Ecological processes and dynamics ANIMALS: (death) LO Decomposition of organic matter Carbohydrates, fats, proteins COp in air ae 4 ss -——* PLANTS: COg in water and Carbohydrates, fats, proteins soil; bicarbonates Volcanoes FIG. ||-2 The carbon cycle. nitrites (NO») and nitrates (NO 3) through the ac- tion of autotrophic bacteria; the process is called nitrification. Other types of bacteria act on ammonia in the process of denitrification, by which nitrogen (Ne) is liberated into the atmosphere. Nitrogen is removed from the air by the nitrogen-fixing bacteria which live either freely in the soil or as symbionts in the root nodules of legumes and some non-legumes ; Ceanothus, Elaeagnus, Alnus, and Myrica, among others. Some blue-green algae, fungi, and yeasts also fix nitrogen. Nitrates and perhaps also the simpler nitrogen compounds are absorbed and used by plants for the synthesis of amino acids and proteins. Ammonia compounds, nitrates, and other substances are added to the soil in small amounts with rainfall; sources of these nitrogen compounds are volcanic eruptions, terrestrial decomposition, and atmosphere nitrogen fixed by lightning. An attempt to estimate the quan- tities of nitrogen involved in the different parts of the cycle has been made by Hutchinson (1944). Carbon cycle Animals obtain much of their carbon, as well as nitrogen, from plants, although some forms are also able to fix carbon directly from salts dissolved in water (Hammen and Osborne 1959). In photosyn- thesis, carbon dioxide obtained from the air and from dissolved bicarbonates in the substratum is combined with water to form carbohydrates, a portion of which may be converted to fats. Plants combine carbon with oxygen, nitrogen, hydrogen, and sulphur to form proteins. Carbon dioxide in the air comes chiefly from the respiration of animals, but small amounts arise from the respiration of plants, the decay and fermentation of organic matter, springs, volcanic action, and solution of sedimentary rock. Volcanoes SN Limestone, coal, oil carbonates, etc. were probably the original providers of carbon diox- ide to the biosphere. Organisms tie up carbon diox- ide as carbonates in skeletons and shells. Carbon is also tied up in the formation of peat, oil, shale, and coal. When limestone and other carbonaceous sedi- ments are exposed to water erosion, the carbonates may be hydrolized to bicarbonates and thus become a source of COs. The concentration of carbon dioxide in the air is stabilized at 0.03 per cent by the buffering action of bicarbonates and carbonates in the oceans and fresh-water bodies (Hutchinson 1948) : COs — H,O 2 HCO; 2 HCO;— 2 COmm> On the other hand, oxygen in the air (20 per cent) is derived almost entirely from the photosynthesis of plants. Other elements In addition to oxygen, carbon, hydrogen, and nitrogen, animals require at least 13 other elements that are all derived from the soil: calcium, phos- phorus (Hutchinson 1948), potassium, sodium, chlo- rine, sulfur, magnesium, iron, copper, manganese, iodine, cobalt, and zinc. Only traces of some of these elements are required, but calcium is required in large amounts for skeletons, shells, antlers, and other or- gans, and in the metabolism generally. Phosphorus is a constituent of nucleoproteins, phospholipids, and skeleton. Goiter occurs in mankind and some animals in regions deficient in iodine. These elements are ob- tained from food, drinking water, salt licks, and grit taken into the stomach. A salt lick is a local, usually clayey, area characterized by a high concentration of salts where deer and other animals foregather to lick the soil for the salt. Soils deficient in or lacking these various elements support sparse animal populations ; individuals are in more or less poor health ; reproduc- tion rates are low (Albrecht 1944, Crawford 1950). Reactions, soil formation, and cycles 16/7 A2 FIG. I1-3 (a) profile of a podzol (mor) from sandy glacial till under coniferous forest in Maine (courtesy Charles E. Kellogg) (b) profile of a gray-brown podzolic soil (mull) formed from loess under oak-hickory in lowa. (c) profile of a chernozern formed from glacial drift under prairie in South Dakota—white spots are calcium carbonate. (d) profile of a sierozem, a desertic soil, derived from alluvium under sagebrush in Nevada. (e) profile of a latosol derived from gneiss under broad-leaved evergreen forest in Brazil. Pinholes and larger channels, formed by roots and insects, extend to 6 feet. Scale in feet and inches (courtesy Roy W. Simonson). The lettering along the margins of the profiles indicate the soil horizons (see page 170). An excess of some elements is harmful. Too much fluorine in drinking water causes mottling of teeth and possibly pathological changes. Selenium in soils of arid plains becomes dangerous when it reaches 0.5 ppm, since some grasses, asters, and cer- tain legumes absorb and retain it in concentrations that can be highly injurious to herbivorous animals. Wild animals have apparently learned to avoid eating these particular plants, but domestic stock blunder into them, eat them, and die (Knight 1937). Certain plants concentrate specific elements, a factor which may affect the food habits of animals. Black tupelo concentrates cobalt, and inkberry concentrates zinc to a much greater extent than do other species growing in the same areas (Beeson et al. 1955). 168 Climate Water, temperature, and wind are important weather factors affecting soil formation. Water is an agent of rock erosion and transportation, sorting, and deposition of soil-building erosion products. Water freezes and expands in cracks and crevices of massive rock structures, breaking them into frag- ments and particles. Daily and seasonal heating and cooling cycles produce cracking because of different coefficients of expansion of the minerals in the rock. Wind erosion is particularly devastating in arid re- | gions ; fine soil particles may be lifted and transported — many miles. Weathering of rock is a chemical as well as physi- Ecological processes and dynamics | cal process. Hydrolysis of some rock materials brings absorption of carbon dioxide and the formation of soluble bicarbonates. Hydration softens and increases the mass of some minerals, so that physical weather- ing of the rock bearing them is facilitated. Oxidation discoloring many rocks, especially those containing iron, is symptomatic of chemical changes in progress ; binding materials are weakened and crumbling oc- curs easily. Finally, many substances simply go into solution and are carried away. Where precipitation is frequent, water percolating through the soil carries soil nutrients to greater depths than where precipi- tation is light. In hot dry climates, organic matter may oxidize completely and so quickly its nutrients are lost to plants and microfauna. Reactions, soil formation, and cycles The climate prevailing there is a determinant of the kinds and prosperity of plant and animal life in an area. The biota has much less effect on soil for- mation in arid climates than in humid climates. By so much, desert vegetation is usually quite as locally distinctive as local soils are distinctive, but in humid regions, where many plant stages succeed one an- other, climax vegetations may be essentially the same regardless of whether the sere originally started on limestone, sandstone, or in a pond. Because of the interactions of parental rock, biota, and climate, dif- ferent soil profiles are formed, each characteristic of a specific climatic region and type of climax vegeta- tion. An understanding of soil profiles is prerequisite to understanding vagaries of animal distribution. 169 90 60 Wire grass Grama grass and 30 buffalo grass Bluestem sod-qrass Bluestem bunch-grass NUNN a me) WN il "4 y ; y LW oO jh si ie MOS We ay re Heike W/; TAI M WW ave ; 79) apn oy a) CAEN BI ROTI RARE Noa NITTANY i a coN RINK (an yy Mm 4 ae Tay NRE is f 30 ancien moist — = \\ | 2 Vi a i |W Naa ' vit iar ae S) (s{o) /; Wy yy el th ") us wh wi i hs ui : ae : Cy es nt ow? =u 90 eee, . eT Permanent ary : 120 Permanently ass 150 FIG. 11-4 Relation of lime hardpan to types of prairie vegetation extending from west to east in central North America (Shantz 1923). SOIL PROFILES As a result of the specific circumstances of weathering, biotic reactions, and climatic influences it has experienced, a mature soil has a definite struc- ture characteristic of its different environment (Lutz and Chandler 1946). The living plant draws nutrient materials from the deeper layers of the soil, but those materials are deposited on the surface soil, where the dead plant decays. Rain falling on the ground surface carries the nutrients and salts back down into the soil, at least as far as the water percolates. This sequence of events produces a definite layering of the soil. Each layer is called a soil horizon, and the series of horizons characteristic of a soil is called the soil profile. Horizons The A horizon is one of organic decomposi- tion and leaching; the B horizon, one of precipita- tion of materials carried down from above; the C horizon is the layer of parent soil material; the D horizon is the underlying stratum of rock or sedi- ment. Horizon A can be subdivided to reflect phases in the decomposition continuum. Ago, the L horizon, is fresh litter or litter only slightly altered. Ay com- prises an F horizon of fermentation where plant ma- terials are partly decomposed but still identifiable, and an H horizon where decomposition has brought the organic material into an amorphous mass. The A, horizon, directly below, is dark-colored and of flocculent texture, a mixture of organic material and the mineral soil. A» is light-colored and of coarser texture, since leaching is maximum. Occasionally an Az 1s recognized as a transition to B. The B horizon is sometimes also subdivided ; suffice it to say that the salts and humus leached out of A are deposited here. The horizon is often brownish or yellowish in color and columnar in structure. The A horizon is com- monly called topsoil ; the B horizon, subsoil. A mature or fully developed soil profile is char- acteristic of climax or late seral stages of a succes- sion. Horizons are not fully expressed in early seral stages, so these profiles are called immature or un- developed. Mature profiles are found only under virgin vegetation, for erosion and cultivation disturb horizons. Profiles are best developed in humid cli- mates where abundant precipitation carries humus and salts well into the soil. Hardpan In arid regions, evaporation may be in excess of rainfall. Moisture in the soil has no opportunity to percolate downwards; rather, it rises to the surface of the ground and is lost. Where rainfall is inade- 170 Ecological processes and dynamics quate for efficient leaching, hardpan may form in the B horizon as the result of deposition here of ferric oxide, alumina, colloidal clay, or calcium salts. This layer becomes so compact and hard that it is im- pervious to root penetration and the burrowing of animals, although during periods of wet weather it disappears temporarily. In arid regions, the hardpan may be at or close to the surface ; but in more humid climates it occurs at progressively greater depths until it disappears altogether. Mull and mor humus Distinction between mull and mor humus is made primarily for forest soils. Both humus types of soil may be subdivided but these subdivisions need not concern us here (Romell and Heiberg 1931, Lutz and Chandler 1946). Mull is a porous, friable humus layer of crumbly or granular structure and only slightly matted, if at all. The A, horizon is well developed; bacteria are abundant, annelids numerous, and_ nitrification oc- curs. Mor is a strongly matted or compacted humus layer. There is no A; horizon; the transition from humus layer to mineral soil is abrupt. The underlying soil is more acid, bacteria are much less numer- ous, and nitrification is usually reduced if not ab- sent. Abundant fungi reduce the raw humus to punky material; thereafter it is worked on by the small arthropods. Decomposition of mor is much less rapid than of mull. Annelids are few or absent, and moles are less common than in mull soils. Snails are scarce in acid soils. In general, the biomass of organisms inhabiting a mor soil is smaller, species are less di- verse, and individuals are smaller than in mull soils. Mor humus is common in cold regions and at high elevations, but is not limited to such climatic zones. It occurs especially under coniferous forests, TABLE |1-! Depth of distribution of soil arthropods in the Adirondack Mountains. The figures indicate approximate num- even in warm climates, and under ericaceous vegeta- tion. It is also found in very wet or very dry habitats where there is accumulation of poorly decomposed or sharply delimited humus layers on top of the min- eral soil, sand, or rock underlying. Mull humus com- monly develops in warm, humid climates and is found especially under hardwood or deciduous for- ests. Patterns of animal and plant distribution cor- relate closely with these two humus types; indeed, the formation of each is a result, in the main, of unique combinations of biota reactions, climate, and mineral soil characteristics (Romell 1935, Fenton 1947). Depth distribution of organisms The small animals in the soil, including proto- zoans and nematodes, are most abundant in the L, F, and H horizons, becoming rapidly less abundant in the mineral soil (Table 11-1). Bacteria, actino- mycetes, and fungi are also most abundant in these top layers, especially in F and H, although they occur well down into the B and C horizons. The depth distribution of soil animals depends on temperature and varies with season. When the top layers freeze during the winter months, much of the fauna keeps well below the frost line, although many species are tolerant of freezing. During the cold months the depth at which most soil insects, mol- lusks, and annelids occur varies from about 9 cm in silty clay-loam to 38 cm in gravelly clay-soil (Dowdy 1944). With the return of warm weather, the fauna ascends to the top horizons. SOIL TYPES The interrelations between the basic min- eral content of the parent substrate, biotic reactions, ber of individuals per m* in the total thickness of each layer (computed from data given by Eaton and Chandler 1942). Soil horizon Depth, cm Mites Springtails All others Mor humus under red spruce and balsam fir L&F 0-5 150,000 19,800 1,400 H 5-25 62,000 19,200 400 A, 25-33 1,500 0 0 B 33-58 1,900 100 0 Mull humus under beech, sugar maple, and yellow birch L&F 0-5 62,000 17,800 2,800 f 5-15 15,000 6,200 600 B 15-55 6,200 5,800 200 Reactions, soil formation, and cycles 17] and climate can be seen through an analysis of the development of the great soil types of the world. For the purpose, we will here adopt the simple, recent classification of Simonson (1957). Podzolic soils are formed in humid temperate climates, under forest vegetation. The A», horizon 1s moderately well developed, for there is sustained leaching. Soils are more or less acid and only mod- erately fertile. Podzols develop under coniferous for- est and have a mor type of humus. Gray-brown and brown podzolic soils are found under hardwood for- ests and have a mull humus. Latosolic soils develop in humid tropical or semi- tropical forested regions. Humus is quickly oxi- dized by action of microorganisms and hence does not accumulate. Chemical weathering of the parental ma- terial is intense. Water drainage through the po- rous soil is rapid, so leaching is extensive. In early stages of its formation, the soil is neutral or slightly alkaline, but as leaching continues, it becomes acidic. The soil has a thin organic layer (Ap and Ay hori- zons) on a reddish, leached soil (As horizon) that extends to great depths below the surface. Chernozemic soils occur in humid to semiarid temperate climates under grass vegetation. The grasses on dying return considerable organic matter to the soil. The A, horizon is consequently dark in color and of great thickness. The soil contains more bases and hence is less acid than in the two types above. The B horizon in humid regions is indistinct, but where there is less rainfall, calcium salts may ac- cumulate to form a hardpan. Prairie soils in temper- ate climates are among the most fertile soils of the world, but fertility decreases in the tropical and desert climates. Desertic soils are characteristic of arid climates and contain very little organic matter. A profile is poorly developed. The surface soil is brownish gray, and grades quickly into the calcium carbonate horizon which usually forms a hardpan just below the sur- face. Wind erosion removes the finer soil particles, leaving the coarser material to form a hard pavement. The soils are but slightly weathered and leached; lacking nitrogen, they are infertile. Mountain and mountain valley soils vary from shallow layers on eroding rocks to deep organic soils of valleys and swampy areas. Tundra soils occur in cold northern areas where the substratum remains continuously frozen and the vegetation of lichens, mosses, herbs, and shrubs makes a peaty surface layer. The region is poorly drained and characterized by many scattered shallow ponds. Alluvial soils, may be important locally. These soils are mostly without a developed profile and are the result of deposition by streams. They are usually very fertile and support luxuriant vegetation. Saline soils are found in dry climates where rapid evaporation of water results in surface deposition and accumulation of salts leached from surrounding up- land areas. These are only the main types of soils, but they are sufficient, however, to show considerable corre- lation with the geographic distribution of the major ecological communities, biomes. In a detailed classi- fication, many subdivisions and intermediate cate- gories would be recognized (Kellogg 1936, Lutz and Chandler 1946). REACTIONS IN WATER Considerable attention has already been paid to the reactions of animals and plants in streams (Chapter 5), lakes (Chapter 6), and ponds (Chap- ter 7). These involve changes both in the chemical and physical characteristics of the habitat and are fundamentally the same as occur on land. Water plants, especially those such as water lilies and water-hyacinths which float, and surface concentrations of both zoo- and phytoplankton reduce light intensities like forest canopies. There is ac- cumulation of plant and animal remains and fecal ma- terial on the bottom of the water bodies just as on land, and this material is worked over by bacteria and a large variety of micro-organisms which differ only in taxonomic composition, not in activities, from those on land (Henrici 1939). Water plants obstruct the flow of water and cause deposition of suspended materials simulating the reduction of wind velocities inside forests. An important physical reaction is the damming of streams by beavers so that ponds are formed. Such beaver activity can sometimes be use- fully co-ordinated with waterflow management (Beard 1953). These ponds eventually fill with silt and organic matter, succession occurs, and so-called beaver meadows are produced (Van Dersal 1937). The nitrogen cycle (Cooper 1937), and carbon cycle in aquatic ecosystems are essentially the same as on land. The absorption of oxygen by organisms and by the decomposition of organic matter in some lakes and ponds causes in the habitat a seasonal change of profound importance. Water conditioning All modifications in the habitat produced through the reactions of organisms represent a condi- tioning effect, whether the habitats are terrestrial or aquatic. The term has been used most commonly, however, in respect to changes produced in small bodies of water, especially under experimental con- trol. Water is conditioned when physical or chemical 172 Ecological processes and dynamics changes occur as the result of organisms living in it. Compared with unconditioned water, these changes may have either a harmful or a beneficial effect on organisms introduced into the water after the original organisms have been removed. Water is said to be homotypically conditioned when the changes were previously produced by individuals of the same spe- cies as being studied and heterotypically conditioned when the changes were produced by a different spe- cies. Harmful effects of conditioning on longevity, growth, or reproduction are more easily explained than are beneficial effects. Harmful effects are often consequences of oxygen depletion, reduction in food resources, accumulation of excreta, or secretion of toxins or growth-inhibiting substances. Over- crowding of frog tadpoles in culture dishes is asso- ciated with the occurrence of peculiar round vacuo- lated cells in the intestinal tract and feces that appear responsible for curtailment of further growth (Rose 1960). Under laboratory conditions, so-called killer stocks of Paramecium aurelia produce a toxin, para- mecin, at the rate of one unit-particle per animal per five hours. One unit-particle is enough to kill one individual of so-called sensitive stock of the same species as well as being lethal to other species of Paramecium (Austin 1948). Conditioning that be- comes unfavorable homotypically may sometimes be favorable, or at least tolerable, heterotypically. Thus in protozoan infusions there is a microsere of one spe- cies succeeding another. Experimental studies, on the other hand, have demonstrated that goldfish grow faster in water that has been homotypically conditioned for 24 hours than in unconditioned water. Both fish and amphibian lar- yae also do better in water conditioned by the pres- ence of mollusks than in unconditioned water (Shaw 1932). The marine flatworm Procerodes wheatland1 will survive much longer when transferred to fresh water conditioned by the presence of either live or dead individuals of the same species or by fresh-water species of flatworms than they do in unconditioned fresh water. The longer survival in toxic solutions, faster growth, and greater reproduction of protozoans, snails, flatworms, cladocerans, amphibian larvae, and fish occurring in aggregations rather than as isolated individuals is attributable to water conditioning. Various factors are involved in producing favor- able conditioning: minute organic particles in sus- pension resulting from excreta, regurgitated food, or disintegration of dead animals previously present may become concentrated in the alimentary tract of the animals and serve as an unsuspected food re- source (Allee and Frank 1949); mucus or slime secreted by organisms may coagulate, precipitate, or reduce the potency of toxic substances ; salts liberated from the body may change the osmotic properties of the culture medium; or there may be liberation of growth-promoting substances from one animal that affects other animals (Allee et al. 1949). Many of these effects, both favorable and unfavorable, are doubtlessly at work in natural habitats and should be carefully studied as part of the internal dynamics of the biotic community. It may well be, for instance, that during the course of evolution organisms have become adapted to tolerate or take advantage of these external metabolites given off by their neighbors with the result that the metabolites have become an im- portant part of their environment (Lucas 1947). SUMMARY The characteristics of soil are determined by the parent rock material, the reactions of plants and animals, and climate. The burrowing of earth- worms, ants and other ground insects, crayfish, and rodents brings subsoil to the surface where it be- comes mixed with humus. Animal metabolic proc- esses aid in the formation of humus by breaking down complex organic matter into simpler compounds which the animals then excrete. Bacteria, actino- mycetes, and fungi are doubtless even more im- portant in this respect. Animals require nitrogen, carbon, oxygen, hydrogen as well as some 13 other elements, and hence are involved in nutrient cycles of these elements in the ecosystem. Climate is directly involved in the weathering of soil particles, insofar as rainwater percolating into the ground carries nutrients into the soil, and indirectly in determining the kind and luxuriance of the vege- tation and animal life that occurs in the area. As a result, mature soils of climax stabilized ecosystems have profiles characteristic both of types of vegeta- tion and of climatic regions. The species composi- tion and density of ground animals vary with the profile horizon and with the various soil-types found in various parts of the world. Nutrient cycles occur in aquatic as well as ter- restrial ecosystems. Organisms may modify or con- dition the chemical and nutrient characteristics of aquatic habitats in various ways to affect the occur- rence of other individuals of the same or different species. Reactions, soil formation, and cycles LF) Cooperation and Diso peration As organisms aggregate in a habitat, they neces- sarily establish interrelations of various kinds with one another. Between organisms, coactions that are beneficial to one or more of the participants consti- tute cooperation (Allee et al. 1949, Allee 1951). Co- operation may occur between members of the same species or between different species. Interspecific co- operation includes mutualism, commensalism, and many other sorts of interrelations within the com- munity. As opposed to cooperation, coactions be- tween individuals or species that are harmful to one or more of the participants constitute disoperations. We will consider parasitism, predation, and competi- tion as disoperations. INTRASPECIFIC COOPERATION An early manifestation of cooperation in the evolution of animals is the grouping of free-living protozoans to form colonies, and the further develop- ment of such colonies into multi-cellular metazoans that thereafter behave and respond as unit organisms. Whether the first gathering of protozoan cells to form colonies developed for better protection from some enemy or environmental condition, improved utiliza- tion of food supplies, or more efficient reproduction, it is impossible to say. The colonial form, however, must have had survival value to persist. Colonization quickly led to division of labor be- tween somatic and reproductive cells, as occurs in Volvox, and later to division of labor between so- matic cells themselves, so that different cells or or- gans became specialized to serve the particular func- tions of digestion, respiration, circulation, and so on. Cooperation between cells, tissues, and organs gave greater metabolic efficiency to the whole individual and resulted in evolution to the highest types of ani- mals. Similarly, the aggregation of individuals must have survival value, because it persists. Hundreds, sometimes thousands, of spotted lady-beetles hiber- nate under leaves at the forest-edge. Mayflies, midges, and mosquitoes swarm for mating purposes. Millions of bats roost together in large caves, notably in the Carlsbad Caverns in New Mexico. The migra- tory locust moves from one locality to another in im- mense hordes, and birds usually migrate in flocks. Highly organized societies are found in such insect groups as termites, ants, bees and wasps, as well as in some breeding colonies of birds and mammals. Benefits derived from aggregating are both physi- ological and psychological. Individual honey-bees are poikilothermal, but when hive temperatures drop be- low 14°C (57°F) during the winter, they form clusters and maintain a mass temperature several de- grees above outside temperatures. This is brought about by increased metabolic oxidation of honey in 1a: i their bodies and by increased muscular activity. Furthermore the compact cluster presents surface area of heat loss that is less than the total surface area of the individuals separately (Milum 1928). When there is danger of overheating, the bees in the hive spread out on the combs and fan with their wings to create a circulation of air. They will also carry water into the hive and place small quantities both outside and inside the comb cells. The forced air circulation evaporates the water and cools the hive. Bees also cool themselves by constantly moving their tongues in and out of their mouths, exposing to evaporation the moisture that is present on them as a thin film (Lindauer 1955). Temperature regulation is less well developed in other social Hymenoptera (Himmer 1932). Coveys of bobwhite quail roost in close circles, at night. Perhaps this enables detection of predators approaching from any direction, but it is certain that the birds can by that behavior tolerate lower air tem- peratures and for a longer time than isolated birds can (Gerstell 1939). Similarly, mice huddle in low air temperatures, a behavior that reduces heat radia- tion and consequent need for frequent feeding (Pry- chodko 1958). Colloidal silver is toxic to fish. Ten goldfish were simultaneously exposed to a liter of water dosed with colloidal silver. They lived an average of 507 minutes each. Fish individually exposed to a similar concen- tration of silver in the same volume of water lived an average of 182 minutes. The slime from the grouped fish was sufficient to precipitate much of the colloidal silver and render the solution less toxic (Allee and Bowen 1932). Photosensitive animals survive longer when exposed to excessive illumination in groups than singly because of partial shading of one by another, but fresh-water planaria exposed to ultraviolet live longer in groups even when no shading is involved (Allee and Wilder 1939). Marine flatworms Pro- cerodes survive longer in fresh water in groups than singly because the first worms that die from the group release calcium into the water, conditioning it and giving protection to the animals that remain (Oesting and Allee 1935). A single muskox or bison may succumb to a pack of wolves. When in a group, the males form a circle facing outward with the females and young in- side, whereby they are usually able to ward off the attack. By the same token, a single wolf has diffi- culty killing a deer; a single coyote, killing a prong- horn antelope. But in packs the wolves can over- power a deer, and by individually taking turns in relay fashion, a pack of coyotes can chase a prong- horn to exhaustion. Whether an animal occurs singly or in groups may affect its learning rate and behavior. The com- mon cockroach and the shell parakeet learn simple mazes less rapidly when other individuals are around than when alone, but goldfishes, minnows, and green sunfishes learn mazes faster in groups; phenomena spoken of respectively as negative and positive social facilitation. Many animals are more active and alert in groups than alone; in groups, individual imitations of others’ behavior are common. Cormorants and pelicans fish more proficiently in groups than alone because group behavior is organized and each indi- vidual plays a certain role (Allee 1957). The beneficial effects of aggregation are lost if the aggregation is either too small or too large. For instance, the longevity of Drosophila is greatest with a population density of 35 to 55 flies per one ounce culture bottle (Pearl, Miner, and Parker 1927). Smaller densities are unable to control the growth of the yeasts on which they feed; greater densities exhaust the food supply and excessive amounts of ex- creta accumulate. Likewise an initial population of 4 Tribolium beetles per 32 g of flour reproduces more rapidly during the 25 days following than smaller or larger initial populations (Park 1932). For all kinds of animals, competition for food and other resources of the habitat becomes more and more intense as populations increase in size above an optimum. The benefits resulting from an increase in the size of ag- gregations up to the optimum represents coopera- tion; the harmful effects resulting from aggregations that are too large is disoperation. The simplest animal aggregations exhibit little social organization, for the individual organisms are brought together more or less ephemerally by chance, by sexual attraction, for reproduction, or because of a similar response to environmental factors. An evo- lution of organization may, however, be traced through intermediate stages to the complex division of labor found in some insect societies. Specializa- tion occurs both in morphology and behavior. The three primary castes of termites and ants are the winged reproductive males and females, the wingless sterile soldiers that possess large mandibles and irri- tating glandular secretions, and the smaller, wing- less, often sterile workers. The soldiers defend the colony against predaceous enemies; this function is assumed by workers in bees and wasps, among which a distinct soldier caste is lacking. In termites, the soldiers may be either males or females; in ants, they are females. The worker caste in ants usually fe- males, but in higher termites it may consist of either males or females. In primitive termites the nymphs of other castes substitute for the workers. The work- ers collect food, cultivate gardens of fungi, take care of domesticated aphids or coccids, feed the other castes, and build shelters. The earliest organized social life of primitive man was perhaps neither so highly organized nor so far advanced in an evolution- ary sense as these complex societies of insects, even Cooperation and disoperation 175 FIG. 12-1 Model of a royal cell of the termite, showing different castes. The queen has an enlarged abdomen; her head is turned to the right. The king is in the left center. Two soldiers with though it was from the greater psychological potenti- alities of primitive man that modern civilization arose (Allee et al. 1949, Allee 1951). In these social relations, indeed in all sorts of symbiotic relations between individuals, one or both partners must have specialized behavior to effect and maintain the relationship. Chemical stimuli are im- portant in this respect and have received much study to date but physical stimuli, such as color, shape, texture, temperature, and so on may also have pri- mary integrative importance as releasers for specific behavior responses, the products of long evolution (Davenport 1955). MUTUALISM Mutualism is an association between two or more species in which all derive benefit in feeding or in some other way. The term symbiosis has often been applied to this relationship, but symbiosis prop- erly refers to the intimate association of two or more pointed heads are in the upper right. Most of the rest are workers (courtesy Buffalo Society of Natural Sciences). dissimilar organisms, regardless of benefits or the lack of them, and hence includes mutualism, com- mensalism, and parasitism. Mutualism, as is true also with commensalism and parasitism, may be facultative, when the species involved are capable of existence independent of one another, or obligative, when the relationship is im- perative to the existence of one or both species. Con- siderable study and experimentation is sometimes re- quired to decide whether a particular relationship is facultative or obligative, or even whether it is truly mutualistic. Mutualism is sometimes considered as, fundamentally, reciprocal parasitism. Many examples of ecological interest of mutualism, commensalism, and parasitism are cited by Pearse (1939) and Allee et al. (1949) ; only a few will be given here. Mutualism in plants is demonstrated in the asso- ciations of fungi and algae to form lichens, of nitro- gen-fixing bacteria with the roots of legumes, and of fungal mycorrhizae with the roots of many flowering plants. 176 Ecological processes and dynamics There are many intimate relations between plants and animals (Buchner 1953). Mutualism is sus- pected in the presence of photosynthetic algal cells in the protective ectoderm of green hydra, and those associated with turbellarians, mollusks, annelids, bryozoans, rotifers, protozoans, and the egg capsules of salamanders. The algae give off oxygen, bene- fiting the animals, which in turn supply carbon di- oxide and nitrogen to the plants. The thick growth of algae often found on the carapace of the aquatic turtles is important mostly as camouflage for the turtles (Neill and Allen 1954). Certain beetles, ants (Bailey 1920, Weber 1957), and termites cultivate fungi for food. Bacteria in the caeca and intestine of herbivorous birds and mammals aid in the digestion of cellulose. The cross-pollination of flowers by the agency of insects and birds seeking nectar and pollen is of such great importance that many structural adaptations in both plant and animal fit the one to the other to insure the success of the function (Robert- son 1927, Dorst 1946). Animals, especially birds and mammals, are of great importance as agents of plant distribution (Mc- Atee 1947). Seeds, fruits, even entire plants become attached to feathers or fur, or ingested seeds are eaten and eliminated unharmed with the feces. When bare seeds are eaten they are usually macerated, digested, and entirely destroyed unless they have very hard coats. But fruits are fed upon primarily for pulp, and most of the seeds pass through the alimentary tract unharmed. Animal transportation of ingested seeds is perhaps the most important means by which fruit species are dispersed (Taylor 1936). Furthermore, germination of the seeds is frequently improved by mechanical abrasion in the stomach and thinning of the seed coat by digestice juices, making it more permeable to water and oxygen (Krefting and Roe 1949). Germination of acorns and nuts is improved if they are buried in the ground rather than left lying on the ground sur- face. Squirrels, chipmunks, wood rats, and some birds, particularly jays (Chettleburgh 1952) and woodpeckers, cache acorns and nuts as a winter food supply, hiding them in cavities and nooks or burying them in the soil. Perhaps most are recovered and eaten ; Cahalane (1942) found that 99 per cent of the acorns buried by the fox squirrel in a locality where the animals were numerous were recovered by the animals, largely through the sense of smell. One per cent of the thousands of nuts produced by it during the lifetime of a tree that are buried but not recovered would be adequate to insure the continuance of the forest. Invasion of oak and hickory trees into sandy areas is greatly accelerated by, and is sometimes de- pendent on, this coaction of squirrels (Olmsted 1937), and the dispersal of forests up the slopes of mountains against gravity may also depend in large part on transportation of the heavy seed by animals (Grinnell 1936). The interesting concept involved here is that plants have evolved fruits and nuts that are highly attractive to animals as food substances. However, the production of prodigious numbers of fruits and nuts during their lifetimes insures that at least some will escape consumption and will be more widely and effectively dispersed. Large populations of such herbivores as rabbits and deer sometimes do considerable damage to new propagation of herbs and trees, but the effects of over- browsing cannot be dismissed as all bad (Webb 1957). Removal of the lower branches of established trees by deer may not seriously affect the vigor of the trees; indeed, such pruning may actually increase their value as lumber. Deer pawing the leaf-litter may thereby plant, so to speak, some seeds that would not otherwise become established. Thinning dense stands of young trees may allow residuals to grow more rapidly; much of new growth is doomed any- way because of root competition, and shading cast by established trees. Some species of shrubs and trees actually produce more annual growth under heavy than light browsing. Other species, however, may be killed when small by heavy browsing, although they tolerate considerable browsing when mature. Detri- mental effects of both browsing and grazing become evident in an area in the form of excessive invasion of new species which are little used as food, and dis- appearance or stunting of the food species that are desirable (Graham 1954). Some tropical acacias have evolved foliar nectaries or other food bodies as well as enlarged hollow stip- ules, spines, or other structures to attract stinging ants. In return, the plants obtain protection from herbivorous mammalian and insect enemies (Brown 1960). Interspecific mutualism is nicely demonstrated by the flagellate Trichonympha, an obligate in the gut of several species of wood-eating termites (but not in the family Termitidae) where it digests cellulose (Cleveland 1924). Trichonympha and related species also occur in the alimentary trace of the wood-eating roach Cryptocercus (Cleveland 1934). The termite and roach reduce the wood to small fragments, pass- ing them through the alimentary canal to the hind- gut where the protozoans digest the cellulose, chang- ing it into sugar. The host benefits the protozoa by removing harmful metabolic waste products and maintaining anaerobic conditions in the intestine (Hungate 1939). The ruminant stomach and the horse coecum con- tain enormous numbers of ciliates and bacteria, some of which digest cellulose. The micro-organisms repro- duce the equivalent of their biomass each day. This provides the host with about 20 per cent of its nitro- gen requirement (Hungate 1960). Cooperation and disoperation Lit COMMENSALISM Commensalism defines the coaction in which two or more species are mutually associated in activities centering on food and one species, at least, derives benefit from the association while the other associates are neither benefited nor harmed. It is often difficult to establish definitely the nature of the relations between species ; and phenomena considered at one time to be commensalism have been later found to be parasitism or mutualism. The concept of com- mensalism has been broadened, in recent years, to apply to coactions other than those centering on food ; cover, support, protection, and locomotion are now frequently included (Baer 1951). The remora fish are remarkable for having the spinous dorsal fin modified to form a sucking disk on top of the head by means of which they become at- tached to the body of the shark, swordfish, tunny, barracuda, or sea turtle. They are of small size and are not burdensome to the host. The host benefits the remora, however, for when the host feeds, the scraps of food floating back are swept up by the rem- ora. Many small animals become attached to the out- side of larger ones, such as the protozoans Tricho- dina and Kerona on Hydra, vorticellids on various other aquatic organisms, branchiobdellid annelids on crayfish, and so on. Commensals may also be in- ternal ; consider, for instance, the harmless protozoans that occur in the intestinal tract of mammals, includ- ing man. The pitcher of the pitcher plant found in bogs furnishes a breeding site or home for certain species of midge flies, mosquitoes, and tree toads. Many kinds of micro-organisms, both plant and animal, live in the canal system of sponges. The nest of one species often furnishes shelter and protection for other species as well. Ant nests may contain guest species of various other insects. Large hawk nests sometimes have nests of smaller species tucked in their sides (Durango 1949); some birds place their nests close to wasps, bees, or ants for the protection offered by these insects (Hindwood 1955). Woodchuck burrows are used also by rabbits, skunks, and raccoons, especially in the winter. During dry periods the water in crayfish burrows, a meter below the ground surface, often teem with entomostraca (Creaser 1931). COMMUNITY ORGANIZATION The final stage in the evolution of coopera- tion is the biotic community. Analogous to a multi- cellular individual, the community is composed of or- ganic units, in this case organisms and species rather than cells and tissues. It has a definite anatomy in its stratification, niches, and food chains. The com- munity, too, is a thing born, and it exhibits the same characteristics of growth and old age as do individ- uals. There is succession of stages to the climax com- munity like the series of instars in the life cycle of an insect. If the community is injured, it heals the wounds in its structure through secondary succession. The community is self-sustaining in that it absorbs energy from the sun and metabolizes it at various trophic levels in order to do work. There is division of labor, analogous to the functions of the various organs in the body of a single individual; plant spe- cies manufacture the food that animals need, and dominant species create environment conditions within the community suitable for the existence of other species. There is transmission of stimuli, in- tercommunication, between individuals and species by voice, odor, sight, and contact. There is control over the numbers of individuals of each species in the bal- ance of nature. The result is that the biotic commun- ity is a highly integrated recognizable unit in which species exhibit various degrees of interdependency. The existence of each component depends to a certain extent on coooperation between them all, so that the community responds and behaves as an organic en- tity. That such complicated interrelationships have come about through evolution indicates that they have survival value for the component species involved. PARASITISM Parasitism is the relation between two in- dividuals wherein the parasite receives benefit at the expense of the host; parasitism is therefore a form of disoperation. Parasitism is mainly a food coac- tion, but the parasite derives shelter and protection from the host, as well. A parasite does not ordinarily kill its host, at least not until the parasite has com- pleted its reproductive cycle. Were the parasite to kill its host immediately on infecting it, the parasite would be unable to reproduce and would quickly be- come extinct. The balance between parasite and host is upset if the host produces antibodies or other sub- stances which hamper normal development of the parasite. In general the parasite derives benefit from the relation while the host suffers harm, but tolerable harm. Classification Parasites are commonly classified as ectopara- sites, those which live on the outside of the host, and endoparasites, those which live in the alimentary tract, body cavities, various organs, or blood or other tissues of the host (Baer 1951). Ectoparasites may 178 Ecological processes and dynamics be parasitic only in the immature stages—the hair- worm larvae, parasitic in aquatic insects; only the adults parasitic—fleas, on birds and mammals; or both larvae and adults may be parasitic—the blood- sucking lice and flies, biting lice, mites, and ticks that occur on birds, mammals, and sometimes reptiles, and the monogenetic trematodes on fish. Similar re- lations obtain among endoparasites, although it is more common to have all stages parasitic: entozoic amoebae, trichomonad flagellates, opalinid ciliates, sporozoans, pentastomids, nematodes, digenetic trem- atodes, acanthocephalons, cestodes, and some cope- pods. Animals may also be parasitic on plants. Nema- todes infest the roots of plants. Galls are formed by wasps or gnats especially on oaks, hickories, willows, roses, goldenrods, and asters. Mites stimulate forma- tion of witches’ brooms in hackberry. A variety of insects the larvae of which are leaf miners, wood borers, cambium feeders, and fruit eaters, should be included here. Plants themselves may be parasites either on other plants or on animals. Bacteria and fungi are among the most important disease-produc- ing organisms in animals. Social parasitism describes the exploitation of one species by another, for various advantages. Old World cuckoos and the brown-headed cowbird of North America do not build nests of their own; rather, they deposit their eggs in the nests of other species, abandoning eggs and young to the care of foster parents (Weller 1959). The bald eagle some- times robs the osprey of fish that it has just caught. One species of ant waylays foraging workers of an- other species and snatches away the food they are transporting ; the robber species may deliberately rob another nest of food. Some species of ants make slaves of the workers of other species. Various other types of dependency of one species on another have evolved, not only between ants, but also in other social insects, such as termites, wasps, and bees. So- cial insects are apparently the only animals other than man to have succeeded in domesticating other species, and of cultivating plants, particularly fungi, for food (Wheeler 1923). Evolution and adaptations The ancestors of ectoparasites were clearly free- living forms. It is not difficult to imagine how a small organism living freely in water or vegetation could accidentally have settled on the outside of a larger species and found conditions favorable for sur- vival. There would even be selective advantage in such a niche if the organism found a rich source of food. The biting lice probably evolved from psocid insects that live beneath the bark of trees. They may Cooperation and disoperation have transferred from this niche to bird nests and then to the birds themselves. Most ectoparasitic in- sects probably are derivatives of carnivores, sapro- vores, or suckers of plant juices. Endoparasites may in some cases have evolved from ectoparasites; more likely, they came directly from free-living ancestors or from commensals. For example, free-living nematodes and scavenger beetles both feed upon decaying organic material, and it is easy to visualize how the beetles could have acci- dentally consumed one or more nematodes. Many kinds which have since become parasites, such as protozoans and flatworms, could have had their first entrance into the alimentary tracts of prospective hosts via drinking water, and subsequently invaded other organs in the body. The invaders would have found their hosts abundant food sources, but would have needed some preadaptation to live at the low oxygen concentrations characteristic of digestive tract, to resist being consumed by the digestive juices of the host, and to keep from being carried out with the feces. As succeeding generations of parasites be- came increasingly adapted to live either on or in their hosts, many kinds lost the capacity for a free-living existence. Specialization to internal parasitism has cost the loss of locomotor, sense, and digestive or- gans, none of which are needed, and led to the development of organs of attachment, increased re- productive capacity, and, in several forms, to polyem- bryony, intermediate hosts, and a complicated life cycle (Lapage 1951). Some parasitic species are more highly evolved than others. Many parasites, for instance, pass their entire existence in a single host; others require one, two, even three intermediate hosts. It is of ecological significance that both primary and intermediate hosts of a parasite occur in the same habitat or community. Even then the hazards to successful passage from one host to another are so great and mortality so high that prodigious quantities of offspring are produced to insure that at least a few individuals will complete the cycle. Parasites are transferred from one host to another by active locomotion of the parasite itself; by in- gestion, as one animal sucks the blood of or eats an- other; by ingestion, as an animal takes in eggs, spores, or encysted stages of the parasite along with its food or drinking water; as a result of bodily con- tact between hosts; or by transportation from host to host by way of vectors. As an illustration of vec- tors, the bacteria that cause tularemia in man are carried from rabbit to rabbit by ticks. Man contracts the disease when he handles infected rabbits, but the incidence of infection is greatly reduced in the autumn when cold weather forces the ticks to leave the rabbits and go into hibernation (Yeatter and Thompson 1952). lives. OFHIOTAE; MIA FERSFICUA LIFE CYCL FIG. 12-2 Life cycle of a snake tapeworm. The eggs are voided into the water with the feces of the snake, where they are ingested by the copepod Cyclops (lower right). A procercoid (middle right) develops in the copepod, from the egg. If the copepod is eaten by a fish, the procercoid changes into a plerocercoid (upper right) and becomes encysted in the liver or mesenteries. When the fish is eaten by a water snake, the mature tapeworm develops (upper left). Other intermediary hosts are tadpoles and frogs (Thomas 1944). Host specificity Copepods are of all parasites the most ubiqui- tous in their host relationships, being reported from various invertebrate groups and from fish. Most par- asitic genera, however, are adapted to hosts of one phylum only. The acanthocephalans Gracilisentis and Tanarhamphus are yet more specific, normally found only in the gizzards of shad fish; Octospinifer is found only in catostomids; Eocollis, only in cen- trarchids. Each order of birds possesses its own particular species of tapeworms; this is true even when several orders of birds live in the same habi- tats, as do, for instance, grebes, loons, herons, ducks, waders, flamingoes, and cormorants (Baer 1951). Species of flagellate protozoans that occur in termite alimentary tracts are largely host-specific (Kirby 1937). However, considerable caution needs to be exercised in assigning host specificity to protozoans. Many species have invaded more than one taxonomic host group; and often several species of a single genus of Protozoa frequent the same host species. Some species of gall wasps attack only one species of oak. Where a single species parasitizes two or more host species, the shape and structure of the gall formed around the egg and larva on both hosts is essentially similar. When several insects are found on the same oak, each kind of parasite produces its own characteristic gall form. Apparently the charac- teristics of the gall that develops depend more on the kind of enzyme secreted by the parasite than on differences of host tissues (Kinsey 1930). 180 Restriction of parasites to special niches is demon- strated by species of biting lice restricted to the head and body regions of birds. Some nematode spe- cies are found throughout the body in connective tis- sue, but not in the gut; some occur only in the digestive tract and associated organs; certain species occur in the glandular crop of birds, but others only in the caecum; many species occur exclusively in the lungs or in the frontal sinuses. Such fine restriction of parasites to particular hosts or organs is a conse- quence of precise physiological and morphological adaptations that permit the parasite to survive and complete the life-cycle only under very special condi- tions. Host-specificity can make the taxonomies of many parasites useful for corroborating phylogenetic rela- tionships of their hosts (Kellogg 1913). The South American bird Cariama cristata has been shifted from one order to another, and was at one time even put into a special order. A study of its helminth parasites disclosed two species of nematodes and two genera of cestodes present which occur together elsewhere only in Eurasian bustards. The occurrence of these forms in groups so far removed geographically from one another could be coincidental or the result of parallel evolution, but for a number of reasons it seems more likely that Cariama and the bustards are derived of a common ancestor which became infected with these parasites, the parasites persisting in spite of evolutionary divergence and geographic separation of hosts. It is interesting to note that this relation- ship of the hosts is sustained by recent taxonomic study of them by ornithologists (Baer 1951). Effect on host: disease By disease we mean a condition which so affects the body or a part of it as to impair normal func- tioning. Parasites may not cause immediate mortal- ity, but they cause damage to body structures which, should it become excessive, may cause death. We may perhaps better visualize the role parasites play in producing disease by listing some of the more com- mon agents of mortality in organisms, in addition to predators and parasitoids, which will be described beyond. 1. Worm parasites, such as tapeworms, nema- todes, and acanthocephalans may wander through the host’s body doing mechanical injury as well as de- stroying and consuming tissues. The host may re- — spond by forming a fibrous capsule or cyst around an imbedded parasite. 2. Protozoan parasites are especially important in the alimentary tract and in the blood. A sporozoan _ species of Eimeria damages the walls of the intestine — Ecological processes and dynamics in upland game birds, producing coccidiosis; To.ro- plasma becomes encysted in the brain of rodents; Leucocytozsoon is a blood parasite common among waterfowl and game birds. 3. Bacteria cause a variety of diseases, notably tularemia, paratyphoid, and tuberculosis among birds and mammals, as well as other diseases in lower types of organisms. 4. Viruses are so submicroscopic in size that many kinds pass through the finest filters. They are the potent agents of hoof and mouth disease in deer, spotted fever in rodents, encephalitis and distemper in foxes and dogs. 5. Fungus spores of Aspergillus that occur in moldy pine litter may be drawn into the lungs of ground-feeding birds where they germinate and grow, causing aspergillosis. Fungus may also develop on the external surface of animals. 6. External parasites such as ticks, fleas, lice, mites, and flies do not commonly produce serious mortality by themselves, but they are often vectors transmitting protozoa, bacteria, and viruses from one animal to another. Heavy infestations of external parasites may, however, lower the vitality or vigor of an animal and cause diseases of fur (mange) or feathers. 7. Nutritional deficiencies in vitamins or min- erals, or improper balance between carbohydrates, proteins, and fats, may produce malformations, lack of vigor, even death. Variations in amount, compo- sition, and intensity of solar radiation may affect the vitamin content of the food an animal consumes. Long restriction to emergency foods of low energy content and outright starvation often cause consider- able loss of life during periods of climatic stress. 8. Food poisoning, botulism, occurs when cer- tain foods become contaminated with the toxins re- leased by the bacterium Clostridium botulinum. Many waterfowl are stricken in some _ localities. Waterfowl also often pick up and swallow gun-shot from marshes in which there has been much hunting, and get lead poisoning. 9. Physiological stress (Selye 1955) is a term that has come to be applied to changes produced in the body non-specifically by many different agencies which may accompany any disease. Effects of stress include loss of appetite and vigor, aches and pains, and loss of weight. Internally, the stress syndrome is characterized by acute involution of the lymphatic organs, diminution of the blood eosinophiles, en- largement and increased secretory activity of the ad- renal cortex, and a variety of changes in the chemical constitution of the blood and tissues. Stress gives rise to abnormal conditions, but it simultaneously elicits from the body defense mech- anisms against those abnormal conditions. It is pres- ently believed that the anterior pituitary gland and the adrenal cortex are chiefly responsible for integrat- ing the defense mechanisms. Three stages are in- volved: the alarm reaction, in which adaptation has not yet been acquired; the stage of resistance, in which the body's adaptation is optimum; and _ the stage of exhaustion, in which the acquired adaptation is lost. Characteristic of the exhaustion phase are, among others, hypoglycemia, adrenal cortical hyper- trophy, decreased liver glycogen, and negative nitro- gen balance. 10. Accidents must be included as an important cause of mortality. Organisms that produce disease generally fall into one or two categories. They are either present in the body at all times but not normally virulent, or they are normally absent but are virulent from the moment the host is infected by them. Even the healthiest ani- mals chronically entrain many parasites and noxious organisms in the body, but these organisms wreak overt harm only when they become unusually abun- dant, when virulent mutant strains develop, or if, for one or another reason, the host’s vitality and resist- ance decline to the point where the host is no longer able to withstand the effects of their presence. Any animal suffering an unusually heavy infestation of parasites will show the tax thus put upon its vitality as a loss of vigor and weight, decreased growth rate, and low resistance to vicissitudes of its natural en- vironment. Normally, a more or less mutual toler- ance exists between host and parasite such that the demands of the parasite are in equilibrium with the host’s capacity to meet them. Host-tolerant parasites have been naturally selected for; mutant strains that are exceptionally virulent quickly die out because they kill the host, without which they cannot survive. A single attack, even a mild one, of some diseases often confers a partial or complete immunity from further attacks of the same disease, even though the agent of the disease may still be carried in the body of the recovered victim. Immunity is an acquired physiological adaptation by which the immune is able to withstand the presence of an otherwise noxious or- ganism, suffering little or no deleterious consequence of that presence. The fact of immunity is demon- strated when parasites not conspicuously harmful to their normal hosts are introduced into a species to which they are normally exotic. The novel host has had no prior occasion or opportunity to adapt im- Cooperation and disoperation 181 FIG. 12-3 Development of a parasitoid black digger wasp. (a) eggs in position on the host larva, (b) the developing larva, (c) the fully grown larva devouring the remainder of the host (courtesy Illincis Natural History Survey). munitively to the alien parasite, and may sicken, even die, of the effects of the parasite’s presence, the same effects in kind and intensity which the normal host easily takes in stride. For instance, a trypanosome that is a natural parasite in many of the larger wild mammals of Africa evokes no spectacular effects in its usual hosts. But when the parasite is vectored by the tsetse fly to man, it causes sleeping sickness; to cattle, nagana. A good bibliography of references to diseases in wild mammals and birds is given by Hal- loran (1955); Davis (1946) discusses diseases of fish, especially trout. PARASITOIDISM Some Diptera and Hymenoptera deposit their eggs in the immature stages of other insects; the larvae on hatching feed on the host until they are full grown. The relation of the larva to its host is fre- quently described as one of parasitism. But it is fundamentally different from parasitism in that the host invariably dies of the larval depredations before the larva emerges, but the larva invariably lives in spite of the host’s death. The relationship resembles that of predator to prey, except that, unlike the true predator, the larva lives within the body of its prey and kills it slowly as it feeds, not suddenly before it feeds. Such larvae are, for these reasons, best thought of as parasitoids. Parasitoids may in turn be infested with hyper- parasitoids. In the Chicago, Illinois, region Samia cecropia, a saturniid moth, suffers the destruction of nearly 23 per cent of its cocoons by an ichneumonid parasitoid, Spilocryptus extrematis, which deposits an average of 33 eggs on the inside of each cocoon or on the surface of the larva. The host larva dies in a few hours after the parasitoid hatches, and the ich- neumonid larva moves about freely, feeding on the cuticle or burrowing into the tissues to drink the body fluids. Another ichneumonid, Aenoplex smithii, was found as a secondary parasitoid, feeding on the larvae of S. extrematis in about 13 per cent of the cecropia cocoons infested by the latter species. A chalcidid, Dibrachys boucheanus, fed both upon S. ex- trematis and, as a tertiary parasitoid, upon A. smithii. Another chalcidid, Pleurotropis tarsalis, infected co- coons containing D. boucheanus and eventually killed the larva as a quarternary parasitoid (Marsh 1937). To have five links in an inverted parasitoid food chain is perhaps unusual, but hyperparasitoidism is com- mon and of importance in controlling the size and interrelations of animal populations. Predation is a form of disoperation, at least in point of immediate effects, since one animal kills an- other for food. Predation is important in community dynamics in so many ways that we will postpone dis- cussion of it until we consider food coactions (Chap- ter 13), productivity (Chapter 14), and regulation of population size (Chapters 16, 17). COMPETITION Competition is the more or less active de- mand in excess of the immediate supply of material or condition exerted by two or more organisms (Clements and Shelford 1939: 159). The materials and conditions sought by animals include food, space, cover, and mates. When these materials are in more than adequate supply for the demands of those organ- isms seeking them, competition does not occur ; when they are inadequate to satisfy the needs of all the or- ganisms seeking them, the weakest, least adapted, or least aggressive individuals are forced to do without, or go elsewhere. Competition may result in death for some competitors, but this is from fighting or being deprived of food or space rather than being killed for food as in predation, or by disease as in extreme parasitism. Competition may be either direct or indirect. It is direct where there is active antagonism, struggle, or combat between individuals; indirect, when one individual or species monopolizes a resource or ren- ders a habitat unfavorable to the establishment of other organisms having similar requirements. Direct competition, or interference, is evident in the fighting of bull seals for larger harems and of grouse for a better position in the social hierarchy ; in chasing and 182. Ecological processes and dynamics color displays (a sort of saber-rattling) by fish and birds for defense of territories; and in the singing and calling of birds, some mammals, and frogs as bids for mates. Indirect competition, sometimes called exploita- tion (Brian 1956), is common among plants when certain species monopolize the water and nutrient re- sources of the soil or available light so that competing species cannot maintain themselves (Clements, Weaver, and Hanson 1929). Animals may also ren- der a habitat unsuitable by their excreta for a species which otherwise would occur. Once an area is well saturated with established individuals, it is often more economical of energy for new individuals to seek homes elsewhere, even in less favorable situa- tions, than to intrude. To be successful by indirect competition, a species needs to get established in an area first, or if the invasion of various species is nearly simultaneous, then to have a more rapid rate of reproduction and growth, or a greater longevity, so as to utilize the resources of the habitat to the fullest possible extent (Crombie 1947). Competition is usually keenest between individ- uals of the same species, intraspecific competition, because they have identical requirements for food, mates, and so on, and because they are more nearly equal in their structural, functional, and behavioral adaptations. Interspecific competition occurs where different species require in common at least some ma- terials or conditions. The severity of competition de- pends on the extent of similarity or overlap in the re- quirements of different individuals and the shortage of the supply in the habitat. It is generally the case that the more unlike the kinds of competing organ- isms, the less intense the competition. Yet birds compete with squirrels for acorns, nuts, and seeds; insects and ungulates compete for food in grassland ; the bladderwort plant competes with small fish for entomostraca and other plankton. Competition has five important effects on the ani- mal community : Establishment of social hierarchies Establishment of territories Regulation of population size Segregation of species into different niches Speciation SE a a The first two effects are chiefly intraspecific ; will be considered in this chapter. Regulation of population size involves both intra- and interspecific competition, and many other types of coaction as well, and will be considered in Chapters 15, 16,and 17. The last two effects are interspecific; they will be discussed in Chapters 18 and 19. It is important to realize that, when these effects are fully manifested, there is a decrease in tension and intensity of competition as each individual or species takes its place in the orderly structure and organization of the community. Let us see how this works out in the instances of social hierarchies and the establishment of territories. Social hierarchies When groups of individuals of certain animal species are confined to limited areas, frequent fights or pecking of one another occur. By way of these encounters, the more aggressive and successful indi- viduals establish a hegemony to which the more sub- missive individuals acquiese. A social hierarchy is thus established; the phenomenon was first clearly described for the domestic fowl (Schjelderup-Ebbe 1922). The so-called peck-order in the domestic fowl is a linear one. Close observation of marked individuals showed that, in a flock of 13 birds, one bird became the supreme despot of the whole flock; another bird was submissive to the first but despotic over the re- maining 11; and so it went on down to the last bird, which had the right to peck none but was pecked by all. This type of social aggressiveness or despotism is called peck-right. In practice, certain individuals establish the right to peck others and not get pecked back. In the middle of a series, the order is some- times less fixed, and reversals or triangles occasion- ally occur. Although most easily demonstrated in the crowded conditions of captivity, peck-right has also been observed to obtain under free natural conditions. The peck-right type of social hierarchy has been found to occur in several other species of birds, in several species of mammals and fish, and in a few lizards, crayfish, and insects. Possession of the following characteristics usually gives an individual at least some advantage in gain- ing a high position in the despotic order: strength; good health; maturity ; relatively large size; hegem- ony over own territory; responsibility of acting to protect young; accompaniment by members of his own group when meeting a stranger; male over fe- male, at least during the nonbreeding season; female mated with a strong male; the hormone testosterone ; and innate aggressiveness (Allee et al. 1949: 413- 414). A high position in the social order is advan- tageous to the individual as it gives him priority over food, mates, territory, and other resources of the habitat (Collias 1944) and is sometimes, but not always, correlated with leadership in the group. In some species the social hierarchy is not as overt as that we’ve described. In peck dominance the indi- vidual that is usually subordinate is successful in a certain number of conflicts. Position in the despotic order is a function of ratios of success in continuing conflicts rather than on the results of the initial con- Cooperation and disoperation 183 ce ee 8: Por ~ lO ae BR<—2:0 —_—— FIG. 12-4 Peck-dominance between the lowest four birds in a flock of seven common pigeons. All four birds were dominated by the three other birds of the flock. The ratios show the proportion of times each bird was successful in its encounters with other individuals (Masure and Allee 1934). tact an individual has with each member of the group, Figure 12-4 shows bird BR successfully subdued GW in 21 encounters but was subdued by GW in 4 en- counters. Thus, an individual can occupy any position in the hierarchy as long as he is able to maintain that position against all challengers. The individual who loses a challenge from below is without position, and can gain a position only by successful challenge. The individual who has successfully challenged a higher position from a lower moves up to the higher, leav- ing the lower position open—who shall fill the va- cated position is determined by combat among those eligible to try for it, among whom is the former holder of the higher position. Plainly the positions in the hierarchy are fixed in order, but occupancy of those positions is fluid. A more fluid form of social aggressiveness is supersedence, in which a success- fully challenging individual usurps the position of an- other individual momentarily possessing special ad- vantages in the presence of food or some other thing. This type of relation has been described for the Mockingbird BI: Red-bellied golden-crowned sparrow (Tompkins 1933) and may likely be found in many other species. There have been few studies of social despotism as an interspecific phenomenon (Neuman 1956). The range of aggressiveness between individuals within any species is so wide that strong individ- uals of one species may be despotic over weak indi- viduals of another even though the majority of individuals in the first species are submissive. How- ever, the sharp-tailed grouse is usually dominant over the ring-necked pheasant, and the latter is usu- ally dominant over the prairie chicken (Sharp 1957). The manner in which different species fit into a social hierarchy may be the key to structure and organiza- tion of communities. Territory and home range The establishment of territories, especially dur- ing the breeding season, is another expression of despotism, but a special one. A territory is any area defended against intruders. It may be the entire home range over which the animal is active, or only a small portion around the nest. Although many ani- mals tend to be gregarious during the non-breeding seasons, they frequently take up isolated positions and become intolerant of the close presence of others when undertaking reproduction. A home range is that area regularly traversed by an individual in search of food and mates, and caring for young. The establishment of territories is best developed in birds (Hinde 1956), but also occurs in some other vertebrates (Carpenter 1958), possibly including some amphibians (Sexton 1960), and certain inver- tebrates (wood ant, Elton 1932; dragonflies, Jacobs 1955). There is increasing evidence that most adult animals, except for small aquatic species, establish Robin 9 Cardinal cages I aie tt 9 ~*—— Song Paes ene ING Ground ———~> Chipping dove sparrow sparrow FIG. 12-5 A social hierarchy between species of birds visiting a feeding station during the winter (Dennis 1950). cee Ninge <— Blue jay Ve ——> Tufted warbler titmouse 184 — Ecological processes and dynamics home ranges, if not territories, at least during the breeding season (snails, Edelstam and Palmer 1950; toads, Bogert 1947; fish, Hasler and Wisby 1958; turtles, Cagle 1944; mammals, Seton 1909, 1925-28). Immature animals, species in migration, or shifting populations during non-breeding seasons commonly do not have definite areas to which they confine their activities. An area should not be called a territory unless one can ascertain that it is defended against intruders of the same species. In order to determine that a home range exists, and to measure the size of it if it does, it is necessary to verify continuous occupancy of the suspected area by the same individual animal. Animals are trapped, marked and released as many times as are necessary clearly to establish the shape and extent of the area over which they wander in carrying out their normal activities. Best procedures for arranging the location of traps so as to reveal true home range and for sta- tistical analysis of the records of recapture are re- viewed by Dice and Clark (1953) and_ Stickel (1954). This can only be done if the individual can be identified by some peculiarity in its coloration or body characteristics, or by some system of applied marking (Taber and Cowan in Mosby 1960). Birds are commonly live-trapped and banded with num- bered aluminum bands placed around the legs. Mam- mals may be live-trapped and marked by distinctive toe clipping, ear notching, or tattooing. Snakes may be marked by removing scales from conspicuous loca- tions on the body; frogs and toads may be identified by punctures in the web between the toes, toe clip- ping, or by tags; turtles can be made to unwind a spool of string by which their trail is marked; fish fins can be clipped, or numbered tags attached to the jaw or gill covert or fin; and so on. The trouble with these techniques is that the animal must be trapped and handled to be identified. The ideal marking would be one obviating all this disturbing clumsiness while permitting easy and positive identification. Various methods of marking animals have been developed so that they may be individually recognized without recapture : colored bands ; dyeing parts of the body ; attaching colored feathers to the tails of birds. Luminous paint applied to small aquatic animals al- lows their movements to be traced in the dark (Lock- head 1939). Attempts have been made recently to dose animals with radioactive cobalt or phosphorous and trace their movements with a Geiger counter (Miller 1957). Territoriality has become so ingrained in the be- havior of some types of animal that simple advertise- ment of possession often constituies adequate de- fense. Such advertisement takes the form of song or other vocal expression in birds, some mammals, and some frogs, or the deposition of scent, a characteristic of many mammals (Holzapfel 1939, Graf 1956). If FIG, 12-6 Theoretical relation between home ranges (area en- closed within solid lines) and territories (area enclosed within broken lines). The black dots represent nesting sites (Burt 1943). an intruder persists in invading a territory, however, the owner will variously display bright threatening coloration, scold or growl, give chase, or actually en- gage in physical combat. FIG. 12-7 Travels of a box turtle over its home range during a week, July 7-14 (Stickel 1950). Cooperation and disoperation 185 Maintenance of a definite territory has several benefits: a definite breeding location in which the nest can be confidently established and protected is afforded ; it aids the acquisition of mates; it insures an area of sufficient size to provide food both for the adults and, later, for the young; and it frees the possessor of the onus of despotic interference by other individuals. The extent to which these advan- tages are attained varies with the species ( Nice 1941). Although competition for territory is most keen be- tween individuals of the same species, it also occurs between different species the space requirements of which overlap (Simmons 1951, Sharp 1957). A home range, on the other hand, only provides a breeding location. Possession of territory lessens the pressure of competition during the reproductive pe- riod, particularly for the female, when the entire energy and attention of animals needs to be devoted to the production of offspring. SUMMARY Beneficial cooperation is evident in division of labor between cells, tissues, and organs within the individual, between individuals in societies, and be- tween species living together in communities. Bene- fits derived from cooperation are physiological and behavioral and may affect survival, reproductive suc- cess, and more efficient use of natural resources. Co- operation between species that is intimate and bene- ficial to both participants is called mutualism ; where only one participant benefits, commensalism. These relations may be either facultative or obligative. Where one or more of the participants is harmed there is disoperation, of which parasitism, parasitoid- ism, competition, and predation are the examples. Distinction is made between true parasites, social parasites, and parasitoids. True parasites and their hosts have evolved adaptive interrelations so that coexistence occurs for varying lengths of time. The host is generally weakened, however, and virulent strains of the parasite may cause high mortalities. Causes of mortality or disease among organisms are predators, parasitoids, worm parasites, protozoan parasites, bacteria, viruses, fungi, external parasites, nutritional deficiencies, toxication, physiological stress, and accidents. Competition may be exerted directly through in- terference in the activities of one organism by an- other, or indirectly in the form of excessive ex- ploitation of natural resources. It may be either intraspecific or interspecific. Competition may result in establishment of social hierarchies, establishment of territories, regulation of population size, segrega- tion of species into different niches, or speciation. The over-all effect of competition is to relegate the individual and species to an orderly place in the structure and organization of the community with the result that there is decrease in tension and dis- turbance. 186 Ecological processes and dynamics | Ecological Processes and Community Dynamics: Food and Feeding Relationships Food-getting necessarily involves interrelations between organisms and between species; these inter- relations are among the most important coactions in any community. Animals are adapted variously to capture and utilize certain types of food, and to avoid being captured by other animals. One must under- stand these adaptations and interrelations to appreci- ate properly the role that food-getting plays in the dynamics of the community. FEEDING BEHAVIOR Free-living animals are commonly classi- fied on the basis of normal feeding behavior, thus: Herbivores: feed on living plants Carnivores: feed on animals that they kill Omnivores: feed on both plants and animals Saprovores: feed on dead plants and animals, and excreta These categories are not sharply defined, as few spe- cies are highly restricted in their diet. Plant-feeding forms occasionally eat animal matter, and carnivores sometimes eat fruit or other plant parts, or carrion. The classification is useful, however, and applies to both terrestrial and aquatic forms, and to any taxo- nomic group. The various categories are capable of further sub- division. Thus, herbivores include large cursorial grazers, such as bison, antelope, the muskox, caribou, sheep; small surface-living grazers, such as rabbits, mice, grasshoppers; subterranean-living grazers, such as woodchucks, prairie dogs, kangaroo rats, ground squirrels; browsers, which feed on buds and twigs of trees and shrubs rather than strictly on grass or ground herbs, such as wapiti, deer, moose, grouse, and defoliating types of insects such as the hemlock looper, spruce budworm, and larch sawfly; seed-, nut-, and fruit-eaters, such as squirrels, chipmunks, gallinaceous birds, sparrows; plant-juice suckers, such as aphids, leafhoppers, mosquitoes, chinch bugs ; and cambium feeders, such as bark beetles, gall flies, cynipids (Clements and Shelford 1939). Carnivores are also called predators. Carnivores restricting their food chiefly to insects are called in- sectivores; those limiting themselves largely to fish are called piscivores; and so on. Parasitoids eventu- ally consume their hosts, and hence are a special type of carnivore. Some plants are carnivorous. The pitcher-plant, Venus’ fly-trap, and sundew, that grow in bogs or wet places, and bladderwort, that occurs in ponds, depend for their nitrogen supply largely on animals that they capture and consume. Perhaps the bacteria, fungi, and viruses that cause disease in ani- mals also belong to this classification. 187 FIG. 13-1 Adaptations in the bills of birds. (a) a seed-eating sparrow; (b) an insect-eating warbler; (c) a plant-eating duck; (d) a fish-eating heron; (e) a predaceous hawk; (f) an aerial insect-eating whippoorwill. Many species eat both plant and animal matter, on occasion, or at particular seasons, but animals are considered to be truly omnivorous only if they feed on plants and animals in nearly equal amounts or in- discriminately. Omnivores occasionally also consume dead organic matter. Some aquatic organisms are filter-feeders and may consume everything within a particular size range that passes through their feeding apparatus (Jorgensen 1955). However, filter-feeders may demonstrate selectivity by feeding in neighbor- hoods where certain species predominate. Some copepods select particles of a particular size, reject- ing larger ones, by regulating the distance between the maxillae in the filter mechanism (Hutchinson OSI) Probably most, if not all, animals have chemore- ceptors of some sort, either to discriminate chemical substances dissolved in drinking water or food (taste), or chemical substances that are water- or air-borne (smell). Essential oils and alkaloids in plants are important as conditions of acceptability to insects. Hairiness, other surface features, or the visual stimuli that plants present are also conditions of attractiveness or acceptability of a food item. The food preference of any species depends on chromosomal inheritance, parental training, and per- sonal experience of that species, but the relative sig- nificance of each of these factors has not been evalu- ated for most animals. Young birds and mammals, in their first experiences at independent feeding, may pick up a variety of material but reject those items that are distasteful or indigestible ; they soon learn to distinguish acceptable substances. This process is established as the parents feed offspring only those things traditional to the species, or so direct the feed- ing movements that untraditional food is excluded. Some adult insects lay their eggs on material that will serve the larvae as food. The larvae acquire the habit of feeding on that material, and do not readily change to something else as adults (Brues 1924, Thorpe 1939). FEEDING ADAPTATIONS Among kinds of mammals, teeth show con- siderable adaptive radiation, correlation with type of food consumed. The molar and premolar teeth of in- sectivorous species, such as shrews and bats, are low and have sharp-pointed cusps for crushing weak- bodied prey. The piscivorous toothed whales have largely lost all differentiation in their teeth, which are simple, conical, grasping structures. The teeth of the carnivorous dogs and cats are high-crowned and tubercular, well fitted for shearing flesh. Herbivor- ous ungulates and rodents have teeth that are flat- crowned, suited to grinding harsh grasses and other vegetation. Their jaws are capable of considerable lateral motion. Omnivores may have both grinding and pointed teeth. Saprovores have rather blunt teeth. The ant-eating sloths and their relatives have no teeth, and the mouth is almost tubular in shape. The tongue has become long and prehensile for lap- ping up tiny ants. The bills of birds display great variety in shape and size, adaptations to feeding in numerous quite specialized niches in the environment. The tongues of birds are variously modified to serve as long probes or spears (woodpeckers and nuthatches), as a strainer (ducks), as a long capillary tube for ob- taining nectar from flowers (hummingbirds), as a rasp (hawks and owls), as a finger for manipulating the food in the mouth (parrots and sparrows), and as a tactile organ (sandpipers and herons) (Gardner 1925). The mouth parts of insects are adapted primarily either for biting and chewing or for piercing and sucking. Among marine invertebrates, adaptations for feeding on detritus (Blegvad 1914) include pseudopodia (Foraminifera) ; ciliated epithelium that maintains a flow of water through the animal (sponges, clams); prehensile, often ciliated arms (various polychaetes; holothuroideans) ; and_ soft eversible gullets (various polychaetes, sipunculids). Those that are herbivorous or carnivorous, as well as detritus feeders, have prehensile tentacles armed with nematocysts (hydroids, actinians), radulae in the mouth (mollusks), eversible stomachs (starfish), and masticatory structures in the mouth or stomach (crustacea, diptera larvae). In addition to mouth part adaptations, there are many modifications in other parts of the digestive tract for handling particular types of food. These adaptations occur throughout the animal kingdom, but are especially evident in birds. A crop is present 188 Ecological processes and dynamics in some species but not in others. The walls of the stomach are more muscular in seed-eating birds than in flesh-eaters. Owls and some other species form and regurgitate from the stomach pellets of indi- gestible matter. Gallinaceous birds are able to retain or shift the supply of gravel or grit in the stomach as an aid to grinding seeds. The length of the in- testine varies with the type of food consumed; the caeca are longer in browsing and seed-eating gal- linaceous birds for digesting cellulose. Associated with these anatomical and histological adaptations are adjustments in function and behavior. Obviously, if an animal has morphological adapta- tions for ingesting and digesting flesh, it must be- have as a carnivore and not as an herbivore. The possession of these adaptations and adjustments means that animals are generally restricted to the particular types of food that they can use most effi- ciently. The kinds of food eaten by animals is of fundamental ecological and economic importance. METHODS OF STUDY A common procedure for analyzing kinds of food consumed by organisms is to identify the con- tents of the crop, stomach, cheek pouches, or other parts of the digestive tract (Hartley 1948). The diet may be described in terms of number of items of each kind of food found in one specimen, or percentage of specimens containing a particular item, but it is usually more satisfactory to measure in one specimen the percentage volume of each food item against the total contents (McAtee 1912). This procedure has the advantage of showing ac- curately what an animal has actually ingested, but has the disadvantage that the animal must usually be killed ; thus, the information obtained is on only one meal, or portion of a meal. It also gives no informa- tion on where or how the food was obtained. Never- theless, considerable information on the food habits of animals has been obtained in this manner (Hen- derson 1927, McAtee 1932, Davison 1940, Martin et al. 1951). Improved techniques make it possible to secure the stomach or crop contents without kill- ing the animal. This is done by manual manipula- tion of the crop (Errington 1932), or by use of flushing tubes (Vogtman 1945, Robertson 1945). Artificial beaks with open gapes placed as decoys among nestlings have been used to collect food brought by the parents (Betts 1954). There are usually indigestible parts in all kinds of food, and these indigestible or undigested parts are eliminated from the body. The contents of fecal droppings or regurgitated pellets can often be identi- fied by differences in shape, size, color and texture, or by histological techniques (Dusi 1949). Collection of droppings is not practicable for aquatic animals, or for more than a few of the terrestrial invertebrates. Moreover, the droppings must be relatively fresh, as they quickly disintegrate in wet weather. The analysis of owl pellets is very fruitful, for owls swal- low all parts of their mammalian or avian prey, and then regurgitate the hair, feathers, and skeleton. Hawks, gulls, and shrikes also produce pellets. The considerable advantage of pellet analysis is the possi- bility of continuous diet analysis on the same indi- vidual, or species, through long periods of time, with- out disturbance to its normal behavior (Dalke 1935, Errington 1932). Whatever method is used, field observation of the feeding behavior of animals in the natural environ- ment is desirable. For instance, one series of stom- achs of the house sparrow contained a large number of May beetles, which would suggest that the bird was important for the control of this insect pest. Observations disclosed, however, that the sparrows were picking up dead beetles littering the pavement under street lights. Field observation itself often fur- nishes considerable information concerning the kinds of food consumed, but the results are usually not quantitative, do not disclose the less conspicuous kinds of food taken, and may be inaccurate if not carefully formulated. A hawk visiting a game farm may take not game animals but undesirable rodents that are also present (Kalmbach 1934). Food chains can be determined by correlating the food eaten by different species in the community. Radioactive elements incorporated metabolically into an organism are taken into the predator of that or- ganism ; radioactivity-tracing technique gives promise of more direct tracing of how matter flows through the ecosystem. A number of radioactive elements may be used ; among them, phosphorus-32 and iodine- 131 (Odum 1959). Interesting studies in this con- nection are being conducted at the Oak Ridge Na- tional Laboratory in Tennessee. CHOICE OF FOOD The kinds of food eaten by animals depend on factors of their genetic heritage, parental train- ing, or conditioning while young. Involved in the evolution of the food habits of a species are the ani- mal’s physical adaptations for ingesting and digesting particular types of food, the nutritional values of the food, its palatability, the size of it, its availability or abundance, and its ease of procurement which de- pends in large part on the various protective devices that it possesses. Food and feeding relationships 189 Nutritional values Animals generally require proteins, carbohy- drates, fats, vitamins, minerals, and water. Proteins are used as the basic substance in the composition of protoplasm; carbohydrates and fats are oxidized to furnish energy for the body; vitamins serve as cata- lysts for specific metabolic processes; minerals are needed to regulate osmotic pressure and as constitu- ent elements of various body organs; and water is used as a general solvent, lubricant, and circulatory medium. Species differ, however, in their needs for particular substances. The beetles, Tribolium, Lasto- derma, and Ptinus, for instance, grow slowly but nonetheless satisfactorily on diets lacking carbohy- drates. Hence, they may be distributed more widely than are species which require carbohydrates in their diet (Fraenkel and Blewett 1943). Foods differ in composition. Foods staple to an organism’s diet are those easily digested, and of high caloric and protein content. They are adequate to sustenance of the weight and vigor of the animal, but usually need to be supplemented with vitamins and minerals. The bobwhite and ring-necked pheasant, for example, eat certain cultivated grains and weed seeds, such as corn, sorghum, barley, wheat, rye, soy beans, pigeon grass, and lesser ragweed as staple foods, at least on a mixed diet. Non-staple or emergency foods are not in them- selves sustentative, and animals limited to them grad- ually lose weight and die. Such foods are, however, often abundant and easily procured in emergencies, when staple foods are covered with snow or ice, and furnish sufficient energy to tide the animal over the critical period. In emergencies, the bobwhite and ring-necked pheasant eat black locust beans, fruits of the bittersweet and sumac, rose hips, dried wild grapes, and sweet clover seeds (Errington 1937). During good acorn years, squirrels, deer, and rac- coons feed extensively on the acorns of white and black oak, but almost completely ignore northern red oak. Experiments with fox squirrels show that the animals gain weight on an exclusive diet of white oak acorns, scarcely maintain weight on acorns of the black oak, and lose weight rapidly on acorns of the red oak. The percentage of tannin in red oak acorns is twice that in white oak acorns, and animals are probably able to distinguish red oak acorns by a bitter taste (Baumgras 1944). Vitamins are necessary for the maintenance of good health in wild animals, just as in domestic ani- mals or man. The symptoms of vitamin deficiency, induced experimentally, are similar. Evidence has been difficult to secure, however, that animals suffer from vitamin deficiencies in their natural environ- ments (Nestler 1949, House and Barlow 1958). Animals obtain most of their required minerals from their food and water. Additional salt must sometimes be given caged animals to prevent canni- balism. The gnawing of castoff deer antlers by ro- dents is apparently for additional salts. The use of certain soil deposits and springs as natural “licking sites’ by deer and other ruminants is apparently for sodium salts lacking in their general diet (Stockstad et al. 1953). Some birds, such as the evening gros- beak, are also attracted to sources of salt supply. There is some disagreement as to the need for grit in the stomach as an aid for the grinding of seeds and hard vegetable matter in gallinaceous and other birds (Nestler 1949) ; this grit may be instead a source of minerals. Animals appear to become aware of nutritional deficiencies in their diet through physiological and neurological mechanisms. Experiments with rats show that when the body lacks some necessary ele- ment such as sugar, salt, or a vitamin, the animal consumes more of that particular substance than usual. Discrimination and selection are apparently made by taste, and a special need for a particular sub- stance sharpens the taste for that substance so that it can be detected even when present in food or water in but very small quantities (Richter 1942). Nutri- tional needs are neither the sole nor necessarily the most important factor involved when animals show preference for one type of food over another. Many other factors condition the choice (Dethier 1954). Palatability Different species of animals vary considerably in efficiency of digestion and utilization of particular food substances. Thus, clothes-moth larvae can digest cloth and bird lice can digest feathers, because among other things they have an exceptionally high hydro- gen ion concentration in their intestines. Digestive enzymes occur generally throughout the animal kingdom although less is known about them in the Protozoa. In the lower phyla the enzymes are generalized in respect to the kinds of foods on which they act; in the higher phyla, they become highly spe- cialized (Prosser et al. 1950). A specific enzyme, however, does not differ greatly from one animal group to another. Carnivores have strong proteases and weak carbohydrases, correlated with their meat diet. Herbivores, on the other hand, have weak pro- tein, but active carbohydrate, enzymes. Herbivorous mammals and birds possess a bacterial flora in their digestive tracts that makes possible digestion of cellu- lose. Omnivores have a full complement of enzymes and can utilize a wide variety of foods. Practically all food contains some indigestible matter; ordinarily, that is passed through the di- gestive tract and eliminated in the feces. If the indi- 190 Ecological processes and dynamics TABLE |3-! Quantitative comparison of food organisms eaten by the brown trout, a carnivore, with those present in the fauna of an English fishpond (data from Frost and Smyly 1952). Sprin Summer Per cent Percent Forage Percent Percent Forage Common name Classification eaten in fauna ratio eaten in fauna ratio MUD-LIVING ORGANISMS Midge fly larva and pupa Chironomidae 36 66 0.5 36 48 0.8 Alderfly larvae Sialis lutaria 10 1 10.0 4 1 4.0 Mayfly naiads Caenis sp. + 6 + 20 + 20.0 Fingernail clam Pisidium sp. 17 16 Hew 5 27 0.2 Worms Oligochaeta 0 1 0 0 2 0 Totals (63) (90) (avg. 0.7) (65) (79) (avg. 0.8) WEED-LIVING ORGANISMS ~Caddisfly larvae Leptocerus Sp. 21 + 21.0 21 1 21.0 Caddisfly larvae Limnophilidae 3 + 3.0 1 1 1.0 Caddisfly larvae Polycentropidae 1 1 1.0 2 1 2.0 Mayfly naiad Leptophlebia sp. 7 4 1.8 0 + 0 Damselfly & dragonfly naiads Odonata 3 1 3.0 2 5 0.4 Beetle adults Coleoptera + + 1.0 + + 1.0 Water-boatman Corixidae 1 3 0.3 2 9 0.2 Snail Lymnaea pereger 1 + 1.0 5 + 5.0 Water mites Hydracarina + 1 . - 3 + Totals (37) (10) (avg. 3.7) (35) (21) (avg. 1.7) gestible material is excessive, or if it contains toxic of species in animals’ food divided by per cent of spe- substances, regurgitation or vomiting may result. cies in habitat (Hess and Swartz 1941). A value of unity indicates that the food item is taken in propor- tion to its abundance; a value greater than unity in- Size of food item dicates that it is taken more frequently ; values of less than unity indicate that the item is either inaccessible, The size of the food in relation to the animal is of the wrong size, too difficult to obtain, or is actu- not of major importance to many herbivores or ally avoided. Table 13-1 is an example of such a saprovores as they normally feed in or on the organ- study conducted upon a carnivorous species. It is ism or substance. With carnivores, the size of the apparent that while there is a relationship between prey must be within their power of conquest. Ordi- the relative abundance of various species and the narily the size of prey is less than that of the carni- degree to which they were taken as food, there are vore that feeds on it, but a high degree of ferocity and also several discrepancies. Weed-inhabiting organ- audacity, or pack hunting in the manner of wolves, isms, more accessible to fish than organisms buried often enables the carnivore to take prey larger than in the mud, are accordingly fed upon heavily, in dis- itself. On the other hand a predator cannot profitably proportion to their relative abundance in the total prey on species so small that the energy derived from fauna. The fingernail clam is fed upon heavily in its consumption does not equal the energy expended spring, since it lies on the surface of the mud. But in its capture. Some very large aquatic animals, how- there is no explanation of why it is fed upon less ever, have become adapted to feed with a minimum heavily during the summer. Water boatmen and of effort on very small organisms occurring in dense water mites, however, are generally not acceptable. concentrations through the evolution of a filtering Although the alderfly larva is a mud dweller, it is fed apparatus in their mouth parts. A good example is upon in large numbers, suggesting that there may be the feeding of baleen whales on plankton. something in its behavior that makes it especially vulnerable, or that brown trout have evolved special methods for securing it. The caddisfly larva Lep- Availability tocerus also appears to be easily taken, as it is de- voured in numbers greatly disproportionate to its In order to determine if a species is fed upon relative abundance. The mayfly naiad Caenis is not in proportion to its abundance (McAtee 1932), it is much fed upon in spring, at which time it is buried necessary to find out what animals have been eating in the mud, but it is fed upon in large numbers in of that which is available in a habitat. The relation summer, at which time it comes to the surface of the between the two may be shown graphically (Hamil- water to emerge. Midge flies also become more vul- ton 1940a) and expressed as forage ratio: per cent nerable during the process of emergence. Zooplank- Food and feeding relationships 191 3, HHH tee See ete ee ee eeeeees 2 ‘ HHH hte Beg et et etter sees 4 + tr + + + + + + + + + + + + + + + + + + tr + + + fs + + + + + + + + + + + + +\ + + + + + + + + + + r + + + + + + + % OF STOMACH CONTENTS (By volume) FIG. 13-2 Monthly variation in kinds of food consumed by an herbivore, the mule deer, in California and Oregon (Interstate Deer Herd Committee 1951, courtesy California Division of Fish and Game). ton is of very minor importance in the diet of the fish. Forage fish were not present. The vulnerability to a predator of a prey species is directly proportional to its relative abundance among the other species available. Voles are pre- ferred by the red fox over white-footed mice, but both mice and voles are preferred over moles, shrews, and snakes. Predator preferences among them, how- ever, occur only when all are abundant. When prey species are reduced in numbers and difficult to get, little or no predator preference occurs (Scott and Klimstra 1955). There is some evidence that predators—particu- larly insectivorous birds—when searching for prey concentrate on one or a few species at a time. By a kind of learning, they acquire what may be called specific searching images for these species and thereby mostly disregard other species. When a new species becomes numerous in an area, they feed on it at first only as the result of chance encounters. To obtain a preference for the new species, they must become conditioned to it gradually and must learn the proper cues of where and how to search it out (Tinbergen 1960). The importance of availability is illustrated by seasonal variations in the kinds and amounts of food consumed by animals. Predator species that remain in one habitat throughout the year must adjust their feeding to the kinds of food available in each season. Species that are unable to do this are compelled to migrate, hibernate, or make other adjustments to sur- vive the unfavorable seasons. Thus birds that are strictly insectivorous may occur in a given area only during the warm part of the year, leaving it before insects disappear. Omnivorous species commonly change from an insect diet in summer to a diet of seeds and fruits in the autumn and are often non- migratory. Seeds and fruits are most abundant and easily ob- tained in the autumn. During the winter they decom- pose, become buried in the snow or softened ground, or are consumed. In Michigan, the weed seeds avail- able on agricultural lands in March are only nine per cent of those available in October (Baumgras 1943). The most critical time of the year, as far as food supplies are concerned, is early spring, the time before new vegetation and hibernating prey animals appear. Abundance of seeds also varies with fertility of soil, which thus influences survival, density, and distribution of animal populations. Animals are subject to considerable variation in the abundance and kinds of food available to them from year to year. In a four-year study of the yield of fruit and seeds from 27 species of trees and shrubs in West Virginia, only 33 per cent of the species pro- duced a crop every year, 29 to 33 per cent failed to produce a crop in three of the four years, and 22 per cent failed twice within the four years (Park 1942). When herbivorous species vary in abundance, because of variation in food supply or other factors, carnivo- rous species that prey upon them often vary in direct proportion. Protective devices Few kinds of plants are well equipped with de- fense mechanisms against the grazing and browsing of animals. Trees and shrubs may be deformed or killed by excessive browsing because the tender ter- minal twigs and buds are destroyed. Grasses are not, for they grow from the base of the leaves; they may even be benefited by moderate cropping. Some spe- cies of trees, shrubs, and cacti are protected from browsing by prickles or thorns. This protection is important to the plants for survival in deserts and in grazing subseres of humid regions. Some plants are noxious or toxic, and animals quickly learn to avoid them. When coloration renders it inconspicuous in its normal environment, an animal is said to have con- cealing or cryptic coloration (Cott 1940). When the coloration or markings reproduce the general tone or characteristics of background, it is called protective resemblance ; disruptive, when the markings break up the outlines of an animal and replace it by some ir- regular configuration so that the animal is not recog- nized as prey. The white collar of the killdeer, ob- served casually from a distance, tends to the human eye to separate the head as a distinct object from the rest of the body. Obliterative coloration, or counter- 192. Ecological processes and dynamics shading, describes the condition where the upper side of the body, exposed to the brighter illumination, is heavily pigmented and the lower side of the body, which is in the shadow, is lighter in color. This colora- tion obliterates the effect shadows have of making a body stand out from its surroundings (Thayer 1910). Aggressive resemblance is where the animal closely resembles some particular object rather than the gen- eral environment. The Kallima butterfly of the Orient and the preying mantis of Central America match the shape, markings, and color of leaves when the insects repose with wings folded. The familiar walkingstick resembles a twig. Several insect species look like bird-droppings. Such resemblances doubtless serve the animals to escape the attention of predators only as long as they remain motionless. Even slight move- ments quickly call attention to animals, regardless of any concealing coloration that they may have. Be- havioral orientation is well shown by those caterpil- lars that are lightly colored dorsally and darkly col- ored ventrally. They bring their counter-shading into proper position by coming to rest upside-down along FIG. |3-3 Protective resemblance of an incubating ruffed grouse to her surroundings (courtesy U.S. Forest Service). FIG. 13-4 Disruptive coloration of a killdeer. The white color obscures the connection between head and body. FIG. |3-5 Countershading. (a) the caterpillar, last instar of Dicranura vinula, in normal upside-down position on a willow twig in natural diffuse light. The back of this animal is lighter than the underparts, annulling the shadow. (b) the same cater- pillar, inverted, is much more conspicuous, for the countershad- ing effect is lost. This caterpillar has second and third lines of defense: when touched, it turns a kind of grotesque ‘‘face" toward you; when pressed, it squirts acid (courtesy N. Tin- bergen). Food and feeding relationships 193 FIG. 13-6 Mimicry of (a) the monarch by (b) the viceroy butterfly. plant stems (Ruiter 1955). The tell-tale shadows cast by animals may be eliminated when the resting animal lies lengthwise, rather than crosswise, to the sun, or when they lie pressed close to the ground. Some animals, on the other hand, are vividly marked with strikingly conspicuous patterns or bright colors, and this aposematic or warning colora- tion is accompanied by unpalatableness in certain butterflies, bugs, beetles, ants, and birds; stings, in wasps and bees; a disagreeable odor in skunks; or some other offensive feature (Poulton 1887). There is experimental evidence that such animals are actu- ally avoided by predators (Finn 1895-97, Jones 1932, Cott 1947). However, a hungry predator is less selective in its choice of food than one that has recently fed. Apparently each individual carnivore must have a personal experience with an animal so marked before it learns to avoid it by associating the coloration with the disagreeable feature. There are many examples of mimicry among in- sects (Goldschmidt 1945). Batesian mimicry is the resemblance of a palatable species in external fea- tures to an unpalatable one that in turn possesses warning coloration and is the more abundant of the two species. The palatable species derives benefit from the relation, since predators, especially birds, avoid them as well as the unpalatable ones. The vice- roy butterfly mimics the disagreeable monarch butter- fly and differs strikingly from other members of its own genus (Brower 1958). There is considerable controversy about mimicry, however, and even the classical example of the butterflies is disputed (Urquhart 1957a). In Mullerian mimicry, both model and mimic are unpalatable. Pooling of num- bers between the two species gives more chances for inexperienced birds to learn to avoid them and re- duces the losses per species during the learning proc- ess (Sheppard 1958). Some animals possess bright spots or colors so placed on the body as to be deflective. The attention of a pursuing predator is drawn to less vulnerable parts of the body; for instance, eyespots on the fins or tail of a fish. Eyespots on the wings of some but- terflies and moths are concealed at rest, but when flashed out by spread wings may frighten away an attacking bird or other predator. Concealing coloration and resemblance to other ob- jects are apparently also useful to animals aggres- sively. A carnivore that matches its background can approach its prey undetected more easily than can a conspicuously marked one. Some predators have di- rective markings to confuse their prey as to the loca- tion of their mouths or to allure them in various ways. There has been considerable controversy about the value of concealing coloration. McAtee (1932) minimized its importance because he found both pro- tectively colored and conspicuously colored species in the stomach contents of the birds he examined. Prob- ably few species are entirely immune to predation, but if coloration to match the surroundings, mimicry of some other avoided species, or peculiarities of form or structure render predation even slightly less fre- quent than it would otherwise be, it can well have survival value and evolve as characteristic of a spe- cles. A number of experiments in regard to concealing coloration have been performed by exposing different kinds of insects (Carrick 1936, Isely 1938), fish (Sumner 1935), and mice (Dice 1947) to bird preda- tors, with the result that those individuals that most closely matched the color of their background were taken less frequently than those that did not do so. In a black aquarium in which equal numbers of black and white mosquitofish were exposed to the preda- tion of a penguin, 27 per cent of the fish eaten were black and 73 per cent were white, but in a white aquarium 62 per cent of the fish eaten were black and only 38 per cent were white. RANGE OF FOOD SELECTION There is considerable range in the variety of foods eaten by most species. Outside of parasitic forms, few animals are restricted to a single species for their food. Herbivorous species are often more specific in their feeding habits than are carnivorous forms. However, even herbivorous forms have vari- ous degrees of restriction, as shown in the following analysis of 240 species of plant-juice sucking aphids (Clements and Shelford 1939) : Percentage Species restricted to a single plant species 27 Species restricted to a single genus but feeding on more than one species 40 Species feeding on several different genera 33 194 Ecological processes and dynamics One may argue with considerable justification that animals such as aphids, gall wasps, some bugs, and others that show host specificity are really para- sitic rather than herbivorous in their feeding be- havior, and hence are not good examples of free- living animals of restricted diet. Actually it is very difficult to find proven cases of animals that confine themselves to a single species of food. It is more common to have an animal feeding on a small group of related species, as do aphids. The potato beetle originally fed chiefly on the sand-bur Solanwm rostra- tum in the Rocky Mountains until about the year 1859, when it began to infest the potato Solanum tuberosum and spread across the country. There may be a biochemical reason for the preference of animals for related plant species, recognized by similarity in taste, odor, or nutritional values. Herbivorous species may be classified in respect to the diversity of their food into : Monophagous: restricted to a single food plant Oligophagous: restricted to a few very definite food plants Polyphagous: feed on many species. The restriction of animals to particular foods may be the result of chemicals affecting odor or taste, or to structural adaptations. Chemical stimuli are espe- cially important with insects (Dethier 1947). The crossed bills of certain birds (Loxia sp.) are well fitted for prying seeds from between the bracts of coniferous tree cones. The Siberian nutcracker has special structures in its bill for cracking the nuts of the Siberian cedar on which it depends almost ex- clusively for food (Formosof 1933). In spite of the monophagy exhibited by some spe- cies, many herbivorous species have a wide choice of food ; the bobwhite quail in Georgia is known to feed on 927 different food species, 107 of them regularly. These are mostly seeds and fruits, although about 14 per cent of the food of this species consists of insects and spiders taken chiefly during the summer months (Stoddard 1931). Restriction of feeding to a single or a few species is a specialized behavior. Feeding on a wide variety of substances or prey usually rep- resents the more generalized primitive condition (Dethier 1954). FOOD CHAINS A single food chain should have at least three links to be complete: plant—=»herbivore—> carnivore. Very often, however, a small carnivore or omnivore may be preyed on by a larger carnivore, and so on until four or five links are involved. Rarely are food chains longer than five links. An example of a three-link chain occurring on the North Ameri- can Great Plains is: grass——> pronghorn——> coyote. A four link chain common to deciduous forest com- munities might include tree foliage——leafhopper — vireo— hawk. A five-link chain would have to involve a number of small species as bacteria—> protozoan—>rotifer——~small fish——large fish. A food chain does not need to start with a living plant; consider, for instance: detritus——>snail—shrew —owl. A strict predator need not necessarily be the last link, it could be an omnivore: flowers—> bees——bear. Saprovores do not fit into food chain diagrams very well because they feed on all links of the chain. Food chains occur in all kinds of habitats and communities, even with the micro-organisms of the soil: detritus—+nematodes—>mites—>pseu- doscorpion. The feeding coactions between the many species that constitute the community are seldom as simple as the food chains just described. The rotifer feeds not only on protozoans but on bacteria. The small fish feeds on insect larvae and many other plankton spe- cies besides the rotifer. The shrew in the forest feeds not only on snails but on a variety of insects, and is fed upon in turn not just by owls but also by hawks, foxes, weasels, and others. If all of these feeding re- lations between species in a community were dia- grammed, a complicated web would be formed—the so-called food web. BALANCE OF NATURE Charles Darwin explained, a hundred years ago, that there was a balance in nature between the abundances of plants, herbivores, and carnivores. Were carnivores for some reason to increase unduly in numbers, they would soon exhaust their food sup- ply and die of starvation. On the other hand, were food plants or herbivores to fluctuate excessively, then predators would vary in a similar manner. There is no doubt that marked variations in abun- dance in one link of a food chain will cause variations in the other links. Rodent plagues in a local area will bring an influx of foxes and hawks; a spruce bud- worm oubreak will result in an increase in the bird population; increases and decreases in soil bacteria are correlated inversely with decreases and increases in soil amoebae that feed upon them (Russell 1923). As a case in point, the relation between the mule deer population and its predators on the Kaibab Plateau in northern Arizona is worth citing in detail (Rasmussen 1941). When this area was made a game preserve in 1906, killing of deer for sport was prohibited. At the same time, there was a marked de- crease in grazing by domestic sheep. During the period from 1906 to 1939, there were 816 mountain Food and feeding relationships 195 lions, 863 bobcats, 7388 coyotes, and 30 wolves trapped or shot, mostly by government hunters. The wolves were exterminated, and the other predators were markedly reduced. The deer in- creased from an estimated 4000 animals in 1906 to an enormous herd of nearly 100,000 in 1924. The deer’s habits and the topography of the country pre- vented a scattering of animals to adjacent ranges. The over-populations of deer consumed all new growth of young trees and browsed the foliage of ma- ture trees as high as they could reach, yet the popu- lation could not secure enough food to keep it in good physical condition. The population far surpassed the carrying capacity of the range. In September 1923, it was estimated that 30,000 to 40,000 animals were on the verge of starvation and during the winters of 1924-25 and 1925-26, an estimated 60 per cent of the population died. Plainly the balance of nature was upset in this instance, with dire results. Hunting was again permitted in 1924, and the herd was reduced to about 10,000 by 1939. This history of the Kaibab deer herd is not unique; similar cases have been re- ported in about a hundred other areas (Leopold, Sowls, and Spencer 1947). Over-populations in all areas have followed reductions in the number of predators, although other factors were involved. Constancy of balance When populations of species with differing food habits are in equilibrium, the surplus of prey species resulting from reproduction is destroyed and con- sumed by the predators. If predation does not de- stroy the total yearly surplus, the prey population increases in size; if predation takes more than the surplus the population of the prey decreases in size. Actually, even in entirely natural communities un- disturbed by man, a strict balance of nature is prob- ably never maintained for any appreciable period of time. It is characteristic for populations to vary in size, but these fluctuations tend to vary rather closely around a certain constant population mean. Depend- ing on the length of its life cycle, a species population in an area may fluctuate from day to day, from sea- son to season, or from year to year. Many factors other than food coactions cause fluctuations in the abundance of animals, and preda- tion is not always the most important (see Chapters 16, 17). Close interdependency between populations of prey and predator species occurs most commonly when a prey species has only one or two important species preying upon it, while the predator species is largely restricted to that one species of prey (Pen- nington 1941). It is obvious, however, that an equi- librium of a sort exists between different species, and this is what is referred to by the concept of balance of nature. TROPHIC LEVELS In order to analyze the intricate coactions involved in the food web and balance of nature, it is desirable to simplify the relationships into nutritional or trophic levels (A) (Thienemann 1926, Lindeman 1942, Allee et al. 1949, Odum 1953). The lowest level (P) is composed of photosynthetic plants that are able to use solar energy for the manufacture of food, and certain types of bacteria that use either the free energy of unstable inorganic compounds or are activated by light to synthesize a limited amount of new organic matter. These are the producers. At the second level (C;) come the herbivores, or primary consumers, at the third level, the smaller carnivores, or secondary consumers (Cz) ; and at the fourth level, the larger carnivores, or tertiary consumers (C3). Occasionally there may be quarternary consumers (Cy). The terms “producer” for plants and “con- sumer” for animals were used, and the essential rela- tionship understood by Dumas in 1841. These two groups are also distinguished as autotrophic and heterotrophic, respectively. The levels in subdivision of consumers are not sharply defined, as the feeding behavior of some spe- cies involves them simultaneously in several levels. Actually, the more remote an organism is from the initial source of energy (solar radiation), the more likely it is that it will prey on two or more levels. This need not confuse the essential relationships in- volved (Lindeman 1942). Omnivores overlap be- tween levels C; and any of the higher levels. Large saprovores, the heterotrophic bacteria, and fungi de- rive their nourishment from the excreta and dead bodies of organisms from all trophic levels. Since they are reducers or decomposers, they may for sim- plification be grouped with the autotrophic bacteria, and be called transformers (T), since their total effect is to convert dead organic matter into nutrients that green plants can again absorb. Figure 13-7 illustrates how a complicated food web may be simplified, somewhat arbitrarily, into trophic levels. Detritus, derived from the disintegra- tion of dead organisms and excreta from organisms, is worked over by the bacterial transformers, and the detritus and bacteria represent an independent base of the food web separate from the green plants. A characteristic of trophic levels in most com- munities is that the nearer a level is to the source of energy, the greater the diversity of species involved. Thus in Fig. 13-7, the primary consumers (C;) in- clude some twelve taxa; the secondary consumers (Cs), four; the tertiary consumers (C3), three; and the quaternary consumers (C4), but one. Species in the lower trophic levels have a higher rate of repro- duction than those in the higher levels to compensate greater predation. 196 Ecological processes and dynamics Deep bottom Open water e--- Sheepshead --¢ Leptodora:: ©) +++++ Digptomus:-** eeeeeees Cyclops Gammarus s«Epischura:** penton Daphnia-*++++ aeend Bosmina-** Hexageniaeé / K\¥ Brrr Eurycerus:: -- Phytoplankton -- ©) ©) Detritus ------ ---Detritus-~ FIG. 13-7 Food web in western Lake Erie, leading to the sheeps- head fish. Species are separated into their different trophic levels. The diagram would be even more complicated if other fish species in the C4 trophic level of the community had been in- Food and feeding relationships Shallow bottom Mi; Lie sy ae Sin aes i a 9 ) nats Beetle larvae sce-oms« () ‘} OX \ ME e--- Baetinine mayflies::-«7 Gammoarus******** wreeseeee (QISFIICS 60>. ae CegHi20¢ Respiration — CgHi120¢ ar 605 = (6 COs +6 HsO ar 674 Keal Respiration is going on at all times to furnish energy for the plant’s activities, and this energy is derived from oxidation of the sugars formed in photo- 200 : t r synthesis. Under ordinary daytime light intensities, the amount of sugar formed by photosynthesis greatly exceeds the amount oxidized in respiration. Photo- synthesis ceases during darkness, but loss of sugar at night, because of respiration, continues. Sugars may be converted to starch or fat or upon combina- tion with nitrogen, sulphur, and phosphorus be changed into proteins. The amount of sugar resulting from photosynthesis represents the primary produc- tion of the ecosystem. Accurate measurement of the rate of primary production is one of the most impor- tant problems of trophic ecology, for the activities of all plant and animal organisms in the community de- pend on the energy thus supplied. It is necessary to distinguish between gross production, the total amount of energy captured, and net production, the amount that remains after that used for respiration. Primary production is commonly expressed in terms of glucose or carbon, or indirectly in the amount of oxygen released or carbon dioxide absorbed, all of which can be converted into calories of energy. Use Net energy at the producer level becomes avail- able for use of animals when it becomes transferred to the higher trophic levels through predation, here considered also to include consumption of plants (Riley 1940, Lindeman 1942, Clarke 1946, Clarke, Edmondson, and Ricker 1946, Birch and Clark 1953, Macfadyen 1957). In order to measure the transfer and use of energy at each trophic level (A), it is de- SOO - .550 SS 550 See < Saas oot, + we % one, “ *. - = * + y * Sete , * *, 600 105 * cane % * ae ate, we ate PALLET rrr rrr . Cer aa Py O tesyaees™ Exchanges, productivity, and yield ‘4 * *, ’ '. Va, M7 or BO CTT > sirable to know the size of the standing crop or bio- mass (B) at each level. Energy is acquired by ani- mals only through the consumption of food, which may be indicated as the gross energy intake (I). A good portion of the energy ingested is used for existence, that is for basal metabolism, temperature regulation, procurement and digestion of food, and other normal activities. There is almost continuous loss of heat energy from the body and in homoio- therms this must be compensated for by increased heat production. Energy is used for the production of eggs and sperm, reproductive behavior, and other ac- tivities in the normal life of the individual animal. Even the process of converting raw food into proto- plasm is work and requires energy. In transfer of energy from one form to another, there is always loss of free energy. No transfer is 100 per cent efficient. This is the second law of thermodynamics. The total energy that is utilized to perform work and to produce heat 1s called respiratory energy (R). Transference The gross energy intake, /, less the respiratory losses of energy, R, gives the net production of the consumer levels, the same as at the producer level. The rate at which net production accumulates is net productivity or simply productivity (A). Net produc- tion may be lost to the trophic level in the decompo- sition of excreta and dead animals; it may become evident in the growth and increase of population ; or it may be transferred to a higher trophic level. 500" 500 . FIG. 14-1 Isolines of average solar radiation (g-cal/cm?/ day) received in July ona horizontal surface in the United States during days of average cloudiness. In December, isolines run nearly straight and parallel across the country, and are of values less than 100 in the North and over 250 in the South (Fritz 1957). 201 TERTIARY CONSUMERS (C3) EOWgyl SECONDARY CONSUMERS (C,) / PRIMARY CONSUMERS (C,) lg E.DW TRANSFORMERS (T) PRODUCERS (P) FIG. 14-2 Energy flow through an ecosystem the trophic levels of which are in balance with each other; symbols are explained in text. The weights of the arrows are intended to suggest the relative proportions of energy flow in the various directions, but this is schematic because the proportions vary widely in different ecosystems. Certain of the food taken in, J, is indigestible or may simply be undigested, or if digested and ab- sorbed is not completely metabolized in the tissues, so that it is eliminated in feces and excreta. This is designated excretory energy, E. Assimilated energy is the energy of the food actually absorbed and uti- lized (I — E). These terms may also be applied to the producer level when J represents the total solar radiation reaching the plant, and E is that portion of the radiation not used. Aside from being killed by predators, organisms die from a multitude of other factors such as disease, extreme weather, starvation, combat, old age, and accident. In order for the populations of the different trophic levels to be maintained at a more or less con- stant level, organisms that die non-predatory deaths, D, must be replaced by the reproduction and growth of new individuals. For energy to be transferred from one trophic level to a higher one, organisms must be killed and eaten by predators of the higher trophic levels. When predators consume their prey completely, as do fish feeding on plankton, there is no wastage, but with many predators, the prey killed is so large it must be eaten piecemeal, and much is not used. Predatory kill must therefore be separated into the energy con- sumed by the higher trophic level (7,1) and the energy wasted, W. Energy lost from a trophic level through excreta, non-predatory deaths, and wastage from kills is used by saprovores or transformers, T. This allows for the decomposition and conversion of nitrogen and other compounds into nutrients suitable for reabsorp- tion by plants. Energy from the transformer level recirculates into higher trophic levels when detritus and the transformer organisms, bacteria, fungi, pro- tozoans, and the like, are consumed, as indicated in Fig. 14-2. When populations of different trophic levels are in balance, the total net production of each trophic level, after losses from excreta, non-predatory deaths, and wastage from kills have been subtracted, is con- sumed by predators of the higher trophic levels. In unbalanced populations, predatory consumption may not equal the available net production, so that the population of that trophic level increases. If the predatory kill exceeds the available net production, the population decreases. A change in the biomass, b, of a population may, therefore, be either plus or minus. An entire ecosystem is in balance when the total exchange of oxygen and carbon dioxide between consumers and producers is equal. Increase in biomass comes with the growth of in- dividuals. When an individual organism grows, it increases in size and weight by adding organic mat- ter. When reproduction takes place there is an in- crease in number of individuals, but not necessarily an increase in biomass, which takes place only if the offspring increase in size. Since individuals of most species have limits of growth increase, reproduction increases the potential productivity of a community by adding to the number of individuals capable of growth. MEASUREMENT OF PRODUCTIVITY Productivity may be measured during any reasonable period of time. Because of essential met- abolic differences between day and nighttime, how- ever, the 24-hour day is the smallest practicable unit. Similarly, because of seasonal changes in the environ- ment and in community populations, the measure- ment of annual production is probably most useful. Production is commonly expressed in terms of indi- viduals, biomass, or preferably calories per unit area per unit time. The basic problem is that of analysis 202 Ecological processes and dynamics of how energy is obtained, how efficiently it is used, what portion of it is available for reproduction and growth, and how much of it is passed on to higher trophic levels through predation. These many factors may be brought together in the following equation to show what happens to the gross energy intake, /, at any trophic level : IN=ETR+D+W + lagi tb One can make a number of derivations from this equation; but what is of particular interest and im- portance is the amount of energy of each trophic level that is transferable to the next. This is represented by the equation: ng RD ee 8 PY ET Each factor in the equation must be measured at each trophic level, and for each species. Since primary production is basic and concerns the capture of en- ergy by plants, it will be considered first. Primary production Various methods are employed for measuring primary productivity, each procedure having certain advantages and disadvantages (Ryther 1956). Fur- ther work in evaluating and improving these meth- ods or developing new ones is desirable. A common procedure for analyzing aquatic habi- tats is to take equal samples of green phytoplankton, ordinarily inseparably mixed with bacteria and zoo- plankton, and suspend during daylight hours in both transparent and blackened bottles at the same depth at which obtained. Photosynthesis of course does not occur in the blackened bottle, and there is a loss of oxygen, resulting from respiration, R, and decompo- sition, E + D + W. In the transparent bottle, photosynthesis occurs in addition to respiration and decomposition, bringing a production of carbohy- drates. There will either be an increase in oxygen concentration, or the loss of oxygen will not be so great as in the blackened bottle. The difference in the final oxygen content of the two bottles will be a measure of gross production: Ip. If the oxygen content of the water is measured at the beginning of the experiment, then the loss of oxygen in the blackened bottled subtracted from the difference in oxygen content of the two bottles at the end of the experiment will represent the net pro- ductivity. This net productivity may also be deter- mined from the difference in the oxygen content of the transparent bottle between the beginning and the end. To obtain net production for an entire daily cycle, the consumption of oxygen for respiration and decomposition over 24 hours must be subtracted from the gross photosynthetic output during daylight hours. The use of black and white bottles for meas- uring productivity has been criticized because of a possible difference in the amount of oxygen utilized by bacteria in the two bottles (Nielsen 1952, Pratt and Berkson 1959), but the occurrence of a signifi- cant difference has been denied (Ryther 1956). One may use the amount of carbon dioxide, rather than oxygen, absorbed during a period of time as a measure of photosynthesis, if correction is made for the carbon dioxide given off in respiration and de- composition. Changes in the amount of COs in the water may be calculated from the differences in pH, the hydrogen-ion concentration (Verduin 1956). Net production during daylight hours may be measured by introducing a known amount of COs into a vol- ume of water where the amount of carbon dioxide already present is known. The amount of C'* ab- sorbed by the phytoplankton can be accurately deter- mined by use of counters applied to phytoplankton collected and dried at the end of the period. Then the proportion of the radio-active carbon absorbed to the amount introduced can be applied to the total CO. initially present to get the total amount absorbed (Nielsen 1952). In fertile eutrophic lakes there is a continual sink- ing of dead organic material, derived chiefly from the plankton, from the epilimnion into the hypolimnion. The decomposition of this material absorbs oxygen from the hypolimnion and liberates COs to produce a stagnation period during the summer months. The amount of oxygen deficit, or carbon dioxide incre- ment, and the rate at which it forms can be measured to furnish a rough index of the lake’s net productivity during the period between spring and autumn over- turns (Hutchinson 1938, Ruttner 1953). Such esti- mates are in error by the amount of organic material brought in by streams, and they will vary in com- parative usefulness depending on the volume ratio of hypolimnion to epilimnion (Hutchinson 1957). Since nitrogen and phosphorus are metabolized more rapidly by plants in the manufacture of food during the growing season than they are regenerated from decomposing material, the rate and extent of the depletion of nitrates and phosphates in freely cir- culating bodies of water or in the epilimnion of strati- fied lakes serve as an index of the amount of organic matter produced (Hutchinson 1957). The rate of ac- cumulation and regeneration of these substances in the hypolimnion from the dead organisms that sink into it may also be used to get an approximation of primary production (Waldichuk 1956). These meas- urements are not exact since they do not account for the repeated regeneration and reutilization of the sub- stances in the photic zone during the season nor their transference to and storage in the bodies of animals. The rate of photosynthesis varies in relation to the amount of chlorophyll present and to light intensity Exchanges, productivity, and yield 203 (3.7 gC assimilated/hr/g chlorophyll in marine phy- toplankton at light saturation) (Ryther and Yentsch 1957). The amount of chlorophyll in the plankton may be determined photometrically for all depths and calculated in terms of unit area of surface. Measurement of productivity in flowing waters, such as streams, presents special problems. Most of the vegetation is benthic rather than planktonic. De- termination of the rates of photosynthesis and oxygen use are best made by direct measurement of changes in the concentration of oxygen in the water as be- tween day and night. Because of photosynthesis, there is a net increase in oxygen concentration dur- ing the daytime. At night, photosynthesis ceases, but oxygen use continues, so the oxygen loss gives a measure of the rate of respiration and decomposition, and this pre- sumably remains the same throughout the 24-hour daily cycle. Adding the average hourly night loss to the average hourly gain during the day and multiply- ing by the hours of daylight gives the total gross pro- duction for the 24-hour day. To obtain the net pro- duction for the entire day, the hourly loss at night must be multiplied by twenty-four, and subtracted from the total gross production. Corrections need to be made, however, for the greater diffusion of oxygen from the air into the water at night, when oxygen concentration in the water is lowered, than during the day, when it is higher. There may actually be dif- fusion of oxygen out of the water during the daytime when the photosynthetic rate is high. Additional cor- rections will also be necessary for import of oxygen from ground water and surface drainage and trans- portation of oxygen and carbon dioxide downstream by swift currents (Odum 1956, 1957). On land, the annual net production of herbaceous plants that grow from seed or underground parts is approximately equivalent to the biomass of the vege- tation at its maximum stage of growth, provided no appreciable amount has been lost or consumed by animals. Exclosures may be erected to prevent graz- ing by larger animals. The measurement of total pro- duction is inexact when the maximum standing crop occurs before the terminus of the growing season. The annual crop divided by the length of the grow- ing season gives net productivity in terms of average daily increment. Daily productivity varies, however, with the stage of growth. At one site, the cumula- tive productivity of common cat-tail, from March 29 to July 2, when the largest crop was reached, aver- aged 8.17 grams carbon per square meter per day, but for a short period of maximum growth, May 4 to 28, was 23.48 (Penfound 1956). Seasonal bio- mass production of grasses is increased by a moderaté amount of grazing, so in measuring primary pro- ductivity under natural conditions, the stimulating or inhibiting effect of animal consumption should be given proper evaluation. The annual woody increment of trees and shrubs is proportional to the increase in diameter or width of growth rings. Mature trees may be felled, unit samples of branches, trunk, and roots dried and weighed, and the annual woody production since germination calculated (Ovington 1957). Attempts have been made to measure respiratory losses (Moller et al. 1954), and the annual production of foliage, seeds, acorns (Downs and McQuilken 1944), and nuts, but measurement methods need to be improved (Baldwin 1942). Secondary production When an animal species is represented by a low overwintering population, or an immature stage, the maximum biomass obtained in each generation is the approximate net production for that generation. However, this does not account for continued repro- duction and growth of individuals after the maxi- mum biomass of the population is attained, nor does it account for excreta, natural deaths, or the kill of predators. If the population of the species is main- tained at a more or less uniform level throughout the year, then the mean biomass times the number of generations gives the net production, with again the exception of the factors mentioned above. Lindeman (1941) considered the phytoplankton turnover, or the production of a new generation, to occur every week from May through September and every two weeks through the rest of the year, the zooplankton to re- place itself bi-weekly through the year, Chaoborus to have three generations per year ; midge flies, two; and various aquatic beetles and bugs, one generation per year. Juday (1940) estimated that the mean standing crop of both phytoplankton and zooplankton replaced itself every two weeks throughout the year. To obtain gross productivity, the respiratory rates of these animals must also be measured. Although shortcut methods may often be prac- ticable, we need more accurate data, based on careful field observations and experiments, of food require- ments, reproductive rates, growth rates, mortality, and so on, for individual species. The energy intake and requirements of individual species can often be measured under experimental conditions by present- ing a known amount of food to one or several indi- viduals and determining the amount consumed dur- ing a period of time. This is preferable to measuring the oxygen intake of resting animals. The influence of various environmental factors, size and age of the animals, density of populations, etc., on the food con- sumption can be ascertained and often also the pro- portion utilized for existence, growth, and other ac- tivities (ground animals, Bornebusch 1930; Tubifex, Ivlev 1939; grasshoppers, Smalley 1960; rotifers, 204 Ecological processes and dynamics Edmondson 1946; Daphnia, Richman 1958; fish, Ricker 1946, Gerking 1954; birds, Kendeigh 1949; mammals, Golley 1960). Occasionally, the amount of food required by a species can be determined un- der natural conditions. Beavers in Michigan cut 216 poplar trees, averaging 5.25 cm in diameter, per year per individual, in order to use the bark and cambium as food. With 3750 such trees per hectare (1500 per acre), one hectare (2.5 acres) could sup- port a pair of beavers for about 9 years (Bradt 1938). Mortality from non-predatory causes, D has rarely been measured. Combined rates of mortality from predatory and non-predatory causes may be computed from the size of populations of different age groups, or from recaptures of marked animals. If one knows the population of predators in an area and their average food requirements, non-predatory deaths may be calculated. PRODUCTIVITY VALUES The photosynthetic efficiency of plants in the use of solar radiation under natural conditions is very low, in terms of calories. Only a small percent- age of the available radiation is actually absorbed and used. The growing season of plants is limited to only a portion of the year. Much radiation is reflected back from ground and water surfaces, or is absorbed by the ground or water or other non-living material, and later radiated back into the atmosphere. Some wavelengths of the spectrum are relatively ineffective in photosynthesis, and complete utilization of even the effective portion of the spectrum may be limited by temperature, available moisture, concentration of carbon dioxide, and amount of chlorophyll present. The utilization of solar radiation received at Lake Mendota, Wisconsin, has been apportioned as follows (Juday 1940), last two figures corrected: Melting of winter ice 2.9% Annual heat budget of water 20.4 Annual heat budget of bottom 1.7 Evaporation 24.7 Reflection 24.0 Conduction, convection, radiation 25.9 Gross biological intake 0.4 The extent to which solar radiation is used de- pends considerably on the luxuriance of the vegetation. Deserts and short-grass plains have a gross primary productivity in dry weight of less than 0.5 g/m?/day ; ocean, fresh-water communities, forests, and prairie from 0.5 to 5.0; coral reefs, estuaries, mineral springs, semiaquatic communities, and evergreen forests may produce up to 20 (Odum 1959). Miscellaneous data on secondary productivity are given in Chapters 5-9. S p» moles/liter/hour if <—AM|2 PM— 6 12 HOURS FIG. 14-3 Rate of carbon dioxide removal, photosynthesis, at different times of day in western Lake Erie. Negative values at night mean that carbon dioxide is being added rather than removed (after Verduin 1957). In both freshwater (Verduin 1957) and the sea (Shimada 1958), maximum rates of photosynthesis occur in the early morning. Monthly variations show a relationship between photosynthesis and solar radi- ation, although it is observed that maximum photo- synthesis occurs in June rather than in July for rea- sons that are not clear. Photosynthesis in the sea increases proportionally with light intensity until saturation intensity is attained (Ryther 1959). The consumer trophic levels derive energy from transformers, T, as well as from green plants, P. The number or biomass of bacteria, fungi, saprovores, and detritus feeders depends on the amount of organic matter that accumulates from non-predatory deaths, D, excreta, E,and waste from predatory kills, W. The ratio between predatory consumption (/4+1) and E, D, W doubtless varies in different ecosystems. Adult lions in Africa kill, on an average, 20 kg (44 lb) of food per day, although they require only about half of this amount for existence. The unused 300 oh Oay aici 600 > 500: 2 3 “E 200 400 & 3 8 300 5 € ce a 100 200; = 100 fo) ) J °F MAM Ju AS) O ND ou MONTHS FIG. 14-4 Rate of carbon dioxide removal, photosynthesis, in different months of the year in western Lake Erie correlated with intensity of solar radiation (after Verduin 1956a). Exchanges, productivity, and yield 205 portion of the kill supports a large population of bird, mammal, and invertebrate scavengers (Wright 1960). Harvestmen are predaceous on insects. Males waste nearly three-fourths of their prey, females about one-third. Of the food ingested, about 46 per cent of the energy is assimilated; the remainder is eliminated in the feces (Phillipson 1960). From measurements made at Lake Beloe, nat- ural mortality of Tendipes plumosus and other bot- tom fauna is reported to be twice predation (Ricker 1946). In a Massachusetts spring-pool, however, non-predatory deaths of another species of midge and of planarians amounted to one-fourth predation (Teal 1957). Ina small Indiana lake, non-predatory deaths of bluegills amounted to 64 per cent of the average protein content of the standing crop during the year, but only 29 per cent of the total turnover of protein (Gerking 1954). In an Arctic lake not in trophic balance, the net productivity of the entire community, as shown by the accumulation of bottom deposits, averaged something less than 2 mg organic matter / cm?/yr during the 6000 years since recession of the Pleistocene glacier, compared with 8 mg/cm?/yr in a Connecticut lake during a comparable period of time (Livingstone et al. 1958). The food chain or food web thus has a double base, ~ although this useful energy, made available by the transformers, disappears as it recirculates into higher trophic levels. In balanced communities it is continu- ously replenished from the producers. Feeders on detritus and transformers are important both in aquatic habitats and in the soil. Mud flats on the Cali- fornia coast contain at least 39 g/m* dry weight of bacteria. Assuming that this biomass increases only 10 times per day, there would be 390 g/m? produced per day available for nourishment of the animal pop- ulation (ZoBell and Feltham 1942). Some of the most intensive and accurate studies of productivity are being carried out with phyto- and zooplankton in marine waters (Riley et al. 1949, Riley 1952, Deevey 1952). It is calculated that the total annual fixation of carbon by photosynthesis in Long Island Sound is about 470 g/m?. Over half of this amount (265 mg) is used in the respiration of phytoplankton (56 per cent). Of the 205 g/m? net production, 26 per cent appears to be used by the macro-zooplankton, 43 per cent by the micro-zoo- plankton and bacteria, and 31 per cent by the benthic fauna and flora (Riley 1956). In the sea off Plym- outh, England, it has been estimated (Harvey 1950) that the zooplankton is required to assimilate daily an equivalent of 4 per cent of its dry weight in vegetable matter just to meet respiratory needs, and 7-10 per cent for growth and to offset the amount consumed by other animals. Thus, of the energy intake at this level, approximately 30 per cent was used for respira- tion and 70 per cent for growth. On the other hand, pelagic fish, at a higher trophic level, used 90 per cent of assimilated food for respiration and converted only 10 per cent into body tissue. Primary productivity of coral reefs is far greater on an areal basis, 1800-4200 grams carbon per square centimeter per year, than for most marine habitats, 28-912, but 4650 in an eelgrass bed off Florida. This high productivity is probably due to the luxuriant benthic algae on the reef platforms (Kohn and Hel- frich 1957). There are difficulties in obtaining accurate meas- urements of all factors for complete ecosystems. In his pioneer study of lakes, Lindeman (1942), and later also Dineen (1953), found that the percentage of gross energy intake that became transferred through predation to the next higher trophic level became progressively greater (Table 14-1). He also found that the percentage respiratory loss increased at each higher trophic level, and this has been con- firmed for a terrestrial food chain involving plants, mice, and weasels (Golley 1960). In a Montana reservoir, of the total gross pro- duction of the phytoplankton during the summer months, 17 per cent was dissipated in respiration, 4.5 per cent was converted into increase of phytoplank- ton, 71 per cent was consumed by macro-zooplankton, and 7.6 per cent utilized by bacteria, micro-zooplank- ton, and bottom fauna. The energy intake of the macro-zooplankton was divided 90 per cent for res- piration and 10 per cent for population increase (Wright 1958). Effort should be made to investi- gate these relations more accurately, especially in ecosystems in which all trophic levels are in equi- librium. Analyses of energy relations in complete ecosys- tems have been made by Odum and Odum (1955) for a coral reef community in the Pacific Ocean, and Odum (1957) and Teal (1957) for spring-fed streams and pools. These studies of flowing waters are complicated by the export of energy downstream with the current. SUCCESSION When trophic levels are in balance within an ecosystem, all the net production of one level is consumed by other levels, and there is neither a sur- plus nor a deficit in the total annual production. The biomass of annual plants, the foliage of perennial plants, and the populations of animals with one or more generations per year reach large size at the end of a growing season, far beyond possible immediate consumption by predatious animals at higher trophic levels. Yet this biomass does not accumulate. With 206 Ecological processes and dynamics TABLE |4-| Gross productivity (!,) and efficiencies (1, + 1|)/ (I,) in lakes and ponds. Cedar Bog Lake, Minnesota Trophic level Lindeman (1942) Lake Mendota, Wisconsin Lindeman (1942) Minnesota pond Dineen (1953) g-cal/cm?/yr Efficiency g-cal/cm?/yr Efficiency g-cal/cm?/yr Efficiency Radiation 118,872 -- 118,872 -- 118,872 -- Producers 111.3 0.10 480 0.40% 49.9 0.04 Primary consumers 14.8 13.3 41.6 8.7 9.2 18.4 Secondary consumers 3.1 22.3 2.3 5.5 3.4 36.9 Tertiary consumers --- -- 0.3 13.0 --- -- YIELD the onset of winter these organisms die, and during the following season their dead bodies are worked over by the transformers so that the nutrients and surplus energy that they contain recirculate through the ecosystem until all energy is dissipated. Measure- ments of COs exchange between plants and animals in western Lake Erie indicate that photosynthesis of the producers and respiration of the total aquatic community are approximately equal (Verduin 1956a). In a coral reef community in the Pacific Ocean, the balance in the ecosystem was reached at a level of energy exchange between producers and consumers of approximately 96 Keal/m?/day (Odum and Odum 1955). Such an equilibrium of energy exchanges is found only in some climax communities. It is more charac- teristic of seral communities for the total net pro- duction of all trophic levels to be greater than can be utilized during the course of the year. On the death and decomposition of these organisms, more organic matter is added to the substratum than can recirculate through the ecosystem. This increases the fertility of the soil. In marsh and bog areas this sur- plus accumulation of organic matter may be consider- able. Fixation of nitrogen commonly exceeds de- nitrification in seral communities ; there is absorption of minerals from underlying rock; and in ponds and lakes there is an influx of nutrients from the sur- rounding drainage basin. All these processes increase the carrying capacity of the habitat so that progres- sively larger standing crops can occur. As fertility increases, changes also occur in the species composition of the community. Species that formerly were unable to occur because of low energy resources or other intolerable situations now find conditions favorable. Their invasion forces those resident species that cannot withstand competition to disappear. Oligotrophic lakes become eutrophic, and then marsh. Bog mats change to bog forests and eventually to the climax (Lindeman 1942). Sandy and rock habitats accumulate greater fertility with each succeeding stage. These changes persist until the climax stage is reached, where respiratory loss of energy balances the energy gain of the producers. Exchanges, productivity, and yield In such applied fields of ecology as wildlife management, forestry, animal industry, and agricul- ture, the objective is to harvest the available net pro- duction for human benefit rather than to let it ac- cumulate in the natural habitat or be used by other organisms. Crops of game animals, timber, or food are removed periodically to give a yield. Man is an animal consumer and removes the net production that would ordinarily be taken by the herbivore or carni- vore plus whatever additional growth, b, occurs. If man takes his yield as a primary consumer he will obtain more energy in food per unit area than if this yield is obtained at higher trophic levels (Fig. 13-8d). Since excretory and respiratory losses and non- predatory deaths bring an accumulative dissipation of energy at progressively higher trophic levels, the potential yield of game or food animals decreases the higher the position of the species in the trophic level of the community. The fewer the links present in the food chain, the greater the yield of game or fish. More plankton-eating cisco or herbivorous carp can be harvested in a given area than fish-eating bass, more muskrats than mink, and so on. In human eco- nomics, a land will support larger populations of people if they are content with eating rice, wheat, or corn than if they require the extravagance of beef, pork, and lamb. Yield should never exceed net production, lest with reduction of the standing crop the productive potential become exhausted. On the other hand, hu- man economics make it desirable to harvest the maxi- mum yield that the ecosystem can supply without jeopardizing continued production year after year. The determination of maximum sustained yield or optimum yield in harmony with the productivity and maintenance of a steady state in the ecosystem is one of the most vital and complicated problems in applied ecology (Russell 1931). Maintenance of maximum productivity is also of importance to the organisms themselves as it permits attaining of large populations in each species. There is a point in the growth curve of all popu- 207 lations at which the species is making the greatest use of the energy resources of the ecosystem and growing most rapidly without the depressing effects of intra- and inter-specific strife, predation, and dis- ease becoming excessive. This is the point of inflec- tion between the accelerating and inhibiting phases of the growth curve, the point at which the incre- ment curve reaches its highest point. In most species investigated, this occurs at a population size approxi- mately half that of the asymptote. Theoretically if the yield were so great that the population is kept below this point, total production would be reduced because of the small parent breeding stock that is left. If the population were allowed to go beyond this point of inflection, fewer offspring would be brought to maturity because of increased competition and other factors. It appears that the optimum yield should be such as to maintain the population con- tinuously at this level, and thus balance maximum annual production (Hjort e¢ al. 1933, Ketchum et al. 1949, Scott 1954). The problem of optimum yield of animal and plant species for human use seems to reduce, then, to determining the point of inflection in the population growth curve of the species con- cerned, keeping in mind that productive capacity varies between different species and habitats. It may well be that balanced ecosystems have evolved under natural conditions so that predation is of such intensity as to maintain populations of prey at this level of maximum productivity. In a small pond in southern Michigan, in which no predatory fish were present, the benthos biomass increased two- or three-fold during the season to an upper asymp- tote, after which there was no net productivity. In another similar pond with fish present, the benthos biomass eaten by fish never reached this asymptote, and productivity was maintained continuously at such a high rate that the production during the growing season amounted to 17 times the standing crop (Hayne and Ball 1956). Experimental work has not so far demonstrated a relation between optimum sustained yield and the point of inflection in the population growth curve. In laboratory cultures of flour beetles, productivity in- creased progressively with rates of exploitation that brought the surviving population far below the point of inflection (Watt 1955). With Daphnia pulicaria, maximum sustained yield occurred over a period of time when 90 per cent of newborn animals were re- moved at regular intervals (Slobodkin and Richman 1956). In experimental populations of guppy fish, the standing crop was reduced but the yield was greatest when the tri-weekly exploitation removed 30-40 per cent of the individuals, and the population mass was at one-third its asymptotic level. An ex- ploitation rate of 75 per cent brought extinction of the population (Silliman and Gutsell 1957). The lack of agreement between experimental re- suits and theory may be due in part to the fact that the age distribution of the population after such harvests is not the same as under conditions of nor- mal population growth. In an attempt to keep a nat- ural population of Norway rats in Baltimore at the inflection point, it was found necessary to remove one and a half times as many animals as expected from an analysis of the growth curve. After a few months, however, the populations collapsed, probably because the average age of the females was reduced until they were too young to breed (Davis and Christian 1958). Certainly much more study is required to deter- mine practical means of estimating optimum yield, to understand the factors involved, and to put proper harvesting procedures into operation (Beverton and Holt 1957, Ricker 1958). In North America, several species have been exterminated through overuse ; the passenger pigeon, for instance. On the other hand, there is evidence that in some localities the yield an- nually taken of fish, muskrats, and deer is not as great as populations of these species could support. With organisms that have no specific adult size but continue growth throughout life, such as fish, yield should be calculated in terms of biomass. With these animals there is the additional problem of de- termining the minimum size limit of individuals which would provide the greatest sustained yield in weight for the population as a whole (Saila 1956, Ricker 1958). In undisturbed ecosystems, non-predatory deaths and excreta at all trophic levels return both organic and inorganic nutrients to the substratum in amounts sufficient, when completely regenerated, to maintain the standing crop, and input of solar energy replaces respiratory losses. With the harvesting of plant and animal crops by man, however, there is removal of nitrogen, phosphorus, calcium, and many other min- erals from the ecosystem that can be replaced only very slowly by natural processes. Artificial fertiliza- tion is ultimately necessary, therefore, when yields are taken. Artificial fertilization is often also desir- able in early stages of succession, when the natural supply of nutrients in the soil or water is a limiting factor. Addition of nitrates and phosphates to sterile ponds usually results in a sudden bloom of phyto- plankton. This bloom is later followed by increases in animals at the consumer levels and greater yields of fish. SUMMARY Energy, unlike nutrients, does not circulate indefinitely through the ecosystem. It is continuously dissipated to perform work and produce heat, and hence must be continuously replaced. The chief 208 Ecological processes and dynamics source of energy is solar radiation. This energy is captured by green plants in photosynthesis. Some of this energy is transferred to higher trophic levels through predation, but the amount that is transferred decreases at each higher trophic level in spite of the greater efficiency of predation, until none remains. Gross productivity is the total energy intake per unit area and unit time at any trophic level. It is called primary productivity at the producer level and secondary productivity at the consumer levels. Gross energy intake minus respiratory losses is the net pro- duction. Net production may be lost to the trophic level in excreta and dead animals, produce growth and in- crease of populations, or be transferred to a higher trophic level. Energy lost in excreta, non-predatory deaths, and wastage of predatory kills is utilized by the transformers to activate the nutrient cycles (Chapter 11), and some of it recirculates again through the ecosystem. A variety of methods are be- ing developed to measure quantitatively the various uses and flow of energy through the ecosystem. In seral stages, annual production exceeds total utilization so that accumulation of energy and nu- trients results. This increases the fertility of the sub- stratum. In climax communities total utilization may balance total production so that the ecosystem is at an equilibrium. Productivity of populations is sustained at a faster rate over a longer time if the surplus production above a certain level is removed by predators, or man. Theoretically, the population level giving greatest absolute productivity should come at the point of in- flection in the growth curve of the population, but disturbance of age ratios or other conditions may place the level of optimum sustained yield at some other point. In this chapter we have been primarily concerned with basic principles and methods rather than with results. Data are not available to permit broad gen- eralizations about the total energy relations within ecosystems. Much more work needs to be done. The need for a better understanding of productivity and yield has both theoretical and economic incentives and provides one of the main challenges in the future re- search of ecologists. Exchanges, productivity, and yield 209 | Reproductivity and Population Structure In the previous chapter, we dealt with the pro- ductivity of communities; in this chapter, we will be concerned with the productivity of single species or reproductivity. The rate at which a species repro- duces and the frequency of its population turnover can affect the speed with which it occupies new areas, becomes adapted to new niches, or evolves into new races. In order to analyze the population dynamics of a species, it is necessary to know its life history. This involves the stages in its life cycle, mortality rates of each stage, longevity, sex and age ratios, age at which individuals become sexually mature, fecun- dity, factors causing mortality, and so forth (Cole 1954). The proportion of different ages and sexes gives the population a definite structure. All these essential data may be conveniently summarized in the form of life-tables. FECUNDITY Species vary greatly in the characteristic number of generations, broods, or litters produced per year, and in the sizes of them. Protozoans often divide so rapidly that they produce a new generation every few hours. Plankton organisms, less fecund, may produce a new generation every few days. Many vertebrates breed but once a year ; some large animals only once every two or three years. Several species of small birds and mammals have two or more broods per year. The female woodland white-footed mouse in Michigan may produce three litters between early April and early June, and two more between middle August and early October (Burt 1940). Under fa- vorable environmental conditions, rodents may con- tinue to breed throughout the winter so that their reproductive potential is enormous (Kalabukhov 1935) Innate capacity The maximum size of a litter is determined by the physiological and morphological characteristics of the species. With mammals which produce vivip- arous young, the size of the uterus and body cavity as well as the number of mammary glands for suck- ling the young after birth are limiting factors. With birds there is a limit on the number of eggs that one individual can cover and successfully incubate. In species that do not take care of their eggs after lay- ing, the number produced may be limited only by the energy resources of the parent. This is indicated in part by the inverse relation between number of eggs produced and their average size (Lack 1954). 210 Parental care The number of eggs or young produced per litter is correlated inversely with the amount of at- tention that they require. When parental care is al- together lacking, invertebrates may lay 1,000 to 500,- 000,000 eggs at one maturation; where there is some protection afforded by brood pouches, 100 to 1,000 eggs may be laid; with a high degree of brood pro- tection, 10, or less, to 100 eggs may be laid. Mam- mals seldom have more than a dozen young in a single litter and, in larger species, usually only one. Char- acteristic clutch size among birds varies from 1 to 15; rarely, 20. There is a limit on the size of the brood or litter that adult warm-blooded animals can successfully feed and raise to maturity. There is no advantage, for instance, for starlings to have broods larger than five (Table 15-1). In larger broods, each individual receives less food, and hence has less vigor and weight on leaving the nest. Mortality increases either before fledging or in immediately subsequent months. The larger broods raised during years of abundance fur- ther indicate that food is a critical factor (Lack et al. 1957). The variability in clutch and litter size for most species allows them to take advantage of tempo- rarily improved conditions. There is some evidence that in the tropics the number of young raised may be limited not by food but by increased predation on the larger sized broods (Skutch 1949). W eather Clutches laid by birds during periods of hot weather are usually smaller than those laid when temperature is moderate (Kendeigh 1941). Clutches laid by related species in temperate latitudes tend to be larger than those laid in the tropics (Moreau 1944, Lack 1947-48). The fecundity of white-tailed deer is higher with good forage than with poor forage (Cheatum and Severinghaus 1950). Reproduction is generally more successful after periods of high mor- tality than during years of abundance. The mobilization of energy, usually within a defi- nite period of time, is a limiting factor in warm- blooded organisms. The house wren, for example, lays 5, 6, or 7 eggs per clutch, the total weight of which is 7.0, 8.4, or 9.8 g respectively ; yet the adult female herself weighs only 11.5 g. It is estimated that under average conditions about one-third of the daily energy intake of the bird, above its needs for existence, is deposited in the eggs being produced. Any appreciable change in temperature or rate of feeding thus affects the size of the egg, the number TABLE 15-1 Reproductivity in the starling in relation to litter size (from Lack 1948). Per cent recovered Brood size Brood Number after * per cent size banded 3 months recovered 1 65 0 0 2 328 1.83 3.7 3 1,278 2.03 6.1 4 3,956 2.07 8.3 5 6,175 2.07 10.4 6 3,156 1.68 10.1 7 651 el 10.2 8 120 0.83 9 18 0 0 10 10 0 0 laid, or whether laying is undertaken at all (Ken- deigh 1941). Among invertebrates, clutch size also varies under different conditions and in different localities. The copepod Eudiaptomus gracilis commonly carries 11 eggs in April, 3 in early August, 9 in early Novem- ber, and 5 or 6 over the winter. There is a decrease between spring and summer in the number of eggs carried in its brood pouch by the cladoceran Daphnia. Diaptomus siciloides carries but 4 eggs in mountain lakes of California, as many as 18 in the [Illinois River. These variations appear to be correlated with differences in temperature and food supply (Hutchin- son 1951). Death rate Death rates vary between species and are cor- related with rates of reproduction (Table 15-2). The death rate of a species is influenced by a number of factors, but of fundamental importance is the num- ber of young that are born in relation to the carrying capacity of the habitat. When more young are born than the habitat can support, the surplus must either die or leave the area. When populations are stabil- ized at a constant level, the death rate must fluctuate with the birth rate. Evolutionary adaptation tends to lower the frequency at which the population replaces itself and to raise reproduction to the highest rate compatible with the energy resources both of the spe- cies (Table 15-1) and of the habitat (Lack 1954). SURVIVAL OF YOUNG Success in raising young depends not only on the ability of the adults to care for the young, but also on the vitality of the embryo and on the chance Reproductivity and structure 211 TABLE 15-2 Relation of reproductive to mortality rates per year (Lack 1954). A. Local differences in same or related species. Young Adult produced mortality Species and locality per pair rate Starling Switzerland 5.8 63% England 4.7 52 Blue tit Britain 11.6 73 Spain and Portugal 6 41 Canary Isles 4.3 36 Wall lizard Italian mainland 24 40 Italian islands 11 20 California fence lizard Plains 8.5 80 Mountains 3.3 30 B. Species differences in same locality Average further life Young of half- produced grown young White-footed mice in California per pair in days Peromyscus californicus 6.2 275 Peromyscus truet aloe 190 Peromyscus maniculatus 20.0 152 eee destruction of nests, eggs, or young by storms, wind, floods, predators, accidents, and desertion of the par- ents. Considerable data are available in this connec- tion with Lirds. Nest failures in birds are most frequent early in the nesting cycle and decrease progressively as fol- lows as nesting proceeds: 2.4 per cent per day dur- ing nest-building, 2.2 per cent per day during egg- laying, 1.2 per cent per day during incubation, and 0.5 per cent per day while the young are in the nest (Kendeigh 1942). Location of the nest is a factor in the successful raising of young (Table 15-3). The relatively low percentage of nests that pro- TABLE 15-3 Correlation between type of nest or nest location in birds and percentage of fledglings raised from eggs laid (Kalmbach 1939, Nice in Spector 1956: 93-94). Number of Per cent Category studies successful Precocial gallinaceous species nesting on the ground 17 44 Open nests of altricial species 27 46 Waterfowl in aquatic habitats 22 60 Hole-nesting altricial species 32 66 22. duce young successfully in many species is not a true index of annual reproductivity, since birds commonly make a second or even a third attempt if earlier nest- ings were failures. The ring-necked pheasant in Iowa has maximum nesting success averages of only 41 per cent; yet, before the season is over, by mak- ing repeated efforts, between 70 and.80 per cent of the hens are successful in raising broods. Full repro- ductive success is not assured, however, in raising the young to the stage of leaving the nest. In the study of the ring-necked pheasant, the average number of young hatched in successful nests was 8.7; after 1 to 3 weeks the average size of the brood was reduced to 6.7; after 4 to 5 weeks to 5.9; after 6 to 7 weeks to 5.3; and after 8 to 10 weeks to only 4.9 (Errington and Hamerstrom 1937). LIFE TABLES Species differ widely in the number of young produced each year, in the average age to which they live, and in their average rate of mortal- ity. When sufficient facts about a species are known, a life table that tabulates the vital statistics of mor- tality and life-expectancy for each age group in the population may be formulated (Table 15-4, Pearl 1923). Age is usually represented by the symbol + and is some convenient fraction of a species’ mean life span, such as a year or stage of development. The life table is set up on the basis of an initial cohort of 100, 1000, or 100,000 individuals ; and the number living to the beginning of each successive age interval is symbolized as /,. Plotting these data gives a sur- vivorship curve for the species. The number dying within each age interval is designated as d, and gives a mortality curve. The rate of mortality during each age interval is commonly expressed as the percent- age of the number at the beginning of the interval 100(d,/l,) and as indicated as q,. Survival rate is the difference between the mortality rate and one hundred per cent (100 — q,) and is expressed as Sy (Hickey 1952). Life expectancy (e,) is the mean time that elapses between any specified age and the time of death of all animals in the age group. Life tables are also useful for computing the aver- age longevity of a population, for showing the age composition of a population, for indicating critical stages in the life-cycle at which mortality is high, for showing differences between species, for showing the success of the same species in different communi- ties, for furnishing information of value in game and | fish exploitation (yield), and in control of pests (Quick, in Mosby 1960). Information for constructing life tables may be — obtained from a knowledge of age at death of a ran- dom but adequate sample of the population ; informa- Ecological processes and dynamics I FIRST YEAR: /74 BREEDERS Breeders a Breeding failure, |O fi aa 25 y— Embryo dies, 30 /000 potential eggs Survivors, |O5 —y Overwinter loss, 81 —a Hunter take, 38 —» Brood loss, 337 —" y— Apr-Sep loss, 24 2 — Overwinter loss, 54 +—Hunter take, 26 Nest loss, 374 a SECOND YEAR: ‘— Survivors, 70 oleae /75 BREEDERS FIG. |5-| Average life equation of a stabilized ruffed grouse population in New York State (Bump ef a/. 1947). tion on the age ratios of the living population, pro- ditions ideal ; all members born at the same time, live vided it is stabilized; or from data on a single cohort, adequately identified, followed throughout its life’s span (Farner 1955). Curves of survival plotted from life tables may be of three types (Pearl and Miner 1935, Deevey 1947). In type I, a cohort finds environmental con- out the full physiological life span characteristic of the species, and all die at about the same time. In type II, the rate of mortality is fairly constant at all age groups so that there is a more or less uniform percentage decrease in the number that survive. Type III shows extremely heavy mortality early in life, but Factor respon- x ly sible for dy dy Vx TABLE 15-4 Life table for the 1952-53 generation of the Eggs 1000 Parasites 17 2(-) spruce budworm in New Predators 86 9(-) Brunswick, Canada (after Morris Others 6 1(-) and Miller 1954). Totals 109 11 Instar I 891 Dispersal, etc. 428 48 Hibernacula 463 Winter 79 17 Instar II 384 Dispersal 242 63 Instars III - VI 142 Parasites 51 36 Disease 3 2 Birds 20 14 Others 61 43 Totals 135 95 Pupae 7 Parasites 0.6 8 Predators 0.7 10 Others 1.3 18 Totals 2.6 36 Moths 4.3 Totals for gen- eration 995.7 99.5 Reproductivity and structure 213 NUMBER SURVIVING FIG. 15-2 Schematic representation of different types of sur- vivorship curves. The vertical scale may be graduated, arith- metically or logarithmically. If graduated logarithmically, the slope of the line will show the rate of change; a straight line is indicative of a mortality rate equal at all ages (Deevey 1947). Olin Bos GS G7 & 2 Ie AGE FIG. 15-3 Survivorship curves for (A) the American robin, zero age at November | (Farner 1945a), and for female mule deer (males only are hunted) in (B) shrubland and (C) chaparral in California (Taber and Dasmann 1957). those few individuals that survive have a high life expectation thereafter. Types I and especially III are not often observed under natural conditions. Nearly all survival curves so far obtained are of type II, although they seldom approach a straight line. Prob- ably more curves approaching type III would be found if data could be secured beginning with the fertilized egg, as mortality in early life is often high, especially in aquatic species that spawn many eggs. Evolution of parental care in higher animals gives greater protection and efficiency in raising the young. This evolutionary trend should change survivorship curves from type III towards type I. SEX RATIO AND MATING BEHAVIOR The primary sex ratio at the time the eggs are fertilized should be approximately 50 8 8: 50 9 2 in most species, although it has seldom been measured. This ratio may be displaced in one direc- tion or the other by differential mortality of the two sexes during the period of growth, become manifest in the secondary sex ratio at the time of hatching or birth, and even more pronounced in the tertiary sex ratio of the adults (Mayr 1939). Protozoa, some coelenterates and flatworms are potentially immortal. Recognition of sex and age in living animals is often difficult, although criteria have been worked out for many species (Taber, in Mosby 1960). The sex ratio of the adults is especially impor- tant in understanding mating relations and reproduc- tion potentials. For instance, the adult sex ratio in ducks is often in the neighborhood of 60 ¢ ¢ : 40 2 9 (Johnsgard and Buss 1956). Since these birds are largely monogamous under natural conditions, a population of 100,000 birds does not furnish 50,000 breeding pairs but only 40,000. In polygynous spe- cies, such as pheasants, some grouse, turkeys, deer, and fur seals, breeding potential is probably not di- minished under natural conditions if there are two, three, or even ten times as many females as males. On the other hand, polyandry has become charac- teristic of some tinamous and bustards, and this is correlated with a preponderance of males (Kendeigh 1952) The tertiary sex ratio is not a constant factor. In the California quail it was found to vary monthly from 51 ¢ g: 49 @ @ in early autumn to 53 6 6: 47 9 9 during winter to 56 3 6: 44 2 @ in June (Emlen 1940). Game birds that are monogamous or only slightly polygynous in the wild may become highly polygynous after hunting seasons or in cap- tivity, situations in which there is a preponderance of females over males. Yearly variations in the ratio of © males to females in the house wren are correlated in- versely with tendencies toward polygyny, and posi- 214 Ecological processes and dynamics tively with tendencies toward polyandry (Kendeigh and Baldwin 1937). Mating behavior is therefore to some extent adaptable in order to compensate for lop- sided sex ratios and to maintain high reproductive capacity. BREEDING AGE The age at which young animals first at- tain the ability for reproduction affects the reproduc- tive capacity and rate of growth of populations. Planktonic entomostracans are sexually mature in a few days ; insects, often in a few weeks. Among birds, a tropical sparrow is known to reach full reproductive level in six to eight months (Miller 1959). Small non-tropical song birds commonly nest during spring and summer of the year following that in which they hatched, but banding of nestling house wrens indi- cated that 12 to 18 per cent failed to do so until the second or third year (Kendeigh and Baldwin 1937). Upland game birds probably nest as yearlings ; geese and wild turkeys do not nest until they are two years old; common terns, commonly only after three years. Some lizards and snakes require two to three years to reach sexual maturity ; turtles much longer. Females of the European voles may mate at 13 days, even before they are weaned, and give birth to their first litter when only 33 days old (Frank 1957). Woodland white-footed mice born in spring may pro- duce young late in summer ; but most small and me- dium-sized mammals do not breed until one year old. Beaver, wolf, lion, and whale breed when two years old. Big game mammals, such as deer, bison, and bear, reach maturity only after three years. The elephant is said to require 8-16 years, and the rhi- noceros 20 years (Spector 1956: 115, 119). NON-BREEDING POPULATIONS Although the breeding population of a par- ticular mammal, bird, or other animal is the only fraction of the total population of a species concerned with its reproductivity, there is often present in an area a substantial, though inconspicuous, non-breed- ing population that must be considered in any under- standing of community dynamics (Zimmerman 1932). In a 16-hectare (40 acre) tract of spruce-fir forest in northern Maine, there were, in 1950, 308 in- dividuals (154 pairs) of nesting birds present during the first half of June. By the use of fire-arms the population was reduced to 21 per cent by June 21, and held at this level until July 11. This involved a removal of not only 228 breeding birds plus 49 of uncertain status, but also of 250 new birds appearing to take over the territories and places of the nesting 20 15 - 2 WJ © 10 ea WwW a 5 2 il la eh eal EM ls 10 I2 14 16 18 20 AGE FIG. 15-4 Age composition of a breeding population of com- mon terns (Austin and Austin Jr. 1956). birds that were removed (Hensley and Cope 1951). This surprisingly high non-breeding reserve may not be typical of all species of birds (Bendell 1955). Other studies have shown that the non-breeding pop- ulation, especially of birds, consists principally of young animals that have been slow to reach sexual maturity, of surplus individuals of either sex in monogamous species, and of adults which, for one reason or another, have lacked reproductive vigor or have been unsuccessful in establishing proper breed- ing relations. LONGEVITY AND MORTALITY RATE When protected in captivity, animals are capable of living surprisingly long periods (Spector 1956: 182). Definite physiological limits of life are characteristic of each species and are occasionally realized under natural conditions (Cooke 1942), but invariably the potential longevity of a species is many times greater than the mean longevity actually at- tained by wild populations (Bourliére 1946). Finding the mean length of life for wild popula- tions requires the working out of life tables, accom- plished for only a few species. In birds older than the juvenile stage, it commonly varies from one to five years (Farner 1955), although in some large species it is considerably longer. In rodents, usually not more than 6 per cent of the population reaches one year of age (Blair 1953). The larger Dall mountain sheep has a mean length of life of 7.09 years. Adult barna- cles have a mean life of 12.1 months (Deevey 1947), and different species of rotifers variously from 3 to 35 days (Edmondson 1946). Longevity may often differ between the sexes. Thus in the male flour Reproductivity and structure 215 TABLE 15-5 Relation between annual mortality and longevity in birds (after Lack 1951). Average Average longevity, annual mortality, Bird species years per cent Starling iL 3 63 California quail 1.5 50 Song sparrow iri 45 Lapwing 2.0 40 Barn swallow 2.8 30 European swift 5.1 18 beetle, Tribolium madens, it is 199 days, in the fe- male, 242 days; in the male T. confusum, 178 days and in the female, 196 days (Park 1945). The rate of mortality in many species varies from one age level to another; thus, a mean death rate has only general significance. In birds, however, the death rate is nearly constant once they become adult, and it is then apparent that it varies inversely with adult longevity (Table 15-5). In adult penguins, peli- cans, shorebirds, gulls, and swifts, the annual mor- tality rate is commonly between 12 and 30 per cent; in herons, hawks, and owls it is about 30 per cent; in ducks, doves, and song birds it is between 40 and 68 per cent, while in gallinaceous birds it is the highest, 60 to 80 per cent. These rates for game species in- clude mortalities from hunting (Farner 1955). TABLE 1!5-6 Theoretical age composition of stabilized popula- tions with three different survival rates, assuming that the rate of mortality is the same for each age group. The figures are the percentage or number of animals (Ix) in each age class in a population of 100 (from Nice 1937). Age (x) Survival rate in time a aaeEeeEeeEo————EEeEeEeEeeeSSS intervals 75 50 25 1 PAs) 50 wD) 2 19 25 19 3 14 13 5 4 11 6 1 5 8 3 0 6 6 2 0 7 5 1 0 8 4 0 0 9 3 0 0 10 2 0 0 11 1 0 0 12 1 0 0 13 1 0 0 Totals 100 100 100 Average lifespan 3.8 2.0 1.3 216 AGE RATIOS The life-table gives the number or percent- age of individuals in a brood or litter surviving to the next age level. From such data, as well as by oc- casional direct observation, it is possible to determine the age structure of a population at any one time. Table 15-6 gives the percentage of each age class in populations of adults having three different mean sur- vival rates in all age classes. It is at once apparent that the number of age classes in a population is greater when survival rates are high than when they are low. It is also evident that there is less difference between number of individuals in succeeding age classes when survival rate is high than when it is low. The exact age of the sexually mature adult is usually at best difficult to determine unless one can band or mark the young when they first appear, or unless there are growth rings, such as in the scales and otoliths of fish and in the shells of clams, or other criteria of age that can be used. Immature ani- mals are often distinguishable from adults (Thomp- son 1958) so that adult-young ratios are usually ob- tainable, and often yield important information. The proportion of immature to adults is highest at the end of the breeding season, and then usually declines un- til the beginning of the next period of reproduction, because of the higher mortality rate of the young compared with that of adults. In the California quail, the ratio of immature to adults in October was 70 :30. During the following months the ratio progressively decreased as follows: November and December, 62:38; January, 58:42; February, 56:44; March, 54:46; and the breeding season, April to June, 50:50 (Emlen 1940). Age ratio is of practical value in wildlife manage- ment (Alexander 1958). A low ratio of immature to adults indicates a poor reproductive season and should caution against excessive take or yield, as the population is declining. The precarious state of the whooping crane is indicated in that the entire popu- lation of the species wintering on the Aransas Wild- life Refuge in Texas from 1949 to 1953 has consisted of only 3 to 4 young birds each year compared to 21 to 34 adults annually present. Low ratios of young to adults also occur with overpopulation, but over- population is usually easily detected. Bag limits may ordinarily be increased if the ratio of young remains consistently high. Here again, however, high ratios of immature to adults are characteristic when popula- tions are recovering from catastrophes. When a pop- ulation of rusty lizards in Texas was reduced by drought in 1954, the percentage of one-year-olds changed from 63 in the relatively stabilized popula- tion to 85 in the subsequent expanding population (Blair 1957). When a population is stabilized, the Ecological processes and dynamics Intrinsic Average growth rate longevity Net produc- TABLE 15-7 Compari f - parison o Species per day (7) in days T rT tion rate intrinsic growth rates and other Short-tailed vole 0.0125 141.75 1.772 5.90 data‘on the populations of Norway rat 0.0147 217.57 3.198 25.66 different species (compiled from Flour beetle 0.101 55.6 5.616 275.00 various sources by Evans and Rice weevil 0.109 43.4 4.731 113.56 Smith 1952). Human louse 0.111 30.92 3.432 30.93 young and middle-aged classes are more or less equal in numbers, the decline in size occurring progressively throughout life. ADAPTATION TO NICHE In the stabilized population of any species, whatever the number of eggs or young produced per pair of adults, the number of offspring reaching re- productive status can never be greater than two in sexual forms, which is the number required to replace the parents on their death. With each new generation there is, therefore, a population turnover, with newly born individuals replacing the adults that die. In a stabilized population, the rate of increase of a popu- lation through the course of several reproductive cycles must equal the death-rate, so that the value of one factor is also a measure of the other. Either factor is indicative both of the rate of population turnover and of the intensity of environmental resistance. The intrinsic growth rates for populations of sev- eral species under optimum conditions is given in Table 15-7 by the factor r, which represents the mean rate of increase per individual per day. There is a general inverse relation between growth rate and longevity, T. However, if growth rate were depend- ent only on the longevity of the species, then rT would equal a constant. Obviously this is not true. It appears that different growth rates may correlate with various intensities of environmental resistance in the different habitats occupied by different species. If there were a habitat offering no environmental re- sistance, and all offspring therefore survived, then a female would need to produce only one female off- spring to replace herself when she died. This would be the net production rate of Table 15-7. Actually, the number of female offspring that must be produced to offset mortality caused by the environment is al- ways more than one, attesting the rigor of the natural environment in spite of the species’ adaptations for life in it. The method used in calculating the net pro- duction rate, S/,*m,, the sum of number alive at age x, 1, times rate of reproduction at age x, m, for all age groups, is explained by Evans and Smith (1952). _ It is interesting that as a result of long evolutionary processes, the low net reproduction rates for the herbivorous vole, the omnivorous rat, and the para- sitic human louse indicate that they are in much bet- ter balance with what to them are optimum environ- ments than are the graminivorous flour beetle and rice weevil. The vole and rat are viviparous; the other three, oviparous. It would be very interesting to have similar data on other species to show the degree to which adjustments to particular environments have become perfected and the reproductive strain imposed upon related species for occupying different habitats. SUMMARY Reproductivity is the rate at which a spe- cies reproduces. The number of offspring raised to maturity per unit of time is generally characteristic of a species, and varies with fecundity and survival of the young. Fecundity depends upon the morpho- SEASON WHEN BORN FALL Ww Eas) SPRING 2 SUMMER = a Ww Ww {2} Ww a < a a uw ° a iso Wu oO = 35100 z Aug eee a Dec Jan Feb Mar Bec Dejoiete Juty Aug Sept FIG. 15-5 Monthly changes in the density and age-structure of a population of prairie deer mice in Michigan. As one popula- tion dies out a new one takes its place, and there is a popula- tion turnover (Howard 1949). Reproductivity and structure 21/7 logical and physiological capacities of the species, the amount of parental care that the offspring receives, and weather conditions. Death rates correlate directly with the number of young produced. Reproduction cannot be considered successful unless the young reach sexual maturity. Life tables tabulate, in condensed form, the vital statistics of survival and mortality by time intervals. They provide essential data for calculating longevity and age composition of populations. Survivorship curves show three characteristic survival patterns, but most populations exhibit a relatively high death rate early in life and a lower, more constant death rate thereafter. Sex ratios are often correlated with mating be- havior. The age of full reproductive maturity varies widely between species. Young birds, surplus adults of either sex, and birds unsuccessful in establishing breeding relations sometimes constitute a relatively large non-breeding population in addition to the more conspicuous breeding one. Perhaps this is true also for other animals, but evidence is scanty. Ratios of young animals to adults often indicate whether a population is expanding, contracting, or is stabilized. In stabilized populations the number of offspring reaching reproductive maturity can never be greater or less than the number of adults them- selves. The number of young that must be produced to permit such a population turnover gives a measure of the rigor of the environment, and how well adapted a species is to its niche. 218 Ecological processes and dynamics | Ecological Processes and Community Dynamics: Regulation of Population Size It is seldom possible to measure the total world- wide population of any species, unless it is one of restricted distribution and is readily accessible to censusing. The gannet, a sea bird, nests in only 22 island and sea cliff colonies. From 1819 to 1929 the annual population level of this species was about 340,000 birds. Because of molestation by man, the population dropped to about 106,000 in 1894, but, after some protection of the colonies was established, rose to about 165,600 in 1939 (Fisher and Vevers 1944). For most species population can only be ex- pressed in terms of number per unit area (population density ). The abundance of a species in a geographic re- gion is termed the average or regional density of it. A region, however, usually includes unfavorable habi- tats, from which the species is absent, as well as suit- able niches in which it is populous. The abundance of a species within its niche is called its economic or niche density, never less and almost always higher than its regional density. The regional density of a species depends on the prevalence of its favored niche in the area and the density which the species maintains within its niche. Muskrats may be very numerous in a marsh, but if there are few marshes in the region, their average density will be low. We will here be primarily con- cerned with why an animal species attains a particu- lar level of abundance within its niche. Species obviously vary in the level of abundance that they attain. Springtails may occur in hundreds per square meter, large mature snails as one per square meter, and white-footed mice as only one in- dividual per 400 m?. Principles involved in under- standing these differences in population levels are the size classes of the animals and their position in food chains, pyramids, and trophic levels. The population of any one species may be said to be stabilized when it fluctuates in an irregular but restricted manner from the mean. If environmental conditions temporarily become unusually favorable or unfavorable, population size may fluctuate accord- ingly, but with stabilization there is always the tend- ency to revert again to the average level when the unusual conditions have disappeared. The dynamic resiliency of populations is evident in the high rates of increase that occur with the beginning of recovery after a population has been depleted, and the progres- sive diminution in the rate of increase as the popula- tion approaches its characteristic level. Although at the beginning of the population growth curve, in- creasing numbers may sometimes bring a cooperative effect evident in increased rates of growth and re- production (Odum and Allee 1954), cooperation is soon replaced by disoperation in that the reproductive rate then varies inversely and the mortality rate varies directly with the density of the population in Z\9 450 \ 400 ora (sane \ \ ne al \ \ 1 \ 350 \ \ \ { \ \ i} \ 1) 300 \ \ \ 250 < \ \ \ fo) | 1930] 1931 | 1932 | 1933] 1934] 1935 | 1936 | 1937 FIG. 16-1 Seasonal and yearly changes in the population of bobwhite on 1800 hectares (4500 acres) in Wisconsin. The solid line shows the net reproductive increase each year from terms of mature offspring raised. As a necessary corollary, the mean longevity of a population also varies inversely with its density (Davis 1945). DENSITY-STABILIZING FACTORS The intrinsic growth rate of a population, is limited to the early stages in the sigmoid growth curve of the population. Very soon, environmental resistance restrains the rate of growth more and more sharply until, at the asymptote, the environmental re- sistance equals the biotic or reproductive potential and the population is stabilized. We need now to take a closer look at the proc- esses that produce these effects and determine the levels at which populations reach their asymptotes. These processes may be conveniently divided into two groups; those that are density-stabilizing, and those that are density-limiting. The first group of factors are biotic in that they depend on coactions be- tween individuals within the same population or be- tween populations of different species. Limiting fac- tors, which determine the level at which populations become stabilized, are basically physical and vary in intensity because of influences outside or largely in- dependent of the population or community. All fac- 220 see = free l l | | 1938 | 1939] 1940] 1941 | 1942| 1943| spring to autumn; the dashed line, the mortality over winter (from Errington 1945). tors taken together are commonly considered to con- stitute the environmental resistance, a convenient if not entirely accurate term. Density-dependent factors (Howard and Fiske 1911: p. 107, Nicholson 1933, 1954, Smith 1935, Solomon 1949, Ricker 1954) are those that vary in the intensity of their action with the size or density of the population, but not all density-dependent factors are density-stabilizing. Only if the percentage of a prey species destroyed by predators, for instance, in- creases with the size of the population and decreases as the population declines, is natural control prevent- ing indefinite population expansion, yet preventing extinction, too. This action then tends to stabilize the population size (Table 16-1). If, however, the percentage of prey taken remains approximately the same at all population levels the effect is propor- tional. If the percentage of prey or host affected actu- ally decreases as the population increases, the effect is inverse. This happens occasionally (Tothill 1922). Obviously the proportional or inverse effects of a fac- tor cannot inhibit the continuous expansion of a pop- ulation. Complete quantitative data are required in order to classify and evaluate the effect of any factor. The density-dependent factors that will be considered in respect to their stabilizing effect on population size are competition, reproductivity, predation, emigra- Ecological processes and dynamics Generation Factor 1 2 3 4 Stabilizing (assuming an increase of 10 per cent mortality each generation until the popu- lation becomes stabilized) Size of prey population a 12 32 72 Number destroyed 1 4 14 40 Percent mortality 25 35 45 55 Number surviving 3 8 18 32 Proportional (assuming a constant rate of mortality) Size of prey population t 12 36 108 Number destroyed 1 3 9 27 Percent mortality 25 25 25 25 Number surviving 3 9 27 81 Inverse (assuming a decrease of 2.5 per cent mortality each generation) Size of prey population 4 12 36 116 Number destroyed 1 3 7 20 Percent mortality 25 22.5 20 17.5 Number surviving 3 9 29 96 tion, and disease. The effect of density-dependent factors has been much studied from a mathematical viewpoint, but the present approach will be largely introductory and non-mathematical. Com petition The definition and basic principles of compe- tition have already been considered. We are here concerned with how competition helps to stabilize a population at a particular level. In this respect com- petition is primarily for space, cover, and food. Every terrestrial green plant requires a volume of soil for its root system and a volume of air in which So 300 (937 e 250 538 0/939 @/943 200 9/942 150 @/930 935 @/936 9/940 eo’ ee @/94/ @/93/ 50 1934 8052 e 1933 % 50 100} FT I509 200" 250) S00) 350 APRIL DENSITY FIG. |6-2 Per cent yearly increase in population size of bob- white in relation to April densities (Errington 1945). Regulation of population size 5 6 7 TABLE (6-1 Different effects of density-dependent factors 128 180 180 on the size of animal 83 135 135 populations, assuming 65 75 75 number surviving is com- 45 45 45 posed equally of males and females, and each pair gives birth to six young. 324 972 2,916 The size of the prey 81 243 729 population each generation 25 25 25 includes both the young and 243 729 2,187 adults. Thus, of 3 individuals surviving, the number of young produced (1.5 x 6 384 1,304 4,564 = 9) plus the 3 adults 58 163 456 totals 12. 15 12.5 10 326 1,141 4,108 it can display its foliage to receive solar radiation. In a dense forest the individual tree grows tall because of competition with its neighbors. Trees unable to keep up with this competition become overtopped by other trees and, lacking sunlight, die. Sessile marine animals, such as corals, mussels, and barnacles, may crowd into close physical contact, even growing on top of one another, but there is undoubtedly a limit to the number that can survive and carry on normal activities in an area of restricted size. Competition for space is well demonstrated in those species that defend territories; birds, for in- stance. With increase in number of birds in an area there is, at first, some accommodation as the size of territories varies inversely with the size of the popula- %o 70 60 @/94/ 50 9/936 0/940 @/930 40 30 O 200 400 600 800 1000 1200 1400 MIDSUMMER DENSITY FIG. 16-3 Per cent loss rates of bobwhite from midsummer to early winter in relation to midsummer densities (Errington 1945). Pa Environmental resistance: N / x) Stabilized) population level Biotic potential: ON _ rN Population growth curve: dt ON. py. KN dt kK POPULATION——> ve >; FIG. 16-4 Relation between biotic potential and environmental resistance in determining the population level attained by a species. tion and amount of competition involved. With 7 pairs of bay-breasted warblers on a 10-hectare plot (25 acres), the average size of territories was 3157 m? (0.78 acre). On another plot with 18 pairs the territories averaged 1740 m* (0.43 acre) ; on a plot with 25 pairs, 1497 m* (0.37 acre); and on a plot with 42 pairs, only 1174 m? (0.29 acre) (Kendeigh 1947). With decrease in size of territories, however, comes intensification of competitive singing, scolding, chasing, and fighting. On a 6-hectare (15 acre) area there were no instances of destruction of nests, eggs, or young in the six years through which the popula- tion of male house wrens did not exceed 11, but dur- ing the 13 years when such acts of destruction did occur, the male population had ranged from 11 to 16 (Kendeigh 1941b). A pair of birds requires a spe- cific minimum territory for successful nesting. When an area becomes saturated with territories compressed to this limited size, disturbances occur in nesting and other individuals attempting to invade the area are forced to go elsewhere. Thus the population density becomes limited by the space available. On the other hand, with species possessing only undefended home ranges, competition for space is of less critical im- portance in regulating population size. FIG. 16-5 Competition for space by barnacles; a median longi- tudinal section through a hummock (after Barnes and Powell 1950). Related to the competition for space is the com- petition for the most favorable portions of the niche, those offering maximum food and protection. In Hol- land, three species of tits (Parus major, P. coeruleus, P. ater) prefer mixed woods to pine woods. In years when they are scarce, the species are mostly confined to the mixed woods; when populations increase in size, they do so first in the mixed woods until the birds become intolerant of further crowding. Then they spill over into pine woods to nest but never be- come as abundant (Kluijver and Tinbergen 1953). Food supply is an important determinant of the carrying capacity of any area. When large numbers of animals are present there is, of course, less food avail- able to any one than when there are few individuals present. Competition for food therefore becomes in- tense in large populations. Population size is as lim- ited as food is available sufficient to supply the mini- mum needs of the individuals already present. For fish, the number of individuals may continue to increase in the presence of a limited food supply but each individual becomes stunted in size. There is a tendency for the biomass of a species to be regu- lated by the food supply, with size or weight of indi- viduals varying inversely with number. Thinning the population artificially usually results in increased growth of remaining individuals (Parker 1958). Body size in Daphnia (Frank et al. 1957) and in several species of mammals also appears to be to a certain extent density-dependent in that smaller-sized individuals are characteristic of larger populations (Scheffer 1955). The role of competition in regulating population size is thus directly effective by causing mortalities through fighting, nest destruction, and loss of food supplies. It also results, as will be seen in the discus- sion that follows, in lowered rates of reproduction, increased predation, dispersal into other regions, and decreased health and vigor. Reproductivity A study of the reproductive rate of a species in relation to population density requires separate studies of the number of eggs or young produced, called fecundity or natality, and the number reaching sexual maturity or survival. In cultures of Paramecium, a decrease in the vol- ume of culture fluid for the same initial number of individuals, or an increase in the initial number of individuals for a given volume of fluid, decreases the rate at which cell fission occurs (Myers 1927). Birth rate and growth rate in cultures of Daphnia magna vary inversely with the density of population even when a surplus of food is present (Pratt 1943, Frank et al. 1957). 222 Ecological processes and dynamics Female Drosophila fruit flies crowded into small bottles do not lay as many eggs as they do when not crowded. This has been attributed to the competition of females for space, and to frequent disturbing con- tacts with other flies, so that they do not feed ade- quately. It could also be attributed to their energy being dissipated and to their ovipositing being too often interrupted (Pearl 1932, Bodenheimer 1938, Chiang and Hodson 1950). On the other hand, the reduction in fecundity may not be so much a result of disturbance as one of reduction both in the quantity and quality of food that is available per individual (Robertson and Sang 1945). Experimental studies of populations of the flour beetle Tribolium confuswm show that, as they in- crease in size and modify the flour in which they live, there is a decrease in the number of eggs deposited, an increase in the length of the larval period before pupation, an increase in larval and pupal mortality, and a decrease in the weight of both the pupae and adults. Apparently these effects are produced partly by decreased fecundity of the individual females and partly by cannibalism of larvae and adults upon the eggs and pupae, presumably induced by accumulation of excreta and deterioration of the food supply. When the modified flour is replaced by fresh flour at 48- hour intervals, the rate of reproductivity rises, even when the beetle populations become very large (Park 1934, 1938, Park and Woollcott 1937, Hammond 1938-39, Rich 1956). Similar effects of crowding on weight, length of developmental period, and mor- tality have been demonstrated in sheep blow-flies (Ullyett 1950) and in Drosophila (Sokoloff 1955). In the sheep blow-fly, artificial destruction of a large per cent of emerging adults brings an increased length of adult life, increased fecundity rate per fe- male, and an increased total number of offspring produced (Nicholson 1954). Overcrowding of pink salmon in small impound- ments causes retention of many eggs within the fe- male at spawning, and perhaps also mechanical in- jury to the eggs already deposited from excessive stirring of the gravel (Hanavan and Skud 1954). A study of the European bird, the great tit, car- ried on for five years in 16 different areas, revealed a striking inverse relation between density and fecun- dity (Kluijver 1951). The average number of eggs laid per pair of birds per season varied from 13 to 20 at population densities of 8 to 12 pairs per 40 hectares (100 acres), to only 7 or 8 at population densities of 9 to 19 pairs per 40 hectares. The per- centage of pairs having second clutches during a year varied between 40 and 100 at population densities of less than 16 pairs, but decreased to less than 10 at higher densities. Similar results have been obtained on the North American house wren (Kendeigh and Baldwin 1937). Apparently the lowered fecun- TABLE 16-2 Relation of reproduction to density of laboratory mice during a four-month period (after Retzlaff 1939). Number Litters Offspring Offspring of per per per Groupings groupings female litter female 1d, 12 12 4.7 7.8 35.2 20, 29 6 4.3 Tin 31.1 40%, 49 3 4.0 6.6 26.2 80%, 89 2 3.6 7.4 26.8 120%, 129 1 3.4 6.4 21.9 dity at high population densities in these cases is in part the result of frequent disturbance and conflicts resulting from the crowding of territories and in part to less food available per pair on the smaller- sized territories (Lack 1952). The non-breeding population of birds is doubtless high only when the breeding population is sufficiently dense that it oc- cupies all of the most favorable territories. In the vicinity of Ithaca, New York, during three years of population increase, the average number of embryos per pregnant female in the meadow vole was 6 to 6.2, but in the year of decline following the peak only 4.5 to 5.5 (Hamilton 1937a). In California, the litter size of the montane vole declined as the popula- tion increased, and small litter sizes continued, as in New York, during the following decline in the popu- lation (Hoffman 1958). There is evidence that as snowshoe rabbit populations build up in the upswing of a cycle, litters are larger and more frequent than during the ensuing downswing (MacLulich 1937, Rowan and Keith 1956, Green and Evans 1940). Experimental studies of populations of the labora- tory mouse lend support to high population as a cur- tailing influence on reproduction (Table 16-2). With an increase in the number of mice crowded into cages of uniform size there was a decrease in the number of litters produced, in the size of each litter, and in the total number of young. At the higher densities there was considerable fighting, resulting in serious wounds and even death for some individuals. A so- cial hierarchy was established, and it appeared that only the despots at the top of the bite order were able to reproduce at a normal rate. Those at the bottom of the order produced few young or none at all (Crew and Mirskaia 1931, Retzlaff 1939, Crowcroft and Rowe 1957). It has been shown that in the coccid insect Lepidosaphes ulmi a decrease in fecun- dity at high population levels was not a result of a decrease in number of eggs laid by fertile females but of an increase in the percentage of females that were sterile (Smirnov and Polejaeff 1934). An experimental population of house mice was established in a large enclosure with cover and water supplied in excess but with food allotments held to a constant daily amount. The population increased in Regulation of population size 223 52.0 Number pupating 4.9 41.8 sl.7 e Is, {= = 25) 50— Wing length aos 20| 40 "ea zy) 15} 30 3 10} 20 1.2 5} lO} Percentage pupating Sl. Oo] oO |e | | | | | r ne) O 50 100 150 200 250 NUMBER OF LARVAE FIG. 16-6 Effect of density of larval populations of Drosophila melanogaster on number pupating, percentage pupating, and wing length of resulting female adults (after Chiang and Hod- son 1950). size until the per capita consumption of food was cut by one-fourth as a result of the increased number of animals, and then reproduction stopped altogether. It appears that in times of stress the limited energy resources of animals are diverted from reproduction to individual survival (Strecker and Emlen 1953). In another experiment, food was supplied in excess but space and cover were restricted. Population in- crease was finally limited by litter mortality from cannibalism and desertion. In some of the popula- tions there was a decline in fecundity. This appeared to be the result of a social hierarchy becoming estab- lished so that subordinated individuals failed to get adequate amounts of food, even though a surplus of food was available, and were prevented from complet- ing their mating behavior (Southwick 1955). Re- productivity has also been found to decline in the short-tailed meadow vole in large populations, be- cause of chasing and fighting, when there was a sur- plus of food and water present (Clarke 1955). Fertility of eggs appears to be high as they are laid under natural conditions. Egg viability, the capacity to hatch, has been shown in Drosophila cul- tures to be modified by the same factors that affect fecundity, particularly the amount of food available to the adult (Robertson and Sang 1945). The survival of young is greatly affected by the number of animals present. When larvae of Dro- sophila are reared at different densities in containers of equal size and with equal amounts of food, the percentage that succeeds in pupating drops in an al- most straight line with increase in density of the larvae. However, because of the larger initial num- bers of larvae present, the actual number pupating is greatest at intermediate densities. The size of the adults emerging from the pupae decreases abruptly as density increases. It has been shown experi- mentally that effects produced were due to the ex- haustion of food at progressively earlier growth stages as the population densities increased. The continued growth of some individuals even after the original food was gone was apparently because they devoured dead larvae (Chiang and Hodson 1950). The average growth rate of tadpoles in a limited volume of water is inversely proportional to the num- ber of individuals. However, some individuals grow at normal rates at all densities; the decreasing aver- age growth rate at higher densities is due to the larger number of individuals that become stunted (Rose 1960). This effect of overcrowding is produced through water conditioning. For the grain weevil there is an optimum inter- mediate density for rate of population increase even though the progeny raised per female decreases pro- gressively as the population increases (Table 16-3). There is a similar relationship among fish (Herring- ton 1947). The amount of disturbance of females with suck- ling young in crowded experimental populations of the house mouse decreased the number of litters suc- cessfully weaned. Females abandoned or devoured their young, and when the disturbance factor became sufficiently severe, all successful weaning of litters ceased (Brown 1953). The snowshoe rabbit cycle in Minnesota reached a peak of 200 per 100 hectares (500 per square mile) in 1933 and dropped to a low of only 13 per 100 hectares (32 per square mile) in February 1938. Mortality percentage of the adults, duration of the reproductive season, proportion of females pregnant, and the average number of embryos per pregnant female remained relatively constant throughout this period. The significant variable that appeared to de- termine the cycle was the mortality of the young after birth. At the peak of the cycle, yearling rabbits con- stituted 60 per cent of the entire spring population, but at the bottom of the cycle in 1937 they constituted only 44 per cent. As the cycle began to swing up- wards again in 1938 and 1939, the proportion of young reached 80 per cent (Green and Evans 1940). It is desirable, when considering reproductivity as a density-stabilizing factor, to distinguish clearly between fecundity and success in raising young to maturity. Changes in fecundity are density-depend- ent, at least in some species, but are not usually sufficiently great to be of major importance in stabil- izing the numbers of a species at any definite level (Lack 1954a). However, mortality of the immature 224 Ecological processes and dynamics TABLE 16-3 Reproductivity of the grain weevil at different densities (Maclagan 1932). Grain Weevil Density Weevils per gram of grain 0.25 0.50 1 4 8 16 32 Number of grains per weevil 100 50 25 12.5 6.25 3.12 1.56 0.78 Population size after 64 days 69 95 138 192 77 51 29 Progeny per weevil 17.2 11.8 8.6 5.2 3.0 1.2 0.4 0.1 stages induced by intraspecific competition, preda- tion, and conditioning of the habitat becomes exten- sive with overcrowding and is usually much more important. Predation It is well known that variations in the popula- tion level of predators coincide or often follow closely after variations in the population of prey species, but it is not always certain whether the number of preda- tors depends simply on the abundance of prey serving as food, or whether the predators by their feeding regulate the number of prey animals. Experimental studies amply demonstrate that under certain condi- tions, at least, both true predators and parasitoids greatly affect the numbers of the species on which they feed, and hence similar relationships may be looked for under natural conditions. A study made in California shows clearly that while the long-tailed mealybug increases rapidly on the citrus trees from March through May, their populations are reduced by June or July by the ac- tion of three insect predators, two lacewings and a lady beetle. The predator populations are low com- pared with the prey, but each predator destroys many mealybugs (DeBach 1949). Quantitative determination of the significance of predation in controlling vertebrate populations under natural conditions is difficult to make, since it re- quires accurate measurement of the number of prey per unit area, the number of predators in the same area, and the number of prey taken by the predators. In one of the best such studies (Errington 1937a), carried on in Wisconsin and Iowa between 1930 and 1935, the population of the prey, the bobwhite quail, was expressed in percentage of saturation or carrying capacity of the area, and the extent of predation by the great horned owl was given in terms of percent- age of owl pellets containing quail remains. Although there is considerable variability evident, the general trend is for the percentage of predation to increase with the density of the prey population in the manner of a density-stabilizing factor (Table 16-4). It ap- pears that at densities of prey below the carrying ca- pacity of the area, when the bobwhite can find plenty of cover and food close at hand, predation is very low, but as soon as populations reach densities above the carrying capacity so that surplus individuals are forced to make use of inferior cover or go greater distances in search of food, predation intensifies. Some excellent studies of predator-prey coactions, especially in vertebrates, have been concerned with the relations between a single predator and its vari- ous prey species (Tinbergen 1933, Errington et al. 1940, Errington 1943, Murie 1944, Tinbergen 1946, Fitch 1947, Dunnet 1955, Craighead and Craighead 1956), or between a particular prey species and all its predators (Errington 1945, 1946, Bump et al. 1947, Koford 1958), but a complete understanding of the role of predation in regulating population levels requires a knowledge of coaction between all prey species and all predator species within community limits, since interrelations between any two species are affected by the interrelations of each species with others in the community. Thus when meadow voles are abundant an owl will feed largely on that single species, but when populations of meadow voles be- 700 — 600 |_ z— Mealybugs NUMBERS BS re) oO z— Predators (xI0) 1 fe) = MAR APR MAY JUN JUL AUG SEP OCT NOV DEC FIG. 16-7 Relation of changes in the populations of long-tailed mealybugs and their predators, both living on citrus trees in California during 1946 (DeBach 1949). Regulation of population size 225 TABLE 16-4 Evaluation of horned owl winter predation on bob- white (from Errington 1937). Number of Intensity of predation locality- Density of prey in in per cent of owl winter per cent of carrying pellets containing records capacity quail remains Vi ee Se a ee eee ee 9 36-100 1.9 (0.0-6.3) 10 106-123 6.7 (0.0-16.0) 3 133-150 14.6 (10.4-19.0) 4 155-197 11.0 (0.0-20.0) come reduced, the owl will prey to a greater extent on other species if they are available (Table 16-5). Other species that take the brunt of predation when a perferred species becomes reduced in availability are called buffer species (Bump et al. 1947). Not only is there variation in the food of preda- tors dependent on the availability of several potential prey species, but with any one prey species there is variation in the number and kinds of predators affect- ing it, dependent on its density and vulnerability. During outbreaks of insect or mouse plagues, preda- tor species of many kinds converge on the easily ob- tainable food supply (Piper 1928, McAtee 1922, Kendeigh 1947). Predatory pressure is therefore very flexible, shifting its major impact from species to species and from one locality to another. This makes the evaluation of predation as a density- stabilizing factor particularly difficult, but there can be no doubt that the application of predation pres- sure from all sources often exerts an important regu- latory influence on prey populations. There are too many cases on record of prey species developing dis- astrous overpopulations when predatory species are artificially eliminated to think otherwise (Ball and Hayne 1952). When only a small number of species are involved in the food web, as in arctic communities or with in- TABLE 16-5 Percentage of different prey species taken by wood owls, during a winter (1930-1931) in which populations of European meadow voles were high, compared with the following winter (1931-1932) when vole populations were reduced (from Tinbergen 1933). Winter Winter Species 1930-1931 1931-1932 Rodents: European meadow-mouse 88.0 52.0 House mouse, European woodland mouse 5.8 14.2 European red-backed mouse teil 2.1 Norway rat 0.5 1.9 Birds 4.1 27.1 Miscellaneous 0.5 2.7 sect pests infecting cultivated crops, stability of popu- lation levels is difficult to attain. An increase or de- crease in the abundance of any one species produces changes in all other species. On the other hand, when a large number of species are involved, as in tropical communities or in complex stands of tem- perate zone vegetation, each predator has so much choice of prey and each prey species is subjected to attacks from such a variety of predators that a sudden change in the population level of any one species is absorbed without greatly affecting the stability of the community as a whole (Votite 1946, Craighead and Craighead 1956, MacArthur 1955). Cycles of popu- lation are much more prevalent, therefore, in far northern communities where the variety of species is scanty than in the highly complex communities of southern latitudes. The importance that predation may have to main- tenance of health and vigor in prey populations, aside from regulating their numbers, is of significance. In careful observations of 688 attacks by hawks on other birds, only 7.6 per cent were successful in the cap- ture of the prey, but of these successful captures, over 19 per cent of the victims had previously shown injuries, abnormalities, or unusual behavior (Rude- beck 1950-51). In another similar study, the ab- normal individuals among the victims varied from 14 to 33 per cent (Burckhardt 1953). Wolves have great difficulty catching healthy adult caribou, and even calves are not often overtaken except in the con- fusion of a large herd. Over 50 per cent of the kills that wolves make are of crippled or sick caribou, although the incidence of such individuals is less than 2 per cent (Crisler 1956). Water boatmen with one or more legs artificially amputated were de- stroyed by fish in an experimental setup at a faster rate than were normal individuals (Popham 1942). Apparently predation exerts a selective force and less fit individuals are eliminated in greater proportion than are the fit. The ultimate result of parasitoids is the death of their hosts. They function in curtailing over-popula- tions of host species in nearly the same manner as do true predators. The relations between host and para- sitoid may, however, be more varied and complex (Nicholson 1933) than between a true predator and its prey. The normal relation between host and parasitoid is one of equilibrium where neither becomes overly abundant or overly scarce. This means that the abun- dance of parasitoids must also be controlled by density-stabilizing factors. Parasitoids may in turn be infected with hyperparasitoids, and the relations between the two are similar to those between the pasasitoid and the original host (Nicholson and Bailey 1935). At low host densities, the reproductivity of the 226 Ecological processes and dynamics parasitoid tends to vary proportionally with the dens- ity of the host (Varley 1947). However, it has been shown experimentally that with increase in popula- tion density of the parasitoid, there is decreased re- productivity for it, increased difficulty in finding in- dividual hosts not already infected (DeBach and Smith 1941), and increased competition between duplicate infestations in the same host individual (super-parasitism) so that neither parasitoid sur- vives (Fiske 1910). Of interest in this regard is that in one experiment 50 parasitoids during a lim- ited period of time were able to find and eventually kill 80 per cent of the hosts, but that it required 100 parasitoids to find 95 per cent of the hosts and 200 parasitoids to find them all. This phenomenon is comparable to the law of diminishing returns and is doubtless one reason why a parasitoid rarely ex- terminates a host (DeBach and Smith 1947). In order for a particular parasitoid to regulate the num- bers of a particular host species, it ordinarily needs to have a high intrinsic rate of increase, at least equal to its host (Muir 1914), and to have high searching ability for locating host individuals (Andrewartha and Birch 1954). Even though the density of host or prey popula- tion greatly affects the success with which a parasitoid or predator finds its victim, searching is not random as far as the individuals are concerned. Predators in general have evolved many adaptations in sense or- gans, methods of attack, and special behavior patterns designed to facilitate the finding of specific prey, avoid unsuitable objects, save time, and increase effi- ciency (Thompson 1939). However, searching is largely at random as far as the area covered by the entire population is concerned since the individuals mostly hunt independently of each other and may cover the same or different areas indiscriminately. Many parasitoids avoid placing their eggs inside the bodies of prey that are already infected, but this be- havior tends to break down when the ratio of number of parasitoids to number of uninfected prey is high. The coaction between host and parasitoid may be complicated by differential effect of environmental conditions, such as temperature, on the two species. Experimental studies have shown that the greenhouse whitefly, a homopteran, at temperatures below 24°C lives longer, lays more eggs, has a higher rate of oviposition, and consequently increases more rapidly in abundance than does it chalcidid parasitoid. At 24°C, the rate of population increase is about the same in the two species, but above 24°C, the para- sitoid population increases more rapidly than does the host species. The result is that the percentage of hosts infected increases markedly with rise in tem- perature (Burnett 1949). Similar relations have been demonstrated for other species of hosts and para- sitoids (Payne 1934). Buffer species may be as important with para- sitoids as with true predators. Prior to 1925 in the Fiji Islands, the zygaenid moth Levuana iridescens was a serious pest of coconuts, defoliating trees over extensive areas. Outbreaks terminated only when its supply of food became exhausted. In 1925, the tach- inid fly Ptychomyia remota was introduced from Malaya and within a year reduced Levuana to a rare species, a status which it has had ever since. How- ever, Ptychomyia requires alternate hosts to main- tain its existence when Levuana becomes reduced in numbers. Such alternate hosts, or buffer species, oc- cur on most of the Fiji Islands, but on one island from which they are absent the death rate among the predators became so high that Leuvana has been able again to increase in numbers (Andrewartha and Birch 1954). There are several known cases where crop pests in agricultural regions have been controlled or virtually extirpated by introduced parasitoids and predators (Fleschner 1959, Varley 1959). In the Hawaiian Islands, the sugar cane leafhopper is con- trolled by the capsid bug Cyrtorhinus mundulus, and the sugar cane borer by the tachinid fly Ceromasia sphenophori. The black scale in California is suc- cessfully controlled by the chalcidid Scutillista cyanea imported from South Africa. The silk industry of Italy was apparently saved by importations of Pros- paltella berlesi which controlled the scale that was destroying mulberry trees, on the leaves of which the silkworm feeds (Thompson 1928). Not all cases of supposed control of pests by parasitoids and preda- tors can be substantiated in critical study. Pests most adequately controlled are usually scale-insects, mealy- bugs, aphids, and leafhoppers of the order Homoptera which are sedentary, gregarious, and limited in the number of host species that they attack. Beneficial parasitoids and predators belong chiefly to the He- miptera, Diptera, Coleoptera, and Hymenoptera (Sweetman 1952). The relations between some insect herbivores and plants are somewhat similar as between parasitoids and hosts. Cactoblastis and Dactylopius, introduced from California, are credited with destruction of large areas of tree cactus in the Hawaiian Islands (Huf- faker 1957). Attempts at artificial control of pests with in- secticides often bring disorder and unexpected re- sults. Elm trees on the University of Illinois campus were sprayed with DDT to control Scaphoideus luteolus, a leafhopper vector of phloem necrosis which was causing considerable destruction of the trees. But the spray also caused a high mortality of Aphytis mytilaspidis, a hymenopteran parasitoid of the scale Aspidiotus, and allowed the scale to in- crease and do damage in turn to the trees (Tinker 1957). Regulation of population size 227 Food consumption ae 35 2 150 = Zon 100 fo) IS a 50 a Emigration of mice ons (@) ———_ = ms = lglaolelalzlSlslSiglglél FIG. 16-8 Relation between population size of house mice, as expressed by average amount of food consumed per day, and emigration of mice away from the colony. The colony was started with five pairs of mice in January (Strecker 1954). Emigration The pressure of overpopulations can be re- lieved by mass emigrations of individuals from par- ticular localities as well as by their death. It has been shown experimentally that such emigrations will ac- tually occur under conditions of crowding in a mouse population. It is of interest that those individuals which remained continued their normal rates of re- production. This contrasts with the drastic reduction, even cessation, of reproduction in other colonies from which emigrations were prevented. Two species of aphids placed in their optimum niches, one at the top, the other at the bottom of a single barley plant, multiplied to saturation and dis- persed downward and upward on the plant until both species came to exist side by side. Continued repro- duction and overcrowding forced surplus individuals to emigrate to surrounding plants over 7.5 cm away, leaving the two populations in equilibrium on the original plant. In another experiment where plants were within 3.0 cm of each other, the aphids spread to the preferred sites on the second plant rather than to less favorable spots on the first plant (Ito 1954). Emigrations under natural conditions occur when there is overcrowding in the migratory locust, lem- ming, grouse, snowy owl (Gross 1947), snowshoe rabbit (Cox 1936), Arctic fox (Braestrup 1941), gray squirrel, and occasionally in other species (Heape 1931, Dymond 1947). The emigrations of the European lemming in the Scandinavian countries are spectacular (Elton 1942). Emigrations on a re- duced scale are known to occur also with lemmings in North America (Thompson 1955). Lemming emigrations do not invariably lead to death of whole armies as popularly believed, but to settlement of new areas, leaving the original area populated with reduced numbers. Emigration must have survival value for the species or otherwise the tendency for emigration would have been a weaken- ing factor and disappeared in the course of evolution. It is, of course, population pressure that is re- sponsible in large part for the dispersal of young and extension of ranges into new areas. Under normal conditions adult animals, especially among the higher vertebrates, are well established on their territories and the young are forced to seek homes elsewhere. Among insects, there is a relation between emigration and inherited behavior tendencies. Individual tent caterpillars, both larvae and adults, differ innately in the extent to which they show activity even within the same colony. In the development of populations of excessive size, spread of infestations of the insect into new regions is largely by the more active indi- viduals. The outbreak finally terminates when the proportion of sluggish individuals comes to predomi- nate in the population (Wellington 1960). Disease Although infectious disease in some form is a common cause of mortality, it is less important as a stabilizing factor than the others already considered, because it reduces the population size in an important manner only when epidemics, or more accurately epizootics, occur. The mortality may then be ex- treme so that the population falls way below the level of stabilization, and a period of recovery follows. Whether or not epizootics occur, depends on the virulence of the disease-producing organism, the rapidity with which it is transmitted from individual to individual, and the resistance of the hosts. Worm and protozoan parasites, bacteria, and viruses may be transmitted through body contact of host indi- viduals, by the host ingesting contaminated food or water, or by vectors which are commonly external parasites themselves. It is obvious that ease and ra- pidity of transmission increase with the size of host populations. Overcrowding often also lowers the vigor of the hosts so that they become more sus- ceptible. In the course of time, natural selection tends to evolve tolerable relations between hosts and the dis- ease organisms that they harbor. Mutations of dis- ease organisms to greater virulence result in more rapid or extensive die-offs of the host with the con- sequence that the mutant strains disappear. During upswings in host populations, extra virulent muta- tions may persist for a time, because of the abundance of host individuals to which they can spread, but when the host population declines, only those host individuals will survive that are not infected with the virulent strain or that develop immunity to it. Epizootics among wild animals are often severe, and they usually break out when population densities 228 Ecological processes and dynamics are high. Among mammals, epizootics have been ob- served in voles, lemmings, mice, rats, beavers, squir- rels, rabbits, moles, foxes, deer (Elton 1942), birds, fishes, and reptiles. Incidence of them is often sporadic in that they do not always appear with high densities of host populations nor with declines in cyclic species so that their importance as a regulating factor on animal populations has been difficult to evaluate (Chitty 1954). Physiological stress is not infectious but becomes pathological when extreme and may bring consider- able mortality in the population. States of stress have been produced experimentally by allowing confined populations of albino and house mice and meadow voles to increase to high levels of abundance, even with a surplus of food and water present. Evidence that a state of stress exists is demonstrable in experi- mental populations by increase in the weight of the adrenals, by decrease in the weight of the body, testes, thymus, preputial glands, and seminal vesicles; by decrease in the number of circulating eosinophils in the blood ; and by aberrant maternal behavior (Clarke 1953, Christian 1956, Louch 1956, 1958). An increase in adrenal weights, especially the adrenocortical tis- sue, as population rises, has also been shown in wild populations of the Norway rat (Christian and Davis 1956), and the behavior symptoms usually associated with high physiological stress have been observed in wild populations of European meadow voles (Frank 1953). It seems probable that shock disease as it occurs in snowshoe rabbits is a manifestation of the stress syndrome (Green and Larsen 1938) and occurs when the liver degenerates leaving inadequate reserves of glycogen available for emergencies. Under these con- ditions, any undue exertion or excitement may cause normal animals to go suddenly into convulsions, sink into a coma, and die. Relaxation of density-dependent effects With so many decimating factors acting on populations, one wonders why species do not become extinct more often than they do. The explanation is that there is relaxation in the intensity of action of the factors as the affected population becomes re- duced in size. This relaxation is brought about by the heterogeneity of the environment so that at least some individuals escape the full force of the factor ; hyperparasitoidism or overcrowding reducing the pop- ulation of parasitoids and predators themselves; de- velopment of immunity to or tolerance of the factors involved ; change in behavior so that the decimating factor is avoided: survival of dermant eggs, pupae, or encysted stages after the active stages in the life- cycle perish (Solomon 1949). DENSITY-LIMITING FACTORS Variations in space or cover, favorable weather, and food occur independently of population densities and may cause drastic changes in the abun- dance of animals. Heavy silting of estuaries along the coast from erosion of the surrounding upland may smother oyster spat and reduce the amount of hard surface available for setting quite independently of the number of oysters already there, or variations in oyster abundance from year to year. The amount of solid surface available also determines the population density reached by sessile rotifers (Edmondson 1946). Variations in water level of a stream affects the availability of suitable spawning areas for fish, and consequently their abundance (Starrett 1951). A drought may dry up a marsh, making it unsuitable for muskrats and waterfowl. A severe winter freeze may kill all but a few hardy individuals of any species regardless of the size of the original population. Failure of a food crop, from weather, flooding, or some other physical factor, may deplete populations that depend on it for subsistence. On the other hand, agriculture has provided food, and allowed some species to become abundant that once were scarce; for instance, many insect pests of crops. Although fluctuations in space, weather, and food may directly affect the abundance of animals in an obvious manner, their average or prevailing condi- tion determines the level at which density-stabilizing factors bring populations into equilibrium. With abundant food, cover, and favorable weather, popula- tions will be high. When food is scarce, competition for it becomes acute at a lower population density. It FIG. 16-9 The 1932 emigration of sharp- tailed grouse from northern Ontario and Quebec (Snyder 1935). Regulation of population size 229 TABLE 16-6 Effect of different combinations of temperature and humidity on the levels attained by populations of flour beetles in experimental cultures (Park 1954). Temperature Relative humidity Mean +S.E. 34°C 10% 38.25 + 1.53 34°C 30% 9.61 + 1.07 29°C 70% 50.11 + 3.40 29°C 30% 18.79 + 1.32 24°C 70% 45.15 +2.77 24°C 30% 2.63 + 0.35 has been demonstrated experimentally, for instance, that the density attained by the cladoceran Daphnia obtusa is directly proportional to the food supply (Slobodkin 1954). This principle was known to Thomas R. Malthus back in 1798, and influenced the theory of evolution as developed by Charles Dar- win. If cover is deficient, animals become exposed to predation and bad weather earlier so that the population becomes stabilized at a low level. Like- wise, differences in temperature and humidity affect the level attained by experimental cultures of flour beetles (Table 16-6). Differences between species in their relative de- mands for space, food, and shelter affect the popula- tion levels that they attain. Species of small body size require less space than those of large size. In similar fashion, species that get along in small territories will be more numerous than those requiring large territories. Herbivorous species find more food avail- able in a limited area than do species higher in the food chain and hence will be the most numerous. Hardy species will flourish in climatic areas where less tolerant species are scarce. The limiting effects of space, weather, and food are properly considered to be density-independent. However, some actions of these factors are density- responsive. The amount of space available for addi- tional individuals is, of course, inversely proportional to the space already occupied, so that the greater the population density, the less space there is avail- able per individual. If all favorable cover is occupied, additions to the population are forced into inferior cover where they receive less protection in inclement weather and where they become more exposed to the attacks of predators. Voles feeding in grassland may, when abundant, consume so much of the vegetation that they destroy their cover as well as their food supply. When populations of brown lemming are low over winter, they utilize less than one per cent of the annual production of the grasses and sedges which are their favorite food. At peak populations however, they use nearly 100 per cent of the growth, become greatly exposed to predation, and are subsequently forced to shift to the less palatable forage and poorer cover of moss. The lack of adequate winter forage and cover, concurrent with a reduction in reproduc- tion and increase in predation, results in a rapid de- cline in population level (Thompson 1955a). Outbreaks of spruce budworm do not occur in the coniferous forests of Canada until the succession to white spruce and especially balsam fir develops a sexually mature evergreen canopy overtopping the aspen and birch. Insect larvae newly emerged from hibernation feed on the flowers, especially the male flowers, before the leaf buds open; then they move down to consume the current foliage, eventually de- foliating and killing the trees. Millions of dollars worth of timber is destroyed. Native parasitoids are incapable of preventing outbreaks. The outbreak dies out in a few years by which time all the mature trees are destroyed and the insects’ food supply is ex- hausted. Another outbreak will not occur in the lo- cality until another generation of spruce and balsam matures in the area (Prebble 1954). In stored grain infested with insects the heat produced by the insects may raise the temperature beyond their limit of tolerance and prevent further increase in population density (Solomon 1953). In- sects and rodents at high populations tend to have reduced vigor and health and to be affected by weather conditions which at low population levels are easily tolerated (Chitty 1960, Wellington 1960). It is thus apparent that even the climatic environment may be density-responsive in its effects in special situations. However, it is well to distinguish between density-responsive effects that are relatively passive and limiting and density-dependent effects that are dynamic and stabilizing. INTERCOMPENSATIONS The difficulty of evaluating, under natural conditions, the role of any factor in regulating the size of animal populations is in large part a result of the fact that it seldom acts alone. The time in the life-cycle of an organism at which a factor takes effect influences the importance of it. Normally, the earlier in the life of the organism at which a factor is effec- tive, the more nearly its apparent controlling role is a real one. Thus 60 per cent of the mature larvae of an insect may be fatally infested with parasitoids, but if 82.9 per cent of the original output of eggs have already failed to reach this stage for other reasons, the influence of these parasitoids must be evaluated at only 10.2 per cent, 0.60 « (100 — 82.9), instead of the full 60 per cent (Table 16-7). On the other hand, a 10 per cent apparent mortality resulting from egg parasites which comes at an early stage in the life cycle may produce a nearly equal real mortality (9.5) ; but a 10 per cent parasitization of pupae late in the cycle may cause only 0.47 per cent real mor- tality. 230 Ecological processes and dynamics Apparent Real mortality mortality TABLE 16-7 Evaluation of Stage Factor per cent per cent mortality factors effective at Eggs at deposition Sterility 5.0 5.0 a pigs Bae i ar Eggs after deposition Egg parasites 10.0 9.5 of an insect (Thompson ). Young larvae Intrinsic factors 80.0 68.4 Mature larvae Larval parasites 60.0 10.2 Mature larvae Agricultural factors 30.0 2.05 Pupae Pupal parasites 10.0 0.47 Adults Meteorological factors 54.86 2.30 Total 97.92 The variability of a factor also affects its im- portance. If a factor consistently produces, say, a 60 per cent mortality year after year, it will influence the size of the population any particular year less than will another factor that varies from, say, 20 to 30 per cent. However, a variation of 10 per cent in a factor that averages 60 per cent mortality is more important than the same variation in a factor that usually pro- duces only 20 per cent mortality. An increase in mortality from 60 to 70 per cent reduces the surviving population 25 per cent (40 — 30) /40, but an increase from 20 to 30 per cent reduces the surviving popula- tion only 12.5 per cent (80 — 70) /80 (Morris 1957). Aside from time and variability, the influence of any factor is dependent on the level of population size at which it first comes to exert an effective or critical role. This level represents a threshold of vul- nerability of the population for that particular factor. The threshold of vulnerability varies between species and within the species, depending on the amount of protective cover that is present, the movements and activities of the species, its protective coloration, and the aggressiveness and capabilities of the predators themselves (Craighead and Craighead 1956). There is also an upper limit of vulnerability or escape phase (Voute 1946) above which a factor no longer exerts effective control over a population in- crease. With increase in the number of cocoons of European pine sawfly, predation by small mammals rises to a peak at a density of 2,000,000 cocoons per hectare (800,000 per acre), at which level about 50 per cent of the prey are destroyed. With further in- crease in density of cocoons, the percentage of prey taken by mammals decreases in a density-inverse re- lation. The prey has escaped the stabilizing influ- ence of predation or gone beyond the upper limit of vulnerability to this factor (Holling 1959). Competition or fighting between individuals ap- pears at rather low population levels among verte- brates. With crowding, a social hierarchy may be- come established with disadvantages to those in the lower positions, or territories may become seriously compressed in size. When fighting becomes intense, individuals are forced to leave the area (emigrate) ; failing this, their reproductive activities are disturbed. In other species, for instance insects, competition may Regulation of population size not be sufficient to control the increase in population, so that the population reaches a level at which preda- tion becomes significant. Populations of vertebrates become especially vulnerable to predation when all suitable cover becomes crowded and surplus animals are forced to accept inferior cover or are driven into the open. Once an outbreak surpasses a certain level in spite of predation, predation can on longer take any significant percentage of the species. In fact, bird predation on insects may even become a hindrance at high population levels in that it destroys parasitoids that then become the most effective regulating fac- tor (Strickland 1928, Thompson 1929, Betts 1955). Emigation and epizootics ordinarily do not occur un- less competition and predation fail to hold down the increase in population and very high population thresholds are reached (Severtzoff 1934, MacKenzie 1951). Populations occupying inferior habitats usu- ally never reach densities that render them vulnerable to epizootics, and they ordinarily escape the violent fluctuations in size that occur with the species in habitats of higher carrying capacity (Evans 1942). These various regulating factors affect species dif- ferently. Song birds with well defined territories sel- dom reach densities at which reduced reproductivity or predation becomes important. They are, however, subject to reduced reproductivity in high populations. Game birds, which do not defend nesting territories, ungulates, and insects, in which group competition is relatively ineffective, commonly have their population levels controlled by predation. If predation, reduced reproductivity, and competition do not curtail popula- tion expansion, lemmings undergo emigrations, and all animals become subject to epizootics or physiological stress. For many species the effect of density- dependent factors is cumulative; that is, several fac- tors are involved to varying degrees (Milne 1957). For some insects, no density-dependent force is ef- fective ; the population is never stabilized, and it con- tinues to increase until there is exhaustion of space or food or curtailment by bad weather. A good example of a population that never be- comes stabilized is the rose thrip, that inhabits rose blossoms. Rapid multiplication of the thrip is possible only during a limited period, during spring and early summer. In this period the insect increases rapidly, 23] POSSIBLE LIMITS OF SPACE, FOOD, OR FAVORABLE WEATHER FIG. 16-10 Interrelations of various density-stabilizing and density-limiting factors (‘‘en- vironmental resistance’) in the regulation of population size at Ox ee various possible levels. Consider- able variation occurs between species in the relative importance and position of the different factors. Note that the limits of population growth set by space, food, or favorable weather may occur for different species at ie25 Seka oro: NUMBER OF INDIVIDUALS ————> but so does the number of roses available to them. The favorable period normally ends long before the thrips have time to saturate the niche. Summer drought brings high mortality and a decline to the low densities of the species characteristic of late sum- mer and winter. The rise of the thrip population is a race against time, the increase in density greatest in those years when the favorable period lasts longest ; but it never reaches the point where competition be- comes important. Annual variations in maximum densities in this species are almost entirely the result of density-independent climatic factors (Davidson and Andrewartha 1948). What is true for thrips may apply also to many other kinds of insects and in- vertebrates having annual life cycles; that is to say, climatic factors appear more important than biotic ones in determining the yearly size of the popula- tions (Uvarov 1931, Bodenheimer 1938, Thompson 1939, Andrewartha and Birch 1954). The various stabilizing and limiting factors act in an intercompensatory manner. All stabilizing factors, for instance, are in temporary abeyance following catastrophes of weather, drought, floods, or other factors until there is recovery of normal population levels again (Nicholson 1954a). Ol peer ete pe Mis | alae bead po Percentage effect RODENTS, LAGOMORPHS & =) eS Q BIOTIC POTENTIAL GAME BIRDS, UNGULATES, INSECTS EXP POPULATIONS OF MICE, INSECTS, ETC SONG BIRDS TIME ————>— One of the most thorough studies of stabilizing factors has been made on the muskrat (Errington 1946, 1951). This species is subject to such density- dependent mortality factors as intraspecies competi- tion or fighting; predation, especially by mink and foxes; emigrations from overcrowded habitats; and epizootics. Overpopulations may be reduced by one of these factors singly, or by two or more working simultaneously. If fighting or predation keeps the population at a low level, disease is unimportant ; but if fighting or predation is negligible some one par- ticular year, then disease may reduce the numbers of animals. Emigration to other areas occurs when a marsh becomes overcrowded or drought reduces the carrying capacity. If freezes, violent storms or floods, drought, or trapping reduces the population exces- sively, there is compensation by increased breeding activity, and for a time all other regulating factors are held in abeyance. The fur yield of a muskrat marsh cannot, therefore, be increased simply by de- stroying the predators, for other controlling factors become proportionately more effective. Trapping for fur, if not excessive, is economically profitable and can be carried on year after year, if the animals elimi- nated through trapping are restricted to the numbers 232 Ecological processes and dynamics that would be destroyed anyway by natural factors. The general trend is to maintain the population at the carrying capacity of the habitat. Improvement of yield is brought about only by increase in the carry- ing capacity in respect to food, cover, and space. These concepts are fundamental not only to an under- standing of population dynamics, but also to wildlife management. RELATION TO DISTRIBUTION Variations in abundance of a species are closely related to distribution. Three zones of abun- dance may be recognized. There is an inner zone of normal abundance, where climatic and other condi- tions are ordinarily favorable and high populations of the species are characteristic. Surrounding this inner area is a sone of occasional abundance, where cli- matic or other conditions are usually severe enough to hold populations at a low level, but where occa- sional years occur in which high populations may be reached. On the outside is a zone of possible abun- dance, where the normal environment is such that the species cannot maintain a permanent population but where the species may occur during favorable years by emigration from the inner zones (Cook 1929). Populations can become stabilized only in the innermost zone. Where climate, suitable space or cover, and food continually vary from year to year, as in the middle and outer zones, stabilization is never attained for any appreciable length of time (Swenk 1929). Ordinarily, therefore, one may expect a spe- cies to maintain a stabilized level of abundance only in the center or optimum habitat of its range, and to decline and fluctuate in abundance to an increasing extent towards the limit of its distribution. SUMMARY The regional density of a species depends on the prevalence of its favored niche, and its habitat density within this niche. Populations become sta- bilized by density-dependent factors whose effects increase in intensity as the population level rises and decrease as the population level declines. The most important density-stabilizing factors are competition, fecundity, survival of young, predation, emigration, and disease and physiological stress. The level at which populations become stabilized 700 oa o oO oO °o °o ADULTS PER ROSE PER DAY rs ° °o FIG. 16-11 Control of population size of thrips inhabiting rose blossoms by density-independent factors. The total population tends to increase each year, as indicated by the sigmoid curve, but never reaches saturation of the available niches because of the onset of summer drought. The dotted lines indicate decline from the maximum population size for each of the years 1932- 1937 (Davidson and Andrewartha 1948). is determined by such density-limiting factors as space or cover, prevailing weather, and food supply. These factors are largely density-independent, since their magnitude is primarily determined by the physi- cal conditions of the environment. However, their action is responsive to the size of the population as the amount of space, protection from weather, and food available per individual decreases as the popula- tion increases. The influence of any factor upon a population is determined by the time in the life-cycle of the or- ganisms at which it is effective, its variability, and its threshold and upper limit of vulnerability for the population. Intercompensations occur so that when one factor becomes ineffective in controlling the density of a population, another factor becomes more effective. A species normally attains a stabilized level of density only in the center of its range, where physical conditions are optimum. Towards the periphery of its range, its population density becomes increasingly unstable and fluctuating. Regulation of population size 233 Irruptions, Catastrophes, and Cycles Abundance may change continuously and pro- gressively in one direction over a long period of time, or variations in abundance may take the form of ir- ruptions, catastrophes, or cycles. An understanding of how and why such changes in abundance occur is of considerable academic interest, and is of the ut- most importance for the economic management of fish and game, preservation of wildlife, and in animal husbandry, agriculture, and forestry. Minor fluctuations of less than 2:1 or 3:1 are often the result of sampling errors in estimating the true size of the population. When the ratio of popu- lation sizes from one period to another is greater than can be explained by errors of sampling, the fluctuations have meaning for which we should know the causes. Population ratios from one year to another are commonly of the order of 10:1, 100:1, or in in- sects, up to 10,000:1 or more (Solomon 1949). PROGRESSIVE CHANGE Populations that continue to increase or de- cline over a period of years are said to change pro- gressively. The phenomenon is demonstrated as a species invades a new habitat or region or is becoming extinct. Progressive change in numbers also occurs with seasonal growth of populations. Long-time cli- matic change may produce gradual changes in abun- dance and distribution. Thus the amelioration of winter temperatures in northern Europe since the mid-nineteenth century correlates with the north- ward dispersal and increase in abundance of several species of birds and mammals (Kalela 1949). IRRUPTIONS, OUTBREAKS, PLAGUES The phenomenon of a population suddenly exploding to supersaturate an area is called an irrup- tion, outbreak, or plague. These terms are considered here to be synonymous and to represent the time when an animal is abundant or injurious enough over an appreciable area to be noticed and recorded by untrained observers (Carpenter 1940b). The num- ber of rodents may be in the hundreds or thousands per hectare, of insects in the millions. Outbreaks are known to have occurred since the. beginning of re- corded history in Europe, Africa, and North Amer- ica, especially in insects and rodents. Plagues of European meadow voles were recorded 18 times in France between 1792 and 1931 (Elton 1942). The cause and control of plagues have concerned man since civilization began. Biological control of these outbreaks by introducing parasitoids, parasites, bacteria, and viruses to infect the species concerned has been attempted. Once a foreign insect has be- p3c FIG. 17-!| Onset and subsidence of an outbreak of chinch bugs in Illinois during the 1930's. Areas supporting the densest popu- come an important crop or fruit pest in the United States, investigators are sent to its place of origin, to discover its natural parasitoid or predator enemies and introduce them into the area of infestation. On the whole, this procedure has been successful (Sweet- man 1958). About the turn of the century several attempts were made in Europe, especially in France, to sup- press plagues of meadow voles by starting epizootics of typhoid. This bacterium was found present in dying voles, was cultured and distributed through the fields on food that the voles would eat. The success of the various attempts was always controversial, and when it was appreciated that the disease was also dangerous to man, these procedures were generally abandoned (Elton 1942). Considerable study is be- ing made at the present time of the use of bacteria and other micro-organisms in the biological control of insects (Steinhaus 1960). Myxomatosis has been used for the suppression of the rabbit population in Australia. Caused by a filter- able virus, it is highly contagious among the intro- duced European rabbits, but apparently not trans- missible to man or other animals. The virus is carried between rabbits principally by mosquitoes in Aus- tralia, and by fleas in England. Death occurs about 15 days after exposure. In 1950, extensive field trials with the myxoma virus were undertaken in eastern Australia, and by the end of the year mortalities lations are indicated by the darker pattern (Shelford and Flint 1943). locally as high as 99.8 per cent occurred. Epizootics have continued in later years but with somewhat lower virulence. The virus was introduced into France in 1952, where it spread rapidly; it reached England in 1953. The prognosis of the disease is un- certain. In some regions of Australia, rabbits have recovered from less virulent strains of the virus, or there has been selection of genetically more resistant individuals, so it is possible that some degree of im- munization may arise. The disease, however, may be successful in keeping the population at a low level (Bourliére 1956). Irruptions may occur with almost any kind of animal in any habitat. Irruptions of the bean clam occurred several times between 1894 and 1955 in the intertidal zone at La Jolla, California. The abrupt decline of the last outbreak in 1951-52 was the result of an epizootic associated with a minute unicellular organism of uncertain identity, found in the tissues of the clam (Coe 1955). CATASTROPHES Catastrophes occur at more or less widely spaced intervals and bring marked depressions in the population level of a species. Figure 17.2 shows an- nual populations of the house wren over 41 years, first in Ohio, then in Illinois. Decidedly low points Irruptions, catastrophes, and cycles 235 o oa ° 18° Temperature during 7s, o receding winter a) ° 60° ° “ 5 ul ee nh 5 [-lae S 55° LI3° 4 oc re Ww ee a a = Ng = F50°+ +++++++44+4++4+++4 ++ + + 4+ + + + + + ++ H# $+ 4 /4+-|H++#++-_, Oclts O25 © 50 . S alo ao ul Total individuals, 6 and 9 Territorial 66, Wi 2 lox northern Ohio Trelease Woods, Illinois]! 2 So ee fee nem oeeMMC Ts = i918 L3B 36 30 34 38 42 46 50 54 58 FIG. 17-2 Yearly variations in populations of the house wren. The data for northern Ohio are for total males and females on a |5-acre estate, most of the males being banded and captured at their nest-boxes. The data for Trelease Woods, in central IIlinois, are for territorial males censused on a 55-acre tract by in this curve occur in 1918, 1926, 1940, and 1958. Information from observers indicated that low popu- lations were widespread in eastern North America both in this species and in many other song and game birds during these years. There is considerable evi- dence that these conspicuous variations in abundance, as well as some less pronounced, were the result of severely low winter temperatures. It is of interest that, in England, severe winters causing high mor- tality among such song birds as thrushes, blackbirds, and tits were recorded in 1111, 1115, 1124, 1335, 1407, 1462, 1609, 1708, 1716, 1879, 1917 (Elton 3 oS 735 ‘37 39 4 FIG. 17-3 Fluctuation in the average snow depth in the winter in southwestern Finland and population fluctuations of the par- tridge during the autumns following. The dotted line shows the YEAR the spot-map method. Mean temperatures are for the winter- ing range only, based on monthly weather data for Tampa and Jacksonville, Florida; Savannah, Georgia; Montgomery and Mobile, Alabama (Kendeigh and Baldwin 1937, Kendeigh 1944). 1927). To birds that feed on the ground, the depth of snow is as critical a factor as low temperature in determining the number that survive. Among mammals, fluctuations in the population of the common hare in Denmark have been correlated with the varying effects of summer rainfall, spring temperatures, and the number of days of frost during the winter (Andersen 1957). Catastrophes may occur with practically any type of animal life. The severe winter of 1917-18, for instance, produced a marked reduction in the num- bers of many species of marine invertebrates in the INCHES "43 45 47 49 5 53 55 WINTER (’29=1928-29, etc.) critical snow depth, below which increases (solid circles) and above which decreases (open circle) in population usually occur (Siivonen 1956). 236 Ecological processes and dynamics region of Woods Hole, Massachusetts (Allee 1919). In seeking correlations between catastrophes and weather or other environmental conditions, one needs to determine the period in its life cycle during which the species is most vulnerable, and the weather con- ditions coincident with that period that exert great- est effect. CYCLES Populations are cyclic or oscillatory when they vary in a more or less uniform manner between high and low levels of density. Types Although cycles of different duration have been postulated for many species at different times, the best established cycles are those of periodicities of 34 years and 9-10 years (Speirs 1939, Elton 1942, Dymond 1947, G.R. Williams 1954, Siivonen 1957). The best known 3-4 year cycles are demonstrated in the following species: Birds Mammals Snowy owl European lemming Willow ptarmigan Siberian lemming (northern Europe) Brown lemming Capercaillie Collared lemming Blackgame European meadow vole Hazel grouse Arctic fox Other species that may also vary in a 3-4 year cycle are rough-legged hawk, northern shrike, North American meadow vole, short-tailed meadow vole (England, Scotland), red fox (far North), marten (far North), and sockeye salmon (Pacific coast of North America). Species well recognized as showing the 9-10 year cycle are: Birds Mammals Rufted grouse Snowshoe rabbit Sharp-tailed grouse Muskrat Willow ptarmigan Canada lynx (North America) In addition, a number of other species may have a 9-10 year cycle: rock ptarmigan, goshawk, great horned owl, red fox (South), marten (South), fisher, mink, and Atlantic salmon. Fluctuations in populations, commonly of 5-6 years (Mackenzie 1952), occur in the British Isles in red grouse, rock ptarmigan, black game, and caper- caillie, but there is question as to whether they are regular and definite enough to be truly cyclical. Among invertebrates, insect pests of coniferous for- ests in Germany fluctuate in periods variously from 6 through 18 years (Eidmann 1931) ; grasshoppers in Manitoba, 7 through 16 years (Criddle 1932) ; chinch bugs in Illinois, from about 3 through 16 years (Shelford and Flint 1943). Subjective estimates of damage by the starfish Asterias forbesi on mollusk fisheries between New York and Cape Cod suggest a periodicity of 14 years for this marine species (Burkenroad 1946). It is possible to demonstrate mathematically that an apparent cycle or a series of irregular fluctuations may actually be compounded of several distinct pe- riodicities, each of different duration (Wing 1953). Cycles can thus be postulated in the population fluc- tuations of many species, in the migration of birds, and in human economics (Wing 1935, Huntington 1945; see also J. Cycle Res. 1952 on, and other publications of the Foundation for the Study of Cycles), but the biological significance of these hid- den periodicities remains to be demonstrated. ‘According to an extensive analysis made by Siivonen (1948) of data extending back through one hundred years, the short-term cycle averages 3% years, with two 3-year and one 4-year cycle coming each decade. He believes that the 9-10 year cycle (average 10 years) results from each third short cycle having a greater amplitude than the other two, and that the long-term cycle is therefore only a modi- fication of the more basic short-term cycle. Although there is doubt about the fundamental nature of this relation, it is true that the 3-4 year cycle is better expressed in the far North and the 9-10 year cycle in more southerly latitudes. The 3-4 year cycle may change to a 9-10 year cycle, correlated with latitude, even in the same species (red fox, marten). South of latitudes 45°-50°N in North America and about 60°N in Europe, variations in population size appear progressively less extreme and cyclic, more irregular or random in character. Thus the numbers of four species of gallinaceous birds during peak years di- vided by their counterpart numbers during low years changes from 3.8 in Lapland, to 2.4 in northern Fin- land, to 2.0 in central Finland, to 1.7 in southern Finland (Siivonen 1954). Cycles may be distinct and definite in the far North because only a few species are involved and the environment is rela- tively monotonous and severe ; in more southerly lati- tudes, population fluctuation becomes more irregular and uncertain because of the interaction of many species and a more moderate environment. Are cycles real? To designate fluctuations as cyclic implies con- siderable regularity for them. In mathematical usage, Irruptions, catastrophes, and cycles 257 a cycle is a recurring variation of regular timing or phasing and of constant amplitude. Fluctuations are considered periodic if the phase is constant but the amplitude varies. What ecologists call cycles are really oscillations because both phase and amplitude are inconstant. Justification for calling the fluctua- tions cyclic, rather than random, lies in the demon- stration that the variability that is evident, especially in phase, is less than is to be expected by chance and that reasonably accurate predictions can be made of the course of future variations in population size (Davis 1957, MacLulich 1957). However, there has been considerable controversy concerning the true significance of cycles (Cole 1954, Hickey 1954). The short-term cycle is commonly 3, 4, or 5 years long, although it may be as short as 2 years, or as long as 6 years (Elton 1942). The snowshoe rabbit cycle varies between 8 and 11 years; the lynx cycle, between 8 and 12 years (MacLulich 1937). The coefficient of variation, standard deviation divided by the mean, for different species having the short cycle varies from 30 to 50 per cent, and is of the same order of magnitude for the longer cycle (Cole 1951). It is of interest that by drawing numbered cards from a well shuffled deck or rolling dice (Palmgren 1949, Hutchinson and Deevey 1949) or plotting ran- dom numbers (Cole 1951) short and long cycles may be obtained of about the same relative lengths and vari- ation coefficients as animal population cycles. In comparing the frequency of peaks in popula- tions and in random numbers it has been a common practice to designate any number as a peak which is higher than both the preceding and following num- bers, regardless of the amount of difference between them. This, however, is not justified with natural populations of animals, since minor variations may be due to sampling errors or to secondary factors modifying a major trend. According to criteria used in this text, a peak would not be considered real un- less the size of the population at that time is at least two or three times its size during the preceding and following depression. When only such conspicuous peaks are considered, oscillations in random numbers are lengthened and some of their correspondence to natural cycles is lost (Cole 1954). Extreme fluctua- tions between peaks and lows in population cycles are ordinarily of much greater amplitude than occur in mathematical models (Pitelka 1957). Of course, the criteria by which a particular peak is to be evalu- ated depend on the accuracy to which the population size was measured. These peaks should be deter- mined in as objective a manner as possible. Before oscillations in the size of natural populations are considered cyclic, they should first be tested statis- tically for randomness. Only after that is done is it profitable to look for periodic or automatic mech- anisms that may be involved. The reality of cycles may be further tested by the amount of synchrony that they exhibit. If it is shown that peaks and troughs in the oscillations of different species in a local area are not correlated in time, and oscillations of populations in different re- gions occur independently of each other, one should take warning that a variety of factors may be involved that fluctuate in their action at different times, in different places, and on different species, in essen- tially a random manner. If such synchrony is deter- mined, then some master factor or set of factors must be affecting all populations alike, although it is still necessary to determine whether the action of the master factor on the population is cyclic in its timing and effect, or whether it is being exerted in an irregular manner. We need to examine the extent to which population oscillations are synchronized. Synchrony With the 9-10 year cycle, local areas may show peak populations that are out of phase with other local areas by one, two, or three years. But when large regions are considered, the peak is manifested over three or four years in the course of which most local areas reach maximum populations while the following trough in the regional cycle may spread over five or six years when very few, if any, local areas have large populations (Butler 1953). A sim- ilar relation probably holds between local and regional fluctuations with the 3-4 year cycle. Synchrony is sometimes evident in local populations that are iso- lated by a hundred or more miles from other popula- tions of the species (Brooks 1955). Some variation in cyclic tempo occurs in different parts of the world, although generally they are close to being in phase. Local areas out of phase with the main cycle commonly come back into phase by the time the next peak is reached. The main cycle of grouse and ptarmigan over most of Canada and in northern United States has shown peaks in 1896, 1905, 1914, 1923, 1932-33, 1941-42, and 1950-51. In the maritime provinces of Canada—Newfound- land, New Brunswick, Nova Scotia, and Prince Ed- ward Island—the cycle is advanced 3 years ahead of the main cycle; in Alaska, the cycle lags by 3 years. In Britain, the grouse and ptarmigan cycle has a mean length of 5-6 years, but in Finland and Scan- dinavia it is only 3-4 years, although some of the same species are involved. In North America the grouse cycle is nearly synchronous with the cycle of the snowshoe rabbit, while in Scandinavia it coin- cides with the cycle of lemmings (G.R. Williams 1954). The lemming cycle in Canada is similar to that in Norway (Elton 1927). Recent peaks in the 3-4 year cycle for small rodents in Finland, Norway, and 238 Ecological processes and dynamics Northern Baffin Island Southern Baffin Island Northern Quebec Northern Labrador Victoria Island (East) Victoria Island (West) | ees (Tes eee ee FIG. 17-4 Relation be- tween peaks and lows of lemming cycle at different localities in eastern and northern Canada (Chitty 1950). a 1934 1937 1940 eastern North America have occurred in 1923, 1926, 1930, 1934, 1938, 1941-42, 1945-46, 1948-49, and 1953, with deviations of one or more years for par- ticular regions (Siivonen 1954). A peak occurred in Alaska in 1956. The predator cycle is dependent on the cycle of the herbivorous mammal or bird prey species. The correspondence in the cycles of predator and prey is usually close, although that of the predator some- times lags a year behind that of its prey (Chitty 1950, Butler 1953). The snowy owl emigrates in large numbers from Canada into the United States within a year after the decline in the lemming popu- lation (Shelford 1945). In those parts of Greenland where the fox population lives largely on lemmings, the 3-4 year fox cycle is very pronounced, but this is not true for the coastal areas where the fox de- pends on a variety of food other than lemmings (Braestrup 1941). NUMBER (Thousands) 1850. 1860 1870. 1880 1890 <—- Varying hare +--~ Canada lynx “ie bot 1900 1910 1920 1930 1943 1946 1949 Intrinsic causes According to early mathematical theories of Lotka and Volterra (D’Ancona 1954) and of Nichol- son and Bailey (1935), a population consisting of a single prey species and a singe predator or parasitoid species occurring together in a limited area, with all external factors constant, automatically displays pe- riodic oscillations or cycles in the numbers of both species. As the predator population increases, it will consume a progressively larger number of prey until the prey population begins to decrease. As the num- ber of prey diminishes, there will be less food for the predator, and they will thus decline. After a time the number of predators will be so reduced that the high reproductive rate of the remaining prey will more than compensate for the loss from predation, and the numbers of the prey species will again in- crease. This will be followed shortly by an increase FIG. 17-5 Population cycles of the snowshoe rabbit and one of its chief predators, the Canada lynx, in northern Canada (data adjusted for years 1912 to 1920), based on number of pelts handled by the Hudson Bay Com- pany (from MacLulich 1937). Irruptions, catastrophes, and cycles 239 in numbers of the predator. The cycle would thus continue indefinitely. According to the differential equations, the predator will never be able completely to destroy the prey, nor will the predator species ever completely disappear by reason of starvation. There is considerable controversy concerning this theory (Andrewartha and Birch 1954). Gause (1934, 1935) conducted a test in a classic series of experiments with protozoan cultures. An experi- mental food chain was established: boiled oat- meal—+bacteria—> Paramecium caudatum—Di- dinium nasutum. When five Paramecium were in- troduced one day and three Didinium two days later, the population of Paramecium was exterminated by the predator. The predator, left without food, disap- peared soon after. In another experiment,cover sedi- ment was introduced into the microcosm, in which the Paramecium could hide, thus to escape the attacks of Didinium. The same number of each species was introduced at the same time. The number of preda- tors increased, and they devoured many of the prey. However, the remaining prey escaped into the cover, and the predators died of starvation. When this hap- pened, the prey, now unchecked, increased in an unlimited manner. When, on the other hand, a micro- cosm was prepared in which there was no refuge, and one Paramecium and one Didinium were introduced on every third day, a series of oscillations resulted. It is suggested that continual cycling of prey and predator populations could be maintained only with immigration of individuals from the outside. In other experiments, interrelated cycles of Paramecium and the yeast on which it fed were established (Gause 1935). In experimental greenhouse plots of strawberries, populations of an herbivorous mite, Tarsonemus pallidus, and its predator, another mite, Typhlodro- mus, fluctuated regularly in relation to each other. At low populations, the prey species was relatively secure in the cover offeted by hairs, spines, and leaf crevices, thus avoiding annihilation. The predator species survived because it utilized honeydew and other nourishment as substitute food until the prey species again increased in numbers (Huffaker and Kennett 1956). Reciprocal fluctuations in the density of the azuki bean weevil and its larval parasitoid, a braconid wasp, were sustained experimentally under constant conditions for 112 successive generations. Appar- ently the prey was able to survive in the low of the cycle because of the difficulty the parasitoid experi- enced in finding the surviving individuals; the para- sitoid, however, never became extinct (Utida 1957). These examples indicate that oscillations in the populations of predators and prey can be sustained for relatively long periods of time if such factors as cover, buffer food, or immigration are introduced into 240 the experiment. This background of experimental studies is useful in the analysis of possible causes of the more or less regular oscillations in animal num- bers that are observed under natural conditions. In an area near Point Barrow in northern Alaska, Siberian lemmings were scarce from 1949 to 1951, increased in 1952, and were near or at a peak in 1953. Associated with this cyclic rise in the lemming popu- lation was a marked increase in the number of preda- tors. There was no breeding in 1951 of pomarine jaegers, snowy owls, and short-eared owls; very few were even seen. In 1953, however, breeding pairs were recorded in densities respectively of about 18, 0.3, and 3-4 per 250 hectares (per square mile). Least weasels and Arctic and red foxes increased from scarce or no record to common. Because of this heavy predation, the lemming population was re- duced by mid-July of 1953 to 4% or less of what it had been when the snow cover melted in early June (Pitelka et al. 1955). Cyclic changes between 1929 and 1940 in the collared lemming at Churchill, Manitoba, were ac- companied by marked fluctuations in breeding popula- tions of snowy and short-eared owls and of the rough- legged hawk (Shelford 1943). This rapid build-up of predator populations must be attributed to their abil- ity to shift from one region to another according to availability of local prey. The lemming becomes more vulnerable to predation when large populations con- sume the vegetative cover. Influxes of predators suf- ficient to exert a controlling role in outbreaks of mice (Banfield 1947), ruffed grouse, and snow- shoe rabbit (Morse 1939), and bobwhite (Jackson 1947) have been reported for regions as far south as Toronto, Minnesota, and northwest Texas. The collared lemming breeds in the winter, at least to some extent, as well as during the summer (Sutton and Hamilton 1932), as does, apparently, the Siberian lemming (Pitelka et al. 1955). When lem- mings are exposed to heavy predation during the summer, it is likely that the main population growth comes between August and the following June, dur- ing which time they are protected by a snow cover. When snow is inadequate, heavy predation doubtless continues throughout the year. With the lack of snow insulation, considerable mortality may also result from effects of low temperature (Shelford 1943). There is no evidence, however, that snow cover oc- curs in cyclic harmony with the lemming populations, necessary were snow the critical factor producing the lemming cycle. A number of general theories of possible intrinsic cycle causes (Dymond 1947, Grange 1949, Lack 1954a) have been found inadequate. Particular cycles have been explained for several species, such as for Daphnia under experimental conditions (Slo- bodkin 1954), sockeye salmon in the Fraser River Ecological processes and dynamics (Dymond 1947), and black crappies in the Illinois River (Thompson 1941), but these explanations have limited application. A persistent theory of the general cause of cycles hypothesizes that animal populations build up to a peak, at which time an epidemic disease occurs so reducing numbers that the disease can no longer spread. The cycle then starts over again. Epizootics observed in cyclic species are those caused by the blood-sucking stomach worm Obeliscoides cuniculi in the snowshoe rabbit; the blood protozoan Leuco- cytosoon bonasae in the ruffed grouse ; protozoan in- fection of the brain caused by To.voplasma in rodents ; and so forth. However, these diseases have been en- countered in some cyclic declines but not in others and offer no explanation of the regular recurrence of the cycles. Toxoplasma, for instance, was re- ported in three early population declines of rodents, but was not demonstrably present in more recent declines (Elton 1942). It is generally agreed that the basic cycle is that of the herbivores: rodents, grouse, snowshoe rabbit, and the like. With a rise in number of herbivores, predator populations may eventually increase suff- ciently by reproduction and immigration to reduce the herbivores, but the predator population cycle depends fundamentally upon the herbivore cycle. The herbi- vore cycle may in turn be dependent on interrelations with its plant food supply. This suggests the follow- ing explanation. Northern plants are often unable, in the short season available, to make a luxuriant vegetative growth and produce seed every year. During lem- ming peaks, the animals deplete their usual food plants and are forced to turn to emergency species which are not self-sustaining. A period of two or more years may be required, following an irruption in the lemming population, for full recovery of the vegetation both quantitatively and qualitatively. The relation between herbivores and plants is very similar to that between predator and prey. The difficulty with this hypothesis is the lack of evidence that lem- ming mortality during population declines is actually a result of starvation. Species depending on seeds for food would be similarly affected by the interval be- tween abundant seed crops (Lack 1954a, Thompson 1955a, Watson 1956, Lauckhart 1957). Variations in mineral salts within plants may be involved in the cycles of herbivorous species. Vari- ations in climate may affect the bacterial flora of the soil and consequently the availability of calcium to plants. In Bavaria, it has been possible to correlate the gradual decrease in calcium content of hay with the development of “licking disease” in cattle. Other elements, although needed only in minute quantities in animal metabolism, sometimes lead, if absent, to an upset in the acid-base balance of the body and the development of acidosis and ketosis or other effects (Braestrup 1940, 1942). Inadequate nutrition, either quantitatively or qualitatively, is well known to affect the rate of re- production in animals (Hammond 1955). The size of egg clutches and the vigor of the hatched young in grouse and other tetraonids in Finland seem to depend on whether the females are able to get new green vegetation for food in the critical period just before the start of egg-laying. That this vegetation become available requires that temperature be suffi- cient to melt the snow cover and to initiate plant growth early in the year (Siivonen 1957). Changes in reproductive vigor and health may also depend on the vitamin content of the food con- sumed (Mason 1939) and on the extent of the ani- mal’s exposure to solar radiation. The vitamin con- tent of animal food is known to vary quantitatively from time to time (Lehmann 1953). It is obvious that at times of high population den- sities animals are subject to increased stresses of vari- ous. sorts in their search for food and cover and escaping predators. They may have to go longer distances to find the essentials for existence and to fight with other animals for possession of them. All of this puts an extra drain on their energy resources at the same time that they may be compelled to subsist on inferior food or tolerate nutritional defi- ciencies of one sort or another. The body adapts physiologically to these stresses under the stimulus of increased hormone secretion from the adrenal and pituitary glands, but when the stresses for existence and reproduction become too great, death results. It has been postulated that the die-off at the end of a cycle is due directly to such an exhaustion of the adrenopituitary system: rather than to such external factors as lack of food, disease, or predators (Chris- tian 1950). “Shock disease” is a manifestation of this stress syndrome. It has been repeatedly ob- served in Minnesota during the decline of the snow- shoe rabbit cycle. Symptoms of the exhaustion phase of the stress syndrome have also been observed in wild populations of European meadow voles (Frank 1953). The continuous decline of populations for three to five years may be due to constitutional de- fects resulting from the stress of overcrowding be- ing transmitted to following generations (Chitty 1952), but direct evidence for this is meager. The difficulty in finding a satisfactory explanation of cycles as they occur under natural conditions is that they may be only one manifestation of an under- lying, more fundamental, cycle in physiological vigor which is not easily detected. Changes in population size are not always correlated with changes in physi- ological vigor, since extremes in weather may at times produce catastrophes even in healthy and vigor- ous populations. But changes in physiological vigor Irruptions, catastrophes, and cycles 24] > ne) FIG. 17-6 Changes in population = 600 and reproductive and physiological 2 vigor of muskrats in lowa, eS correlated with the 9-10 year (A) EQ IS grouse-rabbit cycle (peak years wx 1941-1942, 1950-1951) elsewhere a D010 — over North America (Errington S (aa 1957). (A) autumn populations; = (B) grouse-rabbit cycle; (C) size of litters; (D) young breeding in year of their birth; (B) (E) tolerance of crowding; (F) resistance to disease. oe a () 2 =) az 5 (D) ww oO ao Ld Qa (E) VO ae ie (10) —= (By sc a cola wc (ees uJ = become evident with variations of fecundity and suc- cess in raising young, in susceptibility to disease, and in individual behavior (Errington 1945, 1954, 1957). When chinch bugs were cultured experimentally in the laboratory, starting each year with new individ- uals collected out-of-doors, there were marked dif- ferences during the nine-year period of study both in number of generations raised per year and in number of young per generation. The rating of re- productive vigor rose from 1.4 in 1917 to 31.2 in 1919, dropped to 1.4 in 1921, and rose again to 270 in 1925. These differences from year to year were ap- parently not related to the density of population, and could not be otherwise explained (Shelford and Flint 1943). Changes in reproductivity and behavior D7. i Lyi 7S, Yolen 40) 42 re | [| | AANA Gi 4 Cl OME Caen 4S YEAR of muskrats in Iowa did not correlate closely with variations in population size of the species, but did coincide in a general way with the grouse-rabbit cycle in other parts of North America. Cycles in the environment We have considered so far only factors intrinsic to populations; extrinsic factors (Hutchinson and Deevey 1949) may play a contributing role. Cer- tainly the amount of synchrony within a species evi- dent over extensive areas and between different species or events that are otherwise clearly unrelated (Dewey 1960) is greater than can be explained by Ecological processes and dynamics chance alone and suggests that some climatic or ex- traordinary factor may be effective, either directly or in controlling the time schedule at which the in- trinsic factors function. There have been various attempts to show cycles in the weather. Bruckner (1890) examined records since 1700 of temperature, precipitation, and other factors and suggested a cycle of around 35 years. Scarcities of ducks reported in the 1820's, 1860's, 1890's, and 1930's appear correlated with drought and may possibly represent the intervals of the Bruckner cycle (Rowan 1954). Weather is affected by variations in solar radi- ation (Abbott 1931, Clayton 1943). Sunspots, chro- mospheric eruptions, solar coronal disturbances, and ionospheric and geomagnetic disturbances are indices of solar activity, but not direct measurements of it. The agent presumably effecting changes in the at- mosphere and in living organisms may be short- wavelength ultraviolet radiations or emitted charged particles. Emissions from the sun are constantly un- dergoing great fluctuations, with maximum intensi- ties for short periods being a hundred or a thousand times the minimum intensities. Sunspots are the only expression of solar activity that has been measured over a long period of time (Willett 1953). The intervals between peaks in the mean daily number of sunspots per year fluctuate from 7 to 17 years, average 11.2 years. A correlation between number of sunspots and temperature, rain- fall, and cloudiness is sometimes indicated but needs more complete substantiation before it can be fully accepted (Thomson 1936). For instance, an anal- ysis of records spanning 109 years for the period May through October in southern Wisconsin indi- cates lower temperatures, greater precipitation, and less sunshine in years when sunspots were increasing or at a maximum than when they were decreasing or at a minimum (Morris 1947). A similar correla- tion between summer rainfall and the sunspot cycle has been demonstrated for the Toronto, Canada, area (Clayton 1943). The sunspot cycle is as variable in length as are the cycles of grouse and snowshoe rabbit, and there have been repeated attempts to correlate these cycles (MacLagan 1940, Huntington 1945). It is doubtful, however, if such a correlation is real. Since 1750, the sunspot cycle has coincided with the snowshoe rabbit and lynx cycles part of the time, but goes out of phase until one becomes the inverse of the other. If solar radiation is responsible for population cycles, it is clear that the number of sunspots is not a reli- able index for judging the intensity of radiation. Since the growth of trees and the width of the annual rings they form is largely dependent on rain- fall, one may conjecture the weather record back 3,000 years by measuring the width of annual rings lrruptions, catastrophes, and cycles in the giant sequoias. This analysis of tree rings in- dicates the possibility of a variety of weather cycles, some important ones being in the neighborhood of 9-10 years (Douglass 1928). Weather cycles of 3-4 years are difficult to demonstrate, but an analysis of changes in barometric pressure and other charac- teristics of the annual atmospheric circulation over the British Isles indicates that they may exist (Goldie 1936). Cycles or outbreaks resulting from climatic factors are not necessarily absolutely synchronous over large areas. There is a limit to the size of the area over which a change in weather produces a single common effect. Outbreaks of spruce budworm progressed eastward in Canada between 1945 and 1949. These outbreaks were probably less a result of spontaneous dispersal of the moth, although this was a contributing factor, than of the progressive east- ward circulation of favorable polar air masses (Greenbank 1957). Solar radiation may affect animals and plants in other ways than through the weather. The atmos- phere above the earth’s surface is divisible into the troposphere, which extends to a height of 3-4 km (5-6 miles) ; the stratosphere, which rises to about 30 km (50 miles) ; and the ionosphere, which ex- tends beyond. The concentration of oxygen dimin- ishes with height above the earth, and becomes very low in the stratosphere. However, at heights of about 10 to 20 km (15 to 30 miles), there is a thin but concentrated layer of ozone (O3). Oxygen and ozone are responsible for absorbing most of the ultraviolet radiation (below 3200 A) emanating from the sun before it reaches the earth’s surface. Atmospheric gases, especially carbon dioxide and water vapor, ab- sorb most of the infrared wavelengths (over 20,000 A) (Shaw 1953). A little ozone in the atmosphere at the earth’s surface is stimulating to animals, but a high amount is harmful. There is some experi- mental evidence that ionization of the air, i.e., the conversion of neutral gas molecules into electrically charged ions, may affect the health and the vigor of animals. The height of the ionosphere and ozone layers above the earth is controlled by the intensity of solar radiation, and it is possible that cyclic vari- ations in the height of these layers may affect organ- isms in ways that are little understood at the present time (Reiser 1937, Huntington 1941). Fluctuations in solar ultraviolet radiation vary the extent of ionization of air and the rate of ozone formation. The ozone layer serves as a protective blanket which prevents ultraviolet rays from destroy- ing all life on the earth. In small doses, ultraviolet is anti-rachitic, germicidal, and erythemal ; in some spe- cies ultraviolet also affects skin pigmentation (Luck- iesh 1946). Animals obtain vitamin D either as a product of direct radiation of the skin or in the food that they consume. Wavelengths other than ultra- 243 violet are important, of course, to photosynthesis and the trophic cycle, and for heat. Continuous ultraviolet radiation of the sun has been measured only haphazardly in the past, and in only a few localities, but the data available indicate only a general relation to the sunspot cycle. Varia- tions in monthly averages of ultraviolet intensity and sunspot numbers may show an inverse rather than a direct relation for a year at a time (Pettit 1932), and variations in yearly averages do not closely corre- spond (Thomson 1936). In order to establish any correlation between ultraviolet and variations in the size of animal popu- lations, it is necessary first to determine what stage in the yearly cycle of the animal’s activities is sensi- tive to its effect and then to use quantitative data on ultraviolet intensity for only those critical periods. Likewise, other factors, such as rainfall, may mask the effect at certain intensities. Such analyses are complicated, but a few such studies indicate that medium intensities of ultraviolet radiation combined with optimum conditions of rainfall or weather cor- relate with highest populations of chinch bugs, bob- white quail, prairie chickens, pheasants, cottontail rabbits, pronghorns, and the amount of butterfat in cow’s milk (Shelford 1951a, 1952, 1954a). Solar radition is not received in equal intensities in all parts of the world. Its intensity varies because of inclination of the earth’s axis relative to the sun, differences in terrain, amount of cloudiness, and so forth. Solar radiation likewise does not have an iden- tical effect on all species because of differences in their sensitivity to it and because critical periods in their life cycles come at different times of the year. The lack of agreement among scientists as to the cause of oscillations in population size is evident from the number of explanations that are offered. It is clear that much more information on animal popula- tions under both experimental and natural conditions is required to obtain full understanding of the dy- namic forces involved. SUMMARY Populations may increase or decrease in size progressively through a period of time; they may suddenly irrupt when conditions become favorable, or decline precipitately with unfavorable weather, or they may vary cyclically. The most apparent cyclic phenomena are the 3-4 year rodent cycle and the 9-10 year grouse-rabbit cycle. Populations of several predators display correlated patterns. Cycles are most clearly developed in the far North. Many theories have been advanced as to the im- mediate cause of population cycles; coactions be- tween prey and predator, disease, depletion of food supplies, and changes in nutrient value of foods, phys- iological stress, and physiological vigor, but a com- plete and satisfactory explanation is not yet at hand. There is enough synchrony in the timing of cycles over the world as to suggest that variations in wea- ther or solar radiation may be involved. 244 Ecological processes and dynamics Pita ee ae PFVTQIINILCS: Niche Segregation The ecological niche is a particular combination of physical factors (microhabitat) and biotic rela- tions (role) required by a species for the normal course of its life activities. The term and concept was first developed by Joseph Grinnell (1917, 1924, 1928) of California. He considered the ecological niche the ultimate dis- tributional or spatial unit occupied by just one species, or subspecies, to which that species is held by struc- tural and instinctive limitations such as climatic fac- tors, kind and amount of food, suitable nesting-sites, and cover. He recognized the close relation between animal distribution and cover, and he spoke of ani- mals generally and birds in particular as having pref- erence for a particular niche, choosing surroundings consistent with their needs. At about the same time, the Englishman Charles Elton (1927) independently defined the niche in more functional terms as an animal’s place in the biotic environment, its relation to food and enemies. The present-day concept of the niche is an elaboration of these basic ideas, with em- phasis on the relation of the organism -to both the physical and biotic factors of its environment. The restriction of a species to a particular niche depends on its structural adaptations, physiological adjust- ments, and developed behavior patterns. In many respects, the species in its niche is the only finite unit of animal distribution; at least the relationship is the one most subject to objective and concrete definition. The concept of niche in the hier- archy of ecological distributional units is more or less equivalent to the concept of species in the hierarchy of taxonomic units. When we begin to group niches into the higher units of habitat, biocies, biociation, biome, and realm we become as increasingly vague and arbitrary as we are when classifying species into genera, families, orders, classes, and phyla. CHARACTERISTICS OF THE NICHE Microhabitat The physical features of substratum, space, and microclimate are basic determinants of whether a particular niche can be occupied by a given species. The basic differences in marine, fresh-water, and ter- restrial habitats restricting the distribution of com- munities is immediately obvious. The features differ- entiating microhabitats are less apparent, but assume major significance when particular occupants of it, rather than the community as such, are under investi- gation (Prosser 1955). The intimate relation between niche segregation, substratum, and microclimate is well shown in the distribution of four species of ants in a pine forest of Des Scotland (Brian 1952). On decaying stumps, the number of nests of Formica fusca, Myrmica scabri- nodis, M. rubra, and Leptothorax acervorum were found to be in the ratio of 23:20:12:7, but on the ground away from stumps, the ratio was 1 :13:18:1. The apparent restriction of F. fusca and L. acer- vorum to stumps is probably attributable to the fact that they are not mound-builders; rather, they com- monly make galleries in wood. L. acervorum makes its galleries in wood that is too hard for the other species to work; galleries so small that the larger species are excluded. A difference between species in point of position occupied on the same stump was also observed. F. fusca tended to occupy the warmer southerly sites but often extended over the entire stump. MW. scabrinodis was widely distributed, but there was a tendency to concentration on the east side of the stumps. M. rubra occurred principally in the cooler, moister north and west sectors. Away from the stumps, M. scabrinodis was especially character- istic of small knolls resulting from the decay of stumps no longer favored by M. fusca, while M. rubra extended into cooler moister soils. F. fusca was the most aggressive and socially dominant species, un- challenged in its occupancy of the best sites. M. rubra tended to occupy the second-best sites, leaving them only when they became temporarily unsuitable. M. scabrinodis often came into the vacated sites and when once established could not be dislodged. In earlier chapters, we have described how the characteristics of the soil, which the females first test with their ovipositors, determine where grasshoppers and tiger beetles will lay their eggs. Of 18 types of rodents studied in Utah, 4 were found only in rocky situations, 2 only in gravelly soils, and 2 only in sandy soils; the other 10 were less limited by the type of soil (Hardy 1945). In aquatic habitats, spe- cies segregate according to whether the bottom is rock, sand, or mud. The swift current limits the in- habitants of streams to species possessing clinging structures and proper orienting behavior. Congeneric species of ectoparasitic mites and fleas on small mammals commonly are distributed between several different host species. When they occur on the same host, they are segregated by species on dif- ferent parts of the body, a given species is present only when the host occurs in a particular type of vegetation, or they occur at different seasons ( Jame- son and Brennan 1957). The importance of microclimate in niche segrega- tion of species is shown by a study made in Danish bogs (N@grgaard 1951). The low humidities and high temperatures obtaining at mid-day on the sur- face of the peat mat restrict one species of spider to the stalk region of the sphagnum. Another species of spider tolerates these conditions, so the two species divide the habitat between them. 246 Microclimate is often a major factor in determin- ing whether a species can maintain itself against competition in a particular microhabitat. This has been shown experimentally. When equal numbers of two related species of beetles are introduced into the same flour container and placed at a particular combination of temperature and relative humidity, one species becomes established, the other is elimi- nated. The particular species favored in the various microclimates are as follows (Park 1954) : Tribolium castaneum Tribolium confusum Tribolium castaneum Tribolium confusum Tribolium confusum Tribolium confusum 34°C-70 % RH. 34°C-30 % RH. 29°C-70 % RH. 29°C-30 % R.H. 24°C-70 % RH. 24°C-30 % R.H. Similar reversal of dominance has been found to take place at high and low temperatures with dif- ferent species of grain beetles (Birch 1953), Dro- sophila flies both in Europe (Timoféeff-Ressovsky 1933) and in North America (Moore 1952), two in- sect parasitoids using the same host (DeBach and Sisojevic 1960), and turbellarian flatworms (Beau- champ and Ullyott 1932). Usually the species fa- vored by a given micro-climate has a higher rate of population growth at that particular temperature or humidity. There is a positive correlation between high oxy- gen tensions required by trout for saturating their blood hemoglobin and the oxygen-rich waters that they select. The restriction of these fish to cold waters is correlated with the fact that a rise in temperature decreases the oxygen-loading capacity of the hemo- globin. Catfish and carp, common to warm waters of low oxygen high carbon dioxide content, have hemo- globin that loads and unloads at low oxygen tensions and is less sensitive to changes in carbon dioxide tension and temperature (Prosser et al. 1950). Arctic mammal species differ in thickness and density of fur, which insulate against loss of body heat, and this determines whether they can sleep above ground and be active during the winter or whether they must confine themselves to nests and runways below the snow level (Scholander ef al. 1950). In central Illinois, the short-tailed shrew is largely subterranean in habit and occurs in moist habitats ; the woodland white-footed mouse is nocturnal and inhabits the forest floor ; the prairie vole is restricted to grassland and the most arid of the habitats of the three species. There is a connection between the amount of water available in the habitats and the level of water exchanges in the animal. At 19°C, for in- stance, the rate of water absorption and loss in the shrew is twice that of the mouse. The rate of total Ecological processes and dynamics water turnover in the vole is about the same as in the mouse, but water loss through the lungs and skin is much lower, indicating acclimatization to a drier habitat (Chew 1951). Diurnation and aspection When two or more species are competing for the same resources of a single habitat, this competi- tion is reduced or eliminated if one species makes use of these resources at a different time of day, or in a different season, than the other. The white crappie and black crappie are very sim- ilar in habits, food requirements, and local distribu- tion, except that the white crappie is more often found in small rivers and creeks than the black crappie, which prefers hard-bottom lakes. Both species feed most extensively at dusk; there is a smaller feeding activity peak at dawn. Both species feed to some ex- tent during the night. However, the white crappie feeds considerably more during the daylight period than does its more aggressive black cousin, and this slight difference in timing may be sufficient to permit it to occur in the same areas as the black crappie (Childers and Shoemaker 1953). During a winter in England, when birds were com- ing to banding traps for food, it was noted that the European robin did so most frequently just after sun- rise and just before sunset, the European blackbird just before and after midday, while the blue tit had peaks of feeding between the feeding times of the other two species (Lees 1948). The females of the butterfly Colias eurytheme may be either orange or white; all males are orange. There is partial separation of the two color phases in that the white females are relatively more active in the early morning and the yellow females later in the day (Hovanitz 1948). The two grasshoppers Arphia sulphurea and A. anthoptera occupy similar niches except that A. sul- phurea overwinters in the nymph stage, reaching ma- turity from April to late July, while A. xanthoptera overwinters in the egg stage and hence requires a longer time to mature in the spring. The adults of the latter species occur from late July to early November (Blatchley 1920). Three kinds of sockeye salmon occur in Cultus Lake, British Columbia (Ricker 1938) : (1) the nor- mal anadromous stock, whose offspring may either migrate out to sea or remain as (2) residuals, and (3) the land-locked salmon, which remain continu- ously in the lake. The anadromous and residual popi- lations differ in breeding coloration, but both spawn from October to December. The land-locked forms, which closely resemble anadromous sockeye in breed- ing coloration, spawn only in August and September. The tern Sterna virgata nests on the Kerguelen Islands in October and November ; S. vittata uses the same nesting area in January and February. The niche of each species differs somewhat further in that S. virgata feeds to some extent in inland waters, but S. vittata is strictly marine (Murphy 1938). Tropical species of dragonflies and damselflies which have recently extended their range northward confine their main periods of flight to the warmest months and to the middle of the day. Native species that are better adapted to colder climates are active in early spring and autumn and in the twilight hours of the day (Kennedy 1927). Times of emergence of stream insects are also correlated with their an- cestral places of origin and the extent of their acclima- tization to temperature. Shelter and vegetation Animals require shelter or cover as a protection against unfavorable weather and enemies. Caves, overhanging ledges, deep valleys or canyons, or bur- rows in the ground may serve as shelter for terrestrial animals. The darkness of night is a protection against diurnal predators; daylight is a protection against nocturnal ones. Vegetation is an important source of shelter for animals. Some animals cannot tolerate too much solar insolation, hence seek shade. In arid habitats, jack rabbits shift the location of their forms on the ground at different times of the day to stay in the shade of bushes, and it is a common sight in prairie or desert regions to see horned larks, meadowlarks, or other birds lined up in the narrow shadow cast by telephone poles or fence posts. Burrows of all sorts, whether in the ground or in trees, give the animal good insula- tion against both winter cold and summer heat. The foliage of trees, shrubs, and even grasses and vines reduces the amount of heat radiated from the bodies of animals, especially warm-blooded ones, on cool clear nights, and vegetation in general serves as a windbreak. Birds keep to the lee side of exposed patches of woods during cold windy weather. By staying under cover, prey animals may escape notice of passing predators, or if detected, may more easily avoid capture. Dense vegetation, thorny thick- ets, burrows, and other situations impenetrable to predators are sometimes called escape cover. We also speak of nesting cover, winter cover, and roosting cover, depending on the particular purpose which the cover serves. Animals are often protectively colored to conceal themselves better from enemies in partic- ular kinds of cover. When the beetles Tribolium confusum and Ory- saephilus surinamensis are introduced experimentally into a flour medium, Tribolium is ordinarily success- Niche segregation 247 ful, Oryzaephilus is eliminated. But when the flour medium contains pieces of glass tubing of such bore as will exclude Tribolium but let the larvae of Ory- zaephilus enter and pupate, both species survive (Crombie 1946). Most vertebrates and some invertebrates, espe- cially insects and spiders, build nests, usually of plant material. The type of nest it builds is grossly char- acteristic of a species and dependent on inherited be- havior patterns, yet individual nests are uniquely modified to fit into particular situations. Nests pro- tect eggs and young against weather, and are usually well concealed from enemies. Bird species are commonly found at different heights or in particular strata of the vegetation asso- ciated with the characteristic location of their nests (Beecher 1942), where they seek refuge from ene- mies (Dunlavy 1935), where they do their feeding (Hartley 1953), or the location of their song-posts (Kendeigh 1947). Birds nesting in the tree-tops often feed outside the forest (Colquhoun and Mor- ley 1943). Bird feet are as variously adapted to foraging in different strata or in different habitats as bills are variously adapted to procuring different kinds of food. It is often obvious, from the arrange- ment of toes, length of the legs, and other character- istics, whether a bird scratches the ground for its food, gets its food in the air, wades in marsh, is a swimmer and diver, a percher, or a_tree-trunk climber. However, the minor adaptations of legs, bill, and wings in closely related species that enable them to occupy different niches within the same general type of vegetation are more difficult to detect (Dilger 1956). Segregation to a niche may involve, in addi- tion to obvious external characters, many adaptations throughout the body in skeleton, musculature, and other organs (Burt 1930, Richardson 1942). Many types of animals other than birds, for instance mos- quitoes, are segregated by strata to where they most commonly occur (Snow 1955). Warblers are numerous in the evergreen-decidu- ous forest ecotone of eastern North America because they nest and feed in so many diverse niches (Ken- deigh 1945) : Blackburnian warbler—top level of evergreen trees Black-throated green warbler—middle level of evergreen trees Magnolia warbler—low level of evergreen trees Redstart—secondary deciduous growth Black and white warbler—tree trunks Black-throated blue warbler—shaded shrubs Chestnut-sided warbler—sunlit shrubs Canada warbler—wet shaded ground Yellowthroat—wet sunlit ground Ovenbird—dry shaded ground Nashville warbler—dry sunlit ground Louisiana waterthrush—stream margin Northern waterthrush—bog forest Mammalian adaptations to different strata have al- ready been discussed. When given a choice between a grassy habitat and a tree-trunk habitat, the short- tailed forms of Peromyscus mice selected the grassy habitat ; the long-tailed forms, the tree-trunk habitat (Harris 1952). It has been demonstrated that the long tails of some species and subspecies give them a greater proficiency in climbing than their shorter- tailed relatives exhibit (Horner 1954). In the arid country of southern California the giant kangaroo rat is predominant in flat country covered with brush ; on brushy slopes and rolling hilltops the Fresno kan- garoo rat replaces the giant kangaroo rat; the Heer- mann’s kangaroo rat is forced to live on the open plains since it cannot compete successfully with the other two species on brush-covered land (Haw- becker 1951). Considerable evidence was presented in Chapters 7, 8, and 9 to show how animal distribution corre- lates locally with types of vegetation, and more will be presented in Section IV with respect to geographic distribution. Except for a few herbivorous and para- sitic species, animals do not respond to the taxonomic composition of vegetation when they seek cover or food, but rather to life-form of plants; or they re- spond to the micro-climatic conditions established by the vegetation. In northern Europe, the kinglet Regulus regulus occurs with the chickadee Parus atricapillus in spruce and pine forests, but is mostly absent from the birch forests which the chickadee frequents. The kinglet is unable to feed extensively at tips of the pendulous birch twigs because, unlike the chickadee, it is less able to hold itself in an inverted position, because of poor development of certain muscles in the leg (Palmgren 1932). The ovenbird is absent from coniferous forests unless a few deciduous trees are also present, since the bird requires broad leaves for construction of its oven-shaped ground nest. The red-eyed vireo feeds on insects taken from the leaves and the smaller stiff twigs of deciduous trees. It is mostly absent from coniferous forests where the needle-shaped leaves are attached on all sides of flexible twigs and the bird finds difficulty in obtaining a footing (Kendeigh 1945). When birds of different species were given a choice between the branches of coniferous trees and those of deciduous trees, there was evident a direct correlation between length of foot-span, i.e., the dis- tance from the tip of the middle front toe to the tip of the hind toe, and the frequency of perching on the evergreen branches. Birds with small foot-spans 248 Ecological processes and dynamics greatly preferred the branches of deciduous trees (Palmgren 1936). Three species of garter snakes are found together in Michigan, but Thamnophis butleri is restricted to grasses and sedges near water, 7. sawritus prefers bushy areas near water and is a frequent climber, while T. sirtalis occupies a variety of habitats regard- less of proximity to water (Carpenter 1952). The evidence indicates that if the type of cover required by a species is missing, that species will not occur even if all other conditions are favorable. This is of particular concern to the wildlife manager. He must learn to contro] succession, either by accelerat- ing or retarding it, to give species of game the cover that they need (Leopold 1933, Elton 1939). Food and predators Since most organisms select their food from that most easily available to them, it is usually more important in characterizing their niches to indicate the type of food consumed and the stratum or exact microhabitat from which it is obtained than merely to give a list of species that are taken. Thus, fresh- water fishes are best classified as mud-eaters, plant- eaters, plankton-eaters, mollusk-eaters, insect-eaters, fish-eaters, detritus-eaters, or omnivora (Forbes 1914). In a similar manner birds have been cate- gorized into aerial-soaring, or perching insect-eaters ; those which feed on foliage insects, seeds, or nectar ; timber-searchers or drillers; feeders on ground in- sects, or seeds, and predators (Salt 1957). The accurate description of feeding niches re- quires careful attention to details. Two or more spe- cies may feed together in the same community but be segregated from each other because they search for their food from different plant species, from different parts of the same plant, or they take different foods from the same parts of the same plants. Furthermore, species may overlap broadly in their feeding habits during most of the year but be clearly segregated dur- ing periods when food is scarce (Gibb 1954, Betts #955). The difference in type of vegetation inhabited by each of the three species of Michigan garter snakes mentioned a moment ago correlates with differences in kinds of food each consumes: Thamnophis butleri feeds almost entirely on earthworms and leeches; T. sauritus, on amphibians, fish, and caterpillars ; and T. sirtalis, on both earthworms and amphibians, as well as a few mammals, birds, fish, caterpillars, and leeches (Carpenter 1952). Several species of Dro- sophila flies may occur in a single region, especially in the tropics, but each species feeds preferentially on a different species of yeast (Dobzhansky et al. 1956, Cunha et al. 1957). The spider-wasps Anophus semirufus and A. apiculatus have many very similar behavior patterns, but they avoid competition at a critical point. A. apiculatus hunts for its food in the sandy areas where both species nest, while A. semu- rufits moves to woods or shrubby areas to feed and feeds on a different group of organisms at that (Evans 1953). Differences of food habits between related species often correlate with differences in size. Thus the large-billed parrot crossbill of Europe feeds on pine cones, the smaller-billed red crossbill on spruce cones, and the still smaller-billed white-winged crossbill on larch cones (Lack 1944). Two species of fish hav- ing very similar requirements occur in the same type of habitat in British Columbia, but Cottus rhotheus has a larger mouth than C. asper, and feeds on larger kinds of food (Northcote 1954). Size differences even occur commonly between related species of cope- pods that live together in the same body of water (Hutchinson 1951). Male and female differences of size, structural adaptations, and habits may occa- sion distinctive preferences in kinds of food taken and feeding location which make it possible for the two sexes to live together more comfortably within a small area than were their requirements absolutely identical (Rand 1952). For describing the position a species occupies in the food chains of a community, it is necessary to indicate not only the kinds of food eaten but also what species prey on it and the manner in which they do so. Some parasitoids are quite specific as to the kinds of animals in which they deposit their eggs. If a parasitoid that is specific to a prey species is present in a niche, the simple presence of that para- sitoid may determine the success with which the prey species will compete for and fully occupy the niche. BEHAVIOR ADJUSTMENTS Somehow an animal must get into a hos- pitable niche out of the multitude of niches avail- able to it. Doubtless, some animals accomplish this passively by the instrument of random dispersal of spores, eggs, or larval stages, some of which by chance reach favorable locations, there to mature and survive. But higher animals have more complex ner- vous systems, greater intelligence, and their sense organs are more highly evolved. They are equipped to search actively for and recognize niches hospi- table to them either by sight, smell, contact, or other means. For instance, the intricate migratory behavior of birds and other animals is such that they seek out nearly constant climatic environments throughout the year. The apparent ease and speed with which a new generation of individuals discriminates a hospitable Niche segregation 249 Species TABLE |8-|! Percentage evalua- tion of sign stimuli for recogni- Habitat tion of niches in different species of pipits (Anthus) (Svardson 1949). Light and open country High outlooks Green color No vegetation on ground Water nearby Conspecific males nearby Other external stimuli Totals niche means that they must in some way recognize the merits of that niche by definite characteristics of it that may be in the nature of sign stimuli (Table 18.1). Such characteristics are usually prominent, though they need not necessarily be the most essen- tial features of the niche (Lack 1937). Probably most animals exercise a deliberate, al- though not necessarily conscious, evaluation process in choosing one niche from those available. This has been tested experimentally by exposing the animals to gradients of environmental factors, either in the labo- ratory or field (Harris 1952) ; a variety of apparatus and procedures is available for such tests (Shelford 1929). Usually there is a coincidence of the species’ experimentally ascertained preferendum and its nat- ural preferendum. For instance Elipsocus melachlani and E. westwoodi, both psocid insects, occur abun- dantly on larch trees, but E. melachlani frequents those dead branches heavily encrusted with lichens, and E. westwoodi frequents living branches covered with the alga Pleurococcus. Laboratory experiments showed clearly that when each species was given a choice, each selected its customary habitat. Further- more, the feeding of E. westwoodi was restricted al- most entirely to the alga, although FE. melachlani would feed on both the alga and on lichens (Broad- head and Thornton 1955). For certain species, niche preference can be at- tributed to appropriate behavioral patterns alone. Isopod species occur in water and on land but only in places where the humidity is high. What success the group has achieved on land appears to be the re- sult of their avoidance of the rigors of ordinary ter- restrial conditions by means of behavior mechanisms that retain them in these moist cryptozoic niches, rather than to the development of any special morpho- logical or physiological adaptations (Edney 1954). A stereotyped behavior pattern appears to make the magnolia warbler build a nest supported in the interlocking leaves or twigs of a conifer rather than in the vertical fork of a tree or shrub, as the redstart regularly does. The black-throated green warbler 250 A, trivialis A. pratensis A. campestris A. Sspinoletta Mountains, rocky shores Forest-edge Dry meadows Sandy areas 30% 40% 40% 35% 37 14 20 26 20 20 5 5 5 5 30 15 2 15 1 15 4 4 2 2 2 2 2 2 100% 100% 100% 100% originally had a nest-building behavior similar to that of the magnolia warbler, but in some regions it has taken to building in forks, a behavior which has ex- panded its range into both deciduous and coniferous forests. Why is the American robin restricted to lo- calities where it can get mud to put into its nest? Others members of the family Turdidae do not use mud in their nests. There is, on the other hand, an advantage for barn and cliff swallows to use mud in constructing their nests because it enables them to use locations on the vertical sides of cliffs or buildings free of competition from other species. There ap- pears to be no physical reason why a barn or cliff swallow could not build its nest in crevices or holes like other swallows, why a bank or rough-winged swallow could not build a mud nest like the barn or cliff swallow, or why a robin could not build like other thrushes. Such niche segregations are apparently consequences of restrictions imposed by behavior pat- terns alone, although one can never be sure but that each species has some hidden adaptation that keeps its characteristic kind of nest the best nest for it, and its preferred niche the best niche for it. It is possible that certain species of birds are con- fined to coniferous forests because they are of north- ern origin and coniferous forest was the original community available to them; similarly, the broad- leaved deciduous forest is conjectured to have been the original community inhabited by species of south- ern ancestry. Presumably each group evolved herit- able, instinctive behavior patterns which continue to drive it back to the ancestral community in which it belongs, so to speak, even though other types of communities have become available (Lack and Vena- bles 1939). Where two species with similar niche require- ments come into competition, one species must possess better adaptation to it if it gains full possession of the niche to the exclusion of the other species (Lack 1944). Preadaptation or possession of suitable ad- aptations also appears a necessary prerequisite for a species to invade a new niche or habitat and suc- Ecological processes and dynamics cessfully displace another species already occupying it, as frequently occurs (Simpson 1953a). The search for such microadaptations as might give one species an advantage over another in a competition for a par- ticular niche is a real challenge to ecological research. INHERITANCE The fact that all individuals of a species behave almost identically in many of their activities indicates that these behavior patterns or instincts are in some form passed on or inherited from one gener- ation to the next. Behavior that is learned after birth is much more variable between individuals. Behavior patterns are rooted in the structural arrangements of neurons and synapses. Once a stimulus is received a definite action results. Predisposition for a species’ behavior patterns could well be inherited genetically through chromosomes and genes like any other struc- tural or functional characteristic. These inherited be- havior patterns are doubtless subject to evolutionary development as much as are structural and func- tional adaptations; indeed, the one may have de- veloped synchronously with the others (Kendeigh 1952, Spieth 1952). The often large and elaborate nests built by ter- mites are really manifestations of behavior patterns inherent in the species. The nests of higher termites are built specifically by the sterile workers; plainly, whatever js involved in the capacity the workers have to build a certain type of nest must be transmitted genetically, through the sexual forms only. Adaptive modifications in nest structure occur and phylogenetic sequences in nest structure that correlate with phylo- genetic sequences in morphological characters can be demonstrated (Emerson 1938). The two toads Bufo americanus and B. wood- housei differ in rate of embryonic development and embryonic temperature tolerance but interbreed freely. Embryos of hybrids show an intermediate rate of development (Volpe 1952). The call of a natu- rally occurring hybrid toad (Bufo americanus x B. woodhousei) was found to be intermediate between the calls of the two parent species (Blair 1956). Three sibling species of the cricket Nemobius fasciatus differ slightly in structure and color but are scarcely distinguishable except by the songs of the males. Although they do not interbreed under nat- ural conditions, they were induced to do so under experimental conditions. The song of the F; males was intermediate between those of its parents, indi- cating genetic influence (Fulton 1933). The spinning behavior of the flour moth has been shown to be inherited on a Mendelian genetic basis; it is dependent on light and food factors. At least two genes are involved; in the F; generation the non- spinning behavior is almost, but not quite, dominant ; in the Fy generation there is segregation of spinning individuals (Caspari 1951). The segregation of the prairie deer mouse and woodland white-footed mouse into different niches is very definite (Table 2.1), as is also the segrega- tion of related species occurring in chaparral (Mc- Cabe and Blanchard 1950). The same segregation fo different habitats holds even between prairie (bairdii) and woodland (gracilis) subspecies of the same species (Peromyscus maniculatus). It is of in- terest that laboratory-reared individuals not previ- ously conditioned to their natural habitats were given a choice under experimental conditions, bairdii se- lected grass habitat; gracilis, a tree-trunk habitat. This suggests genetic inheritance of habitat prefer- ence. In hybrids between the two subspecies, choice of the grass habitat was dominant over choice of the tree-trunk habitat (Harris 1952). There may be transmission of behavior patterns to succeeding generations by tradition rather than by genetic mechanisms; i.e., training of young, young imitating parents, conditioning or imprinting. It has been demonstrated that some parasitoid wasps lay their eggs only in the same kind of larvae as they themselves became conditioned to during their early growth and feeding. Young birds and other animals become imprinted to their own parents, to their own species, and perhaps to their proper niche, at critical stages in their development (Baldwin 1896, Cushing 1941, 1944, Thorpe 1945). It is very probable that the manner in which niche segregation is passed on to succeeding genera- tions is not the same in all species. We may believe, however, that most behavior of animals has a genetic basis, but may become highly modified through prac- tice, imitation, and experience. INTERSPECIFIC COMPETITION Segregating effect Charles Darwin stated in The Origin of Spe- cies the case for interspecific competition as an instru- ment segregating species into different niches as follows (Crombie 1947) : 1. The reproductive capacity of organisms is greater than the carrying capacity of the envi- ronment. 2. The range of an organism's tolerance of physi- cal conditions and choice of food is limited. 3. The failure of an organism to survive, or be born at all, may be a result of the direct action of unfavorable habitat, predators, parasites, or competitors. Niche segregation 25] ABUNDANCE ———>— ENVIRONMENTAL NICHES FIG. 18-1 Species | finds optimal conditions in niche C, and reaches greatest abundance in that niche. It is also able to utilize niches B and D, but with less efficiency, and niches A and E only very poorly. Species 2 cannot utilize niches A, B, and C at all, finds D only partially suitable, but E and especially F very favorable. Species | and 2 overlap in niche D, but species 2 prevents any occupancy of niche E by species |. The absence of competition in niche A makes it open to a species evolving adaptations to it (from Mayr 1949). 4. When competition occurs, it is severest be- tween organisms with the most similar require- ments. 5. In general, the closer the taxonomic relation- ship between them, the more similarity there is in needs and habits of species. 6. When new forms appear in a given locality, either by evolvement there or by invasion after evolutionary divergence elsewhere, they will either eliminate or be eliminated by their near- est relatives if they compete with them, unless 7. Each form becomes adapted to a different niche, in which case competition between them will cease, and they may occur in proximity. Evidence that interspecific competition is the most critical factor confining a species to this or that niche is available with the expansion of the species beyond the usual limits of its niche when this competition is removed. This expansion is often evident in geo- graphic differences in the niche characteristics of a species. In Scotland, the mountain hare occurs at high elevations, the common hare at lower ones. In Ireland, which was isolated as an island before the common hare could reach it, the mountain hare oc- curs at both high and low elevations and is differ- entiated into distinct subspecies (Huxley 1943). In the Canary Islands, the chaffinch Fringilla teydea breeds only in pine forests. The closely related F. coelebs usually breeds in broad-leaved forests above and below the pine, but not in the pine forests. On certain islands, F. teydea is missing, and on those islands F. coelebs occurs in the pine as well as in the broad-leaved forests (Lack 1944). Other examples of a similar sort are given, for birds, by Moreau (1948). Success in competition between species of turbel- larian flatworms depends on temperature and water current, but when the competing species is absent, the remaining species disperses far into the micro- habitat usually occupied by its competitor (Beau- champ and Ullyott 1932). Under natural conditions in habitats favorable to it, the male Anna’s hummingbird is usually success- ful in maintaining a high population and forcing the male Allen’s hummingbird into less favorable periph- eral territories. The success of the Anna’s humming- bird in competition with the Allen’s hummingbird is attributable to its establishment of defended terri- tories earlier in the season, by reason of which it is more familiar with the terrain and alert to intrusions. It sings persistently to warn off competing males, and the larger size and flashier coloration of the Anna’s hummingbird give it authority (Pitelka 1951). In other habitats, however, the male Allen’s hummingbird may consistently displace the Anna’s hummingbird (Legg and Pitelka 1956). The general effect of interspecific competition is restriction of a population more closely within its op- timum niche. Intraspecific competition exerts pres- sure impelling individuals to disperse into less favor- able situations. The relative pressure exerted by these two forces determines whether at any particular time the species is contracting or expanding its range (Svardson 1949). Interspecific competition is reduced or eliminated altogether when the combined requirements of all species are less than the supply of materials available. Land snails, insects, aquatic clams, and copepods sometimes occur together in a considerable profusion of species but with little evidence of competition be- tween them (Boycott 1934, Fryer 1957, Ross 1957). Voles, during upswings in population, may become superabundant for periods of two or three years. During such periods several species of hawks and owls may feed in the same field without competing because there are more than enough voles to supply all. During downswings in the rodent population, however, competition does occur, and some or all the predator species are forced to turn to other prey (Lack 1946). Parasites or predators may keep the populations of competing species below the level which available food resources of the habitat can sustain so that com- petition is reduced or disappears. For instance, when two species of weevil are placed together in a limited amount of food, one species is eliminated by the other in about five generations. However, when a wasp 252 Ecological processes and dynamics: equally parasitic on the larvae of both species is intro- duced into the mixed culture, the populations of both species decline markedly below their normal levels in single pure cultures, and they both continue to exist together indefinitely (Utida 1953). Closely related insect species may differ in their adaptations to climatic factors. As the weather dif- fers between different localities during different months or from one year to another, two or more species may reach high abundance at different times, correlated with prevailing weather conditions, in what otherwise appears to be the same ecological niche (Ross 1957). Niches of different species may overlap during most of the year but be clearly defined during critical stages in the life cycle. Plethodontid salamanders wander around freely most of the time, but during the reproductive period they are sharply segregated. In the North Carolina mountains, Desmognathus quadramaculatus is predominant in clear rocky streams, D. phoca lays its eggs in muddy streams on the under surface of rocks or logs, while D. fuscus carolinensis, which also occurs in muddy streams, deposits its eggs in the moss and mud on the upper surfaces of logs (Noble 1927). A microhabitat may be occupied simultaneously by two or more species if their combined populations do not exceed the carrying capacity of the micro- habitat. This may occur when the populations of the species involved are limited by conditions in some other part of their niches. Thus the niches of house wren, bluebird, black-capped chickadee, tufted tit- mouse, white-breasted nuthatch, and tree swallow overlap because they all nest in small cavities in trees or boxes. Usually the number of nesting sites is ample to the demand, so that there is no competition for them. The population of each species is restricted by factors other than available nest sites, perhaps by food supply, unfavorable climate, parasites, or preda- tors. The wren feeds mostly on the ground under bushes, the bluebird in open fields, the chickadee on the smaller tree branches, the tufted titmouse on the larger branches and on the ground, the white- breasted nuthatch on the trunk of trees, and the tree swallow in the air. Competition occurs between them only when one or more species temporarily increases in abundance so that there are not enough nest-sites to go around. When competitive species occupy the same micro- habitat, they sometimes set up mutually exclusive ter- ritorial relations, based on responses to similarity of body form and behavior (Simmons 1951) or call notes (Dilger 1956, Lanyon 1957). This divides the space and reduces competition. Because of parasites, unfavorable weather, preda- tors, or other causes, species do not continuously satu- rate their habitats. It is only when they do that com- petition becomes clearly evident. Competition in the house wren takes various forms including destruc- tion of eggs and young and even killing of adults. In a 19 year study of a 6-hectare (15-acre) plot, such drastic competitive acts, both intra- and interspecific, occurred only when the number of male birds was more than 10. When the number of breeding house wrens was reduced to 10 or less, in consequence of heavy over-winter mortalities, no such competition occurred (Kendeigh 1941b). The niche relationships and the amount of competi- tion between species sometimes varies geographically. The vertical range of the salamander Plethodon glut- imosus in the southern Appalachians is sharply defined from those of three different subspecies (jordani, shermani, metcalfi) of Plethodon jordani, but com- pletely overlaps the ranges of two other subspecies (clemsonae, melaventris) (Hairston 1951). During a spruce budworm insect outbreak in the coniferous forests of northern Ontario, three species of warblers—bay-bfeasted, Tennessee, and Cape May—greatly increased in numbers, partly because of their aggressiveness and partly because they were more accustomed to feeding on this type of food. Other warblers were held in check and one species, the magnolia warbler, actually decreased in numbers because of competition with the three favored spe- cies. Competition between birds for song posts and territorial space was considerable with nearly a third of all conflicts observed occurring between individuals of different species (Kendeigh 1947). It is probable that niche segregation of species becomes established at times of stress or crises like this, and after the be- havior pattern once becomes fixed in the species, only sporadic attacks of one species on another are there- after sufficient to check random variations away from the standard pattern. At times other than those of stress, direct conflict between species is not often observed so that its importance in segregating species to particular niches is occasionally not fully appre- ciated (Lack 1944, Udvardy 1951, Andrewartha and Birch 1954). Gause’s rule The evidence presented in this chapter demon- strates the important concept that has come to be known as Gause’s Rule or, more recently, the “com- petitive exclusion principle” (Hardin 1960) : an eco- logical niche cannot be simultaneously and completely occupied by stabilized populations of more than one species. In other words, two or more species with closely similar niche requirements cannot exist indefi- nitely in the same area. Two species with expanding populations attempting to occupy the same niche will sooner or later come into competition for possession Niche segregation 253 80 “som 60 eee, culture s 20|— Mixed culture > =k So tious! Soo) earary P ee a z ole Pure culture “—_mixed culture (op) (eo) aS {e) 20 10) a 4 6 8 lOy 25 14) 165518 DAYS FIG. 18-2 Growth of populations of Paramecium caudatum and P. qurelia, cultivated separately and in mixed populations (from Gause 1934). of it. Rarely, if ever, will they be equally adapted, and ordinarily the one with the better adaptations or greater aggressiveness will win out and occupy the niche to its full carrying capacity. The basic idea of this rule was understood from observations of natu- ral distribution long before it was verified experi- mentally (Grinnell 1904, Jordan 1905). In experimental cultures, Paramecium caudatum and P. aurelia maintain separate populations at a high level, but when the two species are mixed, they quickly come into competition. As long as the food supply is ample, both species increase in biomass, but as the food supply approaches exhaustion, P. aurelia persists and P. caudatum declines until it finally dis- appears. An analysis of the relative adaptation of the two species shows that P. aurelia is capable of faster population growth and is more resistant to the ac- cumulating waste products than is P. caudatum (Gause 1934). Similarly, Daphnia pulicaria in mixed cultures causes the extinction of D. magna when oxygen and food become limited (Frank 1957), and Tetrahymena pyriformis persists while Chilomonas paramecium disappears in mixed cultures of these protozoans (Mucibabic 1957). When two species of flour beetles with similar re- quirements for food, space, and other conditions are cultured together in the same volume of flour, one species always becomes extinct and the surviving species then establishes a stabilized population. In 2 cultures free of parasites, Tribolium castaneum is the successful species, but in cultures containing the sporozoan parasite Adelina tribolu, T. castaneum be- comes extinct and T. confusum persists, since it is less susceptible to the parasite (Park 1948). Para- sites or predators may then influence the success of competition between species by affecting one more than the other (Crombie 1947). It appears that when two species with similar niche requirements meet in competition under natural conditions, one of three things will happen (Lack 1944): 1. One of the two species will be so much better adapted that it will spread rapidly through the range of the other and exterminate it. 2. One species will be better adapted to a portion of the range, in which it will eliminate the other species, but the other species will be better adapted to the remainder of the range and will occupy it exclusively. Thus the two species will occupy adjacent geographic re- gions with perhaps a zone of overlapping occu- pancy. 3. Each species will be better adapted to a dif- ferent portion of the niche, to which it will become restricted; with this separation, each species will then spread through the range of the other. Advantages of niche segregation Probably the major advantage animals gain by occupying different niches is escape from continuous intense competition. It is also true that the niche oc- cupied is favorable to the species physically in fur- nishing suitable substratum and micro-climate, al- though many species have the ability to live elsewhere were competition not involved. Automatic segre- gation of a species into its niche through inherited behavior patterns avoids the great expenditure of energy and loss of time that would be required if this segregation had to be worked out anew each year or each generation. Segregation into niches avoids con- fusion of activities between organisms in the com- munity and permits a more orderly and efficient life- cycle on the part of each species. Furthermore, the segregation of each species into different niches per- mits the occupancy of the area by a larger number of species, since they will better divide the available resources between them. Similarly the more distinct the niche of a species is, the more it can avoid con- flict with its neighbors and lead a life that is orderly, productive, and efficient. Competition is thus a potent factor in giving ecological structure to the commu- nity. Ecological processes and dynamics TAXONOMIC COMPOSITION OF COMMUNITIES Predominance It is characteristic of the taxonomic structure of communities that a few species furnish the greatest bulk of the population entirely out of proportion to the rest of the species. Thus in stream riffles, two species make up 85 per cent of the total riffles popula- tions and another two species constitute a similar per- centage of the mud bottom pool populations (Table 5-1). In the littoral zone of Lake Erie, one species furnishes 85 per cent of the population on cobble and gravel bottoms, and another species makes up 68 per cent of the population on mud bottoms (Table 6-2). In populations of 79 species of birds nesting on a large tract including advanced stages of succession, 5 per cent of the species (4 species) included 37 per cent of the individuals, 10 per cent (8 species) for 56, and 50 per cent (40 species) for 96 per cent (Evans 1950). The abundant species are ordinarily of small size, herbivorous in their food habits, and at the bottom of the food chains and pyramids. Variety of species When a barren area is first colonized, a variety of species may invade and become temporarily estab- lished because competition is negligible. As the popu- lations increase in size and individuals are crowded together, competition for the limited resources sets in, and the less well adapted species are eliminated (Yount 1956). In general, all over the world, established com- munities in extreme or impoverished habitats con- sist of fewer species than do communities in fertile and more favorable habitats. The tropical rain forest is noteworthy for the great variety of species that it possesses, while arctic areas have, comparatively, greatly reduced faunas. Pioneer communities newly established on bare areas regularly consist of fewer species than later seral and climax ones (Tables 7-4, 8-3). The reason for this is that infertile pioneer communities and those in extreme habitats can sup- port little variety in the way of niches, while com- munities in rich habitats are able to develop a great diversification of them. When an area occupied by a community is broken up into a large number of small, different micro- habitats and niches, few of them can be very exten- sive, hence their average population is generally less than that of species in extreme habitats where the variety of niches is smaller. Tropical and climax communities characteristically have a rich variety of species, but relatively low populations of most spe- cies. In pioneer and extreme habitats, to which rela- tively few species are adapted to survive, the species present may become enormously abundant. In spite of occasional exceptions, there tends to be an inverse relation in the composition of communities between the number of species and the average number of individuals per species. Segregation of related species Closely related species usually have somewhat similar adaptations and niche requirements ; this may affect their distribution in communities in relation to each other. In a study of 55 animal and 27 plant communities from a wide variety of habitats (Elton 1946), it was found that 86 per cent of the genera of insects and other animals and 84 per cent of the genera of plants were represented by a single species. The average number of species per genus of animal and plant in each community was 1.38 and 1.22 respectively, compared with an average of 4.23 species per genus for 11 large insect groups that occur in the British Isles as a whole. This means that while closely re- lated species may occur in the same regional fauna, related species are more apt to be segregated into adjacent communities or habitats than into the same one. This conclusion has been disputed on a mathe- matical basis because of the small size of the com- munities and the small number of species involved (Williams 1947) but is supported by other studies (Lack 1944, Bagenal 1951). It is true that in larger habitats or more diversi- fied communities the larger number of niches ordi- narily present should permit related species to occur. Yet, where related species could occur in large trop- ical communities of Africa, bird species belonging to the same genus occurred together in the same com- munity in only 26 per cent of the communities, and if the weaver finches Ploceidae are excluded, only 16 per cent of possible overlap in habitat distribution is realized. This segregation of related species into dif- ferent communities applied to an equal extent with species belonging to the same family (Moreau 1948). There are some genera of birds and other animals, however, in which a large variety of related species may be found together; notably, certain wood war- blers Dendroica, buntings Emberiza, white-eyes Zos- terops, whistlers Pachycephala, weavers Ploceus, hawks Accipiter, and the insects Drosophila, Anoph- _clesAedes(Mayr 1947). Certain habitats, such as in Africa) are unusual in possessing a great variety of closely related aquatic species (Brooks 1951). The tendency for related species to be segregated | [ Niche segregation 255 into different communities or regions is an old con- cept and may be stated: given any species in any region, the nearest related species ts not likely to be found in the same region nor in a remote region, but in a neighboring district separated from the first by a barrier of some sort (Jordan 1905). The principles and reasons underlying this law require an under- standing of speciation, which will be taken up in the next chapter. Ecological equivalents Communities in general are of fundamentally similar organization, and the facts we have already considered about food chains, trophic levels, and pyramids of numbers and biomass attest this. Con- sequently, a coniferous forest has many niches similar to those available in a deciduous forest, a grassland has some niches that are much the same as forest niches, a pond has some of the same niches as do lakes. The niches in a forest, a grassland, or a pond on one continent are very much the same as those to be found in similar communities on other continents. Communities could well be analyzed, compared, and evaluated in respect to the niches of which they are composed rather than the species they contain. Spe- cies are taxonomic units, niches are ecological units. Similar niches in different communities or in dif- ferent regions are commonly occupied by species pos- sessing similar but not necessarily identical habits, adaptations, and adjustments (Table 8-3). Such species are called ecological equivalents (Friedmann 1946, Dirks-Edmunds 1947). Equivalent species are not necessarily closely related taxonomically. The explanation of why a particular species has come to occupy a particular niche in a particular part of the world requires a knowledge of where it orig- inated, how it dispersed, and how it evolved its present adjustments and characteristics. When popu- lations invade new regions and environments and become geographically isolated, they commonly differ- entiate into new species. Each species thus becomes the product or visible expression of a particular com- bination of environmental factors, interactions, and locality, and as such the species is the practical taxo- nomic unit with which the ecologist must deal. The process of speciation is thus of great ecological inter- est since it is an adaptive process that leads to the fill- ing in of empty niches and to the most efficient and complete utilization of the environment (Mayr 1949). SUMMARY The ecological niche is a particular com- bination of microhabitat and biotic relations required for the existence of a species. The niche of a species is defined by the features of the substratum and mi- croclimate to which that species is peculiarly ad- justed, the time of day and season of year when it is mainly active, the type of shelter or cover it requires, the manner in which it uses vegetation in its repro- ductive performance, the type of food it consumes, and the predators that prey on it. Animals, particularly higher types, have more or less stereotyped behavior patterns associated with their restriction to particular niches. These behavior patterns may be genetically inherited and subject to evolutionary development, or they may be trans- mitted to succeeding generations through training or conditioning (imprinting) of the young. Segregation of species into different niches is doubtless the result of interspecific competition. Ac- cording to Gause’s rule, an ecological niche cannot be simultaneously and completely occupied by stabi- lized populations of more than one species. Occu- pancy of different niches reduces interspecific com- petition, furnishes the species with microhabitats to which they are especially adjusted, reduces confusion and disturbance, and permits a greater variety of spe- cies to occur in the same region. Communities commonly have a predominance of a few species composing the bulk of the populations. Communities in extreme or impoverished habitats have fewer species than those in fertile or favorable ones. Closely related species tend to be segregated into different but adjacent regions separated by bar- riers. Since communities in various parts of the world are fundamentally similar in organization and struc- ture, species occupying similar niches in them make similar adjustments. Although not necessarily re- lated taxonomically, these species are ecological equiv- alents. Understanding how a particular species has come to occupy a particular niche in a particular part of the world requires a knowledge of speciation, which will be considered in the following chapter. 256 Ecological processes and dynamics Ecological Processes and Community Dynamics: Speciation Speciation is the process of evolutionary differen- iation between populations, which may result in one older species becoming split into two or more new ones. Speciation usually entails divergence of the new species into different niches. An understanding of the basic principles of speciation is therefore a prerequisite to an understanding of animal distribu- tion. TAXONOMY As the term is used by taxonomists, a popu- lation is a local aggregation of individuals that differs slightly, but characteristically, from other local aggre- gations of the same species. Geneticists define a population as a reproductive community sharing a common, characteristic gene pool. Every local population is different from every other one ; but they are not easily distinguishable from one another, and therefore are given no formal no- menclature. What a population includes may vary from the entirety of a species to but a few individuals, according as the rate and extent to which individuals interdisperse between localities to make a common gene pool. Among bisexual forms, a species is a group of populations capable of successfully interbreeding, re- productively isolated under natural conditions from other such populations. Species are usually morpho- logically distinct, but the distinguishing character- istics are sometimes barely discernible. Hybridiza- tion may occasionally occur between clearly defined species under captive or unnatural conditions, but does not occur with any significant frequency under natural conditions. Fossils found separated in different geological strata, or living populations separated in space, are considered capable of successful interbreeding, and therefore to be of the same species, if essentially sim- ilar structures, functions, and behaviors can be ad- duced for them. It is unfortunate that species cannot presently be recognized by entirely objective means, but even if this were readily possible there would be difficulties, inasmuch as populations may be at vari- ous stages in the differentiation of complete repro- ductive isolation. Species are distinguished by the familiar bionomial nomenclature standardized by in- ternational rules (Mayr et al. 1953). A subspecies is a geographically defined group of populations which differs in color, size, or some other taxonomic characteristic from other popula- tions within the same species but nonetheless inter- breeds with them freely, regularly, and successfully where their ranges come into contact. Subspecies are commonly distinguished by a trinomial nomenclature, although the desirability of this has been questioned Wes %| (Wilson and Brown 1953; Syst. Zool. 3, 1954: 97- 126, 133). The taxonomic differences between sub- species are usually less pronounced than those be- tween even closely related species, but they are genet- ically fostered differences nonetheless (Sumner 1924, Huxley 1943). Race and variety are terms some- times used as synonyms for subspecies, or for popu- lations even less well defined than a subspecies. It is difficult to determine whether populations that differ phenotypically are subspecies or species when their ranges do not verge, for in the absence of opportunity for them to commingle, it remains uncertain whether interbreeding would or could occur. The assignment of taxonomic rank under these circumstances is dependent upon considerable subjective inference. Boundaries between adjacent subspecies are fre- quently arbitrary in that they fit differences between the populations for some characters but not others. Differences between subspecies or populations are often correlated with differences in topography, soil, climate, or vegetation, and, at least in some instances, appear to be adaptive to these differences in the envi- ronment. If a subspecies becomes isolated by a barrier so that it is prevented from interbreeding with the rest of the species, variations in taxonomic characters may accumulate and the population evolve so distinctively as to pass beyond the rank of subspecies into that of species. However, not all or even most subspecies change into species, a process which depends on effec- tive, permanent reproductive separation and on the forces of natural selection. Speciation, more pre- cisely, is the process of differentiation between popu- lations of the same species in consequence of repro- ductive isolation (Simpson 1953). Populations that do not differ by clearly defined or conspicuous taxonomic characteristics, but never- theless do not interbreed because of physiological or behavioral differences, are described as sibling species. Sibling species have been noted especially among Dip- tera (Drosophila, Anopheles), Hymenoptera (ants), Lepidoptera (especially moths), and Protozoa (Para- mecium) (Mayr et al. 1953). It is apparent that the evolution of physiological and behavioral differences often precedes the differentiation of recognizable taxonomic distinctions (Krumbiegel 1932, Thorpe 1930). Two species the distributional ranges of which do not overlap (i.e., there is geographic isolation be- tween them) are said to be allopatric. Two species are said to be sympatric when their ranges overlap, even though they may locally be ecologically segre- gated into different habitats; for example, the situ- ation in which one species of snail is limited to flood- plain and another species to upland habitats, or one species of rodent confined to the foothills and a closely related species to a higher zone on mountains only a short distance away. Populations should be con- sidered sympatric if they occur within the common dispersal ranges of their young, so that there is at least possible a continuous and appreciable inter- breeding and flow of genes among them. ISOLATION OF POPULATIONS Sympatric species do not interbreed be- cause one or more isolating mechanisms keep them separated. We will proceed to examine what iso- lating mechanisms are, and how geographic isolation permits them to arise. Isolating mechanism Isolating mechanisms that prevent sympatric species from freely interbreeding are largely or en- tirely biotic factors (Mayr 1942, Huxley 1943, Allee, Emerson, et al. 1949, Dobzhansky 1951, Patterson and Stone 1952). They are the following types: Ecological: segregation of species into different habitats, communities, or niches by reason of structural adaptations, physiological adjust- ments, or behavior responses. Ethological: difference in sign stimuli or be- havior patterns required for successful spe- cies and sex recognition and for mating. Mechanical: lack of physical conformity of sex- ual organs, chemical incompatibility of sperm and egg. Genetic: hybrid sterility or decrease of fertility. Ecological isolation was described and illustrated in the discussion of niches (Chapter 18). Animals may fail to find breeding partners; i.e., they may remain ethologically isolated. Careful stud- ies in all vertebrate groups and in such invertebrates as insects, spiders, crustaceans, and snails indicate that animals identify the sex of individuals of their kind only by actively recognizing special clues or sign stimuli (Tinbergen 1951). These clues may be spe- cial color markings, shape or outline, scent, song or call-notes, touch, behavior patterns, or some combi- nation of these. Courtship leading to copulation is often complex and involves a number of steps, each step in the behavior serving as a releaser for the next (Fig. 2-7). If a step is not performed properly and in its sequence, the courtship performance ceases forth- with, and there is no sexual consummation. Different species may be effectively isolated from interbreeding simply because the sexes of one possess different sign stimuli than the sexes of the other, and the sexes of one kind characteristically pursue patterns of pairing 258 Ecological processes and dynamics behavior that are not stimulating to the sexes of the other. Ethological isolating mechanisms (Spieth 1958), effective under natural conditions, often lose efficacy when those conditions are disturbed ; by so much they are difficult to work with experimentally. Where their home ranges overlap, two species of mice, Pero- myscus leucopus and P. gossypinus, frequently occur in the same habitat. Yet, very few hybrids have ever been found. When brought into the laboratory, how- ever, the two species not only hybridize freely, but produce fertile offspring. It appears that ethological factors keep them separated under natural conditions (Dice 1940). Two closely related budworm species, Choristo- neura fumiferana found on balsam fir, and C. pinus found on jack pine, are isolated ecologically on dif- ferent host trees, and because the first species com- pletes its mating season before adults of the second species appear on the wing. Ethological isolation oc- curs when occasionally their mating periods overlap. Females will mate only with males of their own spe- cies, even though males of both species attempt to mate indiscriminately (Smith 1954). Species related but of different sizes may be un- able to fit their copulatory organs together, because of structural incompatibilities. Such hindrance to inter- breeding is a mechanical isolation. Failure of male toads to clasp females of larger or smaller species results in reproductive isolation between species of Microhyla (Blair 1955). Polygyrid snails of the genus Stenotrema have definite behavior patterns prerequisite to copulation. A careful study of several species in this genus (Webb 1947) showed that dif- ferences in these behavior patterns are sufficient to keep some species separated, but that in other in- stances it is differences in the structure of the copu- latory organs that apparently prevents interbreeding. Genetic isolation occurs when there is inability to produce offspring because of incompatibility of spermatozoa and eggs, abnormalities of growth, or the offspring are sterile. Sperm of the sea urchin Strongylocentrotus franciscanus sufficient to give 73 to 100 per cent fertilization of eggs of the same spe- cies produced only O to 1.5 per cent fertilization of S. purpuratus eggs. Eggs of one species may some- _times be successfully fertilized by sperm of another, but all sorts of disturbances may occur in the zygote, such as chromosome elimination during cleavage, ar- rest of gastrulation or organ formation, and death of embryos in advanced stages. A well-known example of a usually sterile hybrid is the mule, the result of a cross between a male ass and a female horse. Genetic and mechanical isolation usually furnish more certain reproductive separation between species than do ecological or ethological isolation. The latter two forms probably represent early steps in the proc- ess of speciation ; the former two, the culmination of speciation, Geographic isolation Physiographic barriers such as land masses, mountain ranges, and bodies of water can effect com- plete or nearly complete isolation of populations. Population segments of a species may become geo- graphically isolated when reproductive individuals cross a barrier by chance. For instance, individuals may be blown by storms or carried by rafts to outly- ing islands. A barrier may arise subsequent to the dispersal of a species, such as when species disperse into a new area by way of land bridges which later disappear. When barriers are only partially effective so that gene flow between adjacent populations is hindered but not stopped, and natural selection goes on inde- pendently in each area, some differentiation of the populations may occur but not above the level of subspecies. Species that are widely dispersed over a continent often display several local centers where differentiation is occurring. Individuals dispersing outward from these centers, however, meet and inter- breed so that intergradation of characters occurs, and there is gene flow from one end of the species’ range to the other. The rate of gene flow is directly proportional to the rate at which individuals disperse from birthplaces, in the face of any intervening bar- riers. This is true even of birds and insects. Less than 5 per cent of young pied flycatchers surviving to one year of age return to their places of birth in order to nest; the other 95 per cent disperse widely. Only 4 subspecies have differentiated in this species, since there is widespread promiscuous interbreeding between individuals. On the other hand, 63 per cent of young song sparrows surviving to sexual maturity return to nest in the vicinity of their birthplaces ; only 37 per cent disperse elsewhere. This results in a limited flow of genes from one locality to another and is correlated with the development of 28 sub- species (Diver 1939, Marshall 1948, Haartman 1949). When the rate of gene flow is slow, mere distance, even over territory unbroken by physiographic bar- riers, may permit populations to vary enough for sub- speciation, or potentially even full speciation, to occur. In the range from Maine to Florida, populations of leopard frogs readily interbreed with adjacent populations. But when individuals from Maine are brought together with individuals from Florida, the two cannot interbreed successfully (Moore 1946). There is a progressive change from North to South in several characteristics of the leopard frog, and it is of interest that if populations intermediate in the Speciation 259 O o | | | | | a i Rd = ew wae gene frequency si species ‘ : ok average ; aN trait cline selection gradient c FIG. 19-1 (A) local plus and minus variations in any charac- teristic of a species occupying a uniform habitat eventually dis- appear because of gene flow throughout the population (pan- mixia). (B) when the habitat characteristics of a geographic range occupied by a species gradually change from one extreme to another, a selection gradient is established that partially counteracts the tendency toward panmixia, producing a cline in the characteristics of the species. (C) when partial barriers range should be eliminated, the populations at each of the two extremes of the range would be considered separate species. As it is, however, at least some gene flow takes place throughout the range, and all popula- tions must be considered as belonging in the same species. Only through geographic isolation can popula- tions differentiate into distinct species. There is no established case in which any change in habitat, be- havior, structure, or genetics acting singly or in combination has, in the absence of geographic isola- tion, been sufficient to prevent at least some signifi- cant gene exchange with the rest of the species, with resultant preservation of the species. With one ap- parent exception, biotic factors apparently cannot by themselves bring full differentiation of new species. The exception is the simultaneous development of polyploidy in certain individuals, which renders them sterile with normal members of the species, although not with each other. Polyploidy rather commonly gives complete genetic isolation in plants, but it is rare in animals. In the presence of geographic isolation, genetic variations and natural selection may bring the af- fected population to a different course of evolution than in the parental species, especially if adaptation 260 to otherwise free gene flow occur in an environment or selection gradient, a stepped cline (a) produces distinct races or sub- species. If the barriers become complete, the isolation of popula- tions may result in divergent evolution and complete speciation so that later, after the barriers disappear (b), biological isolat- ing mechanisms prevent the populations from interbreeding (after Womble 1951). to a new environment is also involved. Biotic iso- lating mechanisms may develop in the process. If the geographic barrier formerly separating the population should disappear, and the hitherto isolated population again comes into contact with the rest of the species, interbreeding will not then occur. This is the process of speciation, the details of which we will now exam- ine more carefully. VARIATIONS IN POPULATION CHARACTERISTICS Observable differences in structure, func- tion, and behavior between individuals belonging to the same species are common. Actually, no two indi- viduals, except perhaps identical twins, have exactly similar characteristics. Early Mendelian geneticists believed that speciation occurred as the instantaneous result of major mutations (macro-evolution) (Bate- son 1894, Goldschmidt 1940). Modern geneticists are nearly unanimous in the view that it is the gradual accumulation of many small variations over many generations (micro-evolution) which — eventually gives a population reproductive isolation and, conse- quently, species identity. Ecological processes and dynamics Non-heritable variations Not all variations of organisms are of direct significance in speciation. Non-heritable changes in body structures, functions, and behavior are common. If muscles are used continually and intensively, they become thicker and stronger; if one kidney is re- moved, the other becomes hypertrophied; skin sub- jected to frequent rubbing or pressure thickens and becomes horny ; and so on through a lengthy catalog. Animals progressively exposed to ever more severe temperatures or lower oxygen concentrations will tol- erate extreme conditions, which, had they been sud- denly presented, would have been fatal. Insect larvae transferred to a new type of food often become so conditioned to it that they produce a strain that pre- fers that food to other more usual food of the species. .Chimney swifts in wilderness North America nested in hollow trees, but with settlement of the country and the construction of chimnied buildings during the last two centuries, the species has changed its behavior almost completely, accepting chimneys as a satisfactory nest-site location. Many phenotypic adaptations persist genera- tion to generation, either as similar responses made by each generation to constant environmental condi- tions; as the result of imitation of parents, condi- tioning of young, or imprinting (Cushing 1941, 1944, Thorpe 1945); or because the particular genes re- sponsible for these characteristics have been sorted out (canalized) from the general gene pool of the species (Waddington 1957). One would expect a change in behavior or function, arisen in consequence of exposure to the new conditions, usually to presage the evolutional development of a new structure, for natural selection cannot bring about the structural adaptation or perfection of an organ unless the organ is already being used for the new purpose (Prosser 1957). According to the so-called Baldwin effect, a mutation which affects established behavioral or functional adjustments is more likely to become per- manently fixed in the germplasm than if the popula- tion were not already so adjusted phenotypically (Baldwin 1896, 1902, Simpson 1953a). Polymorphism When individuals of a population can be grouped into several color phases, distinct body sizes, or other character variations, the population is said to display polymorphism. Polymorphism of a char- acter arises in a species when heterozygotes persist in an environment in the face of natural selection and homozygotes are reduced or eliminated. For instance, individuals with certain characteristics may be better _picta a oregonensis Intergradation area platensis _xanthoptica croceater Ep hh klauberi. FIG. 19-2 Speciation through distance in the salamander species Ensatina eschscholtzii in California. The coastal subspecies picta may represent the ancestral type and demarcate the center of dispersal from which clinal lines became dispersed southward in the coastal and interior mountains separated by a barrier. Re- cently, the subspecies xanthoptica crossed this barrier. Inter- breeding occurs between adjacent subspecies, but partial re- productive isolation obtains between xanthoptica and platensis, and complete isolation similar to that of species obtains between eschscholtzii and klauberi (Stebbins 1949). adapted to environmental conditions during the spring, while individuals with other characteristics are superior in summer or autumn. This results in a mixture of types in the population more or less segre- gated by seasons. Likewise populations may vary in characteristics as adaptations to local habitat condi- tions. An increase or decrease in the frequency of a given characteristic appears a result of variations in the selective pressure of the environment, permit- ting individuals with certain characteristics or gene combinations to survive at one time or place; other individuals, at other times and in other places. In an environment that presents the same set of selective pressures year after year, there commonly occurs a stability in the ratios of the different forms in which a character is manifested (Ford 1940, Dobzhansky 1951, 1956, 1958, Sheppard 1958). Most species differ not by single genes but by hundreds certainly, possibly by thousands, of genes. When panmixia (free interbreeding) obtains in a Speciation 26] 60— PER CENT wW (e) | ty JUN JUL AUG SEP OCT MAR APR MAY FIG. 19-3 Seasonal changes in relative frequency of the third chromosome with the standard (black), arrowhead (stippled), and Chiricahua (cross-hatched) gene arrangements in a natural population of Drosophila pseudoobscura (Dobzhansky 1951). species, these genes may be arranged in all sorts of combinations to form an almost infinite mixture of character modifications (Caspari 1951). Heterozy- gotes are, therefore, much more flexible in adaptively responding to the environment than are homozygotes. The more characters for which an individual is het- erozygous, the more adaptable its offspring are likely to be. Adaptive polymorphic populations are more efficient in exploiting the environment than are ge- netically uniform ones. Conversely, species that are widespread geographically through many habitats are genetically more diversified than are those restricted to few or specialized habitats. There are limits, how- ever, beyond which a character cannot change. The continual tendency for characters to fluctuate around a mean or intermediate condition gives a population genetic homeostasis (Lerner 1954). Genetic drift Although certain characters may result from the action of a single gene or pair of genes, many, perhaps most, characters within a species are poly- genic; 1.e., they are affected by a multiplicity of genes. The exact form in which a character is expressed de- pends on the particular combination of genes which the individual or population possesses (Waddington 1957) If a fertilized female, a single pair of animals, or at most a few hundred individuals become separated from the rest of the species, there will be represented in them a considerable decrease in the number of genes available to the main body of the species, since no individual or small group of individuals can pos- sess all the genes that occur within the species’ pool. Inbreeding within small isolated populations may thus bring into prominence traits which are expressed only irregularly and inconspicuously within the spe- cies as a whole. Establishment thus of restricted genotypes in small population by loss of genes or acci- dental changes in frequencies at which certain genes occur is called genetic drift, or the Sewall Wright effect (Wright 1931). Character variations formed in this manner often appear to be non-adaptive, and there is controversy as to whether such characters are important in species formation. The possibility that genetic drift is a significant factor in speciation under some conditions is shown, however, by ground finches of the Galapagos Islands (Lack 1945), species of which differ chiefly in the size and shape of the bill. Species belonging to the same genus consume the same kind of food. Differ- ences in bill characteristics apparently developed as small populations became isolated on different islands within the Galapagos group, even though the islands were similar in climate, vegetation, and habitats gen- erally. The particular bill characteristics that the various species possess apparently resulted from a chance combination of genes that became segregated on each island. With inbreeding, these bill character- istics became genetically fixed. It is of interest that the peculiar bill characteristics came later to have a secondary significance in furnishing sign stimuli in courtship and in territorial defense. This has given ethological isolation to some species and prevented interbreeding where species, thitherto dispersed and isolated, have come again into contact. Similar fixation of special characteristics may occur in species subject to catastrophic or cyclic re- ductions in abundance. The genotypes of the few sur- vivors will determine the genetic makeup of the entire new population that develops in the area (Elton 1930, Timoféeff-Ressovsky 1940). For example, the arctic fox is a cyclic northern species possessing a white and a so-called blue color phase. Over most of the fox’s range, the blue phase is much less common than the white, but on certain islands only the blue phase occurs. It is possible that at some time in the past, at the bottom of a population cycle, only homozygous blue foxes survived; reproduction of these animals and their offspring rendered the entire new popula- tion blue. This does not mean, however, that blue color is necessarily non-adaptive or that it may not be genetically linked with a character that is adaptive. Mutations Mutations may be the result of chemical changes in the individual gene or to chromosomal 262 Ecological processes and dynamics ¥, BODY LENGTH ° TOOLE, UTAH 104 12 TROPIC, UTAH FIG. 19-4 Variations in body and wing measurements between two local populations of the gall wasp. Abscissas are in microm- aberrations, the latter in the form of changes in the number of chromosomes (haploidy, polyploidy) or of arrangement of genes on the chromosomes (dele- tion, duplication, translocation, inversion) (Dobzhan- sky 1951). The rate at which any one gene mutates varies greatly from one kind of gene to another, but the average rate is of the order once in every 100,000 or 1,000,000 individuals. Between 0.4 and 10 per cent of the individuals in each generation may possess mu- tated genes. Natural populations may therefore be well supplied with small mutations of differing potential values to the organism (Schmalhausen 1949). Mutations of different sorts apparently occur haphazardly and are not influenced by environmental conditions ; only accidentally do they give special ad- vantages to an organism. Adaptations of a species to a particular habitat or niche is effected through natural selection of the favorable mutations out of the many that occur. The size of the population and the rate at which a particular gene mutates affect the odds that a mu- tation of that gene will become established in a popu- lation. In a stabilized population, two offspring must survive and mature to replace the parents on their death. If in one parent gene A mutates to the non- lethal gene A’, the odds are 1:1 that genotype AA’ will appear in one of their two offspring and that the mutant gene will be transmitted to the following gen- eration. If both offspring are heterozygous, the odds for continuation of gene A’ into the next generation are increased to 3:1. On the other hand, if the parent THORACIC LENGTH UL LA WING LENGTH THORACIC WIDTH 44 30 170 10 AI Dah 42 20 160 8 eter scale units; ordinates, in percentages of the population in each measurement class (after Kinsey 1942). carrying the mutant gene fails to reproduce or if all the offspring die, the mutation is lost. The odds that a single mutation will persist through 127 generations is estimated to be only 1:67 (Fisher 1930). If the mutant gene A’ is a dominant, the character is immediately expressed in the phenotype; if it is recessive, the character will not appear in the pheno- type until two heterozygous individuals mate to give rise to the homozygous recessive A’A’. In a small population, inbreeding between heterozygous indi- viduals will quickly produce both homozygous domi- nant and homozygous recessive genotypes, as well as the heterozygous line, and provide a variety of pheno- types upon which natural selection may work. In a large population, mating between heterozygous indi- viduals will be less frequent because these individuals will constitute a lower proportion of the total popula- tion. However, if the gene mutates repeatedly in different individuals, the high mutation rate will greatly increase the number of individuals carrying the gene and increase the chances that the mutant character will become expressed in the population. Hybridization The critical test of whether or not speciation has occurred comes when a barrier between two geo- graphically isolated populations breaks down, so that the formerly isolated populations again come into contact. If speciation is complete, they will not inter- Speciation 263 breed; if it is not, hybridization will occur. The de- tection of hybridization between two populations may be difficult, for introgression commonly occurs; that is, the hybrids backcross with either or both parental populations, and the backcrosses resemble the par- ental populations very closely. The result of intro- gression is the gradual intrusion or transfer of the characters of each population into the other, so that all distinction between them disappears (Anderson 1949). If two populations differing in habitat require- ments interbreed, the first generation hybrids will often show best adjustment to an intermediate envi- ronment. A second generation, if it occurs, will then consist of individuals of both ancestral and hybrid types, each of which requires its own peculiar habitat for optimum development. If there is a paucity or absence of intermediate habitats, the intermediate forms will be selected against, and die out. If the hy- brids are sterile or have a lower reproductive capacity than the parental populations, they also will be se- lected against. Any ecological, ethological, or genetic divergence between the two populations that reduces the gamete wastage in hybrids will be selected for, with consequent reinforcement of the isolating mecha- nism. It happens often, therefore, that niche segre- gation or differences in behavior between closely related species will become decidedly more pro- nounced in the overlap zone of their ranges than else- where. The result, of course, is continued divergence of the two populations until interbreeding between them ceases, and they form distinct species (Brown and Wilson 1956). On the other hand, there are circumstances in which fertile hybrids are not selected against. If the area in which the hybrids are formed offers a variety of habitats, some of which are different than the habitats occupied by the parent population, the hy- brids may find themselves fully as well adapted to them, and by so much be fully as well adapted to that area as are their parents. Under these circumstances the hybrids will survive, introgression will occur, and the formerly isolated parental populations will fuse into one. Introgression between populations has been observed and recorded in areas where man by remov- ing forests or producing other disturbances in the environment has destroyed barriers that maintained a sharp ecological isolation of populations; the phe- nomenon doubtless occurred repeatedly in the geo- logical past with changes in physiography, climate, and vegetation (Anderson 1948, Blair 1951, Sibley 1954, Hubbs and Strawn 1956). A third possibility exists. Where hybrids obtain some advantage or show better adaptations than their parents, they will be selected for, and may eventually replace both parental populations. This appears to be taking place with Colias butterflies in northern Can- ada at the present time (Hovanitz 1949). Asexual and self-fertilizing forms Asexual and self-fertilizing organisms include half or more of the Protozoa and many invertebrates and plants. These organisms offer problems of spe- cies recognition and evolution that are in many re- spects different than those in bisexual forms. Each individual is reproductively isolated from every other individual, giving rise to offspring that are genetically alike by fission, budding, sporulation, or self-fertiliza- tion. Mutations, however, arise, and if favorable, may transform a strain or clone as the result of natural selection. Local clones may differ therefore in pheno- typic and genotypic characters in the same manner as bisexual populations, even though there is no oppor- tunity for variation to occur through assortment and recombination of the genes. Clones that are genet- ically distinct are in the nature of sibling species. Such clones, however, are for convenience considered as belonging to one and the same taxonomic species, if they show similar morphological characters that are readily distinguishable (Meglitsch 1954, Boyden 1954, Sonneborn 1957). NATURAL SELECTION Natural selection is a continual force ex- erted on each successive generation. Before natural selection can take place, however, there must be phenotypic variations between individuals, from which selection can be made. In order for these se- lected variations to have significance in speciation, they must be genetically fixed and heritable. In most species, many more offspring are pro- duced than can possibly survive. Because of this overproduction, there is competition between the off- spring for the necessities of life, which, together with the strife between predator and prey and between organisms and their physical environment, creates a struggle for existence. There is differential survival in this struggle for existence because some individuals have structural, functional, or behavioral variations that give them advantages over individuals lacking those variations. The superior genotypes will make a relatively larger contribution to the gene pool of the next generation. The result of differential survival and differential re- production is popularly known as the survival of the fittest. The accumulation of favorable variations in a population brings the species generally to a better 264 Ecological processes and dynamics adaptation to the physical conditions of the habitat, avoidance of predation, more efficient physiological functioning, and new behavior patterns. Natural se- lection favors those variations that are adaptive, and thereby fosters the continued existence and improved reproduction of the species. If the population under- going these changes is geographically isolated so that the favored changes do not spread throughout the species, then differentiation of characters leading to speciation occurs. The actual mechanics of natural selection are disputed, although understanding of the general: processes involved is steadily increasing. When the ratio of one character to another changes from 1.00:1.00 to 1.01 :1.00 in each genera- tion, the character is being selected for; more indi- viduals with the character are surviving than are in- dividuals without it. With a selective advantage of 1 in 100, a dominant character will become estab- lished in 99 per cent of a population in about 1200 generations (Huxley 1943). This rate is considered rapid evolution. A selection pressure of even 1 in 1000 represents fairly rapid change, but when selec- tion is decreased to 1 in 1,000,000 or more, evolution is relatively slow. A good mathematical analysis of selection pressures is given by Li (1955). The action of natural selection is evident in the following examples. Scale insects, flies, and other in- sect pests are commonly controlled by hydrocyanic gas or DDT sprays. However, after many applica- tions, surviving populations display immunity. Ap- parently, normal populations are mixtures of resistant and non-resistant genotypes. In the absence of the fumigant, there is no selection between the two geno- types, but with continuous fumigation, the resistant types survive and reproduce until they become pre- dominant in the population (Dobzhansky 1951). Natural selection presumably functions in much the same manner to make the color of local populations of small mammals and snails match the color of the soil or vegetation as protection against predators (Dice and Blossom 1937, Blair 1951a, Sheppard 1954) ; to correlate the pigmentation of butterflies with local differences in temperature, moisture, and solar radiation (Hovanitz 1941); to bring parallel variants in many kinds of fish when in the same kind of environment (Hubbs 1940); and generally to establish the many other adjustments and adaptations of organisms to their particular niches. Natural selection is relatively less effective in small populations than in large ones. Small popula- tions may be locally restricted, come into conflict with few competing species, and experience only a favorable habitat. Because of the low selection pres- sure, chance combination of genes (genetic drift) may produce characters of little or no adaptive value that yet have a good chance to persist while really adaptive characters may be lost. In the Hawaiian and Society Islands, there is a great variation in the characteristics of snails that variously occur on the different islands and locally in different isolated val- leys or regions on the same island (Gulick 1905, Crampton 1932). This is apparently a result of the fixation of random variations in small populations not subjected to any considerable selective pressure. In populations that are increasing rapidly in size, in the upswing of cycles, say, there is little selection, and non-adaptive variations may persist as well as adaptive ones. As populations come to saturate hab- ited niches and disperse into new or less favorable habitats, competition, predation, and parasitation in- creases, and individuals become exposed to evermore severe physical and climatic conditions. Natural se- lection then functions, and characteristics that are adaptively advantageous will tend to persist while those less favorable or even harmful are eliminated. Mutations upon which natural selection works are often recessive. They do not become fully expressed in the phenotype except when the individual is homo- zygous for the character. Nevertheless they are im- portant, and in stabilized populations tend to persist indefinitely in constant proportion to the dominant alleles (Hardy-Weinberg law). A recessive charac- ter will become more prominent in a population if that particular gene continues to mutate toward the recessive, or if the homozygous recessive phenotype has adaptations that give it selective advantages. In this latter case, natural selection may ultimately re- sult in complete suppression of the dominant allele so that the hitherto recessive allele becomes perma- nently fixed in the population as the only gene for the character. Since emergent species usually entrain adapta- tions to new environments, it would appear that those characteristics by which we distinguish species and higher taxonomic categories generally are such as serve some useful purpose to the organisms either structurally, functionally, or in point of behavior. It is often very difficult to determine a useful function for all distinctive characteristics of a species, yet one can seldom be positive that a seemingly minor character does not serve, say, as a releaser for some critically necessary behavior or is not vitally important in other unsuspected ways. However, not all characters that distinguish species or higher taxonomic catego- ries are necessarily adaptive (Robson and Richards 1936, Simpson 1953). Some characters originally adaptive may have lost their usefulness, although they persist in the organism. With natural selection no longer acting on them, they usually retrogress and may eventually disappear as have skin pigmentation and eyes in many cave animals, for instance. Other characters may even have a slightly unfavorable Speciation 265 peer Ea ECe7e piacere 70 | Reef corals ie) 5 10 15 20 25 30 MILLIONS OF YEARS FIG. 19-5 Age and time of origin of present-day species of reef corals and mollusks as shown by their percentage of occurrence in fossil faunas of the East Indies (after Umbgrove 1946). value, but be closely linked genetically with selec- tively preferred favorable characters and thus con- tinue in the organism. Before a population can occupy a new region or even expand its niche it must show at least some pre- adaptation for it (Allee, Emerson, e¢ al. 1949, Simp- son 1953). Preadaptation may take the form of a wide range of tolerance that can encompass the con- ditions of the new habitat as well as the old; or it may take the form of a new use for a structure, dif- ferent from its original function (Bock 1959). The European rabbit certainly showed a good deal of preadaptation to the Australian environment; it be- came a local pest within three years of the intro- duction of 24 individuals in 1859. We have earlier described why preadaptation must have been neces- sary for the origin of parasitism. Preadaptation per- mits individuals to exist in new habitats or to perform new functions, but subsequent perfection of an adap- tive trait depends on the accumulation and selection of additional favorable genetic variations over many subsequent generations. ADAPTIVE RADIATION When a species bypasses or surmounts a dispersal barrier, it may penetrate an area having a variety of niches novel to the species. A plastic spe- cies may quickly differentiate adaptively into a num- ber of new species, each becoming established in an unoccupied niche or, if sufficiently aggressive, displac- ing an original but less adaptive occupant. Such adap- tive radiation is known to have occurred in the case of 266 Geographic distribution of ancient marsupials, which crossed from Asia to Aus- tralia and differentiated into the variety of species now found there. The invasion and occupancy of the Hawaiian Islands by snails, insects, and birds is of special in- terest. There are some 3722 insect species endemic to the islands. All of these species appear to be de- rived from some 250 ancestral forms that arrived in 14 separate invasions since Pliocene time (Zimmer- man 1948). The ancestral prototype of the honeycreeper birds reached the Hawaiian Islands sometime within the last five million years (Baldwin 1953). Different populations became isolated on different islands, as a result of which there arose the so-called red and black nectar-eating species that are grouped in the subfamily Drepaniinae. As the nectar-feeding niches became fully occupied, a population diverged in its behavior, feeding more heavily on insects than on nectar. The new niche allowed redispersion of the population through the various islands, and there ensued a second burst of speciation yielding the so- called green insect-eating forms belonging to the subfamily Psittirostrinae. Additional speciation pro- duced short- and long-billed species of insect-eaters. Somewhere in the lineage of the latter group, the birds acquired seed- and fruit-eating habits, and the long bill also became a thick bill. Rapid evolution in this family still appears to be in process. RATE OF EVOLUTION There is evidence that, under natural con- ditions, variations in local populations may sometimes be manifested within a surprisingly few generations (Huxley 1943). Melanistic forms of butterflies now occur in industrial areas of England where vegetation has become coated with dark-colored debris, although 100 years ago such butterflies and the industrial soot as well were virtually absent (Kettlewell 1956). House mice isolated on a sandy island have become within 100 years a distinctly paler population than the one on the adjacent mainland (Jameson 1898). The period of time required for evolution from one taxonomic level to another varies enormously between different kinds of organisms. It has been estimated that the rate of change in characteristics of several lineages of mammals since Pleistocene time, when it can be measured quantitatively, for instance in the length or breadth of the skull, is of the order of 0.2 per cent per 1000 years (Kurten 1958). Ap- parently, subspecies commonly require 10,000 years to become well defined, and may continue to evolve for 500,000 years before rising to the species level. The evolution of a fully defined species usually re- quires at least 50,000 years and frequently a very communities much longer time. Some living mammal species are 1,000,000 years old, and some lower vertebrate and invertebrate species have persisted relatively un- changed for 30 million years (Simpson 1949, 1953). The rate of change, divergence, or evolution of populations into new species depends partly on the rate at which new mutations are occurring in the species’ gene pool, and partly on the rate and extent to which the environment is changing. In a long- continued uniform environment, a species becomes stabilized in a favorable relationship with the habitat and community. The various ecological niches are effectively occupied, and little evolution occurs. New mutations can add little to perfected adaptation. If the habitat changes, however, established adaptations may no longer be appropriate, and variations hitherto rejected might now prove beneficial. A mutation se- lected for in one or more species may initiate a chain of events that alters the internal balance of the whole community, with resultant rapid evolution (Olson 1952). For instance, there has been considerable dif- ferentiation of animals into subspecies during and since the Pleistocene glaciation, but probably most of our present-day species originated in pre-Pleistocene time. SUMMARY Speciation is the process of evolutionary differentiation often leading to species formation, a process usually also involving separation or diver- gence of populations into different ecological niches. Sympatric species do not interbreed because of eco- logical, ethological, mechanical, or genetic isolating mechanisms. Geographic isolation of two populations of the same parental species appears prerequisite to com- plete speciation. As long as a significant amount of gene flow occurs between populations, they diverge no further than subspecies. Populations may show different characteristics as nonheritable variations, heritable polymorphism, genetic drift, mutations, and hybridization. With natural selection, those individuals possess- ing adaptive variations obtain a greater chance for survival and reproduction, and consequently contrib- ute more to the gene pool of the population. This leads to a change in the characteristics of the popula- tion and possible speciation. Exposure to new habi- tats or niches may thus bring adaptive radiation into a variety of new species. Speciation 267 Z Distributional Units Through the study of geographical ecology we at- tempt to understand how organisms are distributed over the world, and what forces brought about this distribution. Most organisms are of restricted dis- tribution—only a few groups of animals, notably cyprinid fish, frogs of the genus Rana, colubrid snakes, passerine birds, rodents, and man, can be called even nearly cosmopolitan. In order to under- stand existing distributions, we must consider the histories of species, where they originated, how they got where they are now, and why they are not found in other parts of the world. This takes us into cli- matology, zoogeography, and palaeontology. In this section we will be dealing with the broader problems of the composition, characterics, and origin of geo- graphic units in the distribution of organisms, rather than with the local units of communities and habitats discussed in Section II. These distributional units, like communities, are distinctive groups of organisms spatially distinct from other groups, but they do not always show functional integration as do communities and are sometimes characterized by genera, families, and orders rather than species. We will begin our discussion with zoological realms, areas defined largely by the past and present relations of the larger land masses to each other. Then we will proceed to a consideration of how the physiographic peculiarities of those land masses affect the origin and dispersal of organisms. Finally, we will consider the major ecological units, biomes, determined by climate and vegetation. ZOOLOGICAL REALMS AND REGIONS Geographic faunal divisions are character- ized by the distinctiveness and uniformity of the taxonomic groups represented. They are relatively self-contained units isolated from other similar units so that there is no free and easy intermingling of species. There have been many attempts during the last hundred or more years to divide the world into such units (Kendeigh 1954). Probably the best sys- tem was that devised by Sclater (1858), modified by Huxley (1868), and extended by Wallace (1876). Although present-day students of special animal groups are inclined to modify the system further to fit best the distributions of those organisms with which each particularly deals, it commonly takes the form demonstrated in Fig. 20-1. The regional divisions approximately coincide with continents. That the continental land masses should serve as the basis for the first major subdi- vision of the fauna of the world is to be expected, since they are mostly separated from each other by large bodies of water that serve as effective barriers. However, different continents have become isolated 268 HOLARCTIC = > PALE aRerTic oor enn, Po a, esi. XX ae ey ORIENTAL SS — SS HOLARCTIC / NEARCTIC? aoe ™ (eeeeeeeeen seem HOLARCTIC o of ' ! ' f See eeeee en ! -* ballet tet FIG, 20-! Zoological regions of the world. Adapted from Map No. 201 HA, Goode Base Map Series; copyright by the University of Chicago. Zoological realms Regions Notogaea Australo-Papuan Neogaea Neotropical Arctogaea (Megagaea) Ethiopian Oriental Holarctic from each other at different times in the geological past and hence are not entirely equivalent in point of faunal distinctiveness. Australia has been sepa- rated from Eurasia for a very long time. Australia lacks a rich fauna, but that which it has is very dis- tinctive, in recognition of which it and neighboring islands are delineated as the Notogaeic (meaning Southland) realm. South America has for long been far distant from Eurasia, where evolution of modern orders and families has proceeded most rapidly, and was long isolated from North America. Its fauna is unique, and it has therefore been given the name of Neogaeic (meaning new land) realm. The Ethiopian, Oriental, and Holarctic regions do not individually have such marked distinctiveness from each other as do the Australian and the Neotropical regions. Hence they are grouped into the Arctogaeic (meaning Northland) realm. We must now look at the char- acteristics of these realms and regions in more detail (Newbigin 1950, Beaufort 1951, Darlington 1957). General location Australia, New Guinea (Papua), New Zealand, and neighboring islands South and Central America, West Indies Africa, Madagascar Tropical Asia, with associated continental islands Eurasia and North America Australo-Papuan region The Notogaeic realm and Australo-Papuan re- gion may be subdivided into the Australo-Papuan, New Zealand, and Oceanic Islands sub-regions. (Some students raise these sub-regions to the rank of regions because there are in fact important differ- ences between them.) Included in the Australo- Papuan sub-region are Australia, Tasmania, New Guinea and the nearby Aru Islands, the Bismarck Archipelago, and the Solomon Islands. These areas, except for the Bismarck Archipelago and the Solo- mon Islands which lie close and to the east, were united by dry land bridges in times past, for they are situated on the same continental shelf. New Zealand and the Oceanic Islands of the South Pacific are dis- tant; the latter, at least, were always isolated. The fauna of the Polynesian Islands is certainly derived from New Guinea. It is a matter of controversy whether an actual land connection ever existed be- Distributional units 269 tween Australia and the Oriental Region or whether there was only a series of stepping-stone islands by which dispersal took place. The Celebes, the Moluc- cas, and the Lesser Sunda Islands lie intermediate between the Oriental and Australian Regions and derive their fauna from both directions. The Australo-Papuan region is marked by the absence of most groups of placental mammals, al- though some have been introduced. The original mammalian fauna consisted chiefly of monotremes, a diversity of marsupials, murid rats and mice, and bats. Well represented among birds are parrots and parakeets, cockatoos, lories, honey-eaters, birds of paradise, pigeons, megapodes, and kingfishers. Note- worthy is the presence of the emu (Australia), cassowary (New Guinea and adjacent islands), lyre- bird, scrub-bird, and kiwi (New Zealand), and the absence of woodpeckers. Reptiles are well repre- sented in the region, including Sphenodon, in New Zealand. Salamanders and true frogs are absent or scarce, but tree frogs occur in a great variety of species. The fresh-water fish fauna is scant, but in- cludes the dipnoan lungfish Epiceratodus. Among the invertebrates, relict crayfishes of the family Para- stacidae and a variety of land snails are of interest. Except some bats, there are no native mammals in New Zealand. The bird fauna is poor, with several flightless forms. In general, as one proceeds east- ward and northward away from Australia and New Guinea the variety of animal life found on the iso- lated islands progressively decreases, but because of the isolation of the islands, many peculiar endemic forms are found. Neotropical region The Neotropical region making up the Neo- gaeic realm includes all of South America and the West Indies, and extends through Central America to include the southern lowland part of Mexico. The fauna of this region includes a large number of endemic forms. Among the edentates, the sloths, armadillos, and anteaters are largely peculiar, al- though one species of armadillo extends north to Texas and east to Florida. There is a variety of marsu- pials present that doubtlessly entered South America from the north, although only one living marsupial occurs at present in the United States. The hystricoid rodents include the tree-porcupines, guinea pigs, agoutis, chinchillas, and others, of which only the porcupine has spread into North America. Prehen- sile-tailed monkeys and marmosets are characteristic, and tapirs occur here as well as in the Oriental region. Insectivores are largely absent. The avifauna is rich and includes the tinamous, hoatzin, trumpeters, sun-bitterns, cariamas, seed- snipe, rhea, puffbirds, toucans, hummingbirds, and several unique families of passerines. Altogether, the Neotropical region has about 2500 species of breeding birds ; Ethiopian region, 1750; Palaearctic sub-region, 1110; Nearctic sub-region, 750; Australia and Tas- mania together, 650. Reptiles are well represented, and among am- phibians the tree frogs Hylidae reach their greatest diversification of species. Toads are present, but only a few true frogs and a few salamanders among the tailed forms. Among fishes, the catfishes, characins, and the eels are well represented, and one of the three remaining genera of lungfishes, Lepidosiren, occurs. Minnows are absent. Islands in the West Indies have a reduced fauna, species of which appear to be variously derived from Central and North America. Of mammals, only ro- dents, bats, and the peculiar insectivore Solenodon are found. Fossil evidence, however, indicates a richer mammalian fauna in the past, which apparently arrived by an over-water route, perhaps on rafts (Simpson 1956). Except in Trinidad, the fresh- water fish that occur in the West Indies are also tolerant of salt water. The occurrence of many related forms in South America, Australia, and Africa, and the absence or poor representation of those forms, even as fossils, in North America and Eurasia has made the ex- planation of how they became distributed difficult. Land bridges across the South Atlantic and South Pacific have been postulated, but the idea is not gen- erally accepted at the present time. The continental drift theory attempted to explain the distributions. An intervening connection at least of South America, Australia, and New Zealand at some time in the geological past with a large continent, Antarctica, in the south polar regions has been suggested (Glenny 1954). There is some evidence that Antarctica at one time had a warmer climate, but its secrets are now largely buried under many meters of snow and ice and must await development of new exploratory tools. Ethiopian region All the rest of the world, outside of the Noto- gaeic and Neogaeic realms, belongs to the Arctogaeic realm. Especially characteristic of this realm are the presence of ungulates, insectivores, catarrhine mon- keys, and ganoid and cyprinoid fishes. Arctogaea is divided into three main regions. In the Ethiopian region is included all of Africa south of the northwest corner ; and, since the Red Sea is of relatively recent origin and is not an important barrier, the southern part of Arabia. In the northeastern quarter of the realm the fauna reflects a mingling with the fauna 270 Geographic distribution of communities of the Oriental region. Madagascar and other islands of the Mascarene group are sometimes considered a region distinct from continental Africa because they have a number of peculiar forms (Rand 1936). At the levels of order and family, there are some similarities between the forest and savanna faunas of the Ethiopian and Oriental regions; at genus and species levels, however, differences are conspicuous. The similarities at higher levels are explained by geo- logical history. During the Miocene and Pliocene, Africa, Arabia, and India shared a rather uniform, moist climate. Continuous land bridges supporting a rich uniform vegetation connected the three areas, and animals moved freely between them. An ocean barrier then developed between Africa and Arabia on the west and India on the east, and drying of the climate in northern Africa and Arabia interposed arid grassland and desert between the forests of central Africa and India. Separation of the forest fauna into two isolated populations has permitted evolution of distinct species and genera in those families and or- ders that the two regions share in common (Moreau 1952). The Ethiopian region is remarkable in having neither deer nor bears. Except for the guinea-fowl, there are few gallinaceous species, so common in the Oriental region. It has several endemic bird families, however, including the Musophagidae and Coliidae ; Africa also has the ostrich. Hornbills occur both in Africa and the Oriental region. Peculiar to Africa, but extending north into Palestine and Syria, are the mammalian hyracoid coneys. Africa is noted espe- cially for giraffes, antelopes, zebras, elephants, and other ungulates that wander around in large herds. The presence of the large hippopotamus and rhinoc- eros should be mentioned and also the number of cats (lion, leopard) and dogs (jackals, foxes, hunting dogs, and others). The tiger of the Orient is absent. Edentates include the aardvark and scaly anteaters. Among the primates are found lemurs, baboons and macaques, the chimpanzee, and the gorilla. Rodents and insectivores are well represented. It appears that most of the mammals, except for the elephants, coneys, and aardvarks, had a Holarctic origin and entered the Ethiopian region from the North. Rep- tiles are well represented in Africa; the chameleons are highly varied in Madagascar. Salamanders and the tree frogs are absent from the amphibian fauna. The fish fauna in the Ethiopian region contains a great proliferation at the species level. Several relict species, such as the lungfish Protopterus, occur: and several families of primitive teleosts are endemic. Otherwise there is considerable relationship between the fish fauna of the Ethiopian and Oriental regions. The Madagascar or Malagasy sub-region has been isolated from Africa certainly since the Eocene and possibly since early Mesozoic. Most modern families of mammals have evolved since early Ter- tiary and would have had to enter Madagascar by the sweepstakes route, it is no wonder then that such groups as ungulates, rodents, carnivores, and mon- keys are so poorly represented, at best, in this sub- region. On the other hand, peculiar insectivores (Centetidae), carnivores, rodents, and primates (especially lemurs) occur here, the derivatives of primitive stock which is poorly represented if not extinct on the continent. A similar situation obtains among groups other than mammals. The avifauna in- clude several families, genera, and species peculiar to the sub-region. Bird groups show affinities both with continental Africa and the Oriental region ; and it is possible that some forms may have been driven hither by heavy winds. In general, the avifauna dif- fers more from the Ethiopian region than the latter does from the Oriental region (Rand 1936). Mada- gascar has no poisonous snakes, but there is a good representation of other reptiles. It is of special inter- est that Madagascar, the Comoro Islands, Bourbon, Mauritius, Rodriguez, Aldabra, Admiralty Islands, and Seychelles in the Indian Ocean all support pecul- iar species or subspecies of large land tortoises. The only other place in the world where large tortoises occur is on the Galapagos Islands off the west coast of South America, but these are not related, being independently derived from small forms. There are no strictly limited fresh-water fish on Madagascar, although fresh-water forms also tolerant of salt-water occur. Parastacid crayfishes are found in Madagas- car, South America, and in the Australian region, but nowhere else. Oriental region The Oriental region includes tropical Asia, con- spicuously demarked from the Palaearctic to the North by the Himalayas. To the West it gradually gives off into the Ethiopian region. An inexact boundary line (for our purposes, we may adopt Weber’s line here) separates it from the Australo- Papuan region to the Southeast. Java, Sumatra, Borneo, and the Philippines belong to the Oriental region and, according to Gressitt (1956) so properly do many of the outlying Pacific islands. The Indian peninsula is an old land mass separated from conti- nental Asia during the Eocene by the Tethys Sea, which intruded eastward from the Mediterranean through much of southern Asia before the uplift of the Himalayas. India may have been connected with Africa at various times in the past, most recently during the Miocene and Pliocene as discussed earlier. Tree-shrews ; the gibbon, orangutan, and tarsier ; fresh-water tortoises (Platysternidae) ; and the slen- der-nosed fish-eating gavials are exclusive to the Distributional units a | Oriental region. Other interesting forms character- istic of this region, although not exclusive to it, in- clude two kinds of loris ; the pangolins, an elephant, a tapir ; two rhinoceroses, several species of deer and antelopes, wild pigs, many rodents, a porcupine; the tiger among several other cats; bears, several kinds of monkey; and, among birds, many pheasants, the bee-eaters, rollers, broadbills, bulbuls, and sunbirds. Many kinds of poisonous snakes occur, and lizards are well represented. True frogs and toads occur ; tree frogs and salamanders do not. The most primi- tive spadefoot toads, Pelobatidae, occur here, other forms being found in Europe and North America. The Apoda also occur throughout the Oriental re- gion; elsewhere, only in the Neotropical and Ethi- opian regions. A varied fish fauna exists. There is a marked difference between the fauna of northern and southern India; probably a conse- quence of the extensive volcanic eruptions that oc- curred in early Tertiary which devastated extensive areas in central India, the so-called Deccan traps, and formed an effective East-West barrier that persists even at present to some extent. Southern India and Ceylon have a fauna related generically to that of Siam, Indo-China, the Malay peninsula, the East Indies, and the Philippines. The extensive Tertiary intrusion of the sea between southern India and Ceylon and the eastern portion of the region, has permitted the extensive evolution of distinct species in the areas. The various islands of the East Indies that fall within this region have received their fauna during the various times they were connected by dry land with each other and with the mainland. Holarctic region The broad Bering land bridge connected North America and Asia through much of the Tertiary, and there was considerable movement of animals back and forth between the two continents. There is simi- larity at least in the genera of many animals in the northern portions of the two continents, and many species are found in both areas. Salamanders are largely limited to the Holarctic; edentates and pri- mates, other than man, are mostly lacking. The horse and camel evolved first in North America and then spread to Eurasia and Africa but became extinct in North America. The horse was reintroduced into North America within historic time by the white man. As one progresses southward on the two conti- nents below the Arctic tundra and coniferous forest, the fauna changes (Udvardy 1958) enough to sepa- rate the Palaearctic and Nearctic sub-regions. The chief differences distinguishing the two are at the level of species and genera and a few families. Dif- ferences are more pronounced among birds and rep- tiles than with mammals. Antelopes, sheep, goats, and certain other groups are in greater variety of species in the Palaearctic; water moles of the sub- family Desmaninae are unique to the sub-region. On the other hand, the Nearctic has several families of birds, such as the vultures, turkeys, mockingbirds, vireos, and wood warblers that have not spread into Asia and Europe. Rattlesnakes, salamanders (Am- bystomidae) , suckers, and catfishes common to the Nearctic are either absent in the Palaearctic or poorly represented. FAUNISTIC SYSTEMS When populations become isolated geo- graphically, they tend first to differentiate into sub- species differing in superficial characteristics, then into new species that would not interbreed even if the isolation were lost, and finally into genera and units of still higher rank. There is a general rela- tion between the size of the area and the extent to which taxonomic differentiation proceeds. Continen- tal masses isolated from one another are usually pre- requisite to the development of orders and families, and the analysis of geographic distribution of animals at these taxonomic levels is recognized in the realms, regions, and sub-regions already discussed. Each of these major geographic areas is subdivided into units of lesser size, wherein genera, species, and subspecies become differentiated. There has been considerable study of different methods of recognizing, evaluating, and classifying these lesser units in the distribution of animals which has resulted in different fauwnistic systems. The more important of these systems need now to be analyzed, especially in reference to the Nearctic sub-region. Faunal areas The first attempt to subdivide North America into geographic units of biological significance was made by Schouw in 1823, for plants. His work stim- ulated zoologists to undertake a number of similar efforts (Kendeigh 1954). J. A. Allen (1892) devel- oped one of the best and most realistic of the faunistic systems. The northern hemisphere was divided into circumpolar arctic and north temperate realms and an American tropical realm separate from tropical realms in the eastern hemisphere. The north tem- perate realm he divided into North American and Europaeo-Asiatic regions, and into cold and warm temperate sub-regions. There is considerable merit in this system: in the Holarctic, a close similarity of the North American and Eurasian faunas obtains in 272 Geographic distribution of communities / = ") ~AUBTRO-OGCIDENTAL ACAPULG BALSAN — SIERRAS DEL SUR | TEHUANTEPECAN tor a | ESPERAN| | BIOTIG PROVINCES _ | SCALE | ! : 120 —— 7 w 100 | L FIG. 20-2 Biotic provinces of North America (Blair 1950, Dice 1943, Miller 1951, Smith 1949). the tundra (arctic realm) and coniferous forests (cold temperate sub-region), but greater and greater intercontinental divergences become apparent as ex- amination progresses southward (Udvardy 1958). Allen considered humidity to become more important than temperature southward in North America, be- cause it determined the prevalent type of vegetation with which animal distribution was correlated. So he separated the warm, temperate sub-region into an eastern humid province that was heavily forested and a western arid province containing grassland and desert. These provinces were characterized by the presence or absence of prominent generic types of animals. Allen’s minutest subdivision was the faunal area, characterized by a combination of species not found elsewhere. Distributional units ale LONGITUDE WEST OF 70 GREENWICH Life-zones The life-zone system was developed between 1890 and 1910 by C. Hart Merriam (Kendeigh 1954). Merriam postulated that animal life in North America had dispersed during past geological time from two great centers; one in the far North, the boreal, and the other in the Southwest, the sonoran. Boreal animals dispersed southward along the higher elevations in the mountains, thence over the conti- nent generally as they became acclimatized to the progressively higher temperatures of lower altitudes and more southerly latitudes. Sonoran forms dis- persed northward through the lowlands as they be- came acclimatized to cold. Believing, contrary to Allen, that temperature was more important than moisture and types of vegetation in controlling the distribution of animals, Merriam extended the faunal areas indicated as Alleghanian, Carolinian, and Louisianian on Allen’s map westward as belts to the Pacific and re-designated them the Transition, Upper Austral, and Lower Austral life-zones. The boun- daries of the life-zones coincided closely with iso- therms. Each life-zone supposedly contained boreal and sonoran organisms in a characteristic and defi- nitely proportioned mixture. Each of these three life- zones he then divided, at about the 100° meridian, into eastern humid and western arid faunal areas to indicate the secondary role played by moisture and vegetation. Flaws in this system have become apparent in recent years. For one thing, the life-zone, as a trans- continental belt, is not a unit that can be recognized everywhere by a characteristic and uniform faunal composition. The Lower Austral zone in Georgia or Florida, for instance, is composed of animal and plant species almost totally different from the composition obtaining in the same zone in Arizona. This has led to the realization that while temperature is of un- doubted importance in controlling animal distribu- tion, differences in type of vegetation, in moisture and terrain, as well as in geological history of community groups are factors usually equally as important as tem- perature, sometimes more so, in determining what species will be present. Furthermore, while the past history of some genera and species can be traced di- rectly to boreal and sonoran distributional centers, it is clear that many species and especially subspecies evolved in various smaller centers (biotic provinces) elsewhere over the continent. Finally, evaluation of local, especially mountainous, areas in terms of biotic communities has tended to confuse the life-zone sys- tem with the biome system. Ze Biotic provinces A concept of biotic provinces first began to attract attention about 1911, and the provinces were defined and mapped by Dice in 1943 and again in 1952a. A biotic province is a continuous geographic area that possesses a fauna distinguishable, at the spe- cies and subspecies levels, from the fauna of adjacent areas, at least to a certain degree. The boundaries of biotic provinces are more likely to coincide with physiographic barriers than with types of vegetation. Unlike faunal areas, life-zones, and biomes, biotic provinces never occur in discontinuous geographic fragments since they are intended to show regional areas of differentiation. A biotic province which includes a mountain may have several types of vegetation or life-belts, each serving as a center of differentiation for its charac- teristic fauna when compared with similar life-belts in other provinces. The biotic province system is be- ing used at the present time by some mammalogists, ornithologists, and herpetologists in the study of par- ticular taxonomic groups, but there has been no gen- eral synthesis of these studies and of plant groups to render the provinces truly “biotic” in nature. Faunal groups or elements In all the systems described, faunal distribution has been analyzed in terms of geographic areas, and the chief criterion for determining the limits of an area has been that of distinctiveness between the fauna of different areas. Recently, largely from the impetus of herpetofaunal studies by Dunn (1931) ; avifaunal studies by Stegmann (1938), Mayr (1946), and Miller (1951); of Simpson’s studies of mam- malian fauna (1947) ; and the plant studies of Wulff (1943) and Cain (1947), a different analytical ap- proach to faunal and floral problems has developed. By this more recent approach, those species with sim- ilar centers of origin, dispersal routes, geological histories, and habitat preferences are the elements of a fauna or flora which are analyzed. The objec- tives are to learn the place where and time at which these groups of species originated, how the groups came to occupy a particular part of the world, be- came adapted to live in new environments, and how they evolved into the present-day living species. A type of vegetation, life-zone, or biotic province may contain species from several different faunal elements that have come to live together. Thus in the bird composition of California one can recognize boreal, Geographic distribution of communities — “(1h6l eFeMypuJOY] pue yoojsuew -Nig) P]HOM eyF yo seyeuujo jedioulid yo uolyNgidjsiq ¢-QZ ‘S|4 asovs qwnizeu3¢ [TI] 4 veowna [MM 3 volvs Ha a owy [=] 3 amnuens [F] 0 ours WU) ve BEE ON3937 A , WIG Y ip 1... MAA , i CN AG, Y) WM WY wf “ ZB 7 Y, , ig 4 YY iy “ay q SR LL aye 3 a Wy, 7’, vir Gi Wa | Vi, 4 p : at. Yiy Y, YY Sa ais 2 ay o sists Ss 2 A} — ty. vs 3 My ee | : r gg ~ : pe a ul uty eT oT ll \ = SS B a i at owvinas fe] O PID Distributional units Great Basin, Sonoran, and authochthonous elements (Miller 1951) (see also Table 10-1). THE ECOLOGICAL SYSTEM A still different approach for the study of animal distribution is the ecological one, involving the biome concept (Clements and Shelford 1939). The other systems described are zoogeographical in their attention to centers of origin, dispersal routes, and evolution. The ecological system emphasizes en- vironmental relations and community dynamics. This does not mean that zoogeographers do not consider the relations of climate and terrain in controlling dispersal and evolution, nor that ecologists are not concerned with the geological history of the forms with which they deal, but the viewpoints and objec- tives are different. Basic to the understanding of the ecological sys- tem is the recognition of communities composed of characteristic combinations of animal and plant spe- cies, of successional relations between communities, and that succession in all local habitats eventually ends in a climax community pattern, the most im- portant ecological characteristic of a geographic area (Cain 1939, Whittaker 1953). The development of an understanding of these ideas has been a major concern of this book, especially Chapters 5 to 9. The major unit in the ecological system is the biome. A biome is a biotic community, characterized by distinctiveness in life-forms of the important cli- max species. On land, the most important climax species are usually plant dominants that occur in dis- tinctive vegetation and landscape types; in the ocean, the important organisms that define biomes are usu- ally the predominant animals, which are sometimes also dominants. Seral communities are developmental stages. They are as much a part of the biome as develop- mental instars are a part of a species of insect. The animal and plant constituents of seral stages are, however, more widely distributed than are species belonging to climax communities, since the habitats in which they belong are more nearly alike in dif- ferent parts of the world than are the habitats that contain the climax. Seral species are not generally useful, therefore, to defining the limits of biomes. The majority of animal constituents of the climax community, however, are characteristic only of the climax vegetation or habitat and therefore of re- stricted distribution. The principal biomes of the world, insofar as they have been identified, are the following: 276 Marine Oceanic plankton Terrestrial Temperate decidu- ous forest and nekton Coniferous forest Balanoid-gastropod— Woodland thallophyte Chaparral Pelecypod-annelid Tundra Coral reef Grassland Desert Tropical savanna Tropical forest The vegetational portion of the biome is some- times called the plant formation (Weaver and Clem- ents 1938). A plant association is a subdivision of a biome or formation, distinguished by uniformity in the species composition of the climax plant dominants. The associes is the equivalent seral plant community, regardless of whether it belongs to the same or a different type of vegetation than the climax. The im- portant point to remember here is that the biome is distinguished by the life-forms of the climax dom- inants, but subdivisions of the biome are recognized principally by taxonomic composition. The type of climax in a terrestrial area is deter- mined mainly by the conditions of climate, although secondary correlation also occurs with major soil groups. Were climate the only factor involved and the terrain uniform, the climax community would be monotonous in its composition and structure, except as one community graded into another with change in climate. Where the composition and character of the prevailing vegetation varies more or less perma- nently with changes in physiography, soil, or fire fac- tors, we may speak of physiographic, edaphic, or fire subclimaxes or faciations. This is in agreement with the monoclimax view- point; that is, that there is only one true climax in an area, determined by the prevailing climate. An opposing concept is that of polyclimaxes. Proponents of the latter viewpoint give nearly equal significance to climate, soil, topography, and other factors, and believe that each major variation in composition or structure of the mature vegetation should be con- sidered as equally important. Hence, several cli- maxes, or at least a complex community pattern that varies in structure and species composition from one site to another, may be claimed for an area (Whit- taker 1957). The controversy is largely one of em- phasis and semantics. A biociation is a subdivision of a biome distin- guished by uniformity and distinctiveness in the spe- cies composition of the climax community, particu- larly of the animal predominants. The biocies is the Geographic distribution of communities = *(L96| Uosuowis) PjJuom ayy yo sdnosB jlos jedioulsd ay} yo uolyngisysig $-0Z “S|4 FOIANAS NOLLVAHESNOO T1108 (mo0}|8,4 pue ‘aBue, ‘SaBues aly ‘a: * Wddrssissipy 84) Se S@AU yeauB YOns Buoje @pMaje| PUR VOReAsie Uyym NBA U>VUM “VOR SBE pur 9} e.W)/2 PUBIEP "S)JO¥ BADGE #/0W 10 BU 10 SUCHEN PUY Waim (s}OSOUITT) $108 AUOIS ~ SNIVINNOW 4O STIOS SO1EWH!D (EHDOAQNS PUe j#2)04 AUP jem pue PIWNY JO SHOE paverod eUUEAES PUR pe}seI04 = STIOS IMOSOLY? hej AeuB yep pur ¥: Alp Jem 40 S}/05 BWOS SEPN/IU) *PIWNUGNS JO $)/08 pasen0>s8"/D - STIOS DINIZONHIHD i108 2ueB10 40 Seose AUeW Sepnjruy “seyewII> Mevedw9} "piUINY JO $I}0% paI¥8/04 = STIOS DMOZGOd S)}O8 peserod ssOW PUR QnIUS EMG ~ dVW TIOS SILVW3HOS GvOuSs t { AYOLINOMOY AO LNAWLY Vdd S11 units ZT Distributional PBUUBAES UIayINOS © BUULAERS UAY]ION ® prae asauepns © pire ysamyynos ®© plie 1TewWog ©) jearedeyo pue pue[poom yuuears [eoidory Bapun} eutdye pue ysar0} ]BAL0g Bapuny oo1y play vapun) auldiy ill 4 TY Ca “(0561 POOM ‘BE6] Uew Bays 'Gyb| uoysuyor Pue YHWS 'ZG61 NEE4O/W ‘E61 42/420) “bbb! 1442S ‘0461 519g) seounos snolea wosy pasedeid dew ‘saucy -099 JO ‘selpiuNWWOD PexiW 40 @alpeoipul ase 'syseiof snonp!oap pue jpeiog usemjeq se 'sjoq -whs Buiddejiead “pjiom aut fo suoipeisoig pue ‘sewolg ‘solprunw -woo jeujue solew S-02 “O|3 equivalent seral community. With the biome, we recognize the primary importance of the life-form of the principal climax organisms for establishing the major units into which the geographical distribution of organisms are naturally divided. The biociation concept does not regard differences in the species com- position of plant dominants, such as are recognized in the plant associations, as establishing the funda- mental subdivisions of the biome for animals. So long as they are of the same life-form, variations in the plant species composition produce only minor differ- ences in dominance, insufficient, for the most part, to induce striking changes in the species composition of the rest of the community. The relation between biocies, associes, and changes in vegetation that occur with succession (Fig. 8-4) obtain also between bio- ciations, associations, and changes in vegetation that occur geographically. Actually, the animal ecologist has no absolute need to identify the species compo- sition of the plant dominants if he can describe the vegetation accurately in other ways (Dansereau 1951). If one wishes not to define plant communities on the basis of the taxonomic composition of the dominants alone, the concept of biociations may be equally useful for the analysis of distribution of plant species. The so-called natural areas of Cain (1947) are a step in this direction. A biociation may originally derive its species com- ponents from several faunal elements (Table 10-1), but once the community constitutes a unit, it may thereafter serve as an element itself, and a geographic area may be described in terms of the biociations and biocies it contains. Biociations differ from biotic provinces in that the latter are geographic areas, rather than communities, and in mountainous areas may contain several life-belts or different types of community. Furthermore, biotic provinces may be characterized in part by the presence of particular subspecies. Subspecies are not used in defining bio- ciations. The faunal system of Allen bears some resem- blance to the biome system. Allen’s barren-ground fauna is equivalent to tundra ; the cold temperate sub- region together with the Pacific Coast district equates with coniferous forest; the warm temperate sub- region with deciduous forest; the Great Plains dis- trict with semi-arid grassland; the Great Basin dis- trict with arid grassland and sagebrush; and the Sonoran sub-province with desert. The different faunas within these major divisions may correspond to biociations. Although the terminology is different and the refinement of details is greater, the system of biomes and biociations is manifestly built upon the basic foundation laid by a long line of zoogeographers (Kendeigh 1948, 1954). It is clear, then, that two sets of factors control local and geographic distribution of organisms. The first set is ecological, including the physical factors of nature of the substratum and climate for terrestrial organisms and the composition of the water for aqua- tic forms ; the biotic factors, especially of food, cover, reproductive requirements, competition, and preda- tors; and the psychological factors of behavior ad- justments, inherited mores, and specific niche re- quirements. On the other hand, zoogeographic factors include the considerations of place of origin ; dispersal pathways, rates, and barriers; and evolutional acqui- sition of new structural, functional, or behavioral adjustments that permit invasion of new areas, sur- mounting of old barriers, or incorporation into new communities. The present-day distribution of ani- mals and plants into different community units is the result of all these forces at work, both at the present time and for untold generations in the past. SUMMARY There are three zoological realms and five zoological regions into which the world is divided. Free dispersal between regions is prevented by the major barriers of oceans, mountain ranges, and des- erts; each region is thus characterized by distinct orders and families of organisms. Within each region secondary barriers separate divisions of lesser rank characterized by genera, species, and subspecies. Of the Nearctic region of North America, such differentiation has prompted the recognition of faunal areas, life-zones, and biotic provinces. In contrast to the analysis of fauna in terms of geographic units is analysis in terms of the elements that it possesses. A faunal element is a group of species coming from the same center of origin and having similar geological histories. Zoogeography is the study of animal distribution in terms of centers of origin, dispersal routes, and evolution. The ecological system emphasizes envi- ronmental relations and community dynamics. The principal unit in the ecological system is the biome. A biome is a major biotic community char- acterized by distinctiveness in the life-forms of the important climax species. Seral communities are de- velopmental stages of the biome. The biome is di- vided into plant associations, distinguished by uni- formity in the species composition of the climax plant dominants, and into biociations, identified by uni- formity and distinctiveness in the species composi- tion of the climax community, particularly of the ani- mal predominants. Some nine terrestrial and four marine biomes are recognized, to the study of which we now proceed. Distributional units 219 yl Paleo-ecology Now that we have adopted the biome system as the point of departure for analysis of animal distribu- tion, it is essential to learn something about the geo- logical history of these community units: how they were first formed, when they first became well de- fined, how they dispersed over the world, and why they came to occupy their present locations. When we know the origin and geological history of vegeta- tional communities on land, we will be better able to understand the origin, differentiation, and present- day distribution of the animal species that are com- ponents of these communities (Epling 1944). A re- view of geological succession generally will be helpful, although we will be mostly concerned with trac- ing the origin and historical development of the bi- omes during the Tertiary Era alone (see Table 3-1). PHYSIOGRAPHIC CHANGES At the beginning of the Tertiary, some 60-70 million years ago, the interior of the North American continént was still widely inundated by the epicontinental seas of the Cretaceous period. As these seas gradually receded, the continent acquired its modern topographic appearance. The Mississippi embayment area is an extension of the coastal plain that continues around the Gulf of Mexico and north- ward along the Atlantic coast. This coastal plain emerged progressively throughout the Tertiary Era. Its general character is much the same now as it has always been— tidal salt marshes and estuaries inter- mingled with shallow lagoons bounded by off-shore bars. The Appalachian Mountain System first appeared near the end of the Paleozoic era and had become eroded to a peneplain by the beginning of the Ter- tiary. A new uplift then occurred, and erosion again followed. In the Miocene, only the Schooley pene- plain, a nearly level surface only slightly above sea level, remained. However, monadnocks, hills of re- sistant rock rising some hundreds of meters, were left projecting out of the Schooley peneplain. Mount Monadnock, the White Mountains, Great Smokies, Cumberland Mountains, among others, are such for- mations. The Schooley peneplain subsequently un- derwent a series of archings and uplifts until it reached some 1200 m (4000 ft) above sea level along the central axis to give the region its present- day character. In New England, Pleistocene glacia- tion covered these mountains, rounded them off, scraped away the old soil, and left a poorer soil full of boulders. The Ozark and Ouachita Mountains were also formed at the close of the Palaeozoic, underwent sub- 280 sequent erosion, and experienced minor uplifts. Else- where in the central interior between the Appalachian Mountains and the Great Plains, peneplanation was the dominant force throughout the Mesozoic and Ter- tiary. Low coastal marshes extended around the Mississippi embayment, and marshes and swamps were frequent elsewhere. During the Pleistocene, glaciers moved tremendous quantities of soil and rock from Canada southward and from mountain ridges into valleys. The retreat of the ice front pro- ceeded haltingly with alternating retreats and ad- vances. When it was stationary but melting, the glacier formed concentric terminal moraines; when in active retreat, the glacier left a thick layer of till in its wake. The Laramide orogeny, which occurred at the end of the Cretaceous period, formed a series of mountain ranges in the Rocky Mountain system, in- cluding the Big Horns, Wind River, Black Hills, Uintas, and the series of more or less parallel ridges in the Great Basin. Some of these mountains were high enough to support glaciers during the Paleocene. Rapid erosion filled the deep basins between the mountains with debris. By the Eocene, the moun- tains were much reduced, and by the Oligocene pene- planation was complete, although the surface was several hundred meters above sea level and monad- nocks remained. This peneplain extended to the Pa- cific Ocean. Beginning in the Miocene, increasing in intensity through the Pliocene into early Pleistocene, but de- creasing since, mountain formation was extensive not only in the Rocky Mountains but also in the Appa- lachians, Ozarks and QOuachitas, Cascades, Sierra Nevada, and Coast Ranges. Volcanic action was ex- tensive in the West, especially in Oregon and Wash- ington. Highly fluid basalt welled out of long fissures in the earth’s crust, filled valleys, altered drainage systems, and formed sheets up to 1500 m (5000 ft) thick over 80,000 sq km (200,000 sq mi). Mount Rainier, Mount Hood, and Lassen Peak were among these volcanoes. During early Tertiary, the Sierra Nevada and the Cascade Mountains were probably only low ranges which were peneplaned by the Mio- cene, but then both the Sierra and Cascades were up- lifted by faulting and tilting so that the eastern edge of the block was 4000 m (13,000 ft) above sea level. During early Tertiary, the area of the present Coast Ranges was in part an island archipelago, sepa- rated from the coast by a deep sea trough that is now the interior of California. Folding and faulting in the Coast Ranges began in the Miocene and were most active in the Pleistocene. Such activity is still going on as evidenced by the recent earthquakes. The Coast Ranges are the youngest mountains in North America. CLIMATIC CHANGES The climate 60-70 million years ago can only be deduced from sediment types and plant and animal fossils. After peneplanation of the western mountains there was little to obstruct the warm, moisture-laden, westerly Pacific winds sweeping across the conti- nent. Rains were heavy, and fell frequently through- out the year. The Mississippi embayment helped to maintain a uniform oceanic climate. Tropical conditions extended as far north as the Dakotas and Vermont, and temperate conditions obtained nearly to the North Pole. With the elevation of the western mountains in the Miocene and Pliocene, especially the Sierra Nevada and Cascades, the westerly winds were forced to high elevations, cooled, and lost much of their moisture as precipitation on the windward western slopes during the winter; dry seasons pre- vailed during the summer. On the lee eastern moun- tain slopes, arid conditions developed because the winds, warmed as they descended the mountain, re- tained what moisture remained in them, thus pro- ducing a rain-shadow. Dry plains and desert thus developed in the Great Basin. More moisture precip- itated as the winds crossed the Rocky Mountains. Mixing of the westerly winds with winds from the North and South, however, produced less aridity east of the Rockies than in the Great Basin, and east of the Great Plains the western mountain rain- shadows had no effect. Concurrent with increasing aridity over the con- tinent was cooling of the air. This began in middle or late Oligocene and brought a gradual southward shift of climatic belts which culminated in the very severe glaciation of the Pleistocene. The actual cause of the glaciation is obscure, but there is no doubt that the glaciation was accompanied by a drop in average temperature, and an increase in annual precipitation (Ewing and Donn 1956). EARLY TERTIARY FLORAS The geological record of Tertiary plants is good, particularly in the western United States. Vol- canic ash, lake deposits, coastal swamps, and river basins preserved fossils well, more or less in situ. These fossils indicate that there were three principal floras, geofloras, or groups of plants that maintained identity together over wide ranges of space and time (Chaney 1947). It was from these floras and the faunas that they contained, that the modern vegeta- tion types, biomes, plant associations, and biociations differentiated. Doubtless there was some latitudinal and altitudinal zonation in the early floras, but the development of present-day community units is the re- Paleo-ecology 28] sult of later; rigorous climatic zonation, and a more extensive physiographic diversification over the con- tinent than prevailed in the early Tertiary. Instead of evolving new tolerance limits, species, with some exceptions, dispersed into those regions that con- tinued favorable to them and became extinct in re- gions that became unfavorable. Species with similar ranges of tolerance thus came into association and, as interdependent coactions became established, into closely knit communities. Neotropical-tertiary flora This flora is known from several Paleocene, Eocene, and Oligocene deposits (Berry 1937). It was composed of tropical and subtropical plants now limited largely to southern Florida, Mexico, and the tropics. Its counterpart, the Paleotropical-tertiary flora, occurred in western Europe. Trees character- istically had broad, thick, evergreen leaves. The laurel familly, Lauraceae, was particularly well devel- oped in North America, and some modern descend- ants are found in the temperate flora. The Neotrop- ical-tertiary flora extended from the tropics as far North as at least 49° latitude in the West and 37° latitude in the East. With the drying and chilling of the continent that began in late Oligocene and Mio- cene, this flora retreated southward and eastward to the localities in which it is found today. Arcto-tertiary flora This flora completely encircled the North Pole, except for the Atlantic Ocean, with little variation in composition or character. Plants migrated freely across the Bering land bridge between North Amer- ica, Asia, and Europe. Probably no land bridge ex- isted directly between North America and Europe during the Tertiary (Lindroth 1957). On the Arctic islands and in northern Siberia, the flora reached North to within 8° latitude of the North Pole. How these species tolerated the long seasons of darkness is a problem; possibly, daily photoperiods, as well as temperature, were different then than now. An eco- tone with the Neotropical-tertiary flora began at 57° latitude on the Pacific coast and 51° in southern British Columbia, Alberta, and Saskatchewan. In Asia, an ecotone began at 42° latitude in Manchuria ; in central Europe, at 48°—50°. Much of the present similarity between eastern North American and Eurasian floras and faunas may be traced to the continuous and extensive distribution of the Arcto-tertiary flora during the early Tertiary. Types that occur prominently in eastern North America and eastern Asia, for instance, are, among the plants, tuliptrees, magnolias, sweetgums, sassa- frasses, witch-hazels, and partridge-berries; among the animals, paddlefish, alligators, fresh-water tur- tles, lizards (Leiolopisma, Eumeces), snakes (Na- trix, Opheodrys, Elaphe, Agkistrodon), hellbender (Schmidt 1946); as well as various birds and mam- mals. The chief differentiation of the Arcto-tertiary flora was latitudinal into boreal and temperate units. The boreal unit contained such trees and shrubs as the M/etasequoia, baldcypress, pines, spruces, willows, birches, and hazels. The temperate unit included maples, alders, birches, hornbeams, chestnuts, dog- woods, hawthorns, beeches, ashes, walnuts, pines, sycamores, poplars, oaks, willows, baldcypress, bass- woods, elms, and Metasequoia. Although there was some mixture, deciduous species predominated in the temperate unit; the boreal unit contained relatively more coniferous species. These same latitudinal rela- tions obtain at the present time. The climate of the temperate unit was probably humid, with summer rainfall, and with moderate temperatures not regu- larly falling below freezing (Chaney 1948). With the elevation of the western mountains in the Miocene and Pliocene and the drying and cooling of the climate, the Arcto-tertiary flora retreated from the far North, and the American portion lost its con- tact with Asia. In North America, the main move- ment of the temperate unit was to the South and East, but a secondary movement of broad-leaved evergreen and deciduous trees and shrubs proceeded southward along the moister mountain slopes, val- leys, and Pacific coast. Beech, basswood, elm, and hornbeam disappeared from the West, probably be- cause of the lack of summer rain there, but were prominent in the movement to the Southeast. Meta- sequoia, on the other hand, became limited to the Pacific coast in North America, and evolved into our present day redwood forests. Living Metase- quoia are still to be found in central China. The boreal unit followed closely behind the temperate unit; and in the higher elevations of the mountains, it extended as far southward as the temperate. There were changes in the taxonomic composition of the Tertiary forest during this long period. Some genera were lost entirely, some new ones were added, others were changed by evolution; but all these changes were conservative, and the present-day mixed meso- phytic forests of the Cumberland Mountains of east- ern Tennessee have nearly the same composition and appearance as did the ancient forests. A related Antarctic-tertiary flora, derived from the Cretaceous flora and containing both conifers and deciduous trees, occurred in the Southern Hemisphere, but its history during the Tertiary was entirely independent of the Arcto-tertiary flora. 282 Geographic distribution of communities Madro-tertiary flora The Madro-tertiary flora had its center of origin on the Mexican plateau in the region of the Sierra Madre, perhaps beginning in the Eocene in scattered dry sites on the lee sides or rain-shadows of high ridges and mountains. Its history previous to the Miocene is obscure. The flora contained a variety of small trees, shrubs, and probably some grasses, although the fossil record of grasses is poor. These species seem to have been derived principally from the Neotropical-tertiary flora in response to increasingly arid environments (Axelrod 1958). FIG. 2|-| North America dur- ing the early Eocene, showing the configuration of the con- tinent (tinted area, superimposed on an outline of the continent at present) (Schuchert 1955), and floral units (from information given by Chaney 1947). The Madro-tertiary flora was begin- ning to differentiate in small scattered areas within the general area indicated (Axelrod 1958). Minor elements of this flora extended into the Great Basin area, but its main movement northward oc- curred in the Miocene and Pliocene. During the latter epochs, the flora came to occupy large areas in southern California, the Great Basin, and the Great Plains, areas which were being vacated by the other two Tertiary floras because of the increasing aridity. Derived in large part from this flora are the present- day communities of woodland, chaparral, sagebrush, subtropical scrub (thorn forest), desert, and arid grassland. These communities are relatively young, as distinct entities ; the North American desert biome, for instance, is probably not older than Upper Plio- cene (Axelrod 1950). Paleo-ecology 283 THE PLEISTOCENE Physical conditions The Pleistocene was marked by a series of great climatic fluctuations throughout the world (Flint 1957). Thirty-two per cent of the land area of the world was buried under glacial ice ; 10 per cent still remains ice-covered. In places, this ice reached a thickness of at least 1500 m (5000 ft), roughly the thickness of the ice sheets now on Greenland and Antarctica. Outwash from the glaciers carried sedi- mentation hundreds of kilometers beyond the farthest ice boundaries. Sea-level fell to 138 m (450 ft) below the present level as water became bound in glacial ice. This resulted in exposed coastal plains all around the world to an extent greater than at present (Russell 1957). There were widespread back and forth move- ments of animal and plant species as glaciers alter- nately advanced and retreated. There is good reason to believe that North Amer- ica was connected by a land bridge to Siberia over the Bering Sea and Strait at times of glacial advance, but not during interglacial periods (Hopkins 1959). The climate over this bridge was probably similar to that associated with tundra vegetation and not warm enough for forests to develop. According to one theory (Ewing and Donn 1956), the Arctic Ocean remained open water and largely free of ice during the time of the glacial advance and its eventual freezing over, thus cutting off the supply of moisture as precipitation, brought the glacier to a halt and then to retreat. A reconstruction of the appearance of North America at the time of the last (Wisconsin) glaciation is shown in Fig. 21-2. Possibly the Arctic Archipelago was not covered with ice (Antevs 1929). It is conjectured that the shores of the Artic Ocean may have had a reasonably mild climate during the advance of the glacier. Of the rest of the world, it is interesting to note that the British Isles were con- nected at this time to continental Europe (Antevs 1929) ; the Indo-Malayan Archipelago extended as dry land to include Sumatra, Borneo, and Java; and New Guinea was connected to Australia (Mayr 1944). There were a number of successive thrusts of glacial ice, both in North America (Nebraskan, Kansan, Illinoisan, Wisconsin) and in Europe (Gtinz, Mindel, Riss, and Wirm in the Alps; Pre-Elster, Elster, Saale, and Weichsel in the northern coun- tries), that came at different times and extended dif- ferent distances southward. Glacial and interglacial stages were probably synchronous in North America and Europe. During the interglacial periods the biota reoccupied the newly uncovered areas as the glacier melted back, only to be driven out as the glacier again advanced. It is possible that at the present time we are in an interglacial period, and in a few thousands of years the northern parts of the continent will again be covered with ice. We are chiefly concerned with the last glaciation, which began perhaps 60,000 years ago and reached its maximum extension 18,000 or more years ago. In North America, the Wisconsin glaciation is di- vided into the Iowa, Tazewell, Cary, Mankato, Vald- ers, and Cochrane substages. Each substage repre- sents a separate glacial advance, one separated from its predecessor and successor by warm periods dur- ing which the glacial front retreated various dis- tances. The first two glacial stages were the most extensive ; the Mankato (at peak about 13,000 years ago) and the Valders (about 10,700 years ago) advances reached as far as the Great Lakes; the Cochrane glaciation (at peak about 7000 years ago) reached only slightly south of James Bay. Glaciation was extensive in northern Europe, but not in north- ern Asia, and occurred southward in the higher moun- tains. It is estimated that, in northern Ohio, the ice ad- vanced during certain stages of the Wisconsin glacia- tion at the rate of 100 m (350 ft) per year, and in southern Ohio at from 12 to 33 m (38 to 108 ft) per year (Goldthwait 1959). There is evidence that the advancing ice lowered the temperature suffi- ciently ahead of it—for a distance of 800 m (0.5 mile )—to decrease the annual growth of spruce and other coniferous trees but not to kill them until the glacier actually overrode and destroyed the forest (Burns 1958). The drop in mean annual temperature over tem- perate North America is estimated to have been 5° to 10°C, but was probably greater at the edge of the glacier (Dillon 1956). Superficial oceanic water lay- ers in the tropics dropped approximately 6°C (Emil- iani 1955). The gradient from low to higher tem- peratures at the glacial front was probably steep. Storm tracks in North America during maximum glaciation extended from the west and southwest to the east and northeast; thus, warm winds were brought against the front of the glacier. There is controversy as to how far south of the glacier, the high pressure anticyclonic conditions developed by the great ice mass were felt (Hobbs 1926). Precipitation appears to have been comparatively heavy during the glacial stages over much of the world. Accumulation of unmelting snow in the North, as the result of increased precipitation, built up the great glacial masses. Even in areas where continental glaciation did not occur (Africa and Aus- tralia, for instance), variations between pluvial and interpluvial periods probably coincided with the gla- cial and interglacial periods and produced far-rang- ing effects on the geographic dispersal of organisms 284 Geographic distribution of communities nl i 3 FIG. 21-2 Conjectural map of vegetation in North America at the time of the maximum Wisconsin glaciation (from informa- (Moreau 1933). Lakes Bonneville and Lahontan, as well as many smaller ones, were formed in the Great Basin of North America during these pluvial periods (Meinzer 1922, Hubbs-and Miller 1948). There was considerable alteration of drainage pat- terns over the northern part of the continent. Old river valleys were filled or dammed by ice or mo- raines. New outlets were formed. Rivers previously separated became connected. The retreat of the gla- cier left vast level areas without drainage so that many lakes, swamps, and bogs remain in northern glaciated areas. In other places the large quantities of melt water cut new channels or widened old val- leys, through which the surplus water was trans- ported to the sea. Silt, sand, and gravel were spread out in outwash plains, from which winds picked up the finer material and deposited it elsewhere as loess in layers up to 2.5 or 3 meters thick (100 in.) over hundreds of square kilometers. The treatise of ‘Thienemann (1950) is an extensive account of what happened to the fresh-water fauna. = Deciduous forest IllIII| Coniferous forest “eee, Desert Glacier 23% Grassland HB Lokes, rivers HHH Mixed forest = Woodland, chaparral afe"s Tundra tion given by Flint 1952, 1957, Meinzer 1922, Hobbs 1950, Braun 1950, Hansen 1947). Terrestrial biota and communities Virtually all the fossil plants and mollusks dur- ing the Pleistocene are represented by living relatives (Baker 1920). Changes of ecological significance are, for the most part, in point of geographic dis- tribution rather than organic evolution. But insects, especially beetles, which are well represented in the fossil record evolved rapidly into new forms. Many mammals became extinct. Large mammals present during early stages of the Pleistocene, but no longer occurring in North America, include camels, horses (one species later re-introduced), ground sloths, two genera of muskoxen, peccaries, a giant bison, a giant beaver-like animal, a stag-moose, several kinds of cats, mammoths, and the mastodon. By the Pleistocene, there was doubtless a broad zone of coniferous forest across the northern part of the continent, and perhaps some tundra. Deciduous forest covered the eastern states; grassland occurred in the central part of the country; and coniferous Paleo-ecology 285 forest was extensive in the western mountains and on the Pacific slope, much as at present. Glaciation destroyed the coniferous forest over vast areas in the north, but there is considerable con- troversy as to the area south of the glacial boundary in which the deciduous forest was thus affected. In- terpretation of the probable climate, of past and pres- ent distribution of plants and animals, and of the pollen record in bogs indicates that, in North Amer- ica, the deciduous forest was not extensively dis- placed, but that it became mixed with coniferous species to varying distances south of the margin of the glacier (Epling 1944, Hansen 1947, Braun 1950, Thomas 1951). Enormous amounts of cold water, melted from the glacier during the summer months and, perhaps, carrying chunks of ice, drained down the Delaware and Susquehanna Rivers in the East, the Ohio, Wabash, Illinois, Missouri, Platte, and Mississippi Rivers in the central part of the continent (Hobbs 1950), and the Snake and Columbia Rivers in the Northwest. Water filled these wide river valleys from the present bluffs on one side to the bluffs on the other, and doubtless extended the boreal microclimate for many kilometers (Wolfe 1951), perhaps permit- ting the establishment of coniferous trees and other northern species on their banks. The Atlantic coastal plain was exposed by the falling sea level. Cold gla- cial waters draining southward between the coast and the Gulf Stream probably created a microclimate of a type permitting northern species, including conifers, to invade the coastal plain as far south as Florida. A tree line existed above 1200-1500 meters (4-5000 ft) on the higher Appalachian peaks. Conif- erous forests that are now limited to the higher ele- vations of the Appalachian Mountains descended the mountain slopes perhaps as much as 600 meters (2000 ft) and covered large areas. In the moun- tains all over the world, the snow-line (Klute 1928) and biotic zones (Murray 1957) were at least 500 meters (1600 ft) and in some humid localities possi- bly as much as 1500 meters (5000 ft) lower than they presently are. The occurrence of pollen and the re- mains of spruce and fir in bogs and glacial deposits in Texas, Louisiana, Florida, North Carolina, and elsewhere on the coastal plain can probably be ex- plained as the result of southward boreal forest intru- sions that did not completely displace the deciduous forest. The ranges of many animal species also ex- tended farther south during the height of glaciation than they do at the present time. Loess was deposited extensively from Wisconsin to southwestern Indiana and west into Nebraska and Kansas (Flint 1957). This buff-colored, homogene- ous, porous, calcareous, non-stratified deposit forms only in arid or semi-arid regions, and is indicative of grassland abutting directly on the glacial front. There is no evidence that extensive coniferous forest existed in front of the glacier during any of its ad- vances in this area. The occurrence of snails in Pleistocene deposits in Kansas indicates that during Wisconsin time, for instance, open prairie occurred on the upland and deciduous woodland along streams much as they do at the present time (Frye and Leonard 1952). With the greater precipitation that was generally prevalent, some of what is now desert in the Great Basin and the Southwest was probably grassland then. Evidence is scanty for the existence of tundra in North America south along the ice front during its advances, although tundra occurred in Alaska dur- ing the last Wisconsin stage. Tundra mammals, such as the muskox and woolly mammoth, are well represented as fossils along the old glacial margins, and a few fossils of these species have been found as far south as Texas, Mississippi, and Florida ( Potzger 1951). These species, however, are believed to be derived from grassland forms, and it appears that the distinction between grassland and tundra faunas did not become sharply defined until late in the Pleisto- cene (Hibbard 1949). In North America the coniferous forest survived glaciation in four separated refugia (Adams 1905, Halliday and Brown 1943). These refugia were in the region of the northern Appalachians, the northern Rockies, the Pacific slope of the Cascades, and Alaska (Hultén 1937). The Appalachian refugium during the Wisconsin epoch extended westward south of the Great Lakes, but was separated from the Rocky Mountain refugium by grassland. Except for the unglaciated pocket in southeastern Minnesota, southwestern Wisconsin, and northwestern Illinois, it appears that forests were absent along the ice front all the way from Illinois to the Rocky Mountains. The Rocky Mountain refugium was separated from the Pacific refugium by the Great Basin, the high peaks of the Cascades and Sierra, and by seasonal differ- ences in precipitation. Probably this separation was only partially effective as a narrow belt of coniferous forest extended around the north border of the Great Basin. Coniferous forest in the Pacific refugium probably extended some hundreds of kilometers far- ther south than it does at the present time. The Alaskan refugium was probably connected by a land bridge across the Bering Sea with unglaciated areas in Asia. Fossil remains found in frozen muck and silt indicate the probable occurrence in Alaska during glacial periods of woolly mammoth, muskox, reindeer, and many other forms (Flint 1952). Our description of how biotic communities were affected by the Pleistocene glaciation is not univer- sally accepted. The southward extrusions of conif- erous forest along the Mississippi River and the At- lantic coastal plains are conjectural. According to 286 Geographic distribution of communities *($56)] neesow '7G6) |JO4, Pue jezuesy Aq ueaib UOIFEWOJU! WoJ}) UOlyeIDe/H aUar0jsiajq ee) WNWIxewW ayy jo Owl} O4f 4 Llseiny ul uolyeyaboa yo dew jeinjoeluod £-|7 “O|4 vapun}-}se105 Ree 18910} poxTw Hit purisseip Bae rayoVID jzeseq {S910} SnosesqUOD 189.10} paaval-prorg 287 ecology Paleo FIG. 21-4 The largest black area shows the present main range of the eastern four-toed salamander. The smaller spots in the southern states represent boreal relicts of a wide southern dis- tribution during the height of the glacial advance. The isolated group in Nova Scotia may also represent a relict from a more northern dispersal of the species during the post-glacial climatic optimum (Smith 1957). Deevey (1949) and Dorf (1960), the climate every- where south of the glacial front was considerably re- frigerated, deciduous forest was driven into refugia in Florida and Mexico, and tundra and coniferous forest prevailed everywhere in between. Griscom (1950) believes that continental refrigeration ex- tended well into Mexico, causing extensive south- ward dispersal of northern birds and the elimination of the South American element previously present. Speciation supposedly occurred in populations of certain birds (Huntington 1952) and amphibians, reptiles, and mammals (Blair 1958) that became fragmented and isolated from each other in the Southeastern and Southwestern refugia. There is a likelihood, however, that these animals were segre- gated into southeastern and southwestern popula- tions, not by effects of cold climate, but by the southward extension of grassland to the Gulf. It is probable that, during the Pleistocene period, there was either grassland or a tenuous savanna type of habitat in Texas that séparated the forests of Mexico and those of the southeastern states (Martin and Harrell 1957). The broad cold waters of the Mississippi River may also have split and isolated eastern and western populations of some animal species at the times when melting of the glaciers was at its height. In Europe and Asia, westerly winds were di- verted south into the Mediterranean region, and high anticyclonic barometric pressures developing over the glacier brought dry, cold, northeasterly and east- erly winds. Loess was deposited in a broad belt from western France east into the Balkans and northeast into Russia (Zeuner 1945). True tundra graded into loess and bush tundra and coniferous and deciduous forests are thought to have been forced into refugia in Spain, Italy, and the Balkans; there is some doubt, however, that the forests were displaced so far south (Hare 1953). The Mediterranean climate was, how- ever, probably cooler and moister during the height of glaciation than it is at the present time (Zeuner 1945). POST-PLEISTOCENE Retreat of the glacier The melting of the ice was quite rapid, per- haps 134 m (440 ft) per year in the Great Lakes region (Flint 1957). Melt water filled depressions to form vast pro-glacial lakes. Sea-level rose 1 m (3.5 ft) per century between 18,000 and 5000 B.c., but the sea apparently has risen very little since then (Russell 1957). The Great Lakes, in their early stages, had outlets down the Hudson and Mississippi Rivers and had different interconnections than at present. Still later, Lakes Agassiz, Ojibway-Barlow, and others were formed in the north. A knowledge of these lakes and the history of past drainage sys- tems is prerequisite to interpretations of present-day distributions of aquatic organisms. With the melting back of the ice, local glaciers were left in the Catskill Mountains of New York, on Mount Katahdin in Maine, in the Shickshock Moun- tains on the Gaspé Peninsula, in Newfoundland, and in Labrador. The glacier receded faster in the west- ern interior of Canada between the Rocky Mountains and Hudson Bay than it did to the East. There is evidence that the glacier disappeared from the Hudson Bay region while still persisting over the highlands of Quebec and the Labrador Mountains. The last of the glacier still remains on the mountains and plateaus of Baffin, Devon, Ellesmere, and Axel Heiberg Islands, and in Greenland (Flint 1947). Identification of kinds of pollen and comparative counts of pollen grains from various depths in peat bogs gives us a picture of the predominant vegeta- tion, and consequently the climate, in the vicinity of the bogs at various times in the past (Sears 1942, Deevey 1949). A chronology is given for North America and Europe in Table 21-1. The time scale is determined, in part, by measuring the radioactivity of carbon, C!*, obtained from samples taken at various depths in glacial deposits. Radioactive carbon disin- tegrates in non-living matter at a progressive rate; 288 Geographic distribution of communities its half-life is 5760 years. The age of any sample can be determined on the basis of the extent to which it has degenerated (Libby 1960). The clisere As the glacier retreated, vast areas were freed for reinvasion by plants and animals (Adams 1905, Gleason 1922). The land must have been a barren, sterile expanse of raw parent soil material, deficient in nitrogen. The first plants to invade were probably species the root nodules of which bore bacteria, fungi, or actinomycetes capable of fixing nitrogen from the air, thereby enriching the soil (Lawrence 1958). The recession rate of the glacier was probably faster than the advance of vegetation and animal life. A belt of tundra developed; coniferous forest broadened to a much greater width than existed at the peak of glacia- tion. Deciduous forest, requiring a better soil, ame- lioration of climate, and competitive displacement of the already established coniferous forests, moved northward rather slowly. There is evidence that this northward movement of the biota is still in progress, and that the great belts of vegetation are not yet stabilized in respect to each other and to the climate. Fossil or pollen evidence for the existence of tundra along the retreating glacial front is scant in North America, except for certain areas in Maine; existence of tundra is better established in Europe. Special difficulties are involved in the identification of tundra pollen in core samples from bogs. Further- more, deep kettles in which bogs later formed sus- tained large blocks of ice, well insulated by being nearly buried in glacial till, for a long time after the main mass of ice had withdrawn northward. De- posits of pollen could not settle in the kettles until the ice blocks had melted, which was usually not until spruce-fir coniferous forests had become the pre- vailing vegetation of the region. This is doubtless the reason for the almost universal occurrence in North America of spruce-fir pollen in the deepest layer of peat cores. The tundra belt may have been 160 km (100 mi) wide as the ice retreated through New England, but was probably much narrower West of the Appalachians. Presumably, however, forest vege- tation continued to advance onto the tundra along its southern margin, but at a rate slower than the tundra expanded northward as the glacier retreated. Tundra gradually, therefore, became more extensive in the North, and permitted its fauna to expand to its pres- ent-day form. The occurrence of pine pollen in peat cores above the spruce-fir indicates the emergence of a warmer climate, drier as well. Beginning with the first ap- pearance of deciduous tree pollen, there is differen- tiation of the pollen spectrum in different parts of the Lake Chicago ——> Tilinois River FIG. 21-5 The Great Lakes at the time of the Valders glaciation (Hough 1958). country, the nature of which apparently reflects a drier climate in the Mid-western states and a more humid one in the East. During the warm moist climatic optimum, when conditions for forest growth were most favorable, eastern hemlock spread from the northern Appala- chians and became firmly established in New Jersey, New York, New England, and, to a lesser extent, in Ohio. Beech appeared early in New Jersey and spread through New York and Ohio. Animal species extended their ranges northward and withdrew from prep slgeiri, ii Lig i J G Glacier FIG. 21-6 Glacial Lakes Agassiz (A) and Ojibway-Barlow (O-B), and the outlets of each (after Flint 1947). Paleo-ecology 289 TABLE 21-1 Late glacial and post-glacial chronology (modified from Deevey 1949, Deevey and Flint 1957, Flint 1957). Climate Sub-boreal warm, dry (xerothermic period) Atlantic oak, hemlock warm, moist (climatic optimum) Hypsithermal (thermal maximum) 7,000 BC 8,000 BC Pre-boreal spruce, fir 9,000 BC cool, moist pine, oak 10,000 BC spruce, pine, birch 11,000 BC Sub-arctic 12,000 BC deglaciation 13,000 BC 14,000 BC the South. When later forced to withdraw from over-extended ranges to the North, some species left relict populations, which persist to the present time. Following the climatic optimum came a warm, dry climate, called the xerothermic period. The for- est vegetation prevailing from the Mid-west into New England consisted of aridity-tolerant oaks and hickories. The most part of beech withdrew from Ohio, but became well established in the East, where Northeastern Wisconsin and Wasa: Ohio Minnesota ee oak, hickory oak, hickory oak, beech (hemlock) Boreal warmer, dry spruce, fir park-tundra park-tundra Baltic North Germany Basin Mya Sub-Atlantic return of beech, oak Baltic cooler, spruce at some Sea moister localities Limnaea pine declining Littorina Sea Ancylus Lake pine, hazel hemlock suffered for want of moisture. A prairie peninsula penetrated at least as far as Ohio, and probably scattered patches of prairie occurred be- yond. Grassland animals penetrated far to the East (Schmidt 1938, Smith 1957). Boreal forest retreated northward; sugar maple-basswood forests extended far into Manitoba (Jenkins 1950). In Saskatchewan and Alberta, the northward withdrawal of coniferous forest left groves of aspen trees in the moister and 290 Geographic distribution of communities more sheltered locations, while grassland invaded the drier areas (Moss 1944). The numerous lakes in the Great Basin shrank in size or entirely dried up, and desert biota spread both far to the North and high up onto the mountains. Northern species were eliminated from the tops of many southern moun- tains. With the coming of cooler, moister climate in the subsequent Sub-Atlantic period, the prairie peninsula receded, leaving populations of biota, relict today, behind. Beech once again spread westward, followed by hemlock ; hemlock re-established its dominance in the Northeast. In the northern states there is some evidence that spruce again spread southward. Many of the lakes of the Great Basin refilled with water. Within historic time, smaller fluctuations in cli- mate are known to have occurred. These have been determined from growth rings of the giant sequoia trees, lake levels, records of past civilizations such as that of the Maya of Yucatan, as well as inferences from historical documents. In western United States these fluctuations have been dated as follows (Brooks 1949) : Wet, 500-250 B.c. Dry, 250-100 B.c. Wet, 100 B.c—a.p. 200 Dry, a.p. 300-800 Wet, a.v. 900-1100 Dry, A.D. 1100-1300 Wet, a.v. 1300-1400 Dry, A.d. 1450-1550 Wet, a.pv. 1550- Mountain glaciation, especially in the Alps and Iceland, was extensive between 1600 and 1850, but glaciers all over the world have been shrinking since then at a very rapid rate. During the last hundred years, mean annual temperatures have increased 0.5° to 2.2°C, and the sea level has risen about 6 cm (2.5 in.) (Flint 1947, Baum and Havens 1956). This amelioration of climate has permitted the north- ward dispersal of birds and other animals in recent years into Ontario (Urquhart 1957), Iceland (Gud- mundsson 1951), northern Europe (Kalela 1949, Haftorn 1958), and in the sea (Taylor et al. 1957). Other species will doubtless follow in the future; northern communities are not presently saturated with the variety of species they could support. This is true of aquatic communities as well as terrestrial ones. For instance, the fresh-water fish fauna of North America is most highly developed in the Mis- sissippi River system. The impoverished variety of the fish fauna northward and northeastward is in large part due to the failure of fish species to bypass land barriers and to disperse into otherwise suitable waters in these regions since the retreat of the glacier. A northward movement of fauna may be expected to continue until the carrying capacity of the ecosystems is reached, or until there is another reversal in the climate. —<— $5 Aspen parkland IIIll| Coniferous forest =} Deciduous forest HHH Ecotone PE Grassland #@ Southern pine-oak FIG. 21-7 Conjectural map of vegetation during the xerothermic period in eastern North America (from information given by Transeau 1935, Clements 1942, Jenkins 1950, Halliday 1937). The broken line shows the probable extent to which the prairie mas- sasauga dispersed eastward during this period (Schmidt 1938). SUMMARY At the beginning of the Tertiary Era, 60-70 million years ago, the North American continent was widely covered with epicontinental seas, marshes, and lakes. Scattered mountain ranges occurred in the Rocky Mountain region, but these had been greatly eroded by Oligocene time. Rainfall was heavy and temperatures mild. Tropical conditions extended across the continent to 49° North latitude in the West, and 37° North latitude in the East; temper- ate climates obtained nearly to the North Pole. Three principal floras occurred during early Ter- tiary time. The Neotropical-tertiary flora was co- extensive with the tropical climate. The Arcto-ter- tiary flora consisted of a temperate unit, largely deciduous forest, and a boreal unit, preponderantly coniferous species; this flora extended to within eight degrees latitude of the North Pole and across the Bering land bridge into Eurasia. The Madro-ter- tiary flora first appeared during the Eocene in scat- tered dry sites on the lee sides of high ridges in northern Mexico and southwestern United States, but did not become well developed until the Miocene. Beginning in the Miocene and increasing in in- tensity through the Pliocene into early Pleistocene, Paleo-ecology 29] FIG. 21-8 Postulated post-glacial dispersal movements of two sub- species of the chorus frog, Pseudacris triseriata, in (a) the climatic optimum, (b) early xerothermic, (c) xerothermic maximum, (d) post-xerothermic, (e) the present. The population of triseriata left behind in New Jersey has recently been recog- nized as the subspecies kal/mi (Smith 1957). mountain-building was extensive in the Rocky Moun- tains, Appalachians, Ozarks and Ouachitas, Cascades, Sierra Nevada, and Coast Ranges. The epicontinen- tal seas receded. The climate in the rain-shadows of the mountain systems became increasingly arid, par- ticularly in the Southwest, Great Basin, and on the Great Plains. Concurrently, the climate became pro- gressively cooler, a trend culminating in the severe glaciation of the Pleistocene. As a result of these changes in physiography and climate, the Neotropical-tertiary flora retreated to the present tropics to constitute the tropical forest and tropical savanna biomes of today. The Arcto- tertiary forest withdrew southward and eastward to form the temperate deciduous forest, coniferous forest, and tundra biomes. Into the areas vacated by these two floras the Madro-tertiary flora expanded to form the woodland, chaparral, grassland, and desert biomes. At maximum glaciation during the Pleistocene, the tundra biome was greatly restricted in North America and the coniferous forest biome was mostly destroyed, except in refugia in Alaska, the northern Appalachians, the northern Rocky Mountains, and on the Pacific coast. Everywhere it extended to ‘AN ——>> SS Fa lower elevations in the southerly mountain areas. The deciduous forest was modified by intrusion of coniferous forest species, but was not otherwise greatly disturbed, either in its extent or its composi- tion. Because of heavy precipitation, grassland was more widely distributed through the Great Basin and the Southwest. With the retreat of the glacier in post-Pleistocene times, the tundra and coniferous forest biomes re- occupied most of northern North America. In the northern states from Minnesota and Illinois east- ward, pollen data indicate changes of climate from cool-moist to warm-moist to warm-dry, then back to the cooler, moister conditions of the present time. Accompanying these climatic changes was a succes- sion of vegetation from spruce-fir to pine to oak- hemlock-beech to oak-hickory and the prairie penin- sula, then back to oak-beech. Comparable changes in climate and vegetation occurred in Europe throughout the Tertiary and Quaternary eras. These changes in climate and veg- etation had a profound effect both in Europe and North America on the evolution and dispersal of animals; and, by so much, on the development of present-day animal communities. 292 Geographic distribution of communities Tem perate Deciduous Forest Biome In this and the following chapters we will try to gain an understanding of the geographic distribu- tion of animals as it occurred in primeval time before the colonization of the continent by white man, using biomes as our units of analysis. Each biome will be considered in respect to its distribution, vegetation, and plant associations ; the constituents of its various biociations ; the relative abundances of the principal animal species, especially mammals and birds; the adaptations and adjustments, especially behavioral, to the biome as demonstrated by the predominant animals ; and human uses made of it. We will devote most of our study to biomes of North America, but the rest of the world will not be neglected. A general reference which the reader will find invaluable is The Naturalist’s Guide to the Americas (Shelford 1926). The temperate deciduous forests of North Amer- ica, western Europe, eastern China, and Japan are related as developments of the Arcto-tertiary flora which at one time was practically continuous around the world in North Temperate climates. In North America, the deciduous forest is best developed in the Eastern United States, although elements of it are mixed with conifers in the North, West, and through the mountains of Mexico into Guatemala (Sharp 1953). The deciduous forest of southern Chile is derived from the Antarcto-tertiary flora, dis- cussion of which we must forgo. Mean annual precipitation for the biome in North America varies from 75 to 125 cm (30-50 in.) ; mean annual precipitation for the Gulf states is occasionally as high as 150 cm (60 in.). For the most part, rain falls periodically throughout the year ; in many places, precipitation also falls as snow in wintertime. Mean monthly temperatures from North to South vary from January minima of —12° to 15°C (10° to 60°F) to July maxima of 21° to 27°C (70° to 80°F). Average mid-day relative humidities during July range from 75 per cent in the East to 50 per cent where the biome contacts prairie in the West. The annual frost-free period varies from about 150 days in the North to as much as 300 days in the South (Kincer 1941). The climax of the deciduous forest biome is a community dominated by broad-leaved trees that are leafless during the winter over most of the area. In the South, the dominant trees are mostly evergreen. The trees usually form relatively dense forests with a closed canopy, but where the biome verges on prairie, the forest gives way to savannas containing scattered groves. The shrub stratum is often but poorly developed within the forest because of the deep shade there, but is well-formed at the forest- edge. The herb stratum has a rich variety of flower- ing plants, which are especially conspicuous in the spring. All seasonal aspects are well defined. The leaves of the trees and shrubs, as well as those of most 293 FIG. 22-1 Frequent ground fires prevent the southeastern pine forests from succeeding into a deciduous forest climax (cour- tesy U.S. Forest Service). herbs, are intolerant of freezing temperatures over- winter and hence are shed in the North during the autumnal aspect. Consequently there is considerable seasonal change in forest microclimates, to which animal life must respond. The growing season is sufficiently long to permit full development of new foliage and maturation of seed each year, although the size of the seed crop, upon which many animals depend, varies greatly from year to year. PLANT ASSOCIATIONS IN NORTH AMERICA The principal plant communities are the following (Braun 1950, Shelford MS). Liriodendron-Quercus association: mixed mesophytic forest. Centrally located on unglaciated Appalachian Plateau. Contains a rich mixture of tree species, white basswood and yellow buckeye are best indicators of the association. Quercus (Castanea) association: formerly called oak-chestnut forest but chestnut now largely destroyed by blight and its place in canopy taken by oaks and other species, best developed in Appalachian Mountains (Woods and Shanks 1959). Quercus-Carya association: oak-hickory forest. Center of distribution in Ozark and Ouachita Moun- tains but radiating far into the prairie along river valleys and into Gulf and South Atlantic states. Fagus-Acer association: beech-maple forest. Mostly northern in distribution ; two principal climax dominants only. Acer-Tilia association: maple-basswood forest. Occurs mainly in Wisconsin and Minnesota and southward to northern Missouri. Tsuga-Pinus-northern hardwoods ecotone: mixture in southern Canada and in the Appalachian Mountains of beech, sugar maple, and basswood with eastern hemlock, various northern species of pine, and yellow birch. Pinus-Pinus associes: southeastern pine forest. Southern species of pines, often mixed with oak. Form extensive subclimax stands in the south At- lantic and Gulf states. Where fire is prevented, this community is succeeded by oak-hickory, beech, or magnolia-oak forest. Magnolia-Quercus association: magnolia-oak forest. Found in southern portions of Gulf states and most of Florida. Dominant trees are coriaceous, broad-leaved, and evergreen; forests often dense, with deep shade, with Spanish moss and other epi- phytes hanging from branches; grading southward into tropical forest (with royalpalm) in Everglades and Florida Keys. Early seral stages include fresh- water marshes and cypress swamps, pine flatlands, scrub oak, patches of prairie, coastal dunes, and salt marshes (Davis 1940). 294 Geographic distribution of communities Ty doy it Wnt OCF Gi = EOD" Aie 0 iy @) CH 6 at oN ii rail ) WT, muy AREER ine ¥ af co TN Nadal iy \ \ eas i Ain ‘ali ental ey Neil Len) FIG. 22-2 View of an idealized mountain and valley of the Great Smoky Mountains, looking east: (Whittaker 1956). GB grassy bald H hemlock forest HB heath bald OCF oak (chestnut) forest BG beech gap forest CF cove forest F Fraser fir forest ZONATION Climate varies with altitude in kind as it does with latitude; most notably, air temperature varies inversely with altitude. Because of this, there are corollary changes in vegetation such that con- spicuous zonation is apparent. Zonation of vegeta- tion and differences in climate profoundly affect ani- mal distributions. In the Great Smoky Mountains of eastern Ten- nessee, there are two zones, differentiated essentially by temperature. Each is characterized by a circum- ferentially heterogeneous vegetation different from that of the other. On north slopes of the mountains the demarcation between them is approximately the 1400 m (4500 ft) elevation. The lower zone is mostly deciduous forest, grading laterally from moist mixed mesophytic or cove forest on the north slopes through oak-hickory and oak (chestnut) to southern pine forest and grassy balds on the warm, dry, south slopes (Fig. 22-2). The vegetation of the upper zone also changes, north to south, as moisture conditions change: gray beech forest on the north and in the moist mountain gaps gives way to spruce-fir forest, which in turn changes into heath balds on the ex- posed southern slopes (Whittaker 1952, 1956). Contrastingly, each of the several zones of New York’s Catskill Mountains is characterized by a cir- cumferentially homogeneous vegetation different from that of the other zones. Below 230 m (750 ft) deciduous forest prevails ; between 230 m and 610 m (2000 ft) there is an ecotone of beech-maple-hem- lock ; then comes a zone where hemlock drops out and the forest is principally gray beech, sugar maple, and OCH oak (chestnut) heath ROC red oak (chestnut) forest OH oak-hickory forest S spruce forest P pine forest and SF spruce-fir forest pine heath WOC white oak (chestnut) forest yellow birch. Above 980 m and extending to 1280 m (3200 to 4200 ft), the deciduous forest is replaced by spruce-fir coniferous forest (Kendeigh 1946). ANIMAL COMMUNITIES North American deciduous forest biociation This biociation occurs in the climax and late seral stages throughout the deciduous forest proper. It extends into the pine-hemlock-hardwoods ecotone, although locally within the ecotone there is rather sharp segregation of many animal species according as they are frequenters of deciduous or coniferous forest (Kendeigh 1946, 1948). The community is repre- sented as a biocies in the aspen-birch seral stage of the boreal forest. The biociation penetrates well into the magnolia-oak association in the Gulf states, but becomes progressively more impoverished southward as species drop out. To the West, the community occurs in the wider strips of forest along the streams, but as the forest diminishes in density westward, the forest-edge biociation replaces the forest biociation. Mammal species that occur or formerly occurred through the deciduous forest biociation include: Eastern mole Mountain lion Southern flying squirrel Eastern chipmunk Bobcat Raccoon Gray fox Oppossum Black bear Short-tailed shrew Gray squirrel White-footed mouse Temperate deciduous forest biome 295 FIG. 22-3 Two predators of the deciduous forest biociation: the timber rattlesnake, and the great horned owl (courtesy U.S. Forest Service). | . a ‘ The mountain lion, bobcat, and black bear are also common in other bibmes but the other species listed are characteristic inhabitants of the deciduous forest. Seton (1909) estimated original populations of mountain lions and bears at one per 26 sq km (1 per 10 sq mi), and gray foxes at one per 10 sq km (1 per 4 sq mi). Gray squirrel populations vary greatly by time and place, but when common may average 2.5+ per hectare (1+ per acre). Chip- munks vary in numbers from year to year, depending on the abundance of nuts and seeds that they can store in their underground burrows to supply them over winter. In beech-maple forests of northern Ohio they average 25 or more per hectare (10 per acre) during the autumnal aspect of good years (Williams 1936). The combined autumn populations of mice and shrews vary from less than 25 per hectare (10 per acre) in poorer forests having little ground humus to ten times as many during good years in a good habitat. A gradient of increasing populations, from West to East, depending largely on moisture availability as well as abundance of humus, is marked in shrews (Wetzel 1949). Birds prominent in the deciduous forest biociation include, in declining order of abundance, Ovenbird Black-capped chickadee Red-eyed vireo Yellow-throated vireo Redstart White-breasted nuthatch Wood thrush Eastern wood peewee Tufted titmouse Cerulean warbler Scarlet tanager Great crested flycatcher Acadian flycatcher Downy woodpecker Hairy woodpecker Red-bellied woodpecker Whip-poor-will Ruffed grouse Barred owl Great horned owl Pileated woodpecker Broad-winged hawk (formerly ) The ovenbird and red-eyed vireo are usually the two most abundant species in deciduous forest stands. An average population of each is 35 to 40 pairs per 40 hectares (100 acres). A 40-hectare plat of aver- age deciduous forest supports approximately 200 pairs of birds, representing all species, as an average. The breeding ranges of most of the species listed above coincide rather closely with the deciduous forest, although some species, such as the downy woodpecker, are distributed more widely and are represented by different subspecies in other biomes (Pitelka 1941). Reptiles and amphibians are represented by: Marbled salamander Slimy salamander Red-backed salamander Common newt Wood frog Tree frogs Timber rattlesnake Copperhead Black rat snake Red-bellied snake Five-lined skink Box turtle 296 Geographic distribution of communities Invertebrates are too numerous and varied to be mentioned specifically (see Chapter 9). Snails and slugs are especially abundant in the moist mixed mesophytic forests of the southern Appalachians, but decrease in abundance and variety as the forest be- comes drier and approaches the prairie (Shimek 1930). Millipedes are numerous in the rich humus of the forest floor. Insects and spiders are repre- sented by a multitude of species in all strata. North American deciduous forest-edge biociation Eastern North America, prior to white colo- nization, had thousands of kilometers of contact between deciduous forest and prairie, with tongues of forest extending far into the prairie along the river valleys. Deciduous forest even bordered the prairie on the north where the aspen grove ecotone intruded in front of the boreal forest. A characteristic forest- edge type of vegetation and distinct animal commu- nity occurs along these contacts and where the forest confronts ocean or large lakes. The forest-edge com- munity also permeates the deciduous forest in the role of a seral community or biocies on rock, sand, aban- doned fields, and around water (Chapters 8, 9). A different faciation of the forest-edge biociation occurs west of the Great Plains. As the interior of the continent grew arid in the Miocene and Pliocene, many species of deciduous trees together with their associated animals were able to persist in local habitats throughout the western part of the country. A distinct plant community—riparian woodland—of willows, cottonwoods, sycamores, aspens, alders, and other broad-leaved deciduous trees presently occurs along streams, bodies of water, and elsewhere. It ap- pears to be seral to coniferous forest or woodland over most of the West, but reaches out into grassland and desert in a manner similar to the tongues of forest in the East, thus greatly extending the linear distance of the forest-edge. The animal species composition reflects the rela- tionship obtaining between the riparian woodland in the West and the forest-edge community in the East. Nearly half of the species listed below pervade both faciations, albeit represented by different subspecies. Several species are confined to one or the other facia- tion as indicated. Speciation among forest-edge forms was doubtless encouraged by the virtual isola- tion of both faciations when the grassland biome evolved. Common species (Ingles 1950, Miller 1951) : Mammals Eastern mole (East) Long-tailed weasel Gray wolf Wapiti Red fox Mule deer (West) Temperate deciduous forest biome FIG. 22-4 White-tailed deer in forest-edge habitat (courtesy U.S. Forest Service). White-tailed deer (East) Fox squirrel (East) Woodchuck (East) Eastern cottontail (East) Striped skunk Birds Turkey vulture Sharp-shinned hawk Cooper’s hawk Red-tailed hawk Swainson’s hawk ( West) Red-shouldered hawk (East) Sparrow hawk Bobwhite (East) Mourning dove Yellow-billed cuckoo Black-bellied cuckoo (East) Screech owl Common nighthawk Chimney swift (East) Ruby-throated humming- bird (East) Hummingbirds (several spp., West) Red-headed woodpecker (East) Yellow-shafted flicker (East) Red-shafted flicker (West) Eastern kingbird (East) Western kingbird (West) Cassin’s kingbird (West) Barn swallow Violet-green swallow (West) Common crow Blue jay (East) Black-billed magpie (West) House wren Catbird (East) Brown thrasher (East) Eastern bluebird (East) Robin Chestnut-backed chicka- dee (West) Cedar waxwing Loggerhead shrike Starling Warbling vireo Bell’s vireo (West) Yellow warbler Yellowthroat Yellow-breasted chat Brown-headed cowbird Bullock’s oriole (West) 29, Brewer’s blackbird Black-headed grosbeak (West) (West) Indigo bunting (East) Chipping sparrow Rufous-sided towhee American goldfinch Field sparrow (East) Song sparrow Reptiles Brown snake Garter snake Ribbon snake Blue racer Smooth green snake Milk snake Southeastern mixed biocies A number of animal species have their centers of distribution in the south Atlantic and Gulf states and are associated with the southeastern pine forest, the magnolia-oak forest, or with seral stages. There is, doubtless, more than one community involved, but until more detailed analysis can be made the species may conveniently be listed together. Common terres- trial vertebrates of the southeastern mixed biocies are: Mammals Southeastern shrew Florida least shrew Eastern spotted Golden mouse Florida mouse Pine mouse skunk Hispid cotton rat Florida skunk Marsh rice rat Southeastern pocket Eastern wood rat gopher Round-tailed muskrat Eastern harvest mouse Marsh rabbit Oldfield mouse Swamp rabbit Cotton mouse Birds Black vulture Mockingbird Swallow-tailed kite (formerly ) Mississippi kite (formerly ) Turkey Carolina parakeet (formerly) Chuck-will’s widow Red-cockaded wood- pecker Ivory-billed woodpecker Scrub jay Fish crow Carolina chickadee Brown-headed nuthatch Carolina wren Blue-gray gnatcatcher White-eyed vireo Prothonotary warbler Swainson’s warbler Parula warbler Yellow-throated warbler Pine warbler Prairie warbler Hooded warbler Orchard oriole Boat-tailed grackle Summer tanager Cardinal Painted bunting Seaside sparrow Bachman’s sparrow Reptiles and Amphibians Rough green snake Chicken snake Corn snake Kingsnake Southern hog-nosed Brown skink snake Chameleon Eastern fence lizard Spadefoot toad Six-lined racerunner In addition to these species, many of those listed for the deciduous forest and forest-edge are also common, but frequently represented here by different subspecies than occur in the North. The seral rela- tions of many of the mammals (J.C. Moore 1946, Pournelle 1950), birds (Nelson 1952), as well as cer- tain insects (Rogers 1933, Friauf 1953) have been worked out for various areas of northern Florida. Several of these species of mammals, birds, rep- tiles, and amphibians have dispersed from the South- east far into the deciduous forest and forest-edge communities. Their distributional ranges, in many cases, extend westward into Texas and southward into Mexico. The closest related forms of some of the more restricted species also lie to the West and South, for instance the scrub jay. The evidence is inferential that this biociation and the corresponding plant associations did not originally belong to the deciduous forest biome. It seems more likely, rather, that they belonged to the sclerophyllous woodland and pine forests, derived from the Madro-tertiary, and to the Neotropical-tertiary floras. During the Pliocene or earlier, Madro-tertiary biota may have been continuous around the north side of the Gulf of Mexico (Pitelka 195la), but later separated into eastern and western portions by the development of grassland through Texas to the Gulf of Mexico. The terrestrial fauna indigenous to the southern tip of Florida is predominantly deciduous forest-edge species; species of the southeastern mixed biocies are represented, and there has also been some inva- sion of tropical species. Among birds, the white- crowned pigeon, zenaida dove, smooth-billed ani, gray kingbird, black-whiskered vireo, as well as races of nighthawk and yellow warbler, are recent new- comers from the West Indies (Robertson 1955). There is also a rich and varied aquatic avifauna that is for the most part tropical in origin. The manatee and the alligator formerly extended from Florida around the north side of the Gulf of Mexico; the crocodile was limited to southern Florida. European deciduous forest biociation Dominants of the plant associations in Europe are different species of the same genera that occur in North America, particularly beeches, maples, oaks, hornbeams, and basswood. Many mammals and birds of the European deciduous forest and seral stages also belong to the same genera as North American species. The similarity in genera may be traced back to the 298 Geographic distribution of communities continuity of the Arcto-tertiary forest between the two continents during the Tertiary; the dissimilarity of species to divergent evolution since the two com- munities became separated. Mammals common to the European forest and forest-edge include both the common and_ white- toothed shrews, European mole, common hare and European rabbit, several mice, wolf (same species as in North America), red fox (perhaps the same species as in North America), weasels, wildcat, wild boar, two deer, and European bison. The bird fauna (European fauna of Stegmann, 1938) includes some falcons, kites, and eagles, a pigeon and a cuckoo, owls, several woodpeckers, a jay, crows, several tits, a nuthatch, a creeper, a wren, several thrushes, a rich variety of Old World war- blers only poorly represented in North America, an Old World flycatcher not found in the New World, an oriole, and various finches. Absent are the tyrant flycatchers, vireos, and wood warblers that are so prominent in the North American deciduous forest (Lack and Venables 1939, Turéek 1951, 1952, 1955). Lists of invertebrates, especially of ground animals, are given by Kiihnelt (1944). It is possible that Pleistocene glaciation disturbed this biociation much more than that in North America (Moreau 1954). Asiatic deciduous forest biociation The broad-leaved deciduous forest of eastern China, Formosa, Korea, and Japan contains many species of plants and animals belonging to the same genera as occur in Europe or North America. Dur- ing the early Tertiary, this forest was in direct con- tact, via the Bering land bridge, with that in North America, and deciduous trees still maintain a narrow and tenuous contact along the southern edge and in seral stages of the coniferous forest with the decidu- ous forest of Europe. In addition, there are some endemic genera of animals confined to the area. A number of Indo-Malayan species penetrate into the biociation as far as northern China and Japan. Steg- mann (1938) gives a list of bird species occurring in this area that belong to what he calls the Chinese fauna, but he does not distinguish between those characteristic of forest, forest-edge, and seral com- munities. ANIMAL ADJUSTMENTS Animals are adapted structurally, func- tionally, and behaviorally to live in or under trees. They may use the trees directly as lookouts, singing posts, nest-sites, for cover and protection, and as a source of food; or they may simply take advantage of the rich humus created by the annual fall of leaves, or the shade, greater humidity, and equable temper- atures of the forest habitat. Some animals, for in- stance the eastern chipmunk, die within a few minutes if exposed directly to the sun. Snails and slugs are most active and carry on their reproductive activities during the moist vernal aspect, but may be con- spicuous throughout the summer when they are able to maintain the necessary water balance. Special adaptations for arboreal habits and for climbing are the sucking discs on the toes of tree frogs, the sharp claws and opposable toes in wood- peckers and squirrels, the prehensile tails of oppos- sums and white-footed mice, the parachutes and bushy tails of squirrels as well as the movable scales of some of the snakes, the many legs of the millipedes, and the slimy feet of slugs and snails. Hearing and voice are well developed in many forest animals, although vision is less perfected since visibility is limited anyway. The rich and almost con- stant singing of forest birds throughout the breeding season is well known, but the voice, or songs, of squirrels, chipmunks, and wolves are also well de- veloped for mammals. The loud singing of tree frogs is noteworthy, and the nightly chorus of insect voices, especially those of orthopterans, is remarkable. Most of these sounds serve to attract mates or advertise territories. The regular and pronounced changes in photo- period and temperature bring full development of the breeding season of most animals to its peak dur- ing the spring and early summer. Deer, bats, and a few others, however, characteristically mate during the autumn, and some of the squirrels and owls dur- ing the winter. All species must meet the severe winter condi- tions of short days, low temperatures, and scarcity of food in one way or another. In those forms of mammals and birds that remain active over winter and in those insects that hiberate in exposed situa- tions there is considerable increase in resistance to cold by internal physiological adjustments, and they live either on kinds of food that are not usually con- cealed by snow or on food cached when it was plenti- ful. Mammals den up in hollow logs or trees during short severe cold periods, coming out again when the weather is mild. Flocking is common in most birds during the winter season in contrast to their isolation in territories during the breeding season. Flocks commonly seek shelter on the lee side of forest areas or in river valleys to get protection from cold winds. Populations and variety of birds are supplemented during the winter as northern species come South. Those species of birds, mammals, reptiles, am- phibians, insects, and snails that cannot maintain activity in winter conditions either migrate or hiber- nate. Migration among birds commonly reduces the Temperate deciduous forest biome 299 population to less than one-third the number of indi- viduals present during the early summer, but during the spring and autumn migratory periods, popula- tions are temporarily greatly increased. Various in- sect species, including the monarch butterfly, migrate many miles southward. Other species move much shorter distances from open country into the forest- edge preparatory to hibernation (Weese 1924). Woodchucks, bats, and possibly chipmunks hiber- nate in the true sense; the black bear enters a pseudo- hibernation state, remaining quiescent over winter but maintaining temperature and other body functions at near normal. Reptiles and amphibians bury them- selves in decaying stumps or logs, in the ground below the frost line, or in the mud bottom of ponds. Nearly all insects and other invertebrates migrate out of the trees, shrubs, and herbs to the forest floor where they hiberate. Some species move up and down in the soil to keep below the frost line. Other species overwinter only in the egg or some other immature stage. Further south, especially in the magnolia-oak forest where there is less need, hiberna- tion and migration of populations that breed in the region are much less pronounced. The original condition of the forest and its wild- life has, of course, been greatly modified by man. The American Indian should probably be considered a native inhabitant of the deciduous forest, and the modifications he produced (Day 1953) as a normal influence comparable to that of other large mammals. The white man, however, is equipped with a large variety of tools that renders his influence extreme. As a consequence, some forest and forest-edge spe- cies, such as the mountain lion, gray wolf, eastern bison, wapiti, passenger pigeon, Carolina parakeet, probably the ivory-billed woodpecker, and others have become extinct. With agriculture and lumber- ing, seral stages have become more prevalent, so that there has been considerable shift in the relative abundance and importance of species from what oc- curred originally (Bennett and Nagel 1937, Allen 1938). HUMAN RELATIONS White man finds in the climate of the de- ciduous forest biome conditions favorable for the highest efficiency of his various activities, for his greatest health and energy, for maintenance of high population densities, and for high development of modern civilization (Huntington 1924). The chief and most profitable occupations of man in the de- ciduous forest biome are agriculture, mining, and industry. In eastern Asia, the broad-leaved decidu- ous and evergreen forests are occupied by Mongo- lians, and like the white man this yellow race early developed a high degree of civilization and large populations. Forests early became essential to white man as a source of lumber, fuel, and raw materials of industry. Trees furnish him shade from the hot summer sun and protection from the cold winter winds. In the early settlement of North America, forests were cleared for farming purposes with difficulty, but forest land was considered more fertile than grass- land because it grew trees instead of grass. As man dispersed westward across North America into the grassland biome, he first built his home in the fringes of forest along the streams or in outlying groves (Hewes 1950). As settlement increased, however, surplus people were crowded onto the prairie as they were crowded also into other biomes. It is of interest that in his invasion of grassland man planted trees around his home and thus tried to bring the forest environment with him. SUMMARY The temperate deciduous forest biome is derived from the Arcto-tertiary forest and is best de- veloped in eastern North America, western Europe, and eastern Asia. In those places, precipitation is moderate and temperatures mild during the summer growing season, but the winter season is generally unfavorable for the activity of most organisms. Animal communities of major significance are the North American deciduous forest biociation, North American deciduous forest-edge biociation (often a biocies), southeastern mixed biocies, European de- ciduous forest biociation, and Asiatic deciduous for- est biociation. Animals are adapted and adjusted in various ways to live in and under trees. Reproduction takes place principally in the spring and early summer. The severe winter season is adjusted to by increase in physiological hardiness, hiberation, or migration. 300 Geographic distribution of communities Communities: Coniferous Forest, Woodland, and Chaparral Biomes CONIFEROUS FOREST BIOME The coniferous forest is a continuous, often dense, forest of needle- or scale-leaved evergreen trees. The sclerophyllous leaves prevent excessive evaporation of water during winter and dry periods, and are adapted to withstand freezing. The ever- green leaves take full advantage for photosynthesis of short summer growing seasons, intermittent warm periods of autumn and spring, and the warm winter rains of the Pacific coast. The flexible branches bear snow-loads without breaking; snow-loads tumble easily off the cone-shaped tree. The dead, dry nee- dles which cling to the trees feed devastating crown fires, much more common in coniferous than decidu- ous forest. DISTRIBUTION AND ORIGIN Coniferous forests are largely confined to the northern hemisphere. They are transcontinental in Canada (Halliday 1937) and in higher elevations on the mountains through Mexico and Guatemala, into Honduras and Salvador. In Eurasia there is also a northern transcontinental coniferous belt with dis- junct patches of coniferous forests on all higher mountains southward. The main mass of coniferous forest species is doubtless derived from the boreal element of the Arcto-tertiary flora, and is much older geologically than is the deciduous forest. There is some evidence, however, that the eastern hemlock is a segregate from the temperate rather than the boreal unit of the Arcto-tertiary flora (Braun 1950, Oosting and Bourdeau 1955, Whittaker 1956), and that the western arid-tolerant ponderosa pine and Mexican pines come from the Madro-tertiary flora. CLIMATE In the transcontinental forest of North Amer- ica, precipitation varies between 38 and 100 cm (15-40 in.) and is mostly summer rain. Mean monthly temperatures vary from a winter low of about —30°C to a summer high of 20°C (—20° to +70°F). The summer period between killing frosts varies from 60 to 150 days. On the Pacific slope of the high western mountains, because of the westerly winds coming from the warm Japanese current, pre- cipitation is higher (125 to 225+ cm, 50 to 90+ in.) ; most of it falls as winter rain. Mean monthly temper- atures are more uniform (2° to 18°C, 35° to 65°F) and the frostless season is 120 to 300 days long. Hu- midity is high, and fogs are frequent in this region. In the northern Rockies, Cascades, and Sierra Ne- 301 vada, heavy winter precipitation falls as snow that accumulates to several meters in depth; winter tem- peratures are considerably lower. Snowfall is not as heavy in the central Rockies, and declines steadily, southward. PLANT ASSOCIATIONS OF NORTH AMERICA Pinus-Tsuga association (pine-hemlock forest) : Eastern hemlock is the climax, but eastern white, red, and jack pines are of wider distribution ; northern white-cedar and yellow birch are prominent. The forest has been badly disturbed by logging and fire, factors which, with climatic succession, have permitted a wide penetration of hardwoods to form an ecotone between deciduous forest and boreal for- est. The association extends from Minnesota to New England, and south into the Appalachian Mountains. Picea-Abies association (boreal forest) : White spruce and balsam fir most prominent (related spe- cies in Appalachians), but black spruce and tamarack also prominent; extends across southern Canada to the northern Rocky Mountains, north into Alaska, and south in Appalachian Mountains; alder thickets common in wet areas and heath shrubs in forest openings ; quaking aspen and paper birch occur ex- tensively as seral stages. Aspen groves, or parklands, form a broad ecotone between forest and grassland from Minnesota to the Rocky Mountains (Bird 1930). In the northern coniferous forest reaches lies a zone extending to the tree line in which the forest decreases in height and density, its floor carpet of lichens and mosses increases in depth and extent, and it becomes interspersed with numerous bogs or mus- kegs. Lichen woodland is especially well developed east (Hare and Taylor 1956) and muskegs west of Hudson Bay. This whole area is forest-tundra, as distinguished from the denser, taller boreal forest ; it is equivalent to the Hudsonian zone of Merriam et al. (1910). Picea-Pinus association (petran subalpine for- est): Extends southward at higher elevations in Rocky Mountains to Arizona, New Mexico, and higher peaks of Mexico; contains Engelmann and blue spruces, subalpine fir, and several species of pine. Tsuga-Pinus association (Sierran subalpine forest) : Occurs chiefly in Cascade Mountains and Sierra Nevada; mountain hemlock as well as various pines, subalpine larch, and red fir prominent; trees tall and narrowly cylindrical at lower elevations but dwarfed, gnarled, and misshapened at tree-line ; aspen and lodgepole pine extensive as seral stages after fire in both Sierran and petran subalpine forests. Pinus-Pseudotsuga association (petran mon- tane forest) : At lower elevations in the Rocky Moun- tains ; ponderosa pine, Douglas-fir, and white fir most important ; ponderosa pine most aridity-tolerant; trees often widely spaced with grass stratum underneath, sometimes forming savannas. Pinus-A bies association (Sierran montane for- est): Contains species listed for petran montane forest and also sugar pine, incense-cedar, and giant sequoia (central Sierras) ; chaparral develops after fire. Pinus-Pinus association (Mexican pine for- est): An extension of montane forest, chiefly pines, at higher elevations in Mexico. Thuja-Tsuga association (coast forest): A luxuriant humid forest on the Pacific slope of moun- tains from northern California to Alaska; western hemlock, western redcedar, Alaska-cedar, Douglas- fir, Sitka spruce, and redwood most characteristic ; trees sometimes 90 m (300 ft) high and to 6 m (20 ft) diameter ; deep shade in climax forest but in open- ings there may be dense tangles of shrubs, lianas, tall ferns ; moss often thick over ground and fallen logs; forest in North extends to west slopes of Rocky Mountains in Idaho, Montana, and British Columbia to form a Coast forest ecotone with petran montane and subalpine forests, in which grand fir, western white pine, and western larch are prominent. ANIMAL COMMUNITIES There are three principal biociations in this biome, two in North America and one in Eurasia. There is overlap in their species compositions. Spe- cies occurring in seral or climax stages of both North American biociations, although less common in the Mexican pine forests, include Mammals Water shrew Deer mouse Snowshoe rabbit Porcupine Red squirrel Gray wolf Northern flying squirrel Black bear Birds Goshawk Yellow-bellied sapsucker Pigeon hawk Hairy woodpecker Ruffed grouse Black-backed three-toed Great horned owl Saw-whet owl woodpecker Traill’s flycatcher 302 Geographic distribution of communities Olive-sided flycatcher Gray jay Common raven Red-breasted nuthatch Brown creeper Winter wren Hermit thrush Swainson’s thrush Golden-crowned kinglet Ruby-crowned kinglet Solitary vireo Nashville warbler Wilson’s warbler Purple finch Pine grosbeak Pine siskin Red crossbill Lincoln’s sparrow North American boreal forest biociation This biociation extends from the Atlantic Ocean to the Rocky Mountains in Canada and south on the Appalachian Mountains to northern Georgia (Shelford and Olson 1935, Kendeigh 1947, 1948, Munroe 1956). There is a broad overlap or fusion between the boreal and montane forest biociations in the northern Rockies where species of one biociation penetrate into the other (Rand 1945, Drury 1953). Characteristic mammals that occur generally through the boreal and pine-hemlock forests, in addi- tion to those listed in the above section, are: Meadow jumping mouse Woodland jumping Arctic shrew Masked shrew Smoky shrew mouse Pigmy shrew American marten Star-nosed mole Fisher Hoary bat Least chipmunk Northern bog lemming Gapper’s red-backed mouse Ungava phenacomys Rock vole Coniferous forest, Ermine Least weasel Wolverine Lynx Moose Woodland caribou FIG. 23-| Left, montane forest in Oregon—a virgin stand of ponderosa pine. Below, forest, tundra in northern Manitoba, composed of spruce and tamarack with the ground covered with a thick layer of moss and lichens (courtesy W. P. Gillespie). Bird species found in this biociation are listed in Table 23-1. This biociation is especially notable for the large representation of wood warblers in the avi- fauna, each with its own specialized niche (Mac- Arthur 1958). In northern Ontario, warblers con- stitute 69 per cent of the breeding bird population in the spruce-fir forest; in northern Maine, 63 per cent. As one proceeds south from Ontario and Maine into Minnesota, Michigan, New York, and along the Appalachian Mountains to Tennessee, species both of mammals and birds drop out, apparently as they reach limits of tolerance to climatic factors. Perhaps woodland, and chaparral biomes 303 TABLE 23-| Comparison of avifaunas and population densi- ties (number per 40 hectares, or 100 acres) of breeding birds in the Black Sturgeon Lake area of northern Ontario (Kendeigh 1947), in Aroostook County, northern Maine (Stewart and Aldrich 1952), and in the Great Smoky Mountains of eastern Tennessee (Fawver 1950). 304 Bird species Goshawk Broad-winged hawk Pigeon hawk Spruce grouse Ruffed grouse Yellow-shafted flicker Pileated woodpecker Yellow-bellied sapsucker Hairy woodpecker Downy woodpecker Arctic three-toed woodpecker American three-toed woodpecker Yellow-bellied flycatcher Acadian flycatcher Least flycatcher Gray jay Blue jay Common crow Black-capped chickadee Boreal chickadee Red-breasted nuthatch Brown creeper Winter wren Robin Wood thrush Hermit thrush Swainson’s thrush Veery Golden-crowned kinglet Ruby-crowned kinglet Solitary vireo Red-eyed vireo Black and white warbler Tennessee warbler Nashville warbler Parula warbler Magnolia warbler Cape May warbler Black-throated blue warbler Myrtle warbler Black-throated green warbler Blackburnian warbler Chestnut-sided warbler Bay-breasted warbler Ovenbird Mourning warbler Canada warbler Scarlet tanager Rose-breasted grosbeak Evening grosbeak Purple finch Pine grosbeak Pine siskin Slate-colored junco White-throated sparrow Totals Northern Ontario spruce- fir COUMMOwW NNOKRKB DONNY NN H+ SP NV+ t+ + moO OFM OM + COON OCONODDM WODWDMO WHWNOINM NWOKK 18 319+ Geographic distribution of communities Eastern Tennessee Northern Maine spruce- Spruce- fir fir 0 0 + 0 0 0 + 0 + + 2 0 + 0 de 0 2 3 + 0 of: 0 0 0 5 0 0 0 + 0 + 0 3 0 + 0 4 74 8 0 8 20 + 38 4 34 6 3 0 0 4 0 21 0 + 18 12 38 8 0 9 24 6 + 9 8 5 55 28 2 20 i) ron CONN OF +H + NWOF NG _ 349+ =" iS) onmooo cooooco COoONOCOO OMeCCeo CCC oO 310+ Eastern hemlock Paws ON1}CT CO OHtOOO OworteK +O 000 i) wo for) Bao OFOOO COCOOrFMO ONATOCO NY Po > i) SCWOODOO ONDAD COO i) 389+ the elimination of these competing species, or possi- bly the change in climatic conditions, makes other species more abundant. This is especially noticeable among birds—the red-breasted nuthatch, brown creeper, winter wren, golden-crowned kinglet, soli- tary vireo, black-throated green warbler, black- burnian warbler, Canada warbler, and slate-colored junco attaining much larger populations in the Smoky Mountains of Tennessee than in northern Ontario. In addition the veery, black-throated blue warbler, and often black and white warblers become numerous. This constitutes a variation in the boreal forest biociation (Stewart and Aldrich 1952) which may be designated the Appalachian faciation. When hemlock, which reaches its best develop- ment in the Appalachian Mountains, and spruce-fir forests occur in the same region, some bird species adaptable to both show a definite preference for one over the other. In Table 23-1 it is evident that the red-breasted nuthatch, brown creeper, winter wren, veery, golden-crowned kinglet, and _ slate-colored junco prefer the spruce-fir forest, while the black- capped chickadee, possibly the solitary vireo, black- throated blue warbler, black-throated green warbler, blackburnian warbler, and Canada warbler prefer hemlock forests. A similar differentiation of bird populations in these two forests is also evident in Algonquin Provincial Park in southern Ontario (Martin 1960). This may be a reflection of the dif- ference in paleo-ecological history of hemlock and spruce-fir forests. The long association of hemlock with deciduous forest may also have permitted the invasion into the former of the Acadian flycatcher, wood thrush, ovenbird, and scarlet tanager. Spruce-fir forests occur at higher elevations in the mountains, and in some areas at least, as in the Catskill Mountains of New York, a zone of deciduous forest intervenes the hemlock and spruce-fir forests. Elsewhere, as in the Cheat Mountains of West Vir- ginia (Brooks 1943), the two coniferous forests come into direct contact. Apparently because of the close interrelations between hemlock and the northern FIG. 23-2 Species common in the coniferous forest biome: (a) porcupine, (b) gray jay, (c) moose, boreal forest, (d) wapiti, western forest (courtesy U.S. Forest Service). Coniferous forest, woodland, and chaparral biomes 305 ty’ ia: SE nutcracker’ MY 2 Yl. : red breast Din 4 , LOTR gyruby cr. kin Ue YY 7 YJ 7 Up } aiff, gm Me pare 5 |) I FO MIP os, Calas | ep ee y Gy GLI W jz yyy, Wig fh alk hermit they tian Shipping sparrowy] Lado £4) i> J VL j Yirran_f pecker’ spine grosbea j Ure es © A, a cassin finch 4 Bwaineon : MW obi : lary ah P Sale eY aa Inson x Lh 4 robinw . Bo Ps EE ju So the a Tet ake vaccinium A hey Poh 8.4) thrush: \ i - ahoe rides aia Hieky/ Nis engelmann Jonicera rubus ? spruce amelanchier % SPRUCE-FIR FOREST FIG. 23-3 Foraging niches of birds in the western forest biocia- tion of the central Rocky Mountains (Salt 1957). hardwoods, several warblers and other species nor- mally characteristic of coniferous forest have learned to occupy niches in the deciduous forests as well, and attain high populations therein (Saunders 1936, Brooks 1940, Kendeigh 1945). The species composi- tion of foliage insects in the coniferous forests of the Smoky Mountains is essentially similar to that of the deciduous forest (Whittaker 1952). Such gen- eral intermingling of species in an ecotone is to be expected, and may be considered characteristic of the Appalachian faciation. Perhaps some species listed above reach larger populations in a seral shrub or forest-edge biocies (Adams 1909). The Philadelphia vireo. palm war- bler, Wilson’s warbler, rusty blackbird, and Lin- coln’s sparrow are largely limited to shrubs or second growth; the northern waterthrush occurs in bogs; the savannah sparrow, in marshes and grassy areas; and the white-winged crossbill, irregularly through the climax. These species extend to the northern tree-line. Seral aquatic stages in the boreal forest contain beaver, muskrat, and nesting horned grebe, black duck, common goldeneye, Canada goose, and the common and hooded mergansers (Hanson et al. 1949). Actually, the coniferous forest does not develop a recognizable forest-edge along its southern border because these borders grade by steps into deciduous forest, aspen parkland, wood- land, and chaparral. The closest resemblance to an edge are shrubby openings within the forest or the subseres that develop in bogs, burns, and logged areas. The aspen parkland contains a fauna in which 306 boreal, grassland, deciduous forest, and deciduous forest-edge biociation species are represented (Bird 1930) and is essentially an ecotone. Invertebrate composition of the seral stages bears a strong resem- blance to that occurring in seral stages of the decidu- ous forest. Along its northern border, the coniferous forest comes in direct contact with open tundra to form a broad ecotone. Boreal forest biociation species reach their northern limits of distribution and tundra species begin to appear. There are no distinctive mammals, but several birds are characteristic of this subarctic, lichen woodland and muskeg, Hudsonian, or (most apt) forest-tundra faciation (Manning 1952, Harper 1953, 1956, Preble 1908) : Solitary sandpiper Blackpoll warbler Lesser yellowlegs Pine grosbeak Rough-legged hawk Hoary redpoll Common redpoll Tree sparrow Harris’ sparrow White-crowned sparrow Fox sparrow Boreal owl Hawk-owl Great gray owl Northern shrike Gray-cheeked thrush Bohemian waxwing (west) The pine grosbeak, white-crowned and fox sparrows extend to the southward at tree-line on the western mountains and the gray-cheeked thrush and _black- poll warbler extend southward at high elevations in the northern Appalachians. Geographic distribution of communities North American montane forest biociation This biociation, considering the forest-interior and forest-edge together, occurs principally in the coast forest (Storer, et al. 1944, Miller 1951, Macnab 1958) and is less developed in the montane and sub- alpine forests of the Rocky Mountains, Cascades, and Sierra Nevada. There appear to be no important subdivisions related to the several plant associations that it covers (Rasmussen 1941, Hayward 1945, Munroe. 1956, Snyder 1950). Because of the moun- tainous terrain and the many possibilities for popu- lations to become partially or wholly isolated from each other, there are many local subspecies and spe- cies of mammals and birds (Findley and Anderson 1956). The following lists include only common species of wide distribution through the biociation. Mammals Long-tailed vole Western jumping mice Grizzly bear Western marten Mountain weasel Shrews Mountain beaver Yellow-bellied marmot Golden-mantled ground squirrel Western chipmunks Wolverine Douglas’ squirrel Mountain lion Bushy-tailed wood rat Bobcat Red-backed mice Mule deer Heather vole Wapiti Birds Golden eagle Blue grouse Flammulated owl Pygmy owl Calliope hummingbird Williamson’s sapsucker White-headed wood- pecker Hammond's flycatcher Western flycatcher Western wood pewee Steller’s jay Clark’s nutcracker Mountain chickadee Pigmy nuthatch Varied thrush Mountain bluebird Townsend's solitaire Audubon’s warbler Townsend’s warbler Hermit warbler Western tanager Evening grosbeak Cassin’s finch Oregon junco Gray-headed junco Additional species from the chaparral biociation penetrate this community, particularly into shrubby stages. The orange-crowned warbler is noteworthy in this respect. In general, the population of breeding birds is less than one-sixth what it is in the boreal forest (Snyder 1950). In the northern Rockies there is con- siderable mixture with species from the boreal forest biociation, both in birds and mammals, but these species drop out progressively southward and very few of them cross the Cascades into the Coast forest. The woodland caribou, for instance, ranges only to northeastern British Columbia and the moose to central British Columbia, eastern Idaho, and western Wyoming. The western facies of the deciduous for- est-edge biociation penetrates widely as a seral stage through the western forest biociation, and certain of its species may persist into the climax. Eurasian boreal forest biociation The dominants of the Eurasian plant associa- tions are different species but the same genera of pines, firs, larches, spruces, poplars, and birches that occur in North America. This biociation is best de- veloped in Asia, from whence the biota is dispersed across the northern part of the continent into Europe (Berg 1950, Jahn 1942, Kalela 1938, Palmgren 1930, Pleske 1928, Stegmann 1932, 1938, Haviland 1926, Schafer 1938, Soveri 1940, Turéek 1956). The mammal fauna contains shrews, a varying hare, flying and red squirrels, a chipmunk, red- backed mice, the wolf and red fox, a brown bear, martens, weasels, wolverine, lynx, a moose, and a deer. Several of these species (wolf, red fox, wol- verine, lynx) are considered by some taxonomists to be conspecific with North American forms (Rausch 1953). This biociation is equivalent to the Siberian bird fauna of Stegmann (1938) and includes several spe- cies of grouse, owls, woodpeckers, crows and jays, and tits, a creeper, several thrushes, several Old World warblers, kinglets, a wagtail, waxwings, and several finches or sparrows. The wood warblers, abundant in the boreal forest of North America, are absent. PALEO-ECOLOGY In early Tertiary we may suppose that the boreal unit of the Arcto-tertiary forest had a fairly uniform animal composition from eastern Canada into Asia and Europe. As the forest progressed southward during the middle and later Tertiary, a large segment became separated in consequence of the submergence of the Bering land bridge, becoming the Eurasian biociation. Forms now peculiar to the Eurasian and to the North American biociations must have evolved after this separation took place (Udvardy 1958). In North America, as the Arcto-tertiary biota retreated southward with the progressive chilling of the continent, it was separated into two portions by the northward invasion of grassland over the Great Plains, except as it had contact through the boreal forest across Canada in the north. During the Pleis- tocene even this northern contact was broken (Fig. 21-2) with each major advance of the glacier. Fur- thermore, the western part of the continent was Coniferous forest, woodland, and chaparral biomes 307 thrown up into mountains beginning in the Miocene, and the climate there became more diversified and rigorous. Plant and animal species tended to segre- gate into either the western or the eastern section of the forest, depending on where habitat conditions and community coactions were more favorable to them, and isolation encouraged divergent speciation. The western section continued to have sporadic contact with the Eurasian biociation, especially during the Pleistocene, but the eastern section was too far away. Hence came the differentiation of the boreal forest biociation in the eastern lowlands of the continent and the western forest biociation in the western mountains and on the Pacific coast. In this connection it is of interest that 52 per cent of the breeding bird species in the boreal forest bio- ciation are of Old World origin and 30 per cent of North American origin, compared with 65 and 17 per cent, respectively, for the western forest biocia- tion in Colorado (Snyder 1950). The difference is even more striking when comparison is made between the breeding populations. In the boreal forest bio- ciation, only 20 per cent of the breeding pairs belong to species of Old World origin while 79 per cent belong to species of North American origin. In the western forest biociation of Colorado the percentages are 98 per cent of Old World origin and only 2 per cent of North American origin. Pleistocene glaciation enhanced the differentiation of boreal and western forest biociations since it al- lowed independent subspeciation and even speciation in the four refugia (Fig. 21-2). The boreal forest be- came compressed with each glaciation into the Appa- lachian refugium, but the western forest was segre- gated three ways into the Rocky Mountain, Pacific, and Alaskan refugia. At these times the Alaskan refugium was probably connected by the Bering land bridge to Asia. The present-day distribution of the four subspe- cies of moose suggests that they were isolated during at least Wisconsin glaciation in the Appalachian, Rocky Mountain, and Alaskan refugia, and in the unglaciated area of Wisconsin, Minnesota, and Illinois. This area probably served also as the refugium for the western subspecies of the wood- land caribou, while the eastern subspecies was isolated in the Appalachian refugium (Vos and Peter- son 1951). Different subspecies of arctic shrews had refugia to the south and east of the glacier and in Alaska. The American marten and the red squirrel apparently survived in the Appalachian refugium ; the western marten and Douglas’ squirrel, in the Pacific refugium. Of the red-backed mice, Clethri- onomys gappert has apparently dispersed from the Appalachian refugium, C. dawsoni from the Alaskan refugium, and C. wrangeli from islands off the coast of British Columbia (Rand 1954). Various subspecies or closely related species of birds apparently differentiated as populations were isolated in one or more of the four refugia. This seems to have occurred with the spruce grouse, sap- sucker, gray jay, boreal chickadee, myrtle and Audu- bon’s warblers, slate-colored and Oregon juncos, and white-crowned sparrow (Rand 1948, Drury 1953). In each interglacial period, the coniferous forest fauna previously isolated in the Appalachian, Rocky Mountain, Pacific, and Alaskan refugia doubtless dis- persed centrifugally from each center until they came into contact with each other. Such segregation and dispersal of the biota must have occurred four times during the Pleistocene; the dispersal from refugia after the Wisconsin glaciation is still going on. The four subspecies of moose have come into contact with each other (Peterson 1955), the least chipmunk has entered Ontario and Quebec (Peterson 1953), and the evening grosbeak has spread across Canada from the western mountains only within the past hundred years. Some of these changes in range may have been hastened as a result of human interference. The woodland caribou was formerly the principal large ungulate present in the boreal forest (Vos and Peter- son 1951), but as logging and fires opened up the forest, the caribou has greatly declined in numbers and the moose has become more abundant. The white-tailed deer has also spread from the deciduous forest well into the boreal forest in recent years, and other species of mammals and birds appear in the process of doing so. ANIMAL ADJUSTMENTS Animal adaptions for life in coniferous forest are similar in many ways to those for life in deciduous forest. Ecological niches in these two for- ests are similar, although the species that occupy them are different. Important differences are the stiff needle-shape character of conifer leaves and their arrangement around all sides of the twigs, which hinder the movements and feeding of some birds, and the poor decomposition of the shed leaves that accu- mulate on the ground, not favorable to high popula- tions of many species of small animals. In contrast to the aspects of the deciduous forest, the vernal and autumnal aspects are less well developed since most of the trees retain their foliage throughout the year. The woodland caribou is largely restricted to the climax forest where it feeds on reindeer moss, a ground lichen, and on tree lichens. Moose are found throughout seral stages as well as in the climax. During the summer they commonly feed on water lilies, pondweeds, sedges, and grasses; during the 308 Geographic distribution of communities me 1 gigas 2 andersont 3 shirasi 4 americana (intro- duced into Newfoundland) 5 Southern limit of Wisconsin glaciation FIG. 23-4 Present distribution of four subspecies of moose in North America. Post-Pleistocene dispersal routes from distributions at the time of Wisconsin glaciation are shown by arrows (from Peterson 1955). winter, on the tips of birch, aspen, cedar, balsam fir, and various other shrubs and small deciduous trees (Shelford and Olson 1935). Small mammals are abundant. In northern Michigan, the populations of two species each of mice, chipmunks, and shrews varied from 6.2 individuals per hectare (2.5/acre) in jack pine to 12.5 (5.0/acre) in black spruce, 16.0 (6.4/acre) in hemlock, 19.5 (7.8/acre) in a white- cedar swamp, and 28.2 (11.3/acre) in white birch (Manville 1949). Perennial animals that remain active over winter have a high tolerance of low temperatures and use food not readily obscured by snow (Snow 1952). The large mammals become browsers in the winter. Wapiti chew bark patches off aspen trees when other forage is difficult to find. Scars thus formed are ideal sites for the development of fungus disease (Packard 1942). Birds feed on seeds extracted from the cones of the coniferous trees, on buds, and on bark insects. When the seed crop fails, large numbers of pine siskins, pine and evening grosbeaks, red crossbills, and white-winged crossbills emigrate southward into the United States. Small ground and subterranean animals are well insulated under the snow where temperatures even in the far North may drop only a few degrees below freezing (Pruitt 1957). Some birds, such as the grouse, roost at night in cavities formed in snowbanks. Less than half of the nesting bird population of the western forest biociation migrates for the winter, and then only to lower altitudes on the mountains. In contrast, the birds of the boreal forest are acclima- tized to warm climate, and over 80 per cent migrate hundreds of kilometers to the south. A few mam- mals also migrate, such as the hoary bat in the East and the wapiti and mule deer down the mountain slopes in the West. Insects virtually dominate the forest, at times. Vast numbers of mosquitoes and flies force moose to spend much of the summer submerged in water, and are generally annoying to other animals and man. The larch sawfly has spread across Canada and the Coniferous forest, woodland, and chaparral biomes 309 FIG. 23-5 Gallery pattern of a bark beetle in lodgepole pine: (a) nuptial chamber, (b) egg gallery, (c) egg niche (Reid 1955). northern states during the last 75 years and caused considerable defoliation and destruction of tamarack (Coppel and Leius 1955). The spruce budworm (a lepidopteran larva) feeding on the leaves has killed balsam fir and spruce trees on vast areas at repeated intervals in the past: 1807-18, 1870-80, 1904-14 (Swaine and Craighead 1924), and again in the 1940’s. Several kinds of bark beetles, wood borers, and long-horned beetles are also destructive forest insects. Beetles, ants, aphids, jumping plant lice, leaf- FIG. 23-6 Seasonal history of a bark beetle (Reid 1955). OVER WINTERING hoppers, and _ spiders, and invertebrates—notably snails, annelids, and millipedes—are not numerous over most of the biome (Rasmussen 1941, Hayward 1945, Blake 1945). Most ground invertebrates have higher population densities in the seral aspen and birch stages than in the coniferous climax (Hoff 1957). Reptiles are few, only the garter snake ex- tends very far north. The northern wood frog, leop- ard frog, and mink frog are widely dispersed in suitable habitats throughout the boreal biociation. Because of its greater humidity and more equable temperatures, invertebrates and cold-blooded verte- brates are generally more numerous in the Coast for- est than elsewhere through the biome. HUMAN RELATIONS Only the lower, warmer portions of the coniferous forest biome are permanently inhabited in large numbers by white man throughout the year. Logging for pulpwood and lumber is an important occupation. Over the more rugged northern portions of the coniferous forest, the population is scattered and, in North America, there are more Indians than whites, at the present time. The Indians engage in hunting and fur-trapping for support. Larger settle- ments of white men occur where minerals may be mined or oil obtained. These regions, as well as the higher conifer-clad mountains, are resorted to for fishing and other recreational activities during the warm summer months. WOODLAND BIOME In contrast to forest, woodland is an open stand of trees with an intervening good growth of grasses or shrubs. The trees are usually short, 6 to 15 m high (20 to 50 ft), but may have a dense crown. In favorable local habitats, the trees form a closed canopy, but in arid situations they are scattered. The trees vary widely in leaf structure, but nearly all species are evergreen and tolerant of low moisture. In North America, woodland of different types €x- tends from Washington and Wyoming well down into Mexico. A similar type of broad-leaved, ever- green sclerophyllous woodland, together with chapar- ral, occurs around the Mediterranean Sea in Eurasia and Africa. CLIMATE In Utah, precipitation in this biome ranges from 4 to 6 cm (10 to 15 in.) per year, and mean monthly temperatures from ==5°@ to 21 (Ce (Gagers 69°F) (Woodbury 1947). Precipitation is often 310 Geographic distribution of communities higher in Mexico, but this is offset by a higher rate of evaporation. West of the Sierra Nevada precipita- tion comes principally during the winter months, as it also does in the Mediterranean region. PLANT ASSOCIATIONS IN NORTH AMERICA Pinus-Quercus association: pine-oak woodland. On mountain slopes of central and northern Mexico; oak scrub, mostly evergreen, at lower elevations grades into pine-oak woodland with some juniper at higher elevations and then into the Mexican pine forest; contains a rich variety of spe- cies (Gentry 1942, Leopold 1950, Marshall 1957). Quercus-Quercus association: oak woodland. Broad-leaved mostly evergreen oaks with Digger pine in certain habitats (Miller 1951) ; mostly west of the Sierra Nevada but extending north into Oregon and Washington. Pinus-Juniperus association: pifon-juniper woodland. Pines and junipers of several species from eastern slopes of Sierras and Cascades across Great Basin to Wyoming and New Mexico (Wood- bury 1947, Woodin and Lindsey 1954). PALEO-ECOLOGY The various types of woodlands in North America were probably derived from the mixed pines and oaks of the Madro-tertiary flora. The pifon- juniper woodland is a segregation that became adapted to the cold winter climates of the Rocky Mountains and Great Basin. It was more widely dispersed and found at lower altitudes during the pluvial Wisconsin glacial period than it is at present. The oak woodland during lower Pliocene was widely distributed over the central and southern portions of the Great Basin but with the trend toward colder winters and decreased rainfall, the oak woodland came to be restricted to the moister mountain habi- tats within the desert and to Pacific coast regions with winter rain and mild temperatures (Axelrod 1950, 1957). There is also some evidence that wood- land vegetation including oaks extended around the north side of the Gulf of Mexico as far as Florida (Pitelka 1951). WOODLAND BIOCIATION The animal life of woodland communities in western North America is not highly distinctive. The trees, being broad-leaved or needle-leaved, at- FIG. 23-7 Pinton-juniper woodland in Utah. tract species from the adjacent deciduous forest-edge (riparian woodland) and montane forest biociations. Since the trees are sometimes scattered, interspersed with grass or shrubs, chaparral, grassland, and desert species may penetrate well into the community. In respect to species composition, therefore, the woodland in North America to a large extent is an ecotone. Mammals Of larger mammals, the mule deer, mountain lion, and coyote commonly occur during the winter months in the pinon-juniper woodland of Utah and Arizona, although most of these species spend the summer high in the mountains. The bobcat also oc- curs and the grizzly bear was formerly not uncom- mon. The rock squirrel, cliff chipmunk, desert and dusky-footed wood rats and pifion mouse are found in both the pifon-juniper and the petran bush but show preference for broken country, rocky hillsides, and cliffs (Woodbury 1933, Rasmussen 1941). In southern New Mexico, four species of mice—deer, brush, rock, and pimon—occur more or less together (Dice 1942). The open floor of the oak woodland in California is relatively devoid of mammal life, with only the California and brush mice common in the vicinity of brushy growth. The western gray squirrel is probably most common in this community (Vaughan 1954). Birds Certain bird species appear to be more charac- teristic of woodland than are mammals. Species oc- curring rather widely in northern Arizona (Rasmus- sen 1941), Utah (Hardy 1945), California (Miller 1951), and Mexico (Marshall 1957) are: Coniferous forest, woodland, and chaparral biomes aH FIG. 23-8 Petran bush in Utah. Common bushtit Blue-gray gnatcatcher Scott’s oriole Western bluebird Hutton’s vireo Black-throated gray warbler Grace’s warbler Olive warbler Painted redstart Hepatic tanager Lawrence’s goldfinch Band-tailed pigeon Acorn woodpecker Lewis’ woodpecker Nuttall’s woodpecker Ladder-backed wood- pecker Ash-throated flycatcher Gray flycatcher Coues’ flycatcher Scrub jay Pinion jay Plain titmouse Rattlesnakes, lizards, and horned toads invade from the desert but are not particularly characteristic of the woodland itself. Invertebrate populations are relatively low, and consist principally of spiders, ants, termites, jumping plant-lice, and a sprinkling of ich- neumonids, flies, leafhoppers, beetles, and banded- wing locusts (Rasmussen 1941). CHAPARRAL BIOME Chaparral, in the strict sense, consists of xeric broad-leaved evergreen bushes, shrubs, or dwarf trees, usually not more than 2.5 m (8 ft) high, and occurring in more or less continuous stands. Be- neath the bushes and shrubs there may be abundant ground litter. Chaparral is less dense where there are rock outcroppings and grass. Most species readily produce sprouts after their tops are destroyed by fire, provided fire does not occur too frequently ; germina- tion of some seeds is hastened by the heat of the fire. Chaparral tends to spread as a seral stage into areas of montane forest and woodland when the latter is destroyed by fire. Although chaparral is doubtless seral over much of its range, it appears to be climax over fairly large areas in southern California and northern Baja California, and a narrow belt on the slopes of the Sierra Nevada and southern Rockies. Broad-leaved evergreen chaparral also occurs, with woodland, around the Mediterranean and elsewhere on other continents. PLANT ASSOCIATIONS IN NORTH AMERICA Coastal chaparral occurs from southern Oregon to northern Baja California and eastward into Nevada and Arizona (Weaver and Clements 1938). This region is one of winter rains, and con- sequently the vegetation consists chiefly of ever- green bushes and shrubs with leaves that are glu- tinous, odorous, or hairy. Coastal chaparral occurs in more massive stands than does petran bush. Petran bush occurs as a lower zone on the moun- tains from South Dakota to Texas and westward into Nevada and Arizona. This association has been called petran chaparral, but the shrubs and bushes are mostly deciduous. Both associations are derived from the Madro- tertiary flora (Davis 1951) and have a phylogenetic history similar to the oak woodland and pinon- juniper woodland respectively, with which they are closely associated. CHAPARRAL BIOCIATION There are no mammals peculiar to the chaparral in North America, although in Utah the bobcat, rock squirrel, and cliff chipmunk reach rela- tively large populations in the petran bush (Hay- ward 1948). In California, the brush rabbit and the dusky-footed wood rat are numerous in heavy brush, along with other mammals also found in woodland (Vaughan 1954). There may be 6 to 12 occupied houses of the white-throated wood rat per hectare (2-5/acre) in southern Arizona (Hanson 1957). The mule deer becomes common (about 10/sq km or 25/sq mi) during the winter when it migrates down from the higher elevations in the mountains. The chaparral fauna, like that of the woodland, is largely ecotonal between montane forest and grassland or desert scrub. The coastal chaparral is extensive enough, how- ever, so that these birds show preference for it (Miller 1951) : Mountain quail California quail Anna’s hummingbird Allen’s hummingbird Wrentit Bewick’s wren California thrasher *Orange-crowned warbler *MacGillivray’s warbler *Lazuli bunting *Rufous-sided towhee Brown towhee Rufous-crowned sparrow Black-chinned sparrow 312 Geographic distribution of communities TABLE 23-2 Climate in different zones on the west slope of the Wasatch Mountains, Utah (Price and Evans 1937). Depth of Total Per cent Eleva- snowon precipi- precipi- Frost-free Mean temperature tion, March 1, tation, tation period, May-Oct. Oct.-May Annual Community meters cm cm as snow days AS (6; ( G} Pifion-juniper woodland 1,700 3.8 29.7 45 - 17.0 2.0 8.3 Petran bush 2,333 63.2 44.4 - 90 14.4 -0.2 5.9 Petran mon- tane forest 2,698 120.1 74.9 70 87 Nl -2.6 3.3 Petran sub- alpine forest 3,078 134.4 (flea 80 80 8.7 -5.7 0.3 Those species marked with an asterisk are found also in the petran bush of Utah and, in addition, the broad-tailed hummingbird, gray vireo, Virginia’s warbler, and green-tailed towhee are characteristic there (Hayward 1948). The petran bush, even more than the coastal chaparral, contains many species from the deciduous forest-edge and montane forest biociations. No snake or lizard is particularly characteristic of chaparral, although at times they may be nu- merous. For the most part, the reptiles occurring in chaparral belong more properly to the desert or grassland and reach their upper altitudinal limits in this biome. Among invertebrates in the petran bush, mites, ants, leafhoppers, locusts and grasshoppers, beetles, aphids, and flies are conspicuous, and large numbers of parasitic and gall-forming hymenopterans depend on the oaks for completion of their life-cycles. A few millipedes and centipedes are to be found under rocks, but snails are scarce (Hayward 1948). In the coastal chaparral of southern California, the period of greatest activity for most invertebrate spe- cies comes in March and April, towards the end of the winter rains, and is correlated with the flowering season of plants and the period of greatest soil mois- ture. During the hot dry summer many invertebrates aestivate (Ingles 1929). ZONATION With increase in elevation there is a de- crease in temperature and increase in wind velocity, depth of snowfall, and total precipitation (Table 23-2). Mean annual temperature tends to drop about 0.6°C (1.0°F) for each rise of 100 m (325 ft) ; hence, by going up a mountain a few hundred meters one encounters similar, but not identical, climates and biota as occur at lower elevations many kilo- meters northward on the continent. In deep valleys and canyons, on the other hand, there is often cold air drainage at night, so that colder types of vege- tation and fauna occur than on the nearby ridges. Alpine tundra and coniferous forest at the higher elevations represent southward dispersal of the Arcto- tertiary flora, while woodland, chaparral, grassland, and desert at lower elevations represent northward dispersal from the Madro-tertiary flora. These dis- persals doubtless began as these western mountains became elevated in mid-Tertiary time. During the Pleistocene period, glaciation occurred extensively in the higher mountains and forced all communities to lower elevations. Pluvial climates generally accom- panied glaciation so that grassland and desert in the lowlands were succeeded by coniferous forest. More continuous zones of forest in foothills and through valleys encouraged wide latitudinal dispersal of ani- mals. With recession of the glaciers and rewarming of the climate, the forest and alpine tundra again withdrew to higher elevations, and mountain ranges assumed the isolation from each other that we see at the present time. This isolation has induced con- siderable speciation, but the similarity or relation- ship between animal life in different mountain re- gions is explicable from the paleo-ecological history of the area. SUMMARY Coniferous forests are largely confined to the Northern Hemisphere. Over most of the area, the summer growing season is short and the winter long and cold. On the Pacific coast of North America, however, precipitation occurs mostly during the win- ters, which are mild. Some mammal and several bird species occur widely through the biome, but the following animal communities are recognized: North American boreal forest biociation, North American montane forest biociation, and Eurasian boreal forest biociation. Appalachian and forest-tundra faciations of the North American boreal forest biociation are well-marked for birds. In early Tertiary, the boreal unit of the Arcto- Coniferous forest, woodland, and chaparral biomes 313 tertiary forest doubtless had a fairly uniform fauna from eastern Canada across the Bering land bridge to western Europe. This fauna became differentiated into the three biociations with the disappearance of the Bering land connection between Alaska and Asia, and the separation of the North American fauna into western and eastern sections by the northward pene- tration of grassland and the southward movements of the Pleistocene glaciers. Differentiation at the sub- species and species levels was further encouraged by the segregation of the North American fauna during Pleistocene glaciation in the Appalachian, Rocky Mountain, Pacific, and Alaskan refugia. Animal adaptations are similar to those in the deciduous forest, but often more extreme. Boreal species, for instance, must have greater physiological tolerance to cold to remain active over winter. Mi- gration is also more extensive, although in mountain- ous areas migration is largely altitudinal rather than longitudinal. Browsing is more common, for the ground during winter months is generally covered with snow. Insects often cause considerable damage LocALiry AUTHORITY Desert or grassland Northern Arizona Merriam 1890 below 1830 m (6000 ft) to forest trees, and mosquitoes and flies are annoying both to animals and man. White man is relatively less numerous in this biome in North America than are Indians. Chief oc- cupations are logging, hunting and trapping, and mining. Woodland and chaparral biociations have some distinctive animal species but in many areas tend to be ecotonal between coniferous forest and grassland or desert. The zonation of communities on mountains de- pends principally on decrease in temperature with increase in elevation. In western North America, desert or grassland, woodland or chaparral, conifer- ous forest, and alpine tundra occur at successively higher elevations. In the Western mountains the following exam- ples of zonation of communities are of interest, the elevations given being the mean lower limits of the communities. It is evident that identical communities occur at lower elevations in the North than in the South. Idaho Larsen 1930 300 m (1000 ft) Pinon-juniper woodland 1830 m (6000 ft) absent Petran montane forest 2135 m (7000 ft) 300 m (1000 ft) Coast forest ecotone absent 615 m (2000 ft) Petran subalpine forest 2800 m (9200 ft) 1700 m (5500 ft) Alpine tundra 3500 m (11,500 ft) 2300 m (7500 ft) LocaLity California Washington AUTHORITY Hughes & Dunning 1949 Taylor 1922 Coast forest absent sea level Grassland below 150 m (500 ft) absent Oak-woodland and coastal chaparral 150 m (500 ft) absent Sierran montane forest 760 m (2500 ft) absent Sierran subalpine forest Alpine tundra Syiet 1980 m (6500 ft) 3350 m (11,000 ft) 1385 m (4500 ft) 2000 m (6500 ft) Geographic distribution of communities 24, Geograph ic Distribution of Communities: Tundra Biome Tundra typically extends from tree-line to the line of perpetual snow and ice, both in the far North and at higher mountain elevations. It is essentially similar in North America and Eurasia (Berg 1950). Very little tundra occurs on Antarctica. CLIMATE, SOIL, AND TOPOGRAPHY Arctic regions in North America have an annual preciptation less than 4 cm (10 in.), although east of Hudson Bay it occasionally reaches 8 cm (20 in.). Most of it comes as rain during the sum- mer and early autumn; snowfall is generally light (Koeppe 1931). Humidity is high and evaporation low during the summer. Mean monthly temperatures vary between ex- tremes of ~—35°C and +13°C (—30° and 55°F). The July isotherm of 5°C (41°F) is sometimes used to separate the so-called high arctic and low arctic, and the isotherm of 10°C (50°F), which corresponds closely to tree-line, to separate the low arctic from the sub-arctic. Frost may occur at any time in the North, but there is usually a frost-free period of about 60 days in the South. During the summer, the surface of the ground commonly thaws to a depth of only a few centi- meters; permanently frozen soil, permafrost, under- lies (Ray 1951). The soil becomes wet and soggy, and the accumulation of water in depressions forms numerous shallow ponds. Freezing and thawing are potent forces in arctic regions, since they may occur daily for long periods of time. This action fragments large boulders into small rocks ; forms polygon shapes on level ground surfaces varying in diameter from a few centimeters to several meters; develops large ground ice or peat mounds or smaller hummocks (frost heaving) ; causes downward slumping of soil on slopes to form terraces, or, a gradual creep of rocks and soil downslope with the consequent round- ing off of ridges and other irregularities in the topog- raphy. The general moulding of the landscape by frost action is called cryoplanation and is of eco- logical importance because it makes the soil unstable and limits the kind of vegetation that can develop on it. During the winter, the soil freezes down to the permafrost, except under streams, on stream banks and narrow flood-plains, and in sandy areas. Lakes are frozen for nine months of the year with ice a meter or more thick. Under the ice, oxygen almost disappears so that conditions are usually critical for animal survival (Andersen 1946). Ponds less than one or two meters deep freeze to the bottom. In the spring, absorption of solar radiation causes the mean temperature of the surface of the soil to rise above freezing three or four weeks before the mean temperature of the air, and it is at this time that 32) plants and ground animals renew their growth and activity (Sorensen 1941). Below the surface, ground temperature rise lags behind rising air temperature. Temperature just below the surface is higher than at greater depths in the summer, but lower in the winter, the turnover coming in April or May in the spring and September or October in the autumn (Beckel 1957). Photoperiods north of the Arctic Circle vary from zero hours during mid-winter to twenty-four hours during mid-summer. Elsewhere, the length of periods depends on latitude. Even in the summer, however, light intensity is low compared with trop- ical latitudes. In contrast to the arctic plains, alpine habitats on the higher mountain slopes are on rugged, often precipitous, terrain. Each mountain top is an island isolated from other mountain tops by intervening forested lowlands. High plateaus may have more or less level surfaces, but the size of such tundra-covered areas, except for the Tibetan plateau of Asia, is gen- erally very limited. These alpine habitats also lack a permafrost in the subsoil, except in the far North, and the extreme change in photoperiod during the year that arctic regions experience. Soils are thin and unstable except in small pockets on the slopes, in valleys, or on protected flat surfaces. Average tem- perature is low, but the range between daily maxi- mum and minimum during the summer is sometimes as great as 32°C (58°F). North slopes are colder than south slopes. The length of the growing season between killing frosts is similar to what it is in the arctic. Precipitation and humidity are commonly high, and the mountain tops are frequently shrouded in fog. Snowfall in some areas, as in the Cascades, may reach 18 m (60 ft) and is generally much greater than in the arctic. Water runoff is rapid because of the severe topographic relief. Winds are strong. On clear days, light intensity, notably ultra- violet, and evaporation may be high because of the thin air. A unique characteristic of the alpine habitat not shared by the arctic is low barometric pressure and oxygen concentration, which probably does not affect plants as much as it does some animals. Alto- gether, the alpine environment imposes greater se- verities on plant development than does the arctic environment (Bliss 1956), and this is doubtless true for animal activities also. VEGETATION Tundra has the appearance of short-grass plains, but differs in that the vegetation consists of sedges, rushes, lichens, mosses, ericaceous or de- cumbent shrubs, and flowering herbs as well as grasses. The plants are generally of small size, stunted growth, and compact structure adopted to resist desiccation and mechanical abrasion from wind, snow, and sand. Germination of seeds is poor, and most species require several years to produce the first flowering. Most tundra plants are therefore peren- nials, and vegetative reproduction is important. Seral vegetation varies in composition depending on whether it develops around ponds, in low wet places, or on clay, sand, gravel, or rock. Flowering herbs are often abundant with different species coming into bloom progressively during the year (Sorensen 1941, McClure 1943). Succession has been studied in only a few areas, and the true nature of the climax is unknown over much of the arctic. There is doubt as to whether or not a stable climax, as understood for southern lati- tudes, actually develops (Raup 1951, Sigafoos 1951, Britton 1957). This is due to the instability of the soil, varying depths to which the soil thaws out in the summer, depth and duration of the snow cover, exposure to wind, and grazing and trampling by animals. Although tundra associations are not recog- nized in this book, we do make a primary division of the biome into arctic tundra and alpine tundra. Arctic tundra The so-called barren grounds of the far North are divisible into four significant types in respect to animal distribution. Bush or mat tundra contains dwarf trees, decumbent shrubs, or heath, usually mixed with mosses and lichens. Near Churchill in northern Manitoba, much of the area is muskeg ; but climax vegetation is interpreted as a mixture of low Ericaceae heath and Cladonia lichen growing in a mat of sphagnum and other organic material 7 to 10 em (3-4 in.) thick. This climax develops on wet ground, clay, sand, and on gravel and rock ridges (Shelford and Twomey 1941). A variety of dwarf shrub-lichen-grass-sedge types have been described in Alaska (Hanson 1953, Churchill 1955) and in the eastern arctic (Polunin 1934-35, 1948, Holttum 1922). Grass tundra is largely limited to deeper mineral and organic soils. The soils are more fertile, and in places thaw out in the summer to a depth of one meter. Different species of grasses and sedges are dominant in recognizable seral and climax stages (Hanson 1951). Lichen-moss barrens (Tanner 1944, Hanson 1953) have been called desert tundra or rock desert by various investigators. The soil is thin, and there is much exposed rock. Vegetation is scant and consists of crustose and foliose lichens, mosses, and scattered short herbs or very small shrubs. In the low arctic, this may be a seral stage, but in the high arctic it is often the only vegetation able to tolerate 316 Geographic distribution of communities the severe climate. In the extreme North there is perpetual snow and ice, a polar desert; vegetation is practically absent, and animal life is restricted to marine forms along the ocean coast. The slight de- velopment of tundra in the antarctic is of the lichen- moss barrens; most of the antarctic continent is cov- ered with ice (Lindsey 1940). Alpine tundra Tundra extends into the tropics on the high Western mountains and into New England on a few of the higher Appalachian peaks. This alpine tundra consists chiefly of grasses and sedges without con- spicuous development of Ericaceae or the great masses of foliose lichens and mosses found in parts of the arctic (Cox 1933, Daubenmire 1943, Hay- ward 1945). About 37 per cent of 170 vascular spe- cies collected in the alpine tundra of the Colorado Rockies also occur in the arctic, and about half of these are circumpolar in distribution. Most of the remaining species are endemic to North America, and many species are uniquely endemic to the Rockies (Holm 1927). The taxonomic composition of alpine vegetation varies greatly from place to place, but most of the plants are perennials. The dwarfness of the shoots in proportion to the flowers and fruits that they bear is very striking. As one ascends the moun- tain slopes, the grass tundra gives way to lichen- moss barrens, and then to perpetual snow and ice. On the downslope side, there is often bush tundra and at tree-line the trees are dwarf and misshapen (krumholz) from the wind and cold. Flowering herbs are often abundant and conspicuous. The occurrence of krumholz is evidence that trees have extended up the slopes of mountains as far as they are able under present climatic conditions (Griggs 1946). The alpine tree-line is usually very irregular. Outlying trees may occur at some distance in advance of the forest proper if they can secure the protection of an embankment, or find other suitable microhabitats. In some mountain areas trees advance to higher altitudes on ridges than in valleys because snow accumulates to greater depths in the valleys and takes longer to melt. Origin The origin of the tundra flora is uncertain (Raup 1941), but the species involved may be segre- gates from seral stages, especially bogs, of the Arcto- tertiary flora that were tolerant of arctic and alpine environments. With the cooling of the continent and the coming of the glaciers we may suppose that these species were left behind when the rest of the flora FIG, 24-1 Tree-line in Rocky Mountain National Park, Colorado. Sub-alpine forest below, alpine tundra above. retreated southward. During Pleistocene glaciation, these species survived in the Alaska refugium (Hul- tén 1937), in possibly unglaciated islands of the Arctic Archipelago, and along the southern margin of the glacier. Pollen profiles indicate that climate and vegetation in the Alaska refugium were not greatly different then than now (Livingstone 1955). During the warm interglacial periods, tundra was probably limited to far northern regions and high mountains, with forests covering much of what is now the low arctic. When the glaciers retreated in post-Pleistocene time, there appears to have been a period when many tundra species were continuous in distribution from the arctic plains onto the mountain slopes in the northern Rockies. Probably at this time also arctic species were able to disperse farthest southward, as alpine tundra occurred more extensively at lower ele- vations, and intervening forests were less extensive (Daubenmire 1943). As the forests dispersed north- ward through the valleys and lowlands and then gradually up onto the mountains, alpine vegetation retreated to the higher elevations and became sepa- rated from the arctic tundra proper. This northward dispersal of coniferous forest was especially rapid during the thermal maximum period, but in some localities, as in Alaska, it is not yet complete (Griggs 1936). On the other hand, the tree-line may be re- treating southward at the present time in other areas (Raup 1941). The coming of the forests interrupted the complete colonization of alpine slopes by arctic species, but the forests brought a new element into the mountain flora derived from refugia south of the glacier (Raup 1947). Since the coniferous forest contained seral grassy stages with species intruding from the grassland biome, some of these species also penetrated the alpine vegetation and became part of it. Furthermore, tundra and grassland probably came into direct contact during the glaciation periods, so there is intermingling of tundra and grassland species in arctic as well as alpine regions (Hayward 1945). Tundra biome S iby ARCTIC TUNDRA BIOCIATION Origin The tundra fauna probably began to evolve late in the Tertiary, along with the tundra flora, as the continent cooled and the Arcto-tertiary flora re- treated southward. The true arctic fauna is appar- ently derived from previously wide-ranging forms able to tolerate cold climates (Johansen 1956-58) and from coniferous forest and grassland, fresh-water marshes, the seacoast (Stegmann 1938), and moun- tainous or upland regions (Larson 1957). Adapta- tions in the avifauna proceeded along lines either of toleration of the many hours of darkness and severe cold of the winter climate, or to the development of extensive migrations, sometimes far into the southern hemisphere as in some shorebirds and terns. During glacial periods of the Pleistocene, the tundra of the Alaska refugium was isolated from the tundra south of the glacier but in direct contact over the Bering land bridge with the unglaciated tundra of Asia. During this time there was doubtless an exchange of fauna with emphasis on invasion of Eurasian forms into North America. With recession of the glacier and disappearance of the land bridge, the Alaska tundra again became isolated from Asia and connected to the North American tundra. This has allowed Asiatic species to disperse over North America to varying degrees (fresh-water triclads: Kenk 1953, birds: Cade 1955). Some species, such as the grizzly bear, doubtless invaded the tundra from the western coniferous forests, perhaps since Pleisto- cene time. Other species now on the tundra are prob- ably derived from refugia in the Arctic islands and from south of the glacier (Rand 1954, Johansen 1956-58). Composition There is enough uniformity in the animal life of the arctic tundra in North America and Eurasia so that only one biociation is presently recognized. Circumpolar distribution is characteristic of both vertebrates (Udvardy 1958) and_ invertebrates (Netolitzky 1932). Common animals of the tundra are those listed below, which are conspecific or repre- sented by equivalent species on the two continents. Marine or strictly coastal species and those of more limited distribution are omitted (Bailey 1948, Ban- field 1951, Bee 1958, Harper 1953, 1956, Manniche 1910, Manning 1946, 1948, Porsild 1943, Preble 1908, Rausch 1953, Salomonsen 1950-51, Soper 1944, 1946, Stegmann 1938, Taverner 1934, Taverner and Sutton 1932) : Mammals. Masked shrew Grizzly bear Arctic shrew Polar bear (limited to Arctic hare coast ) Ermine Wolverine (glutton) Barren ground caribou (reindeer ) Peary’s caribou (limited to North) Muskox Arctic ground squirrel (suslik) Tundra vole Brown, European, Siber- ian lemmings Collared lemming Gray wolf Arctic fox The muskox, formerly of wide distribution, is now restricted to North America and Greenland. The tundra in North America is richer in species, both of mammals and birds, west of Hudson Bay than it is eastward, and richest in Alaska. The most abundant mammals on the tundra are lemmings, and in peak years their numbers are enor- mous. Among the larger animals the caribou form large herds and are important in the food and eco- nomics of Eskimos and Indians. Seton (1912) esti- mates their original number at 30 million, but they are much reduced at the present time. There are also fewer muskox now (C.H.D. Clarke 1940). Birds Sanderling Baird’s sandpiper Pectoral sandpiper Yellow-billed loon Arctic loon Red-throated loon White-fronted goose Purple sandpiper Oldsquaw Dunlin Common scoter Red phalarope Rough-legged hawk Gyrfalcon Peregrine falcon Willow ptarmigan Rock ptarmigan Sandhill crane Semipalmated plover Northern phalarope Pomarine jaeger Parasitic jaeger Long-tailed jaeger Herring gull Glaucous gull Arctic tern Black-bellied plover Snowy owl American golden plover Horned lark Long-billed dowitcher Common raven Whimbrel Water pipit Ruddy turnstone Lapland longspur Knot Snow bunting In addition to these species, the North American faciation contains the whistling swan and snow goose in the north; Canada goose and semipalmated, least, and white-rumped sandpipers rather generally dis- tributed; and in the west, the Eskimo curlew (now probably extinct), Hudsonian godwit, stilt and buff- breasted sandpipers, and Smith’s longspur. The Eurasian faciation also contains some species limited to it: two species each of swans and geese, several 318 Geographic distribution of communities plovers and sandpipers, and another pipit and bunt- ing. Less than one-third of the above species of birds are entirely terrestrial in their life requirements. Many species get their food from the fresh-water ponds and lakes or on the margins of these bodies of water, so characteristic of at least the low arctic tun- dra. On Banks Island in the high arctic, one study (Manning et al. 1956) gave the following population per square mile (260 hectares) : 38 Lapland long- spurs, 15 sandpipers and plovers of several species, 4 horned larks, 1.5 ptarmigans (2 species), and less than one snow bunting. Reptiles and amphibians are poorly represented where not absent, and the invertebrate fauna is com- paratively restricted in variety. In the ponds on the west side of Hudson Bay occur a stickleback fish, a flatworm, a leech, an annelid, a few snails, a couple of phyllopods, a few species each of Cladocera, Copepoda, Ostracoda, and Amphipoda ; a good repre- sentation of dytiscid and hydrophilid beetles, and an abundance of midge fly larvae. Since the lakes and rivers thaw out for only a few weeks, annual produc- tivity is low (Frey and Stahl 1958). Fish are more numerous in rivers, and are largely migratory sal- monids. Pond life in the Alaskan tundra is essen- tially similar to that near Hudson Bay (Johansen 1922). On land, the snails Succinea and Vertigo are found in wet tundra on the west side of Hudson Bay. Spiders and mites are well represented. Springtails and flies are especially numerous among the insects, and there are a few species of Lepidoptera, Cole- optera, and Hymenoptera, but species of Hemiptera, Homoptera, Orthoptera, Odonata, and Neuroptera are scarce or absent. Ants are scarce on the tundra but bumblebees are conspicuous. Especially noteworthy are the vast devastating hordes of mosquitoes, black flies, and deer flies that reach a peak of numbers in mid-July (Seton 1912, Shelford and Twomey 1941, McClure 1943). The invertebrate life of western Greenland is essentially similar (Longstaff 1932). Quantitative studies of the soil fauna in eastern Green- land showed that springtails and mites, especially Oribatidae, reached populations of 780,000 per sq m in bush tundra, but only 3000 per sq m in the lichen- moss barrens (Hammer 1937). Seven different so- cieties of invertebrate fauna have been differentiated here (Macfadyen 1954). In comparison to the arctic, the antarctic supports a limited fauna. Among the invertebrates, one study found several peculiar species of Protozoa, 16 species each of rotifers and tardigrades, two fresh-water crustaceans, mites, and at least 18 species of insects. Vertebrates are primarily marine although several species, especially birds, nest on land (Lindsey 1940). Food is more abundant along the shores of north- ern oceans; the association of sea and land provides niches for various species not found abundantly in- land (Freuchen and Salomonsen 1958). During the winter the sea is covered with ice, there is little or no light, and phytoplankton is scarce or absent except for reproductive spores and eggs. However, nutritive salts, such as nitrates and phosphates, accu- mulate in large supply, so that in May when the ice disappears and light returns there is an almost explo- sive development of phytoplankton followed by micro- crustaceans and other zooplankton. This is the key to the teeming abundance of fish, sea birds, and ma- rine mammals that occur at this time. Large colonies of fulmars, cormorants, auks, murres, guillemots, gulls, and others nest on ledges of precipitous cliffs or in some cases on islands or shores down close to the water. Eider ducks and other waterfowl are fre- quently numerous. Vegetation is best developed in and around these colonies because of the rich nutrient added to the soil from the excreta of the birds. One of the most common seals is the ringed seal which remains over winter, even in the high arctic, by keep- ing blow-holes open through the ice. The harbor seal and harp seal overwinter in the more open waters of the low arctic. Other species of seals, whales, walruses, and polar bears occur during the summer throughout most of the maritime areas of the arctic. Animal adjustments White coloration is common, especially over winter, in several mammal species (artic hare, col- lared lemming, gray wolf, arctic fox, polar bear, ermine, Peary’s caribou) and in a few birds (willow and rock ptarmigans, snowy owl). When the ground is covered with snow, white coloration, of course, conceals both the prey and predators. Many of these species acquire darker coloration during the months FIG. 24-2 Rock ptarmigan males: left, summer plumage; right, winter plumage (courtesy Bert Babero). Tundra biome 319 between May and September. The white winter color apparently does not give special protection against heat loss from the body, as has sometimes been thought (Hammel 1956). A major habitat problem that tundra animals must solve is tolerating or avoiding the long severe cold of the winter season. Cold-blooded animals are generally acclimatized so that they remain active at temperatures down to freezing much better than their relatives in temperate and tropical zones. This is particularly true for aquatic species (Scholander et al. 1953, Bullock 1955). Invertebrates commonly pass the winter in the larval or pupal stage that is especially resistant to freezing, although beetles, spiders and some other forms may overwinter as adults. Rotifers, tardigrades, midge fly larvae, and dytiscid and hydrophilid beetles may be frozen in the ice for months or even years, yet resume activity immediately on thawing (Lindsey 1940, McClure 1943, Andersen 1946). Because of the short growing season and slow development, many tundra insects require two or more summers to complete their de- velopment. The larger mammals and over-wintering birds have good insulation in long, dense pelage or plum- age, and in fat. Heat production in their bodies is not greatly increased until very low air temperatures are reached (Scholander et al. 1950). The tarsi and legs of ptarmigan and snowy owls become well feathered in the winter. Voles, lemmings, and ermines escape the winter cold by staying in their runways and nests under the snow. The ptarmigan also digs tunnels into snow- banks where it roosts protected from the cold (Wet- more 1945), sometimes for days at a time. Only the arctic ground squirrel truly hibernates. This it does by excavating burrows into sandbanks or hills which, because of exceptional drainage, possess an area that remains unfrozen between the deep permafrost and the winter frost at the surface (Mayer 1953). The bears den up during the cold weather but remain ac- tive to the extent of giving birth to their young in the middle of the winter. Those species that cannot tolerate or escape the winter cold and lack of food migrate. The barren- ground caribou on the mainland migrates in long strung-out armies to the southern portions of the tundra, even well beyond the tree-line into the forest- tundra, to pass the winter, and their trails remain con- spicuous throughout the year (C.H.D. Clarke 1940, Harper 1955). The caribou on Greenland, Spitz- bergen, and the northern islands of the Canadian Archipelago are necessarily resident throughout the year. Migration of the bird fauna in this biociation is nearly complete; during the winter only an occa- sional hawk, ptarmigan, raven, or owl will be en- countered over the land, although marine birds occur wherever there is open sea. When the birds return in the spring they are quick to get nesting started. Often they are already mated, carry through their nesting cycle quickly, and then leave promptly again for southern latitudes. In general, the melting of the snow and the break- up of ice is the signal of transition from winter to summer. Although it is possible to recognize all of the four aspects (Sgrensen 1941), the change from winter to summer and back again to winter is so rapid that all aspects are abbreviated except the hiemal and aestival. May is the usual month of par- turition among the larger mammals, and the peak of bird nesting comes in late June and early July. Owls, hawks, water birds, and some passerine species do not breed in those years in which scarcity of food or delayed freeing of nesting grounds of snow and ice are detrimental to survival. Failure of hawks and owls to breed occurs especially in years when the lemming population is low (Marshall 1O52))e Ptarmigan, hare, voles and lemmings, and their predators, particularly the fox and snowy owl, are subject to oscillations in abundance that come at intervals of either 3-4 or 9-10 years. These oscilla- tions are more pronounced in the arctic tundra than in any other biome. The food coactions of the herbivorous animals are of interest. Caribou feed on lichens, including rein- deer moss, especially in the winter, which they un- cover by pawing through the snow. During the sum- mer they also consume shrubby growth and sedges (Harper 1955). Grass is the principal food of the muskox, but it also eats willow brouse and, less fre- quently, lichens and mosses (Jackson 1956). Ptar- migan feed on plant material left behind by these large animals. The chief food of ptarmigan, how- ever, is the buds, leaves, and tender branches of willow and other shrubby plants not easily obscured by snow. Their winter food appears to be richer in fats and contains less protein than the food they con- sume during the summer (Gelting 1937). Gulls are known to eat warble fly larvae rising from the skin of caribou (Scalon 1937) but depend mainly on dead fish that they find in open water, or on carrion. Most of the small passerine birds are seed-eaters or mixed seed- and insect-eaters. Berries become abundant in August, and are much sought after by birds. Exclu- sively insectivorous land birds, such as warblers, would find great difficulty in surviving and reproduc- ing on the open tundra. Shorebirds are largely in- sectivorous but depend largely on aquatic forms for food. Although the continuous arctic summer light permits activity throughout the twenty-four hour day, most mammals and birds need periods of rest, which they take at any time. Birds appear to rest 320. Geographic distribution of communities most frequently in the hours before midnight, but the periods of rest are shorter than in southern lati- tudes (Armstrong 1954, Cullen 1954). In one study conducted above the Arctic Circle, adult robins fed their young for 21 hours per day and the young birds grew so rapidly that they left the nest in 8.8 days instead of the 13 days usual in more southerly lati- tudes (Karplus 1949). During the winter, there is at least a faint glow of light for an hour or two at noon. It is during this period that ptarmigan and probably other birds feed most heavily (Gelting 1937). As a rule, northern birds also tend to lay larger clutches of eggs than their closest relatives in southern latitudes (Lack 1947-1948). Related to their nesting is the tendency for many species to have flight songs, sometimes given high in the air, for announcing the possession of territories and for so- liciting mates. This development of behavior is re- lated to the lack of high song posts and is similarly developed in the grassland biome. A final characteristic of these northern animals is their fearlessness of man. The few Eskimos, Indians, and white men who inhabit the tundra are so scat- tered that animals in general have not learned to fear them. To a certain extent this is true also of some boreal forest species; snow buntings, snowy owls, grosbeaks, crossbills, and other species may be ap- proached closely before they are moved to flee. Among mammals, individual arctic foxes not infre- quently linger close to human habitation for days at a time. Human relations The arctic is the home of Eskimos in North America and of the Lapps in Eurasia (Hadlow 1953, Freuchen and Salomonsen 1958). The Eskimos are concentrated along the coast as much of their food comes from the sea: fish, walrus, seal, and polar bear. During the summer, caribou flesh, bird eggs, and berries are eaten. Caribou fat and seal oil are burned in the Eskimo igloos to furnish light and heat, and pelts from these animals are made into clothing and blankets. Meat is eaten either cooked, dried, or raw, and some of it is frozen and buried in the ground for the winter days of scarcity. The Eskimo gets his transportation during the summer in light boats made of sealskin stretched over frameworks of driftwood or bone, and during the winter in sleds drawn by dogs. Fur trapping is the chief source of income. The Lapp lives much like the Eskimo, although he more commonly lives in a tent made of reindeer skin than in an igloo made of snow. The reindeer is used by the Lapp for pulling his sled, for meat, and for milk. The Lapp may also keep goats. Lapps move up and down the mountains in summer and winter in search of pasture for their animals, north- wards and southwards with the seasons for fishing ; below tree-line they occasionally grow meager crops. ALPINE TUNDRA BIOCIATION Tibetan faciation The largest alpine areas of the world lie on the Tibet Plateau and in the adjacent Himalayan Moun- tains of Asia (Hingston 1925, Schafer 1938). Oc- curring here are related forms of pikas, pipits, rosy finches, and horned larks also found in mountain areas of the western hemisphere, while the marmot and sheep may be conspecific with North American forms (Rausch 1953). Stegmann (1938) gives a long list of birds espe- cially characteristic of the Tibetan fauna. The Tibet Plateau may represent an important center of origin of alpine species, some of which then became dis- persed into the higher mountains of Europe and North America. The Tibetan fauna evolved inde- pendently from that of the Arctic tundra and there are few or no bird species common to the two. There is some overlap of species, however, with the Asiatic grassland (Mongolian fauna), Asiatic deciduous for- est (Chinese fauna), and Ethiopian desert (Medi- terranean fauna) biociations which suggests their possible remote derivation. North American faciation Because of its small total area, rugged terrain, and discontinuity between mountain peaks, there are only a few species characteristic of the alpine tundra in North America. Mammals are conveniently divided into two groups. Those occurring in the high tundra from Alaska to British Columbia are the collared pika, hoary marmot, singing vole, barren ground caribou, mountain goat, and Dall’s sheep. Species limited to the southern mountains are common pika, yellow- bellied marmot, and mountain sheep. The common pika is differentiated into over thirty subspecies in the various mountain areas. Shrews, bears, coyotes, weasels, badgers, mice, wapiti, and mule deer of the western montane biociation range up into the alpine tundra during the summer. Ground squirrels and pocket gophers reach this community south of Canada by extending their ranges from the low elevation grasslands through seral stages in the intervening coniferous forests. Of birds, the white-tailed ptarmigan, water pipit, and gray-crowned rosy finch are characteristic and widely dispersed. In the central and southern Rocky Tundra biome 37 FIG. 24-3 Columbia ground squirrel in alpine tundra, Glacier National Park (courtesy R.L. Day). Mountains the brown-capped and black rosy finches replace the gray-crowned rosy finch. Rock wrens and horned larks are occasional summer visitors. In the alpine meadows of the far North, savannah spar- rows and upland plovers are common, although they belong principally to the prairie biociation and grassland seral stages of the boreal coniferous forest. Also in the North, around ponds, occur lesser yellow- legs, herring gulls, short-billed gulls, and Bonaparte’s gulls (Drury 1953). Except for the caribou and pipit, the alpine and arctic tundra biociations have no important species in common. There appears less taxonomic relation to arctic tundra with animals than plants. | Probably most of the species listed above are of northern origin and, entering North America over the Bering land bridge, dispersed southward on the mountains as they became elevated and the alpine tundra differentiated. Reptiles and amphibians are uncommon. In con- trast to the arctic tundra, flies are comparatively few except in the vicinity of ponds, but there is an abun- dance of springtails, ground-dwelling beetles, leaf- hoppers, grasshoppers, true bugs, butterflies, ants, bumblebees, mites, and spiders (Hayward 1945, 1952) Alpine ponds have an impoverished fauna. In a small pond at 3507 m (11,500 ft) in the Colorado Rockies of maximum depth one meter and which freezes solid in the winter, plankton was scant during the summer after the ice melted, but midge fly larvae reached populations of over 1900/m?, Pisidium fin- gernail clams, 1470/m?, and tubificid worms, 168/m?. A fairy shrimp was the most characteristic metazoan in the open water, although a small number of aquatic insects occurred along the shoreline (Neldner and Pennak 1955). Phyto- and zooplankton tend gen- erally to be represented by fewer species and a smaller number of individuals than in temperate or tropical lakes (Thomasson 1956). Animal adjustments As in the arctic tundra, most of the residents in the alpine tundra that remain active over winter are white in color: mountain goat, mountain sheep, Dall’s sheep, and white-tailed ptarmigan. The pikas inhabit masses of loose rock rather than the climax meadow itself and also occur well below tree-line. During the summer they gather stacks of tundra vegetation and during the winter subsist on this hay. Pocket gophers spend most of their existence in underground burrows feeding on roots and bulbs and hence are well protected from winter cold. The hoary and golden-mantled marmots and ground squirrels hibernate. Wapiti and deer mi- grate to the lower mountain slopes and valleys for the winter as do most of the birds. There is often some downslope movement of mountain sheep and goats for the winter, but it is not so extensive as with the wapiti and deer (Hayward 1952). The be- havior and interrelations of wolves, Dall’s sheep, cari- bou, and other species in the high tundra of Alaska are described in detail by Murie (1944). It is of in- terest that subspecies of mountain sheep formerly occurred regularly on the Great Plains and in desert regions at low altitudes (Buechner 1960). Low temperature slows up the development of insects and other invertebrates and reduces the num- ber of generations possible during the year. Animals, however, are generally acclimatized to be active at low temperatures. For example, springtails are sometimes abundant on the snow, where they freeze at night and thaw out during the day. An interesting food-chain occurs with their feeding on conifer pollen falling on the snow and being fed upon by mites. Because of the usually strong winds, insects and even birds stay close to the ground and fly as little as possible. Many insects are wingless. Birds commonly feed and build their nests on the sheltered side of ob- stacles, or they crawl into holes and crevices. When in the open, they persistently face the wind (Hingston 1925). In the Colorado Rockies, grasshoppers are among the most numerous species. However, of 28 species recorded, only 11 are truly alpine species, the rest being found only as adults which have flown or been blown up from lower altitudes and do not repro- duce successfully there (Alexander 1951). In Tibet, grasshoppers go as high as 5540 m (18,000 ft) ; bees, moths, and butterflies to 6460 m (21,000 ft) ; and spiders to 6770 m (22,000 ft) (Hingston 1925). During the late spring, south slopes on the moun- tains become free of snow before the north slopes because they get direct radiation of the sun. Conse- quently, plant and animal life become active on south slopes before they do on north slopes. Individuals of the same species will also begin growth and repro- 322 Geographic distribution of communities duction on the lower slopes before they do on the higher slopes. Birds appear to construct more com- pact nests, as insulation against cold, at high than at low altitudes, and they build these nests on the exposed south sides of thickets or trees where they benefit from the heat of the sun (Heilfurth 1936). The low oxygen pressure at high altitudes ap- pears to be more critical for the warm-blooded mam- mals than it is for birds, which are adapted to fly at high elevations anyway, or for invertebrates and plants that have much lower rates of metabolism and oxygen requirements (Hall 1937, Kalabuchov 1937). Mammals moving up to high altitudes may become temporarily acclimated through increases in rate of respiration, in rate of heart beat, in number of red blood cells, and in hemoglobin, but these adjustments are seldom as effective as in those species which are permanent residents at high altitudes. Species accli- matized to low oxygen pressures are affected in a reverse manner when they move to low altitudes. Some mammals that have a wide altitudinal range, particularly the pocket gopher Thomomys, are, like plants, smaller in size at high than at low elevations with a continuous gradation between the extremes (Davis 1938). Human relations There are few humans living permanently in the high mountainous regions of North America, but white man goes up there, taking sheep and goats to summer pasture, and for recreational or sight-seeing purposes. Man does occupy the Tibetan plateau of Asia. There, he depends on the yak to plough his fields and to furnish meat, butter, and milk. Although much of the plateau is too rugged and cold for crops, millet, corn, and wheat are grown in sheltered valleys. SUMMARY The tundra biome extends beyond the tree line in the far North and on high mountains. It has low precipitation, low temperatures, a short growing season, and, in the arctic extreme seasonal changes in length of day and night and a permafrost in the ground. Vegetation is bush or mat tundra, grass tundra, or lichen-moss tundra. Perpetual snow and ice occur in extreme areas. The tundra flora is prob- ably derived from seral stages in the Arcto-tertiary flora which became segregated as the continent cooled in the Pliocene and Pleistocene. The arctic tundra biociation is fairly uniform faunistically in North America and Eurasia, although two faciations are distinguishable. Mammals, birds, mosquitoes, and flies are the most conspicuous ani- mals with springtails and mites predominant in the soil. White color is common among mammals and birds, especially during the winter. Acclimatization to cold is highly developed in many resident forms. Some small mammals remain active under the in- sulating cover of snow during the winter months. Most birds migrate. Cycles of abundance are pro- nounced in several species. The Eskimo and Lapp mainly hunt and fish for a living. The alpine tundra biociation is best developed in the Tibetan plateau in central Asia. Only a few spe- cies of mammals and birds are peculiar to the bioci- ation. Mammals are physiologically adjusted to the low oxygen pressure. Man finds habitation in this area difficult except during the summer months, when he brings his sheep and goats for pasture or comes for recreation. Tundra biome Siies: Grassland Biome Grassland presently occurs on all the continents, and at one time covered 42 per cent of the earth’s surface. Grasslands everywhere possess marked simi- larities in points of climate, physiognomy, and ani- mal mores. In Russia this community is termed the steppe, in Hungary, the puszta, in South Africa, the veld, and in South America, the pampas. (Carpenter 1940). In North America the tall, dense grasslands with their rich fertile soils in the eastern portion of the biome are called prairie; in the West, the short grasses and shallow soil characterize the plains. In North America, grasslands extend from north- ern Saskatchewan and Alberta and central British Columbia to central Mexico, and from Indiana to California. The eastern portion is a huge expanse, continuous except for forest strips in the river valleys, but the continuity of the western portion is broken by the many mountain ranges. In general, the ter- rain is flat or rolling, green in summer and brown in autumn and winter (Weaver and Clements 1938, Clements and Shelford 1939). CLIMATE Precipitation in North America may be as high as 100 cm (40 in.) per year adjacent to decidu- ous forest, but trees cannot spread into the grassland because of high rates of evaporation, late summer and autumn droughts that are particularly severe and prolonged during some years, and intermittent fires that kill seedling trees but from which grasses quickly recover. Rainfall decreases and becomes more irreg- ular and evaporation increases in a gradient from East to West or Southwest, because of the general pattern of air circulation over the continent (Kincer 1923, Borchert 1950). An isohyet of 2-3 cm (5-8 in.) precipitation separates grassland from desert. Few species of grasses can tolerate the entire range of precipitation, and differences in the moisture requirements of species is the main reason for the subdivision of the biome into its various plant asso- ciations. There is little snow even in northern por- tions of the grassland, but winds are normally heavy, and there occasionally are severe winter blizzards. Temperature is not as critical a factor as mois- ture, as is evident in the great North-South distribu- tion of grasslands. Temperature, however, helps to separate temperate grasslands from tropical grass- lands. In the North, mean monthly winter temper- atures drop to —15°C (5°F), while summer temper- atures in the South may exceed 32°C (90°F). There are great seasonal and daily ranges of temperature. The frostless period in the North may be only 100 days long, but in the South frost rarely or never occurs. In the California and bunch grass prairies, B25: most of the rainfall comes in the winter months, and in southern California some grasses start their growth in late autumn and come into bloom in De- cember, although others wait until spring. VEGETATION Grassland owes its characteristics to the perennial grasses that constitute the dominant climax vegetation. Annual grasses are largely confined to seral stages. These climax grasses may be tall (1.5-3 m), mid (0.5-1.5 m), or short (less than 0.5 m) and grow in bunches or as sod. Forbs occur mixed with the grasses, and variation in the time of blooming of these broad-leaved and mostly perennial herbs as well as of the grasses gives the grassland a variety of as- pects like the forest (Weaver 1954, Weaver and Albertson 1956). The prairie has beauty, character, and a history all its own (Craig 1908a, Weaver 1944), Grasses grow quickly after the onset of warm and rainy weather and are adapted for long quiescent pe- riods of dryness and cold. The leaves or tops of the grasses die down during unfavorable seasons, but underground buds regenerate new growth during the next favorable period, even if this be delayed for some years. After dry-season fires in tropical cli- mates, new shoots sprout from perennial grass bases. These provide a sparse forage for grazing animals, a forage that, except for the fires, would not be present until the rainy season ( Vesey-Fitzgerald 1960). The grasses and forbs are deeply and extensively rooted, except in arid climates, where a hardpan occurs near the surface (Fig. 11-4). Competition is primarily for the limited water supply and only secondarily for light. Since grasses grow from the base of the leaf, they can tolerate considerable grazing by large her- bivorous animals, and this is an important factor in their dominating the prairie. Several of these, the most important genera of grasses in the North American grasslands, occur also in other parts of the world (Clements and Shelford 1939) : Mostly tall and mid grasses Andropogon—blue stem Aristida—triple-awned Agropyron—wheat grass grass Elymus—wild rye Bouteloua—grama grass Festuca—fescue Buchloé—buffalo grass Koeleria—June grass Panicum—panic grass Poa—blue grass Sporobolus—drop-seed Stipa—needle grass Mostly short grasses PLANT ASSOCIATIONS OF NORTH AMERICA Stipa-Sporobolus association (true prai- rie): Mostly tall and mid grasses in a long strip extending north and south in the eastern more humid part of the biome next to deciduous forest. Much of this prairie was marshy and poorly drained before white man came (Hewes 1951). Oak-hickory forests occur as scattered groves in better drained areas on hills, in sandy areas, and along streams making a savanna. The coastal prairies of Texas constitute a faciation of this community. Stipa-Bouteloua association (mixed prairie) : Mid grasses confined to the moister low areas; short grasses, to drier hillslopes. Bouteloua-Buchloé association (short grass plains) : On Great Plains east of Rocky Mountains. The climate here is so dry that mid grasses are incon- spicuous except during wet years. Pronghorn and bison in former days consumed mid grasses as fast as they appeared, in preference to the short grasses, and hence should be considered co-dominants along with the grasses (Larson 1940). Agropyron-Festuca association (bunch grass prairie) : In northern half of the Great Basin and into British Columbia, mostly isolated from rest of biome by mountains and desert. Precipitation comes chiefly as snow and rain during winter months. Dominant species mostly mid grasses which grow as bunch grasses. Overgrazing by domestic animals has per- mitted the less palatable sagebrush and related spe- cies to spread widely and give character to the land- scape. Stipa-Poa association (California prairie) : Lo- cated in central valley of California almost completely isolated from rest of grassland, dominants are mostly mid and bunch grasses. This is a region of winter rains. Much of area is now cultivated or overgrazed and contains many weedy annuals and exotic species. Aristida-Bouteloua association (desert plains) : Most arid of grasslands, composed mostly of short and bunch grasses. It occurs from southeastern Texas to southern Arizona and extends well down into Mexico. Because of overgrazing and control of fire, desert and tropical shrubs, such as mesquite, creosote bush, Opuntia cactus, are conspicuous throughout the association. The sugary pods of mes- quite are eagerly eaten by cattle although the bony seeds resist digestion and are dispersed widely. Grassland biome 325 PALEO-ECOLOGY Grasses did not evolve until the Upper Cretaceous period of the Mesozoic, and did not be- come important in North America until the elevation of the Western mountains in mid-Tertiary produced a semi-arid climate in the middle of the continent. Several of the mid grasses are circumpolar in dis- tribution and may have constituted seral stages in the Arcto-tertiary flora. They probably segregated out to form the true, mixed, and bunch grass prairies when the forests belonging to this flora retreated southward and eastward. The close relation of grass species on the prairies of North America and north- ern Eurasia is doubtless due to their similar deriva- tion from the Arcto-tertiary flora. The tall grasses of the Andropogoneae may be of tropical origin. The short grasses were probably derived from the Madro- tertiary flora to form the short grass and desert plains. Although one cannot be sure because of the paucity of grasses in the fossil record, it appears that the grassland is of mixed and relatively recent origin (Axelrod 1950, 1952). During portions of the Pliocene and Pleistocene (Fig. 21-2), rainfall was heavy; grassland probably extended through the Great Basin and into the Mo- have desert. There is little information available to indicate whether the California grassland was ever in broad contact with the rest of the biome, but there was probably a narrow and irregular contact through mountain valleys at either or both the southern and northern extremities. During the post-glacial xerothermic period, grass- land doubtlessly retreated in the southwestern states and in Mexico as the desert biome became extended, but it is difficult to draw boundary lines for the ex- tremes of these advances and retreats. In the eastern part of the continent during the xerothermic period, prairie advanced as a peninsula far into the deciduous forest (Fig. 21-7). Relict patches, including the Illinois prairie, still remain after later cool and more humid climate permitted the forest to recover much of the area it had previously lost. Doubtless the prai- rie also advanced northward into the area now domi- nated by the boreal forest of central Canada during the xerothermic period, where relict patches of grass- land may still be found. Reinvasion of prairie areas was slow at first because of frequent fires caused by lightning and Indians and to poor drainage, but since settlement of the area by white man and artificial lowering of the water table it has become rapid’ (Gleason 1922). In spite of its vast extent, only a single grass- Jand biociation can be recognized at present in North America, although it varies in composition between different regions. NORTH AMERICAN GRASSLAND BIOCIATION The North American grassland biociation extends in reduced form as a biocies or seral stage into the deciduous and coniferous forest biomes. Common species are the following, sometimes re- placed locally by related species: Mammals Harvest mice Deer mouse Masked shrew White-tailed jack rabbit (North) Northern grasshopper Black-tailed jack rabbit mouse (South) White-throated wood rat Eastern cottontail (South) (East) Meadow vole Desert cottontail (West) Nuttall’s cottontail (South ) Black-tailed prairie dog Ground squirrels Northern pocket gopher Prairie vole (East) Meadow jumping mouse Coyote Swift fox Long-tailed weasel Black-footed ferret Plains pocket gopher Badger Pocket mice Prairie spotted skunk Banner-tailed kangaroo Pronghorn rat (South) Bison Birds Western meadowlark Dickcissel (East ) Lark bunting Savannah sparrow Grasshopper sparrow Vesper sparrow Lark sparrow McCown’s longspur Ferruginous hawk Great prairie chicken Lesser prairie chicken Sharp-tailed grouse Long-billed curlew Upland plover Burrowing owl Short-eared owl Horned lark (North) Sprague’s pipit (North) Chestnut-collared long- Bobolink spur (North) In addition to the more strictly grassland spe- cies above, many species from the deciduous forest- edge biociation extend their ranges varying distances into the open country. Many desert species also ex- tend their ranges into the grassland biome, especially where sagebrush, mesquite, and other shrubs come into overgrazed areas. Because of these various in- fluences, the long North-South range through vari- ous temperatures, and the isolated nature of some portions of the grassland, several faciations may be recognized (Carpenter 1940). They have not yet been clearly defined but may correspond to the biotic provinces described by Dice (1943) and Blair (1954) ; viz.: Illinoisan in the East ; Saskatchewan in the northern Great Plains; Kansan in the central 326 Geographic distribution of communities TABLE 25-! Equivalent species in grassland communities around the world (modified from Allee ef a/. 1949: 470-471). North America Ecological niche 1. Saltatorial (leaping) herbivores a. grasshoppers Melanoplus Schistocerca spretus paranensis Melanoplus differentiales Melanoplus maculipennis Melanoplus ponderosa b. mammals jackrabbits 2. Fossorial (burrowing) herbivorous mammals a. feeding in herb ground viscacha stratum squirrels pampas cavy prairie dogs b. feeding in sub- pocket gophers | tucotucos terranean stratum 3. Cursorial (running) rhea herbivorous birds 4. Cursorial gregar- ious herbivorous mammals pampas deer guanaco bison pronghorn 5. Cursorial predators a. snakes blue racer bull snake prairie rattler b. mammals swift fox coyote (wolf) (cougar) Pampas cat red wolf Great Plains; Texan and Comanchian in the South; Navahonian in contact -with the Southwestern desert ; Palusian corresponding with the bunch grass prairie ; and Californian, including the California prairie. The student should re-read Chapter 9 which gives additional data on the grassland fauna especially in regard to reptiles and invertebrates. South America Cyclagras gigas Rhadinea merremii Locusta migratoria Locusta migratorioides Stauyronotus macroccanus Schistocerca peregrina springhaas Australia red kangaroo souslik African ground wombat hamster squirrels European bobak rabbit (in- troduced) mole rat golden moles marsupial mole ostrich emu saiga quagga pig-footed goitered gazelle | Burchell’s zebra bandicoot and other eland gazelles springbok maral gazelles (several wild ass species) wild horse black wildebeest blue wildebeest bubal bontebok 30 other genera of antelopes common cobra puff adder death adder Asian moccasin | black-necked tiger snake elaphe cobra rock python Pallas cat lion Tasmanian corsac fox serval wolf cheetah caracal (mar- jackal cheetah supial) Cape hunting dog OTHER BIOCIATIONS Grasslands in other parts of the world (Brehm 1896, Haviland 1926, Stegmann 1938) are occupied by species ecologically equivalent to those that inhabit the grasslands of North America. The parallel evolution of adjustments and behavior in Grassland biome a2 FIG. 25-1 Grassland ani- mals. Clockwise, bison, pronghorn, coyote, badger (courtesy U.S. Forest Service). animals that occupy similar habitats, although often quite unrelated taxonomically, is of particular interest and may best be shown in table form (Table 25-1). ANIMAL ADJUSTMENTS It is in the grasslands throughout the world that the large herbivorous ungulates reach their largest populations. Their adaptations for feeding on grasses and the high productivity of grasses, which in fact is stimulated by moderate grazing, gives an effi- cient food coaction of high energy utilization. A for- est cannot support such large populations of grazing animals since the herb layer is less luxuriant, com- posed more of the broad-leaved herbs, and shrub and tree foliage cannot tolerate continued browsing. The evolutionary development of these large ungulate populations had to await the evolution of these ex- tensive grasslands in mid-Tertiary time (Stirton 1947). Previous to the formation of the grasslands in North America, we may suppose that the ancestral forerunners of bison and pronghorn were largely lim- ited to seral grassy stages in the Arcto-tertiary and perhaps Madro-tertiary floras. It is common for these large ungulates to feed in large herds. The primitive population of bison in- North America is estimated at 50-60 million animals, perhaps an average of 6 per sq km (15 per sq mi). Few occurred west of the Rockies even in colonial times, but to the east and north the species penetrated far into forested areas. Herds containing 20,000 indi- viduals were common and an occasional herd reached a population as high as 4 million animals (Seton 1909). When attacked by wolves or other predators the bulls formed a circle facing outward with the cows and calves inside. The animals shed their fur in the sum- mer and were greatly annoyed by flies, mosquitoes, and the penetrating seeds of the needle grass. They relieved their miseries by rolling in wallows, covering themselves with mud. Some calves were killed by wolves, coyotes, bears, and mountain lions. Adults died in consequence of bogging down in sloughs or swamps, breaking through thin ice when crossing rivers in winter, and of old age. Disease apparently was never common. The animals regularly grazed until mid-morning, traveled sometimes 16 km (10 miles) to a water hole, rested and chewed their cud during mid-day, and grazed again in the evening. A herd would graze an area intensively for several days or weeks, then move to some other area. In these movements and migrations, the animals commonly traveled in single file. There is question, however, whether north-south migrations were very regular and extensive (Roe 1951). The pronghorn antelope occurs in the drier grass- lands, including California, and their primitive popu- lation is estimated at 4 per sq km (10 per sq mi). They traveled in herds of 100-200, sometimes 2000. When they were scattered in feeding, the approach of a predator was quickly signalled from one to the other by raising the hair in the white rump patch so that it flashed like a tin pan reflecting the sun. Safety for these animals depended largely upon fleetness of foot, and some of the fastest-running animals in the world occur in this biome (Craig 328 Geographic distribution of communities 1908a, Visher 1916). Joined with this ability was long-range vision to discern the approach of danger from a distance. Coyotes could not run as fast as pronghorn, but since the pronghorn usually ran in wide circles, the members of a pack of coyotes would sometimes run in relays until the pronghorn fell exhausted. Rodents and lagomorphs constitute the other principal groups of mammals. Ground squirrels, pocket gophers, mice, and jack rabbits are common nearly everywhere and sometimes reach plague pro- portions. Prairie dogs form towns, some in former days large enough to cover several square miles, with each animal feeding on the grasses and herbs only in the vicinity of his own burrow. Prairie dogs are most numerous on the Great Plains where the grasses are shorter, and a century ago they probably numbered in the billions. They as well as other species of ro- dents have numerous predatory enemies such as coyotes, badgers, ferrets, foxes, weasels, owls, and rattlesnakes. They have good vision, however, and quickly plunge into their extensive underground bur- rows at the approach of danger (Koford 1958). Jack rabbits, other small mammals, prairie chick- ens, as well as most of the small birds, grasshoppers, and other insects rely considerably on their well developed protective coloration—they freeze in the deep grass to escape notice of predators. If the preda- tor comes too close, they take to running, jumping, or flying with such a burst of activity so as to startle the intruder momentarily and give them a head start in their flight. The development of hopping locomotion among Grassland biome 329 grassland and desert animals is of special interest. We see it expressed in jack rabbits, kangaroo rats, pocket mice, and grasshoppers in North America ; all these species have short forelegs and long, strong, hind ones. Hopping enables the animals to get above the level of the grass for locomotion and is of advan- tage also in allowing greater visibility. There is a nearly complete absence of animals above ground in the winter. Bison formerly shifted their main populations from the northern to the southern portions of the grassland, pronghorns sought shelter in the pifon-juniper woodlands and petran bush in the valleys and foothills of the mountains, and most of the birds left for the South. Ground squir- rels, prairie dogs, and jumping mice hibernate in underground burrows, reptiles from over large areas aggregate into deep holes or crevices in the sides of hills, and invertebrates pass the winter in a dormant or inactive condition in the soil humus. Many of these animals also become inactive or aestivate dur- ing dry seasons, especially in late summer and autumn, and sometimes aestivation proceeds directly into hibernation so that the animals are active for only a short time each year. Grassland birds are direct, strong fliers and can withstand the fierce winds prevalent in open country. They are adroit in walking and running on the ground. They also appear better able to tolerate the continuous direct rays of the sun, but nevertheless on hot sunny days they commonly seek the shade of the tall grasses, scattered bushes, or line up in the narrow shadow of a fence post or telephone pole. With the absence of trees, many birds make them- selves conspicuous during the nesting season by de- veloping loud flight songs which they give high in the air. Flight songs are much more characteristic of grassland than of forest birds. Contrary to the herd instinct in mammals, flocking is not particularly char- acteristic of grassland birds, although prairie chickens and grouse exhibit gregariousness in. their mating performances. Birds are usually widely spaced with their nests well concealed in the grass (Kendeigh 1941a). The more humid portion of the grassland is studded with small ponds or potholes. These ponds are surrounded with marsh vegetation. Here are found numerous ducks, grebes, herons, bitterns, coots, rails, terns, and other marsh birds as well as muskrats. Animal life is especially concentrated around these water holes (Brehm 1896), although larger species roam widely through the upland for feeding purposes. HUMAN RELATIONS Tall grass prairie is highly productive agri- cultural land for corn, wheat, soybeans, and cereal 330) crops in general. This is evident both in North America and Europe. Hogs are raised on the corn that is grown. The more arid short grasslands, such as are found on the Great Plains and the northern Great Basin of North America and on the Russian steppe, are more hazardous to cultivate, as crops, mostly wheat, often fail during the dry years of cli- matic cycles. Plowed or overgrazed ground, destitute of grass or crop cover, is whipped up by strong winds to produce the great dust storms that have become so well known in recent years. Dry farming for wheat is a common practice in some areas, where land is cropped only every two or more years and left idle between times to accumulate ground moisture, or there may be crop production under irrigation (Weaver 1927). Man has not always used grassland intelligently (Shelford 1944) especially during the war years when the demand for food supplies was so great, because he has plowed up land where the grass cover should have been left intact. Arid grassland in the Great Plains had best be used only for stock raising, especially of beef cattle and sheep, and this is done extensively on our western ranches. In the arid parts of Asia, many different peoples have a nomadic existence in a never-ending search of pasture for the cattle, sheep, goats, camels, and horses that serve their everyday needs (Hadlow 1953). The history of early exploration and settlement on the North American grassland is of considerable in- terest (Malin 1947). Before the advent of white man, Indians were scarce over the grassland because of difficulties in transportation and in hunting large game animals. With the escape of horses from the early Spaniards and their rapid multiplication, the Indians soon learned to use them, and several tribes took to the prairies and plains. The white man was, at first, somewhat reticent about invading the prairies and kept his settlements to the forested areas along the streams. The prairies that he first encountered along the forest-edge in the East were flat, very wet in the spring, and poorly drained. The grass roots made a tough sod difficult to plow with the primitive equipment then available. There were difficulties in obtaining drinking water. Prairie flies were a nuisance, and the prairie fires that occasionally swept across the country were danger- ous. Furthermore, he was accustomed to using tim- ber for buildings, fences, and fuel, and the forest gave him protection from cold winter winds (Vestal 1939). In the course of time, however, and under the pressure of increasing populations, he learned to sur- mount these difficulties. Ditches were dug to drain the land and the streams were deepened. Better plows and other farm equipment made cultivation easier. Construction of roads, bridges, and railroads brought building equipment for homes, supplies, and other comforts of life. At the present time the tall Geographic distribution of communities grass prairie is the so-called bread basket of our modern civilization. SUMMARY Grasslands occur on all continents and are marked by low amounts of precipitation and high rates of evaporation. Climax grasses are mostly per- ennial and are characteristically tall, medium, or short in stature. Since their leaves grow at the base, they tolerate grazing by animals, and it is in this biome that ungulate mammals and rodents attain large population densities. The large herbivorous ungulates commonly go in herds, are fleet on foot, and have long-range vision. Some species of rodents form large colonies and dig extensive underground burrows. Locomotion — by hopping occurs in several mammals and some insects. Protective coloration is well developed in many kinds of animals. Upland birds are strong fliers and com- monly have flight songs. Scattered ponds surrounded by marsh have concentrations of many nesting spe- cies of birds and are the source of drinking water for the large mammals. Migration is well developed among birds and hibernation among small mammals, reptiles, amphibians, and invertebrates so that there is a nearly complete absence of animals above ground during the winter months. Only one biociation is recognized in North America and in each of the other continents, The more humid grassland areas make very fer- tile and productive agricultural land for man while the drier portions are best used for grazing of do- mestic animals. Grassland biome 331 2 Desert Biome Extreme desert is arid wasteland, with practically no vegetation. In the ecological sense, however, des- erts also include arid regions which contain consider- able vegetation in the form of bushes, shrubs, and trees especially adapted to tolerate hot, dry climates. Deserts are unique in possessing a large number of different life-forms among the dominants (Shreve 1942). Deserts, like any other biome, occur in belts at similar latitudes north and south of the equator around the world and they cover about one-fifth of its surface. Prominent deserts occur in southwestern United States and northwestern Mexico; in Sahara, southern Africa, Arabia, central Asia, Australia, and in a narrow strip along the west coast of South America. All these areas are at low elevations. In spite of its arid character, the desert, like other biomes, has a fascination and charm for one who becomes familiar with its inhabitants and their prob- lems (Jaeger 1955, 1957). CLIMATE Deserts around the world generally occur on the lee side of high mountains and continents, with respect to the prevailing winds. Average annual precipitation in the desert scrub of North America is usually not more than 5 in. (13 cm) and snowfall is slight (Jaeger 1957). Because of the high rate of evaporation and lack of penetration into the soil, Weather Bureau statistics are not indicative of how much moisture of precipitation is actually available to organisms. Long drought periods are typical, and the little rainfall that occurs is often in the form of short, violent storms or cloudbursts. The ground surface is generally baked hard, and most of the rain runs off; flash-floods are not infrequent (Lowder- milk 1953). Precipitation is slightly greater and evaporation less in the Great Basin. Where rainfall is so slight, dew formation assumes great significance. In the deserts of Israel, dew forms 120-240 nights of the year (Duvdevani 1953). The yearly evaporation from a pan of water may be 7 to 30 times the actual amount that falls on an area of similar size (Buxton 1923). The high evapo- ration rate correlates with the low relative humidity which at noon averages less than 25-30 per cent. There is little cloudiness, and the actually re- ceived percentage of possible annual sunshine aver- ages close to 90. Ultraviolet radiation reaches the ground in high concentrations. Winds are more or less continuous. The mean annual temperature in the Great Basin is approximately 10°C (50°F), but it is over 20°C (68°F) in parts of the desert scrub. Daily maximum summer temperature in the desert is 40°C (104°F) S13) or more. Differences between daily maxima and minima are greater than in any other biome. Frosts are limited to mid-winter ; the frost-free period aver- ages more than 280 days. Frost and snow are com- mon, however, in the Great Basin (Kincer 1941) ; consequently, the Great Basin area is sometimes called the cold desert in contrast to the hot desert adjacent to the south. The Great Basin is not a single large basin; rather, it is broken up into a number of small ones separated by low mountain ranges running in a north-south direction and seldom exceeding heights of 1800 m (6000 ft). These ranges support pifon- juniper woodland. Water drains from the surround- ing hills into these small basins from which there are usually no outlets. High rates of evaporation make the basin lakes very salty or may dry them up to produce alkaline flats. A characteristic topographic feature of the desert is the alluvial fan of erosion products washed down mountain slopes by the torrential rains. Broad flat basins occur between adjacent mountains. Extensive sand dunes occur in some portions of the desert, and sand and dust storms are spectacular features. VEGETATION Desert bushes and shrubs in North Amer- ica are seldom more than 1-2 meters high and are spaced 3-10 meters apart (Fig. 26-2). Joshua-trees, paloverdes, and saguaros are, however, more con- spicuous. The shrubs seldom form a canopy except along washes. The intervening ground between the shrubs is usually a wind-swept desert pavement of either fine texture soil, gravel, or rock, and always with very little humus. The shrubs and bushes have shallow, wide-spreading, many-branched root sys- tems adapted quickly to absorb any surface moisture. There is very little moisture in the subsoil. Stems and branches often bear prickles or thorns and inter- twine to form a dense tangle. A rich variety of thorny, succulent cacti occurs in the Western Hemis- phere only, and is divisible into tree, cholla, and barrel types. Between the shrubs, a few short annual grasses may grow, but after rains, the ground often becomes thickly covered with a carpet of flowers and grasses that quickly mature, seed, and then disappear in the dry weather that follows. These desert plants have sclerophyllous adapta- tions to retard transpiration and survive long periods of drought. Foliage becomes greatly reduced, even absent altogether, during long periods; stems con- tain the chlorophyll necessary to carry on photo- synthesis. Cell walls are thick, highly ligneous, and have thick cuticles. The cell sap increases in osmotic pressure and hydrophilous colloids. Cacti store con- siderable water in their stems as a reserve for use when there is no water in the soil. Many other kinds of adaptations occur in these xerophytes (Weaver and Clements 1938, Zohary, in Cloudsley-Thompson 1954). Biotic succession is not conspicuous in the desert because of the low rate of reactions by organisms upon the habitat (Shreve 1942). When the vegeta- tion is disturbed it is usually replaced directly by the same type without intervening seral stages (Muller 1940). Physiographic succession is evident, however, depending upon distances from water chan- nels, differences in elevation, leaching of salts out of the alkali flats, and in sand dune areas. There is considerable variation in the distribution of different species because of local differences in the chemical and physical nature of the soil, soil moisture, and so forth (Shantz and Piemeisel 1924). PLANT ASSOCIATIONS IN NORTH AMERICA Covillea-Franseria association (desert scrub) : Creosote bush, Covillea glutinosa, is generally distributed and with bur sage, Franseria, sometimes forms a monotonous, uniform growth on the flat inter-montane plains, relieved only by the larger aca- clas, saguaros, paloverdes, and mesquites along the washes. The richest variety of vegetation is on the outwash plains and lower mountain slopes, where there is greater penetration and retention of soil moisture. The desert scrub has three faciations (Shreve 1942, Axelrod 1950). The Mohave desert to the west is a rolling plain with a monotonous uni- form cover of low shrubs, interspersed conspicuously with the curious Joshua-tree. The Sonoran desert, sometimes subdivided into Colorado and Arizona deserts (Benson and Darrow 1944), or as many as six sub-units (Jaeger 1957), is much more diversi- fied, with tall shrubs, trees, and succulent cacti of many forms, especially along the washes, and a few grasses. The Chihuahuan desert to the east is almost completely separated by mountain ranges from the rest of the desert, and some of the western species are replaced by new ones that come in. The yucca-like sotol is conspicuous (Jaeger 1957). Atriplex-Artemisia spinescens association (shadscale association); Artemisia tridentata- Agropyron association (sagebrush association) : Shadscale, Atriplex confertifolia, and bud sage, Ar- temisia spinescens, as well as other small (less than 1 m high), widely scattered, more or less spinescent, microphyll shrubs are dominant in the southern part of the Great Basin and have contact with desert scrub. Greasewood and a few grasses are important Desert biome 333 GREAT BASIN SONORAN FIG. 26-1 Faciations of the desert biome in North America (after Axelrod 1950, Jaeger 1957). in some localities. Sagebrush, Artemisia tridentata, often occurs in nearly pure stands, but where grazing is limited, several species of perennial grasses, espe- cially wheat grass, Agropyron spicatum, become in- termixed to form a continuum leading into the bunch grass prairie to the north. Only a few opuntia cacti occur. Sagebrush is widely distributed as a subcli- max, because of overgrazing, in the bunch-grass prairie and short-grass plains. Various species of Artemisia also extend into central and southern Cali- fornia and together with associated species have been called coastal sagebrush. It may be subclimax to FIG. 26-2 Joshua trees inter- spersed through low shrubs of the Mohave desert in California (courtesy U.S. Forest Service). chaparral. The original vegetation of these two asso- ciations has become greatly modified as result of overgrazing by domestic animals and increased ero- sion with reduction in grasses and edible shrubs and introduction of exotic species (Fautin 1946, Cottam 1947, Billings 1949). A tropical thorn forest occurs on the west coast of Mexico, in northern Venezuela, and in other scat- tered localities. It is made up of a dense scrubby growth of small, often thorny and leguminous trees. Cacti are common. Some authors distinguish a thorn forest and a short-tree forest, but there is consider- able intergrading between the two (Gentry 1942) as well as with the tropical deciduous forest. PALEO-ECOLOGY During early Tertiary, the present desert regions in North America were largely dominated by tropical and warm-temperate forests. Following Eocene, rainfall gradually decreased, and forests were replaced first by grassland, then by desert. Des- ert began to appear during middle Tertiary on the lee side of high mountain ranges, but did not become extensive until middle and late Pliocene time. The present deserts in North America are therefore of comparatively recent origin. Desert plants have apparently originated through gradual adaptation to arid climates of more hardy species belonging to all three Tertiary floras. Ar- temisia, Atriplex, Eurotia, and Suaeda of the two Great Basin associations are thought to have Arcto- tertiary affinities. Related species in the same genera occur in the Eurasian deserts at the present time. Species of the desert scrub appear to be derived from 334 Geographic distribution of communities the Neotropical and Madro-tertiary floras, and are for the most part unrelated to Eurasian forms. Re- lated species and genera are found, however, in the arid regions of South America. The vegetation of the present Great Basin, Mohave, and Sonoran des- ert regions was largely woodland and chaparral through much of the Miocene and into the Pliocene. Desert vegetation became differentiated with the in- creasing aridity of the mid-Pliocene. The Mohave and Sonoran deserts became distinct as cool winters in late Pliocene and Pleistocene restricted the less hardy succulent species to the Sonoran desert. It is probable that the origin and development of deserts elsewhere over the world has followed the same gen- eral pattern (Axelrod 1950, Clements 1936). DESERT SCRUB BIOCIATION Large mammals, such as the bison and pronghorn, are mostly absent from the desert scrub. The mule deer is present in small numbers both in the desert scrub and in the basin sagebrush, and the mountain lion, bobcat, and badger penetrate to some extent. The most common species of animals are the following (Dice 1939, Huey 1942) although addi- tional species occur farther south in Mexico (Burt 1938, Van Rossem 1945, Baker 1956) (species with asterisks in this and following lists also found in basin sagebrush biociation) : Mammals *Black-tailed jack rabbit | *Botta’s pocket gopher Desert cottontail Desert pocket mouse Rock squirrel Rock pocket mouse Spotted ground squirrel Merriam’s kangaroo rat Round-tailed ground Desert kangaroo rat squirrel *Canyon mouse FIG. 26-3 In the Sonoran desert of Arizona. Saguaro (tree cactus at left), paloverde trees (|in middle distance), tree cholla (cactus at right center), organ pipe cactus (upper right), creo- sote bush (the taller bushes in the foreground), and bur sage (the smaller bushes in the foreground). Cactus mouse *Coyote *Deer mouse *Kit fox Southern grasshopper Gray fox mouse White-throated wood rat *Desert wood rat *Western spotted skunk Collared peccary Mountain sheep In the mesquite vegetation of New Mexico, the mouse and rat populations are highest in May with about 8.5 individuals per hectare (3.4/acre) and with the kangaroo rats the most numerous species. These small desert mammals tend to have larger home ranges than do comparable species in deciduous forest and grassland (Blair 1943). Several of these species extend their ranges well south through the Chihua- huan faciation (Dalquest 1953). FIG. 26-4 Sagebrush with sparse grass in Nevada (courtesy U.S. Forest Service). Desert biome 335 PLEISTOCENE Neotropical Tertiary | Flora Madro—Tertiary Flora FIG. 26-5 Changes in the relative proportions of different types of vegetation in the Mohave desert region since Miocene time (from Axelrod 1950). Birds Wied’s crested flycatcher Vermilion flycatcher Verdin Cactus wren Bendire’s thrasher Curve-billed thrasher LeConte’s thrasher *Red-tailed hawk Harris’ hawk Caracara Gambel’s quail Mourning dove White-winged dove Ground dove Roadrunner *Crissal thrasher *Great horned owl Black-tailed gnatcatcher Elf owl Phainopepla *Loggerhead shrike Lucy’s warbler Abert’s towhee *Black-throated sparrow Lesser nighthawk Costa’s hummingbird Gilded flicker Gila woodpecker Ladder-backed wood- pecker In addition to these, several species from the de- ciduous forest-edge, woodland, and chaparral pene- trate into the desert. Bird populations are very low in the open desert (0-37 pairs/40 hectares or 100 acres) but may reach 108 pairs per 40 hectares in washes or near water where there is a greater diver- sity of vegetation (Miller 1951, Hensley 1954, Dixon 1959). Reptiles Crested lizard *Leopard lizard Gopher turtle Banded gecko 336 Chuckwalla Zebra-tailed lizard Fringe-toed lizard Spiny lizards *Side-blotched uta Long-tailed uta Tree uta *Desert horned toads Desert gila monster Night lizard *Whip-tailed lizard Western blind snake Boa snake Whip snake Leaf-nosed snake *Bull snake Common king snake Western shovel-nosed snake Mojave rattlesnake Diamond rattlesnake Sidewinder rattlesnake The list of reptiles is compiled from the studies of Mosauer (1935), Dice (1939), Huey (1942), and Johnson et al. (1948). Amphibians are not common, but the red-spotted toad occurs in small ponds. Little quantitative investigation has been made of the in- vertebrate populations of desert scrub, but grass- hoppers and other orthopterans are especially con- spicuous (Tinkham 1948), and the scorpion and tarantula spider are much in evidence. BASIN SAGEBRUSH BIOCIATION This community inhabits both the shad- scale and sagebrush associations. There is consider- able overlap of species between the desert scrub and Geographic distribution of communities basin sagebrush biociations, as indicated by the spe- cies marked with an asterisk in the above lists, but there is a sufficient difference to warrant calling the two areas faunistically distinct. At the subspecies level, the contrast between the two animal commu- nities is more striking. In addition to a strong intru- sion of grassland species, the basin sagebrush has the following noteworthy species (Linsdale 1938, Hall 1946, Fautin 1946) : Mammals Nuttall’s cottontail Long-tailed pocket Pigmy rabbit mouse Townsend's ground Chisel-toothed kangaroo squirrel rat White-tailed antelope Ord’s kangaroo rat squirrel Dark kangaroo mouse Northern grasshopper mouse Sagebrush vole Least chipmunk Little pocket mouse Great Basin pocket mouse Rodents, exclusive of ground squirrels and pocket gophers, average about 40 per hectare (16 per acre) in western Utah with deer mice and kangaroo rats most numerous. Ground squirrels are widespread and numerous although sometimes locally restricted. Black-tailed jack rabbits are important constituents of the community and average numbers seen in dif- ferent plant communities range from less than 0.1 to 0.5 per hectare. The pronghorn was once numerous in the Great Basin sagebrush, but not the bison (Fautin 1946). Birds There is greater contrast in the avifauna be- tween the desert scrub and basin sagebrush biocia- tions than in the mammalian fauna. However, more species enter this community from the grassland and forest-edge biociations, such as the Swainson’s hawk, prairie falcon, burrowing owl, and horned lark, than venture into the desert scrub. Bird populations are low during the breeding season, averaging only about 25 pairs per 40 hectares (100 acres). The principal avian species in the basin sagebrush in addition to those listed above are (Fautin 1946, Miller 1951) : Sage grouse Poor-will Sage thrasher Sage sparrow Brewer’s sparrow Reptiles Collared lizard Long-nosed snake Sagebrush lizard Prairie rattlesnake Striped whip snake Lizards are numerous and conspicuous. Counts of only those seen above ground gave an average for the summer season of 6.5 per hectare (2.6/acre). A few amphibians, particularly the western spadefoot toad and western toad, occur near bodies of water (Linsdale 1938, Fautin 1946). Invertebrates Actually, only two strata occur in this com- munity, the shrub and ground, since herbs are few and scattered most of the year. In the shrub stratum, arachnids, cicadellids, fulgorids, coccids, chrysome- lids, and mirids are most numerous. Grasshoppers feed on the foliage and lay their eggs in open areas of the ground. Arachnids, tenebrionid beetles, and ants are the most conspicuous ground invertebrates. The harvester ant and honey ant build conspicuous mounds, the number of mounds of the former aver- aging over 15 per hectare (6.2/acre) in the sage- brush community. Invertebrates are most numerous in sagebrush and greasewood and least abundant in shadscale. Maximum populations occur in May on most of the vegetation, after which they decline as temperature increases, but on the greasewoods, which retain their leaves and remain green, populations remain more constant throughout the summer (Fau- tin 1946). OTHER BIOCIATIONS The vegetation and animal life of extreme deserts around the world are impoverished, but in semi-deserts, similar to those in southwestern North America, ecologically equivalent species occur, al- though they show little taxonomic relationships with each other. These organisms are derived from ad- jacent, more humid floras and faunas and have many similarities in adjustments and adaptations. Rodents, for example, are generally numerous everywhere. In North American deserts the genera Dipodomys and Perognathus of the family Heteromyidae are espe- cially important; in the Eurasian deserts, Gerbillus, Meriones, and Dipodillus belonging to the family Muridae are found; in South Africa Pedetes, family Pedetidae, occurs; and in the Australian desert the family Muridae is represented by Notomys. All are bipedal in locomotion and have elongated hind legs (Schmidt-Nielsen in Cloudsley-Thompson 1954). It is also of interest that tenebrionid beetles are repre- sented by different subfamilies in different deserts of the world, but, contrary to the prevailing desert col- ors, these diurnal beetles are predominantly black (Brehm 1896, Buxton 1923, Haviland 1926, Kach- karov and Korovine 1942, Bodenheimer 1953). Desert biome 3a7 Stegmann (1938) recognizes a distinct Mediterranean avifauna of desert grassland, chaparral, and wood- land that is best developed in northern Africa but ex- tends north through Spain, Italy, the Balkans, Tur- key, and southwestern Asia. ANIMAL ADJUSTMENTS The characteristic animals of the desert are the small herbivorous rodents and the reptiles. Large animals, including the carnivores, are relatively scarce, and population levels of the rodents appear determined more by the availability of food and water than by predation. Among birds on the Arizona desert, insectivorous species are most numerous, then seed-eaters, and lastly carnivores (Hensley 1954). Most adjustments grassland species make to their environment continue to be expressed in the desert and additional ones become conspicuous (Buxton 1923, Sumner 1925, Heim de Balsac 1936, Linsdale 1938, Fautin 1946, Hensley 1954, Schmidt-Nielsen in Cloudsley-Thompson 1954). The two most critical environmental factors are the high temperatures, especially during mid-day, and the lack of water. Reptiles have some advantage in that their scaly skin is adapted to prevent rapid evaporation. Moist skinned amphibians and snails are absent except in the immediate vicinity of springs or other bodies of water. Animals tend to avoid extreme high temperatures rather than to tolerate them for any length of time. They do this in various ways. The small mammals, snakes, and even insects are largely nocturnal. Birds are active chiefly in the cooler hours of early morn- ing and evening and tend to remain quiet and con- cealed during the middle of the day. Lizards are the most conspicuous animals during the day. Nearly all animals spend their time above ground in the shade cast by the scattered shrubs or rocks, and it is here that they have their burrows or nests. Bird nests occur most frequently on the east and northeast side of plants, where they are shaded from the hot after- noon sun. The intervening ground, fully exposed to the sun’s rays, heats up much higher than the air temperature and may not cool down completely even at night, so that small mammals and other animals scurry quickly from the protection of one bush to an- other in their travels for food. Ground surface tem- peratures go well above the upper limit of tolerance of snakes, but some lizards can hold their bodies away from contact with the ground on their long thin legs. Even the grasshoppers come to rest in bushes to avoid the hot ground surface as much as possible. Grasshopper species confined to hot sandy areas have, like the lizards, long slender legs that hold their bodies away from the ground. Many mammals, rep- tiles, and insects (ants, crickets) burrow deeply into the ground and thereby avoid the surface heat; for example, the burrows of kangaroo rats penetrate 50-65 cm below the surface near Tucson, Arizona (Sumner 1925). On one day when the maximum air temperature in the shade reached 42.5°C (108.5°F), and the temperature of the ground sur- face was 71.5°C (160.7°F), at a depth of 10 cm in the burrows the temperature was only 40.1°C (104.2°F), at 30 cm 29.8°C (85.6°F), and at 45 cm 27.9°C (82.2°F). The amount of moisture in the air inside these burrows is also more favorable, being 3 or 4 times higher than it is outside (Schmidt- Nielsen in Cloudsley-Thompson 1954). The percent- age of mammal species that burrow increases from 6 in forest communities to 47 in short-grass plains, ‘to 72 in deserts. This is in contrast to the decrease in percentage of mammals that are active on the ground level; from 68 to 53 to 28 (Bodenheimer 1957). Many desert animals are adapted to go a long time without drinking water, but those species that depend on drinking water, which probably includes many of the larger mammals, are restricted to the vicinity of springs, lakes, or ponds. Dew is often a source of water in the early morning. Much of the desert vegetation, particularly the cacti, is succulent and is a source of water to animals. The development of an armor on the plants in the form of thorns and prickles serves as a defense against excessive brows- ing by animals. The flowers and fruits of such plants as the saguaro are important sources of water to birds and other animals. The blood and body fluids of prey furnish ample water for carnivores. Metabolic water obtained with the oxidation of fats and carbo- hydrates in the food eaten is apparently sufficient for many species of small size. Even some of the larger game mammals of Africa find green pasture sufficient for satisfying their moisture needs if they can also obtain shade (Vesey-Fitzgerald 1960). Water is conserved in the bodies of birds, insects, and many desert reptiles by kidney wastes excreted as solid uric acid salts rather than as urine. The urine of mammals is more highly concentrated than in non- desert species, and feces are egested in a dry condi- tion, the excess water having been reabsorbed in the large intestine (Schmidt-Nielsen and Schmidt-Niel- sen 1952). After rains sufficient to soak the soil or to refill the shallow ponds, a rapid cycle of events occurs. Herbaceous plants become abundant and bloom. Snails come out of aestivation in the mud. Immature insects and crustaceans become abundant in the water. Termites and ants produce winged forms and mate, and other insects appear in large numbers. Frogs come out of their underground burrows and deposit their eggs; tadpoles hatch quickly, grow rap- 338 Geographic distribution of communities idly, and metamorphose into adults. Animals previ- ously aggregated around water holes scatter widely over the surrounding country. However, as the rains stop and the ponds again dry up, populations contract sharply, and many species go back into aestivation, sometimes for years, until the next wet period occurs. Not only invertebrates but also birds and mammals largely confine their reproductive activities to the rainy season (Buxton 1923). In the northern cooler portions of the desert, hibernation over winter is necessary for many cold- blooded forms. Lizards and snakes may hibernate one-half meter or more in sand, under rocks, or in burrows of other animals (Cowles 1941). Rattle- snakes hibernate in natural cavities on hillsides or elsewhere. Although much of the desert soil is a hard, gravelly pavement, loose soil and sand dunes are not uncommon. Many reptiles have special structural and behavior adaptations to cope with sandy habitats so that a good herpetofauna occurs in such places (Mosauer 1935). The texture and hardness of the soil, its depth, slope, and color influence the niche segregation of small mammals (Hardy 1945). The light gray, yellow, and brown tone of desert soils is reflected in the pale coloration of many desert birds and mammals. Desert animals are generally less heavily pigmented and are smaller in size than close relatives in humid regions (Gloger’s rule). It is not certain how much of this is a response to the arid climate (Buxton 1923, Sumner 1925) or high light intensities (Meinertzhagen 1950), and how much is a response to the color of the soil. Deer mice and pocket mice occurring in the White Sands National Monument of New Mexico are very light in color, but a few kilometers away, in the Tularoosa lava beds, their color is very dark (Dice and Blossom 1937). This blending coloration doubtless furnishes protec- tion from the attacks of predators on moonlit nights. Even in humid climates, the dark coloration of ani- mals may be an adaptation to the darker colored vegetation and ground litter (Bowers 1960). The fish of the desert present features of special interest. The small ponds and pools are widely iso- lated from each other and are without outlets to the sea. The salt concentration in some of them is high as a result of centuries of continuous evaporation of water. Some spring-fed pools contain only a few dozen or a few hundred individuals, but because of their isolation these individuals have evolved into distinct varieties or species found nowhere else. These various fish populations, however, show a relation to each other. In many cases, particularly ‘in the Great Basin, the ponds are deep holes that per- sisted after the drying up of large shallow Pleistocene lakes, such as Lake Lahontan and Lake Bonneville. The fish in these ponds are descendants, therefore, of populations formerly widespread throughout the Pleistocene lakes (Hubbs 1940a, Hubbs and Miller 1948). HUMAN RELATIONS In semi-deserts, there is production of cattle, sheep, goats, horses, and camels, but the carry- ing capacity of the land is low. The stock needs to have access to springs, ponds, or rivers which are of course widely scattered. Nomadic primitive people roam the semi-deserts of Arabia, Africa, and Aus- tralia in search of pasture for their herds. Since the scanty rainfall does not wash away the salts, the soil is fertile where irrigation is possible. Vegetable and other crops and fruit can be raised advantageously. Where the ground water table comes close to the surface locally, oases of vegetation occur, even in otherwise extreme desert, and may support small settlements. On the whole, however, man does not find the desert an amenable habitat. SUMMARY Deserts, like grasslands, occur in all conti- nents. They develop in areas with very low annual precipitation and generally high temperatures. The vegetation consists of scrubby, sclerophyllous, small- leaved, often widely-spaced shrubs, bushes, or cacti, which are commonly covered with prickles or thorns. Succession is not conspicuous and the prevailing vegetation varies considerably with local soil and moisture conditions. Desert scrub and basin sage- brush biociations are distinguished in North America. These communities commenced to emerge as distinct biotic entities in mid-Pliocene, as organisms became adapted to the increasing aridity of the climate. The most prominent animals of the desert are small herbivorous rodents and reptiles. Animals tend to avoid extremely high temperatures by becoming nocturnal, spending much of their time in shady places, or burrowing into the ground. Many desert animals are able to go a long time without drinking water, getting what they need from succulent foods and the oxidation of fats and carbohydrates. There is reabsorption of much water from urinary wastes, or uric acid is excreted instead of urea. After periods of rain, there is temporarily a rich expansion of plant and animal reproductive activities. Many reptiles have structural and behavioral adaptations to cope with sandy habitats. There is a general tendency for desert animals to be lighter in color and smaller in size than close relatives in humid regions. Since ponds and lakes are widely isolated, speciation among fishes has developed extensively. Desert biome 339 Z Tropical Biomes Tropical communities and habitats vary from rain forest to desert. The largest continuous mass of tropical evergreen forest lies in the Amazon valley of South America and extends from lower Mexico across northern South America from the Pacific to the Atlantic Ocean. It is interrupted by tropical de- ciduous forest and savanna, as well as cloud forests in the Andes Mountains. Elsewhere tropical vegetation of various types covers extensive areas in central and western Africa and almost the whole of the Oriental Region. Tropical vegetation also occurs in Australia, New Guinea, and the Pacific Islands. CLIMATE The conspicuous features of tropical cli- mate are high, even temperatures throughout the year ; uniform lengths of day and night ; and seasonal variation in rainfall (Richards 1952). Mean monthly temperatures do not drop below 18°C (64°F) and may rise to 32°C (90°F) or higher. Lowest mean temperatures usually but not always occur during the wet season, but the difference between monthly means may be less than 1°C (1.8°F) and is seldom more than 13°C (23°F). There may be a greater range in temperature at different times of day and night than in mean monthly temperature throughout the year. Mean daily minimum and maximum tem- peratures are seldom below 10°C (50°F) or above 43.3°C (110°F). As one ascends mountain slopes there is, of course, a drop in temperature. On the equator, the length of day and night are approximately 12 hours each throughout the year. The seasonal variation increases away from the equa- tor, both north and south, but in an opposite manner. The shortest daylength in the Tropics is about 10.5 hours, the longest about 13.5 hours. In the rainy season, the actual amount of sunshine is low, aver- aging only five or six hours per day. In contrast to the uniformity of temperature and the length of daylight is the considerable diversity in rainfall and humidity in different parts of the tropics. Deserts with insufficient moisture to support any vegetation occur at one end of a climatic gradient, while at the other end large areas exist where annual precipitation is between 250 cm (100 in.) and 400 cm (160 in.). Rainfall is largely convectional and results from the cooling of the air that rises from heated land surfaces. Sudden showers are often ac- companied by lightning and thunder and may bring a sudden drop of as much as 4°C (7°F) in temper- ature. These storms commonly occur in the after- noon and may come regularly day after day. Because of this influence of the sun, the rainy season typically occurs when the sun is directly overhead. Adjacent to the equator, there is considerable 340 rainfall every month in most areas. Between latitudes 3° and 10°-15°, North and South, the two periods of the year when the sun is at its zenith are far enough apart so that there are two rainy and two dry seasons each year. At still higher latitudes there is only one wet and one dry season. What constitutes a dry season is arbitrary, but in the wetter parts of the tropics, it is considered the period when the rain- fall is less than 10 cm (4 in.) per month. Under extreme conditions no rainfall may fall during the dry season, while in the rainy season some localities may receive over 100 cm (40 in.) in a single month. During the dry season the soil may become desic- cated while during the wet season it may become waterlogged. Grass fires frequently occur during the dry season. The periodic monsoons of India and southeastern Asia result from the outflow of dry winds from a high pressure area that persists in central Asia during the winter and the inflow of moisture-laden winds from the surrounding oceans toward a continental low pressure area during the summer. In the wettest parts of the tropics, relative hu- midity is always very high. It seldom drops below 60 per cent of saturation during the hottest part of the day, and may average over 90 per cent for the entire day. On tropical mountains, mean relative humidity rises with increase in elevation until at 1000 m (3300 ft) in some localities there is almost continuous fog and drizzle. 20° Long ary ' season 152 10°;— 5e N Equctor S 5S Sho ary season 10° Long dry season 159 Where rainfall is scant throughout the year (Fig. 20-3), there occurs desert scrub and tropical thorn forest belonging to the desert biome. Climax tropical savanna occurs where rainfall ranges from 90 to 150 cm (36 to 60 in.), but there is a dry season that lasts four or five months. Tropical deciduous forest re- places savanna where the dry season is shorter and less severe. Probably an annual total of 160 cm (64 in.) is the minimum that permits development of tropical broad-leaved evergreen forest. Occasional months may have as little as 6 cm (2.4 in.) but there is no true dry season. In the so-called cloud forests on the mountains, rainfall may not be particularly high, but this is compensated for by almost continu- ous fog and drizzle, condensation of moisture on all the vegetation, and the very low rate of evaporation. VEGETATION Tropical vegetation has been described in detail by Richards (1952). It is essentially a con- tinuum from desert to savanna to tropical deciduous forest to broad-leaved evergreen or rain forest. There is no true lowland climatic grassland in the Tropics except in relatively small areas (Pendleton 1949). Treeless grassland is the result of excessive burning, cultivation, grazing, or unfavorable soil con- ditions for the growth of trees. Tropical savanna is, however, extensive. Much Long FIG. 27-| Wet (indicated by the ary gray tint) and dry seasons in the season Tropics in relation to latitude (from Richards 1952, after E. de Martonne). rt season {fete | | ome I ee JA ES a Saahoe Tropical biomes 341 FIG. 27-2 Tropical rain forest on slopes of Mt. Aoyo, Republic of the Congo (courtesy S. Glidden Baldwin). of the savanna in Africa probably represents a cli- matic climax, but in those areas with a high pre- cipitation, the savanna may be subclimax due to fire, biotic, or edaphic conditions. This latter is the com- mon situation in South America (Beard 1955). Savannas are communities having a dominant stra- tum of more or less xeromorphic herbaceous plants, of which grasses and sedges are the principal com- ponents, and with scattered shrubs, trees or palms sometimes present (Beard 1953). The grasses in the savanna may be tall, mid, or short and related taxonomically to those in temperate regions. The grasses and forbs die down in the dry season at the same time that the trees shed their leaves. Sedges are more common than grasses where there is more rainfall. The trees may form a dense narrow stand along rivers—the so-called gallery forest—or may be more or less uniformly scattered through the grass- land to give the appearance of a park or orchard (Burtt 1942). The trees are often thorny or xerophi- lous, crooked in growth, and seldom over 20 m high. There is very little shrubby undergrowth; lianas and epiphytes are scarce. Probably savanna is in- creasing in extent at the present time because of human influence in destroying or opening up closed stands of the deciduous and evergreen forests. Tropical deciduous forest, including monsoon for- est, is more or less leafless during the dry season, is less lofty than the broad-leaved evergreen forest, but has a higher and more continuous canopy than sa- vanna forest. The forest is rich in woody lianas and herbaceous epiphytes, but not in woody epiphytes. There are usually two tree strata; in the upper stra- tum the trees are scattered and strictly deciduous. A proportion of the trees in the lower stratum is ever- green, and the number of these broad-leaved ever- green trees increases in both strata as the climate be- comes more humid and less seasonal. The deciduous forest is less susceptible to burning than is the sa- vanna, while the evergreen forest is practically im- mune. At its height, the tropical broad-leaved ever- green forest is nearly completely evergreen, hy- grophilous, 36-55 m (100-180 ft) high, and rich in thick-stemmed woody lianas and in both woody and herbaceous epiphytes. It is commonly called a rain forest because of the continuous high humidity. Seasonal changes are minimal, the aspect being per- petually that of mid-summer. There are usually three strata of trees, one of shrubs and giant herbs, and one of low herbs and undershrubs. The trees are extremely varied in size, but the dominants tend to be tall, slender, and unbranched except at tops. The bark is thin, smooth, light- colored, and often covered with lichens. Tree bases are commonly provided with plank buttresses or stilts. Palms and tree ferns may be frequent. There is an extreme variety of species; for instance, there are seldom less than 15 and sometimes over 30 spe- cies of trees over 30 cm diameter in a single hectare. The Indo-Malayan rain forest is richer in species than either tropical America or Africa; the African forest, the poorest and most uniform in flora. The undergrowth is not a thick jungle as is popularly supposed. Because of the dense shade cast by the several tree strata, shrubs and herbs are scat- tered, there is little or no moss on the forest floor, and one can walk through this forest as easily as through one in a temperate climate. It is only where trees are blown down or the forest is undergoing secondary succession that increased light reaches the ground and jungle growth develops. At Barro Colorado Island in the Panama Canal Zone, light intensity under the dense forest canopy is less than one per cent of the direct rays of the sun, but there are numerous sun-flecks where the intensity is greater. Full intensity of sunlight may reach 20,000 foot-candles. The temperature near the forest floor out of the sun-flecks is nearly constant. In the tree-tops, temperature rises rapidly from a nightly minimum not very different from that on the forest floor to a maximum in the early afteroon. Air move- ments within the forest are almost nil, and evapora- 342 Geographic distribution of communities tion is only about one-half what it is in the open. Animals in the lower strata live in remarkably con- stant environments, although by climbing into the tree-tops they can enter into environments compara- ble to those of open plains (Allee 1926, Moreau 1935a). Tropical America has the richest variety of epi- phytes; Africa, the poorest. These epiphytes include lichens, algae, mosses, liverworts, as well as vascular plants. The epiphytes are relatively small in size, have high light requirements, and tolerate a precari- ous water supply and lack of soil. Often they are more numerous than flowering herbs on the ground. They are important in the community dynamics be- cause their closely overlapping leaves, especially in the bromeliads, enclose large masses of humus and water where mosquitoes and other aquatic animals breed. On the mountains, the tropical evergreen forest of the lowland gives way to a submontane and then to a montane rain forest. The trees are still ever- green but lower in stature, simpler in structure, and poorer in species. In addition to strictly tropical species, the forest may include many genera and species both of plants and animals that are of tem- perate origin (Miranda and Sharp 1950, Martin and Harrell 1957). Tree ferns are common. On ex- posed peaks and ridges, the trees become still more dwarfed and crooked but remain covered with many epiphytic ferns, mosses, and lichens; the whole is aptly, if picturesquely, described as elfin forest. Lianas are scarce in the montane rain forest as com- FIG. 27-3 Wildebeest in the tropical savanna of Tangan- yika (courtesy S. Glidden Baldwin). pared with the lowland rain forest, the shrub layer is dense, and there are only two tree strata. Although plant ecologists are gradually working out a detailed classification of tropical vegetation (Beard 1955), we will base our recognition of major communities on the physiognomy of that vegetation which is of importance to animals. Tropical vegetation may be divided, then, into desert, tropical savanna, and tropical forest biomes. Deserts may be extreme with little or no vegetation present or may be cov- ered with scrub or thorn forest, as already discussed. Savannas represent a forest-edge community in which animals make use both of the trees and in- tervening grassland to various extent in different species. Animals may well distinguish by their segre- gation into niches between park savanna and gallery forests and between grassland composed of tall or short bunch-grasses or sedge (Beard 1953). There appears to be no critical dividing line between tropical deciduous and rain forests, but together they give a continuous closed forest contrasting with the open forest or scattered groves of the savanna, and this is of importance to animals. Geographically the trop- ical forest is separable into American (Dansereau 1947, Leopold 1950), African, and Indo-Malayan units. The family predominance, for instance, of the Dipterocarpaceae in the Indo-Malayan rain forest, and Leguminosae in the American rain forest, and the Meliaceae in some of the west African rain for- ests is noteworthy. This geographic distinction be- tween the continents in plant life is reflected also in the animal life. Tropical biomes 343 ge "Saha FIG. 27-4 Zebras in Tanganyika (courtesy S. Glidden Baldwin). BIOCIATIONS Brief attention has already been given to desert and grassland (Table 25-1) animals in tropical regions. We may recognize one or more tropical savanna biociations in Africa to include both the more strictly grassland animals and those of the forest- edge. On the basis of differences in bird distribution, Moreau (1952) recognizes Sudanese arid, Somali arid, southwest arid, northern savanna, and southern savanna faunal areas. Among mammals, this com- munity is especially characterized by extensive num- bers of wildebeest, zebras, gazelles, antelopes, and lions (Table 24-1) that occur in open country; and the elephant, hippopotamus, rhinoceros, giraffe, wart- hog, and African buffalo that spend considerable time in thickets, swamps, and forests. The habits of many of these animals have been described by Selous (1908), Chapman (1922), and Darling (1960). Moreau (1935, 1937) has worked with the birds. Termites are world-wide in distribution in tropical regions and their large mound nests are conspicuous in some parts of the savanna. Another noteworthy insect of the savanna is the tsetse fly, the vector of a parasite which is a scourge to both man and beast (Buxton 1955). Perhaps another tropical savanna biociation may be distinguished in Australia where a variety of marsupials are characteristic, and there are ecological equivalents for birds and other animals. However, the savannas of South America lack these herds of large mammals so conspicuous in Africa, and this is doubtless due to the savannas of South America being seral in nature and relatively recent in origin. We do not at present recognize a climax savanna biociation in South America. Forest and forest-edge communities have been distinguished for the tropical avifauna of Mexico (Edwards and Tashian 1959). The animal inhabitants of the tropical forest biome differ so greatly in taxonomic composition on the Ay Lees RF EO ee Oy different continents that they are placed in three different zoological regions. We may therefore desig- nate the Indo-Malayan, African, and American trop- ical forest biociations. It would be worthwhile at this point to re-read the discussion of these three zoolog- ical regions (Chap. 20) the better to become more acquainted with the more conspicuous kinds of ani- mals that occur in each. Noteworthy studies of the animals of the American tropics are those of Bates (1864), Belt (1873), Beebe et al. (1917, 1947), Haviland (1926), Allee (1926), Strickland (1945), Goodnight and Goodnight (1956). PALEO-ECOLOGY The tropical forest is of very great age and, along with associated habitats, may represent the center of origin for many modern groups of both plants and animals (Darlington 1957). In past geo- logical ages both on the Western and Eastern Hemi- spheres, the tropical forest has expanded over great areas when climates were warm and moist and con- tracted when they became dry and cool. Perhaps, if one were to work out a phylogenetic tree, all other biomes could be traced back in origin to the tropical biota of Mesozoic and Paleozoic times. Arctic com- munities have been derived from temperate ones, and temperate communities from tropical ones, as organ- isms became adapted to occupy climatic and environ- mental conditions that differed more and more from the primitive optimum of tropical regions. This, however, is conjecture. In the Andes of South America it is apparent that the fauna of the lowland rain-forest (tropical zone) long antedates the elevations of the mountains. After the mountains were formed, they were in- vaded by those species of plants and animals adapted to the lower temperatures and other differences in habitat moving gradually upward. The animal life 344 Geographic distribution of communities of the submontane rain-forest (subtropical zone) is derived almost entirely from the rain-forest at lower elevations. The montane rain-forest (temperate zone) is of more recent origin than the sub-montane forest and consequently derived part of its fauna from it. In addition, the montane forest contains many species of both animals and plants that have dispersed into it from higher latitudes both to the north and south where these species occur in temperate climates at lower elevations. Some very high peaks extend above the tree-line to produce alpine meadow (paramo zone). The fauna and flora of this community are derived almost entirely by lateral dispersal of or- ganisms from extreme southern South America (Chapman 1917). ANIMAL ADJUSTMENTS Like the flora, the fauna in the tropical rain forest is very rich in species (Table 27-1). Bates FIG. 27-5 Lions in the Amboseli Game Refuge, Kenya (courtesy S. Glidden Baldwin). (1864) tells of finding 18 species of swallow-tailed butterflies within 10 minutes’ walk of his house in tropical South America. This is to be compared with only about 20 species in all of North America north of Mexico. Apparently, however, the density of in- dividuals in tropical species is low. This is under- standable, since the greater the number of species in an area, the greater becomes the competition among them for living space, and each is forced to withdraw TABLE 27-! Diversity of fauna at various latitudes (Dobzhansky 1950). Birds Snakes Number of Number of Region species Region species Labrador 81 Canada 22 New York 195 United States 126 Panama 1,100 Mexico 293 Colombia 1,395 Brazil 210 Tropical biomes 345 FIG. 27-6 A column of army ants moving across the forest floor, and, right, a bivouac of army ants, Barro Colorado Island, Panama Canal Zone (courtesy T.C. Schneirla). into those niches to which it is best adapted. The great number of tree and plant species provides a variety of niches, but the availability of niches of each kind is limited so that their animal inhabitants are accordingly reduced in numbers. In contrast with temperate regions where animal adaptations are so largely concerned with the physical environment and getting food, there needs to be little or no adaptation in the tropical rain forest to winter cold, inclement weather, lack of food, or desic- cation. Inter-species competition and struggle, how- ever, is harsh and exacting, and evolutionary forces tend to perfect specializations that enable organisms to fit better into their niches or invade new ones and thus avoid much of the competition and predation (Allee 1931, Mertens 1948, Dobzhansky 1950). One specialization in this connection is the ability to hang from trees—animals suspended in mid-air are almost inaccessible to attack by predators. Many birds, particularly flycatchers, orioles, and honey- creepers build pendant nests, as do certain solitary wasps. The cocoons of many moths as well as chalcid wasps are suspended by thin threads. Spiders in their webs suspended off the ground are immune from attacks of army ants. An unusual number of birds nest in holes in trees—trogons, motmots, parrots, toucans, wood- peckers—but this may be to protect them from the 346 Geographic distribution of communities ee * fies ~ reoe di y heavy showers as much as from marauders. Sting- ing ants inhabit the hollow stems of sapling cecropia trees and the swollen bases of leaf petioles of the leguminous tachigalia tree, and for this protection are supposed to defend the trees against the attacks of the leaf-cutting ants. Many other interesting coactions exist. Collared peccaries on Barro Colorado Island in the Canal Zone make narrow trails through the bushy tangles and dense undergrowth and proceed single-file from one place to another. These well-defined paths are utilized also by the coati, octodonts, and several kinds of marsupials which probably could not otherwise penetrate these areas (Enders 1935). The tapir is a trail-maker in the South American jungle, and the elephant in Africa ploughs its way through the forest by sheer strength. The trails it breaks are followed at later times by the hippopotamus, rhinoceros, buffalo, lion, leopard, hyena, pig, and baboon, which in turn make the trail more passable for lesser forms (Hesse, Allee, and Schmidt 1951). In contrast with the sporadic occurrences of most species, ants and termites are abundant in the Amer- ican tropics. The leaf-cutting ants and the insect- eating army ants are especially characteristic (Belt 1873). Termite nests occur in all strata, and the wood-eating habits of these insects hasten the de- struction of woody materials. The Hymenoptera, Diptera, and Coleoptera are, in general, the best- represented groups among the insects (Briscoe 19521): With temperatures high and uniform throughout the year, the developmental period of cold-blooded animals is shortened, and there is a general speeding up of the life-cycle. Insects possessing only a single generation per year in the north temperate zone may complete their life cycle in 3-4+ weeks in the Tropics, and may have several generations during the year. On the other hand, high uniform temper- atures are depressing for the metabolism and ac- tivities of warm-blooded animals, and the pace of their activities is comparatively slow. Cold-blooded animals, particularly reptiles and arthropods, reach their largest adult sizes in the tropics. In the Amazon forest there is a spider large enough to catch and feed on small birds that are caught in its web (Bates 1864). Some moths have a wing-spread of 30 cm; a millipede reaches a length Tropical biomes 347 FIG. 27-7 East face of meridian termite nests in Australia. It is characteristic of these nests to be oriented north and south, their broad surfaces presented one to the rising, the other to the of 28 cm, and a slug 20 cm. Birds and mammals, however, are usually smaller than their relatives in temperate regions. Birds also lay fewer eggs in a clutch than they do in temperate regions, but this may be due to the shorter days that tropical birds have for feeding. Since there is little variation in the duration of light per day throughout the year, photoperiodism is largely absent. In desert regions, the gonads of birds may remain inactive and reproduction in- hibited for a succession of seasons during a prolonged drought, but their sexual cycles respond quickly to rainfall, and nesting may begin within a few days after heavy precipitation (Serventy and Marshall 1957). In regions where wet and dry seasons are not developed, bird species may breed throughout the year, although individual birds, after breeding, need a period of rest before they can breed again (Chapin 1932, Baker 1938, Miller 1955). In the evergreen rain forest of Borneo, where precipitation is ex- tremely high, the onset of breeding among mammals appears correlated with the period of the year when precipitation is minimum rather than maximum (Wade 1958). There is no definite period of dormancy or migra- tion. Movements are largely localized and in quest of ripening fruit or other food supplies. Away from the immediate vicinity of the equator and toward the periphery of the rain forest, where wet and dry seasons become important, the annual cycle of breed- ing, migration, and other activities becomes more pronounced and important (Baker et al. 1936, Davis 1945, Wagner and Stresemann 1950, Moreau 1950). In dense rain-forest on the equator, the daily rhythm of animal activities is striking. Many natu- ralists have commented on the great hush of the forest setting sun, and the narrow top edge to the hot midday sun (courtesy G.F. Hill and A.E. Emerson). during the middle of the day. The forest appears empty of both birds and mammals. There is an occa- sional note of a bird, but birds do not have the varied and conspicuous songs that they do in temperate re- gions. They may be glimpsed in the tree-tops or searched out in the undergrowth moving through the forest in loosely formed groups, each group com- posed of a few individuals each of several species. These social groups occur at all seasons, although nesting birds must withdraw temporarily from them (Davis 1946). The cicada chorus is often loud and persistent; with the onset of darkness, other orthop- teran insects burst into song to which tree frogs, night birds, monkeys, and others add their voices. Nectar feeding is well developed in some tropical birds, and many flowers depend on birds for their pollination. Hummingbirds are numerous in the Western Hemisphere. Many insects are also nectar and pollen feeding, and these species are largely limited to the tropics. Fruits are an important food for many birds and mammals. Fruit-eating bats are confined to the tropics. Sloths and ant-eaters have powerful claws and long sticky tongues with which they open and plunder the nests of ants. Army ants moving in large numbers over the forest floor in their search for prey often attract a large and varied group of birds, but these birds are after the other insects that the ants stir up rather than the ants themselves (Johnson 1954). As in most other terrestrial biomes, animals occur in the greatest variety and numbers in the forest floor. Here they are much more numerous during the wet than the dry season. In the Panama Canal Zone during the wet season there are between 4000 and 10,000 animals per m?*, representing 294 species. Of this fauna, mites constitute 25 per cent, springtails 348 Geographic distribution of communities Terrestrial species Mammal family Cursorial Amphibious Marsupiala Chironectes (1) Rodentia agoutis (2) capybara (1) paca (1) paca (1) spiny rats (2) rats and mice (8) Edentata armadillos (4) anteater (1) Carnivora huntingdog (1) others (2) Ungulata peccaries (2) tapir (1) deer (2) Primates 34 per cent, ants 25 per cent, and all others 16 per cent. Planaria-like flatworms, a leech, and a land crab are found, although in temperate climates they are usually limited to aquatic habitats. Many dif- ferent kinds of millipedes are a characteristic feature of the fauna. Centipedes and snails are not abundant (Williams 1941). Many animals of all sorts have developed an arboreal-living habit although their close relatives outside the tropics are ground-dwellers (Table 27-2). These arboreal species tend to be limited in size and possess opposable toes and prehensile tails. The New-World monkeys have prehensile tails but not the Old-World monkeys and apes. Porcupines, climbing ant-eaters, the coatis, and the kinkajou also possess prehensile tails (Haviland 1926). Some sloths and lemurs hang upside down as they climb around through the branches of the trees. Tree- dwelling snakes and lizards are either long and whip- like or heavy-bodied and with prehensile tails. Para- chutes, similar in function to those of the flying squirrels of temperate forests, have developed in such diverse forms as marsupials, lizards, and frogs. Some frogs are entirely arboreal and have sucking discs on their toes to aid in climbing. Some species of frogs lay their eggs in the trees in sacs made of leaves, others glue their eggs to leaves, still others carry them on their backs and the tadpole stage is passed through before hatching. Snails climb to the topmost branches of the trees. Some butterflies fly continuously about the tree-tops and appear never to alight on the ground. There is a group of tree-dwell- ing tiger-beetles, Odontocheilae. Termite nests lo- cated in trees are often connected to the ground by covered passages. Leeches climb into bushes to get onto the bodies of warm-blooded animals more easily. During the wet season, the mosquito Anopheles gambiae spends most of its time below 7 meters in the forest, but its close relative A. africanus is most Arboreal species TABLE 27-2 Stratal distribution of mammals in British Guiana porcupine (1) (Haviland 1926). squirrels (2) sloths (2) anteaters (2) cats (5) raccoon (1) coati (1) kinkajou (1) tayra (1) grison (1) monkeys (6) marmoset (1) abundant at heights of 13 to 25 m. Species of Anopheles and Culex are mostly crepuscular or noc- turnal in activity while the sabethine group and cer- tain dédes are diurnal (Bates 1949). Many of the epiphytes, especially the bromeliads, hold small quantities of water within the clump of leathery leaves high up in the trees. These small reservoirs usually contain protozoans, rotifers, flat- worms, leeches, annelids, snails, isopods, copepods, ostracods, onychophorans, centipedes, millipedes, scorpions, spiders, a great variety of insects, and small frogs which may spawn here (Haviland 1926). The true forest inhabitants keep well within the forest shade and, like the monkeys, are quite sensitive to direct exposure to the sun. Animal life is less abundant, however, in the depths of the forest interior than it is on the forest margin. It follows that the fauna is richer both in species and in numbers in the tropical savanna than in the tropical rain-forest itself. HUMAN RELATIONS The tropics are the native home of the black or negroid races of man. The death rate of white man in some parts of the humid tropics is in- creased ten times over what it is in temperate regions, and he can seldom spend more than a few months at a time there without impairment of health and vigor. Relatively little effort is required by the native of the tropics to secure food, and the need for clothing is minimal. The biological environment, however, is harsh and exacting. He must guard against malarial plasmodia, hookworms, and a variety of skin para- sites (Dobzhansky 1950). African natives in tropical savanna are nomads and have herds of cattle, goats, and other animals which furnish them with milk, meat, and blood meals. Tropical biomes 349 This region may some day become a big cattle-raising country if the diseases of nagana and rinderpest can be controlled. Some tribes practice a primitive form of agriculture. Occasional locust swarms devastate both the native vegetation and the cultivated crops. The monsoon region of India supports a very large population. The land is divided into tiny plots, plowed by donkey, ox, or water buffalo, and culti- vated by hand tools. Elephants do some of the heavier work. Tea leaves are harvested from bushes, rice is grown, and teak lumber obtained from the forest. Cattle and goats supply milk. Natives in the tropical rain forest make their liv- ing by hunting and fishing. They live in rude huts made of branches and leaves. Their hunting is done with bows and poisoned arrows, blow pipes, and pits dug in the ground. In better developed equatorial lands they grow manioc from which tapioca and flour are obtained, yams, sugar cane, pineapples, bananas, and cocoa. Cocoanuts are important food in some places. Coffee is cultivated extensively in South America. Sap from which rubber is made was orig- inally collected from scattered naturally growing trees in the forest, but rubber trees are now grown extensively in plantations. The tropics undoubtedly are a potentially rich productive area, but this pro- ductivity will not be fully realized until the natives, who are best adapted to live in the area, can be edu- cated and acquire the skills to develop it (Hadlow 1953). SUMMARY Noteworthy of tropical climates is the uniformity of temperature and length of daylight throughout the year. Rainfall varies from a distinct seasonal distribution in some regions to constant and very heavy in others. Correlated with the rainfall gradient is a vegetation continuum from desert, to savanna, to tropical deciduous forest, to tropical rain and cloud forests. | The biomes recognized are those of desert, tropical savanna, and tropical forest. Aside from deserts there are one or more tropical savanna biociations in Africa and in Australia and the Indo- Malayan, African, and American tropical forest bioci- ations. The tropical forest flora and fauna are of great age and continuity, and it may well be that all other biomes can be traced back in origin to the trop- ical biota of mesozoic and paleozoic times. The flora and fauna of the tropical forest is marked by the richness of their species compositions. Correlated with this is the harshness and severity of interspecific competition and predation. On the other hand, except for ants and termites, few species reach high levels of population density. Large herds of un- gulate mammals occur, however, in the tropical savanna. With uniform climate throughout the year, cold- blooded animals may have several generations per year and species of birds, though not the same indi- viduals, may breed during every month. There is no hibernation or period of dormancy, nor is there mi- gration, except in regions of pronounced wet and dry seasons. Cold-blooded forms, especially reptiles and arthro- pods, reach a large size in the tropics, but birds and mammals are generally smaller than their relatives in temperate regions. Although animals occur in great- est numbers and variety on the forest floor, many different groups have evolved members largely re- stricted to the arboreal stratum. The tropics are the native home of the black races. Originally they made their living by hunting and fishing, grazing of domesticated animals, and a primi- tive form of agriculture. The tropics are potentially a rich productive area, but this productivity will not be fully realized until disease can be controlled and the natives better educated. 350 Geographic distribution of communities Geographic Distribution of Communities: Marine Biomes The geographic distribution of organisms in the sea depends on their responses to currents, temper- atures, and physical barriers; local distribution is affected by waves and tides, type of bottom, salinity, and depth. Marine ecology is concerned with envi- ronmental factors and problems of organismic ad- justments quite different from those on land and also different in many respects from those in fresh- water, Animals are relatively more conspicuous than plants. Succession is less evident, but such ecological processes as represented by chemical cycles, cooper- ation and disoperation, food chains, productivity, population dynamics, niche segregation, speciation, and dispersal are fully as important as on land. Distinct self-contained community units are more difficult to recognize in the sea than on land because of the apparently greater interrelation of benthic species and the freer movement with circulating cur- rents of plankton and nekton. Plankton is every- where a basic link in food chains, but the general distribution and importance of plankton species in the sea is no more remarkable than that of soil organ- isms in terrestrial biomes. To consider the entire ocean community as a single biome, as has been sug- gested by some investigators, is stretching the concept beyond its usefulness. Since we identify biomes by differences in the life-forms and functional adjust- ments of the conspicuous dominant or predominant organisms, we may properly recognize biomes that occur in the open ocean, on eroding rocky shores, on muddy and sandy beaches, and composing the coral reefs and atolls. Each of these biomes may be subdi- vided by the taxonomic composition of the predomi- nant organisms into secondary communities equiva- lent to the biociations that we have recognized on land. Much of the early literature on marine com- munities has been reviewed by Gislen (1930). We can only hope in this chapter to present a brief summary of some of the more salient features of marine ecology. For more thorough treatments, the reader is referred to the publications of Sverdrup et al. (1942), Ekman (1953), Harvey (1955), Hedgpeth (1957), and Moore (1958). HABITAT The marine biocycle is considered to have benthic (bottom) and pelagic (open water ) divisions. The littoral zone of the ocean shore extends between the limits of high and low tides. The sublittoral zone covers the continental shelf to a depth of about 200 m, the approximate depth at which maximum wave ac- tion produces any effect. The average depth of the ocean is about 3800 m, but oceanic trenches (hadal zone) extend much deeper; the Marianas Trench in the Pacific Ocean to approximately 11,600 m. The Soh neritic biochore is above the continental shelf and is commonly 16-240 km (10-150 mi) wide. The oceanic biochore is subdivided vertically with the boundary between the epi- and mesopelagic zones, depending on the extent of effective light penetration. Tides The level of water in the ocean rises and falls usually twice each day or at an interval of 12 hours and 26 minutes. In some parts of the world the tides are less regular or there may be but one daily. Flood- tide is the period in which the level is rising and covering more and more of the shoreline; ebb-tide is the period in which the waters are receding. In the open sea the change in water level is less than a meter, but the change may be much more than this on the shore, depending on its configuration. The Bay of Fundy opens broadly to the sea and tapers to a narrow head landward, and tides may be 6 to 10 or even 15 meters. On the other hand, when bodies of water have only a relatively narrow connection with the sea, as does the Gulf of Mexico with the Atlantic, the range in water level is less than 30 centimeters. Even lakes have a tide, but it is hardly perceptible except in the larger lakes where it may amount to a few centimeters. Tides are caused by the attraction of the moon and, to a lesser extent, the sun. When the sun’s at- traction is added to that of the moon, as occurs twice each month at times of full moon and new moon, the fluctuations of the tides are unusually high and unusually low. These are called spring tides. When the tidal influences of sun and moon are opposed as happens twice each month, the tides have the least amount of flow and ebb and are called neap tides. Tides have their greatest effect on animals on the seashore, because of the associated pounding of waves and the alternate submergence in water and exposure to the air. However, the organisms appear well ad- justed to this rhythmic submergence and exposure (Flattely and Watson 1922, Korringa 1947). For instance, as the stones on which the chiton occurs become exposed, the animals react positively to grav- ity and negatively to strong light, and move down- wards. They travel at maximum speed while the stone is still moist and become aggregated on the damp lower sides of the stones. When the stone again becomes immersed by the returning tide, the animals lose their geotatic orientation, and, since illumination becomes more or less equal on all sides of the stones, they move about at random until they reach the upper surfaces again. On the other hand some ciliated and flagellated protozoans and diatoms in inter-tidal habitats are active only when the tide is out and become encysted or inactive and attached to surfaces when the tide is in (Fauré-Fremiet 1951). These rhythms in inter- tidal organisms may persist for days, even when the organisms are placed experimentally under constant conditions (Brown 1959). Substratum The pounding action of waves on rocky shore may have tremendous force, estimated in one instance at 15,000 kg/cm. Animals occupying exposed rocky shores in the surf belt must be strongly protected and firmly attached (Flattely and Walton 1922). The conical-shaped limpets present a minimum of surface to the waves. Barnacles are protected by heavy shells and grow fast to the rocks, snails and chitons hold themselves by powerful suction apparatus on their feet, mussels like Mytilus have a glandular byssus, while some species of sea urchins bore shallow craters into the rock. Advantage is taken of nooks, crannies, and spaces underneath stones and rocks (Glynne- Williams and Hobart 1952). Large depressions in the rock retain water at ebb tide to form tidal pools and thus may contain the more delicate species be- cause of the protection they afford. The various sea weeds absorb some of the wave shock for the animals living with them. The shape, form, and size of corals, sponges, and other colonial types are affected by the amount of wave action to which the animals are ex- posed. A shell of the Mytilus mussel may weigh 58 g where the animal is exposed to a heavy surf but only 26.5 g in more quiet waters. Sandy beaches occur only where the force of waves is reduced by being spread over a more gentle slope. Even here, especially during storms, the sand makes a very unstable substratum and not many animals except mollusks and some of the echinoderms can keep from being smothered or buried. Mud but- toms occur only in relatively quiet waters. Burrows made in mud hold their form better than in sand, so larger populations of animals can occur in mud. The sea-floor at greater depths is covered with a variety of sediments. Terrigenous deposits of min- eral and organic matter derived from the land and from the littoral and neritic biochores are relatively rich in nutrient substances and extend into the bathyal zone. All other deposits on the sea-bottom are pelagic, being derived, in part, from the skeletons of dead plankton and other organisms. In the long slow journey of these dead organisms to the bottom of the sea, much of the organic matter decomposes, releasing carbon dioxide, nitrates, phosphates, and the many other elements in the composition of the protoplasm. Even various amounts of skeletal mate- rial may dissolve, but enough of the organisms reach the bottom to create a substratum of loose flocculent 352 Geographic distribution of communities ——— PELAGIC SSS EVE MTHS Wie =— ==> EPIPELAGIC a ° >|| MESOPELAGIC 500 a 1000 +1500 BA a eeeenie 2000 APHOTIC + 3000 ~4- 4000 ABYSSOPELAGIC Vy FIG. 28-! Subdivisions of the ocean biocycle (Hedgpeth 1957). ooze and to furnish food for the living animals that spend their lives in this habitat. Pelagic deposits that contain less than 30 per cent of organic remains are known as red clay. These de- posits are the most widely spread of all, especially at the greater depths of the ocean. They are probably derived from wind blown desert dust, terrestrial vol- canic dust, and submarine eruptions. The very hard earbones of whales and teeth of sharks are regularly found in red clay. Animal life is scant, consisting only of shellless holothurians and worms, probably because of the poor nutrient content and great depth. Organic deposits are either calcareous or silice- ous, the former being derived from the shells of foraminiferans, small pelagic mollusks, or flagellate coccolithophorids, and the latter from skeletal mate- rial of diatoms and radiolarian protozoans. Pressure There is an enormous increase in the pressure of water upon the bodies of animals at great depths. This is not, however, an important limiting factor in the vertical distribution of animals in general, as internal pressures closely counterbalance external pressures and life is known to exist at the greatest depths. Adjustments of internal pressures are not so rapid, however, to prevent injury in many species that are dredged at great depths and quickly hauled to the surface. Furthermore, individual species have different limits of pressure tolerance. Temperature The temperatures of surface waters vary be- tween the freezing point (—1.9°C) in polar regions and 25°-30°C in the tropics. Seasonal variations are small in polar and tropical waters but somewhat greater in the temperate zones. Temperature varies with depth, more so in the tropics than elsewhere. At 60°N latitude in the At- lantic Ocean, the mean temperature of the warmest and coldest months at 0 meter is about 10°C, while at a depth of 2000 meters it is 3.5°C. On the equator the temperature at 0 meter is approximately 26°C, at 200 meters 13°C, at 400 meters 7.5°C, at 1000 meters 4.5°C, and at 2000 meters 3.3°C (Ekman 1953). This temperature decrease, known as a ther- mocline, is a permanent feature of the tropics. A permanent thermocline also exists at mid-depths in temperate and subtropical waters. Superimposed upon it is a seasonal thermocline that develops near Marine biomes 353 the surface during summer and is destroyed in autumn and winter when vertical mixing creates a layer of relatively uniform temperature in the upper 20 to 300 meters. Currents moving towards the poles from the equator consist of warm water, and currents moving in the opposite direction of cold water. Cold waters flowing towards the equator tend to be deflected to the right and hence bathe the North Atlantic coast of North America and the North Pacific coast of Asia. In the southern hemisphere they come into contact with the west coasts of both South America, Africa, and Australia. Warm waters, on the other hand, bathe the west coasts of Europe and North America and the east coasts of Australia, Africa, and South America. Organisms living in the intertidal zone on the shore are ordinarily exposed to great variations of temperature twice during each day as they are alter- nately flooded by the tides and exposed to the air and direct solar radiation. Unusually severe cold spells during the winter have been known to produce exten- sive mortality of fish and invertebrates in shallow waters off the coasts of Texas and Florida (Gunter 1941). On the other hand, one of the characteristic features of the deep-sea habitat is its low and almost constant temperature. Light The character of the radiation, as well as its intensity, varies with depth. Even in the clearest waters and at maximum radiation, the red, orange, and ultraviolet are absorbed in the first 20 m. Green, yellow, and blue wavelengths penetrate farther, de- pending on the water color. When the sun is not at the zenith, light penetration is reduced, and the maxi- mum penetration in the winter at high latitudes is much less than during the summer (Clarke 1939, Jerlov 1951). The compensation point, or the depth at which the amount of oxygen released in photosynthesis by algae just balances the oxygen needs of the plants for respiration over 24 hours, has been found to vary during the daytime between 1 and 100 m, according to the locality, turbidity, and season. The upper illu- minated layer where photosynthesis exceeds respira- tion is often called the photosynthetic zone (Harvey 1O55))e Salinity The salinity of sea water varies from place to place depending largely on the amount that it is di- luted by the inflow of fresh water from rivers or melting glaciers or the amount that it is concen- trated by evaporation. The Red Sea, for instance, has a salinity of 40°/00 (40 g dry salts in 1000 g sea water) while in some polar seas the salinity is less than 30°/00. The average salinity of the oceans as a whole is commonly given as 35°/00 of which the chloride ion constitutes about 19°/00 and the sodium ion a little over 10°/00. The various major salts occur nearly everywhere in definite and constant pro- portions. As one would expect, the pH of sea water is high, averaging about 8. There is some similarity in relative proportions and concentrations of the vari- ous ions in sea water and in the blood or body fluids of many invertebrate organisms. This may indicate that the sea is the habitat in which living forms first evolved. The contrast in salinity between sea water (35,000 ppm) and fresh water (15-660 ppm) re- quires important differences in physiological adjust- ment of organisms to occupy these two habitats. The problem is one of osmotic regulation (Black 1951). Most marine invertebrates are poikilosmotic in that they are nearly isotonic with sea water, they are highly permeable to water, and gain or lose water according to the concentration of the medium. A few marine segmented worms, flatworms, and crabs and all marine fish and mammals have at least some in- ternal osmotic regulation and tend to be homoios- motic. All except the elasmobranch fishes maintain body fluids hypotonic to sea water in various ways. The skin has decreased its permeability to the free movement of water back and forth, the necessary water is obtained by swallowing, surplus salts are secreted outside the body, especially through the gills, and there is a general decreased function or atrophy of water secreting organs such as the kid- neys. The practical absence of insects and amphibians from the sea is largely due to their inability to secrete salts outwardly. The high osmotic concentration found in elasmobranchs is the result of huge quan- tities of urea retained in the body tissues and fluids. Fresh-water organisms, in contrast to marine forms, maintain body fluids hypertonic to the sur- rounding medium by excretion of water through con- tractile vacuoles in lower organisms or highly func- tioning kidneys in higher ones, active absorption of salts from the surrounding water by special cells in the gills, and reabsorption of salts from the urine. There is no swallowing of water, as sufficient amounts are absorbed by osmosis through the gills and mouth surfaces and incidentally with feeding. Probably the most extensively utilized of the dis- solved substances in the sea are the nitrogen com- pounds (nitrates, nitrites, ammonia salts), phos- phates, calcium salts, and silicates. Nitrates and phospates are particularly important as nutrient ma- terial for phytoplankton. Calcium is required in large 354 Geographic distribution of communities 2155) "(PEG UOS|seD pue uopbuljunpy Jayje) sued20 ayy jo spuasuND a>DejING 7-87 “S|4 "Wiad PUM 38eM “0% gs anos WH - =— — — *‘qUeIIND [el1ojenby yyNog “ST — ad SLN3YY ND NV390 a & *quaIIND [ej1oyenby yWION ‘ST Marine biomes “yueIIND nied “LT ‘JUSIIND BIUIOJITVO “OT ‘que1Ing ueder ‘scT *JUBIIND BI[eSny ISeq “FT *JU9IAIND UPTTeENSHy ISeA\, “ET *queIIND [els1ojenby *ZT *yuaIIND 19}zUNOD UPIpPU] “TT ‘WYlaiq uoosuowm ‘OT *yuea1IMD enbiquiezow “6 “‘quering Belenzuag °“g *‘quaIIND []zZBIg “1 “querINO [el10yenby yynog *9 ‘quadIIND [el1oyenby yVION “Ss “qUsIIND SB}IBUBD “F “Wid OMUBity WAION “€ “UBa§ FIND “Z "‘JuaIIND IOpesqey ‘T FIG. 28-3 Characteristic holoplankton (Sverdrup ef al. 1942). (A) Protozoa: a) foraminifera Globigerina, b) dinoflagellate Gymnodinium, c) tintinnid Stenosomella, d) tintinnid Flavella, e) radiolarian Protocystis, f) another radiolarian, g) dinoflagellate Noctiluca. amounts for the shells of mollusks, the skeletons of corals, some protozoans and worms, certain algae, the other organisms and may be precipitated out of the water by bacteria. Silicon is required by sponges, some protozoans, and the phytoplankton diatoms. These salts keep cycling through the ecosystem, but additions to the supply come continually from the land, being washed into the oceans by the rivers. Neritic waters are especially fertile and support a great mass and variety of animal life because of this land drainage and the pattern of water circulation on the continental shelf. Biological productivity de- creases progressively from shallow waters over the continental shelf, to deeper waters, to the open ocean, but is also high over offshore banks and in areas of upwelling. Substantial amounts of nitrogen salts are also swept out of the air by precipitation, and there is nitrogen-fixation by bacteria. FIG. 28-3 (B) Coelenterates and ctenophores: a) comb-jelly Pleurobrachia, b) siphonophore Velella, c) jellyfish Ag/antha, d) siphonophore Diphyes. 356 It is of interest that atoms of phosphorus, nitro- gen, and carbon occur in sea water in ratios of 1:15:1000 and in plankton in ratios of 1:16:106. This means that there is an overabundance of carbon available in the sea for absorption by the phytoplank- ton, but phosphorus and nitrogen may be limiting for further increases in the population of organisms (Redfield 1958). Oxygen The oxygen supply of sea water comes by diffu- sion from the air at the surface and from photosyn- thesis of green plants down to the compensation point. It is continuously used at all depths in respira- tion of animals and plants and in the decomposition of organic matter. The oxygen content of sea water (Hedgpeth 1957) is seldom limiting for the occurrence of ani- mals, except in the deeper waters of the brackish Black and Caspian Seas where it is practically ab- sent. Oxygen concentration is especially high on shores where there is splashing of waves. Surface waters of the Atlantic Ocean commonly have 4.5 to 7.5 cc/1 and abyssal regions may run over 5 cc/1. Oxygen is somewhat less abundant in the Pacific and Indian Oceans. Oxygen may be reduced to lower concentrations between 100 and 1500 m, because of its use in animal respiration and in decomposition, than at lesser depths where there is photosynthesis or at greater depths where the abundance of animals is greatly diminished. Marine animals have a variety of mechanisms and adaptations for respiration (Flattely and Walton 1922). Greatest difficulties occur in shore animals at low tide when they are exposed to the air, but the need for oxygen at this time is decreased in many forms by curtailment of activity. Some crabs, barna- cles, snails, and fish have become almost amphibious in being capable of respiring in air, although at re- duced rates, as well as in water. Pure mud bottoms may present anaerobic conditions a short distance below the surface, but mud bottoms mixed with sand contain an abundant and diversified fauna. PLANKTON Composition The plankton of the sea includes a great variety of forms, even more than in fresh water (Biglow 1926, Hardy 1956). Rotifers, however, are uncom- mon in marine plankton and cladocerans are much less important. The nannoplankton consists mostly of flagellates, Geographic distribution of communities algae, bacteria, and a few fungi. The bacteria are largely periphytic, in that they are attached to the surfaces of floating plants, animals, and to particles of organic detritus. Very few occur freely suspended in the water (Harvey 1955). Bacteria occur at all depths but are especially abundant in or close to the bottom. They are generally more numerous in the winter than in the summer. The green phytoplankton is composed primarily of diatoms, dinoflagellates, and small unarmored flagellates, but several other kinds of algae are pres- ent and occasionally important. The dinoflagellates Noctiluca and Ceratiwm are luminescent and in some regions may give a glow at night to the entire sea. Bioluminescence is not limited to these organisms, however, but occurs also in various forms of bac- teria, radiolarians, sponges, coelenterates, cteno- phores, nemertineans, worms, crustaceans, brittle- stars, mollusks, balanoglossids, tunicates, and fish (Harvey 1952). The most important groups of protozoan zoo- plankton, other than the green flagellates which are usually considered with the phytoplankton, are the thizopod Foraminifera, the actinopod Radiolaria, and the ciliate tintinnids. They may be enormously abundant at times. Among the Coelenterata, many hydrozoans have medusae and larval floating stages in their life cycle, but only the siphonophores, the best known example of which is the Portuguese man-of-war, are pelagic throughout their life cycle (holopelagic). The true jellyfish of the class Scyphozoa are often conspicuous and ctenophores of the related phylum are often abundant. Some of these forms are so large they are called macroplankton. The various phyla of worms are represented in the plankton by only a few forms, of which the chaetognath Sagitta or arrow worm and the poly- chaete Tomopteris are often abundant. Many benthic worms, however, produce larvae that are temporarily part of the plankton (meropelagic). Many molluskan and echinoderm species are meropelagic and it is by means of their larvae that heavy, slow moving benthic forms become widely dispersed. Two groups of snails are holopelagic: the heteropods that inhabit tropical and subtropical waters and the pteropods which occur in cold waters and are important food for whalebone whales. Crustaceans form one of the principal groups of the net plankton, and of these the holopelagic cope- pods are by far the most abundant (Digby 1954). Calanus finmarchicus is one of the most noteworthy species and is the principal food of the commercially important herring fish. Other important crustaceans that enter the plankton either as larvae or adults are ostracods, cumaceans, amphipods, mysidaceans, eu- phausiaceans, decapod shrimps and prawns, and stom- FIG. 28-3 (C) Crustaceans: a) euphausiid Euphausio, b) ostracod Conchoecia, ¢) copepod Calanus, d) amphipod, Phronemia, in empty mantle of the pelagic tunicate Sa/pa. atopods. Many of these forms are also benthic or nektonic during a part of the life cycle. Among the chordates are the remarkable and sometimes abundant tunicates. The eggs and imma- ture stages of many fish are pelagic in that they absorb just enough water shortly after being spawned to have almost precisely the same density as the sur- rounding water. The eggs of skates and rays, some of the sharks, and some other fishes, such as the herring, however, sink to the bottom where they remain until they hatch. Flotation mechanisms The specific gravity of sea water is 1.02 to 1.03, while that of naked cells or protoplasm varies from 1.02 to 1.08. The specific gravity of the entire FIG. 28-3 (D) Miscellaneous: a) arrow worm Sagitta, b) annelid Tomopteris, c) nemertean Nectonemertes, d) pteropod mollusk Limacina, e) tunicate Oikopleura, f) pteropod mollusk C/ione. Marine biomes 357 organism may be considerably higher if it possesses a skeleton or shell. Organisms have various devices to remain afloat, aside from swimming: absorption of large amounts of water to form jelly-like tissues or sap, storage of gas or air bubbles within the body, formation of light- weight fat deposits in the body or oil droplets within the cells, increase of surface area in proportion to body mass thereby increasing frictional resistance. Increase in the relative amount of body surface is achieved by decrease in size, flattening, attenuation of body form, extensions of body parts as antennae, spines, tentacles, or cerci, surface hairs or tubercles of various sorts, surface sculpturing, or formation of colonies (Marshall 1954, Davis 1955). These de- vices result in many strange shapes among plankton organisms. When the organisms die, the protoplasm disintegrates, special flotation mechanisms are usu- ally destroyed, there is a loss of swimming move- ments, and what is left of the organism sinks to the bottom. Abundance The actual abundance of plankton varies greatly from place to place and from one season to the next. Smaller species tend to be more numerous than larger ones. The mean annual abundance of diatoms is commonly in the tens of thousands per liter and for shorter periods during the year algal blooms may increase the population to hundreds of thousands of cells per liter (Ricketts and Calvin 1948). Zooplankton is, however, much less abun- dant. It has been repeatedly noted that large popula- tions of phyto- and zooplankton do not occur in the same place at the same time. Various explanations have been offered: one, that it is due to the feeding of the zooplankton on the phytoplankton (Harvey 1934), and another, that it is due to the phytoplank- ton, when abundant, producing conditions that are inimical or toxic to the zooplankton (Hardy and Gunther 1935). The total net zooplankton per unit volume of water is some 16 times more numerous in the neritic coastal waters off the Atlantic coast of North Amer- ica than in the Sargasso Sea (Clarke 1940). The abundance of plankton is generally higher in cold than in warm ocean waters, correlated with the greater amount of phosphate present in colder waters (Harvey 1955). Annual productivity is probably less in cold waters, however, because there are fewer generations per year. Cold-water plankton tend to be of larger individual size. There is generally a greater variety of species in most taxonomic groups in warm waters than in cold. Abundance of plankton responds to, and has an effect on, the chemical con- tent of the water. 358 Yearly cycle In general, winter is characterized by minimum levels of plankton. Nitrogen and phosphorus salts in- crease in surface waters because of the decomposition of organisms that have died during the preceding months, to the lesser absorption by phytoplankton, and to the greater mixing of waters from various depths accompanying the loss of thermal stratifica- tion. During the spring, as the result of increasing light, reduction of vertical turbulence of the water, and rising temperatures, the phytoplankton increases to a maximum for the year. In temperate and boreal regions the phytoplankton consists largely of diatoms. The zooplankton at this time abounds in immature stages. By summer, nitrogen and phosphorus become diminished in the surface waters because of their use by phytoplankton and lack of replenishment from greater depths with the re-establishment of thermal stratification. Phytoplankton consequently declines rapidly, lacking nutrients and being consumed by the increasing zooplankton population. Zooplankton reaches its maximum during the summer but as it exhausts its phytoplankton food supply, it also de- clines. Decomposition of dead plankton in shallow waters and the destruction of the seasonal thermo- cline with vertical mixing of waters in the open ocean again returns nitrogen and phosphorus to surface in the autumn and this usually allows a second smaller maximum of plankton to develop. High arctic seas usually have a single maximum of short duration in the summer. In the tropics, on the other hand, there is generally no conspicuous peak, although plankton tends to be more abundant during the winter months (Bogorov 1958). In the Indian Ocean, physical oceanographic changes asso- ciated with the monsoons create a seasonal plankton cycle. Diel movements Many zooplankters vary in the depth at which they are most highly concentrated at different times during the 24-hour diel cycle. The daily movements to the surface waters at night and to greater depths during the day is especially marked in the copepods (Clarke 1934), and occurs in euphausiaceans, my- sidaceans, amphipods, ostracod and decapod larvae, pteropods, chaetognaths, polychaetes, siphonophores, and tintinnids. Even some nektonic animals, such as herring and squid, show vertical diel migrations. Most phytoplankton are confined to the upper lighted zone, although dinoflagellates have been shown to have short, vertical, daily movements in response to light (Hasle 1950). The vertical movements of these Geographic distribution of communities organisms and of small fish are probably responsible for the shifts in the position of the deep scattering layer evident in the reverberation of high frequency sound waves sent out from the surface (Eyring et al. 1948, Backus and Barnes 1957). NEKTON Mollusks, fishes, birds, and mammals make up the nekton of the sea. Mollusks are represented by the squids; fish, by the sharks, flying fish, her- rings, mackerels, as well as many others including numerous varieties of small species; and mammals, by the seals, porpoises, dolphins, and whales. The distribution of fish is irregular, but in general they occur more abundantly in neritic waters than in the open ocean. Likewise they are much more numerous in the epipelagic than in lower strata. Most pelagic fish, except sharks, possess a swim-bladder useful for maintaining hydrostatic equilibrium at the depth where they occur; those fish that lack one are com- monly bottom forms (Marshall 1954). In Arctic waters, fish are less abundant, and mammals rela- tively more important, than is the case farther south. Birds, like many other marine animals, are more numerous in the neritic biochore than in the open ocean. In the oceans far from land occur only pen- guins, albatrosses, shearwaters, and petrels, and even these species become more common shoreward. Other marine species in neritic waters are tropic- birds, pelicans, gannets, boobies, cormorants, frigate- birds, ducks, gulls, terns, skimmers, auks, and murres. These marine birds may spend many days or weeks feeding and travelling over the water, but all must search out some shore, cliff, or isolated island on which to nest. Here they sometimes concentrate in enormous numbers during the nesting season be- cause of the limited number of suitable nesting loca- tions available. BENTHOS Benthos is of much greater variety in ma- rine than in fresh-water habitats. These animals are very abundant in the littoral zone and decrease in numbers with depth until only scattered individuals are found in the deep ocean trenches. Benthos con- sists of sessile forms, the sponges, barnacles, mus- sels, oysters, crinoids, corals, hydroids, bryozoans, and some worms; creeping forms such as crabs, lob- sters, certain copepods, amphipods, other crustaceans, many protozoans, snails, echinoderms, some bivalves, and some fishes ; and burrowing forms including most clams, worms, and some crustaceans. Sessile and creeping forms are often grouped as epifauna and the burrowing forms as infauna. Epifauna in the littoral zone decreases in variety toward the Poles since it is subjected to cold and ice erosion, but the species composition of infauna remains about the same. OCEANIC PLANKTON AND NEKTON BIOME This biome is characterized by the pre- dominance of organisms possessing life-forms adapted to keep them afloat. Plankton and nekton predomi- nate, although the deep-sea benthos may also be con- sidered as belonging to this biome. Seasonal aspec- tion may bring drastic changes in species composition, especially in plankton. Dominance, in the sense used for terrestrial communities, probably does not exist, except possibly in the Sargasso Sea where the float- ing vegetation establishes the habitat. The ecosystem is self-contained, however, since energy is derived from the sun and nutrient material continues to re- circulate with little or no dependence on terrestrial resources. The Sargassum community of the Atlantic Ocean is of special interest. The floating Sargassum alga accumulates and is held within a limited area by circular ocean currents. This plant belongs to the in- tertidal zone of the Caribbean islands but is torn loose in large amounts along with attached animals during the hurricane season. It continues to grow thereafter, but does not reproduce. The fauna that it contains is a truly littoral one, rather than pelagic, but because the alga accumulates in fresh amounts as fast as old plants die, the animals reproduce and maintain a continued existence far from any shore. Composition and characteristics The species composition of this biome varies consistently with depth so that a series of overlapping secondary communities may be recognized (Murray and Hjort 1912, Ekman 1953, Marshall 1954, Bruun in Hedgpeth 1957). The epipelagic community or stratal society has the greatest abundance of plankton, nekton, and birds as already described. The aquatic animals are gener- ally colorless, transparent, or of a blue cast. In the mesopelagic community the fishes are usu- ally small, laterally compressed, often silvery or grayish in color, with very large or telescopic eyes, and usually provided with luminescent organs. Some velvety black or brown fishes also occur here. Inver- tebrates are reduced in number and variety and tend to be reddish in color. Since red rays do not pene- trate to the depths where these animals live, they are essentially invisible. Marine biomes 359 FIG. 28-4 Representative deep-water fishes (Sverdrup ef a/. 1942). The bathy- and abyssopelagic communities are considered as one. The fishes are slender and dark- colored. Pelagic invertebrates include a few endemic species of radiolarians, jellyfish, ctenophores, nemer- tinians, ostracods, copepods, amphipods, euphausi- aceans, mysidaceans, shrimps, and squids. Red color is more common than at intermediate depths. Benthic animals commonly have flat bodies, very long legs, or other means of distributing their weight over the loose, flocculent ooze. Many species rise above the ooze on stalks. The fragile glass sponges, long- stemmed crinoids, and long-legged crabs are possible only in very quiet waters that occur at great depths. Skeletons of all animals are fragile because of the difficulty of forming lime at low temeperatures. Abundance decreases with depth, but even at 8300 m in the hadal zone, some twenty species have been found, chiefly holothurians, polychaetes, and sea ane- mones. Bioluminescence is exceptionally well developed among deep-sea forms. In some invertebrates, light- producing organs are scattered over the body. In other invertebrates and in pelagic fishes, there are special luminescent organs. It is estimated that two- thirds of the bathypelagic fish species and over 96 per cent of the individuals are luminous. Although several species of organisms occurring in surface waters are luminous, bioluminescence is more highly developed in the twilight zone, between 300 and 800 m, and occurs at still greater depths in the complete absence of natural light. The adaptive significance of 360 bioluminescence is highly speculative. It may serve, in part, for attracting and seeing prey. Luminescent display may also serve for species and sex recogni- tion as does color in many surface animals. Joined with this bioluminescence is often the development of large eyes and special structures to permit vision at the very low light intensities that are produced. Perhaps as compensation for the difficulties of vision is the extensive development of antennae on some crustaceans and the very long rays in the fins of some fish which may serve for contact reception. In those fish where the eyes are small there is a reciprocally large development of olfactory organs. The benthos and pelagic forms of the greater depths are doubtless derived from intermediate-depth forms, and these in turn from forms occurring on the continental shelf. Species have come to live in the deeper waters only as they became progressively adapted to this rigorous habitat. Relatively few forms have reached the hadal zone. The deep-sea habitat has existed relatively un- changed since very early geological time except for the increasing deposition of bottom sediments and for some fluctuations in temperature. This uniform habitat has allowed some very ancient forms to per- sist to the present time. The recently discovered coelacanth fish Latimeria, the mollusk Neopilina, and certain crustaceans belong to taxonomic groups that supposedly became extinct many millions of years ago. The examination of deep-sea bottom cores will doubtless give us information as to what kinds of animals were present in past ages. Determination of ratios of different oxygen isotopes and of different minerals in the composition of the fossil skeletons in these cores may make possible the determination of water temperatures and salinities at the time these fossil organisms were living (Ladd 1959). Food chains As in aquatic and terrestrial communities, bac- teria in the sea are largely responsible for the final decomposition of excreta and dead bodies to make their essential nutrients available for reabsorption by the green phytoplankton (Ketchum 1947, Harvey 1955) Nitrogen and phosphorus are least concentrated near the surface of the ocean, since this is the stratum in which they are most rapidly absorbed by the photo- plankton. Excreta and dead organisms sink during the process of decomposition, so nitrogen regenera- tion is most evident at depths of 500 to 1500 m. The organic matter that remains undissolved ac- cumulates on the sea bottom. Numerous species of invertebrates depend on it for food and on the bac- teria that it contains (ZoBell 1952). The deep-sea Geographic distribution of communities fishes feed on these invertebrates or are carnivorous on other fish. Many of them have very wide mouths, distensible stomachs, and formidable teeth. In addi- tion to these food coactions, it is also likely that many deep-sea fish and larger invertebrates undergo ver- tical migrations so that they obtain food by preying on living organisms at more moderate depths. Much that is known about the life histories of these deep benthic species has been summarized by Marshall (1954). Especially fertile regions of the open ocean occur when there is deep mixing of waters by turbulence and upwelling. Vertical water currents bring nutri- ents up to the surface from intermediate depths where they had accumulated. Prominent regions of upwell- ing occur around the Antarctic continent, off the coasts of California, Peru, and Somali, and off the west coasts of both north and south Africa. The net zooplankton feed predominantly on the nannoplankton, probably including bacteria, and on the phytoplankton (Clarke 1934). Particulate or- ganic matter, only partially decomposed, may also be important. Most animals depending on these small organisms and organic detritus have various filter- feeding mechanisms for straining food out of the water. They do not actively search and catch indi- vidual items through directed actions. Invertebrate animals may also be able to absorb some essential salts and dissolved organic compounds to build their skele- tal structures and for general metabolism, but there is considerable controversy on this point (Collier 1953). The baleen or whalebone whales (Mysticeti) are toothless but possess large plates in their mouths that strain out the plankton (especially copepods, euphausiaceans, mysidaceans) that they use as food. Only occasionally are small fish or other invertebrates ingested. Some whales reach tremendous propor- tions, and the differential in size between these ani- mals and their food is one of the most remarkable in the animal kingdom. Much more common is the feeding on plankton by squids, the young stages of most fishes and such adult fishes as sardine, anchovy, menhaden, herring, and mackerel. The menhaden is unique in having such fine-mesh gill-rakers that it can feed extensively on diatoms, which because of their smaller size cannot be readily secured by other large marine animals (Clarke 1954). Small nektonic species are in turn preyed on by larger species. Sharks commonly hold the last link in the food chain. The pelagic birds are also fish- eaters or depend on floating carrion for their food. Productivity Most studies of plankton productivity have been conducted in the neritic zone. In the English FIG. 28-5 The filter feeding apparatus of the California sardine: a) gill cover and gills removed to show one side of branchial sieve formed by gill rakers; b) enlarged drawing of a section of the branchial sieve; c) a small copepod, Ojithona plumifera, drawn to the same scale as b); d) a medium-sized copepod, Calanus finmarchicus, drawn to the same scale as b) (Sverdrup et al. 1942). Channel, the mean annual standing crops of phyto- plankton, zooplankton, and pelagic fish in dry weight of organic matter are 0.4, 1.5, and 1.8 g/m?. This is unusual in giving a larger biomass of herbivores and carnivores than of photosynthetic plant material. However, the daily productivity of phytoplankton makes up for this because it is over 100 per cent, while that of zooplankton is only 10 per cent and that of fish 0.09 per cent. The productivity ratio of phytoplankton: zooplankton: fish is approximately 280:100:1 (Harvey 1950). The daily net productivity of phytoplankton in the upper 20 m of Block Island Sound near the eastern end of Long Island has been estimated at 26 per cent of the standing crop in excess of that consumed by zooplankton and bacteria in the surface layer. The zooplankton consumes not more than 4 per cent of the phytoplankton per day. Most of the excess daily production (19 per cent) in the surface waters settles downward and is used by animals and bacteria on or near the bottom, with the rest (7 per cent) becom- ing laterally dispersed into adjacent areas (Riley 1952). The daily productivity of zooplankton in this same area was calculated at 17 per cent of the stand- ing crop (Deevey 1952). Phytoplankton productivity varies with the time of the year. Near Kiel, Germany, in August there is a surplus of phytoplankton production over the amount consumed by animals; the productivity amounts to 350 mm*/m?/day while animal consump- tion is 60 mm*/m*/day. During February, the pro- ductivity of plankton is only 10 mm*/m*/day while the food requirements of animals is 18 mm*/m*/day. This deficiency in food production is correlated with the decrease at that time in the standing crop of both plants and animals (Sverdrup et al. 1942). The average net phytoplankton of the Sargasso Sea is only one-quarter to one-third what it is in the more productive temperate waters (Riley 1957). Productivity is especially high in those parts of the ocean where there is upwelling. It is in these areas Marine biomes 361 Table 28-| Vertical zonation of mollusks on a rocky shore at Cape Ann, Massachusetts. Numbers given represent the density of individuals per square meter (after Dexter 1945). Mya Littorina Littorina Mytilus arenaria_ Littorina Thais Acmaea Anomia Crepidula Zone saxatilis littorea edulis (seed) obtusata lapillus testudinalis aculeata fornicata ere beet A SS eee eee a ee High tide level 4 4 105 cm lower 23 112 115 cm lower 248 2,225 23 Yat 131 cm lower 58 1,339 116 81 248 35 140 cm lower 31 387 132 0 151 8 156 cm lower 704 31 8 341 15 174 cm lower 1,300 0 163 70 8 184 cm lower 813 0 77 15 15 15 8 199 cm lower (near 387 15 45 Spring low-tide level) that the yield of commercial fish of economic interest to man is the greatest. BALANOID-GASTROPOD.- THALLOPHYTE BIOME This community extends from high to be- low low tide levels on rocky shores. Benthic animals and attached algal plants are conspicuous and im- portant. The benthos is mostly epifauna as the sub- stratum is too hard to permit development of exten- sive infauna. When the tide is out, the organisms are subjected to drying, the occasional inflow of fresh water, higher temperatures, and greater light inten- sities. Organisms avoid desiccation when the tide is out by variously crawling under stones or thick algal growths, closing thick shells or operculae, retreating into crevices, or secreting a mucous seal. Most or- ganisms are also faced with the pounding action of waves. Various holdfast or anchoring devices have developed, and many species protect their more deli- cate structures with a hard shell. The adaptations for life on the seashore are many and varied (Yonge 1949). The plankton and nekton associated with the benthos include many species not common to the oceanic biome. Zonation Vertical zonation of species on rocky shores is usually conspicuous (Table 28-1), although individ- ual species may extend widely into adjacent areas (Hewatt 1937, Yonge 1949, Stephenson 1949, South- ward 1958). Beginning on the landward side there is a supra- littoral zone mostly above the action of tides and in- habited as much by land as by marine animals. This 362 Geographic distribution of is followed seaward by a supralittoral or Littorina fringe which is wetted by the highest tides and by the splashing of waves. Because of the presence of either Myxophyceae or lichens, this zone is often discolored ; commonly, black. The fringe is especially characterized by large numbers of small snails and sometimes isopods. Next below this fringe is the midlittoral or balanoid zone. It is strictly inter-tidal, being covered and uncovered every day, and is occupied character- istically by acorn barnacles. This zone is often di- vided into subzones with the barnacles predominant in the upper portion, while polychaets, colonial hy- droids, or other forms are relatively more important in the lower part. The subzonation of algae is often also well marked. The lowest zone ever exposed, and then only at extreme low tides, is called the infralittoral fringe. It is a transition area. The entire area between extreme high and low tides, including the mid- littoral zone and its supralittoral and infralittoral fringes, when considered as a unit, may be referred to as the littoral, eulittoral, or tidal zone to distin- guisk it from the infralittoral or sublittoral zone that extends from the lowest of low tides to the edge of the continental shelf. Zonation is brought about in large part by dif- ferences between species in tolerance to length of exposure and submergence. Animals get into the proper zones by one of several ways (McDougall 1943). Motile species move in and out of favorable areas in direct response to stimuli. In sessile forms, however, it is the motile larvae which are dispersed uniformly, but die off in unfavorable microhabitats. In some forms the larvae become aggregated into a certain area before settling because of response to environmental factors, but the exact nature of the factors responsible for the aggregation of these pe- lagic larvae remains obscure. The presence of or- ganisms already there may exert an influence on communities SUPRALITTORAL ZONE Upper limit of L/frorina snails, etc. EXTREME HIGH WATER LEVEL SUPRA— LITTORAL Upper limit of Lam/naria algae EXTREME LOW WATER LEVEL Upper limit of barnacies FRINGE LITTORAL ZONE ~ > MIO— LITTORAL ZONE INFRALITTOR— AL FRINGE INFRALITTORAL ZONE the species of larvae which will settle, but this needs more study. Young periwinkles are transported by wave action to the lower margin of stony beaches, and further shoreward movement is mainly loco- motive. They achieve their proper zonation by the end of the first year of life (Smith and Newell 1955). Littoral zone Brown algae form thick masses and give pro- tection to those animals that find shelter in or under them. A fauna of copepods, ostracods, water mites and young littorinids inhabit these seaweeds. In England, the numbers of individuals per 100 g of seaweed vary from about 44 on brown algae to over 13,000 on lichens (Colman 1940). The animal life on rocky shores is varied and luxuriant. Several species of acorn barnacles, snails, marine limpets, marine mussels, goose barnacles, sea anemones, chitons, sponges, hydroids, bryozoans, flatworms, annelids, amphipods, isopods, crabs, sea urchins, starfishes, tunicates, and insects are present. Total abundance of animals may run into tens of thousands of individuals per square meter (Allee 1923, Newcombe 1935, Dexter 1947, Yonge 1949, Stephenson 1950, 1952, 1954, Shelford et al. 1935, Hewatt 1937, Ricketts and Calvin 1948). Sublittoral zone This community is not subjected to exposure by tides or to the pounding of surf, but is affected FIG. 28-6 Diagram illustrat- ing terminology of zonation on rocky coasts (Stephen- son 1949). considerably by wave action and the complete circu- lation of water. Animals move around somewhat more freely and there is less need for strong holdfast structures. Most organisms lack physiological toler- ance for long exposure to the air and hence differ fundamentally in structure and mores from the com- munity described above. Laminarias or kelps are the largest of the brown algae and occur commonly in this community. They have root-like holdfasts attached to the bottom and their stalks, which are often several meters long, bear leaf-like branches that float at the surface in the larger species. A long list of animals find shelter and food in the kelp beds and especially in the pro- tection of the holdfasts (Andrews 1945). Polychaete worms are particularly abundant in these holdfasts (Colman 1940). Filamentous red algae (Rhodophy- ceae) are also prominent. Abundant characteristic animals on the Pacific coast are sea urchins, sea cucumbers, starfishes, snails, rock oyster, chitons, limpets, scallops, mussels, nudibranchs, barnacles, crabs, hermit crabs, hydroids, tunicates, shrimps, and various fish. Distribution of fish species correlates strongly with the type of bot- tom or benthos that is present (Popov 1931). The variety of animals in this community is great, both in genera and species, but the density of any one benthic species is seldom greater than 10/m? on the Pacific coast of North America (Shelford et al. 1935), in contrast with the littoral zone. Off the coast of California, the average fresh weight of the standing crop of plants decreases at depths of 1.5 to 22 m from 4667 to 606 g/m?, while animals increase from 125 to 377 g/m? (Aleem 1956). Marine biomes 363 Tidal pools Sea-water is often retained in depressions or pools in the littoral zone and hence organisms here are never completely exposed to the air. They are, however, subject to high light intensity and increases in the temperature between tides (Klugh 1924). Tidal pools are usually rich in both plant and animal life, and some species are largely restricted to them. Red algae and kelps prefer the more shaded, cooler pools ; the green algae and some of the smaller brown algae predominate in the well-insolated pools. Ani- mals of both the sublittoral and littoral zones are found here. Food chains The basic food elements in these rocky shore communities are the free-floating plankton and de- tritus in the water, the algae, and the organic debris adhering to the rock surfaces (Dexter 1947). Many organisms have straining mechanisms that automat- ically collect food materials out of the large volume of water with which they have contact. Snails crawl over the rocks and seaweeds, scraping away at the algae and plant tissue and eroding the rock surfaces. Crabs and fish have a wealth of invertebrates upon which to feed, and are generally at the top of food chains along with birds that feed along the shore. Marine shore animals have developed many de- vices to protect them from predators (Flattely and Walton 1922). Some crabs are concealed by sea- weeds, hydroids, or other organisms growing on their carapaces. Hermit crabs take refuge in the shells of snails. Protective and warning coloration is com- mon. Protective armor occurs in the form of shells, chitinous exoskeletons, spicules, spines, setae, bristles, and constructed tubes. Various forms of weapons have evolved, some of them poisonous, as the nemato- cysts of coelenterates, stylets of some gastropods, spines of the king crab, and chelae of crustaceans. Autotomy, or the ability to throw off an appendage grasped by an enemy, is highly developed in crabs, lobsters, and echinoderms. Well developed powers of regeneration of lost parts occur in these forms, while in worms, sponges, hydroids, and other groups, re- generation of entire new bodies from small fragments is often possible. Dominance and succession Dominance is exerted by those organisms that compete most successfully for the space that is avail- able. When they become established, they largely control the presence of other species. This is true both with the seaweeds and with the more abundant and successful animals. In describing competition on wharf pilings at Beaufort, North Carolina, McDougall (1943: 367) states: So many barnacle larvae, for example, may settle on a small area of clean surface that only a fraction of one per cent of their number will ultimately find space to grow to full size. Incrusting bryozoans, such as Schizoporella, spread over and smother barnacles and other low- growing species in their vicinity. Colonial hydroids, Sponges, and ascidians often form densely matted tangles which accumulate quantities of sediment and effectively smother barnacles, oysters, bryozoans, and other species less luxuriant than themselves. The colonial hydroid Tubularia crocea dominates the pil- ings in April and May but during June, with water temperatures becoming higher, the animals die and slough off, taking with them many associated species. In the bare areas thus exposed, various other sessile species become established. By the end of August, two other colonial hydroids, Pennaria tiarella and Eudendrium carneum, and a colonial bryozoan, Bugula neritina, become dominant. In late October, as water temperatures fall, these summer species die and Tubularia again becomes active and reproduces abundantly. Low winter temperatures temporarily curtail its activities. It is apparent that true succes- sion actually occurs but is passed through quickly and may be obscured by seasonal changes in the spe- cies composition of the biota (Redfield and Deevey 1952))r PELECYPOD-ANNELID BIOME Habitat This biome develops on depositing sand and mud bottoms in contrast to the biome just described that occurs on eroding rocky shores. There is still a good deal of wave action over sandy bottoms. Fine sand particles shift about almost continuously, and animals have difficulty in preventing their burrows from collapsing. In general, the water over muddy shores is shallower, quieter, and warmer. The mud forms a soft, compact bottom, but is also easily moved or shifted around by storms and wave action. Animal burrows in mud are more permanent. Species tend to segregate depending on the fineness of the soil particles and on the amount of organic matter pres- ent. Shores of high mud content may be low in oxy- gen because of decaying organic matter, so animal pop- ulations tend to be largest and most varied in a mix- ture of mud and sand. Tidal currents are weaker, and change in the level of water less pronounced on sand and muddy shores than on rocky ones. 364 Geographic distribution of communities Composition and characteristics Important plants in this biome are the marine eelgrass which is a seed plant, and green algae, par- ticularly the sea-lettuce, which grows in sheets either attached to the substratum or lying fragmented over large areas, and Enteromorpha, which grows in tufts or tangles. Occurring on eelgrass and sea-lettuce may be several kinds of epiphytic algae. These plants form extensive stands and are important to animals for attachment, shelter, and food. Eelgrass was al- most eliminated from the Atlantic coast in 1931-32, possibly because of a protozoan disease. This dis- turbance had a profoundly deleterious effect on the abundance of many animals, including the brant, a bird that depended on it almost exclusively for food, and on scallop and other coastal fisheries. Twenty years later there was evidence that eelgrass was re- covering much of its former abundance (Cottam and Munro 1954). Predominant animals are pelecypods, polychaete worms, particularly Arenicola and Nereis, starfishes, brittle-stars, sea cucumbers, crabs, amphipods, and snails. Populations may run to several thousands of individuals per square meter. A variety of small fish occur here. Birds include sandpipers, plovers, and herons. The biome is world-wide in distribution, but the characteristic life-forms are represented by different species locally. Thus a number of secondary com- munities (biociations) may be recognized (Petersen 1914, Jones 1950, Thorson in Hedgpeth 1957). Many of the animal constituents in this biome are burrowing forms. The substratum of mud and sand holds considerable water and when the tide is out on exposed flats, pelecypods, worms, and other animal constituents retract their fleshy organs into their burrows or shells and remain in a water satu- rated environment (Hesse, Allee, and Schmidt 1951). They are thus never rhythmically exposed to the atmosphere with changes in the tide even on the shore. Furthermore, most forms are generally toler- ant of low oxygen and high carbon dioxide con- centrations. In order to maintain respiration when retracted in their underground burrows these animals have long siphons, sometimes longer than their bodies, or long tubes or canals that extend to the surface. Through these they maintain a circulation of water, often by means of special pumping organs. Burrowing crustaceans have setose appendages, modified for digging, and small eyes. Those that re- main near the surface have long antennae and robust bodies ; those that live in deep burrows have short antennae and slender bodies. For the same reason, burrowing clams that remain near the surface have heavy shells while those that burrow deeper have more fragile shells. These clams either have wide, slimy feet and small shells and crawl with ease through the sand, or have a slender foot that can expand at the end to give enough anchorage so that the animal can pull itself downward into the sand or mud (Pearse et al. 1942). The infauna also includes many microscopic forms, such as nematodes, flatworms, copepods, os- tracods, foraminiferans, and other protozoans. These small organisms may be enormously abundant in number of individuals. In respect to the ciliated pro- tozoans, some species are ubiquitous, but other species are characteristic of this habitat (Fauré-Fremiet 1950). Some species of ciliates occur in the inter- granular spaces of the sand and mud, other species are associated with the surface film of diatoms (Webb 1956). In contrast to the great variety of infauna in this biome, the epifauna is more restricted, although some species are abundant. The fiddler crab browses in great armies on beaches left exposed by the tide, but retreats into its burrow and plugs up the opening when the tide comes in. The characteristic ghost crab of sandy beaches of middle latitudes spends most of its time above high tide level but must return occa- sionally to water to dampen its gill chamber. Zonation Zonation is less conspicuous than on rocky shores because of the prevalence of burrowing forms and of running and swimming species that move up and down with the tides (Brady 1943). Some evi- dence for zonation occurs on sandy beaches with crus- taceans (Dahl 1953) and with pelecypods and annelids (Stephen 1953). A more pronounced change in spe- cies composition occurs in the sublittoral zone; in the North Pacific this change comes at a depth of 3m (Shelford et al. 1935). Pelecypods and annelids still predominate (Holme 1953, Sanders 1956). Food coactions and productivity Bacteria are more abundant on mud bottoms than in the open sea (ZoBell 1946). On mud flats they may average 10 million cells/per cc of mud, or with a biomass of nearly 40 g/m3. Bacteria are, of course, vital for the decomposition of organic debris, dead organisms, and wastes, but may also be used directly as food. Assuming that the bacteria divide rapidly enough to increase their biomass 10 times per day, this gives a 24-hour production of 400 g/m’, mostly concentrated in the 5 cm beneath the surface. Many protozoans and zooplankters feed on bac- teria and detritus, and a large number of mud and sand dwelling invertebrates of larger size, such as Marine biomes 365 pelecypods, annelids, and crustaceans, are deposit- feeders, or have special straining devices in the form of sieves, brushes, and hairy or mucous nets for col- lecting bacteria, organic particles, and small plankton organisms suspended in the water (Blegvad 1914, Yonge 1953). Deposit-feeders tend to predominate where the bottom is composed of fine sediments, while suspension-feeders predominate where the bot- tom is made of coarser materials (Sanders 1956). Snails, amphipods, and others feed on the plant tis- sues of eelgrass, sea lettuce, and algae. Crabs, echino- derms, and fish feed partly on organic debris and partly on the smaller invertebrates. Fish feed on the invertebrates and larger fish feed on smaller ones. On sand and mud flats on the coast of California, only 5 per cent of all animal species are strictly car- nivorous. Only a few species of animals feed directly on plants, but plant tissue becomes more available after its death and partial decomposition. The prin- cipal food chain is plants: detritus and bacteria: detritus and bacteria feeders: animal feeders: birds (MacGinitie 1935). At low tide, feeding and other activities are at a minimum as the bottom forms retract into their bur- rows and motile forms retreat seaward. However, some of the snails continue to feed on plants, and insects and birds come into the exposed area in search of debris and small animals for food. As the tide returns, the insects and birds retreat landward, but pelecypods extend their siphons, annelids rise from their burrows, shrimps, crabs, and fish move about over the surface, and the whole community becomes a scene of bustling activity. Petersen (1918) and Jensen (1919) of Denmark were concerned with measuring the productivity of invertebrates in the sea, especially benthos, as a source of food for commercially important fish. More accurate calculations in the English Channel (Harvey 1950) give the ratio between the mean annual dry organic weight of bottom invertebrates and the bot- tom-dwelling fish as 15:1. Annual productivity of the invertebrate fauna is twice as great as that of the fish, so the productivity ratio between the two groups is) 30:1" Dominance In contrast to rock-bottom communities, compe- tition for space is not an important factor in mud- and sand-bottom communities. Actual evidence pur- porting dominance to exist is not convincing (MacGinitie 1939). Eelgrass and algae do not ap- preciably react on the physical characteristics of the habitat except to increase the supply of oxygen. Starfishes and brittle-stars may exert control to a limited extent where they are numerous by feeding on and preventing pelecypods from becoming estab- lished in the community (Clements and Shelford 1939). It is possible that fishes may at times modify the species composition of an area, but for the most part it appears that the presence and distribution of species is controlled directly by the physical condi- tions of the habitat (Jones 1950) and that only popu- lation size is modified by predation and competition. CORAL REEF BIOME Coral reefs are formed by the accumulation of the calcareous skeletons of myriads of organisms. They extend from the sea bottom at depths of 46 m, or rarely 74 m, to slightly above low tide level. The best formation of coral reefs is confined to warm waters above 18°C, although individual species may extend into colder regions (Vaughan 1919, Wells in Hedgpeth 1957). Predominant organisms involved are commonly the anthozoan stony corals and organ corals and the hydrozoan milliporids. Some reefs, however, are formed principally by Foraminifera and still others by calcareous algae. All massive coral structures employ calcareous algae as cement. These algae not only thrive in the pounding surf on the windward side of the reef, but by their growth are able to repair damage to the reef caused by storms. Most typical reef-building animals are colonial and of shapes varying from closely compact, globose, or encrusting, to loosely branched or dendritic, depend- ing in part on their exposure to wave action. Each polyp in a colony secretes its own calcareous skeleton and when it dies the next generation builds on top of the old so that the accumulation of a lime structure is fairly rapid. The bright yellow or red colors of corals near enough to the water’s surface for adequate light penetration are the result of algae, the zooxanthellae, which are either embedded in the body wall or free in the internal cavities. In addition, there are bands of green filamentous algae growing to a depth of 2 or 3 cm in the pores of the inert coral skeleton that may have a biomass sixteen times that of the zooxan- thellae. In their photosynthesis, these algae probably absorb carbon dioxide and nutrients from the animal tissues of the coral and liberate oxygen and provide nourishment of value to the animals (Odum and Odum 1955), although this has been disputed. Per- haps because of this symbiotic relationship which re- quires solar radiation, living corals are largely con- fined to the upper, shallower waters. Coral animals actively ingest zooplankton, but apparently not phyto- plankton, from the surrounding water (Hand 1956, Yonge 1958). Reefs may be either fringing, barrier, or atoll 366 Geographic distribution of communities FIG. 28-7 Three types of coral reefs and their possible manner of origin, according to Charles Darwin: a) fringing reef, b) barrier reef, c) atoll. (Fig. 28-7). Fringing reefs are in direct contact with the shore; barrier reefs are separated from the shore by a lagoon of varying width; atolls are annular or horseshoe-shape, surrounding a lagoon that does not contain any central land mass. The Great Barrier Reef that extends for great distances off the east coast of Australia is a good example of the second type of reef, and atolls are numerous in the South Pacific. According to a theory first proposed by Darwin, barrier and atoll reefs form from fringing reefs either as the land subsides or the water level rises (Vaughan 1919). Atoll islands are formed either when the water level falls or when waves break off and pile up chunks of coral limestone to build the reefs a few feet above the high tide level. As the stone dissolves or becomes pulverized, it forms a soil on which plants can grow and terrestrial communities of animals invade. Reefs are often not continuous because the organisms are intolerant of fresh-water brought down by streams, and because they are very sensitive to smothering by mud or sand. On the ex- posed ocean side there is generally a zonation of different species from the shore outward. (Odum and Odum 1955, Goreau 1959, Wells in Hedgpeth 1957). The coral organisms, particularly the algae and the coelenterates, are true dominants in this biome since they build the substratum that makes possible the development of the community and the occurrence of other organisms. Competition for space, light, and protection from wave action is keen. There is a great variety of secondary species asso- ciated with the corals. These include many kinds of alcyonarians; numerous brittle stars, crinoids, and holothurians ; a great variety of chaetopod, echiurid, and sipunculid worms ; crustaceans, including hermit crabs; mollusks; and large numbers of brilliantly colored, strikingly-marked fish. The many crevices, holes, and cavities in and between the coral provide excellent hiding places and refuge from predators so that the impressive development of color among the fishes may be due, in part, to lack of predation pres- Sea level sure. The fishes have a variety of food habits and are represented in all consumer trophic levels (Hiatt and Strasburg 1960). At the Eniwetok atoll in the South Pacific, the average dry weight biomass of the living photosyn- thetic plant material is estimated at 703 g/m?, that of the herbivorous and carnivorous animals at 132 g and 11 g, respectively. These weights exclude the dead skeletal materials associated with the proto- plasm. The ratio between plants and herbivores is 5.3:1, between herbivores and carnivores 11:1, or a composite ration of 64:12:1. The total primary pro- ductivity per year as the result of photosynthesis was estimated at 12.5 times the biomass of the stand- ing crop. This is sufficient to balance approximately the total plant and animal energy needs of the reef and thus render the coral reef a self-contained steady- state ecosystem (Odum and Odum 1955). SUCCESSION TO LAND The three great biocycles—ocean, fresh- water, and land—come into contact with each other around the margins of the seas. The change in the physical nature of the habitat from salt water to freshwater is a drastic one, but not more drastic than the change from salt water to land. The transition of animal and plant life is abrupt, and a zonation or physiographic succession of communities can be recognized. This transition from the ocean to fresh- water and from the ocean to land as we see it today is of special interest since it parallels the probable evolution and dispersal of life in past ages. Life is generally believed to have originated in the littoral region. Apparently no great groups [phyla] of animals originated except in the ocean. The routes by which animals probably left the ocean and reached fresh-water and land have been various. Some animals probably migrated directly across sea beaches; others probably ascended rivers, passed through marshes and swamps, or burrowed through Marine biomes 367 soil. Some animals were transferred from the ocean by land elevations which isolated them in bodies of water which gradually became fresh. . . . Emigra- tion from the sea did not take place at any one time. It has occurred many times in the past and is slowly progressing on many shores today. ... The most successful animal colonizers of the land have been: (1) the arthropods, which have in many cases de- veloped book-lungs or tracheae for breathing air; (2) the vertebrates, with lungs and dry skins; and (3) the snails, with slime and spirally coiled shells to prevent desiccation. ... There are at present many examples of animals which are in the midst of their transformation from marine to fresh-water ani- mals, or from marine or fresh-water into land ani- mals. Not only have plants and animals enugrated from sea to land, but there are countless instances when migrations have taken, and are taking place in the opposite direction. Grasses, insects, reptiles, birds, and mammals have left the land for the sea. . . . Fishes began in fresh-water, but now range through the ocean at all depths (Pearse 1950: pp. 9-10, 14). On rocky shores and cliffs there is a splash or supralittoral zone above high tide level. Green algae occur here and scattered individuals of marine snails, acorn barnacles, limpets, amphipod sandfleas, and flatworms, as well as insects, especially Diptera and other forms that come from the land. Above the in- fluence of splashing, the rocks may be covered with lichens and mosses, representing the initial stages in the terrestrial rock sere. However, salt spray is often blown inland a considerable distance to affect con- spicuously the development of normal terrestrial vegetation and its accompanying animal life. Cliffs along the ocean, as well as sandy beaches and islands, are favorite nesting places for large numbers of pelagic birds. Above water action on sandy shores, the wind may blow the sand into dunes with the consequent development of the dune sere. On muddy flats there is typically a development of salt marshes, particu- larly in protected embayments or along the margins of outflowing rivers. The high marshes are flooded completely only during the spring tides, but the ground water is more or less continuously saline. As sediment accumulates the marsh eventually becomes dry land (Steers 1959). The seashore snails (Littorina), the marsh snail (Melampus), mussels (Brachidontes, Mytilus), crabs (Carcinides, Cancer), amphipods (Gammarus, Or- chestia), and isopods (Philoscia) occur through the extensive salt marshes on the Atlantic Coast of North America and there are numerous flies and mosqui- toes. Killifishes are abundant and devour many mosquito larvae. Herons, plovers, sandpipers, ducks, rails, bitterns, redwinged blackbirds, marsh wrens, 368 and sharp-tailed sparrows feed or nest. Muskrats and meadow voles, as well as other species of mam- mals, occur in salt marshes but are not particularly characteristic of them (McAtee 1939). In tropical regions mangroves may develop in- stead of marshes on mucky, poorly aerated bottoms. The red mangrove has an extensive prop root system and grows in deep water not ordinarily exposed even at low tide. The mangroves protect the shore from erosion and aid in the accumulation of deposits of peat and mud that build up the shore and form islands. The black mangrove at higher levels usually produces erect roots that stick up through the mud and serve as pneumatophores. Mangroves are usu- ually heavily populated beneath by crabs and other marine species. SUCCESSION TO FRESH WATER Where rivers flow into the ocean on low coastal plains and there are extensive embayments or estuaries, as along the Atlantic Coast, there is a very gradual change from salt water to brackish water (salinity : 0.5-30°/00) to entirely fresh water. This habitat gradient fluctuates back and forth with the tides. Since fresh water is less dense and often warmer, it flows over the top of the salt water with the result that strata with different physical charac- teristics are formed and these different strata are in- habited by different kinds of fish and other organisms. Species of marine organisms extend towards fresh water as far as permitted by their tolerance of reduced salinity. Since this tolerance varies between species, the marine flora and fauna become impover- ished as the fresh-water flora and fauna become enriched. There are, however, many more marine species than fresh-water species in estuaries, although productivity in brackish water is considerably less than in the sea. A few species find optimum environ- mental conditions in brackish waters and decrease in abundance both toward fresh water and towards the open sea. Economically important brackish and shal- low water species on the Atlantic coast are the blue crab, lobster, American oyster, scallops, hard-shell clam, soft-shell clam, and such fishes as the Atlantic croaker, striped bass, American shad, scup, weakfish, and others. Of special interest are fish that perform long mi- grations between fresh and salt water for spawning purposes. Anadromous fish, principally salmon, shad, striped bass, and some trout, come from the ocean into fresh-water streams; catadromous fishes, like the fresh-water eels, reverse the process. The chum salmon spends several years in the sea until it be- comes sexually mature, then it ascends fresh-water streams to their cool, gravelly-bottom headwaters to Geographic distribution of communities —_ — aquasaAUOD INI41FVUy aouad1aAuoD [eal0g-nuy uenbewen upauiny ueyAniag sodedelen o}weuEd oNueTy -o}oed Seq UEIpUy 1SAK UEJUEI} NEW upaues1a}|paw UEyUE}ISNT revenseuenet® ® “ urojiawy aUeNY YON uredoing dose UY purleaz Man uvyjeasny aloe [esUaD uEyyeMey “€S6l uewy3 Aq uealb uolpeusojul Woy pesedaig ‘soiyouepuy ey} 40 SIB}LM BOELINS PjOD GY4 MOjaq 'fUefUOD 4/eS Japeaib sey} fo esned -Oq "UIs SIaZeM BdejINS WEA a1e4M sPate fuasaidas senuebiea -U0d r14dUepUy puke jPedog-1}Uy e4) “+945 jepueulyuod 9} uo UOl{NgIijsip jeune} ayf Ul suoibeigns pue suoibey g-3z “O|4 BUIOJITED JO JIND UEBTUIOFIED asaueder oneisy uennary o1jl0eq YW4ION D1j19kq ISA9MYIION uajanasay o04b uy ISeq OT ue}[eaysny yyNoS upAeleW-Opuy adea 990 UEIPUI ed 1S9M\-Opul ag Pay o1osty -onuely oy) 04-4810 oy ory ARNO TN Or DH 369 Marine biomes spawn, after which it dies. The Atlantic salmon, however, has a shorter developmental period and may spawn more than once. Tagged salmon and shad are commonly known to ascend the same streams in which they were hatched, and recent studies indi- cate that they recognize their home waters by chem- ical stimuli (smell), memory of which is retained from very young stages (Hasler 1954). Apparently salmon eggs must be laid in fresh water as otherwise the outer membranes do not harden and the number hatching is reduced (Black 1951). Furthermore, young salmon cannot tolerate sea water until they have developed chloride-secreting cells in the gills. The adult female eel spends 5 to 20 years in the fresh-water streams that drain into the Atlantic Ocean in both the Western Hemisphere and in Europe. They often ascend these streams far into the interior of the continents. The male, however, remains in the brackish water of the bays and estu- aries. It is here that mating takes place as the female returns to the sea to spawn. The journeys of the females have been difficult to follow but apparently both the American and European species spawn in the depths of the tropical sea northeast of the West Indies. The females then die. It is here that the smallest immature forms occur in mixed popula- tions in the open sea. It is still a mystery how the young of the two species become separated and get to their respective continents. ZOOGEOGRAPHY There have been several attempts to recog- nize taxonomic and geographic divisions in marine communities, beginning with Petersen (1914), but probably the best and most complete is that of Ekman (1953). Ekman divides marine life first of all into faunas, based partly on temperature and partly on geography, then into regions and subregions (Fig. 28-8). Subregions may be still further divided into provinces (Stephenson 1954). Families and genera that are endemic or restricted in their distribution have been most useful for distinguishing the major geographic divisions. These divisions are faunistic ones and each division may contain two or more of the biomes that we have just described. Pelagic biome There is enough interchange of water between the different oceans to give considerable uniformity in the taxonomic composition of pelagic organisms. Ubiquitous species found in all oceans and in both equatorial and polar regions include species of sipho- nophores, ctenophores, polychaetes, copepods, chae- tognaths, and amphipods. The principal division of the epipelagic commu- nity is into a warm-water fauna lying between sum- mer isotherms of 14°-15°C north and south of the equator and into Arctic and Antarctic faunas. A large number of species in the warm-water fauna are worldwide. In general, the fauna of the Indian and Pacific Oceans is richer in species than that of the Atlantic Ocean. The arctic and antarctic faunas contain several characteristic and endemic species, some of which may at times become very numerous. The blue and fin whales of the antarctic have long been sought by man for their oil. There is essentially only one region of abyssal- benthos with many genera of animals widely dis- tributed. Doubtless this is due to the considerable uniformity of environmental conditions in the various ocean bottoms. Subregions can be recognized, how- ever, on the basis of relative proportions of endemic species. Biomes of the continental shelf and coral reefs The shelf fauna, made up of benthos and asso- ciated organisms, is divisible into warm-water, tem- perate, Arctic, and Antarctic faunas. The poleward limits of the warm-water fauna are correlated with minimum yearly temperatures of 16°-18°C. This fauna may be further divided into tropical and sub- tropical sub-faunas along an isotherm of 20°C. The Arctic and Antarctic faunas are limited towards the equator by summer isotherms of 4°-7°C. Temperate faunas lie between the warm- and cold-water faunas and in turn are sometimes divided into warm and cold temperate sub-faunas. The tropical sub-fauna is by far the richest in species and contains numerous endemic elements which do not penetrate extensively even into the sub- tropical zone. Coral reefs are found only in the tropics. The variety of forms making up each fauna becomes progressively less poleward. The tropics have been the center of origin, differentiation, and dispersal of these cold-blooded organisms, and spe- cies have invaded colder waters only as they have been able to acclimatize to them. Many tropical genera and families are circum- tropical in distribution ; that is, they are found in the Indian, Pacific, and Atlantic Oceans, although repre- sented by different species in each area. There are, however, a few species that are also circumtropical, including the brittle-star (Amphipholis squamata), certain crabs (Grapsus grapsus, Planes minutus, Plagusia depressa), the hammerhead shark (Zygaena malleus), the porcupinefish (Diodon hystrix), and nearly all the marine turtles. 370 Geographic distribution of communities —~ ee — ae It is not possible here to describe the fauna found in the various regions and subregions nor to analyze the interesting paleo-ecological history of each re- gion. In general the Jndo-West Pacific region and specifically the Indo-Malayan subregion have the greatest abundance, variety, and distinctness of ani- mal life. This is expressed in many different taxo- nomic groups with significant percentages of fam- ilies, genera, and species being exclusive to the region or subregion. This may represent the ancient pro- fusion of forms that during the early Tertiary ex- tended more or less around the world. This ancient fauna persisted here because the region was not sub- jected to the cooling of the climate and waters that occurred elsewhere during late Tertiary and the Quaternary and which brought impoverishment of the fauna. The West Indian subregion of the Atlanto-East Pacific region ranks next to the Indo-Malayan sub- region in size and richness of fauna. The shelf fauna of the North Pacific region and adjacent Polar- Arctic, especially on the American side of the Pacific Ocean, is much richer than that of the North Atlantic region. Both the Arctic and Antarctic regions have a number of endemic forms, but in general the Ant- arctic fauna, especially of invertebrates, is much richer in species. There are a few species of crabs, Cancer, a starfish, Ctendiscus crispatus, and some other organisms that occur in both polar regions with continuous intermediate distribution. However, the species occur in shallow waters in the polar re- gions and only in the deeper cooler waters of the tropics. Various other species or related forms are found in the two opposite polar or temperate regions only, with presumably the interconnecting tropical linkage having become broken sometime during past geological time. APPLIED ECOLOGY Although 71 per cent of the earth’s sur- face is occupied by oceans and only 29 per cent by land, nearly all of the food and raw materials used by man is derived from the latter. This is in spite of the fact that agricultural soil is only a few inches thick and must be cultivated, protected from erosion, and fertilized, while the ocean with its chemical fertility, its photosynthetic production of basic plant food, and its fisheries appears almost inexhaustible. Because of its high productivity, the plankton of the sea represents an important potential food supply for man (Davis 1955). Its energy value is approxi- mately 4 Cal/g dry weight, and it is more or less palatable (Clarke and Bishop 1948). However, there are difficulties involved in securing significant amounts, poisonous species sometimes occur, it is not easily digested and assimilated, and consequently it has not as yet proved to be a feasible diet. The energy value of the plankton is used by man at the present time primarily as it is transferred into higher links of the food chain. Aside from the fishes, the chief marine organisms used as food are the oysters and other mollusks, shrimp, crabs, lobsters, and sea tur- tles. These are mostly animals of the continental shelf and estuaries. There are a number of problems in the use and conservation of marine organisms. Natural beds of American oysters on the Atlantic coast have nearly all been exhausted through over-fishing and _pollu- tion. Because of heavy erosion of the land, silt depo- sition has become excessive in most of the bays and estuaries and there is increasing difficulty for oyster spat to find clean hard surfaces on which to set. Sur- faces that are loose or covered with silt are not suita- ble since the spat is very small and easily smothered. A common practice is to return to suitable areas all shells of mollusks removed or to introduce other suitable hard objects to furnish the necessary sub- stratum for oyster setting. Control of erosion over the watershed would greatly alleviate the problem. The trend is increasing to lease suitable areas of water and to farm oysters in the manner of an agri- cultural crop (Korringa 1952). In spite of their position at or near the top of the food chain, the greatest utilizable food resource of the sea is its fin fishes. The service of transferring the basic fertility of the sea through successive stages in the food cycle to fishes is performed by nature, and man needs only to harvest the final crop. The great bulk of commercial fish is in the families of herrings, codfish, salmons, flounders, and mackerels. Probably most kinds of fish are potentially useful, although some species that occur in coral reefs are inedible or poisonous. Tunas were not widely eaten in the United States until 1928; swordfish, once anathema to fishermen, are now as expensive as steak. Sharks were not used until a few years ago, but are now a major source of vitamins. The loss of elements from the ecosystem with the removal of fish is replaced by the continued inflow of nutrients from the land by way of the rivers. A fishery, temporarily exhausted, will usually become replenished by natural processes if left alone for a period of time. When one realizes that with the same expenditure of effort a man in a year’s time can harvest two and one-half times as much edible fish as he can pork in pigs, it would appear that the ocean community is one that should be more extensively utilized (Taylor 1951, Walford 1958). The situation is different with whales and seals. Whales have been pursued so vigorously for their oils that certain species are in danger of extinction. Seals have been taken extensively for their fur. Interna- Marine biomes 371] tional regulations have now been set up to limit the take of both groups of species. The applied ecologist is also concerned in the fouling of ship bottoms by growth of organisms, par- ticularly those belonging to the balanoid-gastropod- thallophyte biome. This is of major economic im- portance because of reduction of speed and the greater fuel consumption imposed on fouled ships. The prob- lem has stimulated intensive studies of the behavior of the organisms concerned and the searching for chemicals or methods of treatment of ship bottoms to prevent their setting (Iselin 1952). SUMMARY Geographic distribution of marine organ- isms depends on their responses to current, temper- ature, and physical barriers; their local distribution is affected by waves and tides, type of bottom, salin- ity, and depth. Major divisions of the marine bio- cycle are pelagic (open water) and benthic (bottom). Major communities recognized are the oceanic plank- ton and nekton biome in the open sea, the balanoid- gastropod-thallophyte biome on rocky shores, the pelecypod-annelid biome on sand and mud bottoms, and the coral reef biome. Organisms making up the oceanic biome are widely distributed around the world but may be di- vided into warm-water and Arctic and Antarctic faunas. Coral reefs are found only in the Tropics. The two biomes on the continental shelf subdivide into warm-water, temperate, Arctic, and Antarctic faunas and into more restricted regions and subre- gions. The warm-water faunas are richest in species, especially in the Indo-Malayan and West Indian sub- regions. Marine plankton include a greater variety of forms than does fresh-water plankton, although roti- fers are nearly absent; cladocerans, less important. They possess various unique mechanisms for flota- tion. Although abundance varies greatly from place to place and from season to season, plankton is gen- erally much more numerous in neritic coastal waters than in the open sea. Diel movements between the surface at night and greater depths during the day are pronounced. Mollusks (squids), fishes, birds, and mammals constitute the nekton. The taxonomic composition of the fish fauna varies with depth. Bioluminescence is exceptionally well developed among deep-sea nekton and benthos. Benthos includes a great variety of sessile, creep- ing, and burrowing forms. It is very abundant in the littoral zone, and decreases in numbers with depth until only scattered individuals are found in the deep ocean trenches. There is considerable difference in the life-form and species composition of benthos oc- curring on rocky shores and on sand and muddy ones. Zonation of species is more prominent on rocky than on depositing shores. Succession and dominance occurs in some situations, but is less important than in terrestrial communities. Coral reefs have many special features. Food chains in the sea are similar to those in fresh water but different species make up the various links in the different communities. Productivity is especially high in regions where upwelling and tur- bulence bring nutrients from deeper levels up to the surface. The three great biocycles of ocean, fresh-water, and land come into contact around the margins of the seas. 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Bibliography 403 404 Subject Index |Index A Adaptive Radiation: as having occurred with ancient marsupials, 266 conditions of occurrence of, 266 of insect species in Hawaiian Islands, 266 Adjustment(s) to Terrestrial Habitat(s) (see also Behavior Patterns; Morphological Adaptations): absorption of moisture through body surfaces and, 97 body temperature in: constant, as developed by birds and mammals, 98 control of, 98 convergence of sere-stages as, 102 evolution of body functions in: burrowing and dragging to counteract gravity, 96 long-range vision, 99 evolution of body structures in: appendages to counteract gravity, 96 internal air-breathing organs, 99 skeletal framework to counter- act gravity, 96 evolution of body coverings: as protection from solar radia- tion, 99 to counteract gravity, 96 evolution of body surfaces to prevent moisture loss, 97-98 in alpine tundra: birds and insects closeness to ground in face of strong wind, 322 by mammals in altitudes of low oxygen pressure, 323 through freezing and thawing of spring-tails, 322 through white coloration, 322 in arctic tundra: of birds, through development of flight songs, 320-321 of cold-blooded animals, 320 of herbivores, 320 through body insulation, 320 through failure of breeding during phenomena detrimental to survival, 320 through migration, 320 through overwintering, 320 through periods of rest during long summer day, 320-321 through tunneling and burrowing, 320 through white coloration, 319-320 in deserts: after abundant rainfall, 338-339 Adjustment(s) to Terrestrial Habitat(s) (Continued) as Similar to adjustments of prairie species, 338 of fish in ponds, 339 of plants to survive lack of water, 333 through avoidance of hot ground surface, 338 through coloration, 339 through hibernation over winter, 339 through nocturnal activity, 338 through use of shady nesting areas, 338 through utilization of limited water sources, 338 in grassland: during winter, 330 of birds, 330 of bison, 328 of pronghorn antelopes, 328 of small mammals, 329 through development of hopping locomotion, 329-330 through food habits, 328 through protective coloration, 329 through use of ponds and potholes, 330 through vision and fleetness of foot, 328-329 in temperate deciduous and coniferous forests: and migration, 299, 299-300 and time of breeding seasons, 299 as determined by differences between deciduous and coni- ferous forests, 308 during severe winter conditions by active animals, 299 | for arboreal habits and climbing, 299 | in hearing and voice, 299 made by coniferous species in deciduous forests, 305-306 of animals that remain active over winter, 309 of man, 300, 308 of overwintering animals, 300 dl role of trees in, 299 A in tropical rain forests: as not bearing on cold, weather : or food, 346 of cold-blooded animals, 347 ql of organisms living in water u! within clumps of leaves of epiphytes, 349 of reproductive habits of birds, 347-348 of sloths and anteaters, 348 dl of wood- eating insects, 347 Adjustment(s) to Terrestrial Habitat(s) (Continued) through ability to hang from trees, 346 through arboreal living habits of animals normally ground dwellers, 349 through daily rhythms in animal activities, 348 through lack of definite periods of dormancy or migration, 348 through large adult size of cold- blooded animals, 347-348 through nectar and pollen feeding, 348 through nesting in holes in trees, 346-347 through path-making, 347 no need for in areas of little seasonal variation, 101-102 of animal groups: arhythmic, 99-100 crepuscular, 99-100, 100-101 diurnal, 99-100, 100-101, 101 homoiotherms, 98-99 massive, and effect of gravity, 96 nocturnal, 99-100, 100-101, 101 poikilatherms, 98 of organisms in rock seres, 104- 105 of seashore animals submerged by tides, 352 on basis of photoperiodism, 102 on basis of seasonal variations in vegetation and food supply, 101- 102 reproductive, 98 role of foliage in, 102 techniques for resistance to cold in, 99 using ‘‘free water”’ in food in, 97 using ‘‘metabolic water’’ in, 97 water consumption in: drinking, 97 obtaining, 96 water loss in: and excretory organs, 97-98 uncontrolled limitations of, 97-98 excessive, prevention of, 96 Allen’s Rule, 9 Animal Reactions (see also Lakes; Ponds; Streams): chemical, in water, 172 physical, in water, 172 to water conditioning, 172-173 Applied Ecology (see also Forest and Game Management; Lake Management; Pond and Marsh Management, Range Manage- ment; Wildlife Management): and plankton as potential food supply, 371 Applied Ecology (Continued) and use of marine fish as food source by man, 371 artificial fertilization in, 208 determining maximum sustained productivity yield as problem of, 207 in problem of whale and seal extinction, 371-372 need for understanding variations in abundance in, 234 pest control through: crop pests, 227 field mice, 235 Aspection (see also Climate): and changes in species composition of oceanic plankton and nekton biome, 359 and variations in fecundity of invertebrates, 211 as reducer of competition among: dragonflies and damselflies, 247 grasshoppers, 247 salmon, 247 terns, 247 described, 101-102 in arctic tundra, 320 influence of on distribution of plankton, 68-69 in forest communities, 137 climax of deciduous forest biome, 293-294 coniferous forests as compared to deciduous, 308 tropical broad-leaved evergreen, 342 in grassland, and blooming of perennial herbs and grasses, 325 in terrestrial communities, 101- 102 Association(s): defined, 29 plant: defined, 276 in coniferous forests of North America, 302 in deserts of North America, 333-334 in grasslands of North America, 325 in North America, listed, 294 of chaparral, 312 of woodland, 311 on Lake Michigan, 105 Associes, defined, 29, 276 Atmosphere (see also Carbon Dioxide; Moisture; Oxygen): as absorber of solar radiation, 200-201 as diffuser of oxygen into streams, 43 as division of environment, 6 Atmosphere (Continucd) heights of various levels of, 243 ionization of as affecting health of animals, 243 of alpine tundra, 316 ozone in, 243, 243-244 release of oxygen into from lakes, 64 short wavelength ultraviolet radi- ations as causes of changes in, 243 weight of and lake organisms, 59 Autoecology (see Ecology, Subdivi- sions of) B Bacteria: and roots of legumes in mutual- ism, 176 diseases caused by, 181 increase of in change of pond to marsh, 82-83 in food chains of pelecypod-annelid biome of sea, 365-366 intestinal, in mutualism, 177 role of in food-cycle of lakes, 74 role of in nitrogen cycle, 166-167 Banding: development of as technique in animal ethology, 6 in detailed studies of small popu- lations, 37 to determine age of sexually mature adults, 216 to determine breeding age of birds, 215 to determine home ranges of birds, 185 to study dispersal, 149 Behavior Life Histories, defined, 16 Behavior Pattern(s) (see also Innate Behavior): adjustment of through learning, 14, 15 and behavior life histories, 16 and ecological niches, 16 arhythmic activity as, 100-101 as sole determinant of niche preference for certain species, 250 changes in after physiological adjustments to environment, 10 composite periodic activity as, 100-101 correlation of tests of with animal’s behavior under natural condi- tions, 13 endogenous, periodic activity as, 100-101 evolving of to facilitate predation, 227 405 Index Behavior Pattern(s) (Continued) exhibited by lake organisms to decrease specific gravity of body, 60 exogenous, periodic activity as: inheritance of through chromo- somal clues, possibility of, 251 inherited, as factor in responses to environment, 13, 15 in orientation to changes in envir- onment, 12-13 in response to stimuli, 13, 14 motility as consideration in changes i a new, acquiring of as factor in dispersal to new habitats, 150 of poikilotherms in maintaining constant conditions, 98 of stream animals: in orientation to environment, 50, 53 in respiration, 51-52 in response to bottom of stream, 50-51 in response to stream size, 52 preadaptation to niche as advantage in competition, 250-251 transmission of by tradition rather than genetics, 251 trial and error in dispersal and establishment of, 148 Benthos (see Biocies; Biocies, Lake; Biocies, Pond- Marsh; Lakes; Sea) Bergmann’s Rule, 9 Biocenose (see Communities) Biociation(s) (see also Deserts; Forest(s); Grassland, Tropical Biomes; Tundra): as subdivision of biome, 276-279 concept of as disregarding plant dominants in establishing divisions of biome for animals, 276-279 coniferous forest(s): evolution of separate North American and Asian, 307-308 location and number of, 302-303 North American boreal, 303-306 North American montane, 307 North American, species in, 302- 303 deciduous forest(s): Asiatic, 299 European, 298-299 North American, 295-297 defined, 29-30, 30 differences of from biotic provin- ces, 279 in alpine tundra, 321-323 in arctic tundra, 318-321 in chaparral, 312-313 406 |ndex Biociation(s) (Continued) in deserts other than North American, 337-338 in North American Basin sagebrush, 336-337 in North American deciduous forest-edge, 297-298 in North American desert scrub, 335-336 in southeastern North American forest, 298 in tropical forest biomes, 344 in woodland, 311-312 Biocies (see also Deserts; Forests; Grassland; Tundra): defined, 29-30, 30, 276-279 forest-edge, 129 Biocies, Lake (see also Lakes): benthos: described, 66 distribution of in littoral zone, 70-71 distribution of in profundal zone, 71-73 divisions of, 66 fish in, anatomical adaptations of, 74-75 nekton: described, 66 position of limnetic species in lake, 74 segregation of littoral zone species, 74 shrimp as part of, 73-74 neuston, described, 66 plankton: composition of, 66-67 diel movements of, 67-68 effect of annual overturns on distribution of, 69 effect of rainfall on distribution of, 69 in small and medium-sized lakes, 67 irregularity of horizontal distri- bution of, 67 irregularity of vertical distribu- tion of, 67 pulses of distribution of, 69 role of in food-cycle of lakes, 74 times of maximum distribution of, 68-69, 69 times of minimum distribution of, 68-69 seston, defined, 66 Biocies, Pond-Marsh (see also Marsh; Pond(s)): amphibians in, 85-86 benthos in: dwellings of, 82 factors affecting changes in species of, 82-83 Biocies, Pond-Marsh (Continued) 13 relation of biomass of to biomass of vegetation, 82 birds in, 86 infraneuston in, composition of, 81 insects in: air breathing aquatic, respiration of, 84-85 terrestrial, 83-84 mammals in: beaver, 87 mice, 87 mink, 86-87 muskrat: rice rat, 87 shrews, 87 plankton in: distribution of, 82 species of, 81 reptiles in, 86 supraneuston in composition of, 81 terrestrial invertebrates in, 85 use of surface film by, 81 Biocies, Stream (see also Fish; Streams): lack of indigenous species of in sand-bottom pools, 43 most characteristic forms of, overwintering of in sand-bottom pools, 43 plankton in, 43-44 plants in, 43 pond animals in, 43 Biomass (see also Measurement of Populations; Populations): accurate computation of, 85 and net production of a generation of animal species, 204 and productivity values, 206 and reproduction of protozoa and bacteria, 177 as index to trophic levels, 198- 199 as unaccumulated in balanced trophic levels, 206-207 determining of to measure popula- tions, 31 increase in with growth of indi- viduals, 202 in lakes: and productivity of benthos, 76 and productivity of plankton, 76 of benthos, 75, 75-76 of plankton, 75, 75-76 in ponds: as affected by food habits of fish species, 91 as varying with fertility of pond, 90-01 harvest of as key to productivity of fish, 91 of benthos and vegetation, 82 Biomass (Continued) of invertebrates in shallow water, 90 of fish, 90-91 measurement of, transfer of, and use of energy, 201 of bacteria in pelecypod-annelid biome of sea, 365 of birds in forest canopy, 137 factors in, 222 of mud-bottom pools, 55 of organisms in forest communi- ties: birds, 135 mammals, 135, 135-136, 136 soil organisms, 131-132 of organisms in mor and mull soils compared, 171 of streams and chemical compo- sition of water, 56 of vegetation as equivalent to annual net production of energy, 204 per unit area of riffles, 55 use of in calculations of produc- tivity yield of organisms with- out specific adult size, 208 Biome(s) (see also Deserts; Forests; Grassland; Tundra): and monoclimax viewpoint, 276 and polyclimax viewpoint, 276 associes of, defined, 276 biociation of, defined, 276-279 biocies of, defined, 276-297 coniferous and deciduous forests separated as, 138 defined, 29, 30, 276 establishment of subdivisions of for animals as not dependent on plant dominants, 276-279 plant associations as subdivisions of, 276 principal, listed, 276 recognizing importance of life- forms of primary organisms in, 276-279 seral stages of communities not useful in defining limits of, 276 similarity of system of to Allen’s faunistic system, 279 subclimaxes in, 276 system of built on zoogeographical foundation, 279 vegetational portion of called plant formation, 276 Biosphere, as term for ‘‘environ- ment,’’ 6 Biotic Provinces, 272 and biociation of North American grassland, 326-327 differences of from biociations, 279 Biotope, defined, 6 Birds (see also Measurement of Populations; Populations): aquatic, conditions of pond- marsh habitats of, 91 as ‘‘accidentals’’ in area, 156-157 as diurnal animals, 99-100 as more numerous in neritic biochore of sea than in open ocean, 359 as permanent resident of area, 156-157 as showing preference for coastal chaparral, 312-313 as summer residents of area, 156- 157 as transients in area, 156-157 as vehicles for dispersal of insects and small animal life, 147 as winter visitors to area, 156-157 banding of as method for determin- ing home ranges, 185 breeding of: ages for, 215 photoperiods and, 102 singing during season for, 299 capacity of for incubating eggs, 210 categorized according to food habits, 249 differentiation of in four refugia of coniferous forest, 308 differentiation of in North American boreal forests, 305 disease in: and possession of tapeworms, 180 as caused by parasites, 180-181, 181 dispersal of: and number of young, 146 distances of, 149 during Pleistocene era, 286-288 effect of weather on fecundity of, 211 existence of endemic varieties of in Asia, 299 flocking by as adjustment to severe winter conditions, 299 food of on lakes, 75 forest-edge, life-history of bob- white, 142-143 in arctic tundra: and migration, 320 aquatic habits of, 319 food habits of, 320 rest periods of, 320-321 species of, 318-319 in commensal relations with insects, 178 longevity and mortality rate of, 215-216 migration of: annual altitudinal, 158 causes in evolution of, 157-158 Birds (Continued) during seasonal changes in forest, 137 factors in timing of, 158 nest failures of, 212 niches of: as found in same general type of vegetation, 252 choice of according to foot-span, 248-249 in coniferous forests, of decidu- ous forest birds, 246-247 segregation in according to behavior patterns alone, 250 of grassland, characteristics of, 330 origins of species of in boreal and western North American forests compared, 308 pond- marsh: feeding habits of, 86 nesting habits of, 87, 90 population(s) of: and catastrophes, 235-236 and vegetation of forests, 138 censusing of, 36-37 demonstrating nine-ten year cycles, 237 demonstrating three-four year cycles, 237 density of and competition for territories, 221-222 few species as furnishing bulk of, 255 high, and fecundity, 223 in desert, 336 in forest communities, 135 local variations in cycles of, 238 non-breeding, 215 reductions in due to migration, 299-300 in forest, stratification of, 137 sex ratio of and mating behavior, 214-215 small clutches of in tropical rain forests, 347-348 social groups of in tropical rain forests, 348 social hierarchies in as result of competition, 183 special modifications of for feed- ing, 188, 188-189 species of: in abandoned field subseres, 115 in alpine tundra in North America, 321-322 in Australo- Papuan region, 270 in Basin sagebrush biociation, 337 in biociation of North American grassland, 326 in biociation of North American temperate deciduous forest- edge, 297-298 Index 407 Birds (Continued) in biociation of North American temperate deciduous forests, 296 in bogs, 92 in desert scrub biociation, 335- 336 in Ethiopian region, 271 in Eurasian boreal forest biocia- tion, 307 in European deciduous forests, 299 in forest-tundra faciation, 306 in grasslands, 125 in lakes, 74 in Malagasy subregion, 271 in Nearctic subregion, 272 in nekton of sea, 359 in Neotropical region, 270 in North American boreal forest biociation, 303-305 in North American coniferous forests, 302-303 in North American montane forest biociation, 307 in pond- marsh biocies, 86 in sand sere, 110 in stages of clay sere, 113 in southeastern North American forests, 298 in woodland, 311-312 territories of: methods in defense of, 252 well-defined, populations with, 231 best development of ‘‘establish- ing’’ process in, 184-185 Bogs: animal life in: amphibians and reptiles, 92 birds, 92 fish, 92 aquatic fauna of as facies of pond- marsh biocies, 94 changing of dystrophic lakes into, 64-65 characteristics of in Great Lakes region, 92 choice of microhabitat in by spiders, 246 development of, 92 differences of from swamps, 92 distinction of shrub species of, 94 false bottoms of, 92 increase in fertility of through imbalance in energy exchanges, 207 low productivity of, 93-94 peat in, 92, 93-94 pitcher plant in as commensal organism, 178 pollen in: analysis of and climate informa- tion about past, 288-289 408 Index Bogs (Continued) difficulties in identifying tundra pollen in core samples from, 289 pond- marsh invertebrate life in, 92 principal plant organisms in, 92 sere stages in development of: climax forest stage, 92 first plant stage, 93 high shrub stage, 93 low shrub stage, 93 tree stage, 93 temperature of water of, 92 Bottom Organisms (see also Sea; Streams), censusing of, 40-41 Cc Carbon cycle: described, 167 in aquatic ecosystems, 172 Carbon dioxide: and change of pond to marsh, 82-83 and hydrogen-ion concentration: in lakes, 66 in ponds, 79 as absorber of infra-red wave- lengths, 243 concentrations of in air, 99, 167 content of in soil, 165 fixed state of, 65 free state of, 65 half-bound state of, 65 in formation of plant seres on rock, 102-104 presence of in lakes, 65 role of in experiments to determine primary production of energy, 203, 204 Carnivores (see also Food, Food- getting; Mammals): adaptations of teeth of for food- getting, 188 advantages of concealing colora- tion to, 194 and ‘‘balance of nature’’ concept, 195-196 and vulnerability of prey species, 192 avoidance of species with protec- tive coloring by, 194 concentration of on one or few species as prey, 192 deflection of attention of through bright spots or colors on prey, 194 desert, prey of as water source, 338 directive markings on, 194 eating of various species by in proportion to abundance, 191- 192 Carnivores (Continued) Chal effect of variations in abundance | ! of herbivores on, 192 uy enzymes of and food palatability, 190 o food behavior of in streams, 55 f in food chains, 195 0 larger, as tertiary consumers in 0 fourth trophic level, 196 ig plants which qualify as, 187 ua ratio of to herbivores in coral Clip reef biome, 367 seasonal variations in food of, 192 size of prey and energy expendi- as ture of, 191 smaller, as secondary consumers in third trophic level, 196 a8 species included in, 187 Catastrophes (see also Plagues): 4s and environmental conditions as correlated with vulnerability of % species, 236-237 as cauSing variations in species a through survivors, 262 depth of snow as factor in, 235-236 occurrence of at widely spaced intervals, 235-236 di severely low winter temperatures as, 235-236 Chaparral: d as seral stage in montane forest | and woodland, 312 birds which show preference for, d 312-313 at coastal, location of in United 0 States, 312 derivation of from Madro-tertiary | ° flora, 312 a described, 312 t ecotonal character of fauna of, 312 ; in climax stage, location of, 312 invertebrates found in, 313 lack of mammals as peculiar to, 312 U penetration of species from into of montane forests, 307 petran, location of in United States, 312 reptiles of desert or grassland found in, 313 Characteristic Species: and fifty per cent rule, 29, 30 ty as basis for ecological classifica- tion, 20 difficulties in recognizing, 20 in arctic and antarctic faunas of pelagic biome, 370 marsupials as in Australian tropical savanna, 344 | of desert, 338 of eutrophic lakes, 73 of fish: : in dystrophic lakes, 64 in eutrophic lakes, 64 $l Characteristic Species (Continued) in oligotrophic lakes, 64 of microscopic animals in muddy- bottom marine habitats, 365 of North American deciduous forest, 295-296 of oligotrophic lakes, 71-73 of Oriental region, 271-272 of Pacific Coast, 363 Clay, and soil formation, 163-164 Climate (see a/so Aspection; Humid- ity; Microclimate; Precipitation; Temperature; Wind): as basis for faunistic system of J.A. Allen for North America, 272-273 as main influence in type of terres- trial climax, 276 as related to zonation of vegetation, 295, 313 as warm and dry in post- Pleisto- cene era, 289 change in during post glacial xerothermic period as permit- ting forest to regain areas lost to prairie, 326 differing adaptations to by related insect species and reduction of competition, 253 effect of on soil formation, 168-169 historic fluctuations in, 291 in Europe and Asia during Pleis- tocene era, 288 of tundra, arctic, 315-316 of coniferous forests in North America, 301-302 of deserts, 332-333 of grassland biome, 324-325 of North America 60-70 million years ago, 281 of temperate unit of Arcto- tertiary flora, 282, 293 of tropical biomes, 340-341 of woodland biome, 310-311 optimum in for North America, 289-290 succession in, 348 xerothermic period in for North America and flora-fauna spread, 290-291 Clisere, 21, 23-24 Coactions (see also Commensalism; Communities, as organic entities; Competition; Coopera- tion, Intraspecific; Food; Food Chains; Food-getting; Mutual- ism; Parasitism; Predation): by ants in subseres, 115 classifications of, 177 demonstration of in communities, 19 food, in streams, 54-55 in grazing on grassland vegetation, 125-129 Coactions (Continued) in tropical rain forests, 347 Cold-blooded Animals: aggregation of as method of raising body temperatures, 174-175 developmental period of in tropical forests, 347 dispersal of, 148-149, 151-153 in arctic tundra, 320 in desert as hibernating over winter, 339 large size of in tropical rain forests, 347-348 limitations in distribution of, 98 marine, tropics as center of origin of, 370 rate of physiological functions of, and temperature of habitats of, 98 Commensalism (see a/so Mutualism): defined, 178 in small animals attached to out- side of larger ones, 178 internal, 178 nests as sites of, 178 Communities (see also Biociations; Deserts; Forests; Grassland; Sea; Taxonomic Composition of Communities; Tundra): and monoclimax viewpoint, 276 and polyclimax viewpoint, 276 animal: in Asiatic deciduous forest biociation, 299 in European deciduous forest biociation, 298-299 in floodplain, 114 in North American deciduous forest biociation, 295-297 in North American deciduous forest- edge biociation, 297-298 in rock, 104-105 listing of in coniferous forests, 313 naming of, 29, 29-30 number of that can be clearly recognized, 119 slow rate of ecesis in compared to that of plants, 161-162 as easier to recognize on land than in sea, 351 as organic entities, 178 associations in, defined, 29 associes in, defined, 29 balance of affected by selection of mutations, 267 “balance of nature’’ concept in, 195-196 biomes, naming of by characteris- tic form of vegetation, 29, 30 biotic, defined, 18 character of as indicator of type of environment present, 18 Communities (Continued) choice of niches in by animals according to structure of vegetation, 29 climax, characteristics of, 26 climax, restricted nature of dis- tribution in, 276 coaction in, 19 community-stands in, 27-28 community-types in, 27-28 conception of environment of as pattern of gradients, 27 continuing process of change in, described, 21 correlation of soil types with geographic distribution of, 172 criteria for evaluating species in: biomasses and energy require- ments as, 20-21, 21 distributional studies as, 20 food habits as, 20 number of individual present as, 20 secondary groupings as, 20 time and duration of occurrence of a species as, 20 use of zoogeography in, 26 derived from Madro-tertiary flora, 283 dominants in: animals as in water, 18-19 plants as on land, 18-19 role of, 19 extent of, 18 faciations in, 29 facies in, 29 food webs in, 195 forest: bird population in, 135 density of populations and stratification in, 136-137 distribution of soil animals in, 130 division of strata of into two major groups, 136 foliage arthropods in, 135 life-history of millipede as typical animal of, 143-144 microhabitats of soil animals in, 130-131, 134, 134-135 nature of soil and animals in, 130, 130-131, 131-132, 132-134, 134 population ratios in, 130 physiological adaptations of soil animals in, 131-132 seasonal changes in, 137 small mammals in, 135-136 species of soil animals in, 130, 132-134, 134, 135 stratal classification of species in, according to prevailing positions, 136 Index 409 Communities (Conlinucd) forest- edge: biocies of, 129 density of populations in, 129 life-history of bobwhite as typical animal of, 142-143 grassland: dung of larger mammals as microhabitat in, 125 grazing food coactions in, 125-128 lack of suitable web-building sites in for spiders, 124 life-history of meadow vole as typical animal of, 142 number of individuals per square meter in, 124 population variations of spiders in, 124 species of birds found in, 125 species of insects in, 124 species of mammals in, Table 9.3 species of snakes in, 125 structural adaptations of insects for living in, 125 toad as most characteristic species of, 125 grouping of animals in according to size, 198 increases in fertility of, and changes in species composition, 207 individualistic concept of, 27 ‘“influence’’ in, 19 in ponds, distinctiveness of differ- ent strata of vegetation as, 81 in sand, characteristics of, 106- 107 insect, conditions of: for ants, 107-109 for grasshoppers, 107 for spiders, 109 major, defined, 18 “‘members’’ as species of less importance in, 20 merging of to form ecotone, 30 methods of study of food behavior in, 189 minor, defined, 18 more numerous constituents of called ‘‘predominants,’’ 20 naming of on basis of geography, 29 of lakes: dystrophic, 66 eutrophic, 66 oligotrophic, 66 organismic concept of, 26-27 plant: in floodplain, 113 in rock, 102-104 in sand, 105-106 naming of, 29 principal kinds of in North America, 294 410 Index Communities (Conliued) stages of in clay sere, 112 presence of non-breeding animal population in, 215 processes in dynamics of, 155 productivity yield in, 207-208 recognition of: on basis of taxonomic units, 29, 29-30, 30 through dominants and predomi- nants, 27 through physiognomy, 28 seral: character of, 26 species of not generally useful to defining limits of biomes, 276 social hierarchies in, 183-184 strata of as cause for adaptations of life-forms, 8 subclimaxes in, 276 subdominants in, 19, 27 succession in, 21-23 and imbalance in energy ex- changes, 207 as following sigmoid curve, 161 climatic, 23-24 geologic, 24-26 physiographic, 24 plant, and equilibrium with habitats, 163 taxonomic composition of: ecological equivalents in, 256 predominance in, 255 segregation of related species in, 255-256 variety of species in, 255 total, description of formation of from bare areas, 161 trophic levels in, 196-199 biomass as index to importance of, 198-99 vegetation-types in, 28-29 zonation of, 313-314 Communities, Minor, 20 Community Ecology, 3, 4-5 Community- stands, defined, 27-28 Community-types, defined, 27-28 Competition (see also Competition, Interspecific; Predation): among flour beetles for micro- habitats, 246 and specialization in feeding behavior in high trophic levels, 196-98 as barrier to dispersal of species, 149 as cause for morphological adaptations, 9 as negligible during early coloniza- tion of bare area, 255 as unimportant for organisms whose populations are deter- mined by climatic factors, 231- 232 Competition (Conlinucd) between individuals as result of low population levels, 231 compared to predation, 182 defense of territories in by advertisement of possession, 185 defined, 182 difficulty of in area well-saturated with established individuals, 183 direct: defined, 182-183 occurrence of among organisms, 182-183 establishment of territories as area of, 184 for food, and population density, 222 for most favorable portions of niche, and population density, 222 for space and population densities: of birds, 221-222 of fish, 222 of laboratory mice, 223 of plants, 221 of sessile marine animals, 221 indirect: defined, 182-183 occurrence of among plants, 183 in rooting of grassland vegetation, 325 in seral communities, 207 intraspecific as keenest variety of, 183 in tropical rain forests, 345-346 list of effects of on animal com- munity, 183 preadaptation to particular niche as advantage in, 250-251 reduction in: aspection as factor in, 247 diurnation as factor in, 101, 247 through possession of territories during breeding, 186 rise in: with population increase above optimum, 175 with saturation of habitats, 253 severity of dependent on extent of similarity in requirements of different individuals, 183 social hierarchies in, 183 characteristics advantageous for gaining high position in, 183 establishment of among house mice, 223-224 fluidity of movement through, 183-184 peck orders in domestic fowl as, 183 supersedence in, 183-184 Competition, Interspecific (see also Competition; Predation): among grass- eating mammals, 127-28 among island species, 155 among rodents for grassland vegetation, 127 and social despotism, 184 as highly developed in tropical rain forests, 346 Darwin’s view of as instrument for segregating species into different niches, 251-252 due to saturation of habitats, 253 Gause’s rule or ‘‘competitive exclusion principle’’ as describing, 253-254 geographical variations in amount of, 253 increase in during stress or crisis, 253 occurrence of in simultaneously occupied habitats when one species increases in abundance, 253 patterns of with two species of similar niche requirements, 254 reduction of: by differing adaptations to cli- mate, of related insect species, 253 during critical stages of life cycle, 253 or elimination of when require- ments of species less than supply available, 252 through lowering of populations by predation, 252-253 through setting up of mutually exclusive territorial relations, 253 removal of and expansion of species beyond limits of niche, 252 restriction of population to opti- mum niche as effect of, 252 Conditioning (see also Water-Con- ditioning): and transmission of behavior patterns, 251 as ‘“‘trial and error’’ form of learning, 15 of carnivores to new species as prey, 192 of insect larvae and production of new strain, 261 of water, 172-173 Continental Drift Theory: as possible explanation of distri- bution of fauna in Neotropical region, 270 explained, 150-51 Continent(s) (see also North America): animal communities in deciduous forest biociation of European, 298-299 approximate coincidence of with regions, 268-269 Asia: animal communities in deciduous forest biociation of, 299 dispersal of fauna of to North America, 155, 318 Bering land bridge as connecting North American and Asian, 272 connection of to islands by land bridges, 155 distinctiveness of fauna of Australian and designation of Australia as zoological realm, 268-269 evolution and spread of horses across, 272 fauna in of Neotropical region, 270 fauna of African, 271 isolation of prerequisite to develop- ment of orders and families, 272 land bridges between as dispersal pathways, 151 logic of using as basis for first major subdivision of fauna, 268-269 northern: concentration of, 151 periods of aridity and glaciation in, 151 periods of warm, moist uniform climates in, 151 repeated flooding of, 151 South America: dispersal of fauna of to North America, 156 distinctiveness of fauna of, and designation as zoological realm, 268-269 temperature of as influenced by currents of sea, 354 theories about drifting of, 150-151 Convergence, 22-23, 154 Cooperation, Interspecific (see Commensalism; Mutualism) Cooperation, Intraspecific: aggregation by bees as method of raising body temperatures in, 74-75 among muskox and bison, 175 and negative social facilitation, 175 and positive social facilitation, 175 ant societies as examples of, 175- 176 division of labor in colonization as, 174 Cooperation, Intraspecific (Continued) effects of size of aggregation on, 175 grouping of free-living protozoans as, 174 increasing amounts of in increas- ingly complex societies, 175-176 in huddling of mice, 175 in roosting of bob-white quails, 175 in wolf packs, 175 need for specialized behavior in, 176 of aquatic organisms, 175 persistence of aggregation of individuals as, 174 termite societies as examples of, 175-76 Creeks, defined, 42 Cycles (see also Catastrophes; Plagues; Populations): and changes in physiological vigor, 241-242 and vitamin content of food con- sumed, 241 and ultra-violet intensities, 244 as causing variations in species through survivors, 262 better understood as oscillations, 237-238 criteria used to determine real peaks of, 238 defined, 237 epizootics in, and theory of general cause of cycles, 241 explanation of causes of for particular species, 240-241 extreme fluctuations between peaks and lows in, 238 extrinsic factors as necessarily affecting intrinsic factors in, 242-243 five- six year, species occurring in, 237 hidden periodicities in, 237 in heights of ionosphere and ozone layers as affecting populations, 243 in mathematical usage, 237-238 in plankton populations, 358 in predator populations: and ability to shift from one region to another, 240 as dependent on that of herbivores, 241 as dependent on that of prey, 239 in sunspots: and relation to ultraviolet light, 244 and weather, 243 correlated with population cycles, 243 Index 411 Cycles (Continued) in weather: and failure of crops during dry years, 330 and variations in solar radiation, 243 as not absolutely synchronous in large areas, 243 evidence for in annual rings of giant sequoias, 243 length of, 243 thirty-five year, 243 lack of correspondence between, Cycles (Continued) three-four year: as better expressed in far North than southerly latitudes, 237 species demonstrating, 237 use of term justified, 237-238 variations in tempo of in different parts of the world, 238 Cyclomorphosis, 60 D and random number cycles when Desert(s): conspicuous peaks considered, 238 life, critical periods in and effects of solar radiation, 244 nine-ten year: as better expressed in southerly latitudes than in far North, 237 seen as modification of short- term cycles, 237 species demonstrating, 237 nutrition and rate of reproduction in, 241 obtained from plotting random numbers compared to animal population, 238 occurrence of in population of single prey and predator in limited area, 239-240 of herbivores: and relations with plants during, 241 and variations in mineral salts of plants, 241 as basic to cycles of other organisms, 241 of lemmings and predators, 240 periods of among invertebrates, 237 periodicities of best established, 237 role of stress in reduction of populations as cause of, 241 short-term: coefficient of variation in, 238 length of, 238 theories about, 237 sustaining of in populations of predator and prey through introduction of cover, food and immigration, 240 synchrony in: and isolated local populations, 238 as consideration in testing reality of, 238 local and regional fluctuations of, 238 of rodents and lemmings, 238-239 testing oscillations for randomness before designation as, 238 412 Index adaptations in: of animals to lack of water, 338 of animals to soil, 339 of plants to lack of water, 333 age of North American, 283 appearance of during Tertiary era, 334 areas of occurrence of, 332 as usually occurring on lee side of mountains and continents with respect to prevailing winds, 332 avoidance of hot ground surface by animals of, 338 basin sage biociation in: birds of, 337 invertebrates of, 337 overlap of species between desert scrub and, 336-337 populations of small mammals in, 337 reptiles of, 337 species of mammals in, 336-337 subspecies of as contrasting sharply with scrub biociation species, 336-337 biociations of other than North American, 337-338 biotic succession in vegetation of, 333 cause of development of in Great Basin of North America, 281 characteristic animals of, 338 characteristics of soils of, 172 cold blooded animals of as hibernating, 339 description of areas defined as, 332 differentiation of vegetation of during Pliocene and Pleistocene eras, 334-335 distinct nature of species of fish in pools of, 339 evaporation in, 332, 333 inactivity of birds during prolonged droughts in, 348 invasion of organisms from into woodland, 311-312 kinds of vegetation in, 333 most arid grassland association considered as, 325 Desert(s) (Continued) most important environmental factors of, 333 nocturnal habits of animals of to avoid high temperatures, 333 oases in, 339 physiographic succession in vegetation of, 333 plant and animal activity in with abundant rain, 333-339 plant associations of in North America, 333-334 precipitation in, 332 protective devices of plants in, 192 relation of species of to Eurasian forms, 334-335 scrub biociation in: absence of large mammals from, 335 birds in, 335-336 home ranges of small mammals in, 335-336 invertebrate populations in, 336 level of bird populations in, 336 mammals in, 335 reptiles in, 336 semi-deserts, carrying capacity of land, 339 semi-fertility of soil of where irrigation possible, 339 species of as occurring in grass- land, 326-327 temperatures of, 332-333 topography of, 333 tropical thorn forest considered as, 334 use of shady nesting areas by animals of, 338 vegetation of originating from hardy species of Tertiary floras, 334-335 Detritus, 54-55, 55, 58, 73, 74, 87, 92, 114, 130, 188, 195, 196, 356-357 Developmental Life Histories, defined, 16 Diel Rhythms: in marine plankton, 358-359 kinds of, 100-101 Disease (see also Parasites; Parasitism): as stabilizing factor in populations: of muskrats, 232-233 with occurrence of epizootics, 228 as uncommon among primitive bison herds, 328 epizootics of: among wild animals and high population densities, 228-229 as rarely occurring in inferior habitats, 231 defined, 228 factors in occurrence of, 228 Disease (Continued) in cattle due to low calcium content of hay, 241 in elimination of eelgrass on Atlantic coast, 365 introduction of by man: among field mice to control plagues, 235 in rabbit populations to control plagues, 235 mortality of hosts and mutant organisms of, 228 occurrence of and theory of general cause of cycles, 241 physiological stress as factor in cycles, 241 physiological stress as state of, 229 shock in snowshoe rabbit as state of, 229, 241 Dispersal (see also Continents; Dispersal Pathways; North America): accomplishment of primarily by young of species, 149 after changes in the environment, 150 after changes in species, 150 and adaptive radiation, 266 and rate of gene flow from one locality to another, 259 as part of basis in establishment of new faunistic systems, 274-276 barrier(s) to: changes in vegetation as, 149 classification of, 148 climatic, deserts as, 148-149 competition between species as, 149 food-type availability as, 149 humidity as, to moist- skinned species, 148-149 land masses as, to fresh-water organisms, 143 length of season between spring and trosts as, 148-149 mountains as, to low-land species, 148 oceans as, to terrestrial organisms, 148 precipitation as, 148-149 predators as, 149 salt-water as, to fresh-water forms, 148 short photoperiods as, 148-149 solar radiation as, 148-149 valleys as, to mountain species, 148 waterfalls as, to non-flying aquatic species, 148 wide rivers as, to mammals, insects and birds, 148 Dispersal (Continued) broadcasting of eggs in and popu- lation pressures, 150 computing rate of outward diffusion in, 149 defined, 145-146 failure of food supply as reason for, 150 general laws governing, 145 high altitude, vegetation, alpine tundra and coniferous forest as representing occurrence of to south, 313 in non-glacial areas, effect on of pluvial and interpluvial peri- ods, 284-285 in North America as explained in life-zone system, 274 low altitude, vegetation as repre- senting occurrence of to north, 313 manner and means of: as determined by directed move- ments of animals, 147 attachment of eggs of aquatic organisms to debris rafts as, 146 broadcasting of eggs, spores and young in random manner as, 146 passive conveyance in broadcast- ing as, 146 radiation in all directions from home area as, 146 river erosion as, 146 strong winds as, 146-147 trial and error as, 148 use of mild air currents by spiders as, 146-147 use of other animals as vehicles in, 147 northward due to amelioration of climate in post- Pleistocene era, 291 of animals with glacial advance, 286-288 of arctic species southward in post- Pleistocene era, 317 of Asian species in North America due to Pleistocene glaciation, 318 of cold-blooded marine organisms from tropics, 370 of Eurasian bird element in North America, 156, 308 of Eurasian boreal forest biota, 307 of Eurasian forms into South America, 313 of isolated fauna of coniferous forest refugia during inter- glacial period, 308 Dispersal (Continued) of isolated populations as hastened by man, 308 of North American alpine tundra species, 322 of organisms from extreme southern South America to Andes, 344-345 of southeastern North American forests into deciduous forests and forest-edge, 298 of species over continent and local differentiation centers, 259 of Tertiary era flora: Arcto-tertiary, 282, 297 early, into favorable regions, 281-282 Madro-tertiary, 283 of woodland, 311 population pressure as most potent reason for, 150 range expansion as result of generations of, 146 rate of: as low in island forms, 155 as probably similar for all creatures once barriers passed, 145-146 as slow and covering short distances, 145-46 into area previously unoccupied by species, 150 rise of barriers after occurrence of and geographic isolation, 259 Tibet Plateau as important center of, 321 Dispersal Pathway(s) (see also Continents): and continental drift theory, 150-51 corridors defined as, 151 determination of centers of origin of: through continuity and conver- gence of lines of dispersal, 154 through location of greatest diff- erentiation of type, 154 filters, defined as, 151 formation of through connection of island to mainland, 155 from Asia to North America, 155 from Europe to North America, 155 from South to North America, 155- 156 in Australia—Papuan region, 269- 270 in succession from Sea to land, 367-368 land bridges between continents as, 151 of mammals, Eurasia as point of origin of, 151 |ndex 413 Dispersal Pathways (Continued) over advancing and retreating glaciers, 284 ‘‘sweepstakes routes,’’ defined as, 151 taken by cold-blooded vertebrates, 151-153 theories about: based on hypotheses about conti- nents, 150 to explain occurrence of related forms in South America, Australia and Africa, 270 toward southern land extensions during cold period in northern continents, 151 tropical species and Bering land bridge as, 153-154, 154 warm temperate species and Bering land bridge as, 153-154 Distribution: as affected by zonation of vegeta- tion, 295 basic concepts in understanding ecological system in, 285 biotic province concept in, 272 ecological and zoogeographical approaches to compared, 276 factors in of marine organisms, 351, 372 faunistic system for of J.A. Allen (1892), 272-273 in deciduous forest biociation of Asia, 299 in deciduous forest biociation of Europe, 298-299 in deciduous forest biociation of North America, 295 of birds, 296 of invertebrates, 296-297 of mammals, 295-296 of reptiles and amphibians, 296- 297 in deciduous forest- edge biociation of North America, 297 of birds, 297-298 of mammals, 297-298 of reptiles, 297-298 in seral stage communities too wide for definition of biome, 276 life-zone system of, 272, 274 new approaches to study of, 274- 276 of aquatic organisms, knowledge of Distribution (Continued) of mixed biocies in southeastern North America, 298 of organisms in large geographic units and problem of geographic ecology, 268 of organisms into realms and regions, best system of, 268-269 of petran bush, 312 of plant associations of North America, 302 of plant species, concept of biociation as useful for analysis of, 276-279 of temperature, deciduous forests, 293 of tropical biomes, 340 of vegetation in deciduous forest biome, 293-294 of species over wide area and genetic diversification, 261-262 of tundra species in post- Pleisto- cene era, 317 of woodland, 311 restricted in climax communities, 276 summary of aspects of two sets of factors controlling, 279 Diurnal Animals (see also Diurna- tion): birds as, 99-100 color vision in, 101 in tropical rain forests, 349 major period of activity of, 100-101 Diurnation (see also Diurnal Animals): as reducer of competition: among birds, 247 among butterflies, 247 between white and black crappie, 247 Dominance (see also Dominants; Predominants): as exerted through plant reaction, 163 defined in communities, 18-19 expressed through ‘‘coaction,’’ 19 in balanoid-gastropod-thallophyte biome of sea, 364 in pelecypod-annelid biome of sea, 366 in Sargasso Sea, 359 lack of in streams, 43 reversal of because of temperature changes, 246 Great Lakes necessary to under- Dominants (see also Dominance; standing of, 288 of Arcto-tertiary flora, 282 of coastal chaparral, 312 of coniferous forests, 301 of deserts, 332 of grassland in North America, 325 of marine tropical fauna, 370 414 Index Predominants): fifty per cent rule, 29 as key to recognition of communi- ties, 27 as modifiers of effect of environ- ment for other organisms in community, 7 Dominants (Continued) bison as examples of, 7 changes in as affecting community, 21 chestnut blight as example of, 19 defined, 18-19 in coral reef biome, 367 in deserts, large number of dif- ferent life-forms among, 332 in temperate deciduous forest biome, 293-94 life-forms of as determining biome, 276 of Eurasian plant associations, 307 perennial grasses as in climax grassland, 325 plant, as key to recognition of ecesis, 161-162 plant, as most important climax species on land, 276 plant, disregarded by biociation concept in establishing divisions of biome for animals, 276-279 shifts in due to geologic succession, 24-26 Ecad, defined, 7 Ecesis: absolute growth rate in, defined, 160 and plotting of sigmoid curve, 160-161 and relation to dispersal, 159 defined, 159, 162 environmental factors influencing rate of growth in, 161 factors causing inhibiting phase of growth in, 161 favorable conditions for, 159 following of logistic curve in at every level of species organi- zation, 161 growth of populations in as follow- ing sigmoid or logistic curve, 159-160 instantaneous growth rate in, defined, 160 of plant communities, rate of more rapid than in animal communi- ties, 161-162 population growth curves in and productivity yield, 207-208 symmetry in rate of growth of populations in, 160 temporary nature of with migrant species, 159 Ecological Classification: basis for: characteristic species as, 20 cyclic species as, 20 Ecological Classification (Continued) exclusive species as, 20 perennial species as, 20 seasonal species as, 20 ubiquitous species as, 20 “indicator species’’ in, 20 Ecological Life-Histories: defined, 16 ground covered by, 16 of forest organisms, 143-144 of forest-edge organisms, 142- 143 of grassland organisms, 142 of lake organisms, 76-77 of pond organisms, 88-90 of sand organisms, 91-92 of stream insects, 53-54 Ecological Societies, foreign, 6 Ecological Society of America, The: founding of, 6 periodicals of, 6 Ecologist, the: and determining the abundance of species, 31 development of methods to measure size of populations as problem of, 2 importance of preservation of natural areas to, 141-142 Theophrastus as first, 4 Ecology: as challenge to investigator, 2-3 as division of biology, 3 concepts and techniques of not standardized, 6 development of oceanography as branch of, 6 development of wildlife manage- ment in, 6 distinctiveness of as a science, 1 early studies in: during nineteenth century, 4-5 in recognizing community con- cept, 4-5 of succession, 5 establishment of as field of knowl- edge, 1 evolution of animal ethology as branch of, 6 geographic, development of, 5 growth of limnological studies in, 5-6 growth of studies of population dynamics in, 5 importance of preservation of natural areas in, 141-142 journals of, 6 methods for achieving objectives in, 1-2 phenomena studied in, 1 physiological, development of, 6 Ecology (Continued) relation of to morphology, 3 relation of to physiology, 3 role of Theophrastus in history of, 4 rules of, 9, 9-10 subdivisions of, 3, 4 various definitions of, 1 Ecosystem, 3-4, 9 artificial fertilization of, 208 as best unit for study of circula- tion of matter and energy be- tween organism and environ- ment, 18 balanced, conditions in, 202 circulation of salts in, 356 complete, efficiency of use of energy in, 206 defined, 18 energy flow through, 200-201 food chains in and radioactivity tracing technique, 189 lakes as, 63-64 nutrient supply of, 165 oceanic plankton and nekton biome considered as, 359 removal of minerals from with harvesting of crops by man, 208 Ecotone: coast forest, of North America, 302 consideration of woodland as, 311 defined, 30 forest, in New York’s Catskills Mountains, 295 general intermingling of species in, 305-306 in ant communities during sand sere, 109 pine- hemlock- hardwoods, segre- gation of animal species in, 295 Tsuga- Pinus-northern hardwoods, in North America, 294 with neotropical Tertiary flora, location of, 282 Ecotype, defined, 7 Emigration (see also Migration): as method for relieving pressure of overpopulations, 228 from sea to land, 367-368 of aphids after overcrowding, 228 of birds of coniferous forest at failure of seed crop, 309 of lemmings, 228, 231 of species under natural conditions, 228 of young to new ranges, 228 Energy: acquiring of through food consump- tion known as gross energy intake, 201 Energy (Continued) and increase in biomass, 202 as furnished by respiration for plant’s activities, 201 assimilated, described, 202 continuous loss of in ecosystem, 200 drain on of animals suffering stress during high population densities, 241 efficiency of use of: from solar radiation, 205 in complete ecosystems, 206 transformers, 205-206 equilibrium in exchanges of, as characteristic of climax communities, 206-207 excretory, described, 202 experiments with plankton in use of, 206 from predation: consumed by higher trophic level, 202 wasted by higher trophic level, 202 grass production of, 201 heat, continuous loss of from body, 201 high rate of in primary consump- tion, 207 measurement of secondary pro- duction of, 204-205 mobilization of by warm-blooded organisms as affecting fecund- ity, 211 net production of, 201 predatory consumption of: in trophic levels with balanced populations, 202 in trophic levels with unbalanced populations, 202 primary production of: expressed in terms of glucose or carbon, 201 in ecosystem, 201 measurement of, 203-204 respiratory, defined, 201 solar radiation as, basic source of, 200-201 solar, trapping of through forma- tion of sugar by plants that contain chlorophyll, 200-201 transfer of: by replacement of individuals that die non-predatory deaths, 202 from one form to another as always involving loss, 201 measurement of and biomass, 201 to higher trophic levels through predation, 201 Index 415 Energy (Continued) use of by saprovores or trans- formers after loss from trophic levels, 202 use of for processes of existence, 201 Environment (see also Communities; Cycles; Habitats; Microhabitats; Niches): adaptations to: as necessary characteristic of early organisms, 6 by mammals to use energy resources, 8 during orientation, 12, 13 of distinguishing characteristics of species, 265-266 sessility and motility in consider- ing, 7 through learning, 14, 15 and law of the minimum, 12 and law of toleration, 10-11, 11-12 and stimuli, 13, 14 as restraining rate of growth of populations, 220 balance with of organisms and low net reproduction rates, 217 changes in: as affecting period of time required for evolution, 267 physiological adjustments as first responses to, 10 through biotic succession, 21-23 through climatic succession, 21- 24 through geologic succession, 24- through physiographic succession, types of responses to, 6 classification of species in, 20 cycles in: and ultra-violet radiations, 243- 244 of sunspots, 243 of weather, 243 deficiencies in and organisms of, 10 differences between subspecies or populations as correlated with differences in, 257-258 diversification of species con- ditioned by, 6 divisions of, 6 dominance in, 18, 19 effect of on organisms: controlling, 10 directive, 10 lethal, 10 masking, 10 evaluation of species in, 20, 21 experimental testing of factors in, 13 416 Index Environment (Continued) ‘‘influences”’ in and community character, 19 key to offered by kind of commun- ity present, 18 heterogeneity of as helping indi- viduals escape full force of density-dependent effects, 229 individualistic concept of com- munities in, 27 morphological variation an aspect of influences in, 7 most critical factors of in deserts, 338 of alpine tundra as imposing great severities on plant development, 316 organismic concept of communities in, 26-27 reactions to on basis of inherited behavior patterns, 13 relation of mutations to, 9, 262-263 uniformity of in various ocean bottoms, 370 vegetation-types in, 28-29 wide range of intensivity of factors in, 10 Erosion: action of wind in, 168 and formation of lakes, 59 and formation of permanent streams, 42 and silting, 56 as factor in population reduction of oysters, 229 conditions of occurrence of, 56 control of as basic to fish manage- ment, 58 control of to permit oyster spat to set, 371 danger of to animals in streams, 52 effects of animal burrowing on, 165 influence of on kinds of fish in streams, 56 in physiographic succession, 24 methods for prevention of, 56-57 of desertic soils, 172 of grasslands from overgrazing, 128 of river soil as means of dispersal of fresh-water organisms, 146 of rock by water, 168 products of in topography of desert, 333 protection from by mangroves, 368 role of in mountain-building of North America, 280, 281 Ethology: and isolation mechanisms of sympatric species, 258 defined, 14 Ethology (Continued) differences between, and psychology, 14 Evaporation (see also Humidity; Moisture); and humidity in grasslands and forests, 121 and saline soils, 172 decrease in through evolution of moist membranes in body, 99 factors causing, 97 high rate of in deserts, 332 in moist-skinned animals, 101 low rate of in cloud forests of tropical biomes, 341 measurement of, 97 of rainfall termed interception, 121 prevention of excessive amounts of by coniferous forest leaves, 301 use of by bees to cool hive, 174-175 Evolution (see also Adaptation(s) to Terrestrial Habitat(s); Morpho- logical Adaptation(s); Mutations): accumulation of favorable varia- tions and adaptation in, 264-265 adjustment to particular habitat as aspect of diversification of species, 6 and asexual and self-fertilizing forms, 264 and creation of struggle for exis- tence, 264 and hybridization, 263-264 and origin of life, 96, 354 and preadaptation, 266 and succession from Sea to fresh water, 368-370 and succession from Sea to land, 367-368 and survival of the fittest, 264 as lowering frequency with which population replaces itself, 211 differential survival in, 264 elaboration of behavior patterns through, 13 emergent, 26 factors affecting rate of, 267 inheritance of growth-forms as result of, 7 importance of continents in, 151 macro-evolution, defined, 260 manifestations of cooperation in, 27-28 micro-evolution, defined, 260 natural selection in, 9, 264-265 and distinctive characteristics of species, 265-266 and favorable mutations, 262-263 and mutations of disease organ- isms, 228 as fostering improvement of species, 264-265 Evolution (Continued) conditions of occurrence of, 264 recessive character of mutations and, 265 small and large populations compared, 265 of all biomes from tropical forest biome, 344 of bird migration, 157-158 of cold-blooded vertebrates, 151- 153 of color of local populations to match habitat, 265 of DDT-resistant insect pests, 265 of distinct species and genera in fauna of Ethiopian and Oriental regions, 271 of European deciduous forests and Similarity in genera of animal species to North American genera, 298-299 of extensive grasslands as related to evolution of large ungulate populations, 328 of free-living parasites into ecto- parasites and endoparasites, 179 of internal air-breathing organs, 99 of parental care and effect of on survivorship curves, 213-214 of terrestrial forms from fresh- water, 96 of water organisms, quiet-water species prior to swift-water species in, 53 parasites and genetic relations of veographically separated hosts in, 180 period of time required for, 266- 267 population pressure on birds in forest as factor in, 135 role of in low net reproduction rates, 217 role of overproduction in, 264 selection pressures in, 265 speciation in, 26 Exclusive Species: and fifly per cent rule, 29 as basis for ecological classifica- tion, 20 of Oriental region, 271-272 Excretion: accumulation of in water, 172 and nitrate content of soil, 165-166 examination of as method of deter- mining food habits of organisms, 189 non-nitrogenous substances in, 166 of desert animals and conservation of moisture, 338 of food as excretory energy, 202 Excretion (Continued) of fresh-water animals, 354 of indigestible matter in feces, 190-191 of marine animals and salinity of sea, 354 role of in nutrient supply of ecosystem, 165 F Faunistic System(s) (see also Zoogeography): concept of biotic provinces as, 272 explanation of term, 272 in marine communities, 370 in which species with similar centers of origin and dispersal routes are basis, 274-276 life-zone, flows in, 272 life-zone system of C. Hart Merriam, explained, 274 of J.A. Allen (1892) for North America, 272-273 similarity of biome system to Allen’s, 279 Fifty per cent rule, defined, 29, 30 Fish (see also Lakes; Measurement of Populations; Ponds; Popula- tions; Sea; Streams): biomass of, 56, 222 censusing of, 39 choice of habitat by according to oxygen content of water, 246 distinct nature of in desert ponds, 334 effects of limited food supply on, - 222 feeding habits of, 55 and weed- inhabiting organisms, 191-192 as basis for categorizing fresh- water species, 249 in muddy-bottom marine habitats, 365-366 increasing production of in lake management, 77 in lakes: anatomical adaptations of to habitat, 74-75 as principal constituent of nekton, 73-74 feeding habits of, 74-75 life-history of cisco as typical of, 76-77 size of populations of, 64 in ponds: and lack of oxygen, 37 food habits of and biomass, 91 reproduction of, 90 in streams: avoidance of current by, 47, 47-50 Fish (Continued) positive response of to current, 50 species of, 43-44 swimming of in current, 47-50 marine: and food chains, 360-361 distribution of in nekton, 354 in mesopelagic community of sea, 359 marking of as method for deter- mining home ranges, 135 metamorphic migrations of salmon, 15y migration of between fresh and salt water for spawning, 365-370 nesting habits of, 54 of Ethiopian region, 271 of Neotropical region, 270 overwintering of, 43 reasons for impoverished variety of in northern and northeastern United States, 291 repopulation of in fish management, 58 species of in bogs, 92 species of in pond- marsh biocies, toxic effects of colloidal silver on, 175 Floodplain: creation of, described, 113 effects of flooding on animal life in, 114-115 plant communities in, 113-114 rate of ecesis of animal community in compared to that of plant community, 161-162 recognition of six to eight plant stages in sere of, 161-162 species of animal life in, 114 Food(s) (see also Food Chains; Food- getting; Nutritional Values): abundance of in antarctic, 319 as density-limiting factor in populations, 229, 229-230, 230 as excretory energy, 202 as factor in experiments to deter- mine causes of population cycles, 240 as factor in fecundity of fruit flies, 223 availability of and fecundity of birds, 211 “balance of nature’’ concept in supply of, 195-196 classification of animals based on: of birds, 249 of fish, 249 of herbivores, 195 coactions: among herbivores in Arctic tundra, 320 in grassland communities, 125- 129 |ndex 417 Food (Continued) in pelecypod-annelid biome of sea, 365-366 in streams, 54-55 cycles in lakes, 74-75 determining if prey species is used as in proportion to its abundance, 191-192 determining kind of eaten by animals: advantages of field observation in, 189 through examination of digestive tract before digestion, 1389 through examination of excretory matter, 189 through killing, disadvantages of, 1389 through pellet analysis, 189 through securing contents of crop or stomach without killing animal, 189 discrimination of chemical sub- stances dissolved in, 188 effects of decrease in on fish, 222 forage ratio of: defined, 191-192 value greater than unity in, 191- 192 value less than unity in, 191-192 value of unity in forage ratio of, 191-192 indigestible matter in, 190-191 kinds of consumed by garter snakes and differences in niches, 247 measuring percentage volume of each item of in organism, 189 non-staple or emergency, described, 190 of moose in coniferous forests, 308-309 palatability of: and enzymes, 190 and hydrogen-ion concentration of intestines, 190 pelagic deposits on sea bottom as, 352-353 preferences: and attractiveness of food substances, 188 as established by parental feed- ing, 183 factors in for a given species, 138 restrictions: as result of chemicals affecting odor or taste, 195 as result of structural adapta- tions, 195 as specialized behavior, 195 role of in survival of fruit flies, 224 salts in sea as source of, 354-356 418 Index Food (Continued) seasonal variations in, 192 selection of on basis of nutritional needs, 190 size differences between related species and differences in habits of, 249 size of item of and size of animal, 191 sources of for tropical rain forest animals, 348 special adaptations for digestion of, 188-189 staple, described, 190 summary of factors in choice of, 139 supply of and carrying capacity of area, 222 use of in methods to measure secondary productivity of energy, 204-205 variations in from year to year, 192 vulnerability as of prey species, 192 wide range in variety of eaten by most species, 194-196 Food Chains (see also Food; Food- getting): criteria for describing position a species occupies in, 249 determining of by correlation of food eaten by different species in community, 189 double base of, 206 in alpine tundra, 322 in ponds, 87, 95 marine: depths at which nitrogen is regenerated in, 360 filter-feeding mechanisms of organisms in, 361 fish in, 360-361 in balanoid- gastropod-thallo- phyte biome of sea, 364 in pelecypod-annelid biome of sea, 365-366 invertebrates in, 360-361, 361 net zooplankton in, 361 plankton as basic link in, 351 role of bacteria in, 360 role of upwelling water currents in, 361 sharks in, 361 whales in, 361 of five links, 195 of four links, 195 of three links, 195 size of and productivity yield, 207 Food- getting (see also Food; Food Chains): adaptations for: of bills of birds, 188 of mouth parts of insects, 188 Food-getting (Continued) of teeth of animals, 188 of tongues of birds, 188 and formation of food web, 195 by carnivores, concentration on one species in, 192 by forest animals under severe winter conditions, 299 habits of fish in, 191-192 classification of animals according to behavior in, 187 of coniferous forest animals in winter, 309 protective devices of plants in, 192 protective devices of prey animals in, 192-194 Forest(s) (see also Tropical Biomes; Tundra): adjustment to severe winter con- ditions in: by animals that overwinter, 300 by animals that remain active over winter, 299 through migration, 299-300 annual downward migration of insects of, 158-159 annual migration of insects into, 158-159 Asiatic deciduous, biociation of, 299 biociation in Eurasian boreal: best development in Asia of, 307 birds in, 307 genera of plants of same as in North America, 307 mammal fauna in, 307 biociation in North American boreal: Appalachian faciation of, 303-305 characteristic birds in, 303-305 characteristic mammals in, 303 differentiation of bird popula- tions in, 305 high animal populations in seral stages of, 306 lack of forest-edge in southern border of, 306 location of, 303 overlap of with western biocia- tion, 303 similarity of species composition of to that of deciduous forests, 305-306 tundra species in, 306 biociation in North American montane: location of, 307 many local subspecies in, 307 penetration of deciduous forest- edge biociation into, 307 penetration of species from chaparral into, 307 population of birds in, 307 C Forests (Continued) North American temperate Forests (Continued) European deciduous: Forests (Continued) species of birds in, 307 species of mammals in, 307 biomasses of birds in, 135 breeding season in and photoperiod and temperature, 299 climax, equilibrium in decompo- sition of leaves and reabsorp- tion of nutrients in, 165 confining of certain species of birds to either deciduous or coniferous on basis of behavior patterns alone, 250 coniferous: and formation of podzolic soils, 172 changes in range of species of as hastened by man, 308 climate of, 301-302 comparison of to deciduous, 308 description of, 301 destructive insects in, 309-310 dispersal of isolated fauna in during interglacial periods, 308 distribution of, 301 feeding habits of mammals in, as descended from Arcto-tertiary forests, 298-299 birds in, 299 mammals in, 299 similarity in genera of plants and animals of to North American species, 298-299 fertility of as factor in breeding- bird populations, 135 foliage arthropods in, 135 footspan of birds and choice of coniferous or deciduous, 248- 249 general similarity of niches and microhabitats in different types of, 137-138 growth of: during climatic optimum in North America, 289-290 during post- Pleistocene era, 289 during sub-Atlantic period of North America, 291 during xerothermic climatic period in North America, 290- deciduous: birds in biociation of, 295-296 humidity in, 293 invertebrates in biociation of, 296-297 location of biociation in, 95 plant associations of, 295 mammals in biociation of, 295- 296 occurrence of biociation in climax and late seral stages of, 295 precipitation in, 293 reptiles and amphibians in biociation of, 296-297 temperatures in, 293 of North America during Pleisto- cene era, 285-286 outbreaks of spruce budworm in, 230 population densities in and luxuriance of vegetation of, 138 ratio in numbers of individuals per hectare between different animal 308-309 291 groups of, 130 habits of perennial animals that hearing and voices of animals in, relative humidity of compared to remain active over winter in, 299 grassland and forest- edge, 121 309. inability of some species to occupy seasonal changes in: and effect on mammal popula- tions, 137 during vernal aspect, 137 hibernation and, 137 migration of birds and, 137 isolation of animals during both coniferous and deciduous, glaciation into four refugia of, 138 308 inability of to support large grazing isolation of birds during glacia- populations, 328 tion in four refugia of, 308 influence of humus of on species of migration in, 309 soil animals present in, 138 population of soil macrofauna northward dispersal of in thermal life-history of millipede in, 143- during hiemal aspect of, 137 maximum period, 317 144 seral, organic content of floor of, occurrence of mor humus in, 171 light intensities in: 165 origin of, 301 during summer, 122 soil of: plant associations of, 302 vertical gradients in, 123 characteristics of animals in, populations of invertebrates in, logging in, 138 131-132 density of animals in, 130 species of animals in, 130, 132- 134, 134 vertebrates as part of fauna of, 310 measuring annual woody increment refugia for survival of during of trees of, 204 Pleistocene era in North mineral content of leaf fall in, 165 America, 286 modifications of and wildlife of by retention of foliage in, 121 separation of during Tertiary era into North American and Eurasian biociations, 307 uniform animal composition of during early Tertiary era, 307 use of by man, 308 cryptozoa in, 134 decaying logs and stumps of as habitats for soil animals, 134- 135 decomposition of dead leaves in, 121 dry weight of ground litter in, 165 effects of browsing on trees and shrubs in, 177 man, 300 non- breeding bird population in, 135 North American coniferous: boreal and western biociations of as enhanced by Pleistocene glaciation, 308 boreal, compression of into Appalachian refugium, 308 origins of breeding bird species in boreal and western compared, 308 species of birds in, 302-303 species of mammals in, 302-303 western, tripartite segregation of, 308 135 virgin abundance of soil animals in, 130 water in as habitat for organisms, 130-131 southeastern North American: birds in, 298 dispersal of animal species into deciduous forest and forest- edge communities, 298 mammals in, 298 occurrence of subspecies of northern animals in, 298 reptiles and amphibians in, 298 |ndex 419 Forests (Continued) special adaptations of animals in for arboreal habits and climb- ing, 299 species in southern Florida, 298 species of small mammals in, 135-136, 136 strata of: and density of populations, 136- 137 arthropod species in, as carrying on main activities within single stratum, 136 distribution of birds according to, 137 factors in classification of, 136 kinds of in communities, 136 lack of strict segregation between, 136 temperate deciduous: advance of prairie into during post-glacial xerothermic period, 326 advantages of lack of foliage in winter in, 121 as developments of Arcto-tertiary flora, 293 climate of as favoring man’s activities, 300 climax in, described, 293-294 diversity of niches for warblers in, 248 occupations of man in, 300 occurrence of mull humus in, 171 seasonal change in micro-cli- mates of, 293-294 temperature of in summer compared to coniferous, 121 uses of to man, 300 vertical gradients in temperature of, 122, 122-123 zonation of vegetation in, 295 temperature of compared to that of grassland, 121 Tertiary, changes in, 282 tree rings in as evidence for weather cycles, 243 tropical: and formation of latosolic soils, 172 animal adjustments to, 345-349 as possible origin for most modern groups of plants and animals, 344 broad-leaved evergreen, described, 342 deciduous, described, 342 geographic separation of, 343 great variety of species in, 255 in Andes of South America, derivation of fauna of, 344-345 northward dispersal of thwarted by frosts, 159 thorn, described, 334 420 Index Forests (Continued) usefulness of to man, 138 uses animals make of trees in, 297 variations in shade production of tree species in, 121 vertical gradients in relative humidity of, 123 wind-velocity in, 121-122 Forest and Game Management (see also Wildlife Management): erosion control in, 140-141 managing timber on a sustained yield basis, 140-141 methods for preserving forest- edge game animals in, 140-141 preservation of virgin or dense forest in, 140 role of farmer in, 141 Forest- Edge: abundance of game species at, 127 as intermediary between grass- land and forest, 127 biocies of communities of, 129 conversion of forest community into through logging, 138 hibernation in by non-forest species, 137 life-history of bobwhite in, 142- 143 North American temperate deciduous: animal species in as reflecting relation with riparian woodland in West, 297 conditions of occurrence of, 287 distinctiveness of plant commun- ity in Western part of, 297 existence of prior to white colonization, 297 relative humidity of compared to grassland and forest interior, 121 savanna described as, 343 species of as occurring in grass- land, 326-327 temperatures of compared to grassland and forest interior, 121 G Gause’s Rule, 253-254 Gaussian Curve, as normal result of plotting favorable responses to unit intensities of environ- mental factors, 13 Genes (see Genetic Drift; Hybridi- zation; Mutations; Variations in Population Characteristics): Genetic Drift: as possible factor in speciation, 262 Genetic Drift (Continued) conditions of occurrence of, 262 low selection pressure and effect of, 265 survivors of catastrophes and cycles as cause of, 262 Geographic Ecology (see also Ecology): and related sciences, 268 area of study covered by, 268 development of as branch of ecology, 5 Glaciers (see Pleistocene Era; Post- Pleistocene Era; Tertiary Era): Gloger’s Rule, 9 Gradient(s): and formation of mud-bottom ponds, 43 from east to west of North America in evaporation in atmosphere, 324 from low to high temperature at glacial front, 284 in individualistic concept of community, 27 in mammal populations of decidu- ous forests, 296 of environmental factors and choice of niche according to, 250 of habitat from sand to forest, 109 topographic, and rapids, 43 vapor pressure, in measuring water evaporation, 97 vertical: and shifts in microhabitat, 123- 124 in humidity of forests, 123 in light intensities of forests, 123 in microhabitat of grasslands, 122 122 in temperature of deciduous forests, 122, 122-123 sections of, 123 Grassland (see also Grassland Vegetation): absence of animals of above ground during winter, 330 adjustments to: of primitive herds of bison, 328 of pronghorn antelope herds, 328 of small mammals, 327 arid, stock-raising as best use of, 330 causes of aspection in, 325 as possible seral stage in Arcto- tertiary flora, 326 characteristics of birds of, 324 chernozemic soils in, 172 conditions of grazing as in early history of United States, 128- 129 Grassland (Continued) development of hopping locomotion among animals of, 329-330 differences in moisture require- ments of species of as reason for subdivision into plant associations, 324 distribution of during Pliocene and Pleistocene eras, 326 during post- Pleistocene period in North America, 290-291 effect of earthworms on soil of, 164 elimination of predators from and overpopulation, 128-129 extension of in North America, 324 factors in reinvasion of by decidu- ous forests, 326 failure of trees to spread into due to precipitation, 324 first evolution of, 326 fleetness of animals in, 326-327 herbivorous animals favored in, 125-126 ‘‘invasion’’ of forest in by means of man, 300 invasion of over Great Plains as factor in creating western and eastern forest biociations, 307- 308 invertebrate communities in, 124- 125 large herbivorous ungulates as reaching largest populations in, 328 life-history of meadow-vole in, 142 list of most important genera of grasses in North American, 325 marked similarities of on ail continents except for species composition, 324 microclimate of: differences in between north and south facing slopes, 124 distribution of animals in relation to, 124 names for in various countries, 324 North American, biociation of: birds in, 326 extension of into deciduous and coniferous forest biomes, 326 faciations of, 326-327 mammals in, 326 occurrence of ponds or potholes in, 330 origin of short grasses of, 326 origin of tall grasses of, 326 overgrazing in and change in members and kinds of animals present in, 128 precipitation in North American, 324 Grassland (Continued) protective coloration of animals of, 329 recording of rainfall in, 121 relation of North American to Eurasian due to similar derivation, 326 relative humidity of compared to forest-edge and forest, 121 retreat of during post-glacial xerothermic period, 326 segregation of animals by south- ward extension of during Pleistocene era, 286-288 short, hazards in cultivation of, 330 similarity of biociations of in rest of world to North American, 327-328 spread of Indians in after use of horses, 330 tall, high productivity of for cereal crops, 330 temperature of: as compared with that of forest, 121 as factor in separating temperate from tropical, 324-325 range of, 324-325 trampling of soil of by large terrestrial animals, 164-165 use of by man: early difficulties in, 330-331 in past as poor, 330 surmounting of difficulties in, 330-331 through dry farming, 330 vertebrate communities in, 125 vertical gradients in microhabitat factors in, 122 Grassland Vegetation (see also Forests; Grassland; Grazing): ability of to tolerate considerable grazing, 325 and propagation of perennial grasses, 120 and retention of foliage by conifer- ous trees, 121 and shedding of foliage by decidu- ous trees, 121 as consumed by invertebrates, 126-127 as food for big-game mammals, 127-128 bunch grasses, defined, 120 dangers of grazing on by rodents, 127 dangers of heavy grazing on, 125- 126 depth of rooting of, 325 division of into categories, 120 forbs in: as sod formers, 120 Grassland Vegetation (Continued) defined, 120 occurrence of, 325 grazing and protein production of, 125-126 growth of after dry-season fires, 325 growth of and weather conditions, 325 in climax stage, 325 in cold climates, evergreens as part of, 120 in forest-edges, 120 in warm Climates, broad-leaved deciduous trees as part of, 120 on north-facing slopes, 124 on south-facing slopes, 124 percentages of that can be safely used for grazing, 125-126 perennial grasses as dominants of, 325 plant associations consisting of in North America, 325 renewal of each year, 120 stimulation of by grazing, 125-126 strata of, described, 121 terminal buds of not injured by grazing, 125-126 transition area in, 120 Grazing (see also Grassland Vegetation): after dry-season fires in tropical climates, 325 among animals on arctic tundra, 320 by invertebrates, 126-127 deforming of trees and shrubs through, 192 detrimental effects of and brows- ing in forests, 177 effect of on grassland vegetation, 125-126 moderate, as stimulating high productivity of grasses, 328 of primitive herds of bison, 328 overgrazing, as producer of change in kinds and numbers of animals in grassland, 128 overgrazing, prevention of by predators, 128-129 Gravity: adjustments of organisms to counteract, 96 positive reaction of sea-shore animals to, 352 Grinnell, Joseph, and development of concept of ecological niche, 245 Growth-forms, 7 Index 42] H Habitat(s) (see also Adjustment(s) to Terrestrial Habitat(s); Microhabitats): addition of organic matter to, 19 adjacent segregation of related species into, 255 and average densities of popula- tions, 219 and equilibrium with communities through plant succession, 163 as affected by dispersal of propa- gules in biotic succession, 22 carrying capacity of and death rate, 211 characteristics of sand dune as, 106-107 choice of and threshold limits of environmental factors, 10 continuing process of change in, 21 control of by pond- marsh fish, 85 defined, in ecology, 6 expression of dominance in, 19 fertile, wide variety of species in, 255 food resources of as sustaining populations of competing species because of predation, 252-253 forest-edge, in clay sere, 113 genetic inheritance of preferences for, 251 grassland and forests, vertical gradients in, 122-123 grassland, forest-edge, and forest- interior compared as, 121-122 growth rates and environmental resistance offered by, 217 impoverished, little variety in species of, 255 in alpine tundra, rugged terrain of, 316 increase in fertility of during succession, 207 in forest-edge vegetation, 120 in lakes: and littoral zone, 69-71 and profundal zone, 71-73 compared with those in streams, 59 control of in lake management, 27 dystrophic, 64 eutrophic, 64 oligotrophic, 64 in streams: falls, 42-43 in headwaters, 52 invasion of as part of evolution, 53 mud-bottom ponds, 42-43, 43 rapids, 42-43, 43 riffles, 42-43, 43 sand-bottom pools, 42-43, 43 422 Index Habitat(s) (Continued) intertidal: adjustments of organisms of to alternate submergence and exposure, 352 anchoring devices of organisms in, 362 and infralittoral fringe, 362 and midlittoral or balanoid zone, 362 and supralittoral fringe, 362 and supralittoral zone, 362 and tidal pools, 364 avoidance of desiccation by organisms in, 362 effect of water temperature variations on organisms of, 354 mud and sand burrows compared as, 352 proper choice of by animals according to zone, 362-363 protective devices of animals of, 352, 364 species of animals in, 363 large, functioning of natural selec- tion in, 265 less favorable: inhabitants of rarely vulnerable to epizootics, 231 Occupation of in years of popula- tion pressure, 150 local, persistence of flora and fauna in despite mass retreat of vegetation, 297 marine: adjustments of plankton to through flotation mechanisms, 357-358 and characteristics of sublittoral zone of balanoid-gastropod- thallophyte biome, 363 and species in estuaries, 368 and zonation in pelycopod- annelid biome, 365 characteristics of crustaceans in, 365 coral reefs as, 366-367 deep-sea, as unchanged from early geological time, 360 deep-sea, derivation of organ- isms of from intermediate depth organisms, 360 deep-sea, development of bio- luminescence by organisms of, 360 deep-sea, examination of cores from as providing information about ancient forms, 360 deep-sea, persistence of ancient forms in, 360 deep-sea, special structures for vision in, 360 division of sea into, 351-352 Habitat(s) (Continued) filter-feeding mechanisms of organisms in, 361 fragility of skeletons of animals in, 360 gradual changes in to fresh- water habitats, 368 light in, 354 maintenance of hydrostatic equilibrium in, 359 muddy-bottom burrowing as characteristic of animals in, 365 muddy bottom, conditions of, 364 muddy-bottom, epifauna in, 365 muddy-bottom, food coactions in, 365-366 muddy-bottom, microscopic forms in, 365 muddy-bottom, plants in, 365 muddy-bottom, productivity in, 366 muddy-bottom, species of animals in, 365 oxygen in, 356 pressure in, 353 salinity of, 354-356 substratum of, 352-353 temperatures in, 353-354 tidal pools as, 364 tides in, 352 measuring primary production of energy in, 203-204 measuring secondary production of energy in, 204-205 new, invasion of and formation of secondary dispersal center, 154 new, production of and consequent dispersal of species, 150 of beavers, 86-87 of frogs, 90 of muskrats, 86-90 of pond fish, 90 of salamanders, 90 of small mammals in forest communities, 136 of swamp birds, 90 optimum, stabilization of popula- tions as occurring in, 233 origination of in ponds, 79 primary bare areas in, 22 saturation of and rise of competi- tion, 253 secondary bare area in, 22 shape of and sample plots, 32 small, functioning of natural selection in, 265 temperature of and effect on cold-blooded animals, 98 use of term in naming aquatic communities, 29 water conditioning in, 172-173 Hardpan: and chernozemic soils, 172 and rooting of grassland vegetation, 325 conditions of formation of, 170-171 Hardy-Weinberg Law, 259 Herbivores (see a/so Carnivores): adaptations of teeth of for food- getting, 188 and plants similar to predators and prey, 241 as more specific than carnivores in range of food selection, 194 as most numerous organisms in food chain, 230 as primary consumers in second trophic level, 196 classification of in respect to diversity of food, 195 cycles of and variations in mineral salts of plants, 241 cycles of as basic to other cycles, 241 effect of variations in abundance of on carnivores, 192 enzymes of and food palatability, 190 food behavior of in streams, 55 food coactions among in arctic tundra, 320 large ungulate populations of as developing best in grassland, 328 ratio of to carnivores in coral reef biome, 367 size of food not of major im- portance to, 191 species included in, 187 toleration of grasses to consider- able grazing by, 325 wide choice of food for, 195 Heredity, as factor in determining responses to environment, 13 Humidity: absolute, 97 as factor in species of ants in sand, 109 compared in grassland, forest- edge, and forest interior, 121 compared on north and south fac- ing slopes of grasslands, 124 in alpine tundra, 316 in arctic regions of North America, 315 in deserts, 332 in sand dune habitat, 106 in tropical biomes, 341 low relative, as barrier to dis- persal of moist-skinned creatures, 148-149 of temperate deciduous forest biome in North America, 293 relative, 97 Humidity (Continued) vertical gradients of in forests, 123 Humus: amount of in soil, 165 and degree of maturity of soil profile, 170 and mineral content of leaf fall, 165 and production of black soils, 163- 164 as food for earthworms, 132-134 defined, 165 dry weight of material constituting, 165 effect of on number and kinds of animals in soil, 121 formation of and animal excreta, 165-166 formation of and respiration of plant parts and animals under- ground, 165 functions of layers of, 165 kind of dependent on whether leaves are coniferous or deciduous, 138 mor, defined, 171 mull, defined, 171 occurrence of mor layer of, 171 of burned forest, animals in, 130 peat soil not classified as, 165 quick oxidation of in latosolic soils, 172 use of by animals in forests, 299 variations in rate of absorption of organic content of in seral and climax forests, 165 Hybridization: and more pronounced niche segre- gation, 264 and variety of habitats as causing introgression, 264 detection of difficult because of introgression, 263-264 factors in natural selection against products of, 264 occurrence of and necessity for intermediate habitats, 264 occurrence of as indication that speciation does not yet exist, 263-264 products of as superior to parents and consequent selection for in evolution, 264 Hydrosphere, 6 Imitation: and transmission of behavior patterns, 251 as form of learning, 15 Imprinting: and transmission of behavior patterns, 251 in birds, as form of learning, 15 Innate Behavior (see also Behavior Patterns): aggressiveness as form of and position in social hierarchy, 183 and emigration among insects, 228 and releasers, 14 as rooted in nervous system, 251 as subject to evolutionary develop- ment, 251 building of termite nests as exam- ple of, 251 demonstrated in spinning of flour moth, 251 endogenous periodic rhythms as, 100-101 evidence for existence of, 251 in choice of kind of forest com- munity by birds, 250 in choosing niche as saver of energy and time of animal, 254 in cricket hybrids as intermediate between that of parents, 251 in habitat preferences of laboratory animals, 251 in toad hybrids as intermediate between that of parents, 251 type of nest built as, 248 types of, 13 Insects (see also Measurement of Populations; Populations): activities of and temperature of sand dunes, 106-107 adaptation of mouth parts of for food- getting, 188 annual local migrations of, 158-159 as destroyers of coniferous forest, 309-310 as grazing animals, 126-127 behavior of in experimental gradients, 123-124 changes in populations of in grass- lands from overgrazing, 128 daily migration of deciduous forest species, 159 density of populations of in air, 147 emigration of and inherited behavior, 228 evolution of DDT-resistant kinds of, 265 foliage: censusing of, 37-38 in forest communities, 135 species composition of in conif- erous forest as similar to that of deciduous forest, 305-306 in abandoned field sub-seres, 115 in American tropics, 347 in arctic tundra, 319 in floodplain seres, 114 Index 423 Insects (Continued) in grassland communities, 124 in rock seres, 104-105 in soil of forest communities, 130, 131-132, 132-134, 134, 134-135, 135 intra-specific cooperation in societies of, 175-176 mimicry as protective device among, 194 multiple population peaks of, 137 pond, life-histories of, 88-89 populations of, and weather levels of conditions, 230 social, as domesticators of other species, 177 soil, censusing of, 38-39 stream, life-histories of, 53-54 structural adaptations of for living in grasslands, 125 terrestrial, in pond- marsh biocies, 83-84 use of dung by as microhabitat, 125 variations in populations of in streams, 55-56 Insight learning, 15-16 Instinct: as cause of invariable responses to environmental factors, 12- 13,13 defined, 13 migratory behavior as, 158 Irruptions (see Plagues) Invertebrates: adjustment to winter by, 99 as affected by flooding, 115 as asexual and self-fertilizing forms, 264 as found in chaparral, 313 as grazing animals, 126-127 development of in alpine tundra and temperature, 322 inactive condition of during winter in grassland, 330 in antarctic, 319 in burned coniferous forest: in coniferous forest as resem- bling those of deciduous forest, 306 increase in, with advance of clay sere, 112-113 in desert sagebrush biociation, 337 in desert scrub biociation, 336 in forest soil, 130-135 in grassland communities, 124-125 in North American deciduous forests, 296-297 in secondary communities of oceanic plankton and nekton biome of sea, 359-360 in woodland, 311-312 marine: adaptations of for feeding on detritus, 188 424 Index Invertebrates (Continued) as feeders on undissolved organic matter, 360-361, 361 catastrophes and populations of, 236-237 number of eggs laid by and parental care, 211 overwintering of in arctic tundra, 320 population cycles among, 237 populations in coniferous forests, 310 species of in arctic tundra, 319 terrestrial, species of in marshes, 85 variations in clutch size of and weather, 211 variation of between diurnal and nocturnal, 100-101 Islands: competition among species of, 155 dispersal to from mainland by means of land bridges, 155 Hawaiian, adaptive radiation on, 266 inbreeding of animals on and loss of adaptability, 155 large, fauna of compared to that of small islands, 155 long survival of animals on due to lack of competitors and predators, 155 low resistance of species of to invasion of mainland forms, 155 near continents, similarity of fauna on to that of mainland, 155 oceanic, unbalanced fauna of, 155 of East Indies, receipt of fauna by through land connections, 272 Isolation of Populations (see also Niches): and dispersal during interglacial periods, 308 biotic factors listed which function as mechanisms for of sympatric species, 258 during Pleistocene era into differ- ent refugia of coniferous forests of North America, 308 ethological: and development of characteris- tics which prevent interbreed- ing in re-contact of species, 262 breakdown in during disturbance of natural conditions, 259 correct performance of court- ship behavior and, 258-259 failure to find breeding partners and, 258-259 Isolation of Populations (Continued) occurrence of in ecologically isolated budworms when mating periods overlap, 259 recognition of sex of individuals through clues or sign stimuli and, 258-259 genetic, conditions of occurrence of, 259 geographic: and summary of speciation process, 260 as a necessary factor in differen- tiation into distinct species, 260 circumstances of possible occur- rence of, 259 importance of intermediate species in, 259-260 partial effectiveness of barriers in and nature of differentiations, 259 slow rate of gene flow in and possible speciation, 259-260 in North American montane forest biociation, 307 mechanical, through differences in structure of copulatory organs, 257 occurrence of during split of Arcto-tertiary forest into western and eastern sections, 307-308 of arctic tundra due to Pleistocene glaciation, 318 reduction of gamete wastage in hybrids and reinforcement of mechanisms causing, 264 K Kineses, defined, 12 Kinesthetics, as cause of stimuli, 13-14 Krumholz, occurrence of in alpine tundra, 317 L Lake Management: as form of applied ecology, 77 control of erosion as job in, 77 dangers of silting and necessity for control in, 77 improvement of habitats in, 77 increasing fish productivity in, 77 problem of pollution in, 77 varieties of tasks in, 77 Lakes (see also Biocies, Lake; Littoral Zone of Lakes): adaptations of organisms to, 60 Lakes (Continued) amounts of dissolved solids in from runoff of falling rain, 65 as complete ecosystems, efficiency of use of energy in, 206 as differing from streams, 59 autumn overturn in, 62 biomass in: and productivity of benthos, 76 and productivity of plankton, 76 and relation of depth to organisms in, 75 of benthos, 75 of phytoplankton and zooplankton compared, 75 of plankton and benthos compared, 75-76 of seston, 75 chemicals in: amino acids, 65 carbohydrates, 65 fats, 65 color of, 61, 64 compensation level in, 61 cyclomorphosis in plankton found in, 60 deep, circulation of water in, 61 defined, 59 derivation of carbon dioxide in, 65 dimictic, defined, 63 draining of to form mature river, 42 dystrophic, defined, 64 effects of low concentration of oxygen in on organisms of, 64 efficiency of use of solar radiation in, 205 epilimnion of, 61, 61-62 eutrophic: composition of bottom mud of, 73 defined, 64 seasonal variations in popula- tions of, 73 food cycle of: anatomical adaptations of fish in, 74-75 role of bacteria in, 74 role of phytoplankton in, 74 role of zooplankton in, 74 formation of, 59 through water accumulation in large basins, 42 freezing of in tundra biome, 315 gases in: ammonia, 65 hydrogen sulphide, 65 marsh gas, 65 nitrogen, 65 hard-water, 65 heat in: annual budget of, 63 loss of, 63 Lakes (Continued) unpolluted, hydrogen-ion concen- tration of, 66 hypolimnion of: amount of oxygen in, 64 fertility of, 61-62 introduction of exotic species into, 77 land masses as barriers to dis- persal of organisms of, 148 life-histories of organisms in, 76- 77 light in: as establishing rhythm in activi- ties of organisms, 60 measuring intensity of, 60 littoral zone of: differentiation of species distri- bution according to bottom of, 70 divisions of, 69-70 extension of, 69-70 vegetation of and populations in, 70-71 mar! formation in, 65 measurement of primary produc- tion of energy in 203 microscopic animals in, 71 monomictic: cold, 63 warm, 63 occurrence of animals in: amphibians and reptiles, 74 bird species, 74 mammals, 74 oligotrophic, defined, 64 orders of: first, 63 second, 63 third, 63 oxygen debt in, 64 oxygen in: factors in distribution of at various depths, 63 reduction of, 64 penetration of light into: and presence of ice, 60-61 factors affecting, 60, 60-61 pH of: and toleration of organisms to changes in, 66 degree of alkalinity of, 65 determining acidity or alkalinity of, 66 photosynthesis in, 61 physical properties of water of: buoyancy, 60 density, 59-60 pressure, 59 salts in: dissolved, accumulation of, 59-60 inorganic, absence of and popula- tions, 65 Lakes (Continued) nutrient, 65 saturation of water of with oxygen, 63-64 segregation of fish in, 74 Similarities of to ponds, 79 soft-water, 65 spring overturn in, 63 stagnation period in: during summer, 64, 73 during winter, 64 temperature of: during autumn, 62 during spring, 63 during summer, 61-62 during winter, 62-63 terrestrial insects in, 71 thermocline in, 61-62 turbidity in, 60-61 wind action in: currents created by, 61 wave action influenced by, 61 zones of: limnetic, 70 profundal, extension of, 70 trophogenic, 61 tropholytic, 61 Land Bridges (see Continents. Dis- persal Pathways) Larvae: and parasitoidism, 182 and temperature changes, 7 as food for fish, 191-192 as parasites on plants, 179 black fly, toleration of to currents, 50 caddisfly, adjustment of to cur- rents of stream, 46-47 conditioning of to food, 261 diel movements of, 68 digestion of cloth and feathers by, 190 filter feeding by, 55 food preferences of, 188 in lakes, 76 in soil of forest communities, 130, 135 metamorphic migrations of, 159 migration of from lakes during summer, 73 of flour beetle, and cannibalism, 223 of forest millipede, 144 of intertidal organisms, zonation of, 362-363 of pond organisms, 88-90 of stream insects, 53-54 of tiger beetles, 112 populations of in eutrophic lakes, 73 Learning: conditioning in, 15 defined, 14 Index 425 Learning (Continued) habituation as simplest form of, 14-15 imitation, 15 imprinting, 15 insight, 15-16 Life-forms (see also Life-forms of Animals; Life-forms of Plants): as factor in characterizing biotic communities, 7 defined, 7 great variety of among dominants on deserts, 332 morphological adaptation of according to strata within a community, 8 Life-forms of Animals (see also Life-forms): four-footed mammals classified, 8 major types, list, 8 Life-forms of Plants (see also Life-forms): as determiners of physiognomy of terrestrial communities, 28 characteristics of, 7 relation of animal communities to, 29-30 response to by animals in seeking cover, 248 system of classification of for animal ecologist, 8 Life Tables: formulation of, 212 mortality curve in, 212 obtaining information for construc- tion of, 212-213 survivorship curves in, 212, 213- 214 uses of, 212 to determine age structure of population at any one time, 216 with wild populations to find mean length of life, 215-216 Life- Zone System: confusion of with biome system, 272 designation of faunal areas in, 274 development of by C.Hart Merriam, 274 evolution of species in biotic provinces rather than distribu- tion centers of, 272 theory of dispersal centers in, 274 zones in lacking a uniform and characteristic faunal composi- tion, 272 Light: and compensation point in sea, 354 and sunspot cycles, 243 and vitamin content of food, 181 as absorbed by atmosphere, 200- 201, 243 426 Index Light (Continued) as affecting activities of sand dune organisms, 106-107 as cause of blue color of pure water, 61 as controlling height of ionosphere and ozone layers above earth, 243 as not having identical effect on all species, 244 avoidance of by desert animals, 338 efficiency of use of in photosyn- thesis, 205 excessive, as barrier to dispersal, 148-149 exposure of animals to and repro- ductive vigor, 241 in arctic tundra, 320-321 in developing jungle undergrowth, 342-343 infra-red, effects of on animals, 99 intensity: as not equal in all parts of world, 244 as varying on forest floor, 122 extreme fluctuations in, 243 in Arctic Circle, 216 in deserts, 332 in forests, vertical gradients in, 123 in grassland, 122 measurement of in water, 60 measurement of radiation of, 99 monopolizing of by plants, 183 movements of plankton in response to, 67-68, 358-359 on south-facing slopes of grass- lands, 124 penetration: in ice, 60-61 in lakes, 61 in ponds, 79 in sea, variations in, 354 in water, factors affecting, 60, 60-61 lateral, under forest canopy, 122 to deciduous forest floor, 121 reduction of: and change in species composi- tion, 107 by water plants, 172 sensitivity of lake organisms to, 60 ultra-violet: and sunspot cycle, 244 dangers of, 243-244 effects of on animals, 99, 244 fluctuations in, and ionization of air and ozone formation, 243, 243-244 in deserts, 332 Light (Continued) variations in as affecting weather, 243 wavelengths of: and growth rates, 99 characteristics of, 99 Limnology, growth of as a branch of ecology, 5-6 Lithosphere, as division of environ- ment, 6 Littoral Zone of Lakes, 69-71 few species furnishing bulk of population in, 255 Littoral Zone of Sea (see also Sea): abundance of benthos in, 359 algae as protection for animals in, 363 as compared to sublittoral zone, 363 divisions of, 362 epifauna in as decreasing in vari- ety toward Poles, 359 extension of, 351-352 species of animal life in, 363 Locomotor Movements: aided by mucous glands in nema- todes, 130 evolution of appendages to facili- tate, 96 evolution of skeleton to facilitate, 96 hopping as special adjustment of animals to grassland, 329-330 loss of by parasites in adapting to host, 179 of animals in response to stream bottom, 50-51 of organisms in response to changes in environment, 12, 12- 13 of reptiles in sand, 110 of soil animals, 131-132 of swift-water invertebrates, 47-50 M Mammals (see also Measurement(s) of Populations; Populations): abundant varieties of in arctic tundra, 318 adjustment of to low oxygen pres- sures at high altitudes, 323 and insects, relative influence of in community, 20 arctic, choice of microhabitat by, 246 as agents of plant distribution, 177 as demonstrating nine-ten year cycles, 234 as demonstrating three-four year cycles, 237 Mammals (Continued) as retarders of succession in subseres, 115 body size of as density-dependent, 222 breeding ages of, 215 choice of microhabitats on bodies of by congeneric species of mites and fleas, 246 choice of strata in vegetation by, 248 correlation of breeding of with rainfall in tropical rain forests, 348 development of voices of for forest living, 299 dispersal of: and number of young, 146 into Africa and South America, 151 rivers as barriers to, 148 dominance of in geologic succes- sion, 24-25 fecundity of, 210 first evolution of in Eurasia, 151 food of in coniferous forests, 308- 309 four-footed, classification of, 8 grass-eaters among, 127-128 in Alpine tundra in North America, 321 in arctic tundra biociation common to Eurasia and North America, 318 in chaparral, 312 in desert scrub biociation, 335-336 in Ethiopian region, 271 in Eurasian boreal forest biocia- tion, 307 in European deciduous forests, 299 in Malagasy sub-region, 271 in Neotropical region, 270 in North American boreal forest biociation, 303 in North American coniferous forests, 302-303 in North American grassland biociation, 326 in North American montane forest biociation, 307 in North American temperate deciduous forest biociation, 295-296 in North American temperate deciduous forest- edge biocia- tion, 297-298 in pond- marsh biocies, 86-87 in sea, 359 in southeastern North American forests, 298 in stages of clay sere, 113 in tropical savanna biociations in Africa, 344 Mammals (Continued) in woodland, 311 keenness of smell in, 101 lack of in savannas of South America, 344 live-trapping of as method for determining home ranges, 185 morphological adaptations of for food-getting, 8 parasitical diseases of, 180-181, 181 placental, absence of in Australo- Papuan region, 270 of Pleistocene era no longer occur- ring in North America, 285 period of time required for evolu- tion of, 266-267 physiology of and size of litter, 210 populations of: and catastrophes, 236 censusing techniques for measure- ment of, 35-36 in forest as affected by seasonal change, 137 reactions of to flooding, 114-115 recent forms of, as confined to Northern hemisphere, 151 response of to cold, 299 size of litter of and parental care, 211 small: adherence to homesite of, 149 adjustments of to grassland, 329 effect of burrows of on soil structure, 164 species of in forest communities, 135-136 species of peculiar to Basin sage- brush, 336-337 teeth of as adapted for feeding, 188 tundra, fossils of, 286 tundra, insulating mechanisms of for cold toleration, 320 Marsh(es) (see also Biocies, Pond- Marsh): changing of eutrophic lakes into, 64-65 drying of and succession from sea to land, 368 effect of drought in on muskrats, 87 effects of development of pond into, 82-83 increase in fertility of through imbalance in energy exchanges, 202-203 kinds of vegetation in, 79-81 mammals found in, 86-87 muskrats as fur yielders in, 232- 233 occurrence of around margins of ponds, 79 salt, sea organisms in, 368 Measurement(s) of Populations (see also Populations): available methods for unsatisfac- tory, 31 capture per unit-effort method for, 35 capture-recapture method in: description of, 34-35 contagious distribution in, 34 counting of net plankton, 40 counting of surface plankton, 39-40 difficulty of except with restricted distribution, 219 evaluating differences in densities in, 34 methods of marking animals for, 185 negatively contagious distribution in, 34 of all animals in single forest community, lack of, 130 of birds: gannets, 219 methods in use for upland game species, 36-37 size of census plots for predators, 37 spot- map method for smaller species in nesting season, 37 use of airplanes with water-fowl, 36-37 of bottom organisms: and use of dip-nets, 40-41 and use of dredges, 41 use of bottom samplers for, 41 washing of samples of through sieves, 41 of fish: through catch per unit- effort method, 39 through draining of artificial ponds, 39 through electric shock method, 39 through use of nets in ponds and shallow waters of lakes, 39 through use of nets in small streams, 39 through use of poison, 39 of insects in air, 147 of insects on foliage: description of sweep net sam- pling, 37 of small trees, sampling, 37 of tall trees, sampling, 37-38 reliability of sweep-net sampling, 37 of mammals: and influx and departure of mam- mals, 35-36 and methods of ‘‘marking,’’ 36 |ndex 427 Measurement(s) of Populations (Continued) and ‘‘saturation’’ of area with traps, 35-36 distribution of live traps in, 36 nocturnal, use of traps for, 35 reducing boundary of contact with outside area in, 35-36 through radioactive labelling, 36 use of bait in live trapping, 36 value of live-trapping in, 36 of soil animals: earth-worms, hand-sorting of, 38-39 fauna on fallen logs, censusing of, 38 macrofauna, censusing of, 38 megafauna, censusing of, 38 mesofauna, censusing of, 39 microfauna, censusing of, 39 protozoans, censusing of, 39 use of Berlese funnel in, 39 use of flotation method in, 39 plotting of sigmoid curve in- creases, 160-161 relative indices and absolute abundance in, 31 sample plots as method for: empirical method for determining size of, 32 precision as factor in determining number of, 34 randomly located, 32 relation of random distribution to size of, 34 shapes of, 32 systematic arrangement of, 32 variation of size of with species and density, 32 strip censuses, 31-32 trapping in for determining exist- ence of home ranges, 185 usefulness of indices of abundance in, 31 Metabolism: adjustments in with seasonal changes, 102 and biomass, 21 as affecting influence of species in a community, 20 calcium carbonate as essential in, 105 changes of as cause of ‘‘motiva- tion,’’ 14 high rate of in birds as cause of migration, 157-158 increase in and rise of body temperature, 98 measurements of and trophic levels, 199 of nitrogen and phosphorus by plants, 203 428 Index Metabolism (Continued) removal of wastes of in mutualism, 177 requirement of calcium for, 167 Microclimate: and choice of microhabitats by ants, 245-246 and choice of microhabitats by spiders, 246 as factor in competition for particular microhabitat, 246 response to by animals in seeking cover, 248 Microhabitat(s) (see also Habitat(s); Microclimate; Niches): aquatic: on surface film of ponds: oxygen tensions in as factor in choice of by fish, 246 segregation of species according to character of substratum in, 246 amount of water available in and level of water exchanges in animal, 246-247 as setting for traps in censusing, 35 carcasses of dead animals as, 125 characteristics of soil of and egg- laying, 246 choice of by arctic mammals, 246 choice of by four species of ants in relation to substratum and microclimate, 245-246 choice of on bodies of small mam- mals by congeneric species of mites and fleas, 247 defined, in ecology, 6 differentiating features of more easily seen by investigation of particular occupants, 245 division of by competitive species and reduction of competition, 253 dung in grassland as, 125 effect of microclimate on compe- tition for, 246 grouping in and random dispersal, 34 importance of microclimate in choice of by spiders, 246 measurement of physical factors in as method in ecology, 1-2 occurrence of bioseres in, 23 rock as, 104-105 simultaneous occupation of by species to carrying capacity, 253 stones, rotting logs and tree bark as, 134-135 vertical gradient in factors of from above grasses to ground, 122 Migration (see also Birds): annual, described, 156-157 annual altitudinal, described, 158 annual altitudinal, species involved in, 158 annual latitudinal: of bats, 158 of bison, 158 of insects, 158 of locusts, 158 annual local: described, 158 of aquatic organisms, 158 of insects, 158-159 as example of intraspecific cooperation, 174 as method of adjusting to severe cold of arctic tundra, 320 as method of adjusting to severe winter conditions by forest animals, 299-300 classification of, 156 daily: of deciduous forest insects, 159 of plankton, 159 differences of from dispersal, 156 latitudinal, described, 156-157 metamorphic: of aquatic organisms, 159 of insects, 159 no definite period of in tropical rain forest, 348 of birds: annual latitudinal, 156-157 causes for evolution of, 157-158 factors in timing of, 158 of fish between fresh and salt water for spawning, 368-370 of primitive herds of bison, 328 role of in developing arctic tundra fauna, 318 Minimum, Law of the: development of, 12 stated, 12 Minor Communities (see Communi- ties) Moisture: amount of in habitat as correlated with level of water exchanges in animal, 246-247 and absolute humidity, 97 and relative humidity, 97 as cauSing gradients in populations, 296 belief in importance of by J.A. Allen for controlling distribu- tion of animals, 272-273 different requirements of by grass- land species, and division of grassland into plant associations, 324 evaporation of: and relative humidity, 97 Moisture (Continued) and saturation deficit, 97 factors in, 97 measurement of rate of, 97 in food, as ‘‘free water,’ 97 in soil: addition of and re-activation of soil animals, 107 as causing activity in inverte- brates, 313 content of, 165 drying of and decrease in soil animal population, 134 organisms active in, 131 lack of in deserts: adaptation of plants to, 333 as critical environmental factor, 338 loss of: decrease in through evolution of internal moist membranes, 99 prevention of by body surfaces, 97-98 through excretory organs, 97-98 through feces, 97-98 through respiratory surfaces, 97-98 metabolic water, 97 role of in formation of plant seres on rock, 102-104 sources of for desert animals, 338 to organisms, methods of, 97 Morphological Adaptations (see also Adjustment(s) to Terrestrial Habitats; Food; Food- getting): and relation to strata of communi- ties, 8 as basis for division of aquatic organisms, 66 as causing restriction of animals to particular food, 195 establishment of through natural selection, 9 for food-getting, 188-189 for predation and competition, 9 influence of temperature on, 9 in reproductive system, 9 interplay of genetics and environ- ment in inheritance of, 7 of insects for living in grasslands, 125 of parasites in being restricted to special niches, 180 sessility and motility as considera- tions in, 7 variety of for food- getting, 8 Morphology, 3, 16 Mortality: and mean length of life for wild populations, 215-216 at end of cycles due to exhaustion of adrenopituitary system, 241 Mortality (Continued) causes of in primitive herds of bison, 328 environment as cause of and repro- duction of females of species, 217 expression of in life tables, 212- 214 in shallow marine waters due to severe cold, 354 of coral reef polyps and building of reefs, 366 of lemmings during population declines and state of vegetation, 241 of rabbit populations through intro- duction of myxomatosis, 235 of snowshoe rabbit young, 224 of white man in tropics, 349 of young and overcrowding, 224- 225 rate of: as high among young and adult- young ratios, 216 as high in epizootics, 228 as influenced by number of young and carrying capacity of habitat, 211 direct variation in with density of population, 219-220 for animals in captivity, 215 in rose thrip and climatic factors, 231-232 in young and parental care, 211 variation in from one age level to another, 216 variations in between sexes, 215- 216 Motile Organisms: and kineses, 12 and taxes, 12 and tropisms, 12 in grassland, variations in micro- habitat of, 123-124 in intertidal habitats as achieving proper zonation, 362-363 morphological changes in induced by habitat, 7 Motility, as consideration in mor- phological adaptation, 7 Mutations: as affecting period of time re- quired for evolution, 267 behavior patterns subject to, 13 cause of, 263 factors in establishment of in populations, 263 haphazard nature of, 262-263 in asexual and self-fertilizing forms, 264 independence of from environ- mental conditions, 9 Mutations (Continued) permanent fixing of as described in Baldwin effect, 261 rate of occurrence of, 262-263 recessive character of those upon which natural selection works, 265 Mutualism: and animal browsing on trees and shrubs, 177 as demonstrated between plants, 176 as demonstrated by protozoa and termites, 177 between plants and animals, 177 defined, 176 differences from symbiosis, 176 defined, 176 nut-burying by squirrels as, 177 defined, 176 through animal transportation of ingested seeds, 177 through improvement of seed germination by animal diges- tion, 177 N Natural Selection (see Evolution) Nekton (see Biocies, Lake; Lakes; Sea) Niche(s) (see also Aspection; Diur- nation; Microhabitat(s); Shelter): adaptations to and rate of repro- duction, 217 advantages to animals of segrega- tion into, 254 as center of origin for spread of new species, 26 as defined by Charles Elton in 1927, 245 availability of each kind in tropical rain forest limited, 345-346 bill adaptations of birds for feed- ing in, 188 choice of: according to food preference determined by differences in size of related species, 249 as determined by vegetation, 29 aspection as factor in, 247 as sometimes a random matter, 249 by birds according to foot-span, 248-249 by birds according to strata of vegetation, 248 by mammals according to strata, 248 by warblers in evergreen forests, 248 Index 429 Niche(s) (Continued) deliberate evaluation process exercised in by animals, 250 diurnation as factor in, 247 intricacy of equipment of higher animals in, 249 shade as consideration in, 247 through consideration of physical features by given species, 245 competition for: and relationships in as affected by geography, 253 interspecific, as causing restric- tion of species to optimum niche, 252 interspecific, as segregator of species according to Darwin, 251-252 interspecific, reduced by adapta- tions of related species to climate, 253 interspecific, removal of and expansion of species, 252 most favorable portions of, 222 preadaptation as advantage in, 250-251 success in as dependent on parasitoidism, 249 concept of as developed by Joseph Grinnell, 245 concept of in distributional units equivalent to concept of species in taxonomic units, 245 correlation of growth rate with environmental resistance in, 217 defined, 16, 245 different, divergence of new species into in speciation, 257 easy discrimination of by new generation, 264 economic densities in, 219 establishing of segregation in during times of stress or crisis, 253 factors in restriction of species to, 245 features differentiating micro- habitats in for particular species, 245-247 feeding, factors in description of, 249 Gause’s rule or ‘‘competitive exclusion principle,’’ as describ- ing occupation of, 253-254 inhabited by garter snakes, and kinds of food eaten, 249 kinds of cover afforded by, 247 measurement of, 27-28 nests as, 248 occupied by species considered ecological equivalents of other species, 256 430 Index Niche(s) (Continued) of parasites, 179 of soil animals in forest, 130-131 ordinary overlapping in clearly defined during critical stages of life cycle, 253 preferences in for certain species attributed to appropriate behavioral patterns alone, 250 requirements in of different species and community inter- relations, 16 segregation of species into to avoid interspecific disturbances, 98 similarity of from one community to another, 256 similarity of in different forest types, 137-138 special, restriction of parasites in body to, 180 type of and body water balance, 97-98 variety of downstream and number of species, 52 Nitrogen Cycle: described, 166-167 in aquatic ecosystems, 172 Nocturnal Animals: amphibians and reptiles as, 99-100 and diurnal predators, 101 choice of microhabitat by, 246-247 compared to diurnal, 101 in deserts, 338 in tropical rain forests, 349 major period of activity of, 100- 101 mammals in forests as, 99-100 physiological adjustments of for night activity, 101 restriction of body coloration of, 101 North America: amelioration of climate of, and northward movement of organ- isms, 291 Antarctic—tertiary flora of, 282 Arcto-tertiary flora of, 282 Asian origin of fauna in, 130-131 Basin sagebrush biociation in, 336-337 best developed deciduous forest in, 293 birds in arctic tundra of, 318-319 birds in coniferous forests of, 302-303 boreal forest biociation in, 303- 306 building of Appalachian Mountain System in, 280, 291-292 climate of coniferous forest in, 301-302 climatic changes in 60-70 million years ago, 281 North America (Continued) climatic optimum in, flora and fauna during, 289-290 coastal plain of Gulf of Mexico in, character of, 280 Coast ranges of during early Tertiary era, 281 deciduous forest biociation of, animal communities of, 295-297 deciduous forest- edge biociation, animal communities of, 297-298 desert scrub biociation in, 335-336 dispersal of Asian species in due to Pleistocene glaciation, 318 dispersal of isolated fauna of dur- ing interglacial periods, 308 distinctiveness of plant community in west of Great Plains, 297 division of coniferous forest refugia in, 308 effects of cold climate on dispersal of birds in during Pleistocene era, 286-288 emergence of Gulf of Mexico coastal plain in during Tertiary era, 280 existence of plants of Pleistocene era in at present, 285 factors in separation of Arcto- tertiary forest of into western and eastern biociations, 307-308 faunistic system of J.A. Allen for, 272-273 flora and fauna of, during xerother- mic period of, 290-291 flora of during Sub- Atlantic period, 291 forests of during Pleistocene period, 285-286 formation of Ouachita Mountains in, 280-281 formation of Ozark Mountains in, 280-281 fossil and pollen evidence for existence of tundra in during post-Pleistocene era, 289 grassland in: biociation of, 326-327 climate of, 324-325 extension of, 324 in Texas, existence of as possi- ble during Pleistocene era, 286- 288 list of most important genera of grasses in, 325 plant associations of, 325 similarity of to that of rest of world, 327-328 interior of 60-70 million years ago inundated by seas, 280, 291 isolation of animal species in different refugia of coniferous forest of, 308 North America (Continued) isolation of bird species in different refugia of coniferous forest of, 308 lack of similarity between fauna of and that of Europe, 155 Laramide orogeny and formation of mountains in Rocky system, 281 local glaciers in, 288 Madro-tertiary flora of, 283 mammals in arctic tundra of, 318 mammals in coniferous forests of, 302-303 mammals once present in, 285 montane forest biociation in, 307 Neotropical-tertiary flora of, 282 origins of breeding bird species in forest biociations of, 308 physical conditions of during Pleistocene age, 284-285 plant associations in, 294 in coniferous forests, 302 in deserts, 333-334 in woodland, 311 precipitation in arctic regions of, 315 primitive population of bison in, 328 range of time covered by mountain formation in, 325 refugia of coniferous forest sur- vival during Pleistocene era in, 286 scanty evidence for existence of tundra in, 286 separation of coniferous forest of from Eurasian forest in Tertiary era, 284 species characteristic of alpine tundra in, 321-322 South American origin of fauna in, 155-156 southeastern mixed biocies in, animal communities of, 298 temperate deciduous forest biome in: humidity of, 293 precipitation in, 293 temperature of, 293 volcanic action in, 281 Western mountains of, zonation of vegetation in, 314 zonation of communities in, 313- 314 Nutritional Values: and rate of reproduction in animals, 241 awareness by animals of lack of in diet, 190 effects of deficiencies in of foods, 181 of carbohydrates, 190 Nutritional Values (Continued) of fats, 190 of minerals, 190 of non-staple foods, 190 of proteins, 190 of staple foods, 190 of vitamins, 190 of water, 190 oO Oceanography, early studies in, 6 Omnivores: adaptations of teeth of for food- getting, 188 conditions under which animals qualify as, 188 enzymes of and food palatability, 190 filter-feeders as, 188 in food chains, 195 position of in trophic levels, 196 seasonal variations in food of, 192 Optimum: defined, for environment, 10 finding of in population growth curve to determine productivity yield, 207-208 Orientation (see also Behavior Patterns): of animals to streams, 50 of organism to changes in environ- ment, 12, 13 stream organisms’ invasion of habitats and need for, 53 to current, by stream animals, 44, 50 Outbreaks (see Plagues) Overwintering: as characteristic of life-forms of plants, 7 by forest animals, 300 in grassland, 330 of arctic tundra animals, 320 of cold blooded desert animals, 339 Oxidation Debt, 64, 73 Oxygen: absorption of by aquatic insects, 84-85 absorption of by water organisms and seasonal change, 172 abundance of in air as compared to water, 99 and carbon cycle, 167 as absorber of ultraviolet radia- tion, 243 availability of seldom critical for land animals, 99 comparison of amounts of in air and water, 63 dissolving of by soft-shelled turtle, 86 Oxygen (Continued) in lakes, 63-65 and measurement of pH changes, 66 classification of lakes according to amount of, 64 during summer stagnation period, ears effects of low concentrations of on lake organisms, 64 factors in reduction of supply of, 64 of tundra, lack of, 315 in ponds: and seasons, 79 as varying with photosynthesis, 79 decline of in summer, 87 protected from winds, 79 variations in content of from day to night, 79 in sea: adaptations of marine animals for obtaining, 356 and compensation point, 354 concentration of on surface, 356 factors in reduction of concen- trations of, 356 high concentration of on shores with splashing of waves, 356 sources of, 356 in soil, 165 loss of to plants in hot, dry cli- mates, 168-169 low pressure of at high altitudes, and mammal adjustment, 323 percentage of in dry air, 99 reduction of in change of pond to marsh, 82-83 restriction of fish to water rich in, 246 role of in experiments to determine primary production of energy, 203-204 saturation of water with, 63-64 trapping of in soil during floods, and arthropods, 115 Paleo- Ecology: of coniferous forests, 307-308 of deserts, 334-335 of grassland, 326 of North American woodland, 311 of tropical forest biomes, 344-345 Parasites (see also Parasitism): adaptation of to hosts of one phylum, 180 adaptations of to living in or on host, 179 |ndex 43] Parasites (Continued) as producers of disease: bacteria, 181 fungi, 181 protozoa, 180-181 ticks, fleas, lice, mites and flies, 181 viruses, 181 worms, 180 causing food-poisoning, 181 classification of, 178-179 development of immunity to diseases carried by, 181-182 effects of accidental presence of in exotic host, 181-182 effects of lowering of host’s resistance to, 181 endoparasites, evolution of, 179 evolution of varieties requiring intermediate hosts, 178 factors in transmission of, 228 free-living evolution of into ectoparasites, 179 gall wasps as, 180 methods of transfer of from one host to another, 179 mutant strains of, 181 normalcy of presence of in healthy host, 181 restriction of to special niches in body, 180 role of in interspecific competi- tion, 254 species of involved in social parasitism, 179 species of on animals, 178-179 species of on plants, 179 taxonomies of and phylogenetic relationships of hosts, 180 Parasitism (see also Parasites): defined, 178 disease in, defined, 180 host-specificity in, 180 mutants in and host mortality, 228 nutritional deficiencies caused by, 181 role of in physiological stress, 181 social, described, 179 Parasitoidism (see also Parasitism): as example of carnivorous feeding, 187 as factor in determining success with which prey species will compete for niche, 249 as parallel to predation in curtail- ing over-population, 226 attempts at artificial control of pests and disorder in, 227 choice of larvae for egg-laying in determined by conditioning, 251 coactions in as affected by tem- perature, 227 432 Index Parasitoidism (Continued) crop pest control through intro- duction of, 227 differences of from parasitism, 182 difficulty of finding host in and its consequent survival, 240 duplicate infestations in due to increase in parasitoids, 226-227 evaluating effect of on insect larvae, 230 hyperparasitoidism as form of, 182 importance of buffer species in, 227 lack of random behavior in search for host in, 227 low host densities and reproductiv- ity in, 226-227 normal relation between host and parasitoid in, 226 parallelism of hyperparasitoidism to, 226 resemblances of to predation, 182 reversal of dominance in, 246 use of to control plagues, 234-235 Photosynthesis: and amount of oxygen in water, 63- 64 and carbon cycle, 167 and carbon dioxide in lakes, 65 and compensation point in sea, 354 and formation of energy for plant’s activities, 201 and oxygen content of ponds, 79 and respiration approximately equal when trophic levels in balance, 206-207 by lake organisms, 61 effect of curtailing of on fish, 87 efficiency of use of solar radiation in, 205 factors controlling rate of, 12 immediate initiation of as response to environment, 10 in food- cycle of lakes, 74 of algae in coral reef biome, 366 reduction of through heavy grazing, 125-126 reduction of through sitting, 56 reproduction affected by light in, 60, 60-61 role of in experiments to determine primary production of energy, 203, 203-204 Physics, tropisms and taxes explain- able in terms of, 12, 12-13 Physiological Adjustments (see also Adjustment(s) to Terrestrial Habitat(s)): and law of the minimum, 12 and law of toleration, 10-11, 11-12 and threshold of organism, 10 Physiological Adjustments (Continued) as first response to environmental changes, 10 cycles in as correlated with popu- lation cycles, 241-242 immunity to diseases as, 181-182 internal, for resistance to cold by forest animals, 299 made by sea organisms in fresh water, 96 to stress, 181 types of in response to environ- mental factors, 10 Physiological ecology, growth of as a branch of ecology, 6 Physiology: as secondary consideration in ecological life history, 16 distribution of species according to, 27 of organism and initiation of major activity in life cycle, 14 relation of to ecology, 3 Plagues: as providing food for predators, 226 biological control of in United States, 234-235 defined in terms of untrained observers, 234 occurrence of niche segregation during, 253 of bean clams in California, 235 of field mice, through starting epizootics of mouse typhoid, 235 of locusts in tropics, 349-350 of rabbits through introduction of myxomatosis, 235 of small mammals in prairies, 329 of spruce budworm, 243, 309-310 outbreaks of since beginning of recorded history, 234 synonymous terms for, 234 Plankton (see also Sea, Oceanic Plankton and Nekton Biome): absence of in winter in antarctic, 319 abundance of in eutrophic lakes, 64 ancient origin of, 67 as food source for young fish, 75 as food source in ponds, 87 as reducers of light intensity in water, 172 as subjects of experiments in uses of energy, 206 biomass of in lakes, 75, 75-76 censusing of, 39-40 cyclomorphosis in, 60 daily migration of, 159 diel movements of, 67-68 distribution of in small and medium-sized lakes, 67 fecundity of, 210 Plankton (Continued) fresh-water, composition of, 66-67 fresh-water, irregular distribution of, 67 hatching of organisms of from dormant condition, 87-88 in pond- marsh biocies, 81-82, 82-83 in stream biocies, 43-44 kinds of, 66 marine: as. eaten by coral animals, 366 as not more remarkable in importance than soil organisms on land, 351 as potential food source for man, 371 as whale food, 191 diel movements in, 358-359 flotation mechanisms of, 357-358 great variations in abundance of, 358 greater abundance of in cold waters than in warm, 358 greater variety of than in fresh- water, 356 large populations of phyto- and zooplankton as not occurring in same place at same time, 358 lower rate of productivity of in cold waters than in warm, 358 use of nitrates and phosphates by as nutrient material, 354-356 yearly cycle in abundance of, 358 marine, composition of: chordates, 357 coelenterata, 357 crustaceans, 357 echinoderm species, 357 green phytoplankton, 357 mollusks, 357 nannoplankton, 356-357 protozoan zooplankton, 357 worms, 357 measurement of, 39-40 minor importance of in fish diet, 191-192 occurrence of and pH values, 66 productivity of and biomass, 76 rate of reproduction of, 204 release of oxygen by near water, 63-64 role of in experiments to determine primary production of energy, 203, 203-204, 204 role of in food-cycle of lakes, 74 seasonal distribution of, 68-69 uncommon as source of food for stream animals, 56 Plant Ecology (see Ecology, sub- divisions of) Plant Reactions (see also Deserts; Forests; Humidity; Lakes; Light; Ponds; Precipitation; Streams; Temperature; Tropical Biomes; Wind): and ants in soil structure, 164 and conversion of raw organic matter into usable material for re-absorption, 166 and earthworms in soil structure, 164 and water conditioning, 172-173 chemical, in water, 172 decomposition and utilization of humus in soil structure, 165 effect of dead roots on soil in, 164 on rocks and effect on soil struc- ture, 164 physical, in water, 172 prevailing climate as determinant of, 169 role of carbon cycle in, 167 role of nitrogen cycle in, 166-167 Pleistocene Era: alteration of drainage patterns during in northern North America, 285 climate over land bridge to Siberia during, 284 climatic conditions accompany ing glaciation of, 281 cold glacial water flow during and forest development, 286 destruction of coniferous forest by glaciation during, 286 differentiation of desert vegetation during, 334-335 dispersal of isolated fauna of dur- ing interglacial periods, 308 drop in mean annual temperature during, 308 early forests of North America during, 285-286 existence of tundra during, 286 fate of coniferous forest of North America during, 286 glaciation and wind directions in, 284 glaciation of and Schadley pene- plain, 280 glaciation of and zonation of vegetation, 313 glaciation of as disturber of Euro- pean deciduous forest biocia- tion, 299 glaciation of as enhancer of differentiation of boreal and western forest biociations, 308 glacier movement from Canada during, 280-281 heavy precipitation during, 284-285 Pleistocene Era (Continued) heavy rainfall of and extension of grassland, 326 ice advance in Northern Ohio during, 284 isolation of tundra of Alaska refugium during, 284 land bridges in, 284 lower tree lines during than at present, 286 mammals of no longer present in North America, 285 plants and mollusks of still in existence, 285 possible southward dispersion of birds during due to cold climate, 286-288 presence of loess in North America as key to absence of extensive forests, 286 refugia for coniferous forest survival in North America during, 286 sea level during, 284 segregation of animals by south- ward extension of grasslands during, 286-288 substages of Wisconsin glaciation during, 284 successive thrusts of glacial ice in North America and Europe during, 284 survival of tundra flora in Alaska refugium during glaciation of, 282 terrestrial biota in Europe and Asia during, 288 theory about condition of Arctic Ocean during, 284 thickness of ice during, 284 Pollution: of lakes: difficulty of controlling, 77 moderate degree of advantageous, aia of streams: control of as basic to fish management, 58 industrial, 57 methods for determining degree of, 57 organic, 57 through hydrogen sulphide, 65 Polymorphism (see Variations in Population Characteristics) Pond and Marsh Management: advantage of constructing artificial ponds, 91 as challenge to applied ecologists, 92 awareness of relation between available food supply and population in, 91-92 Index 433 Pond and Marsh Management (Continued) control of mosquitoes in, 92 control of turbidity in, 91 control of water level in, 91 increasing fertility of vegetation in, 91 prevention of disappearance of habitat through succession, 91 stocking prey-predator combina- tions of fish, 91 Ponds (see Biocies, Pond- Marsh): acidity of, 79 activity of life in during winter, 87 animal sere in, 81 artificial, construction of, 91 benthic production in and presence of fish, 91 biomass and food habits of fish species in, 91 biomass of and fertility, 90-91 biomass of fish in, 90-91 biomass of invertebrates in shallow water of, 90 changing of eutrophic lakes into, 102-104 characteristic species of, 96 defined, 79 drought in and spores resistant to desiccation, 87-88 freezing of in tundra biome, 315 icy, mortality of fish in, 87 in alpine tundra, fauna of, 322 life-histories of principal pond organisms, 88-90 mature, characteristics of bottoms of, 79 occurrence of in grassland, 330 of desert, distinct nature of fish in, 239 organisms of in arctic tundra, 319 oxygen content of and photosynthe- sis, 79 oxygen content of and seasons, 79 periphyton as food source in, 87 PH values of, 79 predators in, 87 productivity of fish in indicated by biomass harvested, 91 productivity of vertebrates other than fish in, 91 shrinkage of in summer, 87 stages in hydrosere of, 79-81 temperatures of, 79 temporary, egg-laying in by frogs and toads, 88 temporary, inhabitants of, 88 variations in oxygen content of in day and night, 79 young characteristics of bottoms of dg Population Dynamics, growth of as branch of ecology, 5 434 Index Population(s) (see also Catastrophes, Cycles; Ecesis; Isolation of Populations; Measurement(s) of Populations; Sea; Variation(s) in Population Characteristics): average or regional density in, defined, 219 abeyance of stablizing factors in after catastrophes, 232 age ratios in after catastrophes, 216-217 as affected by breeding ages of animals, 215 as influenced by abundance of seeds, 192 average ratios of from years to years, 234 biomasses of in ponds, 90-91 characteristic levels of for species, 219-220 climatic and biotic factors in size of for rose thrips, 231-232 competition as stablizing factor in: amount of food available and, 222 close growth of sessile marine animals and, 221 defense of territories by birds and, 221-222 food supply of fish and, 222 plant struggle for space and, 221 struggle for most favorable portion of niche and, 222 control of in pond and marsh management, 91-92 cooperation and disoperation in growth curves of, 219-220 densities of hosts and reproduc- tivity of parasitoids, 226-227 density-dependent factors in, defined, 220-221 density- independent factors in acting as density- responsive, 230 density- independent factors in, Space, weather and food as, 230 density of in forests, and stratifi- cation, 136-137 density- stabilizing factors in described as biotic, 220 depletion of by catastrophes, 235- 237 difficulty of developing methods for measurement of, 2 difficulty of measuring except with restricted distribution, 219 disease as stablizing factor in, 228-229 distinguishing between passive density-responsive effects and dynamic density-dependent effects in, 230 distinguishing young from adults in, 216 Population(s) (Continued) distribution of in inner zone of normal abundance, 233 distribution of in zone of occa- sional abundance, 233 distribution of in zone of possible abundance, 233 economic or habitat density in, defined, 219 effects of competition in on other life processes, 222 evaluation of species through abundance of, 20 existence of density-limiting factors in, 229 factors determining regional densities of, 219 factors in differences of levels in, 219 factors in production of asymptote in, 220 fluctuations in ratios of and errors of sampling, 234 fluctuations in with predation, 196 forest and luxuriance of vegetation in forests, 138 growth curve of and productivity yield, 207-208 high level of in ground animals in forests, 137 hyperparasitoidism as form of control of, 182 importance of variability in factors affecting, 231 increase in predation with density of prey as density-stablizing factor in, 225 increase in with progression of clay sere, 112-113 increases in parasitoidism due to temperature, 227 individual variations in reactions to factors controlling and need for combined action of factors, 231 in grasslands from which preda- tors have been eliminated, 128- 129 in ponds during different seasons, 87 in streams, 43-44 intercompensations between com- petition and predation in, 231 inverse effects of density-depend- ent factors in, 220-221 inverse relation in between number of species and number of indi- viduals per species, 255 kinds of variations in, 234 level of and overgrazing, 128 level of in relation to number of species and predation, 226 Population(s) (Continued) levels of affected by different species demands for space, food, shelter, 230 limiting factors in described as physical, 220 maintenance of health and vigor in through predation, 226 mean longevity of and density, 219- 220 mean survival rates in, 216 necessity for measurement of, 31 number of age classes in and survival rates, 216 of ants during sand sere, 107-109 of benthos, in lakes, 69-73 of birds: breeding, in forests, 135 differentiation in of North American boreal forests, 305 highest level of in autumn, 156- 157 level of in North American deciduous forests, 296 non-breeding, in forests, 135 of Basin sagebrush biociation, 337 on arctic tundra, 319 of bison in North America, 328 of fish: factors in density of, 55 in dystrophic lakes, 64 in eutrophic lakes, 64 in oligotrophic lakes, 64 replacement of in fish manage- ment, 58 of grasshoppers during sand sere, 107 of host species, factors in regula- tion of by particular parasitoid, 226-227 of insects: as varying with season, 55 density of in air, 147 in abandoned field sub-seres, 115 of invertebrates in Basin sage- brush biociation, 337 of invertebrates in grassland communities, 124 of lemmings, curtailment of through emigration, 231 of mammals: in forest as affected by seasonal changes, 137 level of in North American deciduous forest biociation, 296 small, in forest communities, 135-136 small, in Michigan coniferous forest, 308-309 of marine plankton, 358 of mice and rats in mesquite vege- tation of New Mexico, 335-336 Population(s) (Continued) of muskrats, stabilizing factors in, 232-233 of nekton, in lakes, 73-74 of parasitoids, increase in and duplicate infestation, 226-227 of plankton, in lakes, 66-69 of primitive pronghorn antelopes, 328 of soil animals in forest commun- ity, 130, 131-134, 134-135 of spiders during sand sere, 109 of spiders in grassland communi- ties, 124 of trophic levels, and predatory consumption of energy, 202 order of occurrence of regulatory mechanisms in, 231 predation as a density-dependent factor in, 220-221 predation pressure shifts in species and locality as density stablizing factor in, 226 presence of non-breeding animals in, 215 pressure of as most potent reason for dispersal, 150 prevalence of cycles of and number of species in, 226 progressive variations in, described, 234 proportional effects of density- dependent factors in, 220-221 ratios of individuals per hectare between different animal groups, 130 recognition of characteristic species in, 20 reduction in as causing withdrawal of species to optimum habitat, 150 reduction in level of before limits of range are reached, 11 regulation of through emigration, 228 regulatory factors in, importance of time of occurrence of in life of organism, 230 relation between density-limiting and density-stabilizing factors in, 229-230 relaxation in intensity of action of density-dependent factors with reduction in, 229 reproduction as stabilizing factor in: average growth rate of tadpoles and, 224 density and fecundity of great tit and, 223 disturbance of females and, 224 effects of overcrowding on fecundity of salmon and, 223 Population(s) (Coulinued) fecundity of house mice and, 223- 224 fecundity of laboratory mice and, 223 fecundity of voles and, 223- fertility of egg and, 224 flour beetle fecundity, 223 necessity for distinguishing between fecundity and survival in, 224-225 optimum intermediate density of grain weevils and, 224 paramecium fecundity and sur- vival and, 222 reduction in fecundity of fruit flies and, 223 sheep blow-fly, 223 snowshoe rabbit cycle and, 224 survival rates of Drosophila and, 224 resiliency of after depletion, 219- 220 role of buffer species in para- sitoidism and level of, 227 size of in community classifica- tion, 29 stabilization of as occurring only in innermost zone of abundance, 233 stabilized, age ratios in, 216-217 described, 219-220 term defined by geneticists, 257 term defined in taxonomy, 257 threshold of vulnerability for particular factors affecting, 231 upper limit of vulnerability for particular factors affecting, 231 variations in abundance in accord- ing to fertility of habitat, 255 wild, mean length of life for, 215- 216 Post- Pleistocene Era: amelioration of climate in and northward movement of fauna, 291 analysis of pollen in bogs and determination of climate in, 288-289 climatic optimum in and extension of animal ranges, 289-290 climatic optimum in and forest growth, 289-290 conditions of flora and fauna during xerothermic period of, 290-291 distribution of tundra species during, 317 early forest growth in, 289 evidence for emergence of warmer, drier climate during, 289 existence of tundra in, 289 first plants to appear during, 289 <3) |ndex Post- Pleistocene Era (Continued) growth during sub- Atlantic period of, 291 historic fluctuations of climate in, 291 melting of ice and remaining of local glaciers in, 288 rapidity of melting of ice in, 288 retreat of grassland during, 326 Precipitation: amount of in arctic regions of North America, 315 and ultra-violet rays, effect of on organisms, 99 and vegetation, as factors in dis- persal of species, 148-149 as factor in change of grassland to present desert, 286 as favoring decomposition of organic ground matter, 165 as form of moisture available to organisms, 97 as heavy during glacial stages, 284-285 as heavy during historic climate fluctuations, 291 as heavy during Pleistocene era and extension of grassland, 326 as masking effect of ultraviolet rays, 244 correlation of with sunspot cycle, 243 decrease of as factor in shrinkage of ponds, 87 degree of in North America 60-70 million years ago, 281 effects of on animals in flood- plain, 114 effect of on distribution of plankton in lakes, 69 frequent, as carrier of soil nutri- ents, 168-169 inadequacy of and hardpan forma- tion, 170-171 in alpine tundra, 316 in deserts and consequent plant and animal activity, 338-339 in forests, stem-flow, 121 in North American deserts, 332 in North American grassland, 324 ‘“nterception’’ of, 121 in tropical biomes, 340-341, 341 in woodland biome: lack of as increasing mortality of muskrats, 87 mean annual rate for temperate deciduous forest biome in North America, 293 measurement of through fall in forests, 121 “net rainfall’’ in forests, defined, 121 436 Index Precipitation (Continued) rate of in coniferous forests, 301- 302 runoff of into lakes and streams, 65 Predation: and ‘‘balance of nature’’ concept, 195-196 and population balance of trophic levels, 202 and value of concealing coloration of prey, 194 as a density-dependent factor in populations, 220-221 as force in maintaining maximum productivity yield, 208 as lacking in pressure in coral reef biome and color of fishes, 367 buffer species in, 225-226 by coyotes on pronghorn antelope, 328-329 classification of energy transferred in, 202 compared to competition, 182 concentration of carnivores on one species in, 192 crop pest control through introduc- tion of, 227 cycle of predator dependent on cycle of prey in, 239 difficulties in quantitative deter- mination of significance of, 225 experimental studies in, 225, 225- 226 importance of in maintaining health and vigor in prey populations, 226 in communities with a large num- ber of species, 226 in communities with a small num- ber of species, 226 increase in percentage of, with density of prey population in intercompensation with competi- tion, 231 interdependence between popula- tions of prey and predator species in, 196 interrelations of any two species in as affected by other species, 225-226 lack of random behavior in search for prey in, 227 on lemmings and cyclic changes, 240 on prey species by varying num- bers and kinds of predators, 226 on small grassland mammals, 329 parasitoidism as form of, 182 Predation (Continued) periodic oscillations in numbers of both species, given single prey and predator and limited area, 239-240 protection from by hanging from trees, 346 role of in keeping populations of competing species below level food resources of habitat can sustain, 252-253 transfer of energy to higher trophic levels through, 201 upper limit of vulnerability in, 231 use of to control plagues, 234-235 variations in rate of with carrying capacity of area for prey, 225 vulnerability of prey species in as proportional to its relative abundance, 192 Predominants (see also Dominance; Dominants): and fifty per cent rule, 29 as key to recognition of communi- ties, 27 in ocean as defining biomes, 276 in oceanic plankton and nekton biome, 359 in pelecypod-annelid biome of sea, 365 in taxonomic structure of com- munities, 295 Preferendum: defined, 13 in choosing of niche by species, 250 of forest animals for stratum like native microhabitat: of insects in experimental gradi- ents, 123-124 Productivity (see also Productivity Values; Productivity Yield): and biomass in lakes, 75-76 and biomass in ponds, 90-91 and biomass in streams, 55-56 basic problem in measurement of, 202-203 biological, progressive decrease in from shallow waters to open ocean, 356 calculation of and rate of repro- duction, 217 carrying capacity in, defined, 21 factors in variations in for marine plankton, 361-362 in climax communities, 206-207 in coral reef biome, 367 in seral communities, 206-207 of bottom-dwelling marine organ- isms, 366 of lakes and rivers of Arctic tundra, 319 Productivity (Continued) of marine phytoplankton, zooplank- ton and fish compared, 361 of plankton in Block Island Sound, 361 of tropics as notfully realized, 350 primary: measurement of carbon- dioxide in photosynthesis to determine net amount of in aquatic habi- tats, 203 measurement of oxygen in photo- synthesis to determine net amount of in aquatic habitats, 203 measuring amounts of chloro- phyl! in plankton to determine, 203-204 measuring annual woody incre- ment of trees and shrubs, 204 measuring carbon dioxide incre- ment in fertile eutrophic lakes to determine, 203 measuring net amount of in her- baceous plants on land, 204 measuring of gross amounts in slow-flowing streams, 204 measuring of in fast-flowing streams, 204 measuring of net amounts in slow- flowing streams, 204 measuring oxygen deficit in fertile eutrophic lakes to determine, 203 measuring rate and extent of nitrate and phosphate depletion in aquatic habitats to determine, 203 problems in measurement of in flowing streams, 204 use of photosynthesis to measure gross amount of in aquatic habitats, 203 secondary: calculating by offering food, 204- 205 calculating gross amounts of, 204 calculating mortality from non- predatory causes in determin- ing, 205 determining amount of food required by species in calcu- lating, 204-205 measuring influence of environ- mental factors in calculating, 204-205 use of maximum biomass in calculating net amounts of, 204 standing crop, defined in, 21 terms in which expressed, 202-203 twenty-four hour day as smallest practicable unit for measuring, 202-203 Productivity (Continued) year as most useful unit for measurement of, 202-203 Productivity Values (see also Productivity; Productivity Yield): and double base of food chain, 206 and luxuriance of vegetation, 205 complications in study of in flowing waters, 206 measurement of at Lake Mendota, Wisconsin, 205 of coral reefs, 206 of lakes as complete ecosystems, 206 of plants in use of solar radiation, 205 of reservoirs as complete eco- systems, 206 role of transformers in, 205-206 studies in with use of plankton, 206 Productivity Yield (see also Produc- tivity; Productivity Values): artificial fertilization in mainten- ance of, 208 as objective in applied ecology, 207 calculation of in terms of biomass for organisms without specific adult size, 208 determining point of inflection in population growth curve to find optimum, 207-208 factors in maintenance of in undis- turbed ecosystems, 208 higher rate of when taken by man as primary consumer, 207 increased rate of with fewer links in food chain, 207 in upwelling ocean waters, 361-362 maximum sustained, determining of as problem in applied ecology, 207 need for studies in estimating optimum of, 208 optimum sustained, experiments in relations between, and point of inflection in population growth curve, 208 predation as natural force in maintaining maximum, 208 rate of and net production, 207 rate of and ratio of immature to adults, 216-217 reduction in and position of species in trophic levels, 207 use of life tables in determining, 212 Profundal Zone of Lakes, 71-73 Protective Devices of Plants: paucity of, 192 poison as, 192 prickles and thorns as, 192 shelter and vegetation as, 247-249 Protective Device(s) of Animals (see also Adjustment(s) to Terrestrial Habitat(s)): ability to hang from trees as, 346 aggressive resemblance, described, 192-194 aposematic or warning coloration, described, 194 Batesian mimicry as, 194 concealing coloration as, 192-194 experiments to prove value of, 194 in carnivores, 194 value of, 194 deflective spots and colors, 194 directive markings on carnivores as, 194 disruptive, described, 192-194 evolution of in small mammals and snails, 265 formation of circle by bison as, 328 hair-raising by pronghorn antelopes as, 328 in arctic tundra, white coloration as, 319-320 in desert, burrowing as to escape high temperatures, 338 in desert, coloration as, 339 in grassland, coloration as, 329 in inter-tidal habitats, 352 in trophic levels and rates of reproduction, 196-198 Mullerian mimicry as, 194 obliterative coloration, described, 194 of rocky marine shores, 364 protective resemblance, described, 192-194 species exhibiting aggressive resemblance as, 192-194 R Range Management, restoration of balanced populations of species in, 128-127 Rapids, defined, 43 Realm(s): Arctogaeie or northland, regions composing as lacking markedly distinct fauna, 268-269 distinctiveness of fauna Neogaeie or new land, 268-269 distinctiveness of fauna of Noto- gaeic or southland, 268-269 fauna of northeastern Arctogaeic, 270-271 kinds and location of, list, 269 regional divisions of Arctogaeic, 270-271 Receptors, 13 Reflex, defined, 13 Index 437 Region(s): approximate continental coinci- dence with, 268-269 Australo- Papuan: absence of placental mammals in, 270 fauna of, 270 former land bridges in, 269-270 subdivisions of, 269-270 Ethiopian: anifauna of Malagasy sub-region of, 271 boundaries of, 270-271 few gallinaceous species in, 271 fish and reptiles of Malagasy sub-region of, 271 geological history as explanation of fauna of and oriental region, 271 isolation from Africa of Mala- gasy sub-region of and evolu- tion of mammals, 271 mammals in, 271 of fish fauna of, 271 similarity of fauna of at order and family level to that of Oriental region, 271 Holarctic: differences in fauna of Palaearc- tic and Nearctic sub-regions of, 272 kinds and location of, list, 268 land bridge in and evolution of animals of, 272 Neotropical: area included in, 270 avifauna in, 270 endemic forms in, 270 fauna of West Indies in, 270 theories about distribution of fauna in, 270 Oriental: boundaries of, 271 characteristic species in, 271- 272 exclusive species in, 271-272 marked difference in fauna of northern and southern India in, 272 Releasers: and triggering of actions of organ- isms, 14 concept of as controlling instinctive behavior, as development in animal ethology, 6 minor characters possibly serving as for critically necessary behavior, 265-266 of behavior responses in symbio- sis, 176 varieties of, 14 438 Index Reproduction (see also Populations): adjustments in patterns of for terrestrial habitats, 98 and environmental adjustment, 217 and genetic isolation of species, 259 and geographic isolation of popula- tions, 259-260 and lack of interbreeding among sibling species, 258 and mechanical isolation of species, 259 and population turnover, 217 and production of organic matter, 30 and protective devices of animals in trophic levels, 196-198 and secondary sex ratio, 214 and tertiary sex ratio, 214, 214-215 as affected by attainment of breed- ing age by animals, 215 as affected by erosion, 229 as offset to mortality caused by environment, 217 as variation from random dispersal of species, 34 capacity of habitat for and death rate, 211 changes in physiological vigor and, 241-242 changes in primary sex ratio in, 214 crowding of fruit flies and rate of, 223 curtailment of in arctic tundra during certain years, 320 distinguishing between fecundity and survival in, 224-225 disturbance factor in and female house mice, 224 effect of photoperiodism on, 102 effects of weather on fecundity in, 211 egg fertility in experimental con- ditions of, 224 eggs in, and need for flooding of soil, 115 factors in survival of young in, 211-212 failure of pairing behavior leading up to and consequent species isolation, 258-259 high rate of and wildlife manage- ment, 216-217 increases in density of fruit flies and survival in, 224 in isolation and speciation, 258 innate capacity of species in deter- mining size of litter in, 210 in Pacific salmon, 159 in snowshoe rabbit population cycle, 224 Reproduction (Continued) in territories and lessening of pressures of competition, 186 low rate of and wildlife manage- ment, 216-217 low rate of in areas of mineral deficiency in soil, 167 low ratio of immature to adults in and productivity yield, 216- 217 nestiag habits of stream fishes in, 54 nutritional values in and popula- tion cycles, 241 of aphids and consequent emigra- tion, 228 of aquatic larvae and naiads, 159 of birds, nest failures in, 212 of cold-blooded animals in tropical rain forests, 347 of desert animals as occurring in wet season, 338-339 of fish, methods for increasing in lake management, 77 of fish migrating between fresh and salt water for spawning, 368-370 of fish through stocking of prey- predator combinations, 91 of forest millipede, 143-144 of grain weevil and optimum inter- mediate density of population, 224 of grasshopper, 107 of house mice in stress situations, 223-224 of laboratory mice and high popu- lation densities, 223 of lake organisms, 76-77 of pink salmon and effects on of overcrowding, 223 of pond amphibians, 85-86 of pond- marsh birds, 86 of pond reptiles, 86 of principal pond organisms, 88-90 of snails and slugs, and presence of moisture, 299 of stream insects, 53-54 of tiger-beetles, 112 of toads and frogs in temporary ponds, 88 of voles, 142, 223 overcrowding and tadpole growth rate in, 224 parental care in and number of eggs or young produced per litter, 211 per iods of and adult-young ratios, 216 placement of eggs on stream bot- toms, 51 presence of non-breeding popula- tion during, 215 Reproduction (Continued) rate of and artificial destruction of emerging adult blow-flies, 223 rate of and calculation of secondary net production of energy, 204 rate of and emigration, 228 rate of as equalling death rate in stablized populations, 217 relation between density and fecundity of great tit and, 223 results of overproduction in, 264 survival to next age level of young in, 216 through broadcasting of eggs, spores, and young, 146 times of breeding seasons for in forests, 299 to offset high mortality rates, 9 vigor in and vitamin content of food consumed, 241 volume of culture fluid of para- mecium and variations in rate of, 222 wide variation in fecundity of different species in, 210 Respiration (see also Oxygen): absence of organs for in soil animals, 131-132 adaptations for, in marine animals, 356 and carbon cycle, 167 and photosynthesis approximately equal when trophic levels in balance, 206-207 and reduction of oxygen supply of lakes, 64 as furnisher of energy for plant’s activities, 201 as user of oxygen in photosynthe- sis, 61 measurement of to determine gross secondary production of energy, 204 method of by diving beetle, 84-85 of air-breathing aquatic insects, 84-85 of burrowing animals in muddy and sandy marine habitats, 365 of mosquito larvae, 89 of stream animals, 51-52 role of in measurement of pri- mary production of energy, 203-204 underground, and oxygen-carbon- dioxide content of soil, 165 Riffles: as barrier to dispersal of pond or lake species, 148 change of river to pond and change in population of, 82-83 defined, 43 invertebrate biomass in, 55 Riffles (Continued) two species in as furnishing greatest bulk of population, 255 River(s) (see also Streams): base-level of, defined, 42 character of in maturity, 42 character of in youth, 42 debris rafts of as means of dis- persal of eggs and spores, 146 dispersal of animals of and soil erosion, 146 land masses as barriers to dis- persal of organisms of, 148 Mississippi, as isolator of popu- lations during glaciation, 286- 288 peneplain region of, 42 salt water as barrier to dispersal of organisms of, 148 slow-flowing as essentially elon- gated ponds, 79 waterfalls in as barrier to disper- sal of non-flying aquatic organ- isms, 148 wide, as barriers to dispersal of mammals, insects, birds, 148 Rock: and chemical composition of soil, 163-164 chemical changes in weathering of, 168-169 erosion of by water, 168 on marine shores and succession from sea to land, 368 plant and animal communities in, 102-105 role of in protecting sea-shore animals from pounding action of waves, 352 weathering of through plant reactions, 164 Ss Salinity (see also Salts): of sea: and abundance of populations, 356 as varied by inflow of fresh water, 354 average figures for, 354 factors in maintaining, 356 osmotic regulation in adjustment of organisms to, 354 terms used to indicate range of tolerance to, 11-12 Salts (see also Salinity): absence of and population of lakes, 65 accumulation of during winter in antarctic, 319 Salts (Continued) accumulation of in marshes through sea-wind action, 368 and measurement of pH changes in lakes, 66 and water evaporation in formation of saline soils, 172 as lacking in podzolic soils, 172 calcium, and chernozemic soils, 172 content of in bags, 92 decrease of in ocean during sum- mer, 358 extensive utilization of in sea water, 354-356 high concentration of in desert ponds, 339 high concentration of in ‘‘salt licks,’’ 167 in carbon cycle, 167 increase of in ocean during winter, 358 in desert soil, as not washed away by rainfall, 339 kidneys as critical in maintaining proper concentration of, 97-98 liberation of in water conditioning, 173 nutrient, in lakes, 65 nutritional value of in diet, 190 variations in within plants and cycles of herbivores, 241 Sand (see also Seres): and soil formation, 163-164 animal life in: ants, 107-109 birds, 110 grasshoppers, 107 insects, 109-110 reptiles, 110 spiders, 109 vertebrates, 109-110 as habitat, 106-107 characteristics of, 105 life-history of tiger-beetle found in, 112 plant communities in, 105-106 Saprovores: adaptation of teeth of for food- getting, 188 as users of energy lost from trophic levels, 202 defined, 187 in food chains, 195 position of in trophic levels, 196 role of as transformers, 196 role of in providing energy in food chain, 205-206 size of food of not of major importance, 191 Sea (see also Plankton, Marine; Zoogeography): Arctic, condition of during Pleisto- cene era, 284 AS? |ndex Sea (Continued) areas of occurrence of biomes of, 351 as source of food for man: and chief marine organisms used, 371 and difficulties of use of plankton, 371 and oyster production, 371 apparent inexhaustibility of, 371 through fin fish, 371 average depth of, 351-352 balanoid- gastropod-thallophyte biome of: algae as protecting animals in littoral zone of, 363 anchoring devices of organisms in, 362 and characteristic animals in sublittoral zone of, 363 avoidance of dessication by organisms of, 362 benthos of as mostly epifauna, 362 density of species in sublittoral zone of, 363 dominance in, 364 elements in food chains of, 364 extension of sublittoral zone in, 362 extensions of, 362 infralittoral fringe of, 362 kelp in sublittoral zone of, 363 midlittoral or balanoid zone of, 362 movement of animals into proper zones of, 362-363 protective devices of animals of, 364 species of animals in littoral zone of, 363 sublittoral zone of compared to littoral zone, 363 succession in, 364 supralittoral fringe of, 362 supralittoral zone of, 362 tidal pools in, 364 composition of benthos in, 359 coral reef biome of: as found only in tropics, 370 atoll reefs in, 366-367 barrier reefs in, 366-367 biomass in, 367 calcareous as cement for struc- tures of, 366 colors of corals at water’s surface in, 366 dominants in, 367 formation of, 366 fringing reefs in, 366-367 intolerance of organisms of to fresh water, 366-367 440 Index Sea (Continued) lack of predation pressure and color of fishes in, 367 predominant organisms of, 366 repair of damage in, 366 secondary species in, 367 symbiosis in, 366 depth of trenches of, 351-352 difficulties in recognizing com- munity units in, 351 distribution of benthos in, 359 drop in temperature of in tropics during Pleistocene era, 284 extension of littoral zone of, 351- 352 extension of sublittoral zone of, 351-352 increase in pressure at great depths of, 353 internal pressures in animals as balancing external pressures of, 353 level of in post- Pleistocene era, 288 level of during Pleistocene era, 284 light in: and photosynthetic zone, 287 as varying with depth, 283 compensation point of, 284 nekton of: birds of as more numerous in neritic biochore than in open ocean, 291 composition of, 291 distribution of fish in, 291 species of birds in, 291 oceanic biochore of, 280-281 oceanic plankton and nekton biome of: and daily productivity of plankton in Block Island Sound, 361 and Sargassum Community of Atlantic Ocean, 359 as characterized by organisms with life-forms adapted for floating, 359 as complete ecosystem, 059 aspection and changes in species composition of, 359 bathy- and abyssopelogic com- munities of, 359 bioluminescence of deep-sea forms in, 360 depths of nitrogen regeneration in, 360 derivation of forms in greater depths of from intermediate depth forms, 360 epipelagic community of, 359 factors in variations in produc- tivity of plankton of, 361-362 Sea (Continued) filter-feeding mechanisms of organisms in, 361 fish in food chains of, 360-361 food of net zooplankton in, 361 invertebrates in food chains of, 360-361 mesopelagic community of, 359 productivity of phytoplankton, zooplankton and fish of, com- pared, 361 role of bacteria in food chains of, 360 secondary communities in due to variation of species composition of with depth, 359 sharks in food chains of, 361 undissolved organic matter in food chains of, 360-361 uniformity of habitats in as per- mitting ancient forms to per- sist to present, 360 upwelling water currents and food chains of, 361 special structures of animals to permit vision at great depths of, 360 whales in food chains of, 361 oxygen in: adaptations of marine animals for obtaining, 356 factors in concentrations of, 356 sources of, 356 pelecypod-annelid biome of (see also Sea, Coral Reef Biome of): animal burrows in, 364 areas of occurrence of, 364 burrowing as characteristic of animals of, 365 characteristics of crustaceans in, 365 deposit-feeders in, 365-366 dominance in, 366 epifauna in, 365 feeding habits of fish in, 365-366 high tide in and feeding activities, 366 low tide in and feeding activities, 366 microscopic forms in, 365 mud content of shores in, 364 nature of water action in, 364 predominant animals in, 365 principal food chain in, 365-366 productivity of, 366 recognition of secondary com- munities in, 365 respiration of burrowing animals in, 365 role of bacteria in food coactions of, 365 species of plants in, 365 suspension-feeders in, 365-366 Sea (Continued) zonation in, 365 plankton as basic link in food chains of, 351 Salinity of: adjustments of marine organisms to, compared with adjustments of fresh-water organisms, 354 and abundance of populations of organisms, 356 and pH, 354 approximation of, 354 as providing nutrition, 354-356 forces keeping up level of, 356 osmotic regulation in adjustment of organisms to, 357 variations in as dependent on inflow of fresh water, 354 similarity of concentration of ions in to body fluids of some inver- tebrates and origin of life, 354 southern hemisphere composed largely of, 151 substratum of: and adjustments of animals to pounding action of waves, 352 conditions for occurrence of as sandy beach, 352 pelagic deposits in, 352-353 red clay in, 353 seashore burrowing animals in, 352 terrigenous deposits in, 353 succession from to fresh water: and brackish water species, 368 and development of mangroves in tropical regions, 368 and fish migrating between fresh and salt water for spawning, 368-370 and marine species in estuaries, 368 and origination of life, 367-368 and routes by which animals left ocean, 367-368 and sea organisms in marshes, 368 and spawning of eels, 370 and tolerance of marine organ- isms to reduced salinity, 368 most successful animal colonizers in, 367-368 through gradual change from salt water to fresh water, 3638 through salt spray in supralittoral zone, 368 through wind action, 368 summary of factors in geographic distribution of organisms of, 351 temperature of: and movement of currents, 354 as constant in depths, 354 Sea (Continued) in depths, as varying more in tropics than elsewhere, 353-354 effect of on sea-shore animals, 354 on surface, 353-354 thermoclines in, 353-354 tides of: causes of, 352 ebb-tide, 352 effect of on animals in inter-tidal habitats, 352 flood-tide, 352 in bodies of water with narrow connection with sea, 352 nature of away from shore, 352 neap, 352 regularity of, 352 spring, 352 width of neritic biochore of, 351- 352 zoogeographical system for divi- sion of communities of, 370 Sere(s) (see also Subseres): animal, on bare rock, 104-105 bioseres, grouping of into priseres and subseres, 22 changing of bogs into marshes as result of cliseral succession, 92 classifications of, 21 clay: birds in, 113 climax forest in, 112-113 increase in invertebrate species with advance of, 112-113 mammals in, 113 most abundant animals in, 113 stages of plants in, 112 climax as last stage in biosere, 26 clisere in eastern North America, 23-24 clisere in post- Pleistocene era, 288-291 convergence as end of stages of, 102 development of subsere following prisere, 22 eoseres in physiographic succes- sion, 24 floodplain: eastern U.S., variations in tree- stage of, 113-114 example of, Mississippi in Western Tennessee as, 113 formation of through overflow of streams, 113 geoseres, 24-26 and speciation, 26 insects in, 114 plant stages in, 113 similarity of animal life of to that of deciduous forest, 114 (Sere(s) (Continued) toleration of animals of to flood- ing in, 114-115, 115 length of time of various kinds of, 26 maturity of soil profile in relation to, 170 microsere, stages of in dung on grassland, 125 occurrence of bioseres in micro- habitats, 23 of coniferous forest, species composition of, 306 plant, in bogs, 92 plant, on bare rock, 102-104 pond, stages in and characteristic species, 79-81 pond, succession of animal adapta- tions in, 81 rock, development of through salt spray on marine shores, 368 sand: birds in, 110 changes in mores of spiders during, 109 changes in species of spiders in, 109 characteristics of as habitat, 106- 107 feeding-habits of grasshoppers in, 107 greatest change in species com- position in, 107 insects in, 109-110 Lake Michigan, ideal nature of, 105-106 Lake Michigan, stages in, 105 sand-binding plants in, 105 shifting species of ants in, 107-108 species of grasshoppers in, 107 two major ant communities in gradient of, 109 vertebrates in, 109-110, 110 Sessility: as characteristic of some forms of benthos in sea, 359 as consideration in morphological adaptation, 7 as factor in competition for sur- vival, 221 as factor in morphological adapta- tion of trees, 7 of intertidal organisms and dis- persal of motile larval into proper zones, 362-363 of zooid and change in turgescence, 10 Seston, defined, 66 Sewall Wright effect, described, 262 Shelter (see also Habitats; Micro- habitats; Niches): animal response to life-forms of plants in seeking, 248 Index 44] Shelter (Continued) as protection from passing predators, 247 choosing of in vegetation according to strata by mammals, 248 correlation between length of foot- span and kind of vegetation in seeking, 248-249 factors in choice of by birds in same general type of vegeta- tion, 248 from cold, foliage as, 247 from sun, vegetation as, 247 from wind, vegetation as, 247 in vegetation of deciduous forests for warblers, 248 kinds of cover, described, 247 lack of occurrence of species when required kind missing, 249 nests as, for protection of eggs and young against weather, 248 nightfall as, 247 required by animals as protection against weather and enemies, 247 various uses of trees as, 299 Sigmoid Curve: consistent presence of as describer of ecesis in new communities, 161 intrinsic growth rate as limited to early stages of, 220 plotting of, 160-161 Soil (see also Erosion; Forests; Humus; Sand; Soil Types): acidity of, 165 alluvium, defined, 163-164 and carbon cycle, 167 and nitrogen cycle, 166-167 classification of horizons in, 170 clay, porosity of after addition of organic matter, 164 decomposition of humus in, 165 effect of ant movements on struc- ture of, 164 effect of burrowing by small mam- mals on structure of, 164 effect of crayfish excavations on structure of, 164 effect of dead roots on, 164 effect of earthworms on structure of, 164 effect of temperature on formation of, 168-169 effect of water on formation of, 168-169 effect of wind on formation of, 168 effects of animal burrowing on erosion of, 165 elements in required by animals, 167 excretory matter in composition of, 165-166 442 |ndex Soil (Continued) factors in depth distribution of organisms in, 171 formation of hardpan in, 170-171 freezing and thawing of in arctic tundra, 315-316 harmful effects of excess of ele- ments in, 168 horizon, defined, 170 horizons, and occurrence of organisms, 171 humus formation and carbon- dioxide content of, 165 humus formation and oxygen content of, 165 immature profiles of as found in early seral stages, 170 in grass tundra, 316-317 in marine habitats with sandy and muddy bottoms, 364 listing of factors in formation of, 163 loess, defined, 163-164 mature profiles of characteristic of late seral stages of succes- sion, 170 minerals in as determiners of chemical composition and structure of, 163-164 moderation of temperature of by humus, 165 moisture content in, 165 mull layer of humus in, 171 mor layer of humus in, 171 nature of in alpine tundra, 316 non-nitrogenous substances in fresh litter in, 166 of desert as influencing niche segregations of small mam- mals, 339 of tropical biomes and rainfall, 340-341 profile, defined, 170 reactions of large terrestrial animals on structure of, 164- 165 residual, defined, 163-164 sandy, porosity of after addition of organic matter, 164 sequence of events in layering of, 170 subdivisions of ‘‘A’’ horizon of, 170 till, defined, 163-164 wind erosion of, 168 Soil Types (see also Soil): alluvial, 172 chernozemic, 172 desertic, 172 latosolic, 172 mountain, 172 podzolic, 172 saline, 172 Soil Types (Continued) tundra, 172 Solar Radiation (see Light) Speciation (see also Taxonomy) and adaptive radiation, 266 and development of polyploidy, 260 and segregation of related species into different regions or com- munities, 255-256 as adaptive process which explains niche occupation, 256 as occurring with organisms isola- ted during glaciation, 286-288 as result of competition in animal community, 183 as result of isolation of mountain regions, 313 conditions of occurrence among forest-edge forms, 297 conditions of occurrence of, 26 confusion of with elevation of sub- species into species, 258 encouragement of through split of Arcto-tertiary forest into western and eastern sections, 307-308 ethological isolation as step in, 258-259, 259 factors influencing period of time required for, 267 first-generation hybridization as indication of non-occurrence of, 263-264 genetic isolation as step in, 259 geographic isolation as necessary factor in, 260 mechanical isolation as step in, 259 nature of when geographic barriers are only partially effective, 259 occurrence of as result of catas- trophes and cycles, 262 occurrence of through natural evolutionary selection, 264-265 period of time required for, 266- 267 possibility of genetic drift as significant factor in, 262 possible occurrence of when rate of gene glow is slow, 259-260 process of, defined, 257 repeated mutations of genes and possible occurrence of, 263 resulting from natural selection of hybrids as superior to parents, 264 size of habitat as related to, 265 summary of process of, 260 Stimulus: and behavior responses to, 12 and drives, 14 and kineses, 12 and taxes, 12 and tropisms, 12 Stimulus (Continued) as activator of inherited behavior patterns, 13 as causing motile intertidal species to move into proper zones, 362-363 internal, derivation of, 13-14 prefixes employed in identifying kind to which organisms respond, 12 receipt of as always causing definite action, 251 receptors for receiving of, 13 role of in conditioning, 15 sign, as trigger only for particular actions, 14 to migration of birds, 158 ‘trial and error’’ responses to, 13 Stream(s) (see also Biocies, Stream; Rivers): adjustments to current of: by clinging mechanisms, 44-47 by swimming, 47-50 through avoidance, 47 altitude of and organisms in, 52- 53 amount of food consumed by fish in, 55 amounts of dissolved solids in from runoff of falling rain, 65 and creation of flood-plain, 113 as differing from lakes, 59 biomass of, and fertility and chem- ical composition of water, 56 carnivorous species in, 55 changes in shape and size of organisms in, 53 character of bottom of, 42-43 chemical analysis of to determine pollution, 57 comparative biomasses in kinds of, 55 deposition of alluvial soils by, 172 erosion of, 56-57 filter feeding in, 55 formation of beaver meadows in, 172 formation of mud-bottom ponds in, 43 formation of through erosion of headwaters of, 42 headwaters of as poor habitat, 52 herbivorous species in, 55 impermanence of headwaters of, 42 industrial pollution of, 57 introduction of fish into which did not occur there originally, 58 introduction of invertebrate animals into to determine pollution, 57 life- histories of insects of, 53-54 Stream(s) (Continued) major food substances in for stream animals, 54-55 material in suspension in, 52 measurement of primary produc- tivity in, 204 organic pollution of, 57 organisms in and temperature, 52-53 orientation behavior patterns of animals in, 50 oxygen content of, 51 passive conveyance of eggs and spores in, 146 population densities in, 55-56 principal habitats in, 42-43 problems in measuring productivity values in, 206 repopulation of, in fish management, 58 respiratory equipment of animals in, 51-52 responses to bottom of: burrowing, 51 placement of eggs, 51 segregation, 50 support and locomotion, 50-51 salt content of, 43 silting in, 56 size of, relation to distribution of species, 52 stages in aging of, 43 temperatures of, 43 variation of insect population in with season, 55 velocity of current in, 42-43 Strip Censuses: inaccuracies in, 31-32 method for taking, 31-32, 32 Subsere(s): burns: animals in destroyed coniferous forest, 119 kinds of trees which invade destroyed coniferous forest, 117-119 of longleaf pine, 117 function of as forest-edge in North American boreal forests, 306 in abandoned fields: bird succession in, 115 stages in on Great Plains, 115 stages of in Atlantic and Gulf States, 115 stages of in Michigan, 115-116 stages in prarie region of Oklahoma, 115 succession in retarded by feeding animals, 115 succession rapid in, 115 in pasture: native vegetation as eliminator of domestic animal grazing, 116-117 Subsere(s) (Continued) overgrazing as factor in, 117 resistance of other species by sod in, 116-117 role of fires in creating, 117 Succession (see also Seres; Sub- seres): and changing of lake types, 64-65 biotic: convergence in, 22-23 disappearance of old species in, 21 growth and dispersal rates as contributing factors to, 21 influenced by propagules available in area, 21-22 in terrestrial communities, 21 role of bioseres in, 22-23 changes in species composition with increase of fertility during, 207 climatic: as causing changes of bogs into marshes, 92 changes in environment as cause of, 23-24 in eastern North America due to retreat of continental glacier, 23-24 climax stage in, 26 in arid climates, 120 true nature of for tundra regions unknown, 316 early studies in, 5 forest-edge character of grassland vegetation during, 120 from sea to fresh water: and brackish water species, 368 and fish migrating between fresh and salt water for spawning, 368-370 and impoverishment of marine species in approaching fresh water, 368 and spawning of eels, 370 through gradual change from salt water to fresh water, 368 from sea to land: and development of mangroves in tropical regions, 368 most successful animals in, 367- 368 role of salt spray in, 368 from sea to land routes of, 367-368 sea organisms in salt marshes and, 368 wind action in, 368 geologic: dominance of amphibians in, 24 dominance of giant reptiles in, 24 dominance of mammals in, 24-25, 25-26 dominance of marine life in, 24 Index 443 Succession (Continued) insects in, 24, 24-25, 25 man in, 24-25 role of angiosperms in, 24 role of birds in, 24, 24-25, 26 role of conifers in, 24 imbalance in trophic levels during, 207 in abandoned fields, 115-116 in balanoid-gastropod-thallophyte biome of sea, 364 influence of substratum in, 102 in forests, character of food avail- able from decay of logs influ- ential in, 135 occurrence of on all primary and secondary fare areas, 119 of animal seres in ponds, 81 of animal seres in rock, 104-105 of insect species found in dung, 125 of plant hydrosere in ponds, 79-81 of plants and equilibrium of habitat and community, 163 of plant seres in rock, 102-104 of spider communities in sand sere, 109 physiographic: inundations of sea as causing, 24 mountain erosion in, 24 plant, retarding of to prevent disappearance of habitat, 91 process of, explained, 21 stages in and maturity of soil profile, 170 stages of in humid climates, 120 Swamp(s): as early seral stage of magnolia- oak forest, 294 differences of from bogs, 92 forest, invasion of by swamp facies, 81 forest, stages in sere of dependent or climate of region, 81 forest, trees in, 79-81 shrubs of, 79-81 Symbiosis (see also Commensalism; Mutualism; Parasitism): defined, 176 in coral reefs, 366 Symmetrical muscles, and locomo- tion, 12-13 Synecology (see Ecology, Subdivis- ions of): aT Taxes, defined, 12 Taxonomic Composition of Com- munities: average populations in as affected by number of microhabitats, 255 444 Index Taxonomic Composition of Com- munities (Continued) decrease in variety of species in with extreme or impoverished habitats, 255 ecological equivalents in, 256 few species, as furnishing greatest bulk of population in, 255 great variety in with fertile or favorable habitats, 255 segregation of related species into adjacent habitats and communi- ties in, 255-256 wide variety in during early colon- ization of bare area, 255 Taxonomy: allopatric species defined in, 258 as basis for subdivisions of biome, 276 differentiation in and size of area, 272 difficulty of assigning rank to subspecies in, 257-258 difficulty of recognizing species in, 257 fossils of animals in different geological strata grouped as same species in, 257 isolation of subspecies and con- sequent evolution into species, 258 populations in, defined, 257 position of cloves in, 264 sibling species in, defined, 258 species in, defined, 257 subspecies in, defined, 257-258 sympatric species in, defined 258 use of biotic province concept in study of, 272 Temperature(s): adjustments to by coniferous forest animals in winter, 309 adjustments to by homoiotherms, 98-99 adjustments to by poikilotherms, 98 aggregation by animals as method of coping with, 175 amelioration of since nineteenth century and increase in abun- dance of species, 234 and arctic and antarctic marine fauna, 370 and depth distribution of soil animals, 171 and hibernation, 137 and increased surface area in plankton organisms, 60 and metabolic rate, 10 and poleward limits of warm- water fauna, 370 and reversal of dominance, 246 Temperature(s) (Continued) as affecting absence of animal life from deep water, 42-43 as affecting coaction between host and parasitoid, 227 as creating inaccuracies in sweep net sampling of insects, 37 as factor in life-cycle of soil animals in forest, 131-132 as factor in morphological varia- tion among fish, 7 as lowered by advancing ice of Pleistocene era, 284 as related to increase in elevation, 313 atmospheric, of arctic tundra, 315 average in temperate deciduous forest biome in North America, 293 body, control of by animals other than birds and mammals, 98 body, rise in and increase of physiological functions, 98 compared, of deciduous and coni- ferous forests, 121 compared on north and south facing slopes of grasslands, 124 constant body, development of birds and mammals, 98 correlation between and sunspots, 243 effect of on fecundity of organisms, 211 effect of on population density of insects, 230 effect of with low relative humidity on warm-blooded animals, 10 high, adjustments to by desert animals, 338 high, as critical environmental factor in deserts, 338 importance of in controlling distri- bution of animals according to C.Hart Merriam, 274 influence of on body structure, 9 influence of on location of species, 9 influence of on migration, 9, 158 influence of on number of young, 9 influence of on pigmentation of animals, 9 in grasslands, 324-325 in Great Basin desert, 332-333 in lakes: and annual heat budget, 63 deep, and circulation of water, 61 dimictic, 63 during various seasons, 61-63 effect of on distribution of plank- ton, 69 monomictic, 63 of the first order, 63 of the second order, 63 Temperature(s) (Continued) of the third order, 63 slowness of changes in, 63 in sand dune habitat, 106, 106-107 in winter, methods of adjustment to, 99 lethal effect of, 10 marine: and movements of currents, 354 effect of variations in on seashore animals, 354 of depth waters, 353-354 of surface waters, 353-354 thermoclines in, 353-354 of alpine tundra and development of invertebrates, 322 of chloroplast in photosynthesis, 12 of coniferous forests, 301-302 of deserts as climatic barrier to dispersal, 148-149 of grassland and forest compared, 121 of ponds, 79 of streams, 43, 52-53 of tropical biomes, 340 of water, and selection of micro- habitat according to, 246 of woodland biome, 310-311 range of in alpine tundra, 316 regulation of by bees, 174-175 relation of to reproduction of cisco, 16-77 seasonal, and adjustments to terrestrial habitats, 101-102 snow as insulating factor in, 240 soil, moderation of by humus, 165 soil, of arctic tundra, 315-316 success in competition dependent on between flatworms, 252 terms used to indicate range of tolerance to, 11-12 terrestrial, necessity of organ- isms to adjust to, 98 variation in effects of on organism at different times and condi- tions, 10 vertical gradient in of deciduous forests, 122, 122-123 Territories (see also Birds): advertisement of through sounds, 299 benefits to animal of maintaining, 186 control of populations in, 231 decrease in size of and competi- tion, 221-222 defense of by simple advertisement of possession, 185 defined, 184 differences of from ‘‘areas,’’ 184-185 Territories (Continued) direct competition in defense of, 182-183 establishment of best developed in birds, 184-185 high population densities in and lowered fecundity, 223 home range in, defined, 184 home ranges in, as providers of breeding locations, 186 increase in number of birds in, and effects of competition, 221-222 procedures for determining home range in, 185 Tertiary Era (Continued) separation of Arcto-tertiary forest during into North American and Eurasian biociations, 307 separation of North American coniferous forest during into eastern and western sections, 307-308 Sierra Nevada and Cascade Moun- tains during, 281 similarity of North America floras and faunas as due to distribution of Arcto-tertiary flora of, 282 temperate units of Arcto-tertiary flora in, 282 proclaiming of by non-breeding bird Thermocline: populations in forests, 135 recognition of in nesting of birds as technique in animal ethology, 6 removal of nesting birds from, and consequent re-population, 215 size of and space requirements of species, 230 species establishing, 184-185 Tertiary Era: appearance of deserts during, 334 area of growth of Arcto-tertiary flora during, 282 as period of evolution of tundra fauna, 318 boreal units of Arcto-tertiary flora in, 283 changes in forests of, 282 coast ranges during, 281 contact of Asiatic deciduous forest with North America during, 299 dispersal of Madro-tertiary flora of, 283 early flora of as forming modern vegetation types, 281-282 elevation of mountains during, and beginnings of zonation of vegetation, 313 evolution of grasslands in, 326, 328 geological record of plants of, 281- 282 interior of North America during, 280 movement of Arcto-tertiary flora after drying and cooling of climate in, 282 neotropical flora of, 282 oe of Madro-tertiary flora of, 83 peneplanation during, 280-281 present fauna of Indo-West Pacific marine region as possibly ex- pressing profusion of forms during, 371 as permanent feature in mid-depths of temperate and subtropical waters, 353-354 defined, 61-62 seasonal, in sea, 353-354 Threshold: and efficient functioning of organism, 10 and lower limit of tolerance, 10 and upper limit of tolerance, 10 defined, 10 Toleration: climatic succession occurs as result of reaching limits of, 23-24 of intertidal animals to exposure and submergence and zonation, 362-363 of low oxygen and high carbon dioxide content in marine waters by burrowing animals, 365 of pH changes by lake organisms, 66 terms used to indicate extent of, 11-12 variation among species in limits of to same factor, 11-12 variations in activities of species conditioned by stress rather than limits of, 11 variations in to temperature by different species, 98 Trapping: as method in determing existence of home ranges of animals, 185 use of kill-traps in for censusing small mammals, 35-36 use of live traps in for censusing small mammals, 36 Trophic Levels (see also Energy; Productivity): and biomass, 198-199 and rates of metabolism, 199 animals distinguished as hetero- trophic in, 196 balanced, conditions prevailing during, 206-207 |ndex 445 Trophic Levels (Continued) consumed and wasted energy in, 202 diversity of species and closeness of to source of energy, 196 energy lost from as used by saprovores, 202 fish of coral reef as represented in, 367 herbivores or primary consumers as second of, 196 higher, transfer of energy to through predation, 201 larger carnivores or Tertiary consumers as fourth of, 196 of consumers not shaply defined, 196 plants distinguished as autotro- phic in, 196 position of omnivores in, 196 position of saprovores in, 196 position of species in and reduc- tion of productivity yield, 207 producers as lowest of, 196 pyramids of numbers arranged by and food co-actions, 198 rates of production and growth at, 1199 rates of reproduction of organisms in and protective devices, 196- 198 replacement of individuals that meet non-predatory deaths in, 202 simplification of food web into, 196 smaller carnivores or secondary consumers as third of, 196 solar radiation as basic source of energy for, 200-201 with balanced populations, total net production of energy of consumed by predators of higher, 202 with unbalanced populations and predatory consumption of energy, 202 Tropical Biomes (see also Forests; Tropical Vegetation): agriculture in, 349-350, 350 as harsh and exacting to man biologically, 349 biociations of: African savanna, 344 African tropical forest, 344 American tropical forest, 344 Australian savanna, 344 Indo- Malayan tropical forest, 344 South American savanna, 344 cattle-raising in, 349-350 distribution of, 340 diversity of rainfall in, 340-341 dry season in, 340-341 446 |ndex Tropical Biomes (Continued) even temperatures of, 340 humidity in, 341 hunting and fishing in, 350 length of day and night in, 340 occurrence of climax savanna in, 341 occurrence of deciduous forest in, 341 tropical forests: ability of animals of to hang from trees, 346 Andes, derivation of fauna of, 344-345 arboreal living habits in of animals normally ground- dwellers, 349 as possible origin for most modern groups of plants and animals, 344 birds of, nesting in holes in trees by, 346-347 daily rhythms in animal activities in, 348 developmental period of cold- blooded animals in, 347 epiphytes in as habitats for small water organisms, 349 fauna of rich in species, 345-346 feeding of sloths and ant-eaters in, 348 greatest variety of animals as occurring in floor of, 348-349 harsh and exacting nature of competition in, 346 nectar and pollen feeding in, 348 no definite period of dormancy or migration in, 348 reproduction of birds in, 347-348 sensitivity of animals of to sun, 349 size of cold-blooded animals in, 347-348 trail-breaking by animals in, 347 wet season in, 340-341 wood- eating insects in, 347 Tropical Vegetation (see also Tropical Biomes): as essentially a continuum, 341 broad-leaved evergreen forest: aspection in, 342 comparisons of, 342 described, 342 dominants of, 342 high humidity of, 342 light intensities in, 342, 342-343 strata of, 342 undergrowth of not thick jungle, 342 variety of species of, 342 classified according to aspects of physiognomy important to animals, 343 Tropical Vegetation (Continued) deciduous forest, 342 epiphytes: described, 343 importance of in community dynamics, 373 in tropical America and Africa compared, 373 water in clumps of leaves of as containing insects, 349 montane rain forests, 343 no lowland climatic grassland in, 371 Savanna: as forest-edge community, 343 defined, 341-342 extensiveness of, 341-342 factors in increase of, 341- 342 gallery forest of, 341-342 grasses of, 341-342 sedges of, 341-342 Tropism, defined, 12 Tundra: alpine: acclimation of animals to low temperature of, 322 acclimation of animals to strong winds of, 322 as lacking permafrost in subsoil, 316 atmosphere of, 316 birds of in North America, 321- 322 dispersal of North American species of, 322 earlier activity of animals in spring on southern slopes of than on northern slopes, 323 endemic species in, 317 extension of, 317 factors in small number of characteristic species of in North America, 321 habits of pikas in, 322 high incidence of grasshoppers in, 322-323 impoverished fauna of ponds of, 322 limitations in size of, 316 location of Tibetan faciation of, 321 low oxygen pressure at high altitudes of and mammal adjustments, 323 mammals of in North America, 321 nature of soil of, 316 occurrence of krumholz in, 317 overlap of animal species of with arctic species, 317 overlap of Tibetan fauna of with that of other biociations, 321 Tundra (Continued penetration of grassland species into, 317 precipitation in, 316 protection from cold by animals of, 322 range of temperatures in, 316 rarity of reptiles and amphibians in North American, 322 rugged terrain of compared to that of Arctic, 316 species of as dispersing from Tibet Plateau, 321 species of Tibetan faciation of conspecific with those of North America, 321 taxonomic composition of vege- tation of, 317 uses of by man, 323 white coloring of animals in, 322 antarctic: absence of plankton in during winter, 319 abundance of aquatic organisms in, 319 limited fauna of, 319 nutritive salt accumulation and population abundance in, 319 shores of as more abundant food areas in, 319 arctic: animal adjustments of to severe cold, 320 aspection in, 320 bush or mat type, 316 colonization of alpine slopes by species of as interrupted by forests, 317 dispersal of species of southward in post- Pleistocene era, 317 effects of freezing and thawing on soil of, 315 Eskimos in, 321 failure of breeding in, 320 fearlessness of animals of before man, 321 flight songs of birds in, 320-321 food coactions in among herbi- vores, 320 freezing of lakes of during winter, 315 grass type, 316-317 importance of cryoplanation in, 315 Lapps in, 321 length of photoperiods in, 316 lichen- mass barrens type, 316- 317 migration in, 320 mingling of species of with grass- land species, 317 oscillations in abundance of animals in, 320 Tundra Arctic (Continued) periods of rest of animals in during summer, 320-321 permafrost in, 315 perpetual snow and ice type, 316-317 precipitation in, 315 soil temperature below surface, 315-316 soil temperature on surface in spring and animal activity, 315-316 temperature in, 315 use of by man, 321 white coloration of animals as protective device in, 320 arctic and alpine biociations of, compared, 319 typical extension of, 315 arctic biociation of: aquatic nature of birds of, 319 as including both North America and Eurasia, 318 birds in found in North America and Eurasia, 318 birds in North American faciation of, 318-319 evolution of fauna in from forms able to tolerate cold climates, 318 insects found in, 319 invasion of Eurasian forms into North America and evolution of fauna of, 318 lake animals of, 319 mammals in common to both Eurasia and North America, 318 most abundant mammals in, 318 pond animals of, 319 richness of species of in North America, 318 as representing southward disper- sal of Arcto-tertiary flora, 313 continuous distribution of species of from North to Rockies in post- Pleistocene era, 317 description of vegetation of, 316 factors in composition of seral vegetation of, 316 flora of possibly segregates of Arcto-tertiary flora, 317 kinds of vegetation in, 316 nature of climax in unknown, 316 perrenial nature of plants of, 316 regions of during interglacial periods, 317 seed germination in, 316 species characteristic of in bio- ciation of North American boreal forests, 306 survival of flora of during Pleisto- cene glaciation, 317 U Ubiquitous Species: as basis for ecological classifica- tion, 20 of microscopic animals in muddy- bottom marine habitats, 356 of oceans and polar regions, 370 Vv Variation(s) in Population Charac- teristics (see also Populations): and Baldwin effect, 261 and exploitation of environment, 261-262 and genetic homeostasis, 261- 262 and natural selection, 264-265 as caused by genotypes of survivors of population reductions, 262 due to occurrence of mutations, 262-263 expression of through gene com- binations, 262 genetic drift or Sewall Wright effect, defined, 262 having adaptive significance to food consumed at genus level, 262 inbreeding and consequent restric- tion of genotypes as causing, 262 occurrence of over many genera- tions as causing species identity, 260 of asexual and self-fertilizing forms, 264 polymorphism, described, 261 preadaptation as, 266 relativity in adaptivity of, 265- 266 through hybridizations, 263-264 through panmixia, 261-262 through phenotypic adaptations not genetically heritable, 261 through polymorphism, conditions of occurrence of, 261 Vertebrates: as successful colonizers of land, 367-368 cold-blooded, dispersal of, 151- 153 competition between individuals at low population level among, 231 conditions of susceptibility to predation among, 231 in antarctic, 319 in forest soil, 135 in grassland communities, 125 in sand seres, 109-110 Index 447 WwW Warm-blooded Animals: ability of to live in cold climates, 9-10 compensation for loss of heat energy by, 201 effect of temperature on and latitudinal distribution of, 98 existence energy of, 98-99 fecundity of and weather, 211 high rate of physiological function- ing of, 98 pace of in tropical rain forests, 347 productive energy of, 98-99 relation of energy balance of to air temperature, 98-99 size of litter of and parental care, 211 survival of at high temperatures, 10 Water Conditioning: as producing overcrowding of tadpoles, 224 described, 172-173 favorable effects of, 173 favorable, factors involved in production of, 173 harmful effects of, 173 heterotypical, described, 172-173 homotypical, described, 172-173 through release of calcium by flatworms, 175 Wildlife Management: development of as branch of ecology, 6 in trapping for fur, 232-233 practical value of age ratio in, 216-217 Wind(s): action of in lakes, 61 and movement of sand, 105 as continuous in deserts, 332 as instrument of passive convey- ance of eggs and spores, 146 as transporter of soil, 163-164 circulation of in deciduous forests, 121 condition of in North America 60-70 million years ago, 281 448 Index Wind(s) (Continued) conditions of ponds protected from, 79 direction of and glaciation in Pleistocene era, 284 erosion, of desertic soils, 172 influence of on lake currents, 61 in tropical biomes, 340-341 mild, use of by spiders as means of dispersal, 146-147 populations of insects in, 147 role of in formation of plant seres on rock, 102-104 strength of in alpine tundra and closeness to ground of insects and birds, 322 strength of in grassland, 324 strong, as dispersal mechanisms for organisms, 146-147 vegetation as shelter from, 247 velocity of in beech- maple forest, 121-122 vertical gradients of velocity of above prairie grasses, 122 Woodland: areas of occurrence of, 310 climate of, 310-311 consideration of as ecotone, 311 description of, 310 invertebrates in, 311-312 paleo-ecology of, 311 plant associations of, 311 species of birds in, 311-312 species of mammals in, 311 Zonation: and succession from sea to fresh- water and land, 367 animals in intertidal habitats as guided by adjustments to, 362- 363 in Great Smoky Mountains of eastern Tennessee, 295 in New York’s Catskill Mountains, 295 in pelecypod-annelid biome of sea, 365 Zonation (Continued) in Western Mountains, 314 of vegetation: and dispersal of Tertiary flora, 313 causes for development of, 295 on ocean side of coral reefs, 366- 367 role of climate in, 313 subdivisions in of intertidal habitats, 362 Zoogeography: as foundation of biome-biociation system, 279 as one of two major factors in distribution of organisms, 279 defined, 279 faunistic systems in, 272-276 of marine communities: and abyssal-benthos, 370 and arctic and antarctic faunas in pelagic biome, 370 and circumtropical distribution of species, 370 and distribution of continental shelf fauna as limited by tem- perature, 370 and divisions of continental shelf fauna, 370 and fauna of arctic and antarctic regions, 371 and fauna of North Atlantic region, 371 and fauna of North Pacific region, 371 and fauna of West Indian subregion of Atlanto- East Pacific region, 371 and richness of tropical sub- fauna, 370 and uniformity of pelagic organ- isms in different oceans, 370 and variety of abundance of animal life in Indo- West Pacific region, 371 and warm-water fauna in pelagic biome, 370 as based on faunas, regions and subregions, 370 subject matter of, 26 Species Index A aardvark, Orycteropodidae (mammal), 271 acacia, Acacia greggii, A. constricta (plant), 333 Acanthocephala (parasitic worms), 178-179, 180 Acmaea testudinalis (marine limpet), Table 28-1 actinomycete, Thallophyta (plants), 166-167, 171, 289 adder, death, Acanthophis (snake), Table 25-1 adder, puff, Bitis avielans (snake), Table 25-1 agouti, Dasyproctinidae (mammal), Table 27-2, 270 Alaska-cedar, Chamaecyparis nootkatensis, 302 Alcyonaria, Anthozoa (corals), 367 alder, Alnus, 79-81, 85, Table 7-6, 282, 297, 302 alderfly, Sialidae, 85, Table 3-1, Table 6-2, Table 7-1, Table 13-1 algae, Thallophyta, 364, 366, 367 algae, blue-green, Myxophyceae, 22, 64, 66-67, 69, 77, 87, 166-167 algae, brown, Phaeophyceae, 363 algae, green, Chlorophyceae, 66-67, 77, 87, 177, 356- 357, 365, 366, 368 algae, red, Rhodophyceae, 363 alligator, Crocodylidae (reptile), 282 alligator, American, Alligator mississippiensis, 298 Ambystoma maculatum, Spotted salamander, 90 Ambystoma texanum, small-mouthed salamander, 90 Ambystoma tigrinum, tiger salamander, 90 Amphibia (frogs, toads, salamanders), 24, 38, 74, 92, Table 3-1, 97-98, 99, 100, 101, 114-115, 151-153, 155-156, 173, 184-185, 286-288, 298, 300, 319, 322, 336, 338, 354 Amphipoda (entomostracans), 70-71, 71, 82, 87, 88, 90, Table 3-1, Table 6-2, Table 7-1, 150, 319, 357, 358- 359, 360, 363, 365-366, 368, 370 anchovies, Engraulidae (fish), 361 anemone, sea, Actinaria (coelenterate), 188, 360, 363 ani, smooth-billed, Crotophaga ani, 298 Annelida, Oligochaeta (worms), fresh-water, 43, 57, 64, 70-71, 71, 74, 76, 82, 87-88, 90,Table 3-1,Table 7-1, 147, Table 13-1, 177, 178, 204-205, 319, 322, 349 Annelida, Oligochaeta (worms), land, 38, 38-39, 71, 82, 96, 97-98, 99, 113, 114, 115,124, 125, 130, 132-134, Table 8-5, 164, Table 9-9, 171, 310, Annelida, Polychaeta (worms), marine, 188, 353, 354, 354-356, 357, 358-359, 360, 362, 363, 364, Table 3-1, 370 Anomia aculeata (mollusk), Table 28-1 ant, Formicidae, 99-100, 104-105, 107-109, 113, 124, 134, 135, 136, 164, Table 9-7, Table 9-8, 174, 176, 177, 178, 179, 194, 198, 245-246, 258, 310, 311-312, 313, 319, 322, 337, 338-339, 348-349 ant, army, Eciton, Fig. 27-6, 347, 348 ant, harvester, Pogonomyrmex occidentalis, 115, 337 ant, honey, Myrmecocystus mexicanus, 337 ant, leaf-cutting, Atta, 346-347, 347 ant, stinging, Azteca, 346-347 ant, wood, Formica rufa, 184-185 anteater, Myrmecophagidae (mammal), Table 27-2, Table 27-4, 270, 348, 349 anteater, scaly, Manidae (mammal), 271 antelope, Bovidae (mammal), Table 25-1, 187, 271, 271- 272, 272, 344 antelope (see pronghorn), 164-165, Table 9-5, Table 24-1 ape, Pongidae (primate mammal), 349 Index 449 aphid, Aphididae plant louse), 149, Table 9-7, 187, 194, 196, 227, 228, 310, 313 aphid, cabbage, Brevicoryne brassicae, 160 Aphytis mytilaspidis (hymenopteran insect), 227 Apoda, Amphibia (caecilians), 271-272 Arachnida, see spiders, mites armadillo, Dasypodidae (mammal), 270 ascidian (see Tunicata) ash, Fraxinus (tree), 79-81, 116-117, Table 7-6, 282 ash, white, Fraxinus americana, 113-114 aspen, quaking, Populus tremuloides, Fig. 8-14, 117- 119, 290-291, 297, 302 Aspidiotus (scale insect), 227 ass, wild, Equus hemionus (mammal), Table 25-1 Aster (plant), Fig, 8-11, 168, 179 aster, Aster ericoides, 115 auk, Alcidae (bird), 319 B baboon, Cercopithecidae (mammals), 271, 347 backswimmer, Notonectidae (insects), 50, 84-85, 87, Table 3-1, Table 7-1 bacteria, Thallophyta (plants), 65, 71, 73, 74, 87, Table 3-1, 164, 166, 166-167, 171, 172, 176, 177, 179, 181, 187, 195, 196, 206, 228, 234-235, 235, 289, 356-357, 357, 360, 365 badger, Taxidea taxus (mammal), Fig. 25-1, 128-129, 164, Table 9-4, 321, 326, 335 Balanoglossus (hemichordate), 357 Balanus,(see barnacle, rock) baldcypress, Taxodium distichum (tree), 282 bandicoot, pig-footed, Choeropus castanotis (mammal), Table 25-1 barnacle, Cirripedia (crustaceans), 352, 356, 363 barnacle, acorn, Balanus, Fig. 16-5, 215-216, 362-364, 363, 368 barnacle, goose, Mitella, 363 barracuda, Sphyraenidae (fish), 178 bass, Centrachidae (fish), 54, 55, 74-75, Fig. 5-1 bass, largemouth, Micropterus salmoides, 64, 90, 91, Table 5-2, Table 7-3, 207 bass, rock, Ambloplites rupestris, Table 7-3 bass, smallmouth, Micropterus dolomieui, 50, Table 5-2 bass, striped, Roccus saxatilis, 43-44, 353, 368-370 basswood, Tilia (tree), 282, 298-299 basswood, American, Tilia americana, Fig. 9-4, 105, 112, 113-114, 121, 290-291, 294 basswood, white, Tilia heterophylla, 294 bat, Chiroptera, 101, 155, 165-166, 174, 188, 270, 299, 300, 348 bat, hoary, Lasiurus cinereus, 158, 303, 309 bat, red, Lasiurus borealis, 158 bear, Ursidae, 271, 271-272 bear, black, Euarctos americanus, 140, 215, 295, 296, 300, 302-303, 321 bear, grizzly, Ursus horribilis, 140, 215, 307, 311, 318, 320, 321 bear, polar, Thalarctos marvitimus 318, 319, 319-320, 320 beaver, Castor canadensis, 43-44, 87, 92, 172, 204-205, 215, 228-229, 306 Index 450 beaver, giant, Castoroides ohioensis, 285 beaver, mountain, Aplodontia rufa, 307 bee, Apoidea, Hymenoptera, 109, 124, 164, Table 9-7, 174, 174-175, 179, 194, 322-323 beech, Fagus (tree), 282, 298-299 beech, American, Fagus grandifolia, 23-24, 81, Fig. 3- 3, 105, 113-114, 115, 121, Table 7-6, Table 20-1, 290-291, 291, 294 beech, gray (var. of Fagus gvandifolia), 295 bee-eaters, Meropidoe (birds), 271-272 beetle, Coleoptera, 57, 83-84, 97, 99-100, 109, 113, 114, 115, 124, 125, 134-135, 135, 136, 137, 147, 148, Table 9-7, Table 13-1, 177, 179, 190, 194, 198, 204, 227, 246, 285, 310, 311-312, 313, 319, 320, 322, 347, Table 9-6 beetle, Tenebrionidae, 337, 337-338 beetle, bark, Scolytidae, 135, 187, 309-310 beetle, bark, Pityogenes knechteli, Fig. 23-7, 303 beetle, bronze tiger, Cincindela scutellaris, 110 beetle, crawling water, Haliplidae, 83-84, 87, Table 3-1 beetle, dermestid, Dermestidae, 109 beetle, diving, Dytiscidae, Hydrophilidae, 83-84, 84-85, 85, 87, 89, Table 3-1, Table 7-1, 319, 320 beetle, Donacia, 85 beetle, flour, Tvibolium, 159-160, 161, Table 15-7, Table 16-6, 175, 208, 215-216, 217, 223, 246, 247-248, 254 beetle, green tiger, Cincindela sexguttata, 110 beetle, ground, Carabidae, 85, Fig. 9-11, 109, 114, 134 beetle, histerid, Histeridae, 109, 134 beetle, long-horned, Cerambycidae, 135, 309-310 beetle, May, Scarabeidae, 125, 164 beetle, Passalus cornutus, Fig. 9-8 beetle, potato, Leptinotarsa decemilineata, 196 beetle, riffle, Psephenidae, Dryopidae, Elmidae, 46-47, Fig. 5-7, Table 3-1, Table 5-3, Table 6-2 beetle, rove, Staphylinidae, 109, 114, 134 beetle, snout, Curculionidae, 109 beetle, spotted lady, Megilla maculata, 174 beetle, tiger, Cicindelidae, 85, 109, 112, 114, Table 8-4, 349 beetle, 12-spotted cucumber, Diabrotica 12-punctata, 114 beetle, whirl-i-gig, Gyrinidae, 50, 81, 84-84, Table 3-1, Table 7-1 beetle, white ground, Geopinus incrassatus, 109 beetle, white tiger, Cicindela lepida, Fig. 8-8 beggar’s-tick, Bidens (plant), 113 Belostomidae, water bugs, 83-84 benthos, 40-41, 66, 69-73, 75-76, 82-84, Table 6-3, 206, 208, 359, 360, 362-364 bighorn (see sheep, mountain) birch, Betula (tree), 121, 282, 165, Table 20-1 birch, dwarf, Betula pumila, 92 birch, paper, Betula papyrifera, 117-119, 302 birch, yellow, Betula alleghaniensis, 294, 302 bird, Aves, 9, 24, 24-25, 26, 36-37, 74, 75, 86, 87, 90, 91, 92, Fig. 9-15, Fig. 10-13, Fig. 13-1, Table 3-1, 96, 97, 97-98, 98, 99-100, 101, 102, 113, 119, 106, 106-107, 135, 137, 138, Table 8-2, 146, 148, 151, 151- 153, 155, 155-156, 156, 156-157, 157-158, 158, 165- 166, Table 9-7, Table 9-11, Table 15-3, Table 16-5 174, 177, 178, 180, 181-182, 182-183, 183, 184-185, 185, 188, 189, 192, 194, 195, 198, 204-205, 210, 211, bird, Aves (Continued) 212, 215, 215-216, 216, 226, 228-229, 231, 234, 237, 247, 248, 249, 251, 252, 255, 268, 270, 272-273, 286- 288, 298, 299, 299-300, 307, 308, 309, 318, 319, 320, 320-321, 321, 322, 323, 330, 338, 338-339, 339, 344, 346, 346-347, 347-348, 348, 359, 361, 364, 365, 365- 366, 366, 368 bird, gallinaceous, Galliformes, 187, 188-189, 190, 216, 231, 237 bird-of-paradise, Paradisaeidae, 270 bison, eastern, Bison bison pennsylvanicus, 297-298 bison, European, Bison bonasus, 299 bison (fossil), Bison latifrons, 285 bison, plains, Bison bison bison, 19, Fig. 25-1, 125, 127- 128, 158, 164-165, Table 9-4, Table 9-5, Table 25- 1, 175, 187, 215, 325, 326, 328, 330, 335, 337 bittern, least, Jxobrychus exilis, Table 7-4 bittersweet, Celastrus scandens (vine), 190 blackberry, Rubus allegheniensis, 112, 116-117 blackbird, Brewer’s, Euphagus cyanocephalus, 297-298 blackbird, European, Turdus merula, 247 blackbird, redwinged, Agelaius phoeniceus, 368, Table 7-4 blackbird, rusty, Euphagus carolinus, 306 blackgame, Lyrurus tetrix (bird), 237 bladderwort, Utricularia (plant), 79-81, 183, 187 Blepharoceridae, net-winged midges, 46 blight, chestnut, 19 blow-fly, sheep, Lucilia, Chrysomyia, 223 bluebird, eastern Sialia sialis, 253, 297-298, Table 7-4 bluebird, mountain, Sialia currucoides, 307 bluebird, western, Sialia mexicana, 311-312 bluegill, Lepomis macrochirus (fish), 91, 92, Fig. 7-4, Table 7-3, 205-206 bluestem, big, Andropogon gerardi (grass), Fig. 11-4, 120 bluestem, little, Andropogon scoparius (grass), Fig. 11- 4, 120 boar, wild, Sus scerofa (mammal), 299 boatman, water, Corixidae (insect), Table 3-1, Table 6- 2, Table 7-1, Table 13-1, 226 bobak, Marmota bobak (mammal), Table 25-1 bobcat, Lynx rufus, Table 9-4, 195-196, 295, 296, 307, 311, 312, 335 bobolink, Dolichonyx oryzivorus (bird), 125, 326 bobwhite, Colinus virginianus (bird), Fig. 9-16, Fig. 16- 1, Fig. 16-2, Fig. 16-3, 115-116, 140-141, 142-143, 149, Table 16-4, 175, 190, 195, 225, 240, 244, 297- 298, Table 8-6 bontebok, Damaliscus pygorgus (mammal), Table 25-1 borer, European corn, Pyyausta nubilalis (insect), Fig. 10-4 borer, sugar cane, Rhabocnemis obscura (insect), 227 borer, wood, Buprestidae (insect), 309-310 bowfin, A mia calva (fish), 85, Table 7-3 boxelder, Acer negundo (tree), 113-114 brant, Branta bernicla (bird), 365 briars (see raspberry, blackberry) brittle-star, Ophiuroidea (echinoderm), 357, 365, 366, 367, 370 broadbills, Eurylaimidae (birds), 271-272 bromeliad, Bromeliaceae (epiphytic plant), 349 Bryozoa, fresh-water, 8, 61, 70, 90, 105, Table 3-1, Table 6-2, 177 Bryozoa, marine, 363, 364 bubal, Bubalis buselaphus (mammal), Table 24-1, Table 25-1 buckbean, Menyanthes trifoliata (plant), 92 buckeye, Ohio, Aesculus glabra (tree), 113-114 buckeye, yellow, Aesculus octandra (tree), 294 budworm, pine, Choristoneuva pinus (moth larva), 259 budworm, spruce, Choristoneura fumiferana (moth larva), Fig. 9-10, 125, Table 15-4, 187, 195, 230, 243, 259, 309-310 buffalo, bigmouth, /ctiobus cyprinellus, Table 5-2, Table 7-3 buffalo, Castastomidae (fish), 74-75, 85 buffalo (see bison) buffalo, African, Syncerus caffu (mammal), 344, 347 bug, Hemiptera, 57, 83-84, 87, 88-89, 115, 124, 135, Fig. 7-3, 147, Table 9-6, Table 9-7, 194, 196, 204, 227, 319, 322 bug, Capsidae, 222 bug, chinch, Blissus leucopterus, Fig. 17-1, 187, 237, 241-242, 244 leaf-legged, Coreidae, Fig. 9-9 shore, Saldidae, 85, 109 spittle, Cercopidae, 109 stilt, Neididae, Fig. 9-9 bug, stink, Pentatomidae, Fig. 9-9 bug, tarnish plant, Lygus oblineatus, 114 bug, toad, Gelastocoridae, 85 bulbul, Pycnonotidae (birds), 271-272 bullhead, Ictaluridae, 54, 57, 74-75, 85, 90 bullhead, black, /ctalurus melas, Table 5-2 bullhead, brown, /ctalurus nebulosus, 92 builsnake, Pituophis melanoleucus, 125, 128-129, Table 24-1 bulrush, Scirpus (plant), 79-81, Table 7-6 bumblebee, Bombidae (insect), 319, 322 bunting, indigo, Passerina cyanea (bird), 297-298, Table 8-6 bunting, lark, Calamospiza melanocorys, 151, 326 bunting, lazuli, Passerina amoena, 312 bunting, painted, Passerina ciris, 298 bunting, snow, Plectrophenax nivalis, 318, 319, 321 bur-reed, Sparganium (plant), 79-81 bush, creosote, Covillea glutinosa, Fig. 26-3, 325, 333 bushtit, common, Psaltriparus minimus (bird), 311-312 bustard, Turnicidae (birds), 214 buttercup, Ranunculus (plant), 79-81 butterfly, Lepidoptera, Fig. 9-14, 97, 101, 135, 137, 148, 158, Table 9-7, 192-194, 194, 251, 264, 265, 266, 319, 322, 322-323, 349, Table 9-6 butterfly, monarch, Danaus plexippus, Fig. 13-6, 158, 194, 299-300 butterfly, swallow-tailed, Papilionidae (insects), 345-346 butterfly, viceroy, Limenitis archippus, Fig. 13-6, 194 butternut, Juglans cinerea (tree), 113-114 buttonbush, common, Cephalanthus occidentalis, 79-81, Table 7-6 bug, bug, bug, bug, Cactoblastis, moth, 227 cactus (plant), 25-9, 117, 325, 333, 333-334, 334, 338 cactus, organ pipe, Lemaireocereus thurberi, Fig. 26-3 |ndex 45] caddisfly, Trichoptera, 43-44, 46, 47, 50, 51, 52, 52- 53, 53-54, 55, 85, 90, Table 3-1, Table 5-3, Table 5-4, Table 5-5, Table 5-6, Table 6-2, Table 7-1, Table 13-1 caddisfly, Hydropsychidae, 46, 53, 55, 82-83, Fig. 5-5 camel, Camelidae (mammals), 272, 285 Campodeidae, Thysanura (insect), Fig. 9-6 capercaillie, Tetrao urogallus (bird), 237 capybara, Hydrochoerus (mammal), Table 27-2 caracal, Caracal (mammal), Table 25-1 caracara, Cavacara cheriway (bird), 335-336 cardinal, Richmondena cardinalis (bird), Fig. 12-5, 298, Table 8-6, Table 7-4 cariama, Cariamidae (bird), 270 caribou, barren ground, Rangifer arcticus (mammal), 187, 226, 318, 320, 321, 322 caribou, Peary’s, Rangifer peary?, 18, 318, 319-320 caribou, woodland, Rangifer caribou, 187, 303, 307, 308, 308-309 Carnivora (mammals), 271 carp, Cyprinus carpio (fish), 19, 55, 56, 57, 74-75, 85, 90, 91-92, Table 5-2, Table 7-3, 207, 246 carpsucker, highfin, Carpiodes velifer (fish), Table 5-2 carpsucker, quillback, Carpiodes cyprinus, Table 5-2 cassowary, Casuarius (bird), 270 cat, Felidae, 188, 285, Table 27-2 cat, Pallas, Felis manul, Table 25-1 cat, pampas, Felis pajeros, Table 25-1 catbird, Dumetella carolinensis, 297-298, Table 7-4, Table 8-6 caterpillar, Lepidoptera, Fig. 9-14, Fig. 13-5 caterpillar, tent, Malacosoma pluviale, 228 catfish, Ictaluridae, 55, 74-75, 246, 270, 272 catfish, channel, Jctalurus punctatus, Table 5-2 cat-tail, Typha, 79-81, Table 7-6 cat-tail, common, Typha latifolia, 204 cattle (domestic), 116-117, 126-127, 127-128, 128-129, Table 9-5, 168, 177, 181-182, 241, 244 cavy, pampas, Cavia, Table 25-1 cedar, Siberian (tree), 195 centipede, Chilopoda, 38, Fig. 9-6, Fig. 9-14, 124, Table 3-1, 130, 131-132, 134, Table 9-7, Table 9-8, 313, 348-349, 349 Ceratium (dinoflagellate), 68 cestode (see tapeworm) Chaetognatha (arrow worms), 358-359, 370 chaffinch, Fringilla teydea, F. coelebs, 252 Chalcididae, Hymenoptera (insects), 182, 227 chameleon, Chamaeleontidae (lizard), 271 chameleon, Anolis carolinensis, 298 Chaoborus ghost larva (insect), 64, 68, 71, 73, Table 7-1, 204 Chara, stonewort (green algae), 79-81 characin, Characidae (fish), 270 chat, yellow-breasted, /cteria virens, 297-298, Table 8-6 cheetah, Acinonyx jubatus (mammal), Table 25-1 cherry, sand, Prunus pumila, 105 chestnut, Castanea, 282 chestnut, Arherican, Castanea dentata, 19, Table 20-1, 294 chickadee, black-capped, Pavus atricapillus, 137-138, 253, 296, 305, Table 7-4, Table 23-1 chickadee, boreal, Parus hudsonicus, 137-138, 308, Table 23-1 chickadee, Carolina, Parus carolinensis, 298, Table 8-6 chickadee, chestnut-backed, Parus rufescens, 125, 297- 298 chickadee, mountain, Parus gambeli, Fig. 23-5, 307 chicken, greater prairie, Tympanuchus cupido, 125, 184, 244, 326, 329, 330 chicken, lesser prairie, Tympanuchus pallidicinctus, 326, 329, 330 chimpanzee, Simia satyrus (mammal), 271 chinchilla, Chinchillidae (mammal), 270 chipmunk, Sciuridae (mammal), 177, 187, Table 9-7 chipmunk, cliff, Eutamias dorsalis, 311, 312 chipmunk, eastern, Tamias striatus, Fig. 9-12, 135- 136, 136, 295, 296, 299, 300 chipmunk, least, Eutamias minimus, 303, 308-309, 336- 337 chipmunk, western, Eutamias, 334 chiton, Amphineura (mollusk), 352, 363 chiton, Lepidochitona cinereus, 352 Chlorophyceae (green algae), 92 chokeberry, Pyvus, 92, Table 7-6 chokecherry, common, Prunus virginiana, 105 cholla, tree, Opuntia, Fig. 26-3 Chrysomelidae (leaf beetles), 337 chub, bigeye, Hybopsis amblops (fish), Table 5-2 chub, creek, Semotilus atromaculatus, 521 Fig, 5-1, Table 5-2 chub, hornyhead, Hybopsis biguttata, Table 5-2 chub, river, Hybopsis micropogon, 51 chubsucker, creek, Ervimyzon oblongus (fish), Table 5-2 chubsucker, lake, Evimyzon sucetta, Fig. 5-1 chuckwalla, Sauromalus obesus (lizard), 336 chuck-will’s-widow, Caprimulgus carolinensis (bird), 298, Table 8-6 Cicadellidae (see leafhopper) Cisco, Coregonus artedii (fish), 64, 74-75, 76, 76-77, 207 Cladocera (entomostracans), 60, 66-69, 71, 81, 88, Wi salle 319) 356) Cladonia, ‘‘reindeer moss’’ (lichen), 316 Cladophora (green algae), 43 clam, Unionidae, 7, 43-44, 47, 50-51, 52, 53, 54, 55, 56, 57, 70, 74, Fig. 5-9, Table 3-1, Table 5-3, Table 6-2, Table 7-1, 146, 188, 216, 252 clam, bean, Donax gouldi, 235 clam, fingernail, Sphaeriidae, 69-70, 74, Table 3-1, Table 5-3, Table 7-1, 92, 147, Table 13-1, 66, 70- WAlperley 322 clam, hard-shell, Venus mercenaria, 368 clam, marine (see Pelecypoda) clam, soft-shelled, Mya arenaria, 368 Clethrionomys occidentalis, red-backed mouse, 137-138 clothes-moth, Tineidae, 190 clover, sweet, Melilotus alba, Fig. 8-11, 112, 190 coati, Nasua (mammal), Table 27-2, 347, 349 cobra, common, Naja naja (snake), Table 25-1 cobra, black-necked, Naja nigricollis (snake), Table 25-1 Coccidae (see scale-insect), mealybug Coccolithophoridae (flagellate protozoan), 353 cockatoo, Psittacidae (birds), 270 cocklebur, XYanthium (plant), 113 cockroach (see roach) codfish, Gadidae, 371 Coelenterata, 357, 364, 367 Coleoptera (see beetles) Coliidae, colies (birds), 271 coney, Hyracoidea (mammals), 271 Copepoda (entomostracans), 66-69, 71, 73, 75, 88, Fig. 6-7, 104-105, 178-179, 180, 188, 211, 249, 252, 319, 349, 357, 358-359, 360, 363, 365, 370 copperhead, Ancistrodon contortrix (snake), 296-297 coral, Anthozoa, Fig. 19-5, 206, 206-207, 352, 354-356, 366-367, 370 coral, organ, Hexacorallae, 366 coral, stony, Madreporaria, 366 Corixidae, water boatmen (insects), 83-84, 84-85, 85 cormorant, Phalacrocoracidae (birds), 319 cormorant, double-crested, Phalacrocorax auritus, 175 cotton-grass, Eriophorum, 92 cottontail, Sylvilagus (mammal), 128, 140-141, Table 9-3, Table 9-4, Table 9-7, 177, 179, 187, 215 cottontail, desert, Sylvilagus audubonii, 244, 326, 335 cottontail, eastern, Sylvilagus floridanus, 113, 114-115, 115-116, 178, 244, 297-298, 326 cottontail, Nuttall’s, Sylvilagus nuttallii, 326, 336-337 cottonwood, Populus, 297 cottonwood, eastern, Populus deltoides (tree), 79-81, Fig. 8-7, Fig. 8-11, 105, 113 cougar (see mountain lion) cowbird, brown-headed, Molothrus ater, 179, 297-298, Table 7-4 coyote, Canis latrans, 86-87, Fig. 25-1, 128-129, Table 9-4, Table 25-1, 175, 195-196, 311, 321, 326, 328- 329, 335 crab, Decapoda (marine crustacean), 348-349, 354, 356, 360, 363, 364, 365, 365-366, 366, 367, 368, 370, 371 crab, blue, Callinectes sapidus, 368 crab, fiddler, Uca, 365 crab, ghost, Ocypode, 365 crab, hermit, Pagurus, 363, 364 crab, king, Limulus, 364 crabgrass, Digitaria sanguinalis, 115 crane, Sandhill, Grus canadensis (bird), 318 crane, whooping, Grus americana, 216-217 crappie, Pomoxus (fish), 57, 74-75 crappie, black, Pomoxus nigromaculatus, Table 5-2, Table 7-3, 210-241, 247 crappie, white, Pomoxis annularis, Table 5-2, Table 7-3, 247 crayfish, Decapoda (crustaceans), 43-44, 50, 51, 52, 55, 87-88, 99-100, Table 5-3, Table 6-2, Table 7-1, 164, 178, 183, Table 3-1, 270, 271 creeper, brown, Certhia familiaris (bird), Fig. 23-5, 302-303, 303-305, 305, Table 23-1 creeper, trumpet, Campsis yvadicans (plant), 112 Crepidula fornicata (mollusk), Table 28-1 cricket, camel, Ceuthophilus, Fig. 9-7, 107 cricket, field, Gryllidae (insects), Fig. 9-9, 107, Table 9-7, 251, 338 cricket, Mormon, Anabrus simplex, 126-127 cricket, tree, Oecanthinae, Fig. 9-9 Crinoidea (echinoderms), 360, 367 croaker, Atlantic, Micropogon undulatus (fish), 368 crocodile, American, Crocodylus acutus (reptile), 298 crossbill, parrot, Loxia pylyopsiltacus (bird), 249 crossbill, red, Loxia curvirostra, 249, 302-303, 309, 321 crossbill, white-winged, Loxia leucopltera, 249, 306, 309, 321 crow, common, Corvus brachyrhynchos, 297-298, Table 23-1, Table 7-4 crow, fish, Corvus ossifragus, 298 crustacean, aquatic arthropod, 180, 258-259, 357, 365, 365-366 Ctenophora, 357, 360, 370 cuckoos, Old World, Cuculidae (birds), 179 cuckoo, black-billed, Coccyzus erythropthalmus (bird) 297-298, Table 7-4 cuckoo, yellow-billed, Coccyzus americanus, 297-298, Table 8-6 cucumber, sea, Holothuroidea (echinoderm), 363, 365 Culicidae, mosquito, 57, 85 Cumacea (crustaceans), 357 curlew, Eskimo, Numenius borealis (bird), 318-319 curlew, long-billed, Numenius americanus, 326 Cynipidae (insects), 187 cypress, Taxodium distichum (tree), 81, 113, 294 Cyprinidae (minnow fish), 9, 268, 270, 270-271 D dace, Cyprinidae (fish), 54 dace, blacknose, Rhinichthys atratulus, 50, Fig. 5-1 dace, redbelly, Chrosomus erythrogaster, Fig. 5-1 Dactylopius, Coccidae (insect), 227 daddy-long-legs (see harvestman) damselfly, Zygoptera, Odonata, 43, 50, 85, 87, Table 3-1, Table 5-3, Table 6-2, Table 7-1, Table 7-2, Table 13-1, 247, Fig. 7-3, 319 Daphnia, Gladocera (entomostracan), Fig. 6-1, 204-205, 208, 222, 229-330, 240-241, 254 darter, Etheostominae (fish), 47-50, 50, 54, 55 darter, banded, Etheostoma zonale, Table 5-2 darter, fantail, Etheostoma flabellare, Table 5-2, 51, 55 darter, greenside, Etheostoma blennioides, Table 5-2 darter, Johnny, Etheostoma nigrum, Table 5-2 darter, rainbow, Etheostoma coeruleum, Table 5-2, 51 Decapoda (crustaceans), 357, 358-359, 360, 363, 366, 371 deer, Cervidae, 32, 102, 115-116, 127-128, 167, Table 27-2, 177, 187, 190, 208, 214, 215, 228-229, 271, 271- 212, 299 deer, mule (black-tailed), Odocoileus hemionus, Fig. 13- 2, Fig. 15-3, 158, Table 9-4, Table 9-5, 195-196, 297-298, 307, 309, 311, 312, 321, 322, 335 deer, pampas, Odocoileus bezoarticus, Table 25-1 deer, roe, Capreolus capreolus, 150 deer, white-tailed, Odocoileus virginianus, 136, 140-141 Table 9-4, 211, Fig. 22-4, 110, 297-298, 308 desmid, Thallophyta (green alga), 92 diatom, Thallophyta (plant), 43, 54-55, 66-67, 87, 352, 353, 354-356, 357, 358 dickcissel, Spiza americana (bird), 326 Dicranura vinula, caterpillar, Fig. 13-5 ’ 415) Index digger-wasp, Sphecidae, Fig. 8-9, 109 dinoflagellate, Thallophyta (plant), 92, 357, 358-359 Diptera (see fly) Dipterocarpaceae (trees), 343 dog, Cape hunting, Lycaon, Table 25-1, 271, Table 27-2 dog, prairie, Cynomys, 128, 164, Table 9-4, Table 25-1, 187, 329, 330 dog, black-tailed prairie, Cynomys ludovicianus, 326 dog, Gunnison’s prairie, Cynomys gunnisont, 127 dogwood, Cornus (shrub), 79-81, 282 dogwood, red-osier, Cornus stolonifera, 105 Douglas-fir, Pseudotsuga menziesii (tree), 121, 302 dove, ground, Columbigallina passerina, Fig. 12-5, 335- 336 dove, mourning, Zenaidura macroura, 140-141, 297-298, 335-336, Table 8-6 dove, white-winged, Zenaida asiatica, 335-336 dove, zenaida, Zenaida aurita, 298 dowitcher, long-billed, Limnodromus scolopaceus (bird) 318 dragonfly, Anisoptera, Odonata, 25, 43, 85, 87, 88-89, 90, Fig. 7-3, Table 3-1, Table 5-3, Table 7-1, Table 7-2, Table 13-1, 184-185, 247, 319 Drosophila, fruit fly, 7, Fig. 16-6, 159-160, 175, 223, 224, 246, 249, 258, Fig. 19-3 drum, freshwater, Aplodinotus grunniens (fish), Table 5-2 Dryopidae (see beetle, riffle) Dryops, riffle beetle, Fig. 7-2, 84-85 duck, Anatidae, 87, 91, 92, 214, 243 duck, black, Anas rubripes, 306 duck, eider, Polysticta, Somateria, 319 duckweed, Lemnaceae, 79-81 dunlin, Erolia alpina (shorebird), 318 Dytiscidae (see beetle, diving) E eagle, bald, Haliaeetus leucocephalus, 179 eagle, golden, Aguila chrysaétos, 307 earthworms, Lumbricidae, Megascolecidae (see An- nelida) earwig, Dermaptera (insect), 134 Echinodermata (starfish, sea urchin, jellyfish), 352, 357, 364, 365-366 Edentata (armadillos, ant-eaters, sloths), 272 eel, Gymnotidae (fish), 270 eel, European, Anguilla anguilla, 43-44, 368-370, 370 eel, western, Anguilla bostonensis, 43-44, 368-370, 370 eelgrass, Vallisneria, 79-81 eelgrass, marine, Zostera marina, 206, 365, 366 eland, Taurotragus oryx (mammal), Table 25-1 elephant, Elephantidae (mammal), 160, 215, 271, 271- 272, 344, 347 elaphe, Elaphe dione (snake), Table 25-1 elk (see wapiti) elm, Ulmus (tree), Table 20-1, 282 elm, American, Ulmus americana, 79-81, Fig. 9-4, 112, 113, 113-114, 116-117, 121, Table 7-6, 227 elm, slippery, Ulmus rubra, 79-81, 113-114, 121, Table 7-6 454 Index Elmidae, riffle beetles, 46-47 emu, Dromiceius (bird), Table 25-1, 270 Enchytraeidae (see potworms) Entomostraca (crustaceans), 67, 74, 147, 215 Ephemeridae (mayflies), 44-45, 45 Ericaceae (heath plants), 316 ermine, Mustela erminea (mammal), 303, 318, 319-320 Eskimo, 321 Euphausiacea (crustaceans), 357, 358-359, 360 Eurotia, winterfat (bush), 334-335 EF falcon, peregrine, Falco peregrinus (bird), 318 falcon, prairie, Falco mexicanus, 337 fern, Pteridophyta (plants), 22, Fig. 8-5, 97, 102-104 ferret, black-footed, Mustela nigripes (mammal), 128- 129, 326 Ferrissia, snail, 44, 47 finch, black rosy, Leucosticte atrata (bird), 321-322 finch, brown-capped rosy, Leucosticte australis, 321- 322 finch, Cassin’s, Carpodacus cassinii, Fig. 23-5, 307 finch, Darwin’s, Geospizinae, 155 finch, gray-crowned rosy, Leucosticte tephrocotis, 321- 322 finch, ground, Geospizinae, 262 finch, purple, Carpodacus purpureous, 302-303, Table 23-1 fir, Abies (tree), 23-24, 121, Table 7-6, 286, 289 fir, balsam, Abies balsamea, Table 20-1, 230, 302, 309- 310 fir, California red, Abies magnifica, 302 fir, grand, Abies grandis, 302 fir, subalpine, Abies lasiocarpa, 302 fir, white, Abies concolor, 302 fish, Pisces, 7, 9, 11-12, 24, 39, 50, 52, 53, 56, 57, 58, 60-61, 64, 65, 66, 74, 74-75, 75, 77, 81, 85, 87, 90, 90- 91, 91, 91-92, 92, Fig. 5-10, Table 3-1, 99, 101, 102, 146, 147, 151-153, 155, 156, 172, 173, 182-183, 183, 184-185, 185, 191-192, 194, 204-205, 206, 208, 216, 222, 224, 226, 228-229, 229, 246, 249, 271, 271-272, 291, 319, 339, 367-368 fish, elasmobranch, Chondrichthyes, 354 fish, marine, Pisces, 354, 356, 357, 359, 359-360, 360- 361, 361, 363, 364, 365, 365-366, 366, 367, 370, 371 fisher, Martes pennanti (mammal), 237, 303 fishfly, Chauliodes (insect), Table 3-1 flatworm, Turbellaria, 7, 39, 44, 71, 82, 87-88, Table 3-1, 101, 130, Table 6-2, 173, 175, 177, 178-179, 179) 246, 252, 318, 319, 348-349, 349, 354, 363, 365, 368 flea, Siphonaptera (insects), 178-179, 181, 235, 246 flicker, gilded, Colaptes chrysoides (bird), 335-336 flicker, red-shafted, Colaptes cafer, Fig. 9-11, 297- 298 flicker, yellow-shafted, Colaptes auratus, 297-298, Table 7-4, Table 8-6, Table 23-1 flounder, pleuronectidal (fish), 371 fly, Diptera, 66-67, 114-115, 124, 174-175, Fig. 9-14 125, 135, 137, Table 5-4, Table 5-5, Table 7-1, Table 9-6, Table 9-7, 178-179, 181, 182, 188, 227, 265, 309-310, 311-312, 313, 319, 322, 347, 368 black, Simulium, 46, 50, 54, 85, Fig. 5-6, Table 5-6, 319 . brine, Ephydra gracilis, E. hians, 59-60 cocklebur, Euaresta aequalis, 114 crane, Tipulidae, 115 culicid (see Chaoborus) deer, Tabanidae, 319 flesh, Sarcophagidae, Muscidae, 109 fruit, Drosophilidae, Fig. 16-6, Fig. 19-3, 100-101, 223 gall, Cynipoidea, 187 horse, Tabanidae, Table 3-1, 47 midge, Chironomidae, 57, 70-71, 71, 71-73, 73, 74, 74-75, 76, 81, 85, 90, Fig. 6-12, 114, Table 6-2, Table 7-1, Table 13-1, 174, 178, 204, 205-206, 319, 320, 322, Table 3-1 midge, Tanytarsus, 64, 90 net-winged midge, Blepharoceridae, 53, Fig. 5-7 red midge, Tendipes (formerly Chironomus), 57, 64, 70-71, 71, 73, Table 3-1 red midge, Tendipes plumosus, 57, 205-206 robber, Asilidae, 109 sludge, Psychoda, 57 fly, tachinid, Tachinidae, 227 fly, warble, Oestridae, 320 flycatcher, Acadian, Empidonax virescens, 296, 305, Table 8-6, Table 23-1 flycatcher, ash-throated, Myiarchus cinerascens, 311- 312 flycatcher, Coues’, Contopus pertinax, 311-312 flycatcher, gray, Empidonax wrightii, 311-312 flycatcher, great crested, Myiarchus crinitus, 296, Table 7-4, Table 8-6 flycatcher, Hammond’s, Empidonax hammondii, 307 flycatcher, least, Empidonax, minimus, Fig. 4-3, Table 23-1 flycatcher, olive-sided, Nutiallornis borealis, Fig. 23-3, 302-303 flycatcher, pied, Muscicapa hypoleuca, 259 flycatcher, tyrant, Tyrannidae (birds), 299 flycatcher, Traill’s, Empidonax traillii, Table 7-4, 302- 303 flycatcher, vermilion, Pyrocephalus rubinus, 335-336 flycatcher, western, Empidonax difficilis, 307 flycatcher, Wied’s crested, Myiarchus tyrannulus, 335- 336 flycatcher, yellow-bellied, Empidonax flaviventris, Table 23-1 flyingfish, Exocoetidae, 359 fly-trap, Venus, Dionaea (plant), 187 Foraminifera, Rhizopoda (protozoans), 188, 353, 357, 365, 366 fowl, domestic, Gallus domesticus, 183 fox, Canidae, 271 fox, Arctic, Alopex lagopus, 228, 237, 240, 262, 318, 319-320, 320, 321 fox, corsac, Vulpes corsac, Table 25-1 fox, gray, Urocyon cinereoargenteus, 295, 296, 335 fox, kit, Vulpes macrotis, 128-129, 335 fox, red, Vulpes fulva, 114-115, Table 9-4, 192, 228- 229, 232-233, 237, 239, 240, 297-298, 299, 307 fox, ee Vulpes velox, 128-129, Table 9-4, Table 25- 1, 326 frog, Hylidae (tree frogs), 270, 271, 271-272, 296-297, 299 frog, Ranidae (tree frogs) 270, 271-272 frog, Salientia, 85-86, 87, 90, Table 3-1, 99, 99-100, 158, Table 9-7, 173, 178, 182-183, 185, 268, 338-339, 349 chorus, Pseudacyris triseriata, Fig. 21-8 leopard, Rana pipiens, 92, 259-260, 310 mink, Rana septentrionalis, 310 northern wood, Rana sylvatica catabrigensis, 310 frog, spadefoot, Scaphiopus, 88 frog, wood, Rana sylvatica, 296-297 Fulgoridae (insects), 337 fulmars, Procellariidae (birds), 319 fungus, Thallophyta (plants) 164, 166, 166-167, 171, 172, 176, 177, 179, 181, 187, 196, 289, 356-357 frog, frog, frog, frog, G gale, sweet, Myvica gale (plant), 92 gallinule, common (Florida), Gallinula chloropus (bird), Table 7-4 gannet, Morus bassanus (bird), 219 ganoids (polypterus, sturgeon, gar, bow-fin fishes), 270- 271 gar, Lepisosteidae (fish), 74-75 gar, longnose, Lepisosteus osseus, Table 7-3 gastropoda (see snails) Gastrotricha, Trochelminthes, 66-67, 71 gavial, Crocodylidae (crocodiles), 271-272 gazelle, Gazella (mammal), Table 25-1, 344 gazelle, goitered, Gazella subgutturosa, Table 25-1 gecko, banded, Coleonyx variegatus (lizard), 336 Gerridae (see water striders) gibbon, Pongidae (primate mammal), 271-272 giraffe, Givaffa (mammal), 271, 344 gnat, Cecidonyidae (insects), 179 gnatcatcher, black-tailed, Polioptila melanura (bird), 335-336 gnatcatcher, blue-gray, Polioptila caerulea, 298, 311- 312, Table 8-6 goat, Bovidae, 272 goat, mountain, Oveamnos americanus, 321, 322 godwit, Hudsonian, Limosa haemastica (bird), 318-319 goldeneye, common, Bucephala clangula (bird), 306 goldenrod, Solidago (plant), 179 ; goldfinch, American, Spinus tristis (bird), 297-298, Table 7-4, Table 8-6 goldfinch, Lawrence’s, Spinus lawrencei, 311-312 goldfinch, Carassium auratus, 173, 175 goose, Anatidae (birds), 91 goose, Canada, Branta canadensis, 215, 306, 318-319 goose, grey-lag, Anser anser, 15 goose, snow, Chen hyperborea, 318-319 goose, white-fronted, Ansey albifrons, 318 gopher, Botta’s pocket, Thomomys bottae, 335 gopher, northern pocket, Thomomys talpoides, 326 gopher, plains pocket, Geomys bursarius, 326 gopher, pocket, Geomys, Thomomys, 128, 164, Table 9-4, Table 25-1, 321, 322, 323, 329 gopher, southeastern pocket, Geomys pinetis, 298 gopher, western pocket, Thomomys bottae, 127 325) Index gorilla, Gorilla (primate mammal), 271 goshawk, Accipiter gentilis, 237, 302-303, Table 23-1 grackle, boat-tailed, Cassidix mexicanus, 298 grape, Vitis bicolor (plant), 190 grape, frost, Vitis vulpina, 105, 190 grass, Fig. 9-2, Fig. 8-7, Fig. 11-4, 102-104, 105, 115, 115-116, 117, 120, 122, 125-126, 168, 316-317, 317, 330-331, 341-342 grass, broomsedge, Andvopogon virginicus, Fig. 8-12, 115 grass, buffalo, Buchloe dactyloides, Fig. 11-4, 120 grass, grama, Bouteloua, Fig. 11-4, 120 grass, needle, Stipa, 120 pigeon, Setaria, 190 grass, sandbur, Cenchrus pauciflorus, 105 grass, slough, Spartina michauxiana, 120 grass, triple-awn, Aristida, 115 grass, wheat, Agrvopyron spicatum, 333-334 grass, wire, Aristida, Fig. 11-4 grasshopper, Tettigoniidae, Locustidae (insects), 25, 107, 113, 115, 124, 126-127, Table 24-1, 187, 237, 247, 313, 322, 322-323, 329, 329-330, 336, 337, 338 grasshopper, band-winged, Oedipodinae, 107 grasshopper, long-horned, Tettigoniidae, 107 grasshopper, shield-backed, Decticinae, 107 grasshopper, short-horned, Acridiidae, 107, 128 grasshopper, spur-throated, Locustinae, 107 greasewood, Sarcobatus vermiculatus (shrub), 333-334, 337 grebe, horned, Podiceps auritus (bird), 306 grison, Galera vittate (mammal), Table 27-2 grosbeak, black-headed, Pheucticus melanocephalus (bird), 297-298 grosbeak, evening, Hesperiphona vespertina, 190, 307, 308, 309, 321, Table 23-1 grosbeak, pine, Pinicola enucleator, Fig. 23-3, 302-303, 306, 309, 321, Table 23-1 grosbeak, rose-breasted, Pheucticus ludovicianus, Table 7-4, Table 23-1 grouse, Galliformes (birds), 187, 214, 228, 238, 241, 309 grouse, blue, Dendragapus obscurus, 307 grouse, hazel, Tetrastes bonasia, 237 grouse, red, Lagopus scoticus, 237 grouse, ruffed, Bonasa umbellus, Fig. 13-3, Fig. 15-1, 115-116, 140, 237, 240, 241, 296, 302-303, Table 23-1 grouse, Sage, Centrocercus urvophasianus, 182-183, 337 grouse, sharp-tailed, Pedioecetes phasianellus, 125, Fig. 16-9, 184, 237, 326, 330 pemee spruce, Canachites canadensis, 308, Table 23-1 guanaco, Lama huanacus (mammal), Table 25-1 guillemot, Alcidae (birds), 319 guinea-fowl, Numididae (bird), 271 gull, Laridae (birds), 319, 320 gull, Bonaparte’s, Larus philadelphia, 321-322 gull, glaucous, Larus hyperboreus, 318 gull, herring, Lavus argentatus, 318, 321-322 gull, mew, Larus canus, 321-322 guppy, Lebistes reticulatus (fish), 208 gyrfalcon, Falco rusticolus (bird), 318 Gyrinidae (see beetle whirl-i-gig) grass, ar KS) Index H hackberry, Celtis occidentalis (tree), 113-114, 179 hairworm, Gordiacea, 178-179 Haliplidae (see crawing water beetle) hamster, Cricetus cricetus (mammal), Table 25-1 hare, Arctic, Lepus arcticus (mammal), 318, 319-320, 320 hare, common, Lepus europaeus 236, 252, 299 hare, mountain, Lepus timidus, 252 hare, varying (see snowshoe rabbit) harvestman, Phalangidae (arthropod), Table 9-7, 205- 206, Fig. 9-9 hawk, broad-winged, Buteo platypterus (bird), Table 23-1, 296 hawk, Cooper’s, Accipiter cooperii, 297-298 hawk, ferruginous, Buteo regalis, 326 hawk, Harris’, Pavabuteo unicinctus, 335-336 hawk, marsh, Circus cyaneus, 124 hawk, pigeon, Falco columbarius, 302-303, Table 23-1 hawk, red-shouldered, Buteo lineatus, 297-298, Table 7-4 hawk, red-tailed, Buteo jamaicensis, 297-298, 335-336 hawk, rough-legged, Buteo lagopus, 237, 240, 306, 318 hawk, sharp-shinned, Accipitey striatus, 297-298 hawk, sparrow, Falco sparverius, 297-298 hawk, Swainson’s, Buteo swainsoni, 297-298, 337 hawl-owl, Surnia ulula, 306 hawthorn, Crataegus (tree), 116-117, 282 hazel, Corylus (shrub), Table 20-1, 282 heath, Ericaceae (bush), 302 hellbender, Cryptobranchus (salamander), 282 hellgrammite, Corydalus cornutus (insect), 47, 54, 55, Table 3-1 Hemiptera (see bug) hemlock, eastern, Tsuga canadensis (tree), Fig. 3-3, 23-24, 121, Table 7-6, Table 20-1, 289-290, 291, 294, 301, 302, 305 hemlock, mountain, Tsuga mertensiana, 302 hemlock, western, Tsuga heterophylla, 302 heron, black-crowned night, Nycticorax nycticorax (bird) heron, green, Butovides virescens, Table 7-4 herring, Clupeidae (fish), 357, 358-359, 359, 361, 371 hickory, Carya (tree), 23-24, 112, 113, 113-114, 115, Table 7-6, Table 20-1, 177, 179, 290-291, 294 hickory, bitternut, Carya cordiformis, 105 hickory, shagbark, Carya ovata, 105 hippopotamus, Hippopotamus (mammal), 271, 344, 347 hoatzin, Opiathocomus hoazin (bird), 270 holly, /lex (tree), 92, Fig. 7-5 holly, mountain, Nemopanthus mucronata, Table 7-6 Holothuroidea (sea cucumbers), 188, 353, 360, 367 Homoptera (cicadas, leafhoppers, aphids), Fig. 9-14, 147, Table 9-6, 319 honeycreeper, Drepaniidae (birds), 155, 266 honey-eater, Meliphagidae (birds), 270 hornbeam, Carpinus (tree), 282, 298-299 hornbill, Bucerotidae (birds), 271 hornet, bald-faced, Vespidae (insect), Fig. 9-9 hornet, common, Vespa crabo, Fig. 10-12 horntail, Siricidae (insects), 135 hornwort, Cevatophyllum (plant), 79-81 horse, Equidae, 116-117, 185, 272, 285, 330 horse, wild, Equus przewalskti. Table 25-1 horse-weed, Evigeron canadensis, 115 hummingbird, Trochilidae, 270, 297-298, 348 hummingbird, Allen’s, Selasphorus sasin, 252, 312 hummingbird, Anna’s, Calyple anna, 252, 312 hummingbird, broad-tailed, Selasphorus platycerus, 313 hummingbird, calliope, Stellula calliope, 307 hummingbird, Costa’s, Calypte costae, 335-336 hummingbird, ruby-throated, Archilochus colubris, Table 8-6, 297-298 Hydra, Hydrozoa (coelenterate), 71, 81, 82, 87-88, 178 hydra, green, Chlorohydyra viridissima, 177 hydroid, Hydrozoa, (coelenterate), 211-212, 362, 363, 364 Hydrometridae (see measurer, water), 83-84 Hydrophilidae, (see beetle, diving) hyena, Hyaenidae (mammal), 347 Hymenoptera (wasps, bees, ants), Table 9-6, 147, 182, 227, 3138, 319, 347 I Ichneumonidae (insects), 182, 311-312 incense-cedar, Libocedrus decurrens, 302 Indian, American, 117, 300, 310, 330 inkberry, /lex glabra (tree), 168 insect, Insecta, Table 3-1, 97-98, 101, 104-105, 106- 107, 114-115, 115, 123-124, 124, 126-127, 128, 128- 129, 129, 130, 131-132, 137, Table 8-2, 147, 155, 155-156, 156, 158, 158-159, Table 9-7, Table 9-8, Table 16-7, 177, 183, 188, 192-194, 194, 195, 215, 230, 231, 234-235, 235, 246, 252, 252-253, 253, 255, 258-259, 261, 266, 298, 299, 299-300, 319, 322, 338, 338-339, 347, 347-348, 348, 349, 354, 363, 366 insect, aquatic, 24, 24-25, 25, 37-38, 38, 39, 56, 65, 73, 146 insect, land, 96, 171, 299, 300, 305-306, 335, 338 Insectivora (mole, shrew), 270, 270-271, 271 Isopoda (crustaceans), 52, 87, Table 3-1, 123-124, 124, 134, Table 7-1, Table 9-7, Table 9-8, 250, 349, 362, 363, 368 J jackal, Canis aureus (mammal), Table 25-1, 271 jaeger, long-tailed, Stercorarius longicaudus (bird), 318 jaeger, parasitic, Stercorarius parasiticus, 318 jaeger, pomarine, Stercorarius pomarinus, 240, 318 Japygidae, Thysanura (insect), Fig. 9-6, Table 9-8 jay, blue, Cyanocitta cristata (bird), Fig. 12-5, 297-298, Table 7-4, Table 8-6, Table 23-1 jay, gray, Perisoreus canadensis, Fig. 23-2, 302-303, 308, Table 23-1 jay, pinon, Gymnorhinus cyanocephala, 311-312 jay, scrub, Aphelocoma coerulescens, 298, 311-312 jay, Steller’s, Cyanocitta stelleri, 307 jellyfish, Scyphozoa, 357, 360 jellyfish, fresh-water, Craspedacusta, 66-67 jerboa, Glactaga, Table 25-1 Joshua-tree, Yucca brevifolia, Fig. 26-2, 333 junco, gray-headed, Junco caniceps (birds), 307 Fig. 23-3, 307, 308 110, 303-305, 305, junco, Oregon, Junco oreganus, junco, slate-colored, Junco hyemalis, 308, Table 23-1 juniper, Juniperus (tree), 311 juniper, creeping, Juniperus hovizontalis, 105 K katydid, Pseudophyllinae (insect), Fig. 9-9, 107 katydid, false, Phaneropterinid, 107 kelp, Phaeophyceae (brown algae), 363, 364 killdeer, Charadrius vociferus (bird), Fig. 13-4, 192- 194, Table 7-4 killifish, Fundulus (fish), 368 kingbird, Cassin’s, Tyrannus vociferans, 297-298 kingbird, eastern, Tyrannus tyrannus, 109-110, 297-298, Table 7-4 kingbird, gray, Tyrannus dominicensis, 298 kingbird, western, Tyvannus verticalis, 297-298 kingfisher, Alcedinidae (bird), 270 kinglet, golden-crowned, Regulus satrapa (bird), Fig 23-3, 302-303, 303-305, 305, Table 23-1 kinglet, ruby-crowned, Regulus calendula, Fig. 23-5, 302-303, Table 23-1 kingsnake, Lampyropeltis getulus, 298 kinkajou, Potosflavus (mammal), Table 27-2, 349 kite, Mississippi, /ctinia misisippiensis (bird), 298 kite, swallow-tailed, Elanoides forficatus, 298 kiwi, Apteryx (bird), 270 knot, Calidris canutus (bird), 318 L labrador-tea, Ledum groenlandicum (bush), 92, Table 7-6 lacewing, green, Chrysopidae (insect), 85, Fig. 9-9, Table 9-7 Lagomorpha (rabbit, cottontail), 329 laminaria (see kelp) lamprey, sea, Petromyzon marinus (fish), 77 Lapp (man), 321 lapwing, Vanellus vanellus (bird), 150, Table 15-5 larch, subalpine, Larix lyallie (tree), 302 larch, western, Larix occidentalis, 302 lark, horned, Eremophila alpestvis (bird), 125, 150, 247, 318, 319, 321-322, 326, 337 laurel, Dees (shrub), 92 leafhopper, Cicadellidae (insects), Fig. 9-9, 124, 135; 137-138, Table 9-7, 187, 227, 310, 311-312, 313, 322, 337 leafhopper, sugar cane, Perkinsiella saccharicida, 227 leatherleaf, Chamaedaphne calyculata (bush), 92, Fig. 7-5, Table 7-6 leech, Hirudinea (annelid), 87, Table 6-2, Table 7-1, 319, 348-349, 349 legume, Leguminosae (plant), 166-167, 343 lemming, brown, Lemmus trimucronatus, Fig. 17-4, 228, 237, 318 lemming, Lemmus, Dicrostonyx 241, 318, 320 lemming, European, Lemmus lemmus, 228, 231, 237, 318 (mammals), 238-239, |ndex 457 lemming, Siberian, Lemmus sibiricus, 237, 240, 318 lemming, collared, Dicrostonyx groenlandicus, Fig. 17- 4, 237, 240, 318, 319-320 lemming, northern bog, Synaptomys borealis, 303 lemming, southern bog, Synaptomys cooperi, 136 lemur, Lemuridae (primate mammal), 271, 349 leopard, Felidae (mammal), 271, 347 Lepidoptera (see butterfly) lichen, Thallophyta (plants), Fig. 8-5, 102-104, 164, 176, 316, 316-317, 317, 362, 368 lily, pond, Nymphaea (plant), Table 7-6, 172 lily, water, Castalia, Table 7-6, 172 limpet, Ferrissia (snail), 44, 47, 52 limpet, marine, 352, 363, 368 linden, Tilia (tree), Table 20-1 lion, Felidae, Fig. 27-5, 344 lion, Panthera leo, Table 25-1, 205-206, 215, 271, 347 lion, mountain, Felis concolor, Table 25-1, 109, 295, 296, 300, 307, 311, 335 Liriodendron (see tuliptree) Littorina (periwinkle or marine snail), 362, 362-363, 363, 368, Table 28-1 lizard, Reptilia, 99-100, 110, 125, 155, Table 9-7, 183, 215, 271, 282, 311-312, 313, 337, 338, 339, 349 lizard, Sceloporus, Table 15-2 lizard, collared, Crotaphytus collaris, 337 lizard, crested, Dipsosaurus dorsalis, 336 lizard, desert night, Xantusia vigilis, 336 lizard, eastern fence, Sceloporus undulatus, 298 lizard, fringe-toed, Uma notata, 336 lizard, leopard, Crotaphytus wislizeni, 336 lizard, rusty, Sceloporus olivaceus, 216-217 lizard, sagebrush, Sceloporus graciosus, 337 lizard, spiny, Sceloporus clarki, S. magister, 336 lizard, wall, Lacerta sicula, Table 15-2 lizard, whip-tailed, Cnemidophorus tigris, 336 lizard, zebra-tailed, Callisaurus draconoides, 336 lobster, Homarus (Decapoda), 364, 368, 371 locust (see grasshopper (insects)) locust, banded-winged, Tvimerotropis, 311-312 locust, black, Robinia pseudoacacia, 190 locust, migratory, Locusta migratoria, 158, 174, 228, 349-350 locust, pigmy, Acrydiinae, 85 longspur, chestnut-collared, Calcarius ornatus (bird), 125, 326 longspur, Lapland, Calcarius lapponicus, 318, 319 longspur, McCown’s, Rhynchophanes mccowaii, 125, 326 longspur, Smith’s Calcarius pictus, 318-319 loon, Arctic, Gavia arctica (bird), 318 loon, red-throated, Gavia stellata, 318 loon, yellow-billed, Gavia adamsii, 318 looper, hemlock, Ellopia fiscellaria (insect), 187 loris, Lorisidae (mammal), 271-272 lory, Lorius (bird), 270 rane: Mallophaga (insects), 178-179, 179, 180, 181 louse, blood-sucking, Anoplura (insects), 178-179, 181 louse, human, Pediculus humanus, Table 15-7, 181, 217 Lumbricidae (see Annelida) lungfish, Epiceratodus, 270 lungfish, Lepidosiren, 270 458 Index lungfish, Protopterus, 271 lynx, Lynx canadensis (mammal), Fig. 17-5, 237, 238, 243, 303, 307 lyrebird, Menuridae, 270 M macaque, Cercopithecidae (primate mammal), 271 mackerel, Scombridae (fish), 359, 361, 371 Madrepora, Anthozoa (coelenterate), Fig. 2-1 maggot, rattail, Evistalis (insect), 57 magnolia, Magnolia (tree), 282, 294 magpie, black-billed, Pica pica (bird), 297-298 mallard, Anas platyrhynchos (bird), Table 7-4 mammals, Mammalia, 9, 24, 24-25, 25-26, 35-36, 38, 74, 86, 87, 96, 97, 97-98, 98, 99-100, 101, 102, 129, 130, 135-136, 136, 136-137, 137, 138, Table 8-2, 146, 148, 151, 151-153, 155, 155-156, 156, 158, 174, 177, 181-182, 182-183, 183, 184-185, 185, 188, 204-205, 210, 211, 222, 234, 265, 267, 286- 288, 298, 299, 299-300, 306, 320, 323, 338, 338-339, 339, 347-348, 348, 354 mammoth, Elephas (mammal), 285 mammoth, wolly, Elephas primigenius, 286 man, 24-25, 159-160, 175-176, 181-182, 207, 268, 300, 310, 323, 330, 330-331, 339, 349, 349, 350, 350 manatee, Trichechus latirostris (mammal), 298 mangrove, black, Avicennia nitida (shrub), 368 mangrove, red, Rhizophora mangle, 368 man-of-war, Portuguese, Physalia (coelenterate), 357 maple, Acer (tree), 282, 298-299 maple, black, Acer nigrum, 113-114 maple, mountain, Acer spicatum, Fig. 3-3 maple, red, Acer rubrum, 79-81, Table 7-6 maple, silver, Acer saccharinum, 79-81, Fig. 8-11, 112, 113, 121, Table 7-6 maple, sugar, Acer saccharum, 81, Fig. 9-4, 105, 112, 113-114, 115, 121, Table 7-6, 165, 290-291, 294 maral, Cervus elaphus (mammal), Table 25-1 marmoset, Hapalidae (primate mammal), Table 27-2, 270 marmot, hoary, Marmota caligata, 321, 322 marmot, yellow-bellied, Maymota flaviventris, 307, 321, 322 marsupial, Marsupialia (mammals), 151, 270, 344, 347, 349 marten, American, Martes americana (mammal), 140, 237, 303, 308 marten, western, Martes cauyina, 307, 308 massasauga, Sistrurus catenatus (rattlesnake) 86, Fig. 21-7, 125 mastodon, American, Mammut americanum (mammal), 285 mayfly, Ephemeridae, 43, 43-44, 44-45, 45, 51, 52, 52-53, 53, 55, 82-83, 85, 87, Table 5-1, Table 5-3, Table 5-4. Table 5-5, Table 5-6, Table 6-2, Table 7-1, Table 13-1 Fig. 5-3, 174 meadowlark, eastern, Sturnella magna (bird), 124, Table 8-6 meadowlark, western, Sturnella neglecta, Fig. 9-3, 125, 247, 326 mealybug, Coccidae, 227 m [= a 2 — or — or — |} BS & = mealybug, long-tailed, Pseudococcus longispinus, Fig. 16-7, 225 measurer, water, Hydrometridae (insects), 81, 83-84 megapode, Megapodiidae (bird), 270 Meliaceae, mahoganies (trees), 343 menhaden, Clupeidae (fish), 361 merganser, common, Mergus mergansey (bird), 317 merganser, hooded, Lophodytes cucullatus, 317 Mesoveliidae (see water striders) mesquite, Prosopis juliflova (shrub), 117, 325, 333 Metasequoia, sequoia (tree), 282 midge, net-veined, Blepharoceridae (insect), 46 milfoil, Myriophyllum (plant), 79-81 millipede, Diplopoda (arthropod), 38, Table 3-1, 114- 115, 123-124, 124, 130, 131-132, 134, 136, Table 9- 7, Table 9-8, 296-297, 299, 310, 313, 348-349, 349 millipede, Pseudopolydesmus serratus, Fig. 9-7, 143- 144 millipede, round red, Spivobolus marginatus, Fig. 9-7 millipede, yellow-margined, Fontaria virginicus, Fig. 9-7 milliporid, Hydrocorallina (hydrozoans), 366 mink, Mustela vison (mammal), 43-44, 86, 207, 232-233, 237 minnow, Cyprinidae (fish), Fig. 5-1, 175 minnow, bluntnose, Pimephales notatus, Fig. 5-1, Table 5-2 minnow, Silverjaw, Evicymba buccata, Table 5-2 minnow, steelcolor, Notropis whippli, Table 5-2 minnow, stoneroller, Campostoma anomalum, Table 5-2 minnow, 5-2 Miridae (leaf bugs), 337 mirid, cocklebur, //necora stalii, 114 mite, Acarina, Table 3-1, 113, 131-132, Table 6-2, Table 9-6, Table 9-8, Table 11-1, 313, 319, 322, 337, 178- 179, 179, 181, 240, 246 mite, soil, 39, 104-105, 348-349 mite, water, Hydrachnidae, 47, 66-67, 71, 82, Table 13- 1, 363 moccasin, Asian, Ancistodon balys (snake), Table 25-1 moccasin, cottonmouth, Agkistrodon piscivorus, 86 mockingbird, Mimidae, 272 mockingbird, Mimus polyglottos, Fig. 12-5, 298 mole, Talpidae (mammals), 99-100, 135-136, 136, 228- 229, 272 mole, eastern, Scalopus aquaticus, 198, 295, 297-298 mole, European, Talpa europaea, 299 mole, golden, Chrysochloridae, Table 25-1 mole, hairy-tailed, Pavascalops breweri, Fig. 9-12 mole, marsupial, Noforyctes, Table 25-1 mole, star-nosed, Condylura cristata, 303 mollusk, fresh-water (see also clams, snails), 64, 65, 66, 70, 87-88, 92, 171, 177, 188, 285 mollusk, marine, Fig. 19-5, 188, 352, 353, 354-356, 357, 360, 367 monkey, catarrhine Old World, Cercopithecoidae, 270- 271, 271, 271-272, 349 monkey, prehensile-tailed New World, Cebidae, Table 27-2, 270, 349 Monotremata (mammals), 151, 270 monster, Gila, Heloderma suspectum (reptile), 336 suckermouth, Phenacobius mirabilis, Table moose, Alces americana (mammal), Fig. 23-2, Fig. 23-4, 140, 164-165, 187, 303, 307, 308, 308-309, 309-310 mosquito, Culicidae, 57, 81, 85, 89, 92, Fig. 10-7, 115, 174, 178, 187, 235, 258, 309-310, 319, 349, 368 mosquitofish, Gambusia patruelis, 194 moss, Bryophyta (plants), Fig. 8-5, 102-104, 164, 316, 316-317, 317, 368 moss, Spanish, Tillandsia usneoides, 294 moss, water, Fontinalaceae, 43 moth, Lepidoptera, 101, 182, 194, 227, 258, 322-323, 347-348, Table 9-7 moth, Mediterranean flour, Ephestia kuhniella, 251 moth, pine-leaf tube-building, Eulia pinatubana, 138 mouse, Rodentia, Table 9-7, 35-36, 135-136, Table 27-2, 175, 187, 194, 215-216, 228-229, 240, 246, 299, 308- 309, 321, 329, 335-336 mouse, brush, Peromyscus boylii, 311 mouse, cactus, Peromyscus eremicus, 335 mouse, California, Peromyscus californicus, Table 15- 2,311 mouse, canyon, Peromyscus crinitus, 335 mouse, cotton, Peromyscus gossypinus, 101-102, 259,298 mouse, dark kangaroo, Microdipodops megacephalus, 336-337 mouse, deer, Peromyscus maniculatus, 16, Fig. 9-12, Fig. 15-5, Table 2-1, 136, Table 15-2, 248, 251, 302-303, 311, 326, 335, 337, 339 mouse, desert pocket, Perognathus penicillatus, 335 mouse, eastern harvest, Reithrodontomys humulis, 298 mouse, European red-backed, Clethrionomys glareolus, Table 16-5 mouse, European woodland, Apodemus sylvaticus, Table 16-5 mouse, Florida, Peromyscus floridanus, 298 mouse, Gapper’s red-backed, Clethrionomys gappevi, Fig. 9-12, 136, 303, 308 mouse, golden, Peromyscus nuttalli, 115, 298 mouse, Great Basin pocket, Perognathus parvus , 336-337 mouse, harvest, Reithrodontomys, Table 9-3, Table 9- 4, 326 mouse, house (laboratory, albino), Mus musculus, Fig. 16-8, Table 16-2, Table 16-5, 223, 223-224, 224, 229. 266 mouse, jumping, Zapus, 307 mouse, lemming (see lemming, bog) mouse, little pocket, Perognathus longimembris, 336- 337 mouse, long-tailed pocket, Perognathus formosus, 336- Sirf mouse, meadow (see vole, meadow) mouse, meadow jumping, Zapus hudsonius, 136, Table 9-4, 303, 326, 330 mouse, northern grasshopper, Onychomys leucogaster, 326, 336-337 mouse, oldfield, Peromyscus polionotus, 298 mouse, Old World, Muridae, 270 mouse, pine, Pitymys pinetosum, 114-115, 115, 298 mouse, pinon, Peromyscus truei, Table 15-2, 311 mouse, pocket, Perognathus, Table 9-4, 326, 329-330, 339 mouse, prairie deer, Peromyscus maniculatus bairdii, 109, 109-110, 113, 115-116, Table 9-3, Table 9-4 mouse, red-backed, Clethrionomys, 307, 308 Index 459 rock, Peromyscus nasutus, 311 rock pocket, Pevognathus intermedius, 335 southern grasshopper, Onychomys torridus , 335 mouse, western jumping, Zapus princeps mouse, white-footed, Peromyscus leucopus, 16, 87, Table 2-1, 113, 114, 115-116, 136, 149, 210, 215, 246-247, 248, 251, 257, 295, 296, 299 mouse, woodland jumping, Napaeozapus insignis, 303 mudminnow, Umbra limi (fish), 64, 85, 92 mulberry, red, Morus rubra (tree), 113-114 murre, Uyia (bird), 319 muskox, Ovibos (mammal), 175, 187, 285, 286, 318 muskrat, Ondatra zibethicus, 43-44, 86, 87, 90, 91, 92, Fig. 17-6, 145-146, 207, 208, 219, 229, 232-233, 237, 241-242, 306, 368 muskrat, round-tailed, Neofiber alleni, 298 Musophagidae, plaintain-eaters (birds), 271 mussel, marine, Pelecypoda, 352, 363, 368 Mya arenaria, soft-shelled clam, Table 28-1 Myriapoda (see millipedes, pauropods, centipedes) Mysidacea (crustaceans), 68, 357, 358-359, 360 Mysis relicta, mysid crustacean, 73-74 Mytilus edulis, mollusk, Table 28-1 Myxophyceae, Thallophyta (blue-green algae), 92, 362 mouse, mouse, mouse, N naiad, Najas (plant), 79-81 Naucoridae, creeping water-bugs, 83-84 nekton, 66, 76, 359, 359-362 Nematoda (round worm), 39, 71, 87-88, 104-105, 124, 125, 130, 130-131, 178-179, 179, 180, 365 Nemertinea (ribbon worms), 357, 360 Nepidae (water scorpions, insects), 83-84, 85 nettle, wood, Laportea canadensis (plant), 112 Neuroptera (lacewings, insects), 319 neuston, 66, 81 newt, common, Diemictylus viridescens (salamander), 296-297 nighthawk, common, Chordeilis minor (bird), 297-298, 298 nighthawk, lesser, Chordeilis acutipennis, 335-336 Notonectidae (backswimmers, insects), 83-84, 84-85, nudibranch, Gastropoda (sea slugs), 363 Nuphar, yellow pond-lily (plant), 92 nutcracker, Clark’s, Nucifraga columbiana (bird), Fig. 23-3, 307 nutcracker, Siberian, Nucifraga caryocatactes, 195 nuthatch, brown-headed, Sitta pusilla (bird), 115, 298, Table 8-6 nuthatch, pigmy, Sitta pygmaea, 307 nuthatch, red-breasted, Sitta canadensis, Fig. 23-3, 109- 110, 302-303, 303-305, 305, Table 23-1 nuthatch, white-breasted, Sitta carolinensis, 253, 296, Table 7-4 Nymphaea, water lily (plant), 79-81, 92 O oak, Quercus (tree), 23-24, 112, 113, 113-114, 115, 116- 117, 24, Table 7-6, Table 20-1, 175, 179, 282, 290-2 291, 294, 298-299, 311 460 Index oak, black, Quercus velutina, 105, 121, 190 oak, bur, Quercus macrocarpa, 121 oak, northern red, Quercus rubra, 105, 121, 190 oak, pin, Quercus palustris, T9-81 oak, swamp white, Quercus bicolor, 79-81 oak, white, Quercus alba, 105, 121, 190 Octodontidae (spiny rats, etc.), 347 Odonata (see dragonfly, damselfly) oldsquaw, Clangula hyemalis (duck), 318 Oligochaeta (see Annelida) Onychophora (worm-like arthropods), 349 opossum, Didelphis virginiana (mammal), 109, 114-115, 151, 295 orangutan, Pongidae (primate mammal), 271-272 oriole, Bullock’s, Jcterus bullockii (bird), 297-298 oriole, orchard, Icterus spurius, 298 oriole, Scott’s, Icterus pavisorum, 311-312 Orthoptera (grasshoppers, crickets, cockroaches), Table 9-6, 299, 319 osprey, Pandion haliaetus (bird), 179 Ostracoda (entomostracans), 66-67, 71, 81, 88, Table 7-5, 319, 349, 357, 358--359, 360, 363, 365 ostrich, Struthio camelus (bird), Table 25-1, 271 otter, Lutva canadensis (mammal), 43-44, 86-87 ovenbird, Seiurus aurocapillus, 248, 248-249, 296, 305, Table 7-4, Table 23-1 owl, Strigiformes, 188-189, 299 owl, barred, Strix varia, 296 owl, boreal, Aegolius funereus, 306 owl, burrowing, Speotyto cunicularia, 326, 337 owl, elf, Micrathene whitneyi, 335-336 owl, flammulated, Otus flammeolus, 307 owl, great gray, Stvix nebulosa, 306 owl, great horned, Bubo virginianus, Fig. 22-3, 149, Table 16-4, 225, 237, 296, 302-303, 335-336 owl, pygmy, Glaucidium gnoma, 307 owl, saw-whet, Aegolius acadicus, 302-303 owl, screech, Otus asio, 297-298 owl, short-eared, Asio flammeus, 125, 240, 326 owl, snowy, Nyctea scandiaca, 150, 228, 237, 239, 240, 318, 319-320, 320, 321 oyster, Pelecypoda (mollusk), 229, 364, 371 oyster, American, Crassostrea virginica, 368, 371 oyster, rock, Pododesmus macroschisma, 363 P paca, Agouti, rodent, Table 27-2 paddlefish, Polydontidae, 282 paddlefish, Polyodon spathula, 74-75 paloverde, Cercidium (tree), 26-9, 26-12, Fig. 26-3, 333 pangolin, Mania (mammal), 271-272 panther (see mountain lion) Pape oe Carolina, Conuropsis carolinensis (bird), 298, 00 Paramecium, Protozoa (ciliate), Fig. 18-2, 159-160, 173, 222, 240, 254, 258 parrots and parakeets, Psittacidae, 175, 270 partridge, gray, Perdix perdix (bird), Fig. 17-3 partridge-berry, Mitchella repens (plant), 282 pauropods, primitive (arthropods), 39, Table 9-8, 124, 131-132 peccary, Artiodactyla (mammal), Table 27-2, 285 peceary, collared, Pecari tajacu, 335, 347 Pectinatella, Bryozoa, 44 Pelecypoda (clams, mussels, oysters), 364-366 pelican, Pelecanidae (birds), 175 penguin, Sphenisciformes (bird), 194 pentastomidae (parasitic arthropod), 178-179 perch, Percidae (fish), 57 perch, log, Percina caprodes, Table 5-2 perch, yellow, Perca flavescens, 64, 74-75, 75, 92, Fig. 6-11, Table 7-3 periwinkle (see Liftforina) pewee, eastern wood, Contopus virens, Table 8-6, Table 7-4, Fig. 4-3, 296, Table 7-4, Table 8-6 pewee, western wood, Contopus sordidulus, 307 phainopepla, Phainopepla nitens (bird), 335-336 phalarope, northern, Lobipes lobatus, 318 phalarope, red, Phalayropus fulicarius, 318 pheasant, Phasianidae (birds), 271-272 pheasant, ring-necked, Phasianus colchicus, 115-116, 125, 140-141, 184, 190, 212, 214, 244 phenacomys, Ungava, Phenacomys ungava, (mouse), 303 phyllopod, Branchiopoda (crustacean), 88, 319 phytoplankton, 66-69, 74-75, 75, 203-204, 204, 206, 319, 357, 358, 358-359 pickerel, grass, Esox americanus (fish), Fig. 5-1, Table 5-12 pig, Suidae (mammal), 271-272, 347 pig, guinea, Cavia, 270 pigeon, Columbidae, 270 pigeon, band-tailed, Columba fasciata, 311-312 pigeon, common (rock dove), Columba livia, Fig. 12-4 pigeon, passenger, Ectopistes migratorius, 208, 300 pigeon, white-crowned, Columba leucocephala, 298 pika, collared, Ochotona collaris (mammal), 321, 322 pika, common, Ochotona princeps, 321, 322 pike, Esocidae (fish), 64, 74-75 pike, northern, Esox, lucius, 92, Fig. 5-1, Table 7-3 pike, walleye, Stizostedion vitreum, Table 7-3 Pine, Pinus (tree), Fig. 8-7, Fig. 8-12, 23-24, 105, 121, Table 20-1, 282, 289, 294, 301, 302, 310-311 pine, digger, Pinus sabiniana, 311 pine, eastern white, Pinus strobus, 65, 121, Table 7-6, 302 pine, jack, Pinus banksiana, 105, 117-119, 302 pine, loblolly, Pimus taeda, 115 pine, lodgepole, Pinus contorta, 117-119, 121, 302 pine, longleaf, Pinus palustris, 117, 165 pine, ponderosa, Pinus ponderosa, 121, 301, 302 pine, red, Pinus resinosa, 105, 302 pine, shortleaf, Pinus echinata, 115 pine, sugar, Pinus lambertiana, 302 pine, western white, Pinus monticola, 302 pinon, Pinus edulis, 311 pipit, Anthus (bird), Table 18-1 pipit, Sprague’s, Anthus spragueii, 326 pipit, water, Anthus spinoletta, 318, 321-322, 322 pipsissewa, Chimaphila umbellata (plant) pirateperch, Aphredoderus sayanus (fish), Table 5-2 pitcher-plant, Sarracenia, 178-187 Planayia, Turbellaria (flatworm), 44, 55, 81, 130, Table 6-2, Table 7-1, 175, 205-206 plankton, 39-40, 64, 66-69, 75, 75-76, 81, 82, 87-88, Fig. 6-6, Fig. 6-8, Table 6-3, 172, 210, 322, 351-359, 359-362, 371 plant-louse, Aphididae, Fig. 9-9 plant-louse, jumping, Chermidae, 310, 311-312 Plecoptera (see stoneflies) plover, American golden, Pluvialis dominica (bird), 318 plover, black-bellied, Squatarola squatarola, 318 plover, piping, Charadrius melodus, 109 plover, semipalmated, Charadrius semipalmatus, 318 plover, upland, Bartyamia longicauda, 125, 321-322, 326 Plumatella, Bryozoa, 8, 44 polychaete (see Annelida, Polychaeta) pond-lily, Nymphaea, Nelumbo (plants), 79-81 pondweed, Polamogeton (plant), 79-71 poor-will, Phalaenoptilus nuttallii (bird), 3-37 poplar, Populus (tree), 121, 204-205, 282 poplar, balsam, Populus balsamifera, 105, 117-119 porcupine, Erethizontidae (mammal), Table 27-2, 3-49 porcupine, Evethizon dorsatum, Fig. 23-4, 270, 302-303 porcupine, Old World, Hystricidae, 271-272 porpoises, dolphins and whales, Cetacea (mammals), 359 Potamogeton, pondweed, 92 potato, Solanum tuberosum, 196 potworms, Enchytraeidae, 39, 132-134, Table 8-5 Primates (monkeys, apes, man), 272 pronghorn, Antilocapra americana (mammal), Fig. 8-2, Fig. 25-1, 127-128, 164-165, Table 9-4, Table 9-5, Table 25-1, 175, 244, 326, 328, 328-329, 329-330, 335, 337 Prospaltella berlesi (insect), 227 protozoan, Protozoa, 23, 39, 47, 54-55, 60, 64, 66-67, 67, 71, 74, 82, 87, 87-88, 92, Fig. 10-5, 104-105, 125, 130, 130-131, 166, 173, 174, 177, 178, 178-179, 179, 180, 180-181, 181, 190, 195, 210, 219, 240, 254, 264, 319, 349, 352, 354-356, 356-357, 357, 365 protozoan, Toxoplasma, 241 protozoan, blood, Leucocytozoon bonazae, 241 Protura, telson-tail (insect), Fig. 9-6, 39 Psephenidae (see riffle beetles) pseudoscorpion (arachnid), 131-132, Table 9-7, Table 9-8 Psocidae, Corrodentia (insects), Table 9-7, 250 ptarmigan, rock, Lagopus mutus (bird), Fig. 24-2, 237, 238, 318, 319, 319-320, 320, 320-321 ptarmigan, white-tailed, Lagopus leucurus, 321-322, 322 ptarmigan, willow, Lagopus lagopus, 237, 238, 318, 319, 319-320, 320, 320-321 pteropod (marine snails), 357, 358-359 puffbird, Bucconidae, 270 pumpkinseed, Lepomis gibbosus (fish), Table 7-3 python, rock, Python sebae (snake), Table 25-1 Q quagga, Equus quagga (mammal), Table 25-1 quail, California, Lophortyx californicus, Table 15-5, 216, 312 quail, Gambel’s Lophortyx gambelii, 335-336 quail, mountain, Oveortyx pictus, 312 Index 46] R rabbit (see cottontail) rabbit, black-tailed, Lepus californicus, 128, Table 9-3, Table 25-1, 247, 326, 329, 329-330, 335, 337 rabbit, brush, Sylvilagus ‘bachmani, 312 rabbit, European, Ovyctolagus cuniculus, 19, 149, Table 25-1, 228-229, 235, 266, 299 rabbit, marsh, Sylvilagus palustris, 298 rabbit, pigmy, Sylvilagus idahoensis, 336-337 rabbit, snowshoe, Lepus americanus, Fig. 17-5, 223 294, 228, 228-229, 237, 238, 240, 241, 243, 302- 303 rabbit, swamp, Sylvilagus aquaticus, 278 rabbit, white-tailed jack, Lepus townsendit, 128, Table 9-4, Table 25-1, 247, 326, 329, 329-330 raccoon, Procyonidae Gnamienaly Table 27-2 raccoon, Procyon lotor, 86-87, 109, 114-115, Table 9-7, 178, 190, 295 racer, blue, Coluber constrictor (snake), 110, 125, Table 25-1, 297 298 racerunner, six-lined, Cnemidophorus sexlineatus, 110, 298 Radiolaria (amoeboid protozoans), 353, 357, 360 ragweed, common, Ambrosia artemisiifolia (plant), Fig. 8-11, 113, 115 ragweed, lesser, Ambrosia artemisiifolia, 190 rail, Virginia, Rallus limicola (bird), Table 7-4 Ranatra (see water scorpion) raspberry, black, Rubus occidentalis (briar), 112, 116- 117 banner-tailed kangaroo, Dipodomys spectabilis , 326 black, Rattus vattus, 145-146 bushy -tailed wood, Neotoma cinerea, 307 chisel-toothed kangaroo, Dipodomys microps, 336- 337, 337 cotton, Sigmodon, 128 desert kangaroo, Dipodomys deserti, 335 desert wood, Neotoma lepida, 311, 335 dusky-footed wood, Neotoma fuscipes, 311, 312 rat, eastern wood, Neotoma floridana, 298 rat, Fresno kangaroo, Dipodomys nitratoides, 248 rat, giant kangaroo, Dipodomys ingens, 248 rat, rat, rat, rat, rat, rat, rat, rat, rat, Heermann’s kangaroo, Dipodomys heermanni, 127, 248 rat, hispid cotton, Sigmodon hispidus, 298 rat, kangaroo, Dipodomys, 128, 165-166, 187, 329-330, 335-336, 338 marsh rice, Oryzomys palustris, 298 Merriam’s kangaroo, Dipodomys merriami, 335 mole, Spalax, Table 25-1 New World, Cricetidae, 190, 228-229 Norway, Rattus norvegicus, 145-146, 150, Table 15- 7, Table 16-5, 208, 217, 229 Old World, Muridae, 270 Ord’s kangaroo, Dipodomys ordii, 336-337, 337 rice, Oryzomys palustris, 87 spiny, Echimyidae (mammal), Table 27-2 white-throated wood, Neotoma albigula, 312, 326, 335 rat, wood, Neotoma, 177 rattlesnake, Crotalus, 128-129, 272, 311-312, 339 rattlesnake, diamond, Crotalus atrox, 336 rattlesnake, Mojave, Crotalus scutulatus, 336 rat, rat, rat, rat, rat, rat, rat, rat, rat, rat, ay Index rattlesnake, prairie, Crotalus viridis, 125, Table 25-1, 337 rattlesnake, sidewinder, Crotalus cerastus, Fig. 8-10, 110, 336 rattlesnake, timber, Crotalus horridus, Fig. 22-3, 296- 297 rattlesnake, western diamond, Crotalus atrox, 336 raven, common, Corvus corax (bird), 302-303, 318 ray, Selachii (elasmobranch fish), 357 redcedar, eastern, Juniperus virginiana (tree), 105, 116-117 redcedar, western, Thuja plicata, 302 redhorse, northern, Moxostoma aureolum (fish), Table 5-2, Table 7-3 redhorse, shorthead, Moxostoma breviceps, Table 5-2, Table 7-3 redpoll, common, Acanthis flammea (bird), 306 redpoll, hoary, Acanthis hornemanni, 306 redstart, American, Setophaga ruticilla (bird), 248, 250, 296 redstart, painted, Setophaga picta, 311-312 redwood, Sequoia sempervirens (tree), 282, 302 reed, common, Phragmites communis (plant), 79-81, Table 7-6 reindeer, Rangifer tavandus (mammal), 286 Remora, Echeneidae (fish), 178 reptile, Reptilia, 24, 24-25, 38, Table 3-1, 74, 92, 96, 97-98, 99-100, 101, 151-153, 155, 155-156, 228-229, 270, 271, 286-288, 298, 299-300, 300, 319, 322, 330, 338, 339, 347-348 rhea, Rheidae (bird), Table 25-1, 270 rhinoceros, Rhinocerotidae (mammal), 215, 271, 271- 272, 344, 347 rice, wild, Zizinia (plant), 79-81 roach, cockroach, Blattidae (insect), 134, Table 9-7, 25,175 roach, Cryptocercus, 177 roach, wood, Parcoblatta, 107 roadrunner, Geococcyx californianus (bird), 335-336 robin, Turdus migratorius (bird), Fig. 10-2, 174-175, 150, 250, 297-298, 320-321, Table 23-1, Table 7-4 robin, European, Evithacus rubecula, 247 rodent, Rodential (mammal), 188, 230, 268, 270, 271, 271-272, 329, 337, 338 roller, Coraciidae (birds), 271-272 rose, Rosa (plant), 79-81, 179, 190 rosemary, bog, Andromeda glaucophylla (plant), Table 7-6, 92 rotifer, Rotatoria, 39, 47, 60, 66-67, 67-69, 71, 74, 81, 82, 87-88, 88, 92, 104-105, 130, 147, 177, 204-205, 215-216, 229, 319, 320, 349, 356 royalpalm, Florida, Roystonen elata (tree), 294 rush, Juncus (plant), 79-81, 316 s sage, bud, Artemisia spinescens, 333-334, 334-335 sage, bur, Fvanseria dumosa, F. deltoides (bush), Fig. 26-3, 333 sagebrush, Artemisia tridentata, Fig. 26-4, 117, 333- 334, 334-335, 337 sae Cereus giganteus (tree cactus), Fig. 26-3, 333, 8 saiga, Saiga tatarica (mammal), Table 25-1 salamander, Caudata, 85-86, 90, 134, Table 8-2, Table 9-7, 177, 253, 270, 271, 271-272, 272 salamander, Eusatina, Fig. 19-2 salamander, eastern four-toed, Hemidactylium scula- tum, Fig. 21-4 salamander, marbled, Ambystoma opacum, 296-297 salamander, red-backed, Plethodon cinereus, 296-297 salamander, slimy, Plethodon glutinosus, 253, 296-297 Salmon, Salmonidae (fish), 371 salmon, Atlantic, Salmo salar, 11-12, 43-44, 47-50, 158, 237, 368-370 salmon, chum, Oncorhynchus keta, 47-50, 368-370 salmon, Pacific, Oncorhynchus, 43-44, 47-50 salmon, pink, Oncorhynchus gorbuscha, 47-50, 223 salmon, sockeye, Oncorhynchus nerka, 47-50, 237, 240- 241, 247 sand-bur, Solanum rostratum (plant), 196 sanderling,Cvocethia alba (bird), 318 sandflea, Orchestia (amphipod), 368 sandpiper, Baird’s, Evolia bairdii (bird), 318 sandpiper, buff-breasted, Tryngites subruficollis, 318- 319 Sandpiper, Sandpiper, sandpiper, sandpiper, Sandpiper, sandpiper, least, Erolia minutilla, 318-319 pectoral, Erolia melanotos, 318 purple, Evolia maritima, 318 semipalmated, Ereunates pusillus, 318-319 solitary, Tvinga solitaria, 306 spotted, Actitis macularia, 109 sandpiper, stilt, Micropalama himantopus, 318-319 sandpiper, white-rumped, Evolia fuscicollis, 318-319 sapsucker, Williamson’s, Sphyrapicus thyroideus (bird), 307, 308 sapsucker, yellow-bellied, Sphyvapicus varius, 302-303, 308, Table 23-1 sardine, Clupeidae (fish), 361 Sargassum (brown algae), 359 Sassafras (tree), 282 sawfly, Tenthredinidae, Fig. 9-10, Table 9-7 sawfly, European pine, Neodiprion sertifer, 231 sawfly, larch, Pristiphora (Lyaeonematus) erichsonii, 92, 174, 309-310 Sawgrass, Zizaniopsis, 79-81 scale, black, Saissetia cyanea (insect), 227 scale-insect, Coccidae, 223, 227, 265, 337 scallop, Pecten (mussel), 363, 368 Scaphoideus luteolus, leafhopper, 227 scorpion, Arachnida (arthropod), 349 scorpion, Centrurus, 336 scorpion, water, Ranatra, Table 3-1, Table 7-1, 85 scoter, common, Oidemia nigra (bird), 318 scrub-bird, Atrichornithidae, 270 sculpin, mottled, Cottus bairdi (fish), 55 scup, Stenotomus chrysops (fish) seal, Pinnipedia (mammal), 359, 371-372 harbor Phoca vitulina, 319 harp, Phoca groenlandica, 319 aap ata fur, Callorhinus ursinus, 158, 182-183, 1 seal, ringed, Phoca hispida, 319 sea-lettuce, Ulva, (green alga), 365 sedge, Carex, 79-81, 92, 316, 316-317, 317, 341-342 seedsnipe, Thinocoridae (bird), 270 Sequoia, giant, Sequoia gigantea, 243, 291, 302 serval, Leptailurus serval (mammal), Table 25-1 seston, 66, 75 shad, American, Alosa sapidissima (fish), 43-44, 368, 368-370 shad, gizzard, Dorosoma cepedianum, 14-15, Table 5-2 shadscale, Atriplex confertifolia (shrub), 333-334, 334- 335, 337 shark, Selachii (elasmobranch fish), 178, 353, 357, 359, 361, 370, 371 sheep, Bovidae, 272, 321 sheep, Dall’s, Ovis dalli, 215-216, 321, 322, 322 sheep, mountain (bighorn), Ovis canadensis, Table 9-5, 187, 321, 322, 335 sheepshead, Aplodinotus grunniens (fish), Fig. 13-7 shiner, common, Notropis cornutus (fish), 50, Table 5-2 shiner, golden, Notemigonus crysoleucas, Table 5-2 shiner, redfin, Notropis umbratilis, Table 5-2 shiner, river, Notropis blennius, Table 5-2 shrew, Insectivora (mammals), 35-36, 99-100, 135-136, Table 9-4, Table 9-7, 188, 307, 308-309, 321 shrew, arctic, Sorex articus, 303, 308, 318 shrew, common, Sorex araneus, 299 shrew, Florida least, Cryptotis floridana, 298 shrew, little, Cryptotis parva, Table 9-3 shrew, long-tailed, Sorex, Fig. 9-12, 136 shrew, masked, Sorex cinereus, 87, 303, 318, 326 shrew, pigmy, Microsorex hoyi, 303 shrew, short-tailed, Blarina brevicauda, 87, Fig. 9-12, 113, 115-116, 136, Table 9-3, 246, 247, 295, 296 shrew, smoky, Sorex fumeus, 87, 303 shrew, southeastern Sorex longirostris, 298 shrew, water, Sorex palustris, 302-303 shrew, white-toothed, Crocidura, 299 shrike, loggerhead, Lanius ludovicianus (bird), 297-298, 335-336 shrike, northern, Lanius excubitor, 237, 306 shrimp, Palaemonetes, Table 7-1 shrimp, brine, Artemia gracilis, 59-60 shrimp, fairy, Eubvanchipus, Branchinecta, 88, 322 shrimp, marine (see Decapoda) Sialidae, Megaloptera (alderflies), 88-89 silverbell, Halesia carolina (tree), Fig. 3-3 Simuliidae (see black fly) Siphonophora (coelenterates), 357, 370 Sipunculida, Annelida (marine worms), 188 siskin, pine, Spinus pinus (bird), Fig. 23-3, 302-303, 309, Table 23-1 skate, Selachii (elasmobranch fish), 357 skink, brown, Leiolopiama laterale, 298 skink, five-lined, Eumeces fasciatus (lizard), 296-297 skunk, Mephitis, Spilogale, Table 9-4, 194 skunk, eastern spotted, Spilogale putorius (mammal), 298 skunk, Florida, Mephitis elongata, 298 skunk, prairie spotted, Spilogale interrupta, 326 skunk, striped, Mephitis mephitis, 178, 297-298 skunk, western spotted, Spilogale gracilis, 335 sloth, Bradypodidae (mammal), Table 27-2, 188, 270, 348, 349 sloth, ground, Megalonyx, Megatherium, Mylodon, 285 slugs, Philomycus, Deroceras, Pallifera (mollusk), 114, 124, 130, 132-134, Table 9-7, 296-297, 299 Index 463 smartweed, Polygonum pennsylvanicum (plant), 79-81, 112,113 snail, Gastropoda (mollusk), Table 3-1, 184-185, 265, 286, 338-339, 367-368 snail, fresh-water, 43-44, 47, 51, 52, 53, 54, Table 5-3, 55, 57, 66, 70-71, 74, 81, 82, 84, 85, 87, 89- 90, 90, Table 3-1, Table 6-2, Table 7-1, Table 13-1, 146, 147, 173, 319, 349 snail, land, 9, 38, Fig. 9-7, Fig. 9-14, 96, 97-98, 99, 105, 113, 114, 114-115, 124, 130, 132-134, 134, 136, 138, Table 8-2, 148, 155, Table 9-7, 171, 252, 258-259, 259, 265, 270, 296-297, 299, 299-300, 310, 313, 319, 338, 348-349, 349, 363 snail, marine, 352, 356, 357, 362-364, 363, 364, 365, 365- 366, 366, 368 snake, Reptilia, 110, 114-115, Table 9-7, 185, 215, 249, 268, 271, 271-272, 282, 299, 313, 337-338, 339, 349 snake, black rat, Elaphe obsoleta, 296-297 snake, boa, Lichanura roseofusca, 336 snake, brown, Storeria dekayi, 297-298 snake, bull, Pituophis catenifer, 336, Table 25-1 snake, Butler’s garter, Thamnophis butleri, 249 snake, chicken, Elaphe quadrivittata, 298 snake, commonking, Lampropeltis getulus, 336 snake, corn, Elaphe guttata, 336 snake, eastern hognose, Heterodon platyrhinos, 109-110 snake, garter, Thamnophis sirtalis, 125, 249, 249, 297- 298, 310 snake, leaf-nosed, Phyllorhynchus browni, 336 snake, long-nosed, Rhinocheilus lecontei, 337 snake, milk, Lampropeltis doliata, 297-298 snake, Plains garter, Thamnophis radix, 125 snake, red-bellied, Storeria occipitomaculata, 296-297 snake, ribbon, Thamnophis sauritus, 249, 297-298 snake, rough green, Opheodrys aestivus, 298 snake, smooth green, Opheodrys vernalis, 297-298 snake, southern hognose, Heterodon simus, 298 snake, striped whip, Masticophis taeniatus, 337 snake, tiger, Notechis scutatus, Table 25-1 snake, water, Natrix, 86 snake, western blind, Leptotyphlops humilis, 336 snake, western shovel-nosed, Chionactis occipitalis , 336 snake, whip, Masticophis flagellum, 336 Solenodon, Insectivora(mammal), 270 solitaire, Townsend’s, Myadestes townsendi (bird), 307 sora, Porzana carolina (bird), Table 7-4 sotol, Dasylirion (plant), 333 souslik, Citellus (mammal), Table 25-1 sowbug (see Isopoda) Sparganium, bur-reed (plant), 92 sparrow, Fringillidae (bird), 187 sparrow, Bachman’s Aimophila aestivalis, 298, Table 8-6 sparrow, black-chinned, Spizella atrogularis, 312 sparrow, black-throated, Amphispiza bilineata, 335-336 sparrow, Brewer’s, Spizella brewerii, 337 sparrow, chipping, Spizella passerina, Fig. 23-3, 109- 110, 297-298 sparrow, field, Spizeila pusilla, 297-298, Table 8-6 sparrow, fox, Passerella iliaca, 306 sparrow, golden-crowned, Zonotrichia atricapilla, 183- 184 sparrow, grasshopper, Ammodramus savannarum, 125, 326, Table 8-6 464 index sparrow, Harris’, Zonotrichia querula, 306 sparrow, Henslow, Passerherbulus henslowii, 125 sparrow, house, Passer domesticus, 145-146, 189 sparrow, lark, Chonestes grammacus, 109-110, 317 sparrow, Lincoln’s, Melospiza lincolnii, 302-303, 306 sparrow, rufous-crowned, Aimophila ruficeps, 312 sparrow, sage, Amphispiza belli, 337 sparrow, savannah, Passerculus sandwichensis, 306, 321 322, 326 sparrow, seaside, Ammospiza maritima, 298 sparrow, Sharp-tailed, Ammospiza caudocuta, 368 sparrow, song, Melospiza melodia, Fig. 12-5, 150, Table 15-5, 259, 297-298, Table 7-4 sparrow, Swamp, Melospiza georgiana, Table 7-4 sparrow, tree, Spizella arborea, 306 sparrow, vesper, Pooecetes gramineus, 109-110, 125, 326 sparrow, white-crowned, Zonotrichia leucophrys ,306-308 sparrow, white-throated, Zonotrichia albicollis, Fig.12-5, Table 23-1 sphaeriids (see clams, fingernail) sphagnum, Bryophyta (moss), 92, 316 Sphenodon, lizard-like reptile, 155, 270 spider, Avaneida, 37-38, 38, 81, 85, Fig. 8-9, Fig, 9-14, Table 3-1, 96, 104-105, 109, 113, 114, 114-115, 124, 130, 131-132, 134, 135, 136, Table 7-1, Table 8-3, 146-147, 147, Table 9-6, Table 9-7, Table 9-8, 198, 246, 248, 258-259, 310, 311-312, 319, 320, 322, 322- 323, 337, 347-348, 349 spider, spined, Argiopidae, Fig. 9-9 spider, tarantula, Eurypelma californicum, 336 spider-wasp, Pompilidae, 249 spike-rush, Eleocharis (plant), 79-81 sponge, fresh-water, Spongillinae, 8, 43-44, 44, 54, 55, 66-67, 87-88, Table 3-1, Table 6-2, 188 sponge, marine, Porifera, Table 3-1, 352, 354-356, 357, 360, 363, 364 springbok, Antidorcas marsupialis (mammal), Table 25-1 springhaas, Pedetes caffer, Table 25-1 springtail, Collembola (insects), 39, 81, 85, Fig. 9-6, Table 3-1, 131-132, 134, Table 7-1, Table 9-8, Table 11-1, 319, 322, 348-349 spruces, Picea (tree), 23-24, Fig. 8-14, 121, Table 7-6 165, Table 20-1, 282, 284, 286, 289, 291 spruce, black, Picea mariana, 92, Fig. 7-5, Table 7-6, 302, 209-310 spruce, blue, Picea pungeus, 302 spruce, Engelmann, Picia engelmannii, Fig. 9-5, 302 spruce, Sitka, Picea sitchensis, 302 spruce, white, Picea glauca, 230, 320, 309-310 squid, Cephalopoda (mollusks), 358-359, 359, 360, 361 squirrel, Sciuridae (mammal), 135-136, 149, Table 9-7, Table 27-2, 177, 183, 187, 228-229, 229 squirrel, Abert, Sciurus aberti, 148 squirrel, African ground, Xerus, Table 25-1 squirrel, Arctic ground, Sperophilus undulatus, 318, 320 squirrel, Beechey’s ground, Citellus beecheyi, 127 squirrel, Columbian ground, Citellus columbianus, 24-3 squirrel, Douglas’, Tamiasciurus douglasii, 307, 308 squirrel, fox, Sciurus niger, 113, 114, 140-141, 177, 190, 297-298 squirrel, Franklin’s ground, Citellus franklinii, Table squirrel, golden-mantled ground, Cifellus lateralis, 307 squirrel, gray, Sciurus carolinensis, 114-115, 115-116, 140, 145-146, 228, 295, 296 squirrel, ground, Cifellus, 128, 128-129, Table 25-1, 187, 321, 322, 326, 329, 330, 337 squirrel, Kaibab, Sciurus kaibabensis, 148 squirrel, northern flying, Glaucomys sabrinus, 302-303 squirrel, red, Tamiasciurus hudsonicus, 110, 302-303, 308 squirrel, Richardson’s ground, Cilellus richardsonii, Table 9-4 squirrel, rock, Citellus variegatus, 311, 312, 335 squirrel, round-tailed ground, Citellus tereticaudus, 335 squirrel, southern flying, Glaucomys volans, Fig. 9-12, 136, 295 squirrel, spotted ground, Citellus spilosoma, 335 squirrel, 13-striped ground, Citellus tridecemlineatus, Table 9-3, Table 9-4 squirrel, Townsend’s ground, Citellus townsendii, 336- 337 squirrel, western gray, Sciurus griseus, 311 squirrel, white-tailed antelope, Citellus leucurus, 336- 337 stag-moose, Cervalces scotti (fossil mammal), 285 starfish, Asteroidea (echinoderm), 237, 363, 365, 366, 371 starling, Sturnus vulgaris (bird), Fig. 10-1, 145-146, 150, Table 15-1, Table 15-2, Table 15-5, 211, 297- 298 stickleback, Gasterosteidae (fish), 64, 319 stickleback, threespine, Gasterosteus aculeatus, Fig. 2- 6, 7 Stomatopoda (crustacean), 357 stonecat, Noturus flavus (fish), Table 5-2 stonefly, Plecoptera, 43-44, 45-46, 53, 55, 82-83, 85, Fig. 5-4, Table 3-1, Table 5-3, Table 5-4, Table 5-5 Stratiomyidae (soldier flies), 57 striders, water, Gerridae, Veliidae, Mesoveliidae, 81, 83-84, 87, Table 3-1, Table 7-1 sturgeon, Acipenser (fish), 74-75 Suaeda (shrub), 334-335 sucker, Catostomidae (fish),19,51, 54, 74-75, 85, Fig. 5-1, 272 sucker, hog, Hypentelium nigricans, Table 5-2 sucker, white, Catostomus commersoni, Table 5-2 sugarberry, Celtis laevigata (tree), 113 sumac, smooth, Rhus glabra (shrub), 112, 187 sumac, staghorn, Rhus typhina, 190 sunbird, Nectariniidae, 271-272 sun-bittern, Eurypgidae (bird), 270 sundew, Drosera (plant), 187 sunfish, Centrarchidae, 50, 54, 64, 74-75, 90, Fig. 5-1 sunfish, green, Lepomis cyanellus, Table 5-2, 175 sunfish, longear, Lepomis megalotis, Table 5-2 sunfish, orangespotted, Lepomis humilis, Table 5-2 swallow, Hirundinidae (birds), 250 swallow, barn, Hirundo rustica, Table 15-5, 250, 297- 298 swallow, tree, Iridoprocne bicolor, 253, Table 7-4 swallow, violet-green, Tachycinata thalassina, 297-298 swamp-loosestrife, Decodon verticillatus (plant), 79-81 swan, whistling, Olor columbianus (bird), 318-319 sweetgum, Liquidambar styraciflua (tree), 113, 282 swift, chimney, Chaetura pelagica (bird), 261, 297-298 swift, European, Apus apus, Table 15-5 swordfish, Xiphiidae, 178, 371 sycamore, Platanus (tree), 282 sycamore, American, Platanus occidentalis, Fig. 8-11, 112, 113, 113-114, 116-117, 297 Symphyla (primitive arthropods), Fig. 9-6, 39, Table 9-8 Syrphidae (fly), 57 dh tabanidae (see horse fly) tamarack, Larix laricina (tree), 92, Fig. 7-5, 121, Table 7-6, 302, 309-310 tamerisk, five-stamen, Tamarix pentandyra (tree), 113 tanager, hepatic, Pivanga flava (bird), 311-312 tanager, scarlet, Pivanga olivacea, 296, 305, Table 23- 1, Table 7-4 tanager, summer, Pivanga rubra, 298, Table 8-6 tanager, western, Pivanga ludoviciana, Fig. 23-5, 307 tapeworm, Cestoda (parasitic flatworm), 178-179, 180 tapeworm, snake, Ophiotaenia perspicua, Fig. 12-2 tapir, Tapiridae (mammal), Table 27-2, 270, 271-272, 347 Tardigrada, Arthropoda (water bears), 39, 47, 71, 104- 105, 130, 319, 320 tarsier, Tarsiidae (primate mammal), 271-272 tayra, Tayra tayra, (mammal), Table 27-2 telson-tail, Protura (insects), Table 9-8 Tendipes plumosus (see fly, red midge) termite, Isoptera (insect), Fig. 12-1, 109, 164, Table 9-8, 174, 175-176, 177, 179, 251, 311-312, 338- 339, 344, 347, 349 termite, meridian, Amitermes meridionalis, Fig. 27-7 tern, Sterna virgata, S. vittala (birds), 247 tern, arctic, Sterna paradisaea, Fig. 10-10, 318 tern, common, Sterna paradisaea, Fig. 15-4, 215 Thais lapillus (snail), Table 28-1 Thallophyta (algae), 362-364 thrasher, Benedire’s, Toxostoma bendirei (bird), 335- 336 thrasher, brown, Toxostoma rufum, Fig. 12-5, 297- 298, Table 8-6 thrasher, California, Toxostoma redivivum, 312 thrasher, Crissal, Toxostoma dorsale, 335-336 thrasher, curve-billed, Toxostoma curvirostre, 335- 336 thrasher, LeConte’s, Toxostoma lecontei, 335-336 thrasher, sage, Oreoscoptes montanus, 337 thrip, Thysanoptera (insects), Table 9-7, Table 9-8 thrip, rose, Thrips imaginis, Fig. 16-11, 231-232 thrush, gray-cheeked, Hylocichla minima (bird), 306 thrush, hermit, Hylocichla guttata, Fig. 23-3, 302-303, Table 23-1 thrush, Swainson’s, Hylocichla ustulata, Fig. 23-3, 302, 303, Table 23-1 thrush, varied, Ixoreus naevius, 307 thrush, wood, Hylocichla mustelina, 296, 305, Table 8-6, Table 23-1, Table 7-4 thuja, Thuja (tree), 121 Index 465 tick, Acarina (arthropod), 175, 179, 181 tiger, Panthera tigris (mammal), 271, 271-272 tinamou, Tinamidae (bird), 214, 270 tintinnid, ciliate protozoan, 357, 358-359 tit, Parus (bird), 222 tit, blue, Parus caeruleus, Table 15-2, 222, 247 tit, great, Parus major, 223 titmouse, plain, Parus inornatus (bird), 311-312 titmouse, tufted, Parus bicolor, Fig. 12-5, 253, 296, Table 7-4, Table 8-6 toad, Bufonidae (amphibian), 97, 270, 271-272 toad, Bufo, Fig. 10-6, 109, 125, Table 9-7, 184-185, 185, 251 toad, Bufo americanus, 88 toad, Microhyla, 259 toad, eastern spadefoot, Scaphiopus holbrookii, 298 toad, Fowler’s, Bufo fowlerii, 88, 109-110 toad, red-spotted, Bufo punctatus, 336 toad, spadefoot, Pelobatidae, 88, 271-272 toad, tree, Hyla, 176 toad, western, Bufo boreas, 337 toad, western spadefoot, Scaphiopus hammondi, 337 toads, desert horned, Phrynosoma m’calli, p. platyrhi- nos, P. solare (lizards), 336 toads, Texas horned, Phrynosoma cornutum, 125, 311- 312 topminnow, Fundulus (fish), 44 topminnow, blackstripe, Fundulus notatus, Table 5-2 tortoise, fresh-water, Platysternidae (reptile), 271-272 tortoise, land, Testudinidae, 271 toucan, Ramphastidae (bird), 270 towhee, Abert’s, Pipilo aberti (bird), 335-336 towhee, brown, Pipilo fuscus, 312 towhee, green-tailed, Chlorura chlorura, 313 towhee, rufous-sided, Pipilo erythrophthalmus, Fig. 12-5, 297-298, 312, Table 8-6 tree-porcupine, Erethizontidae (mammal), 270 tree-shrew, Tapaiidae (mammals), 271-272 Trematoda (see flatworm) Tribolium (see flour beetle) Trichoptera (see caddisfly) triclads (see flatworm) trout, Salmonidae (fish), 55, 56, 56-57, 102, 181-182, 246, 368-370 trout, brook, Salvelinus fontinalis, 52-53 trout, lake, Salvelinus namaycush, 64, 74-75, 77 trout, rainbow, Salmo gairdnerii, 52-53 trumpeter, Psophiidae (bird), 270 trypanosome (flagellate protozoan), 181-182 tsetse-fly, Glossina morsitans, 149, 181-182, 344 Tubifex, Tubificidae (see Annelida, fresh-water) tucotucos, Cfenomys (mammal), Table 25-1 tuliptree (yellow-poplar), Liviodendron tulipifera, 113, 113-114, 116-117, 282, 294 tuna, tunny, Scombridae (fish), 178, 371 Tunicata, Hemichordata, 357, 363, 364 tupelo, black, Nyssa sylvatica (tree), 168 Turbellaria (see flatworm) turkey, Meleagrididae (bird), 214, 272 turkey, Meleagris gallopavo, 141, 215, 298 turnstone, ruddy, Avenaria interpres (bird), 318 turtle, Reptilia, 114-115, 158, Table 3-1, 184-185, 185, 215, 282 466 Index turtle, alligator snapping, Macrochelys temmincki, 86 turtle, box, Tervapene carolina, Fig. 12-7, 114-115, 296- 297 turtle, gopher, Gopherus agassizi, 336 turtle, map (geographic), Graptemys geographica, 86 turtle, musk, Sternotherus, 86 turtle, painted, Chrysemys picta, 86 turtle, sea, Cheloniidae, Dermochelyidae, 178, 370, 371 turtle, snapping, Chelydva serpentina, 86 turtle, softshell, Trionyx, 86 U ungulate, Artiodactyla, Perissodactyla (mammal), 183, 18 223, 270-271, 271, 328 urchin, sea, Echinoidea (echinoderms), 352, 363 urchin, sea, Stronglocentrotus franciscanus, S. purpur- atus , 259 uta, long-tailed, Uta graciosa (lizard), 336 uta, side-blotched, Uta stansburiana, 336 uta, tree, Uta ornata, 336 Vv veery, Hylocichla fuscescens (bird), 303-305, 305, Table 23-1, Table 7-4 Veliidae (see water striders) velvet-ant, Mutillidae, 106-107 Verdin, Auriparus flaviceps (bird), 335-336 vireo, Vireonidae (birds), 272, 299 vireo, Bell’s, Vireo bellii, 297-298 vireo, black-whiskered, Vireo altiloquus, 298 vireo, gray, Vireo vicinior, 313 vireo, Hutton’s, Vireo huttoni, 311-312 vireo, Philadelphia, Vireo philadelphicus, 306 vireo, red-eyed, Vireo olivaceus, 138, 248-249, 296, Table 8-6, Table 23-1, Table 7-4 vireo, solitary, Vireo solitarius, 115, 302-303, 303-305, 305, Table 8-6, Table 23-1 vireo, warbling, Vireo gilvus, 297-298 vireo, white-eyed, Vireo griseus, 298, Table 8-6 vireo, yellow-throated, Vireo flavifrons, 296, Table 8-6 virus, 181, 187, 228, 234-235, 235 viscach, Vizcacia (mammal), Table 25-1 vole, European meadow, Microtus arvalis, 19, Table 16- 5, 215, 228-229, 229, 234, 235, 237, 241 vole, heather, Phenacomys intermedius, 307 vole, long-tailed, Microtus longicaudus, 307 vole, Microtus, etc. (mice), 230 vole, montane, Microtus montanus, 223 vole, North American meadow, Microtus pennsylvan- icus, 87, 115, 128, 142, 223, 229, 237, 326, 368 vole, prairie, Pedomys ochrogaster, 113, 142, Table 9- 3, Table 9-4, 246-247, 326 vole, rock, Microtus chrotorrhinus, 303 vole, sagebrush, Lagurus curtatus, 336-337 vole, short-tailed meadow, Microtus agrestis, Table 15- 7, 217, 224, 228-229, 237 vole, singing, Microtus miurus, 321 vole, tundra, Microtus oeconomus, 318, 320 vulture, black Coragyps atratus, 298 vulture, New World, Cathartidae (birds), 272 vulture, turkey, Cathartes aura, 297-298 WwW walkingstick, Phasmidae (insect), Fig. 9-9, 107, 135, 192-194 wallaby, banded, Lagostrophus (mammal), Table 25-1 wallaby, hare, Lagorchestes, Table 25-1 walnut, Juglans (tree), 282 walnut, black, Juglans nigra, 113-114 walrus, Odobenus rosmarus (mammal), 319 wapiti, Cervus canadensis (mammal), Fig. 23-2, 127- 128, 158, Table 9-4, Table 9-5, 187, 297-298, 300, 297, 309, 321, 322 warbler, Audubon’s, Dendroica auduboni (bird), Fig. 23-2, 307, 308 warbler, bay-breasted, Dendroica castanea, 221-222, 253, Table 23-1 warbler, black and white, Mniotilta varia, 248, 303-305, Table 8-6, Table 23-1 warbler, blackburnian, Dendroica fusca, 110, 248, 303- 305, Table 23-1, 305 warbler, blackpoll, Dendroica striata, 306 warbler, black-throated blue, Dendroica caerulescens, 248, 303-305, Table 23-1, 305 warbler, black-throated, gray, Dendroica nigrescens, 311-312 warbler, black-throated green, Dendroica virens, 110, 248, 250, 303-305, Table 23-1, 305 warbler, Canada, Wilsonia canadensis, 248, 303-305, Table 23-1 warbler, Cape May, Dendroica tigrina, 253, Table 23-1 warbler, cerulean, Dendroica cerulea, 296 warbler, chestnut-sided, Dendroica pennsylvanica, 154, 248, Table 23-1 warbler, hermit, Dendroica occidentalis, 307 warbler, hooded, Wilsonia citrina, 298, Table 8-6 warbler, Kentucky, Oporornis formosus, Table 8-6 warbler, Lucy’s, Vermivora luciae, 335-336 Warbler, MacGillivray’s, Oporornis tolmiei, 312 warbler, magnolia, Dendroica magnolia, 248, 250, 253, Table 23-1 warbler, mourning, Oporornis philadelphia, Table 23-1 warbler, myrtle, Dendroica coronata, Fig. 12-5, 110, 308, Table 23-1 warbler, Nashville, Vermivora ruficapilla, 248, 302- 303, Table 23-1 warbler, olive, Peucedramus taeniatus, 311-312 warbler, orange-crowned, Vermivora celata, 307,312 warbler, palm, Dendroica palmarum, 306 warbler, parula, Parula americana, 298, Table 23-1 warbler, pine, Dendroica pinus, 115, 298, Table 8-6 warbler, prairie, Dendroica discolor, 109-110, 298, Table 8-6 aay prothonotary, Protonotaria citrea, 298, Table warbler, Swainson’s, Limnothlypis swainsonii, 298 amar Tennessee, Vermivora peregrina, 253, Table warbler, Townsend’s, Dendroica townsendi, 307 warbler, Virginia’s, Vermivora virginiae, 313 warbler, Wilson’s, Wilsonia pusilla, 302-303, 306 warbler, wood, Parulidae (birds), 272, 299, 303, 307 warbler, yellow, Dendroica petechia, 297-298, 298, Table 7-4 warbler, yellow-throated, Dendroica dominica, 115, 298, Table 8-6 wart-hog, Phacochverus aethiopicus, 344 wasp, Hymenoptera (insects), 124, 164, Table 9-7, 174, 179, 194, 240, 251 wasp, black digger, Tiphia, Fig. 12-3 wasp, digger, Bembex spinolae, Microbembex mono- donta, Fig. 8-9 wasp, gall, Cynipidae, 180, 196 wasp, gall, Biorhiza eburnea, Fig. 19-4 wasp, Indian mason, Eumenes conica, 15-16 waterfowl, Anatidae (birds), 181, 229 water-hyacinth, Eichhornia crassipes, 79-81, 172 water-lily (see Nymphaea) water penny (see beetle, riffle) waterthrush, Louisiana (bird), Sezurus motacilla, 248 waterthrush, northern, Seiurus noveboracensis, 248, Table 7-4, 306 waterweed, Elodea (plant), 79-81 waxwing, Bohemian, Bombycilla garrula (bird), 306 waxwing, cedar, Bombycilla cedrorum, 297-298 weasel, Mustela, Table 9-4, 299, 319 weasel, least, Mustela rixosa, 240, 303 weasel, long-tailed, Mustela frenata, 297-298, 326 weasel, mountain, Mustela arizonensis, 307 weevil, azuki bean, Callosobruchus chinensis (insect), 240 weevil, cocklebur, Apion pennsylvanicum, 114 weevil, grain, Calandra granaria, Table 16-3, 224 weevil, rice, Calandra oryzae, Table 15-7, 217 whale, Cetacea (mammal), 188, 215, 319, 353, 371-372 whale, baleen, Mysticeti, 191, 357, 361 whale, blue, Balaenoptera musculus, 370 whale, fin, Balaenoptera physalus, 370 whimbrel, Numenius phaeopus (shorebird), 318 whip-poor-will, Caprimulgus vociferus (bird), 296 white-cedar, northern, Thuja occidentalis (tree), 92, 105, Table 7-6, 302 whitefish, lake, Coregonus clupeaformis, 64, 74-75 whitefly, greenhouse, Tvialeurodes vaporariorium, 227 wildcat, Felis sylvestris (mammal), 299 wildebeest, Connochaetes (mammal), 344 wildebeest, black, Connochaetes gnou (mammal), Fig. 27-3, Table 25-1 wildebeest, blue, Gorgon taurinus, Fig. 27-3, Table 25-1 willow, Salix (tree), 79-81, 92, 105, 112, 113, 121, 179, 282, 297 willow, black, Salix nigra, 113 willow, sandbar, Salix interior, 113 witch-hazel, Hamamelis virginiana (shrub), 282 wolf, gray, Canis lupus (mammal), 128-129, Table 9-4, Table 25-1, 175, 195-196, 215, 226, 297-298, 299, 300, 302-303, 307, 318, 319-320, 322 wolf, red, Chrysocyon, Table 25-1 wolf, Tasmanian, Thylacinus cynocephalus, Table 25-1 wolverine, Gulo luscus (mammal), 303, 307, 318 wombat, Phaecolomys (marsupial mammal), Table 25-1 Index 467 woodchuck, Maimota monax (mammal), 113, 114-115, 178, 187, 297-298, 300 woodpecker, Picidae (birds), 270, 299 woodpecker, acorn, Melanerpes formicivorus, 311-312 woodpecker, black-backed three-toed, Picoides arcticus, 301, 302-303 woodpecker, downy, Dendrocopos pubescens, 296, Table 8-6, Table 23-1, Table 7-4 woodpecker, Gila, Centurus uropygialis, 335-336 woodpecker, hairy, Dedrocopos villosus, Fig. 23-3, 296, 302-303, Table 8-6, Table 23-1, Table 7-4 woodpecker, ivory-billed, Campephilus principalis, 298, 300 woodpecker, ladder-backed, Dendrocopos scalaris, 311- 312, 335-336 woodpecker, Lewis’, Asyndesmus lewis, 311-312 woodpecker, northern three-toed, Picoides tridactylus, Table 23-1 woodpecker, Nuttalls, Dendrocopos nuttalli, 311-312 woodpecker, pileated, Dryocopus pileatus, 296, Table 8-6, Table 23-1 woodpecker, red-bellied, Centurus cavolinus, Fig. 12-5, 296 woodpecker, red-cockaded, Dendrocopos borealis, 298 woodpecker, red-headed, Melanerpes erythrocephalus, 297-298 woodpecker, white-headed, Dendrocopos albolarvatus, 307 worm, stomach, Obeliscoides cuniculi, 241 worms (see Annelida) wren, Bewick’s, Thryomanes bewickii (bird), 149, 312 wren, cactus, Campylorhynchus brunneicapillum, 335- 336 468 Index wren, Carolina, Thryothorus ludovicianus, 298, Table 8-6 wren, house, Troglodytes aedon, Fig. 17-2, 149, 150, 211, 214-215, 215, 221-222, 223, 235-236, 253, 297-298, Table 7-4 wren, long-billed marsh, Telmatodytes palustris, 368, Table 7-4 wren, rock, Salpinctes absoletus, 321-322 wren, Short-billed marsh, Cistothorus platensis, Table 7-4 wren, winter, Trvoglodytes troglodytes, 302-303, 303- 305, 305, Table 23-1 wrentit, Chamaea fasciata (bird), 312 Yj yak, Bos grunniens (mammal), 323 yeast, Thallophyta, 159-160, 166-167 yellowlegs, lesser, Totanus flavipes (bird), 306, 321-322 yellowthroat, Geothlypis trichas (bird), 248, 297-298, Table 7-4, Table 8-6 yew, Taxus (shrub), 121 Z zebra, Equidae (mammals), Fig. 27-4, 271, 344 zebra, Burchell’s, Equus burchelli, Table 25-1 zooplankton, 66-69, 74, 75, 178, 183, 204, 206, 319, 351- 359, 359-362, 365-366 zooxanthellae, alga-like flagellates, 366 a Hutte pet i ; . i fittest i) sa : iy 4 ee ne ee A a ees eee ee