iii>*.SM: ?■:•.(■;.: M ' ^ ';'!••'.'' i ''■' i ■ ' J ; ' ' • ' !'';;;'';- [fViY'- . ■. » ■( ; ; 1 !('-•] J :'! ;i:'-': ';-'/■ is graphically represented in Diagram 9, on p. 312, where both physiological and genetic relations are indicated. CHAPTER III THE ANIMAL ENVIRONMENT: ITS GENERAL NATURE AND ITS CHARACTER IN THE AREA OF STUDY I. Nature and Classification of Environments (35a, 55, 58)' The environment is a complex of many factors, each dependent upon another, or upon several others, in such a way that a change in any one efifects changes in one or more others. The most important environ- mental factors are water, atmospheric moisture, light, temperature, pressure, oxygen, carbon dioxide, nitrogen, food, enemies, materials used in abodes, etc. In nature the combinations of these in proportions requisite for the abode of a considerable number of animals are called "environmental complexes" (55). It is our purpose to consider animals as inhabiting environmental complexes, rather than to isolate their responses to various single factors. The consideration of environmental complexes in any comprehensive way would consume much space and require extensive and special knowledge of many fields. Accordingly, we can present here only the briefest outline of some of the principles of classification, and the important features. If one is to understand the most elementary principle of the classi- fication of environments, he must recognize the distinction between local and (55, 58a) climatic environmental complexes. Local complexes are often referred to as secondary or minor conditions or as edaphic or soil conditions. The climate, and such features as types of vegetation covering large areas, e.g., steppe, deciduous forest, etc., are commonly regarded as climatic. Opposed to these, and lying within them, are the local conditions, such as streams, lakes, soils, exposure, etc, which are only indirectly dependent upon climate. The idea can be better illus- trated by the desert than by our own region. For example, in the Mohave Desert, the climatic conditions may be characterized as hot and arid. Within this desert are a few streams fed by mountain rain- fall. These streams are local conditions in themselves, and produce others, such as moist soil, and types of vegetation which do not belong to the desert. Within the area about Chicago are represented two geographic complexes, the savanna and the deciduous forest, and lying ' Numbers in the text in parentheses refer to references in the Bibliography (PP- 325-36). 42 FACTORS 43 in and among these are \'arious local complexes. The history to follow applies particularly to the local complexes. The analysis into factors applies to both local and climatic. II. The Important Factors and Their Control in Nature Little experimentation has been conducted with a view to determin- ing the relative importance of different factors in the control of animals within an environmental complex. It is known, however, that moisture (evaporating power of the air), light, and materials for abode are factors important in the life of land animals; carbon dioxide, oxygen, materials for abode (including bottom), and current, in the life of aquatic animals. The evidence for these statements cannot be presented here, but will be given in appropriate places throughout the discussion which follows. I. THE CONTROL OF FACTORS This is related to physiography, surface geology, and vegetation. a) Physiography. — In streams, current and oxygen content are determined very largely by physiographic conditions. Current is a function of volume of water and slope of stream bed. Oxygen content is largely determined by the rate of flow, and therefore is influenced by physiography. In lakes, oxygen content is determined by the depth, the temperature, and winds — ^physiographic factors are again important. On land, moisture and light are in a measure controlled by physiographic features. Slope and direction of facing profoundly affect vegetation, moisture, and light. b) Surface materials and vegetation. — Materials for abode are largely the surface soil or rock or the vegetation. Surface soil or rock influences the moisture. Both moisture and surface materials influence the kind and amount of vegetation. All are interdependent (350). Physiographic features change with time. Erosion changes the gradient of streams, the width of valleys, the steepness of valley walls and cliffs, the ground-water level, etc. The weathering of rock is a process familiar to all. It is the aggregate of processes by which the coarse and hard or massive materials are reduced to clay and soil. This requires time. The fact that vegetation grows upon the so-called sterile, coarse, rough-surface materials, usually scattered or ephemeral at first, but increasing in denseness with each generation, is also familiar (58), Plants add organic matter to the soil. This organic matter holds the water so that moisture increases and plants may increase. With such 44 ANIMAL ENVIRONMENT changes it is obvious that an area of sterile soil will support more animals as time goes on, than at the outset, when the conditions were such that only a few hardy species could live. Here again, then, time is the impor- tant factor in determining the change of the area, so as to be suitable for more species (because more species are adapted to live in the result- ing than in the initial conditions). The length of time which has elapsed since a given set of surface and physiographic conditions became exposed to the atmosphere is very important in governing the number, kind, and distribution of animals in a given area. c) The value of physiographic form. — Physiographic features are classified according to their form and their mode of origin. What is the importance of their forms and modes of origin to the animal ecologist ? Has a kame or an esker or a valley train any significance so far as animals are concerned? So far as anyone has been able to observe, the fact that they possess their particular form is of no significance whatever. Their relations to present ground-water level, their slope, relation to the sun, etc., are significant. The amount of surface soil and the denseness of vegetation are also of very great importance, and conditions in these respects are usually closely correlated with the length of time that the structures have been exposed to the atmosphere. Since age is important, we turn at once to the history of an area in order to learn the relative age of the various features present. We have parted company with the physiographer and his discussion of mode of origin, and are interested in origins only in point of time. III. History of the Region about Lake Michigan (59) I. physiographic history We will give the briefest possible account of the history of the Chicago area, following Leverett (59), Salisbury (57, 60), Alden (61), Atwood (62), Goldthwait (62, 63, 64), and Lane (65). The most important features of our area were shaped during and since the glacial epoch. To us, the only important movement of the ice was that of the last Wisconsin ice sheet. This came to us mainly from the east and north. It spread out over the great basins now occupied by the Great Lakes and thence pushed on to the higher rock to the south of them and reached its southernmost extent in Southern Illinois. In retiring from here (Fig. i) one of the positions in which the edge of the ice halted corresponded to the present Valparaiso Moraine. The crest of this moraine extends from the Fox Lake region (see map) FACTORS 45 ^ W 1 r % •' SHEET /; ^ 1 V-T~.-- ■^ ^ f, :^^ ^r" 4 A.-.. - — The History of the Chicago Region Fig. I. — Showing the region of the Great Lakes when the Wisconsin ice sheet was retreating from its maximum extent (after Atwood and Goldthwait). Fig. 2. — A part of the same area, showing the drainage of the ice sheet by the Kankakee and Huron rivers through Dowagiac Lake (from Lane after Leverett). Fig. 3.— Showing a later stage of the retreat of the ice sheet— the Glenwood stage (from Lane after Leverett). Fig. 4.— a later stage of the same— the Calumet stage of Lake Chicago (from Goldthwait after Leverett and Taylor) . Fig. 5.— a still later stage— probably the Tolleston stage (from Lane after Leverett) . Fig. 6. — A post-Tolleston stage (from Goldthwait and Atwood after Leverett and Taylor). 46 ANIMAL ENVIRONMENT southward around the head of Lake Michigan, nearly parallel with the shore, then northward into Michigan, there turning somewhat more to the east (Fig. 2). Beyond the edge of the ice, early lines of drainage were established and temporary lakes came into existence. All of our south- ward flowing rivers bore the sediment-laden waters from the melting ice. The results of this may be seen in the gravel and sand outwash, valley trains, etc., along the DuPage and other rivers, the more sandy portion usually being farthest downstream. In Southwestern Michigan, these early lines of drainage were by the St. Joseph and the Dowagiac valleys. In the latter a small lake is believed to have existed (Fig. 2). These waters did not flow into the south end of the lake, as at present, but united and flowed down the present course of the Kankakee River. The Kankakee marsh area and the region at the mouth of the Kankakee (Morris Basin) are believed to have been occupied by a lake. These basins are surrounded by sand areas which are probably the oldest in our area of study. Dunes are said to be present to the south and east of "Lake Kankakee," a few being present on the moraine in the extreme southeast corner of our map (frontispiece). The next stage was marked by the retirement of the ice from the position of the Valparaiso Moraine to the present basin of Lake Michigan. The drainage of glacial waters down the Fox, DuPage, and Upper DesPlaines rivers stopped (Fig. 3). The lakes to the south and east probably began to disappear. Later, the St. Joseph and Dowagiac changed their lower courses and flowed directly into Lake Michigan, which found an outlet by way of the lower DesPlaines. Now begins the history treated in the first bulletin of the Geographic Society (60), and Bulletin 7 of the Illinois Geological Survey and else- where (6 1 , 6 2 , 63 , 64) . The predecessor of Lake Michigan stood at a level 55 to 60 feet above the present lake. The stage is known as the Glen- wood stage of Lake Chicago. Cliffs were cut, beaches of sand and gravel were deposited, and dunes were formed. These are our second oldest sand and gravel areas. Their position is shown on the map (facing p. 52). The water then fell to a level of 35-40 feet above the present lake. This is known as the Calumet stage (Fig. 4). Here again cliffs and beaches of sand and gravel were formed, and constitute our third in point of age. These beaches have not been indicated on the map because their distribution within the state of Michigan has not been studied by physiographers. In the vicinity of Waukegan they are very close to the Glenwood beach. FACTORS 47 The lake again receded, probably to a low level, and readvanced to a 2o-foot level known as the Tolleston stage (Fig. 5). Here the develop- ment of beaches continued and the cutting of new clilTs was inaugurated. From these beaches, dunes were developed which are fourth in point of age. The position of these beaches is not indicated on the map. The lake is believed to have fallen after this to a level of 60 feet below the present level of Lake Michigan (60-62), which is known as the Cham- plain stage. At this time the sea came up the Gulf of St. Lawrence as far as Lake Ontario. Since the cliffs and beaches of this stage were again submerged, they are no doubt of some importance to the aquatic life in Lake Michigan, because they affected slope and bottom locally. The water rose again to a level 12 to 15 feet above the present lake, known as the Algonquin or post-Tolleston stage (Fig. 6), which was followed by a retreat to the present level. 2. THE FORMER CLIMATE AND ANIMALS (66) During the ice age, the entire region about Chicago was overridden by the ice, and plants and animals migrated southward. There are at present a few animal species which inhabit glaciers and ice fields, and probably such were the only regular inhabitants at that time. The tundra and coniferous forest were crowded to the southward, and with them the caribou, musk ox, and other northern animals. As the ice retreated north of the southern end of the basin of Lake Michigan and the Lake Chicago stage was inaugurated, a tundra climate no doubt prevailed in the Valparaiso Moraine. It was probably the breeding- place of the present tundra species of birds; the home of the musk ox, the caribou, the snow grouse, and other northern animals. The ponds grew aquatic plants and probably supported hordes of mosquitoes (2) and other aquatic insects in summer. Early Lake Chicago is said to show no evidence of life. If we may judge from Arctic lakes at present, it had a summer fauna, especially of small crustaceans and probably some fishes. As the ice retreated still farther northward, the coniferous forest displaced the tundra, and the musk ox and caribou were presumably only winter visitors; the woodland caribou and the moose were probably regular residents. Conditions in the lake were similar to those of the preceding stage. By this time a relatively rich flora and fauna probably existed. Organic material accumulated in the soil, shade was produced, etc. With the further retreat of the ice, the coniferous forest continued for a long time, but the plants and animals became gradually more and 48 ANIMAL ENVIRONMENT more like those of the southern portion of the coniferous forest (67), and gradually gave way through processes of ecological succession to the species of the present day. Just preceding our period, the mastodon roamed over the site of Chicago. The skeleton of one of these was found in a marsh near Crown Point, Ind., another at Cary, 111. IV. Extent and Topography of the Area Considered' The area which we shall consider has its center at a point 18 miles east of Lincoln Park. It extends 67 miles (108. i kilometers) to the east and to the west and 40 miles (64.4 kilometers) to the north and 40 miles to the south from this point. Measured from the mouth of the Chicago River it extends 85 miles (137 kilometers) eastward, 49 miles (79 kilo- meters) westward, 38 miles (61 kilometers) southward, and 42 miles (68 kilometers) northward. It is 80 by 134 miles (128.8 by 216 kilo- meters) and contains over 10,700 sq. miles (27,820 sq. kilometers). The range of altitude in the Chicago area is not great. The lowest part of the bottom of the lake included in our map is about 80 feet above sea-level. The highest point on the Valparaiso Moraine is 900 feet above sea-level, which gives a range of altitude of 820 feet. The surface of the lake is 581 feet above sea-level. The plain of Lake Chicago is ' See frontispiece map. The term " Chicago Area " has been appUed to regions varying in extent and direction, according to the points of view and interests of various authors. Chicago biologists have as yet written but little concerning the ecology of areas to the east of Millers, Ind. It becomes necessary to go farther from Chicago every year. The areas in Michigan and Northern Indiana offer the only substitute for those nearer to Chicago which are being so rapidly destroyed. The following maps covering the area have been published: 1. Lake Michigan a) U.S. Hydrographic Office, Maps Nos. 1467-75. b) U.S. Lake Survey Maps, Custom House Bldg., Detroit, Mich. 2. Land a) County surveyors often publish maps covering particular counties, e.g., LaPorte Co., Ind. b) lUinois Internal Improvement Committee, The Water-Way Report, Springfield, 1909. c) Topographic sheets of the U.S. Geological Survey (prepared for much of the region covered by our map). d) The U.S. Land Office has maps of the original land surveys which are said to . give roughly the distribution of prairies, forests, and marshes. e) Rand McNally & Co. publish maps of all local counties. /) Brown & Windes' (Chicago) map of the Fox Lake Region. g) Davis, "Peat" (map of marshes), Ann. Rept. Mich. Geol. Siirv., 1906. AREA OF STUDY 49 chiefly between 581 and 600 feet, and presents very little relief. The lowest point of land on our map is in the valley of the Illinois River below the entrance of the Kankakee. This is 480 feet above tide, or loi feet below the level of Lake Michigan. In passing from the lowest point in the lake shown on our map to the vicinity of Lake Zurich, which is the location of one of the high points on the moraine, one would travel 64 miles and make an ascent of only 12 feet per mile on the average. Indeed, if Lake Michigan were to become dry and its bottom a prairie, it would appear an undulating plain. V. Climate and Vegetation of the Area I. meteorological conditions affecting animals (68) The table (I) illustrates the fact that there are some notable differ- ences between the different parts of our area. Extreme points would TABLE I Temperature Mean Rainfall Sunshine Ratio of Rainfall to Evaporation Station April to September Year April to Sep- tember Year April to Sep- tember Year July. 1887, to July. 1888 Mean Mean of Maxima Mean of Minima Mean Chicago . . . South Bend 62.6 653 70.0 76.3 55.6 54-3 48 49 193 18.3 33-4 34-5 1695 hrs. 2616 hrs. .... 95% 105% show greater differences. The evaporating power of the air is probably one of the best indices of conditions which affect animals. The ratio of rainfall to evaporation is the only expression of the evaporating power of the air which has been mapped. Fig. 7 shows this phenomenon in Central North America, with our area indicated. 2. VEGETATION (69, 70) Those features of the vegetation which are called climatic must be discussed first. The two main climatic divisions of vegetation represented in the Chicago area are savanna including the prairie vegetation, and deciduous forest. The prairie, or savanna, as distinguished from steppe, is a strip of country (the forest-border area) a few hundred miles wide, from Athabaska to Texas, where trees, chiefly oak, hickory, basswood, so ANIMAL ENVIRONMENT and elm, occur in groves and along streams. It has the general form of a bow, with its central and most eastern point at Chicago (Fig. 8). To the east of Valparaiso, Ind., the forest is chiefly beech and maple (see frontispiece). The types are believed to stand in close relation to climate, especially to ratio of rainfall to evaporation (Fig. 7).^ The vegetation of local conditions, as indicated on p. 42, is different from that of the region as a whole and we are concerned in part with Fig. 7. — Map showing ratio of rainfall to evaporation in percentages, with area of special study inclosed in rectangle (after Transeau). Compare with Sargent's map of the "Forests of North America" (loth Census Report and. Fig. 8 below). the relations of the animal communities of local conditions to animal communities of the climatic vegetation. VI. Localities of Study In beginning the investigation of any biological subject from the point of view of general principles, the most important step is the selec- tion of the material (animals to be studied). In ecological work we ' A glance at the map shows us that our area of study is in the center of the Forest-Border Region. LOCALITIES STUDIED 51 have not only this, but we must make a still more important choice, namely, that of the locality of study. To make this selection one must possess a good knowledge of animal environments, such as we have touched upon in the preceding pages. I. BASIS OF SELECTION AND SUBDIVISION Such knowledge can be acquired from texts of physiography and plant ecology, and from special works on the area at hand. The basis Fig. 8. — Map showing the location of the plains, savanna (prairie), and forest regions of North America, with area of special study inclosed in rectangle (from Transeau after Sargent). of selection is either that of age or of present conditions, or both. The points selected for study are called stations. Stations are subdivided on the basis of plant and animal habitats into substations. The sub- stations may represent either formations or divisions of formations. For example, a station like Wolf Lake may be divided into sandy shore substation, vegetation of open-water substation, and embayment substation. 2. ENUMERATION OF STATIONS — GUIDE In the study at hand we have made use of a large number of stations which are enumerated below and are referred to in the text. The list Z. R METCALF 52 ANIMAL ENVIRONMENT of stations and accompanying remarks with the Guide Map may serve as a guide to the region about Chicago for field students. List of Stations with Direction and Distance by Rail from the Mouth of the Chicago River, and Transportation A. Aquatic Communities I. Large Lake Communities (chap. v). Station i. The open water, piers at Jackson Park, 6 miles south. Station la. The eroding shore, Jackson Park, introduced rocks. Station 2. The eroding shore, Glencoe, 111., C. & N.W. R.R., 20 miles north. Station 3. The depositing shore, Buffington, Ind., L.S. & M.S. R.R., and P. R.R., 22 miles southeast. Pine, L.S. & M.S. R.R., 24 miles southeast. Boats and launch from fishermen. II. Stream Communities (chap. vi). Station 4. Youngest ravines, Glencoe, 111., C. & N.W. R.R., 20 miles north. Station 5. Youngest brooks, Glencoe, 111., C. & N.W. R.R., 20 miles north. Station 6. County Line Creek, Glencoe, 111., 21 miles north. Station 7. Pettibone Creek, North Chicago, 111., C. & N.W. R.R., 34 miles north. Station 8. Bull Creek, Beach, 111., C. & N.W. R.R., 41 miles north. Station 9. Dead River, Beach, 111., 41 miles north. Station 10. Spring-fed streams and springs, Gary, 111., C. & N.W. R.R., 40 miles northwest. Station 11. Spring-fed streams and springs, Suman, Ind., B. & O. R.R., 52 miles southeast. Station 12. Rock ravine stream, the Sag, Joliet Electric, 22 miles southwest. Station 13. Intermittent headwaters, Butterfield Creek, Matteson, 111., I.e. R.R., 28 miles south. Station 14. Small swift permanent stream, Butterfield Creek, Floss- moor, I.e. R.R., 24 mUes south. Station 15. Larger swift stream and effect of rock outcrop, Thornton, 111., C. &. E.I. R.R., 23 mUes south. Station 16. Permanent headwaters and pre-erosion stream, Hickory Creek, Alpine to Marley, Wabash R.R., 28 to 31 miles southwest. Station 17. Permanent swift stream, Hickory Creek, Marley to New Lenox, Marley (Wabash R.R. only). New Lenox, C.R.I. & P. R.R. or Wabash R.R., 31 to 34 miles southwest. INSERT FOLDOUT HERE LOCALITIES STUDIED 53 Station i8. Sluggish small stream, North Branch of the Chicago River, Schermerville, CM. & St. P. R.R., 21 miles northwest. Station 19. Moderately swift, medium-sized stream, North Branch of the Chicago River, Edgebrook, CM. & St.P. R.R., 12 miles northwest. Station 20. Fine gravel bottom, DuPage River, Winfield, C & N.W. R.R., 28 miles west. Station 21. Gravel bottom, DesPlaines River, Wheeling, 111., W.C R.R., 2,2, miles northwest. Station 22. Sandy bottomed streams, headwaters of the Calumet, Otis, Ind., L.S. & M.S. R.R., 50 miles southeast. Station 23. Larger sandy stream. Little Calumet, Chesterton, Ind., L.S. & M.S. R.R., 42 miles southeast. Station 230. Deep river, E. Gary, Ind., M.C R.R., 36 miles southeast. Station 24. Small and intermittent sandy streams, South Haven, Mich. (4 miles south), steamer, 80 miles northeast. Station 25. Small sandy stream. Deep River at Ainsworth, Ind., G.T. R.R., 46 miles southeast. Station 26. Medium sandy stream, Black River, South Haven, Mich., steamer, 80 miles northeast. Station 27. Large drowned sandy stream with marsh border, Deep River, Liverpool, Ind., P. R.R., 31 miles southeast; boats at saloon. Station 28. Sandy large drowned stream. Grand Calumet, Clark, Ind., P. R.R. (destroyed by industrial waste), 25 miles south- east. Station 29. Sluggish stream of the base-level type. Fox River, Cary, 111.; boats near railroad bridge, C & N.W. R.R., 40 miles northwest. III. Small Lake Communities (chap. vii). Station 30. Wolf Lake (a) Roby, Ind., L.S. & M.S. R.R., P. R.R., electric railway from 63d St., and Sheffield boathouse, 15 miles southeast; {b) Hegewisch, L.S. & M.S. R.R., P. R.R., or South Shore Electric R.R., boats from Delaware House (not practicable at low water). Station 30(7. Small lake. Lake George, Ind. Electric railway from Ham- mond or to Hammond from 63d St., or from Robertsdale, L.S. & M.S. R.R., P. R.R., 18 miles southeast; boats near south end of lake. For information regarding Indiana lakes, boats, etc., see Report of the Indiana Fish and Game Commission for 1907. Station 31. Fox and Pistakee lakes. Fox Lake, 111., CM. & St.P. R.R., 50 miles northwest; boats at all hotels. 54 ANIMAL ENVIRONMENT IV. Pond Communities (chap. viii). Station 32. Young ponds, Pond i, Buffington, Ind., L.S. & M.S. R.R. or P. R.R., 22 miles southeast (i miles east from station). Station S3- Middle-aged pond, Pond 5, Pine, Ind., L.S. & M.S. R.R., 24 miles southeast (pond at rear of station). Station 34. Middle-aged pond, Pond 7, Pine, Ind., L.S. & M.S. R.R., 24 miles southeast (pond to the right in front of station). Station 35. Mature pond. Pond 14, Clark Junction, Ind., P. R.R., 23 miles southeast (the fourth pond south of bridge over P. R.R. tracks). Station 36. Late mature pond, Pond 30, Clark, Ind., P. R.R., 25 miles southeast (pond parallel with main street and east of school) Station 37. Senescent pond, Pond 52, Cavanaugh, Ind., South Shore Electric R.R., 27 miles southeast. Station 38. Prairie ponds, Roby, Ind., 26 miles southeast, east side of Wolf Lake, between second and third icehouses. Station 39. Morainic pond or small lake, Butler's Lake, Libertyville, 111., CM. & St. P. R.R., 36 miles northwest. B. Temporary Pond and Swamp Communities. AQUATIC PHASES (CHAPS. VIII AND x) Station 40. Young artificial temporary ponds. Pine, Ind., L.S. & M.S. R.R., 24 miles southeast (ponds i mile northwest of station). Station 41. Middle-aged temporary ponds, Pine, Ind., L.S. & M.S. R.R., 24 miles southeast (ponds i mile northeast of station). Station 42. Prairie temporary ponds, south of Jackson Park, I.C. R.R.^ South Chicago Branch to Bryn Mawr, 10 miles south. Station 43. Prairie temporary ponds, 8ist St. and Stony Island Ave., electric railway from 63d St. and Jackson Park Ave., south. Station 44. Temporary pond of prairie type, but being captured by shrubs. Pond 90 or 93, Ivanhoe Station, L.S. & M.S. R.R., to Gibson, Ind., and G. & I. R.R. to Ivanhoe (i mile south of Ivanhoe), 36 miles southeast. C. Marsh, Forest Margin, and Prairie Communities Station 45. Low forest margin (see Station 30). Station 46. Intermediate forest margin, Beverly Hills, C.R.I. & P. R.R., 12 miles southwest. Station 47. High prairie, Chicago Lawn, 63d St. electric railway, II miles southwest. Station 48. High prairie (some low prairie). Riverside, 111., C.B. & Q. R.R. or LaGrange electric railway, 12 miles west. Station 49. Temporary forest pond of early stage, Pond 93, near Station 44. LOCALITIES STUDIED 55 Station 50. Strictly temporary forest pond, Pond 92, near Station 44. Station 51. Spring-fed marsh, Gary, 111., C. & N.W. R.R., 40 miles northwest. Station 52. Swamp forest, elm, and ash, Wolf Lake, Roby, Ind., south- east (same as Station 30). Station 53. Swamp forest, wood west of Dempster St., Evanston, 111., C. & N.W. R.R., elevated, or surface cars, 12 miles north. Station 54. Tamarack swamp, Mineral Springs, Ind., South Shore Electric R.R, 46 miles southeast. (For other tamarack swamps, see map.) Station 540. Tamarack swamp, Pistakee, 111., 4 miles south of Fox Lake (see Station 31). D. Dry Forest Communities I. EARLY STAGES (CHAP. XH) Station 55. On rock, Stony Island, L.S. & M.S. R.R., 12 miles south on suburban loop. Also Pullman electric car from 63d St. and Jackson Park Ave. n. ON CLAY (chap. XII) Station 56. Bluff at Glencoe, lU., C. & N.W. R.R., 20 mUes north. Station 57. On sand, moving dunes. Mineral Springs, Ind. (near Lake Mich, and Station 54). Station 58. Lower beach, cottonwood and pine, Pine, Ind. (near Station 40). Station 59. Pine and oak, Miller, Ind., near bridge over the Calumet, L.S. & M.S. R.R., 31 miles southeast. Station 60. Black oak (same as Station 59 but near village). Station 61. Clark, Ind., near Station 28. Station 62. Cavanaugh, Ind., near Station 37. Station 63. Black oak, white oak, red oak, near Station 44. E. Moist Forest Communities (chaps. XI AND xn) Station 64. White oak, red oak, hickory, upland forest, near Station 56. Station 65. Forest on Blue Island, Beverly Hills, C.R.I. & P. R.R., 12 miles southwest. Station 66. Youngest flood-plain forest. New Lenox, III, C.R.L & P. R.R., also Wabash R.R., 35 miles southwest. Station 67. Early flood-plain forest, near Station 15. Station 67a. (Near station 71a). Station 68. Mature flood-plain forest, near Station 48. S6 ANIMAL ENVIRONMENT Station 69. Elm, basswood, oak, hickory forest, Gaugars (near New Lenox), 37 miles southwest, Joliet So. Electric R.R. from Joliet or New Lenox. Station 70. Oak, hickory, beech, maple, Suman, Ind., near Station 11. Station 71. Beech and maple, Otis, Ind., L.S. & M.S. R.R., 50 miles southeast. Station 71a. Beech and maple, Sawyer, Mich., P.M. R.R., 73 miles east (4 mUes southwest). Station yib. Beech, maple, and hemlock. Sawyer, Mich., P.M. R.R., 73 miles east (i| miles northwest). F. Secondary Communities Station 72. Roadsides, Flossmoor, 111., near Station 14. Station 73. South Haven, Mich, (see Station 24). Station 74. Stream contamination, Riverdale, 111., I.C. R.R., 17 miles south. Station 75. Pasturing of forests, Beatrice, Ind., C.C. & L. R.R., 45 miles southeast. Station 76. The growth of a modern city, Gary, Ind.; many lines of transportation; 27 miles southeast. VII. Legal Aspects of Field-Study The student must recognize that legally, when he leaves the public highway, he usually becomes a trespasser, even though he walks in a stream bed or along a lake margin. Public property is scarce. Still, since the cost of prosecution is far greater than the remuneration secured by it in the way of damages, etc., even the most unreasonable owners are not inclined to insist upon the enforcement of the laws concerning trespassing. It should be borne in mind, however, that owners or tenants are entitled to respect, and that as a usual thing they will not object to the student's working on their property if they be treated with courtesy. Damaging gates, fences, etc., should be carefully avoided, and gates should be left as they are found. Small wild animals such as insects, snails, etc., are not property, in the eyes of the law, and an owner would probably not be able to pre- vent their removal from his land except by trespass procedure. Many of the larger animals are considered as public property and are therefore protected by law. In most states nearly all birds are protected by law. It is usually legal to kill certain game birds in season, and certain con- demned birds at all times. Game mammals are protected in accordance with a similar plan. It is usually necessary that a license to shoot be LEGAL ASPECTS 57 obtained before shooting of any sort be carried on. This would apply even to the shooting of snakes, lizards, and such animals, as well as game. Fishes, turtles, and fresh-water mussels are protected in Illinois, as are fishes in nearly all states. The use of seines and nets of all sorts, including hand dip-nets, dynamite, and all other devices for securing fishes, is usually forbidden. The hook and line is the only exception in some states. Forbidden equipment is nearly always confiscatable, and the fines for illegal fishing are usually very heavy. In some states it is possible to obtain licenses or permits to take birds, birds' eggs, and sometimes fishes for scientific purposes. For specific information one should consult the state fish and game warden. CHAPTER IV CONDITIONS OF EXISTENCE OF AQUATIC ANIMALS I. Introduction: Comparison of Land and Aquatic Animals The conditions of existence of aquatic plants and animals are very different from those of land plants and animals. Some of the most important diflferences are as follows: a) Water, the surrounding medium, is about 768 times as heavy as atmospheric air at the sea-level. 6) The necessary gases are in solution in the water and their diffusion is much less rapid than in the atmosphere. c) The necessary inorganic salts are in solution in the surrounding medium. d) The necessary organic food substances for plants and some of the carbon compounds necessary for animals are in solution in the water and are taken directly by the plants and animals (47). e) Vegetation rooted to the bottom is important in most bodies of water. In large lakes like Lake Michigan, however, there are very few attached or rooted plants, and therefore nothing comparable to the vegetation of the land, or to the plant-eating animals which live on it, is to be found. Most of the plants float freely in the water. Such plants are present also, however, where rooted vegetation occurs. II. Chemical Conditions I. DISSOLVED content OF WATER In order to support animals and plants, water must contain certain minerals and gases in solution (71). Salts (carbonates, sulphates, and chlorides) of magnesium, calcium, and sodium and salts of potassium, iron, and silicon are practically always present in solution in water, and their presence in definite proportions is essential to the life of the animals (72). Water without these has been shown to kill fish (71). Dissolved gases in definite proportions are also necessary. Gases. — The chief facts regarding the occurrence of gases in nature and their solubility under experimental conditions are shown in Table II. The standard method of expressing quantity of gas in solution is in cubic centimeters per liter at 0° C. and 760 mm. of mercury (73). All values are therefore given in these terms. 58 CHEMICAL CONDITIONS * TABLE II Showing the Distribution and Solubility of Atmospheric Gases 59 Gas Values in Cubic Centimeters per Liter AT 0° C AND 760 MM. Mercury CoMPosmoN Kind of Water Having Gas Gas OF Air in At Temperature 20° C. 760 mm. Maximum Content Given Percentages Amounts Found in Natural Fish in Preceding Column Water Absorbs Water Absorbs Waters, Springs from Air Pure Gas Excepted Nitrogen, argon, etc. . 79.02 12.32 c.c. 15.00 C.C. 19.00 C.C. Lakes (74, P- 152) Oxygen 20.9s 6.28 C.C. 28.38 C.C. 24.00 C.C. Streams, lakes, win- ter, with green algae Carbon dioxide. . . . 0.03 0.27 C.C. 901. 00 C.C. 30.00 C.C. Ponds Ammonia. . . . Small traces Very large 14.00 C.C. Sewage con- locally quantities taminated Methane Small traces locally 34.00 C.C. 10.00 C.C. Bottom of lake in September (74,p.ioi) Nitrogen has little effect upon animals e.xcept when present in excess. Under these conditions in the laboratory, bubbles of the gas accumulate in the tissues and blood-vessels of fishes and cause death. It is not certain that such conditions exist in nature (Fig. 9). Oxygen is usually necessary to the life of animals. Most animals that have been studied select water with a rather high oxygen content instead of water with little or no oxygen. The resistance of animals to lack of oxygen varies in different groups. It has been found that water with about 6 c.c. of oxygen and 14 c.c. of nitrogen per liter is suitable for brook trout. Mackinaw trout have been taken in water containing but I c.c. of oxygen per liter (6). In general, carbon dioxide is a narcotic in its action upon animals. In small quantities it is a stimulant, especially to respiratory action. In large quantities it produces anesthesia and death. Several workers have shown that carbon dioxide is very toxic to fishes. Most aquatic animals that have been studied turn back when they encounter water containing large amounts of the gas. This turning away from carbon dioxide is much more decided than it is in the case of corresponding differences (24 c.c. per liter) in oxygen content. Fishes, for example, 6o AQUATIC CONDITIONS turn away when they encounter as small an increase as 5 c.c. per liter of carbon dioxide. Since a large amount of dissolved carbon dioxide is commonly accompanied by a low oxygen content as well as other important factors, the carbon dioxide content of water (strongly alkaline waters excepted) is probably the best single index of the suitability of the water for fishes. Fishes do not turn away from ammonia. Ammonia is rarely present in any great amount in nature. The effect of dissolved methane is unknown. Oxygen and nitrogen go into solu- tion from the atmosphere and oxygen is also produced by green plants. The other gases are produced chiefly by organisms as excretory and decomposition products. III. Physical Conditions I. CIRCULATION The distribution of dissolved salts and gases is dependent upon the circulation of the water, as their diffusion is too slow to keep them evenly distributed. The circulation of water in streams is probably such as to keep all dissolved gases and salts about equally distributed. The water of streams has been found to be supersaturated with oxygen (74). Oxygen is taken up by the water near the surface. Nitrogen and carbon dioxide are produced especially near the bottom, and if the water did not circulate they would be too abundant in some places and deficient in others for animals to live. In lakes, during strong winds (74), there is a piling-up of water on the leeward side and a lowering of the level on the windward side. This is usually com- pensated for by a downward flow of the waters along the bottom, as shown in Fig. 10. Small lakes with little exposure to the wind and with considerable depth frequently develop a summer circulation, such as is shown in Fig. 11. Such lakes are without oxygen in the deeper water in summer (74), and will not support the fishes which are known to inhabit the deeper water of Lake Michigan; hence we con- clude that Lake Michigan must have a deep circulation at all times. Fig. 9. — A marine fish affected with gas-bubble disease causing protrusion of the eyes, due to excess of dissolved nitrogen in aquarium water (after Gor- ham). CIRCULATION AND TEMPERATURE 6l We have been able to find no record of the amount of lowering of the waters of Lake Michigan at a given point, by the wind, nor any discussion of the relations of the surface currents to the effects of winds and the vertical circulation. The waves of large lakes rise to consider- able heights, as is familiar to all. They are of much importance in keeping a large amount of gas in solution in the lake waters. The current in streams differs from that in lakes in that it is for the most part in one definite direction, while the lake currents often alternate. There are backward flows and eddies at various points in streams, in front of and behind every object encountered in the current (57, p. 124). On the basis of the current, streams are classified as intermittent, swift, Fig. 10. — Showing the circulation of the water in a lake of equal temperature. W represents the direction of the wind (after Birge). Fig. II. — ^The circulation of the waters of a lake of unequal temperature (after Birge). moderately swift, sluggish, and stagnant or ponded. The current within the same stream differs at different times, and in different places. As we pass across a stream w^e find the current swiftest near the surface in the middle, and least swift at the bottom near the sides. 2. TEMPERATURE Temperature has always been regarded as of great importance in the direct control of the distribution of life in water. The tendency of modern investigation is to show that its influence is of great indirect importance, and the belief in its direct importance is correspondingly weakened. The temperature in a stream is probably about the same at the various points in any cross-section. The extent to which daily, seasonal, and weather fluctuations in atmospheric temperature affect a lake is 62 AQUATIC CONDITIONS determined by the depth. Small lakes with incomplete circulation in summer are cold at the bottom, being heated at the surface only (Fig. ii). Lake Michigan is a deep lake and none of these fluctuations is felt throughout (see Table III below and Table IX, p. 74). In summer the water of the surface is warmed, but if the vertical circulation is what we suppose it to be, all the heat in the waters flowing downward at the leeward side (Fig. 10) must be absorbed above no meters. Table III shows the temperatures recorded by Ward (75); these were evidently taken at the bottom and do not therefore represent the temperatures at the same level in the open water, especially those records made in the shallower situations where the sun's rays can reach the bottom essentially undiminished in intensity. TABLE III Temperature of Lake Michigan Hour P.M. Tempera- Tempera- Temperature at Date Unless Sky ture of ture at Depth in Depth Stated Air Surface Next Column Meters Feet Aug. 16 4:0s Clear 16. 7° c. 18.3° c. 18.3° c. 64.9° F. 5-66 18.6 Aug. 18 9:00 A.M. Cloudy 18.9° c. 17. 2° C. 16. 7° C. 62.0° F. 11.32 37-1 Aug. 18 12:25 Clear- ing 16. 7° C. 17. 5° C. 7.2°C. 44.9° F. 22.63 74.1 Aug. 16 S:io Clear 16. 7° C. 18.3° c. 7.5°C. 45.5°F. 32.06 105.2 Aug. 25 3:2s 20.0° C. i9.4°C. 7.2°C. 44.9° F. 43.38 142.3 Aug. 16 12:05 Clear iS.6°C. 18.3° C. 5.2°C. 41.3° F. 55-93 183.5 Aug. II 10:30 A.M. Hazy 18.9° C. s.i°c. 41 . 1° F. 108. 22 355-0 Aug. 16 1:50 Clear 16. 7'' C. 18.3° C. 4.2°C. 39.5° F. 1 1 2 . 00 367.5 Aug. 18 4:30 Scat- tered clouds 18.9° C. 18.3° c. 4.2°C. 39.5°F. 132.66 436.0 3. LIGHT (76) •Light is an important factor in controlling the distribution and activities of animals. The depth to which light penetrates water is therefore of importance. Forel found that in Lake Geneva, Switzer- land, during the period when the water was clearest, light diminished gradually from 25 to 65 meters, and then decreased rapidly to 115 meters where there was not sufiicient light to affect the photographic plate. No doubt future investigation with more accurate means of measuring light will show that very faint light penetrates much farther. The depth of light penetration in fresh water is usually determined by the amount of sediment in the water. Forel found that in Lake Geneva the depth of light penetration decreased with the melting of the mountain LIGHT AND PRESSURE 63 snows and the beginning of the rainy season. The drainage area of Lake Michigan is very small and has little relief, and the amount of sediment carried in is small at all times. The depth of light penetration is there- fore not so much influenced by these factors as in Lake Geneva. Wave- action is also important in stirring the bottom materials near shore. We would expect the light penetration in Lake Michigan to be least during the rainy and windy seasons, and greatest in calm, dry weather — late summer and autumn.^ All of the surrounding physiographic con- ditions are factors controlling light. Table IV shows the seasonal distribution of rainfall and light penetration in Lake Geneva, and the seasonal distribution of winds and rainfall at Chicago. TABLE IV Showing Depth of Light Penetration in Lake Geneva and Conditions Affect- ing THE Same in Both Lake Geneva, after Forel (76, Vol. 11, p. 439), AND Lake Michigan In the eighth column the results are given in seconds, in terms of the effect on the photographic plate, of equivalent exposures to the sun. Month Lake Michigan Lake Geneva, Switzerland (after Forel) Rainfall Inches January.. . February. . March. . . April May June July August. . . . September October. . . November. December . 2.0 2-3 2-5 2-7 3-5 3-7 3-6 2.8 30 2.6 2.6 2. 1 Centi- meters '''"^l^ir"'* ! Rainfall and Light Miles per Hour 5-2 6.4 6.9 8.9 9.4 9.2 7-2 7-7 6.6 6.6 5-3 17.8 20.0 20.4 19.4 18.3 14.4 14.6 134 16. 7 17.6 19.0 19.9 Meters per Second Prec. in Cm. Light Limit at Depth in Meters Light and Depth Intensity of Light (March) at Depth in Next Column 500 sec. 500 sec. 500 sec. 400 sec. 360 sec. 120 sec. 60 sec. 25 sec. 10 sec. 2 sec. o sec. Depth in Meters 19.6 25. 2 45 -S 5S-S 65.6 75-6 85.7 95-8 105-4 115. 6 4. PRESSURE (76) Pressure in w^ater increases with depth. The results given by Forel are shown in Table V. 'The Lake Michigan Water Commission has reported greatest turbidity in January, February, March, and April. 64 AQUATIC CONDITIONS TABLE V (76) Pressure in Atmospheres I 2 3 5 8 10 20 Depth in meters. 10.328 20.6 30.9 Si-5 82.4 103.27 206 . 49 It will be noted that there is a little more than one atmosphere increase in pressure for each 10 meters (33 feet) in depth because water is very slightly compressible. According to this, animals in the deepest parts of Lake Michigan are living under a pressure of about 375 pounds to the square inch. 5. BOTTOM The character of materials and topography of the bottom are very important to animals living on the bottom, but it has its effect also on free swimming animals as a determining factor in the amount of sedi- ment. The kind of bottom is important because many animals are dependent upon solid objects for attachment and are absent from bottoms made up of fine materials. Others must burrow into mud or creep on sand and gravel. This will be discussed later in special cases, particularly in streams. Topography of the bottom in shallow water is important in lakes locally in affecting wave-action and currents, and through these, bottom vegetation and temperature. Ward (75) noted such effects but did not carry the work far enough to solve any of the problems involved, which are usually local. In lakes, bottom materials are most important in shallow water, because of their effect in connection with wave-action, the amount of sediment in suspension, and the stability of the bottom. The bottom materials of lakes vary greatly locally. Taking Lake Michigan as an example, if we were to see the region about Chicago denuded of all vegetation, we would be able to appreciate the fact that there are bowlder deposits, gravel deposits, sand, clay, and bare rock. Evidently the ice sheet left the same kind of bottom materials strewn with the same irregularity in the bottom of the lake as on the land. Apparently wave-action has not affected them below 25 meters (85 feet). The waves of Lake Michigan are believed not to move sand below 9 meters (30 feet). It is thought that, during the Champlain stage, the lake stood at a level 60 feet below its present level. Along the north shore there is a cliff at this level with sand deposits lying on the side toward the deeper water. Inside of this is an area of clay and then, next VEGETATION AND FOOD SUBSTANCES 65 to the present shore, sand and gravel again. It is seen that this lower level of the lake influenced both the topography and bottom material locally, both of which probably have an influence on the occurrence of certain animals. 6. VEGETATION ^ The amount and kind of rooted vegetation is very important to animals. Of all the aquatic situations with which we have to deal Lake Michigan has fewest attached plants, and these are all algae. Cladophora, Chara, and filamentous algae are the most important. These do not appear to have been recorded below about 25 meters; some of them require solid bodies for attachment, and are probably most abundant on the rock outcrops of shallow water. The vegetation of the younger streams consists largely of holdfast algae like those along the rock shores of the lake. These are of impor- tance to animals. The more sluggish streams have rooted aquatic \'egetation. The vegetation is used as breeding-places. Eggs are stuck into plant tissues by the predaceous diving beetles {Dytiscidae) and by the water scorpions {Ranatra). Eggs are attached to plants by the electric-light bugs {Belostomidae) , back-swimmers, May-flies, ' caddis-flies, water scavengers (Hydrophilidae), long-horned leaf beetles {Donacia), snails, and many fish {Umbra, and probably Abramis). Young animals are often dependent upon plants for shelter, to escape from enemies, etc. Many animals must use plants as a means of reaching the surface for oxygen. The most important of these are the Dytiscidae (adults and lar- vae), the Hydrophilidae (adults and larvae), the back-swimmers, Zaitha, Belostoma, Donacia, snails, Ranatra, and Haliplidae. Some, for example Zaitha and dragon-fly nymphs, lie in the vegetation and wait for their prey. Different kinds of vegetation have different values for animals. The bulrush is barren for the following reasons: (i) hardness makes it a bad place for eggs; (2) there are no clinging-places ; (3) there is little shade; (4) it gives a high temperature in summer; (5) there is no great addition of oxygen by vegetation; (6) it does not afford a suitable place for securing food. Equisetum is unfavorable for similar reasons. Elodea is excellent; Myriophyllum, good; water-lilies and Chara, only fair. IV. Elementary Food Substances (47) Nitrogen, in the form of nitrates, is necessary for the growth of the plants of a pond, lake, or stream, and an insuflScient quantity is secured from mineral soil. Nitrogen can be taken from the air only by nitrogen- 66 AQUATIC CONDITIONS fixing bacteria, such as Azotobacter, an aerobe, and Clostridium, an anaerobe. These bacteria occur on the outside of plants and animals, in the mud of the bottom, etc. Plants and animals provide carbon for the bacteria ; bacteria provide the nitrites or nitrates for the plants. Ammonia, resulting from the decomposition of proteid of the dead bodies of plants and animals, is oxidized to nitrous acid; nitrous acid is oxidized to nitric acid by the bacteria {Nitrosomonas, Nitrobader, Nitro- coccus). This acid unites with bases to form nitrates and nitrites. There are accordingly two sources of nitrate and nitrite. Working against these are the denitrifying bacteria {Bacterium actinopelte [Baur]) which reduce nitrogen compounds to free nitrogen. Their work is influenced by temperature. Baur placed a standard quantity of nitrate infected with Bacterium actinopelte at several temperatures (47, p. 271) with results as follows: 1. Temperature 25° C: Denitrification began 24 hours after inocu- lation; in 7 to II days later the solution was nitrate-free. 2. Temperature 15° C: Denitrification began 4 days after inoculation; in 27 days the solution was nitrate-free. 3. Temperature 4-5° C: Denitrification began 20 days after inocula- tion; process incomplete 112 days after. 4. Temperature 0° C. : Denitrification not initiated. The quantity of life in water has been held by some to be in propor- tion to the available nitrogen. The amount of plankton in the sea is greatest in the polar regions in summer. It has been suggested that the greater retarding effect of low temperature on the denitrifiers, as compared with the producers of nitrates, is a cause of the greater quantity of Ufe in colder waters. Atmospheric nitrogen in solution is important in the building of nitrogen compounds by nitrogen-fixing bacteria. Oxygen is necessary for the life of most organisms, though a few can live for considerable periods in its absence. Carbon dioxide is necessary for starch building by chlorophyll-containing plants and animals. These organisms form the principal (food) basis of all other organisms. Complex foodstuffs, such as proteids, are necessary for most animals. It is only animals which contain chlorophyll in the form of algae living symbiotically in their bodies, or otherwise, that can live without taking in proteid from the outside. Proteids are made only when light for the production of starch, nitrates, and several other inorganic foods are present. Light is then indirectly necessary to animals which can live in darkness. The smaller aquatic animals are commonly either alga-eaters or predatory. The larger aquatic animals are commonly predatory or QUANTITY 67 scavengers. The rooted vegetation is eaten only to a small extent. Small floating or swimming plants and animals, called plankton (Figs. 12-18, pp. 75, 76) are the basis of the food supply of larger animals. We could probably remove all the larger rooted plants and substitute something else of the same form and texture without greatly affecting the conditions of life in the water, that is, so far as the life habits of the animals are concerned. The aquatic plants are commonly covered with a coating of green algae, protozoa, and other small organisms, so that animals such as small snails may rasp the surface of the plants and secure food without eating the plant tissues themselves. Plants in water are of particular use to animals as clinging- and nesting-places. V. Quantity (47) of Life in Water The quantity of living matter in water, so far as it is plankton or floating organisms, has been much studied. The quantity is usually expressed in one of two ways: number of organisms per liter or cubic meter of water, determined by counting a part of a collection; or in cubic centimeters per cubic meter of water. In Lake Michigan (August) Ward (75) found an average of 11 . 5 c.c. per cubic meter in water from the surface to 2 m.; from 2-25 m., 3.9 c.c; 25 m. to bottom, o. 4-1 . 5 c.c. He found that Pine Lake (a small lake) contained relatively less plankton than Lake Michigan, the surface stratum of Pine Lake containing more and the deeper strata much less than the larger lake. Lake St. Clair contains only one-half as much plankton as Lake Michigan. Lake Michigan contains only about one-tenth as much plankton as some of the small European lakes (Dobersdorfer See). Kofoid (77) found 71 .36 c.c. per cubic meter the maximum record for the Illinois River. The average for the year is 2.71 c.c. per cubic meter. The largest amount recorded by Kofoid is 684.0 c.c. per cubic meter (Turkey Lake, Ind.) . Small streams and lakes with large inflow and outflow have but little plankton. Large amount of plankton is commonly associated with high CO2 content, low oxygen content, and a large amount of carbonate in solution. The amount fluctuates from season to season. Kofoid (77) found the maximum for the Illinois River in April to June. The amount gradually decreases until December and January, when the minimum is reached. He also found evidence that the light of the moon increases photosjoithesis and the amount of plankton. The maximum of Crustacea was found by Marsh (78) to fall in July, August, and September, differing in different years. The maximum in Lake Michigan probably is usually 68 AQUATIC CONDITIONS in late summer or early autumn. Smaller bodies of water are similar in this respect. I. LAW GOVERNING QUANTITY (47) Liebig's Law of Minimum, as applied to plants, is stated as follows: "A plant requires a certain number of foodstuffs if it is to continue to live and grow, and each of these food substances must be present in a certain proportion. If one of them is absent, the plant will die; if one is present in a minimal proportion, the growth will also be minimal. This will be the case no matter how abundant the other foodstuffs may be. Thus the growth of a plant is dependent upon the amount of the foodstuff which is presented to it in minimal quantity" (47, p. 234). The amount of plankton is determined by the same law. All food sub- stances must be present in the correct proportions. The amount of plankton may be determined by one substance which is deficient in amount. 2. AGE AND QUANTITY (6 and citations) In bodies of water with small outlet, the quantity of plant and animal life probably increases with the age of the water body. This is because the foodstuffs are washed in by the inflowing water, and because rooted plants absorb food from the soil in which they grow, and when they die and decay these foodstuffs are added to the water. Accordingly, the older the pond and the longer rooted vegetation has grown, the greater the quantity of life. This principle is illustrated by an age-series of ponds at the south end of Lake Michigan to be discussed in detail later. The numbers used indicate relative age. Ponds i, 5, 7, 14, 30, 52, 89, and 95 were studied, but especially i, 5, 7, and 14 (6). Tables VI-VIII give a summary of the results. TABLE VI Showing Quantitative Results of Examination of Factors Related to Quantity or Plankton Pond Numbers — Age-Series No. of Collection Total carbonates in parts per million . . CO2, c.c. per liter* Oxygen, c.c. per liter* Bacteria per c.c 138.800 0.0 6.28 779 160. 200 3-4 3-47 2450 160.300 2.7 2.78 3550 •Average of collections, April, May, June, July, taken over sandy bottom (pond i) or at the top of submerged vegetation (ponds 7 and 14). QUANTITY 69 We note that on the whole the carbonates, CO2, and bacteria are greater in quantity according to age. Oxygen is on the whole less. TABLE VII Showing the Number of Entomoslraca in Approximately 90 Liters of Water Body of Water September 3, 4 April 30, 1910 Average of Collections in Parentheses Relative Age Wolf Lake 213 232 4,115 556 539 2,773 2,900 9,333 19,866 Aug. 28, 191 2 104 1,556 (3) 4,781 (3) 11,991 (3) 874 (6) 927 (6) 2,680 (6) 1 2 Prairie Pond I 3 Prairie Pond II Pond I 14 I Pond 7 7 Pond 14 14 Pond 30 1,039 351 2,600 2,870 ! 11,400 .... 1 2,480 30 Pond 52 52 Pond 89 89 Pond 0 ? 95 TABLE VIII Showing Ratio of Number or Quantity of Different Organisms When the Maximum Is ioo Rooted vegetation Entomostraca .... Midge larvae .... Sphaeridae Gilled snails Lunged snails . . . . A niphipoda Crayfishes Insects Fish Pond Numbers — Ecological Age-Series 20 32 80 o 20 10 50 10 40 100 60 35 80 50 50 50 90 SO 90 87 146 100 100 100 100 100 100 100 100 100 87 The Entomostraca are rated on the basis of actual count of six col- lections. The other figures are estimates (6). Here we note that the number of Entomostraca was greater in the older ponds though some irregularities occur, dependent upon the amount of rainfall. In rainy seasons the increase with age appears almost throughout. As we pass from younger to older ponds we note an increase in the number of animals, excepting fish. These appear to decrease, probably 70 AQUATIC CONDITIONS because of the increasing unsuitability of the ponds as fish breeding- places. The oxygen content decreases, particularly on the bottom. The distribution of the fish present in these ponds, and whose breeding habits were known, was found to be correlated with the distribution of the bottom upon which they breed. This becomes less and less in amount as the ponds grow older. 3. EQUILIBRIUM Each animal prefers certain food. The food relations of pond animals are shown in Diagram 3, below. For purposes of illustration let us suppose the existence of a community composed of the species named only. Pickerel^ Diagram 3. — Showing food relatfons of aquatic animals. Arrows point from the organisms eaten to those doing the eating. For explanation see text. Any marked fluctuation of conditions is sufificient to disturb the balance of an animal community (see chap, i, p. 18). Let us assume that because of some unfavorable conditions in a pond during their breeding period the black bass (79) decreased markedly. The pickerel, which devours young bass, must feed more exclusively upon insects. The decreased number of black bass would relieve the drain upon the crayfishes, which are eaten by bass, crayfishes would accordingly increase and prey more heavily upon the aquatic insects. This combined attack of pickerel and crayfishes would cause insects to decrease and the number of pickerel would fall away because of the decreased food supply. Mean- while the bullheads, which are general feeders and which devour aquatic insects, might feed more extensively upon mollusks because of the EQUILIBRIUM 71 decrease of the former (see chap, i, p. 15), but would probably decrease also because of the falling-off of their main article of diet. We may thus reasonably assume that the black bass would recover its numbers because of the decrease of pickerel and bullheads, the enemies of its young. A further study of the diagrams shows that a balance between the numbers of the various groups of the community would soon result. Diagram s Diagram 4. — Showing the life histories of the animals of the pond community in the form of circles. The heavy, vertical, black lines represent the animals which are dependent upon the most elementary food substances. A represents dead animal matter; B, the protozoa, rotifers, and Entomostraca, the smallest animal food. The black lines come into contact with different numbers of life cycles, but are indirectly connected with all so that any change in the position or rate of movement (meaning number or rate of reproduction and growth) of the rod must effect the entire com- munity; compare with Diagram 3. Diagram 5. — Showing the food relations in the brook communit3^ A repre- sents algae which grow upon the stones. B represents the floating animal bodies and other organic matter. The latter are of small importance because of their small number and the swift current. Under other circumstances, such as the extinction of the black bass, the resulting condition would be entirely different from the original one, but a balance between supply and demand would nevertheless finally be established. The community is said to have equilibrated when such a condition is reached; that is, a new equilibrium is established which may or may not be like the old. 72 AQUATIC CONDITIONS The causes of fluctuations of numbers of organisms are numerous. Cold winters often destroy aquatic vertebrates. Large rainfall dilutes the plankton in streams and carries it away. Too little sunshine causes a poor production of the chlorophyll-bearing organisms which are the food basis of all the others. High temperature favors denitrification. From Diagram 3 and brief discussion above it will be seen that there are in a pond community, close interrelations traceable to certain groups which are closely dependent upon the more elementary food substances. A representation of these relations is given in Diagrams 4 and 5. CHAPTER V ANIMAL COMMUNITIES OF LARGE LAKES (LAKE MICHIGAN) 1. Conditions I. GENERAL (75) Lake Michigan lies between 4i°-4o' and 46°-5' N. latitude. Its total length is about 350 miles and its greatest width is approximately 85 miles. Its area is about 25,000 sq. miles. Its greatest depth is nearly 275 meters (900 ft.) and its average depth is approximately 122 meters (400 ft.). Within the area covered by our map (frontispiece) there are about 3,200 sq. miles. The maximum depth is about 152 meters (500 ft.). It has been estimated that the lake contains 262,500,000,000,000 cubic feet of water. It becomes obvious at once that the lake constitutes one of the most uniform and extensive environments with which we have to deal. 2. CIRCULATION The level of the lake fluctuates from season to season with the amount of rainfall, but we have been unable to find a statement as to the amount of such fluctuation. Changes in atmospheric pressure over part of the lake cause various fluctuations in level, called seiches. In Lake Michigan there is a definite circulation of the surface waters. Here the current moves southward alon^ the west shore (57), around the head of the lake, and northward along the east shore. The rate of flow is 4 to 90 miles per day. II. Communities of the Lake* (80, 81, 82, 83, 84) One of the recognizable animal communities of Lake Michigan is made up of the animals which live freely in the water, either swimming or floating. This community is called the Pelagic or Limnetic com- munity. Other communities are governed directly or indirectly by depth ' The only published account of the invertebrate fauna of the Great Lakes is that of Lake Superior. From this account and from incidental scattered notes found in various publications cited we have been able to bring together enough data to give an idea of the conditions and life which we may expect future investigations to show. The attempts to study Lake Michigan have been ill-fated. In 187 1, the Chicago Academy of Sciences and the United States Fish Commission co-operated in an attempt to study the fauna of the lake. The work on the vertebrates was published 73 74 COMMUNITIES OF LARGE LAKES and bottom. Accordingly the conditions on the bottom at various depths are roughly shown in Table IX. TABLE IX Depth Vegetation Meters Feet Limit of sand-moving waves Limit of daily temperature fluctua- tions; limit of wave action; be- ginning of light decrease; pressure about 2\ atmospheres 8 25 39 54 70 "5 153 274 26 82 128 177 230 377 500 900 Lowest record of Chara Pressure 4 atmospheres; light re- duced to 5 and (75) Cladophora Scanty filamentous algae Seasonal temperature fluctuations less than 1°; light reduced to |; pressure 5* atmospheres Light 5 ; pressure 7 atmospheres . . . No light; pressure ii| atmospheres; no change in temperature; uni- form conditions (75) Nosloc and diatoms (75) No bottom plants recorded No plants recorded Greatest depth in the area con- sidered; pressure 15 atmospheres Greatest depth in lake; pressure 27I atmospheres No plants recorded No plants recorded I. THE LIMNETIC COMMUNITY (Station 1 ; List I) Chicago is famous for its good water supply. However, if one fastens a small sack of miller's bolting-cloth under an open water tap for an hour in summer and examines the contents of the sack with the naked eye and then with the microscope, he will be of the opinion that he has not been straining drinking water but stagnant ditch water. He finds small microscopic plants in great numbers (75), as well as large numbers of small animals, most of the larger ones dead. Every person drinking water from a lake or river drinks the small plants and animals. If every one of the 2,000,000 persons in Chicago drank a quart of unfiltered by the United States Fish Commission, and Doctor Stimpson of the Academy pub- lished a brief note on the invertebrate forms found in the lake, but never gave more than a hint of the work, as the collections were aU burned with the Academy's build- ing. Subsequently, collections were made by the State Laboratory of Natural His- tory, and later by the Fish Commissioners of Michigan. In the siunmer of 1902, the University of Chicago and the Academy of Sciences made a single-day excursion, but no report was ever published. LIMNETIC COMMUNITY 75 city water in a day in August, all together they would be consuming about lo quarts of solid plant and animal substance — enough to make a meal for about forty people. One does not think of the lake as an area of luxuriant vegetation, teeming with animal life, but rather as a barren waste of water. How- ever, if one's vision for small objects were only better, he would see as he passes over the water in a boat, thousands of small animals and plants such as are shown in Figs. 12-18 together with about fifty other forms of protozoa, wheel animal- , // iJ/ -^•^ .y- ■■% '"^■m^ 12 cules, crustaceans, insects, and small fish. Most of these spend their entire existence freely floating or freely swimming. With the exception of the fish and insects they consti- tute the plankton which is the basis of the food of the millions of pounds of fish taken from Lake Michigan every year. From the standpoint of our economic interests, the limnetic formation is of great importance. It deserves comment also because of its scientific interest, and the aes- thetic value of the vari- ous forms of which it is composed. a) Its composition (85, 86, 87, 88, 89). — The minutest animals of this formation are the protozoa. About thirteen species have been found to inhabit the open waters of the lake. Of these the sun animalcule (Actinophrys sol) (Fig. 12) and the shelled protozoan {Difflugia globu- losa) (Fig. 14) are easiest to recognize. Nine of the thirteen common species are mixotrophic in their nutrition (i.e., contain chlorophyll and manufacture their own food) (Fig. 13) and share with the algae and diatoms the important function of furnishing food for the rotifers (wheel animalcules) and the crustaceans. Fig. 12. — A sun animalcule {Actinophrys sol Ehrbg.); 330 times natural size (after Leidy). Fig. 13. — Protozoan {Peridinium tabulatum Ehrbg.); 400 times natural size (after Kent). Fig. 14. — A shelled protozoan {Difflugia globu- losa Duj.); 130 times natural size (after Leidy). 76 COMMUNITIES OF LARGE LAKES About a dozen species of crustaceans are common in the lake. They feed chiefly on the protozoa, diatoms, desmids, and possibly the rotifers (85). Such crustaceans constitute almost the sole food of young fishes and are the first food of the young whitefishes (79). They are divided into copepods and Cladocera (and ostracods, rare). This division of the crustaceans is known as the Entomostraca. The smallest and most Representative Crustaceans and Rotifers of the Limnetic Community of Lake Michigan Fig. 15. — A common copepod {Cyclops bicuspidatus); 25 times natural size (after Forbes). Fig. 16. — A cladoceran {Bosmina); enlarged (from Forbes after Gerstaecker) . Fig. 17. — A cladoceran {Daphne hyalina galeata); enlarged as indicated (after Smith) . Fig. 18. — A pelagic rotifer {Notops pelagicus Jen.); 180 times natural size (after Jennings) . Fig. 19. — The same, side view. abundant of the Entomostraca of the lake is only i . i mm. in length and is slender and colorless. It is the slender Cyclops bicuspidatus, shown in Fig. 15. The commonest C/a(/ocem of the lake are Bosmina (Fig. 16), Daphne retrocurva, and Daphne hyalina (Fig. 17). One other small species {Leptodora hyalina) belonging to this group is a very interesting creature. SHALLOW WATER COMMUNITIES 77 "When in its native element it is almost perfectly transparent and consequently invisible — a true microscopic ghost" (Forbes, 89). The wheel animalcules are as a rule larger than the protozoa and are of a much higher structural organization, capable of making more complex movements. About thirteen species of these may be found in the waters of the lake in midsummer. Notops p'ygmaeus Calm, (see Figs. 18-19) is a characteristic member of the group. In addition to these forms there are also worms, such as round worms, planarians, leeches, etc., found in the limnetic formation either inciden- tally or habitually. None of the adult fishes of the lake belong strictly to the limnetic formation. Fishes such as the whitefish, lake herring, and lake trout are sometimes found in the open water, and the young of some lake fishes may belong there strictly (90). b) Characters. — Specialists in the various groups of animals might be able to pick out some structural characters which would distinguish the forms of such open-water situations from the forms living in among the vegetation or on the bottoms of this or smaller lakes. The only striking structural character is the transparent or translucent color of most of the forms. A large number, if not all, of the limnetic crustaceans are in deep water during the day and come to the surface at night. The behavior of the rotifers is somewhat different. Jennings (87) says: "During the day the limnetic rotifers are found in much greater numbers near the surface than near the bottom, reversing the condition commonly observed for the crustaceans. At night the distribution seems not to be materially changed. The immense numbers of crustaceans obscure the rotifers; but there was no greater number of rotifers near the bottom in the few to wings made at night than in the day time." The most striking characteristic of the limnetic formation is that it is independent of bottom and in its reactions is indifferent to the bottom. Jennings (44) states that pelagic forms have a more simple type of behavior than the attached and bottom forms. 2. BOTTOM COMMUNITIES Forms inhabiting the bottom of lakes and also of the sea in a general way bear the same relation to the water that the terrestrial animals do to the surface of the land. Usually they do not leave it to rise to any considerable height above the bottom. The fishes of lakes correspond to the birds of the land. 78 COMMUNITIES OF LARGE LAKES Other relations are, however, different. As has been stated, there are no truly rooted plants in the bottom of Lake Michigan. Those attached to the bottom are not rooted in the way that land plants are. The things which land plants get from the soil are supplied to the aquatic plants by the water itself. The same is true of the bottom animals; food is floating in the water in quantities and can accordingly be secured without effort, and some animals have the form of plants and simply depend upon the food which may be brought within reach by accident. Classification of bottom formations: Bottom formations are de- termined by depth (and associated phenomena) and bottom. Bottom is of greatest importance in shallow water (less than 8 meters). Its importance is inversely proportional to depth. Within the zone of wave-action conditions are somewhat different than below it. Here the kind of animals is determined by (i) strength of wave-action, (2) erosion and kind of material eroded, and (3) deposi- tion, and animal communities may be classified as those of (i) eroding — rocky or stony — shores, (2) depositing or sandy shores, and (3) pro- tected situations. a) Eroding rocky shore sub-formation (80, 81, 82, 83, 84) (Stations la, 2 ; Table XV). — There are a considerable number of rock outcrops in the bottom inside the 8-meter (26 ft.) line, between Gross Point and the mouth of the Calumet River at South Chicago (61). As we shall see later, these are of great importance to the animals of the lake. However, the communities of such situations are known to us only through the study of the very shallow water in the vicinity of Glencoe. Here, attached to the rocks by their silk, are caddis- worms (Hydropsyche). (Mr. W. J. Saunders has given me specimens of Parnidae (Psephenus) and stone-fly nymphs (Perla) taken from Lake Ontario at Kingston, Ontario.) All these ordinarily live in swift streams. Under the stones and among the algae attached to them are amphipods {Hyalella knickerh-ockeri) and May-fly nymphs (Ephemeridae) , but so far as we have been able to record these are the only forms common here. The animals avoid the waves by creeping under stones or are attached to withstand wave- action. The lake trout (Fig. 20) is known to breed on the rocks off Lincoln Park. These rocks are then of considerable importance to the fish. Some species of small fish may be common here, but they have not been studied. b) Sandy depositing shore sub-formation, 0-8 meters (26 ft.), shifting sand bottom (Station 3; Table XII). — On the open shore inside of 1.5 meters (5 ft.) of water we have found nothing on the bottom. From this SHALLOW WATER COMMUNITIES 79 depth to 4 meters (13 ft.) Sphaerium vermontanum, which occurs rarely in Hickory Creek also, and midge larvae (a red and a white species) appear characteristic. A number of species of small fish such as the blunt-nosed* minnow, the straw-colored minnow, and shiners are likely to be found in from 4-8 meters (13-26 ft.) of water. An occasional Lymnaea woodruffi, is found at this depth. Representative Fishes Belonging Mainly to the Tr.\nsition Belt of Lake Michigan (25-54 m.) Fig. 20.— Great Lakes trout {Cristivomer namayctish); length 3 feet (after Jordan and Evermann). Fig. 21.— The long-jaw vvhitefish {Argyrosomus prognathus); length 15 inches; from the depth of 74 meters (after Smith). c) Communities of protected situations (Table X).— Near Chicago, bays and inlets are rare. Doubtless the mouths of some of the larger rivers before, they were modified for navigation, were of this character. Such places have been studied in Lake Superior (80, ^t,) and the Grand Traverse Bay region. Out of 21 species recorded here, 16 are definitely 8o COMMUNITIES OF LARGE LAKES recorded below 9 meters and not on the open shores. All are found in small lakes and sluggish streams. d) Lower shore formation (8-25 meters) (Station 3; Tables XI, XIII, XV).— The belt immediately below the shore belt is characterized by wave-action sufficient to move only the finest material. Its lower limit is the limit of wave-action; the beginning of light diminution; the lower limit of daily fluctuation in temperature; and the lower limit for most of the species of Mollusca (75, appendix). Practically all the forms that have been recorded here are inhabitants of still, shallow water also. Notable among these are the common still-water amphipod Eucrangonyx gracilis, the little bivalve Sphaerium striatinum, and several species of Amnicola and Valvata which, together with Lymnaea woodruffi, are more characteristic of Lake Michigan than of shallow waters. While a large number of Mollusca are recorded from the lake above 25 meters only the Sphaeridae are found below this limit. Small annelids, midge larvae, and leeches are very abundant north of Gary, Ind., in 1 1 meters of water. This belt is the principal breeding-ground of the whitefish. The eggs are deposited on the bottom and left unguarded. It appears that the young fish stay in the shallow waters for a considerable time. Wher- ever the bottom is firm the lake trout breeds also. Nearly all the fish traps are set in the upper edge of this belt and in the lower boundary of the one above. e) Belt of overlapping: upper deep-water belt (25-54 meters) (Tables XIV, XV). — This belt is characterized as below wave-action, below daily fluctuations of temperature, with seasonal fluctuations not exceed- ing f C. It is intermediate between the belt above and the deep belt, and is the characteristic feeding-ground of the whitefish and the regular home of the long-jaw (Argyrosomus prognathus, Fig. 21). On the other hand, it is the upper limit for some of the deeper-water forms, such as the well-known Mysis relicta and Pontoporeia hoyi (Figs. 22, 23), the deep- water crustaceans which are the chief food of the whitefish. f) Deep-water formation (54 meters to bottom) (Table XV). — This belt is characterized by weak or no light and by seasonal changes in temperature less than i degree. Below 115 meters there are no light and no seasonal changes, and the temperature is 4° C. throughout the year. Off Racine in 82 metefs (265 ft.) the bottom is of reddish-brown sandy mud (82); in 95-125 meters (311-410 ft.) dark-colored impalpable mud, depressions with decaying leaves (82a). In the Grand Traverse Bay region, Milner found decaying sawdust in 183 meters (600 ft.) (81). Except for unimportant variation in bottom, conditions are practically uniform throughout. Milner (81) states that the invertebrates are SUMMARY 81 abundant and evenly distributed throughout the deep-water belt. The principal invertebrates are Pontoporeia hoyi, Mysis relicta, water-mites, midge larvae, and a species of Pisidium. The fish, however, show some noteworthy peculiarities of distribution. The lake trout rarely leaves this belt, except during the breeding season. The blackfin {Argyrosomus nigripinnis) is below 70 meters, except in December, when it has been recorded in 60 meters. Hoy's whitefish Representative Crustaceans of the Deep-Water Community of Lake Michigan Fig. 22. — A schizopod {Mysis relicta); enlarged as indicated (after Smith). Fig. 23. — An amphipod {Pontoporeia hoyi) (after Smith). {Argyrosomus hoyi) is rare, and Triglopsis thompsoni has not been recorded above 115 meters; all accordingly live under uniform condi- tions— no day, no night, no seasons. III. Summary The available data on the conditions and life in the lake are of such a nature as to justify few conclusions of weight. We find only hints here and there which may be useful to those who shall investigate the lake in the future. I. Bottom forms are the most abundant on the open shores which are rocky, and which form good substrata for the attachment of algae and the holdfast organs of animals. 82 COMMUNITIES OF LARGE LAKES 2. The sand-depositing shores are without animals, at least to a depth of 1 . 5 meter, and life is scanty to 8 meters, on account of the shifting character of the bottom. 3. Animals are abundant in protected bays; the species inhabiting these situations are commonly found in sluggish streams and small lakes, and a few of them have been recorded below 8 meters also, which is relatively quiet water. 4. The animals of the upper shore belt, 0-8 meters, are found also in swift streams. 5. The animals of the lower shore and upper deep-water zone are below effective wave-action and are those found in still waters. 6. The animals of the deep-water zone are not found outside of deep lakes, and cannot be compared with any others of our Chicago area. 7. We have, then: swift- water animals in the upper belt, still- water animals in the middle belt, and deep-water animals in the lowest. 8. The fish are migratory and deserve special comment. DISTRIBUTION OF WHITEFISH AND DEEP-WATER FISH IN LAKE MICHIGAN (75) Argyrosomus artedi, the lake herring, is near the surface. Coregonus clupeijormis, the whitefish, lives most commonly between 21 and 36 meters; it spawns in water between 3 and 28 meters, most commonly between 15 and 19 meters. It makes migrations into the 9-meter belt in summer, supposedly on account of bad aeration; has disappeared where breeding-grounds have been destroyed. Argyrosomus prognathus, the long-jaw, is found mainly in from 36-66 meters. Argyrosomus nigripinnis, the blackfin, is found in from 70-80 meters, coming up to 60 in December. Argyrosomus hoyi, Hoy's whitefish, is usually recorded below 115 meters. Triglopsis thompsoni is confined below 115 meters. Cristivomer namaycush, the lake trout, is confined below 25 meters, except during the breeding season. It breeds between 2 and 25 meters on rock or other hard bottom. Loia maculosa, the lawyer, appears to be distributed throughout, but no account is to be found regarding its movements or their causes. An interesting truth is illustrated by the species of whitefishes {Argyrosomus and Coregonus). If a group is to be successful and become extensive in its distribution, it must so differentiate in habits as to bring the different races out of competition with each other. We usually find that different species which are closely related have different habitats. Here we have these species of fish arranged one above the other. The separation in such cases is usually horizontal. ANIMALS OF LARGE LAKES 83 Animals Recorded from Lake Michigan' LIST I Common Entomoslraca Copepods: Cyclops leuckarti Claus, C. biciispidatus Claus, C. prasinus Fischer, Epischiira lacustris Forbes, Diaptomus ashlandi Marsh, D. oregonensis Lil.; Clado- cerans: Daphne hyalina Ley., and D. retrocurva Forbes. TABLE X Animals occurring in protected situations (bays, harbors, etc.) in Lake Superior in from 0-2 meters of water, and known also to occur in Lake Michigan where habitats are not recorded: Common Name Scientific Name Literature Mussel Auodonla grandis Say (75, 83, 91) (75,83,91) (75, 83, 91) (75,83,91) Mussel Auodoiild maTginata Say Snail Amnicola lustrica Pils Snail Valvata Iricarinata Say TABLE XI Animals of the lower shore belt. Those definitely recorded from 8-15 meters of water are marked * and **, the latter indicating that the records are original from II meters of water north of Gary, Ind. (Station 3); f indicates that the animals are recorded from protected bays in 0-2 meters of water (Lake Superior), and H that they occur in inland waters, especially ponds: Common Name Scientific Name tH SnaU tH Snail tH Snail t1f**Snail t1I**Snail t1I**Snail n**Snail **Snail tH Snail t **Bivalve t1[**Bivalve t1f**Bivalve n* Bivalve tH* Bivalve n* Bivalve t1[**Bivalve t1f**Bivalve li* Midge larva. V Leech 1[**Worm Lymnaea stagnalis Linn Planorbis bicar hiatus Say Planorbis exacutus Say Amnicola Umosa Say Amnicola Umosa porata Say Amnicola emarginata Kiister A mnicola lustrica Pils Valvata bicarinala perdepressa Walk. Valvata sincera Say Pisidium idahoense Roper Pisidium scutellatum Sterki Pisidium compressum Prime Pisidium variabile Prime Pisidium ventricosum Prime Pisidium punctatum Sterki Sphaerium striatinum Lamarck Calyculina transversa Say Metriocnemis sp Glossiphonia stagnalis Linn , Limnodrilus claparedianus Ratzel.. . . Literature (75, 83, 91) (75, 83, 91) (75,83,91) (91) (91) (91) (91) (91) (75, 83) (83. 91) (91) (75, 83, 91) (75, ^i, 91) (75, 91) (80) (91) X (91a) X See citation q8. 'The numbers in parentheses in the column headed "Literature" refer to refer- ences in the Bibliography at the end of the bock. 84 COMMUNITIES OF LARGE LAKES TABLE XII Animals on depositing shores in from 0-8 meters of water, * indicating that records are original. Common Name Scientific Name Literature *Blood\vorm *Bivalve *Midge larvae *Snail Long-nosed sucker. . . Common sucker Hog sucker Red-horse *Trout perch Minnow Straw-colored minnow *Shiner *Blunt-nosed minnow. Top minnow Johnny darter Least darter Lake herring Pumpkinseed Bluegill Mud minnow Eel • Chironomid larvae Sphaerium vermontanum Prime (characteris tic) Metriocnemns sp Lymnaea woodruffi Baker (rarely) Catostomus caiostomiis Fors Calostomus commersonii Lac Catostomus nigricans LeS Moxostoma aureolum LeS Percopsis guliatus Ag Notropis hudsoniiis DeW. Clin Notropis blennius Gir Notropis atherinoides Raf Pimephales notatus Raf Fundulus diaphanus menonaj. and C. . . . Boleosoma nigrum Raf Microperca punctulata Put Argyrosomus anedi LeS Eupomotis gibhosus Linn Lepomis pallidus Mitch Umbra limi Kirt AnguiUa rostrata LeS (81, 84) (81, 84) (81. 84) (81, 84) (81) (84) (84) (84) (84) (84) (84) (84) (75- 84) (81) (81) (81) (81, 84) TABLE XIII Animals occurring in from 15-25 meters of water: Common Name Scientific Name Literature Snail. . . . Polyzoan Snail. . . . Snail. . . . Leech. . . Larvae. . Rotifer. . Rotifer. . Amnicola walker i Pils. . . . Plumatella sp Pleuroceridae Lymnaea sp Clepsine sp Neuropteroid insects. . . . Rotifer elongatus Weber . . Dinocharis tetractis Ehrbg (75, 83) (81, 82) (81, 82) (81, 82) (81, 82) (81, 82) (75) (75) TABLE XIV Animals occurring in from 25-54 meters of water: Common Name Scientific Name Literature Bivalve (82) Polyzoan Paludicella ehrenbergii van Ben (75) Polvzoan Fredericella sultana Blum (75) ANIMALS OF LARGE LAKES 85 TABLE XV Showing the recorded distribution of animals occurring in several of the vertical belts of Lake Michigan. The star indicates that the animal is present at the depth indicated at the head of the column in which the star occurs. B indicates that it breeds, and F that it feeds, at the indicated levels. The numbers in the column headed "Literature" refer to the Bibliography at the end of the book. The lower depth limit of many of the fishes listed is somewhat uncertain, as Milner does not indicate their exact distribution inside of 35 meters, but implies that thej^ may occur at the depths indicated in the table. Other records bear out Milner's implications. Common Name Sturgeon Crayfish Crayfish Long-nosed gar Lake catfish Croaker Perch Wall-eyed pike Large-mouthed black bass Small-mouthed black bass Northern moon-eye . . Toothed herring Tadpole cat Carp Pike Brook silverside Stickleback Whitefish Rock bass Amphipod Snail Long-jaw Lawyer Lake trout Hoy's whitefish Amphipod Schizopod Blackfin Small cottoid Scientific Name Acipenser rubicimdiis LeS. Cambarus propinquus Gir. Cambarits virilis Hag Lepisosteus osseus Linn. . . Ameiurus lacustris Wal. . . Aplodinolus grunniens Raf Perca flavescens Mitch. . . . Stizostedion vitreum Mitch. Microplerus salmoides Lac. Micropterus dolomieii Lac. Hiodon alosoides Raf. . . . . Hiodon tergisus LeS Schilbeodes gyrinns Mitch. Carpiodes sp Esox lucius Linn Labidesthes sicculus Cope. Eucalia inconstans Kirt. . . Coregonus clupeiformis Mitch Ambloplites rupestris Raf. Eucrangonyx gracilis Smith Lymnaea lanceata Gld. . . . Argyrosomus prognalhus Smith Lota maculosa LeS Crislivotner namavcush Wal '. Argyrosomus hoyi Gill (MSS) _ Pontoporeia hoyi Smith. . . Mysis relicta Loven Argyrosomus nigripinnis Gill Triglopsis thorn psoni Gir . . Depth in Meters Literature (75, 81) (7S,P.iS) (7S,P-i5) (81, 84) (81, 84) (84) (81, 84) (81) (81) (81, 84) (81, 84) (81, 84) (81) (81, 84) (81) (75: 81) (75, 81) (81) (80) (75, 80) (75) (75, 81) (75, 81) (75, 81) (82, 75) (82, 75) (75, 81) (75, 81) CHAPTER VI ANIMAL COMMUNITIES OF STREAMS I. Introduction The conditions in streams from headwaters to mouth have many features in common with lakes, like Lake Michigan. It is therefore appropriate that they follow the discussion of such a lake. The streams belong to two drainage systems — the Mississippi and the Saint Lawrence. All are tributary either to Lake Michigan or to the Illinois River. The principal tributaries of the lake near Chicago are the Chicago River, the Calumet River, Trail Creek, the Galien River, the St. Jospeh River, and the Black River. The principal tributaries of the Illinois River, with which we are concerned, are the Fox River, the DesPlaines River, the DuPage River, the Kankakee River, Salt Creek (III.), Hickory Creek. The factors of greatest importance in governing the distribution of animals in streams are current and kind of bottom. They influence carbon dioxide, light, oxj^gen content, vegetation, etc. These factors are controlled by age (physiographic), length of stream, and elevation of source above the mouth, all of which are physiographic. The typical stream begins as a gully and works its way into the land (Fig. 68, p. 112). The importance of some of the factors is greater in some stream stages than in others. For example, in the younger stages (a) material eroded, (b) relation to ground water, and (c) slope of stream bed play a more important role than they do in later stages. II. Communities of Streams I. classification The classification of stream communities is based upon physio- graphic history and physiographic conditions. In the early stages of stream development there are two tv^pes to be distinguished: (a) the communities of intermittent streams, and (b) spring-fed streams. As soon as the intermittent stream cuts below the ground-water level, it becomes much like the spring-fed stream. Permanent streams are divided into brooks, swift and moderate, and rivers, sluggish and moder- ate, with communities named accordingly. We undertake a discussion, first, of the history of the communities of streams developing in materials 86 INTERMITTENT STREAMS 87 easily weathered and eroded, containing bowlders, gravel, and occasional strata of hard rock. 2. THE INTERMITTENT STREAM COMMUNITIES (Stations 4-8; Tables XVII, XVIII) There are two types of these — intermittent rapids and pool communities. An Intermittent Stream Fig. 24. — The young stream at Glencoe in spring at high water, showing the leaf-barren trees. Fig. 25. — The same in summer, showing the stream entirely drj-. a) Temporary rapids consocies (Figs. 24, 25). — Small gullies in which water runs only when it is raining do not have any aquatic residents. As soon as such a gully has cut a channel deep enough to stand below ground-water level during a few days or weeks of the rainy season, aquatic insects make their appearance. The species which is usually found in the smallest trickle of water is the larva of the black fly, Simulitim (Figs. 27-32). As the stream grows a little larger, and per- haps even at such a young stage also, we sometimes find the nymphs 88 ANIMAL COMMUNITIES OF STREAMS of May-flies. Such streams have, however, no permanent aquatic resi- dents. These aquatic forms are not aquatic during their entire lives. They require water only during their early stages. If the water is running at the time the female is ready" to deposit eggs and if she is properly stimulated by the conditions, she deposits them without regard to future conditions. If the wet weather continues long enough, the larvae will mature and the other adults will appear, otherwise they die. This type of animals continues after the stream becomes large enough Stream Communities Fig. 26. — The pupal case of one of the caddis-worms (Rhyacopliila) from the rapids of the temporary stream at Glencoe; enlarged as indicated (original). Fig. 27. — The larva of the black fly {Simiiliuni); about 15 times natural size (after Lugger) . Fig. 28. — Pupa of the same (after Lugger). Fig. 29. — Pupa of the same in the pupal case (original). to have permanent pools. At such a stage the number of species is increased, but no two collections are alike (see Table XVII). Clinging to the upper surface of the stones are black-fly larvae, caddis-w^orms (Rhyacophilidae) (Fig. 26); under stones, May-fly nymphs, those col- lected as different times often belonging to different species. On some occasions there are great numbers of unidentifiable dipterous larvae and caddis-worms without gills or cases. Such a stream may possess any or all of these on one occasion, and none or only a few of them on another. INTERMITTENT STREAMS 89 Fig. 30. — The eggs of the black fly, about 15 times natural size (from Williston after Lugger). Fig. 31. — Side view of the adult fly (from Williston after Lugger). Fig. 32.— The same from above (from Williston after Lugger). 90 ANIMAL COMMUNITIES OF STREAMS b) Temporary pool consocies.— As a ^-oung stream grows deeper it often reaches some depression or marsh at its headwaters of which it forms the outlet in the early spring. It is now permanent for a longer period each season of normal rainfall, and small pools usually alternate with the rapids just described. In these pools aquatic insects, crus- taceans, and snails which belong primarily to stagnant ponds make their appearance. The first resident species are the crayfishes. They are found in the pools in the early spring when the water is high. The drying of the stream calls forth behavior suited to the conditions, and in summer their burrows are common in the stream bed. They come out at night and are preyed upon by raccoons, the tracks of which are commonly seen. c) The horned dace, or permanent pool communities. — The first per- manent parts are permanent pools. In these, conditions such as current, sediment, oxygen content, etc., are intermittent or spasmodic. The current in the rapids is distinctly spasmodic and conditions in these rapids are similar to those in the stream before even temporary pools were developed. Streams with permanent pools are represented in the Chicago region by many which enter the lake where high bluffs are present. County Line Creek (Figs. 24, 25) has been studied as an illus- tration of this type (Table XVII). The larger pools possess a practically permanent fauna. The char- acteristic forms are the crayfishes {Cambarus virilis and propinquus). The young are to be found in the pools at all seasons of the year. Water- striders, back-swimmers, and water-boatmen are common. Occasionally one finds dragon-fly nymphs (Aeshna constricta and Cordulegaster obli- quus), dytiscid beetles (Hydroporus and Agabus), crane-fly larvae, the brook amphipod {Gammarus fasciatus) , and the brook mores of the sow- bug {Asellus communis) (Fig. 55, p. 98). These are common among the lodged leaves. They move against water current. The species of fish (Table XVIII) which is most commonly found in the smallest streams (92) and nearest the headwaters of the larger streams is the horned dace or creek chub {Semotilus atromaculatus) (Figs. 33, 34). It possesses certain noteworthy physiological characters. Like many other species of fish, it goes farthest upstream for breeding (50). Its nest is made of pebbles. Often after the breeding season is over, and the adults have gone downstream, the water lowers so that young fishes are left in large numbers in small drying pools. Here they swim about, with their mouths at the top of the water, which is constantly being stirred up by the many tails, and which often contains much blackened, INTERMITTENT STREAMS 91 oxygen-consuming excreta and decaying plant materials. This would cause death to less hardy fishes. AUee (53) found very little oxygen in the waters of such pools. As it is, the pools often dry up, and the fish die. The second fish to enter a small stream appears to have many of the characters of the first. It is usually the red-bellied dace (Chrosomus erythrogaster), which breeds on sandy or gravelly bottom (93) but toler- ates standing water, being found also in some of the stagnant ponds at the south end of Lake Michigan. In some streams, the black-nosed dace (Rhinichthys atronasus) (Fig. 35) is second from the source. These fishes go against the current, but avoid the places where it is most violent. Breedixg Habits of a Pioneer Stream Fish Fig. ^^. — Showing, in longitudinal section, the nest of a horned dace (Semoliliis alromacitlatiis), with male and female fish in the nest. The stream flows in the direc- tion indicated by the arrow at the upper left-hand corner of the picture; | natural size (after Reighard) . Fig. 34. — Male and female horned dace during the spawning act. Each time the male clasps the female she deposits 25 to 50 eggs in the nest. Note pearl organs on the head of the male (after Reighard) . This one also breeds on gravel bottom, and can withstand the stagnant conditions of the summer pools. As the stream lowers its bed, this type of formation passes gradually into a later one. The beginning of the succeeding formation is heralded by the coming of the Johnny darter (Boleosoma nigrum), the common sucker {Catostomns conimersonii) (Fig. 36), and the blunt-nosed minnow (Pimephales notatus) (Fig. 37) (79). d) Characters of the communities. — The intermittent-stream com- munities are made up of animals which are dependent upon water during only a part of their lives and which possess a means of attach- ment and move against current (94) (positive rheotaxis). The pool communities are made up of animals tolerating great extremes of 92 ANIMAL COMMUNITIES OF STREAMS conditions and being also positively rheotactic. The fish are able to meet the current and to withstand the conditions of the stagnant pools. The crayfishes live in the water in the spring and burrow in the 35 .^.^m Pioneer Stream Fishes Fig. 35. — Black-nosed dace {Rhinichthys atronasus) (from Forbes and Richardson) . Fig. 36. — Common sucker {Catostomiis commersonii); length 18 in. (from Meek and Hildebrand after Forbes and Richardson). Fig. 37. — Blunt-nosed minnow (Pimephales notakts); length 2 to 3^ in. (from Forbes and Richardson). dry weather; adults of the aquatic insects creep into moist places when the stream dries. Allee (53) has found that isopods are positively rheotactic and that they can be acclimated to extreme conditions. SWIFT STREAMS 93 3. SPRING BROOK COMMUNITIES (Stations 10 and 11; Table XIX) In glaciated areas many of the streams are fed by springs which have not been produced by erosion, but are the result of porous and impervious layers of till arranged as in regions possessing artesian wells. The presence or absence and numbers of animals in a spring depend largely upon the chemical content of its water. Spring waters commonly have insufficient oxygen to support animals and at the same time may contain sufficient nitrogen and carbon dioxide to be detrimental if not fatal to animals. The mineral matter in solution may be large in quantity and in some cases poisonous also. As the water flows away from the spring it becomes aerated and diluted with surface water so that the animals of the spring brook can live in it. Spring consocies differ in different springs because of variations in the character of the water. In an area where there are springs, they are usually numerous. The little brooks unite to form larger streams. Typically, such streams may not be larger than intermittent streams, but a nearly constant flow at all times of the year is one of the characteristic conditions. Pools and rifiles are not so well defined, but contain some small fishes. The watercress grows abundantly at the sides of the stream and affords a lodging-place for aquatic animals not furnished so abundantly by young streams of other types. The water is colder in summer and warmer in winter than in other streams. Spring brook associations. — ^x\mong the watercress are the amphipods {Gammarus fasciatus), the larvae of Simulium attached to the leaves, beetles, dragon-fly nymphs, and young crayfishes. Here are also found occasional snails (Physa gyrina). The species of the cress association are nearly all found under stones or on stones in the riifies. On the stones are Simulium larvae and Hydro psyche (95), the net-building caddis- worm (Figs. 39, 40, p. 96). Under the stones are the nymphs of the May-fly {Baetis and Heptagenia), the larvae of flies and midges (Chironomus, Dixa, and Tanypus), the brook beetles {Elmis fastiditus) (Fig. 47, p. 98), and occasional amphipods and crayfishes. 4. THE SWIFT-STREAM COMMUNITIES As the spring brooks and the intermittent streams continue to erode their beds, they increase the extent of their drainage systems and become larger streams. Springs tend to disappear in connection with the spring brook and the intermittent stream reaches the ground-water level and becomes permanent. The two sets of conditions converge 94 ANIMAL COMMUNITIES OF STREAMS toward the larger swift stream (Fig. 38). While the conditions in these are like those of the spring brook, the watercress is absent and there are few rooted plants. Pools and riffles are well developed and the flow of water is constant, but fluctuates in volume. These streams differ in size, but the formation mores are practically the same, although larger species commonly inhabit the larger stream. a) Pelagic sub-formation is very poorly developed in the smaller streams and will be discussed in connection with sluggish streams. - ^^^Jubfi9EjKl^^^S■ SH^IPflH ^^^S^^^HkjSHH I^^^^kS^S ^^'snH^^SBBlfl SEv'k^H '^^^?3lHg Ei^Stl'VC^Bn •s* w --.. fti^'Ig^^HM Bjfes}|^M^fty| .^m9B E '^>^Bi^B^n^| ^K*Ntti^M ^^^H **» ( ^^- :. ^aHhH ^^9ci?'^^^BI A, >.jB|Mg^ "' *!jn3^^^^^^^^^^^^^l ■siH^^I H '^NfiflHl^^^^^l ■ 1 ''^'3j Fig. 38. — The permanent swift stream showing the stones in the rapids, and the stiller places below (New Lenox, 111., Gaugars Station) (original). b) Hydropsyche or rapids formations (Stations 14, 15, 17, 19, 20, 21; Tables XX, XXI, XXII). — These are usually due to the presence of coarse material or an outcrop of rock. They are tv-pical in streams with large bowlders and stones of all sizes. Here current is probably the controlling factor. In these streams, we find the best expression of the riffle formation, which we have seen is poorly developed in the smaller streams. This formation includes three ecologically equivalent modes of life, each meeting the current in a different way. These are (i) cHnging SWIFT STREAMS 95 to stones in the current, (ii) avoiding the current by creeping under stones, (iii) self-maintenance by strong swimming powers. Upper surface of stones (stratum i) : Here again we find the black-fly larvae, particularly in the smaller streams. They are provided at the posterior end of the body with a sucker surrounded with hooks (Figs. 27-32). The salivary glands are, as is common in insects, modified into silk glands and the silk is of such a nature that when it is brought into contact with a stone it adheres. The animals are usually found attached to the rock by the sucker, with the head downstream. The fans are extended and serve to catch diatoms and other floating algae. If for any reason the sucker gives way, the animal starts to float downstream. If the mouth can be brought into contact with a stone, the silk is exuded and the animal is held until it can make the sucker fast again. The pupae of this fly are also attached to the stones. They are surrounded with a cocoon. We have removed them from the stream and have found that they cannot make this cocoon in the absence of the current, but make a shapeless tangle instead. The adults deposit their eggs at the sides of the streams (96). On the tops of stones caddis-worms {Hydropsyche sp.) usually have cases made of pebbles stuck together with silk (Figs. 39, 40). They also have a net for catching floating food. The net faces the current (usually upstream) (Fig. 40). The river snail {Goniobasis livescens) (Fig. 54) is common on the upper surfaces of the larger rocks and is distinguished by a strong adhesive foot. These snails are usually headed upstream. When placed in a long piece of eave-trough into which the tap water was running at one end, they nearly all made their way to the upper end within a short time. They are ecologically equivalent to the caddis-worms and the black-fly larvae. Among the stones (stratum 2) : Of the animals living among stones, the darters are most important. Of these the banded darter (Etheostoma zonale) (Fig. 44), the fan-tailed darter (£. flabellare), and the rainbow darter {E. coeruleum) (97) (Fig. 45) live among and under the stones or in the algae which cover the rocks (especially the fantail). With them are some times, found the Johnny darter {Boleosoma nigrum), the black- sided darter {Hadropterus aspro) (Fig. 46), and the small bullhead or stonecat {Schilbeodes exilis). These fish are all positively rheo tactic. They apparently orient because of unequal pressure on the two sides of the body when it is not parallel with the direction of the current. Under the stones (stratum 3): There are many more forms living under and among the stones than on the tops of them. Here are the 96 ANIMAL COMMUNITIES OF STREAMS May-fly nymphs, the flattened Heptageninae, and the more or less rounded Siphlurus (95) (Figs. 48, 49, 50), evidently succeeding well together. This fact makes the value of the flattening as an adaptation appear nil. There are also the larvae of midges {Chironomus sp.) (98) and of horse- flies (Tabanus) (Figs. 51, 52). The adults of the latter deposit their eggs in great masses on the tops of the stones which protrude from the water. The stone-fly nymphs, similar to the Heptageninae May-fly Representative Aquatic Insects of a Rapids Community Fig. 39. — ^The net of the brook caddis- worm {Hydropsyche) seen from the front. Drawn from a specimen which made its case against the side of an aquarium (original). Fig. 40. — ^The same in its case with the net adjoining the opening which faces upstream (original). Fig. 41. — The larva of a caddis-fly {H el ico psyche) with a case made from pebbles, in the form of a spiral; 2I times natural size (original). Figs. 42, 43. — The water-penny larva of the brook, beetle (Parnidae) seen from above and below (43); 2^ times natural size (original). nymphs in form and appearance, are found here also. Perhaps the most bizarre of all are the water-pennies. These are round flat objects adhering to the under sides of stones, and not looking like animals at all. They are the larvae of a parnid beetle (Psephenus). Figs. 42 and 43 show two views of a larva. The old larval back becomes the cover for the pupa. Tha adults live under the stones also and their general appearance is like that of the parnid in Fig. 47. Sessile or attached animals are common in the brooks, but their numbers vary greatly from SWIFT STREAMS 97 year to year. On one occasion the surface of the rocks and stones in Thorn Creek was almost covered with sponge, and while some sponge is always to be found, we have not seen it so abundant again. Polyzoa Representative Fishes of a Rapids Community Fig. 44. — The banded darter {Etheostoma zonale); length 2 in. (from Forbes). Fig. 45. — The rainbow darter {Etheostoma coeriileum); length 2 in. (from Forbes). Fig. 46. — Black-sided darter {Hadropterus aspro); length 3-4 in. (from Forbes). are usually present under the stones. Such animals depend upon foods in solution and small floating jylants and animals. In addition to those rapids which have large rocks, are those in which the bottom is of coarse sand and gravel, with only a few small stones. 98 ANIMAL COMMUNITIES OF STREAMS Representative Animals of a Rapids Community Fig. 47. — An adult brook beetle {Parnidae); twice natural size (original). Figs. 48-50. — Different views of the nymph and adult of the May-fly {Siphlurus allcrnatus); 3I times natural size (after Needham). Fig. 51. — The eggs of a tabanid fly taken from a protruding stone; twice natural size (original). Fig. 52. — Adult fly. Fig. 53. — A water-strider {Rhagovelia collar is), from the margin of the swift brook (New Lenox, Gaugars); twice natural size. Fig. 54. — The common river snail (Goiiiobasis livescens), covered with calcium carbonate secreted by algae; natural size (original). Fig. 55. — An intermittent stream sowbug {Asellus communis); twice natural size (original). SWIFT STREAMS 99 Here we find the caddis- worm {Helicopsyche) (Fig. 41, p. 96), which has a spiral case made of sand grains. These are most abundant where some sand and swift current are both found. There is from time to time some vegetation in such situations and on it we find the brook damsel-fly nymph (Calopteryx maculata), the adult of which is the black-winged damsel-fly. Characters of the formation: The swift-stream formation has a striking behavior character, namely, strong positive rheotaxis. Other physiological characters, such as the toleration of only low temperatures and high oxygen content, and the necessity for current for the successful carrying-on of their building operations, are probably common to the animals. So far as the fishes of the rapids are known, they breed on coarse gravel bottom or under stones. The mores of the formation are, then, current resisting and current requiring, dependent upon large stones or rock bottom for holdfast and building materials. c) Sandy and gravelly bottom formation (pools) (Stations 15-22; Tables XVII-XXV). — The pools of streams with characteristic forma- tions are usually 2 or 3 to 10 feet deep, depending upon the size of the stream. The bottom is sand or coarse gravel. In these we find condi- tions very different from those in the rapids. The pools are the home of the rock bass (Ambloplites rupestris), the small-mouthed black bass (Micropterus dolomieu), the sunfishes (Lepomis pallidus and megalotis), and the perch (Perca flavescens) , together with a number of interesting small fishes whose distribution is shown in Tables XXI and XXII (79, 92). With these are also the mussels (91), frequently as many as nine or ten species, among which are Lampsilis luteola, ventricosa, and liga- mentina, the little Alasmidonta calceola (Figs. 57, 58), and Anodontoides ferussacianus (Figs. 59, 60), the last-named being perhaps the most characteristic of them all. They are often found beneath the roots of willows along the sides of the pools. Mr. Isely found that mussels migrate to shallow water during flood time. Mussels are dependent upon fish for a part of their lives. The young are carried by the adult until ready to attach to the body of the fish (99). When they leave the fish they are able to take care of themselves. Burrowing in the gravel are bloodworms (Chironomus sp.) (95, 98), the burrowing dragon-fly nymph (Gomphus exilis), a burrowing May-fly (Fig. 64a, p. 107), a caddis- worm, and occasionally snails, Campeloma (Fig. 61 or 64c) and Pleuro- cera (Fig. 64J). There are a few plants that grow on the sandy bottom in such places, and among these one finds the snail (Amnicola liniosa), AXIMAL COMMUNITIES OF STREAMS Representatives of the Pool Community Fig. 56. — A long-legged spider taken from a stone out of water in a stream {Tetragnatha grallaior); twice natural size (original). Fig. 57. — Outside of shell of a small mussel from Hickory Creek {Alasmidonta calceola); natural size (original). Fig. 58.— Inside of the same. Fig. 59. — Inside of shell of mussel from Hickory Creek (Anodontoides fenissacianus, subspecies subcylindraceus Lea); natural size (original). Fig. 60. — Outside of the same. Fig. 61. — ^A snail from the stiU water of Thorn Creek {CampeJoma subsolidum); natural size (original). Fig. 62. — A snail from the still water of Hickory Creek (Planorbis bicarinatus) , seen from the left; natural size (original). Fig. 63. — The same seen from the right. SANDY BOTTOMED STREAMS loi occasional aquatic insects, and hair-worms {Gordius). In some localities bivalved mollusks (Sphaeridae) and leeches are numerous. Under primeval conditions beavers are associated with the pool for- mation. They build dams which contribute to the deepening of the water of the pools. For a good account of their habits see citation ggb. An old beaver dam is supposed to have turned the waters of the DesPlaines out of the Chicago River and down the Chicago outlet. Characters of the formation : The mores of the pool formation are dis- tinctly those of partially burying the body just beneath the surface of the fine gravel and moving against the current. The few animals that make cases usually use gravel or sand grains. A single caddis-worm makes its case from small sticks such as commonly lodge in eddies. Some of the fishes breeding in these situations cover their eggs (50). Some fishes orient the body and swim upstream as a result of seeing the bottom apparently move forward below as the fish floats down (94). They behave the same if put into a trough with a glass bottom and the trough drawn forward. Some orient also when their bodies rub against the bottom when floating downstream. 5. the communities of sandy bottomed streams (shifting bottom sub-formations) (Stations 22-26; Table XXIV) We have studied the upper course of the Black River, the upper course of the Calumet River, and the Deep River, and two or three tributaries of Lake Michigan near South Haven. The kind of material eroded is of the greatest importance in determining the mores present in a stream. The streams of the eastern part of our area are in till which is sandy and their bottoms are sandy. This material is always slipping and moving downstream. There are few large stones. The bottom is not suitable for animals. The swift-water animals are almost entirely absent. The forms present are those which belong to moderately swift water. Composition and subdivisions. — Such streams are poorly populated. Their mores resemble those of the formations of the pools of streams eroding coarse material, but the shifting is so much more general and the species found so different, that it has been thought wise to separate the two. In the Michigan streams there are in summer a few scattered plants, which support a considerable number of insects; some of the brook beetles (Parnidae) are found attached to them. The logs and roots that happen to be in the water are important; they are the only I02 ANIMAL COMMUNITIES OF STREAMS places that support any amount of life. From these logs I have taken hundreds of specimens of small Parnidae, and with them predaceous diving beetles (Dytiscidae) which were found hiding in the cracks, also a few scattered caddis- worms (Hydropsyche) . The fauna of the bottom is made up of burrowing and semi-burrowing forms. The little dytiscid (Hydroporus mellitus Lee.) (99c) is characteristic: it has the habit of burying itself in the sand. The bivalved mollusks, especially mussels, are present. From the Deep River (upper course) we have taken nearly a dozen species. The only snail found is a burrowing form also. Animals of such a stream are subject to severe conditions. Many of them burrow. The substratum is very unstable and the logs and parts of trees to which many of them are attached are free to float down- stream with every flood. We know nothing of the reactions of these animals to various stimuli. They are distinctly subjects for investi- gation. 6. THE SLUGGISH STREAM COMMUNITIES (Stations 19, 27, 28, and 29; Tables XVII, XVIII, XX-XXV) There are several phases or types of sluggish stream formations. The most important of these are the sluggish or base-level creek, the sluggish river, and the drowned river. These are all illustrated in the Chicago area. The sluggish creek type is illustrated by the west branch of the DuPage River and its tributaries; the upper course of the west branch of Hickory Creek, Dune Creek, some parts of the Little Calumet south of Millers, and the Kankakee and some of its tributaries. The sluggish rivers are the Upper Fox, the lower St. Joseph, the Grand Calumet, the lower Galien, the lower Black, and others. These constitute a group of streams representative of the sluggish type about the Great Lakes. a) Sluggish creek sub-formations (Stations 16, 18). — The west branch of Hickory Creek has been studied in a cursory manner. The fish are a strange mixture of semi-temporary stream and pond forms. The black bullhead (Ameiurus melas) (79) is probably the most characteristic fish. The golden shiner (Abramis crysoleucas) and sunfish {Lepomis cyanellus) are also found. Baker (100) studied the upper portion of the east-north Chicago River. He recorded the same species of Mollusca as were taken in the upper part of Hickory Creek. He records also the black bullhead. The insects which he mentions are those commonly found in ponds. This SLUGGISH STREAMS 103 community is distinctly of the pond type in its general mores. Stagna- tion and low oxygen content and the partial drying of the stream are tolerated by all the residents. b) Sluggish river formations. — The conditions in sluggish rivers are different from those in smaller swift streams in many respects. The bottom is for the most part of fine materials; there are no rocks. The difference between pools and rapids no longer exists. The river is a gently flowing mass with relatively little distinction as to different parts. The margins of such streams are lined in summer with typical rooted and holdfast aquatic plants. The small bays and out-of-the-way spots, out of the current, support bulrushes and sometimes cattails. We can distinguish several formations in the Fox River: (i) The pelagic forma- tion, (2) the formation of sand and silt bottom (association of sandy bottom where the current drags in midstream or beats against the shore; association of silt bottom where least current is present), and (3) the formation of the zone of vegetation. Pelagic formation: This is well developed in the larger rivers, e.g., the Illinois River (77). While the Illinois no doubt differs from the Fox in many respects, doubtless the general features are much the same. It does not differ greatly from that of Lake Michigan. Burrowing May-fly or sand and silt bottom formations: On the bottom in ten feet of water we have found mussels (Anodonta grandis and Quadrula undulata), the snail (Goniobasis Uvescens), bloodworms (Chironomidae), green midge larvae (Chironomidae). On the old mussel shells were large colonies of the bryozoan Plumatella and occasional caddis- worms (Hydropsyche) (Figs. 39, 40, p. 96). On sandy bottom, conditions near the margin are similar to those on the bottom. We find here also an occasional snail (Goniobasis, Pleurocera, and Campe- loma), the midge larvae and bloodworms, occasional burrowing May- fly nymphs, and a number of mussels {Unio gibbosus and Quadrula rubiginosa being the most characteristic). There is also an occasional specimen of the long-legged dragon-fly nymph (Macromia taeniolata) and the black-sided darter. A considerable number of these species occur in the stillest pools of Hickory Creek, indicating the types that will dominate later. Silt is often found in particular spots. The most characteristic animals in this are the large mussel {Quadrula undulata), the burrowing May-fly nymph (Hexagenia sp.), and the bloodworms (Chironomidae). There are also the worms (Annelida) which burrow in the mud and protrude their anterior ends, often also the common mussel (Lampsilis luteola), the Sphaeridae, and the mud leech (Haemopis I04 ANIMAL COMMUNITIES OF STREAMS grandis). All of the animals of the silt formation burrow and prob- ably require little oxygen. Planorbis bicarinatus formation, or formation of the vegetation: Here we have for the first time the conditions which we find in ponds — . a dense rooted vegetation. With such a growth of vegetation we have a very different fauna: a large number of aquatic insects and pulmonate (lunged) snails. Of these there are a considerable number of species which must come to the surface of air, both in the adult and the young stages. The most important of these are the bugs: water scorpions {Ranatra fused) , the creeping water-bugs (Pelocoris femoraius) , the small water-bug {Zaitha fluminea) , the water-boatmen {Corixa sp.), the still- water brook beetles or parnids {Elmis quadrinotatus), several species of predaceous diving beetles {Dytiscidae) (99c), and water scavengers {Hydrophilidae). The pulmonate snails are Physa Integra, Planorbis bicarinatus (Figs. 62, 63), and often species of Lymnaea. Where the bottom is not too soft we often find numbers of viviparous snails (Campeloma) and an occasional mussel (Anodonta grandis). The crustaceans are distinctly clear- water forms: the crayfish (Cambarus propinquus) (loi), the amphipod (Hyalella knickerboekeri) , and the brook amphipod {Gammarus faseiatus) (102). The gilled aquatic insects are the May-fly nymphs {Caenis and Callibaetis sp.) and the damsel-fly nymphs {Isehnura verticalis) and dragon-fly nymphs (Aesehnidae and Libellulidae) . To practically all of these the vegetation is necessary as a resting-place or clinging-place, or a place to enable them to creep to the surface to shed the larval skin and become adult. Variations of the formation: The Fox is fairly representative of base-level rivers beyond the reach of tide-water except perhaps that the presence of gravel and sand in this stream may not seem fully in accord with this statement. There are, as has been noted, rivers near Chicago in which these conditions, which go along with old age in a stream, are still more marked. The lower Deep River is perhaps a good example of this. It is very sluggish and the bottom in the vicinity of Li\-erpool, Ind., is, so far as we have been able to ascertain, entirely covered with silt, with considerable humus mixed with it. The margins are peaty. The Calumet and the lower Black are similar. In these, sand and gravel areas, and animals which inhabit them, are reduced to a mini- mum and the silt and vegetation associations are better developed. Characters of the formation: The vegetation formation is distinct and clearly marked off from all others. The animals are dependent upon EFFECTS OF DROUGHTS AND FLOODS 105 the vegetation for support. The adult aquatic insects must creep to the surface of the water to renew their air. The forms that have gills are, at least many of them, dependent upon the vegetation for crawling to the surface to molt the old skin. The crustaceans are forms that cling to the vegetation and the snails must come to the surface for air. Doubt- less this formation should be divided into strata, but our data do not justify such division. III. Special Stre.\m Problems (103, 92) The first special problem is that of the relations of animals to seasonal changes, to changes in volume of water, amount of silt, shifting of bottom materials, and the seasonal aspects of the vegetation. The second prob- lem of streams is the historic or genetic, which includes the phenomena of the origin of the animals of the stream, their mode of entrance, and the effect of rejuvenation, drowning, etc. I. SEASONAL CHANGES Streams are more strikingly affected by rainfall and drought than are any other of the aquatic habitats. In extremely dry years streams dry up in the rapids where they have perhaps not been dry for a century. Floods change all the landmarks of the stream bottom and often scatter the animals of the stream over the flood-plain. a) Floods. — We found at the side of the high bank of the stream where the water is quiet at low water, the Johnny darter (Boleosoma nigrum), the little pickerel (Esox vermiculatus) , the tadpole cat {Schil- beodes gyrinus), the crayfish (Cambarus virilis), and an occasional Hydropsyche. Here were also an occasional sphaerid mollusk and one or two leeches. Caught in a mass of driftwood behind the roots of a tree were case- bearing caddis-worms {Phryganeidae), the black-winged damsel-fly nymph {Calopteryx maculata), the larvae of the black fly {Similium sp.), and two species of May-fly nymphs (one Heptageninae). The last two belong to the swift water, the others to the still water or the pools. During floods the still-water fauna and the swift-water fauna become mixed in the still places. At the time of our study there was a growth of rank weeds on the flood-plain. While the stream had been swollen for a long period and had stood higher than at the time of observation, little or no invasion of these weeds by aquatic animals had occurred. Animals evidently react negatively to such bottom and vegetation. io6 ANIMAL COMMUNITIES OF STREAMS We have had but little opportunity to study the swift-water forma- tion during floods, though some of the riffles in Butterfield Creek have been studied when the stream was bank full, but no marked changes were noted. It is obvious that the extreme floods which move large stones crush large numbers of swift-water animals. b) Droughts. — There was an unusual drought in the autumn of 1908. The data on the distribution of fishes in Glencoe Brook and County Line Creek were collected before this date (Fig. 67, p. iii). Table XVI shows the arrangement after the drought. TABLE XVI Showing the Effect of Drought on Fishes The localities i, 2, 3, 4 are indicated on the maps of the North-Shore Streams (Fig. 67, p. III). P=before drought. *P = after drought. Name of Stream and Common Name of Fish Scientific Name I 2 3 4 P P County Line Creek Horned dace Semotilus atromacidalus . . . Rhinichthys atronasus . . . . P *P P *P *P Common sucker *P County Line Creek was entirely dry except the pool nearest its mouth in September, 1908. This is locality 4 in Fig. 67, p. iii. The following spring was one of normal rainfall. The fish proceeded upstream a distance of only three rods. This partially restored the usual arrangement. If this represents the rate, the fish proceed upstream slowly. Glencoe Brook has not recovered its fish. As evidence of upstream migration of Mollusca, the following seems to be important. Frequent examination of a section of the North Branch of the Chicago River at Edgebrook, between 1903 and 1907, showed that Pleurocera elevatum and Campeloma occur in this stream. P/ewrocera was not found during this period (ending November, 1907) above a certain point. Campeloma was found only sparingly above this point. The spring of 1908 was one of heavy rainfall and the streams were in flood from April to June. On July 6 the snail Pleurocera was found in numbers one-fourth of a mile farther up- stream than formerly. Campeloma had gone nearly as far. The sea- son from November to April was not different from other seasons and there is no reason to assume that the migration began before the spring floods. If this is true the snails could make their way toward EFFECTS OF DROUGHTS AND FLOODS 107 the headwaters at the rate of at least a mile per year, if they were intro- duced into a large stream. This must be a response to both water pressure and current. The small value of such single observations is recognized but they are presented here because the opportunity to secure such data is small. In this river there are also notable relations between especially dry seasons and the distribution of other animals. The season in which the riffles were dry (October 31, 1907) the pools presented The Transverse Distribution of Stream Animals Fig. 64. — Shows the form of bottom and size of bottom materials in a cross- section of the North Branch of the Chicago River, a-d, natural size (original). a, a burrowing May-fly nymph {Hexagenia sp.). b, small bivalve {Sphaeriunt stamineum) , two individuals, two views. c, viviparous snail {Campeloma integrum), seen from two sides. d, the long river snail, young and full grown {Pleurocera eJevatiim). Fig. 65. — Cross-section of the stream with reference to a curve. an unusual aspect. The standing pools were choked with water-net. The minuter forms, such as protozoa and fiatworms, were present in the greatest profusion. Hydra was abundant. All this is in marked con- trast to the conditions which one finds when the stream is running. The season following the dry riflles, we found small Hydropsycke larvae, and a few young stone-fly nymphs. The only forms present were those that could be introduced by terrestrial, egg-laying females. Io8 ANIMAL COMMUNITIES OF STREAMS In the autumn of 1906 Professor Child found that the May- fly and stone-fly nymphs were not present in the riffles but were present in the moderately swift and more quiet parts below. The spring of 1906 was a dry spring and the females probably laid their eggs in the moderately swift instead of the preferred swift water. The distribution is deter- mined by the conditions at the time of egg laying. We note that even in the larger streams the weather conditions affect the presence and absence and abundance of animals. The mores, however, remain essentially the same. 2. TRANSVERSE STUDIES Cross-section studies of streams are of interest as showing a hori- zontal arrangement of forms belonging properly to different formations. This is best illustrated in the cross-sections of curves where there is a horizontal gradation of current and in the size of material of the bed. Figs. 64 and 65 illustrate this. The burrowing May-fly nymph, belonging to the silt, is in the finest materials of the inside of the curve; passing toward the center of the stream we next encounter the sphaerid {Sphae- rium) and a little farther in the snail (Campeloma integrum) , with it often mussels {Anodontoides ferussacianus); and still farther into the stream we find, clinging to the larger stones, the long snail (Pleurocera elevatum). While depth of water may be a factor here, the size of bottom material is of first importance. 3. LONGITUDINAL STUDIES (Figs. 66, 67, 68, 69) If one passes from the headwaters of a stream to its mouth, he will usually find either the spring brook formation or the intermittent formation in the upper course, the swift-water formations in the middle course, and the sluggish stream or river formations in the lower course. There are very numerous variations of this and several of them deserve comment. Large streams with a large drainage area and much sedi- ment, and with much of the upper part in a young stage, are subjected to many changes in the lower courses, such as silting-up at the end of the flood periods and washing out later. This often prevents the development of the vegetation formation and favors the shifting sand and gravel formations . a) Rejuvenation, ponding, and retarding of erosion. — Streams are often dammed by some obstruction in their mid course, or erosion is checked at a point by a hard stratum, or the stream which has reached base-level is rejuvenated by a lowering of the water level at the mouth. LONGITUDINAL STUDIES 109 The obstruction of the hard layer encountered always produces local swift water. Above this the water may be sluggish and the area reduced to the general level of the obstruction. In the case of rejuvenation the head of erosion proceeds upstream ; the part of the stream above the point to which erosion has reached is sluggish and is sometimes called the pre- erosion stream. Of the rivers and creeks which we have considered, nearly all the larger ones are sluggish or pre-erosion in their upper courses. This is true of the DesPlaines, which is held in this condition largely by rock at Riverside. Hickory Creek (Fig. 66) is also of this type, the head of erosion being at Marley. In passing from source to mouth of such a stream we find formations arranged as follows: In the upper sluggish courses of all the streams mentioned we find (i) sluggish creek or river formations, (2) chiefly swift-water formation's below the sluggish, (3) chiefly gravel bottom formations below the swift-water formation. .^ // Fig. 66.— Diagrammatic profile of Hickory Creek: A, source; 5, mouth; C, head of erosion; D, rock outcrop. The figures below refer to the columns in Table XXI and represent parts from which fish were collected. and (4) typical sluggish river formations farthest downstream where the vegetation, silt, and sand formations are arranged much as in the Fox River. Tables XVIII, XXI, and XXII and Figs. 67-69 show the longi- tudinal distribution of fishes in six streams. A few moments' study and comparison of these tables will make the following facts evident: a) The only species in the youngest stream of the North Shore series is at the headwaters of all the others. b) The species found in County Line Creek are found in the same order in the upper courses of Pettibone Creek and Bull Creek; additional species are found farther downstream in the larger streams. c) The same species are at the headwaters of Thorn-Butterfield and Hickory creeks and in the upper courses of the North Shore streams. Other species are with them. The species of the North Shore streams are crowded together in these large streams which have permanent no ANIMAL COMMUNITIES OF STREAMS deeper water at their sources (due to springs) and in which the graded series of conditions found in the North Shore streams is wanting. d) The swift-water fishes begin markedly at the head of erosion in Hickory Creek. e) The fish communities differ as to species where the conditions are very similar, for example, in Thorn-Butterfield and Hickory creeks. The general habits of the fishes are the same. /) Larger fishes are found in the larger water course and in the down- stream portions of the smaller streams. g) Fish, when entering a stream, go upstream to a point suited to their physiological constitution, regardless of its physiographic mode of origin. 4. GENETIC ECOLOGY OF STREAMS Several years ago Adams (103) pointed out that the dispersal of aquatic animals is determined by the shifting backward of the head- waters and other conditions in streams as erosion proceeds. The forms that are in the young streams are moved back as the headwaters are moved back and as the river system spreads out into the usual fan shape, the animals that belonged in or near the headwaters move backward as the conditions migrate backward. In a broad geographic way this is unquestioned but details may be studied in the small streams of the bluff between Glencoe and the Wisconsin state line. Fish are the only strictly aquatic forms in these streams that might not have entered by some other method than through the mouth of the stream. We have made a study of the fish of these streams for the pur- pose of determining whether the fish in the headwaters of the large streams are the same as the fish that are found in streams that are just large enough to have a single lish species, and the relation of the animals to stream development. The changes in animal communities which take place at one point are called succession. a) Ecological succession. — Ecological succession is the succession of ecological types (physiological t3^es, modes of life) over a given point or locality, due to changes of environmental conditions at that point. From this point of view we have nothing to do with species, except that names are necessary. However, we may speak of the succession in terms of species whenever their life habits (mores) are not easily modifiable. Succession always involves all the animals of a community but it is often easier to discuss the changes which take place with respect to one group, such as the fishes. It is always to be understood that with changes in the fish communities there are similar changes in the communities of SUCCESS I ox OF COMMUNITIES Other animals living with them. To illustrate the succession of fish in streams we shall consider succession of fish in the North Shore streams. b) Statement of ecological succession. — Succession is a reconstruction. Here it is based on the superposition of all the fish communities (Fig. 67) over the oldest part of the oldest and largest stream. To make this clearer we will state, with the aid of the diagram (Fig. 69), the succession of fish in Bull Creek. This succession will be considered as taking place Fig. 67.— Diagrammatic arrangement of the North Shore streams. The streams are mapped to a scale of one mile to the inch, and the maps are placed as closely together as possible in the diagram. The intermediate shore-lines are shown in broken lines which bear no relation to the shore-lines which exist in nature. Toward the top of the diagram is west. Each number on the diagram refers to the pool nearest the source of the stream which contains fish, as follows: i, the horned dace (Semotilus airomaculatus); 2, the red-bellied dace {Chrosomus erythrogaster); 3, the black-nosed dace {Rhinichlhys atronasiis); 4, the suckers and minnows; 5, the pickerel and blunt- nosed minnow; 6, the sunfish and bass; 7, the pike, chub-sucker, etc. The bluff referred to is about 60 ft. high. The stippled area is a plain just above the level of the lake (see Table XVIII). over the oldest part of the portion of Bull Creek which lies back of the bluff and higher levels of Lake Michigan. This is the point designated as 5. (Table XVIII and Figs. 67 and 69 should be before the reader.) When Bull Creek was at the stage represented by the first stage in our diagram (which is represented by the present Glencoe Brook), its fish, if any were present, were ecologically similar to those now in Glencoe Brook in their relations to all factors except climate. This ecological type is represented by the horned dace alone. As Bull Creek eroded its 112 ANIMAL COMMUNITIES OF STREAMS bed and became hypothetical stage C of the diagram, the fish community of stage I was succeeded by a fish community ecologically similar to the fish communities at the localities marked 2 in Fig. 67. The fish now eco- logically representing this community are the horned dace and the red- bellied dace. The community of the single species, the horned dace, had at such a period moved inland to the point where line i-i (Fig. 69) crosses the curved line representing the profile of hypothetical stage C. As erosion continued, the fish community ecologically represented by the horned dace and red-bellied dace moved gradually inland and was succeeded by a fish community occupying the mouth of hypothetical H G F E D C \B Fig. 68. — A diagram showing the successive stages in the profile (general shape of the bottom) of a very young stream, curved lines, A-B, A-C, A-D, A-E, A-F, A-G, A-H representing the successive profiles. The uppermost horizontal line represents the surface of the land into which the stream is eroding. The horizontal line with the arrowheads indicates the migration of the source of the stream and accordingly of similar stream conditions. The vertical line with arrowheads when followed downward passes through a succession of stream conditions and represents physiographic succession at the locality B. The point A is the mouth of the stream. Opposite this are shown three successive sizes of the stream, and therefore succession at that point. stage D, ecologically similar to that now found at the point 3. This is represented by the three daces and the Johnny darter. As the hypothetical stage D eroded its bed and became stage E, which is represented by County Line Creek, fish community 3 was then succeeded by a fish community ecologically similar to the fish community now present at point 4. This is ecologically represented by the three daces, the Johnny darter, and the young of the common sucker. The fish communities designated as i, 2, 3 have meanwhile moved inland and are arranged in the order which their ecological constitution requires. The continuation of the process resulted in displacing a fish com- munity ecologically similar to the fish community 4 by a fish community SUCCESSION OF COMMUNITIES 113 ecologically similar to the present fish community 5. This is repre- sented in the lower waters of Bull Creek — stage F, Ecological succession is one of the few biological fields in which pre- diction is possible. We may carry this discussion a little farther. We have noted that the developing streams continue to erode their beds, grow larger, and bring down the surface of the land. These processes have not stopped in Bull Creek; it will become larger, contain a larger volume of water at the locality 5, and the fish community of locality 5 Fig. 69. — This figure is based on Fig. 68. The profiles of the streams shown here are separated vertically at the mouth. The curved hnes represent seven stream stages as follows: B, Glencoe Brook; C, hypothetical stage; D, hypothetical stage; E, County Line Creek; F, Pettibone Creek; G, hypothetical stage; H, Bull Creek- Dead River. The hypothetical stages could, no doubt, be found along the shore of Lake Michigan; the difficulty arises from the introduction of sewage into so many streams. The comparative size of the mouth of each stream stage is represented by a stream cross-section at the right. The direction of reading in succession is indicated by the vertical line with the arrowheads pointing downward. The oblique lines marked i-i, 2-2, ^-;^, etc., pass through points in the stream profiles which are in the same physiographic condition and occupied by similar fish communities. will be succeeded by a fish community ecologically similar to that now at locality 6. This stage has been designated as hypothetical stage G in the diagram. With a further continuation of the process, the fish community of stage G, locality 6, will be succeeded by a fish community ecologically similar to that now found at the locality 7 (Dead River) — ■ stage H. The communities of every stream have some such history as we have reconstructed, but the details may be modified by conditions. That branch of ecology which deals with such histories is called genetic ecology. 114 ANIMAL COMMUNITIES OF STREAMS TABLE XVII Distribution of Invertebrates in North Shore Streams The meaning of the numbers is shown in Figs. 67 and 69. 0 = Temporary pool (consocies); 6 = Very young stream and intermittent riffles (ephemeral consocies). Common Name Caddis-worm Mosquito larva Amphipod Isopod Snail Crayfish Black-fly larva May-fly njTnph Crayfish Burrowing dragon-fly Dragon-fly nymph. . . Amphipod Snail Crayfish Crayfish Crane-fly larva Amphipod Snail Dragon-fly nymph. . . Scientific Name Phryganeidae Anopheles Eucrangonyx gracilis Smith Asellus communis Say.. . Lymnaea tnodicella Say. . Cambarus dio genes Gir . . Simulium sp Heptageninae Cambarus blandingi acutus Girard Cordulegaster obliquus Say Aeschna constricta Say.. . Gammarus fasciatus Say . Physa gyrina Say Cambarus virilis Hag . . . Cambarus propinquus Gir Pedicia albivitta Walk (rarely) Hyalella knickerbockeri Bate Planorbis campanidatus Say Tetragoneuria cynosura Say STREAM ANIMALS 115 * TABLE XVIII Showixg the Distribution of Fish (Nomenclature after 79) in the North Shore Streams at the Times Indicated (The numbers refer to Figs. 67 and 69) Name of Stream and Common Name of Fish Date and Scientific Name I 2 3 4 s 6:7 Glencoe Brook Homed dace August, 1907 Semotilus atromaculalus. . . 1907-8 Semotilus atromaculatus. . . Rhinichthys atronasus .... Bolcosoma nigrum Pimephales promelas Pimephales notatus Catoslomus commersonii . . September, 1909, and April, 1910 Semotilus atromaculatus . . Chrosomus erythrogaster. . . Rhinichthys atronasus .... Bolcosoma nigrum. Catoslomus commersonii. . . September, 1909 Semotilus atromaculatus. . . Chrosomus erythrogaster.. . Rhinichthys atronasus .... Catoslomus commersonii. . . Pimephales notatus Esox vermiculatus Lepomis pallidus Micro pterus salmoides. . . . Esox lucius Pomoxis annularis Moxostoma aureolum Erimyzon sucetta Abramis crysoleucas Notropis cornutus Notropis cayuga * * ? * * * * * * * * * * * * * * * * * * * * * * * * * * * * * County Line Creek Horned dace Black-nosed dace Johnny darter Blackhead minnow Blunt-nosed minnow .... Common sucker Pettibone Creekf Horned dace Red-bellied dace Black-nosed dace Johnny darter Common sucker Bull Creek-Dead River Horned dace 1 ] * 1 Red-beUied dace Black-nosed dace Common sucker Blunt-nosed minnow .... Little pickerel * * * * * * Bluegill .... * 1 * Large-mouthed black bass Pike Crappie Red-horse * * * * * Chub-sucker * Golden shiner * Common shiner Cayuga minnow Tadpole cat * Schilbeodes gyrinus * t The lower part of Pettibone Creek has been destroyed by the United States Naval School, other- wise the table would include the records for a point 5 and perhaps a point 6, but probably not 7 Note.— Table XDC follows Table XX. ii6 ANIMAL COMMUNITIES OF STREAMS o pti 3 c X 6 « u O o ■33A p CO 1I!S * « * « -J3JBAV « * H < Pi O O S3p!s * * * * * siooj * * S3EB]a CO S3U01S 3uoniv * * * S3U0}S japufi * * ******** saaojg * Hi < 5 B i o > * * ************ > * fV. fV, a « * * * * »-H * * * * t-H * * * * in a O O t^ ^ ri* ir^ S >-* -S C^ "^ ^ *» »» O w O, Q^ a, o N fi^ CO Cj Ci -=13 <^^ § S ?^ Ci-5 !3 -Si D. « ■S "-r S ft. •?. V »^ coU ^ »***************#*##*##«# ^^^^ •« O !« >; .g!-^ C S C rt SW « 1^2 i !o o ^ t< s' t! r" ,S a Do s a S '3 "^ IS ~ <> = 'r- r^ 2 5 -^ ^ ~ - tu s^ ex, k^ OJ^ -ft. "^ X = 2 S ^ % S S 2^^ tejO 4J (U OJ QJ <1> 6 c c2;333d33333=3 S 6 S O c '^ a 1- ^F= ; bo tc .- . Gjs : §1 :|P . . ugc^^SS^S^SSSc^QS^c^uS^c^c^Qfgc^C^ ii8 ANIMAL COMMUNITIES OF STREAMS TABLE XIX Animals of Springs and Spring Brooks The meaning of the letters in the column headed "Location" is as follows: Cs = Gary spring; Gs = Gaugars spring; Zs = Zion spring; Sb = Suman spring brook; Cb = Gary spring brook. Common Name Amphipod Planarian Planarian Dragon-fly nymph. Midge larva Black-fly lar\'a .... Gaddis-worm Midge larva Fly larva May-fly nymph . . . Bivalve Amphipod Crayfish Snail Damsel-fly nymph. Parnid Scientific Name Gammarus fasciatus Say . . . Planar ia dorotocephala Dendrocoelum sp Aeschna sp T any pus sp Simulium sp Hydro psyche sp Chironomus sp Dixa sp Heptagenia Musculium Eucratigonyx gracilis Smith Cambarus propinqims Gir. . Physa gyrina Say Calopleryx maculata Beauv . Elmis fastiditus Lee Location Gs, Gs Gs Gs Gs, Zs Zs Sb Sb, Cb Cb Cb Cb Cb Cb Cb Cb Cb Cb Cb STREAM ANIMALS 119 ♦ TABLE XXI The Distribution of Fish (Nomenclature after 79) in Hickory Creek (and Its West Branch) in the Summer of 1909 Those starred were in the pool nearest the source. I, the first mile of the stream, measured from the fish pool nearest the source, toward the mouth; II, the third and fourth miles; III, at the head of erosion, five miles from the pool nearest the source; IV, six miles from the pool nearest the source; V, nine miles from same; stream much larger with good riffles and one weedy cove. Common Name Scientific Name Horned dace* Golden shiner* Johnny darter* Stone-roller* Straw-colored minnow*. . . Blue-spotted sunfish* .... Blunt-nosed minnow Common sucker* Mud minnow Top minnow Red-beUied dace Chub-sucker Black bullhead Blackfin River chub Fan-tailed darter Rainbow darter Least darter Sucker-mouthed minnow . Cayuga minnow Rock bass Common shiner Rosy-faced minnow Banded darter Bluegill Long-eared sunfish Stonecat Yellow perch Small-mouthed black bass Hogsucker Common red-horse Semolilus atromaculatus Abramis crysoleucas . . . . Boleosoma nigrum Campostoma anomalum. Notropis blenniiis Lepomis cyanellus Pimephales notatus Catostomus commersonii. Umbra limi Fundulus notatus Chrosomus erythrogaster. Erimyzon sucetta Ameiiirtts melas Notropis nmbratilis .... Hybopsis Kentuckiensis Etheostoma flabellare . . . Etheostoma coeruleum . . Microperca punctulata. . Phenacobius mirabilis . . Notropis cayuga Amblopliies riipestris. . . Notropis cornutus Notropis riibrifrons .... Etheostoma zonale Lepomis pallidus Lepomis megalotis Notiirus flavus Perca flavescens Micropterus dolomieii . . . Catostomus nigricans . . . Moxostoma aureolum . . . Ill IV I20 ANIMAL COMMUNITIES OF STREAMS TABLE XXII The Fish (Nomenclature after 79) of Thorn Creek, Collection Made at the Headwaters in 1908 and 1909 and at Other Points in 1909 AND 1910 A = the first fish pool; B = four miles downstream; C = ten miles downstream. Common Name Horned dace Blunt-nosed minnow Blue-spotted sunfish .... Stone- roller Banded darter Common shiner Striped- top minnow Black-sided darter Johnny darter Mud minnow Cayuga minnow Golden shiner Large-mouthed black bass Small-mouthed black bass Bluegill Crappie Pirate perch Yellow perch Carp Black bullhead Common sucker Short-headed red-horse. . . Pike Scientific Name Semotilus atromaculatus Pimephales notatus .... Lepomis cyanellus Camposlotna anomalum. Notropis umbratilis . . . . Etheosloma zonale Notropis cormitus Fundulus dispar Hadroplerus aspro Boleosoma nigrum Umbra limi Notropis cayuga Abramis crysoleucas . . . Micropterus salmoides . . Micropterus dolomieu. . Lepomis pallidus Pomoxis sparoides Aphredoderus sayanus . . Perca flavescens Cyprinus carpio Ameiurus melas Catostomus commersonii Moxostoma breviceps . . . Esox liiciiis STREAM ANIMALS 121 « TABLE XXIII Animals of the DesPlaines, Chicago, and DuPage Rivers The meaning of the letters in the column headed "Location" is as follows: L = Libert}^4lle (still-silt); W = Wheeling (mud-gravel) ; D = DuPage; R = Riverside (swift-stones). Liberty\'ille is the farthest upstream, and the other situations follow in the order named. C = Chicago River at Edgebrook, which is added without regard to longitudinal order. Common Name Crayfish Snail Dragon-fly nymph . . Snail Snail Crayfish Craj^sh Mussel Mussel Mussel Mussel Bivalve Snail Mussel Snail Snail Snail Mussel Snail Snail Stone-flies Dobson Amphipod Caddis- worm Isopod Damsel-fly nymph . . Dytiscid Newt Polyzoan Pamid Caddis- worm Leech Caddis- worm , Sialid Sphaerid Burrowing dragon-fly nymph Scientific Name Cambarus virilis Hag Lymnaea humilis nwdicella Say Basiaeschna janata Say Ancylus tardus Say Ancylus rivularis Say Cambarus propinquus Gir Cambarus diogenes Gir Anodonta grandis Say Anodontoidcs ferussacianus Lea Quadrula uudiilata Bar Lampsilis luteola Lam Musculium truncalum Lins. . . Goniohasis livescens Mke Alasmidonla calceola Lea. . . . Amnicola limosa Say Planorbis bicarinatus Say. . . . Physa gyrhia Say Lampsilis ellipsiformis Con . . Pleurocera subulare intensum Ant Pleurocera elevatum Say Perla sp Corydalis cornuia Linn Hyalella knickerbockeri Bate. . Hydropsyche Asellus communis Say Argia sp Hydro porus viUatus Lee Diemictylus viridescens Raf . . . Plumaiella sp Elmis Helico psyche sp Haemopis grandis Verrill .... Phryganeidae Sialis sp Sphaerium stamineum Con. . . Gomphus exilis Selys Location R R R R R R D D D D D D D D D D D D C W C W C W W W W 122 ANIMAL COMMUNITIES OF STREAMS TABLE XXIV Mussels of the Calumet-Deep River. Arranged in Order of Longitudinal Succession Beginning with the Upper Parts of the River at Ainsworth The letters indicate place of collection. A = Ainsworth; G = East Gary; M = south of Miller, in the Little Calumet; and C = Clark, in the Grand Calumet. Common Name Scientific Name Location Mussel Symphynota costata Raf A G G G M M M M M M Mussel Mussel LampsiUs ventricosa Bar A Quadrula undulala Bar A LampsiUs luteola Lam 1 A Symphynota complanata Bar. . . A Undo gibbosus Bar Quadrula rubiginosa Lea i Anodonta grandis Say Mussel Mussel Mussel Mussel Mussel C STREAM ANIMALS 123 * TABLE XXV Animals from a Sluggish Portion of Fox River The meaning of the letters in the column headed "Location" is as follows: Gm=gravel in mid river in eight feet of water; G=gravel near shore; S = sand; M = mud or silt; V= vegetation. Common Name Scientific Name Location Snail Mussel Mussel Mussel Red midge larva . . Green midge lar\'a . Caddis- worm Polyzoan Dragon-fly nymph . Crayfish Snail Snail Mussel Mussel Mussel May-fly nymph . . . Fly larva May-fly nymph . . . Amphipod Ma3'-fly nymph . . . Beetle Sialid larva Snail Water-boatman. . . Water scorpion. . . . Amphipod Bug Parnid Creeping bug Back-swimmer. . . . Dragon-fly nymph. Leech Top minnow Snail. Gomohasis livescens Mke. . Anodonta grandis Say. . . . Lampsilis ligamenlina Lam Quadrula undulata Bar. . . Chirouomus C/iironomus Hydro psyche Phimatella Macromia iaeniolaia Ram. Cambanis propinquus Gir. Campeloma inlegrum DeK. Phurocera elevatum Say. . . Unio gibbosus Bar Quadrula rubiginosa Lea . . Lampsilis luteola Lam.. . . Hexagenia Stratiomyia sp Callibaetis sp Hyalella knickerbockeri Ba. Cacnis sp Donacia Chauliodes sp Physa integra Hald Corixa sp Ranatra fusca Beau Gammarus fascialus Say. . Zaitha fluminea Say Elmis 4-notatus Say Pelocoris femoraliis Pal Beauv Notonecta variabilis Fieb . . Ischnura verlicalii Say. . . . Glossiphonia fusca Castle . Fuiidulus diaphanus menona J. and C Planorbis bicarinatus Say . Gm Gm Gm Gm Gm Gm Gm Gm M M M M V V V V V V V V V V V V V V V V V V CHAPTER VII ANIMAL COMMUNITIES OF SMALL LAKES I. Introduction Lakes are difficult to classify on the basis of animal relations. This is because size, shape, exposure to wind, depth, and age are all important in determining conditions that aflfect animals. A classification into coastal lakes and morainic lakes will serve our purposes best, because, other things being equal, it represents age and depth (near Chicago). Morainic lakes are depressions in the moraine due to irregularities of deposition, which stand below ground-water level. They are of various sizes. We shall apply the term lake only to those bodies of water that are large enough to produce an area of at least a few square rods of sandy shore, which supports gilled snails, mussels, etc. The principal lakes included in our area are shown on the map facing p. 52. The largest of these are the Fox, Pistakee, Maria, and Grass lakes in northern Illinois; Hudson, Cedar, Stone, and Flint lakes in Indiana; and Paw Paw and Pipestone lakes in Michigan. The only coastal lakes of any size are Wolf Lake and Calumet Lake. These are located in the old Lake Chicago plain. I. CONDITIONS IN LAKES Depth is important in determining the conditions at the bottom, but is of little importance to the other parts of the lake. Little is known of the depths of our lakes. Exposure to wind is of importance in affecting the waves and circulation of the water (see p. 61), both of which are important to animals. A lake well protected by high hills will be likely to be less affected by wind than others. Shape is also a factor. Long lakes whose long axes are parallel with the direction of the prevailing winds are more strikingly affected by the wind than those with the long axis at right angles to the wind. Waves are never large on small lakes, but are usually effective in determining the kind of bottom by controlling erosion and deposition. The general circulation of all our lakes has not been studied. On account of their small size it is probable that the deeper ones at least have an incomplete circulation like that indicated in P'ig. 11, p. 61. Those that get warmed throughout in summer probably have a complete circulation. The dissolved content of the waters of lakes is usually 124 LIMNETIC COMMUNITY 125 similar to that of the large lakes and rivers. Oxygen is usually abundant in the surface waters, but is often wanting in the bottoms of lakes (74) with incomplete summer circulation. Muck bottoms in deep water or in bays have little or no dissolved oxygen. Dissolved nitrogen is important, but has been little studied. In the open water light and pressure are governed by the same factors as in the large lakes (see pp. 62-64). The bottom in small lakes varies with exposure to waves. Where the waves are eroding, the bottom is stony or sandy; where deposit- ing, it contains silt and humus. There are often deposits of marl, which is a calcium carbonate deposit, frequently reaching a depth of 18 feet in the Indiana lakes. It frequently reaches to the surface of the water, but when it does so is often covered by muck. Muck bottom is common in the deeper water and in bays. The vegetation in such lakes is very much like that in base-level streams. The vegetation of the shores of rivers like Fox River is duplicated in these lakes, and in fact, small lakes are strictly comparable to sluggish rivers in many respects. We have patches of vegetation, patches of sand and gravel bottom, but also much bottom which has more organic matter than river silt. The principal difference is that currents in the lakes vary with the wind, and in sluggish streams are mainly in one direction. II. Communities of Small Lakes (Stations 30, 30a, 31; Table XXVI) These are divided into the limnetic formation, the formations of sandy and stony shores, the formations of muck bottom in shallow water, the formations of the vegetation, and the formations of deep water (anaerobic). I. THE LIMNETIC FORMATION (104) (List II) The limnetic formation of the smaller lakes is very similar to that of the larger lakes. It is made up of the same groups, but with the addition of a few pelagic insects such as the phantom larva {Corethra sp.). The species of crustaceans, rotifers, and protozoa are different. The char- acters of the formation are similar to those of Lake Michigan (p. 75). 2. SHALLOW WATER FORMATIONS a) Terrigenous bottom formation (105). — Vegetation sparse or absent — water 0-3 meters. Crawling over the sandy bottom are usually found caddis- worms {Goera sp. or Molanna sp.) (Figs. 70, 71). These forms 126 COMMUNITIES OF SMALL LAKES belong to different families, but have similar cases and similar habits. This is a good example of what is meant by mores. The forms are very different, but their mores are similar. The Johnny darter, the straw- colored minnow (Fig. 72), and the blunt-nosed minnow are usually found (105) in the shallowest water. The Johnny darter, the blunt- nosed minnow, the miller's thumb, and probably other minnows breed in these situations (105, 106). Crayfish are common here (in Wolf Lake, Cambarus virilis). Snails (such as Pleurocera subulare [Fig. 73], and sometimes Goniobasis livescens) are common on the shoals, crawling over the bottom which is always covered with diatoms, desmids, etc. These algae serve as food for the mussels. Miss Nichols found 16 species of algae on the shell of a specimen of Pleurocera taken from a Wolf Lake shoal. In the deeper waters (3 ft.) we find the same crayfishes and the same snails fewer in number than in the shallower parts of the shoals. Associated with them are the mussels (especially Lampsilis luteola, Anodonta mar- ginata and grandis) . Such sandy and gravelly bottomed shoals in 1-3 ft. of water are especially important to the food fishes. There are many first-class food fishes in all such lakes. Of those in Wolf Lake seven breed in these shallows. There are the large-mouthed black bass (Fig. 74), the bluegill, the pumpkinseed, the green sunfish, the perch (Fig. 75), the speckled catfish, and the crappie. Nearly all in making their nests scrape the bottom clear of all debris; the males guard the nests. The number of food fishes in a lake is related to the area of such shoals, which are accordingly of great economic importance and should be protected from destruction by the encroachment of vegetation and accumulation of debris. Associated with the fish are occasional musk turtles {Aro- mochelys odorata). Shoals are invaded by bulrushes and bare bottom may exist between them. Here the viviparous snail {Vivipara contec- toides) (Fig. 76) sometimes occurs. Fig. 70. — The case of a caddis-worm {Mol- anna sp.), sandy bottom (Fox Lake, 111.) (original) . Fig. 71. — The same from below. BOTTOM COMMUNITIES v»tvvV^V#V**> <>m^fi. ~-h^ 72 73, Representatives of the Bare Saxd Community Fig. 72. — Straw-colored minnow {Notropis hktutiiis) (from Forbes and Rich- ardson) . Fig. 73. — Snail (Pleurocera siibiilare) crawling o\er sand}' bottom; slightly enlarged (photographed in aquarium). Fig. 74. — ^Large-mouthed black bass {Micro pterus salmoides), juvenile; natural size (original). I2i COMMUNITIES OF SMALL LAKES Characters of the formation : The formation is distinctly dependent upon a clean bottom of sand or coarser materials, and is made up of creeping forms and those using the bottom as a breeding-place. Representative Animals of the Submerged Vegetation Fig. 75. — Upper fish, the green sunfish {Lepomis cyanellus); lower fish, the yellow perch {Perca flavescens); both juvenile; slightly reduced (original). Fig. 76. — A viviparous snail (Vivipara contecloides); natural size. Fig. 77. — A winter body or statoblast, of the gelatin-secreting polyzoan {Pectina- tella magnifica); 10 times natural size (original). Fig. 78. — ^A shrimp {Palaemonetes paludosus); twice natural size (original). h) Submerged vegetation association of the open waters. — A lake of the coastal type is separated rapidly from the larger body of water in con- nection with which it is formed, or a morainic lake, when the ice retreats, VEGETATION COMMUNITIES 129 is left with the greater part of its shallow water of the type which we have described. Vegetation is present from the first in the form of floating microscopic plants, and the dead bodies of these and of the animals present are swept into the depressions and protected situations where the waves do not drag on the bottom. Here vegetation grows in the greatest luxuriance and causes the production of more plant debris, which adds to that already in the protected situations. We then have, after a time, a covering of the bottom by the humus and conditions unfavorable for most bottom animals. The animals of the bare bottom shoals are no longer present in numbers. Small, apparently stunted forms of Lampsilis luteola are found for a time, but are soon driven out by the increase of humus and vegetation. The early vegetation is made up of scattered aquatic plants, such as Myriophylluni and Elodea, and in the shallower water usually bulrushes. One of the most distinctive and characteristic forms of such lakes is a transparent true shrimp {Palaemonetes paludosus), about 2 inches long (Fig. 78), which is a close relative of some of the edible marine shrimps. In spring they are found carrying numbers of green eggs attached to the appendages of their abdomens. Another common animal in these situations is the large polyzoan {Pectinatella magnifica). This is a colonial form which reproduces by budding in several directions. It also secretes a clear and transparent jelly. As the number of animals increases the amount of jelly increases on all sides and the animals are arranged on the outside of the more or less spherical mass of jelly; the necessary increase in surface for the growth of the colony is supplied through additional secretion by each new animal added. Some of these masses of jelly reach a size of 6 inches in diameter. They are often attached about a stalk of Myriophylluni as a center. In the autumn they form bodies known as statoblasts (Fig. 77), which are disk-shaped, the center containing living cells and the rim being filled with air-bubbles. The rim of the disk is supplied with hooks which catch onto objects. Probably they must be frozen before they will grow into new colonies for they do so only in the spring. Other characteristic animals of this open-water vegetation are shelled protozoa (Fig. 79), water-mites (Fig. 80), and ostracods (Fig. Si). On the stems of the water plants, such as bulrushes and pickerel weed, are the snails (Ancylus) which belong to the lunged group, but are said to take water into the lung and thus do not need to come to the surface for air. Occasional snails, leeches, and midge larvae occur. Water- mites fasten their eggs to the bases of the aquatic plants. Among the I30 COMMUNITIES OF SMALL LAKES leaves of the divided leaved plants the midge larvae, damsel-fly nymphs, and May-fly nymphs (Callibaetis sp.) are usually numerous. All these are important as fish food. This area is the feeding-place for a number of fishes. Those feeding in the vegetation are the sul^fishes, basses and perches, most of which breed on the barren shoals. With them are also the carp, the chub-sucker, the warmouth bass, the brook silverside (Labidesihes sicculus), and the buffalo fish (84). This part of the lake is also the favorite haunt of the turtles (107), such as the soft shell {Aspi- donectes spinifer), and in the parts with some bare bottom, the musk 80 ^ 81(^ Fig. 79. — Shelled protozoan {Diffliigia pyriformis Perty.) (after Leidy) . Fig. 80. — A red mite {Limnochares aqiiaticus) ; 6 times natural size (after Wolcott). Fig. 8i. — Dorsal view of an ostracod {Cypridopsis vidua); 80 times natural size (after Brady). Fig. 8ia. — The same seen from the side. turtle (Aromochelys odorata), and the geographic turtle {Graptemys geo- graphicus). The mud puppy (Nedurus macidosus) is also found in such situations {fide Mr. Hildebrand). The muskrat {Fiber zibethicus) builds its nest (Fig. 82) in the shallow water adjoining these situations. The musk turtle frequently deposits its eggs on the nest in early summer (105). We have found them in these situations in the month of June. Various aquatic birds feed here (108). This formation may be characterized as belonging to the aquatic vegetation, but practically V EG ETA TIOX COMMUNITIES 131 all the species are relatively independent of the atmosphere and of the bottom. c) Emerging vegetation association of bays. — Such situations as are occupied by this association are found in bays and protected situations in the larger lakes and represent a stage which is last in the history of a lake. Water-lilies, water buttercups, and Myriophylluni are the prin- cipal plants. Filamentous algae are usually very ainnidant. Logs, sticks, and pieces of wood are not uncommon. On the under side of logs, we find such forms as the polyzoan (Plumatella) and sponges (Spongilla sp.). On the under side of the water- lily pads are usually numbers of Hydra together with great numbers of Fig. 82. — A muskrat's nest adjoining the lake border among the biilrushes on sandy bottom. shelled protozoans and rotifers, especially sessile forms. Snails also are common here {Segmentina armigera, Planorbis parvus, Physa gyrina and Integra, Planorbis campanulatus, and some species of Lymnaea). A large number of species of aquatic insects cling in the vegetation vdth. the abdomen near the surface of the w^ater and secure air through various anatomical arrangements which conduct it to the spiracles; the most noteworthy of these are the water scorpion (Ranatra), the electric- light bugs (Benacus and Belostoma), the predaceous diving beetles iPytiscidae) (99c), the water scavengers (Hydrophilidae), and the water- boatmen (Corixa). There are also a number of aquatic insects that are not dependent upon the atmospheric air in their young stages. They require, however, some object which reaches above the surface of the 132 COMMUNITIES OF SMALL LAKES water when they emerge from the larval skin. The prominent members of this group are the dragon-fly nymphs {Anax Junius and Ischnura verticalis) . There are a few insects that are relatively independent of vegetation as a means of attachment. The back-swimmers are an example. They float or swim in the water among the vegetation. The commonest of these are those belonging to the genera Plea, Notonecta, and Buenoa. There are a few fish that have a similar habit. The top minnow (Fundulus dispar), which feeds at the surface, is an example. It invades the pools near shore and devours mosquito larvae. The young of such fishes as the basses and the sunfishes are sometimes taken in these situations. In the mud of the bottom there are but few animals. Some of these are the same species as those found in the bottom in the region of open water and will be discussed later. There are, however, forms that live only on the rhizomes of the water-lily. Certain of the leaf-feeding beetles {Chrysomelidae, Donacia) (109) are aquatic in the young stages. The female eats a hole in the leaves of the water-lilies and reaches through with her ovipositor and deposits the eggs in a semicircle which has the hole as its center. When these eggs hatch the larvae crawl to the rhizomes. They are not provided with gills and do not come to the surface for air. They have a pair of spines adjoining the spiracles. These spines are thrust into the plant and the spiracles which open at their bases come into contact with the holes; the gas in the plant and the gas in the air tube of the insect's body interchange, and the animal is thus supplied with oxygen. When the larva is ready- to pupate it spins a cocoon in some unknown way under water, but when it is completed it is filled with gas, not water, and surrounds the body of the animal. The animal then eats a hole, connecting the cocoon with the air spaces of the plant. It then pupates and is supplied with oxygen by the plant during the entire pupal period. The common painted turtle {Chrysemys marginata) and the snapping turtle are common in such small bays. They come out upon the logs and bask in the sun. The pied billed grebe builds its floating nest, and many other aquatic birds feed in such situations (108). Characters of the vegetation formation: This formation is of the old-pond type which will be especially discussed in the following chapter. There are two characters, one or the other of which is possessed by nearly all the animals. They depend upon the atmospheric air or must have the support of the vegetation, or both. The majority of the ani- mals of this formation stick their eggs either in or on vegetation. Such SUCCESSION OF COMMUNITIES 133 formations are quite similar in many respects to the formations of the \-egetation in sluggish rivers but resist lack of oxygen and stagnant water much better. d) The anaerobic formation. — This is the bottom and deep-water formation. We have already stated that the circulation of water (see Fig. ID, p. 61) is not known for any of the lakes discussed. Old lakes like those about Chicago are usually covered with humus on the bottom. In this humus and probably just above it there is little or no oxygen. Analyses of the bottom water from ponds with humus-covered bottoms showed that it contained no oxygen. The open water of the lakes with the incomplete circulation in summer is without suflEicient oxygen to support life, below the level of circulation (Fig. 11, p. 61). There are, however, numbers of animals that pass the summer under these conditions (no, in). These are protozoa belonging to eleven genera, worms belonging to two genera, one rotifer, one ostracod, and the small bivalve {Pisidium idahoense). Dr. Juday kept these animals in jars without oxygen and observed their activities. The rotifer was always active. The ostracod showed little activity, and the bivalve kept its \'alve closed, showing no activity whatever. There are occasional midge larvae in the mud of such bottoms, but they are rare. Some of these have haemaglobin in their blood and are supposed to be able to use oxygen when it is present in the minutest quantities. In the open oxygenless water there are phantom larvae (Corethra) which are able to carry a supply of oxygen with them from the surface. III. Succession in Lakes The general tendency of succession in lakes has been indicated. The first formation is the bare-bottom type, which is locally transformed to the vegetation of open-water type. This usually begins in the protected situations first; the bays are ecologically oldest. These bays pass rapidly from the third open-lake type to the bay conditions. When such a stage has been reached the situations that have a less degree of protec- tion from waves have reached the second stage and we have lakes as we find most of the larger ones about Chicago. They contain, at various points, the three formations which we have discussed. The lake is reduced in size by filling near its shores and the lowering of its outlet. The older stages are continuously encroaching on the younger. The area of barren shoal is constantly becoming less as the lake fills and the outlet, if it has one, is lowered. Around the shores the development of prairie or forest is usually well begun and one or the other of these types of land vegetation finally displaces the lake. 134 COMMUNITIES OF SMALL LAKES I. THE INFLUENCE OF SIZE AND DEPTH Size and depth have a marked influence on the rate of succession. If the lake is large, like Lake Michigan, its waves beat upon the shores with such force as to prevent the development of vegetation or the establishment of any of the formations just discussed. Smaller lakes have proportionally less efl&cient wave-action, and situations which would not be protected to any marked degree in a lake like Lake Michigan are relatively free from effective wave-action. The formations succeed one another rapidly where wave-action is slight. The various parts of the shore of a small kettle-hole with a regular shore-line would pass through all these stages at nearly the same rate. Depth is an important factor also because the various formations cannot succeed over the deep water until the deeper parts are filled (or drained), which often requires long periods. The rate of succession in lakes is then directly proportional to their size and depth. The small lakes pass through all the stages more quickly than the larger lakes. Those considered here have for the most part, at present, become dominated by the late stages. The lakes of the inland type which are large enough to maintain all the formations discussed are among the most complex of all our habitats. 2. INFLUENCE OF MATERIAL AND MODE OF ORIGIN At the very beginning the kind of material in which a lake is situated is important but as time goes on it becomes less and less important. If the lake is in clay, at the outset there are no sandy areas, but the action of the weaves soon removes the finer material and leaves sand (the finer materials being deposited on the bottom of the lake). Young lakes in rock are probably very different from those in clay, but even here sandy shores are soon formed and occupied by the same animals as sandy shores of different origin. The distinction between lakes and ponds is a purely artificial one. The ponds have the same communities at the outset as the lakes, but the changes proceed so rapidly that very young ponds are rare. All lakes and ponds tend to become ecologically similar, regardless of mode of origin and kind of material. LIST II The following Entomosiraca have been taken from Wolf Lake: * indicates the species is found in Fox Lake; f in Butler's Lake; % in the series of ponds at the head of Lake Michigan: Copepods: X*\ Cyclops serrulaius Fischer; *tJ C. albidiis Jurine; JC. vlridis bremspinosus Herrick. Cladocerans: Acropcrus harpae Baird; % Scapholeberis miicronata Muel.; % Pleuroxus denticidalus Birge; Diaphanosoma brachyuruni Liev.; % Chydorus sphaericus Muel.; Polyphemus pedicidus Linn; Macrothrix ro^ea Jurine; XCcriodaplmia reticulata Jurine; % Simocephalus serridatus Koch; Bosmina obtusirostris Sars. Ostracods: Potamocypris smaragdina Vav. SMALL LAKE ANIMALS 135 • TABLE XXVI Animals from Small Lakes Meaning of letters occurring in the columns is as follows: "Habitat" column: S = bottomof sand; SH = bottom of sand and humus; B = bulrush vegetation; V0 = vegetation of open water; VB = vegetation of bays; in "Lake Where Recorded" column: F = Fox Lake; W= Wolf Lake; G= Lake George; B = Butler's Lake Common Name Scientific Name Habitat from Which CoUected Caddis-worm Caddis-worm Caddis-worm Snail Snail Craj^h Turtle (musk) Geographic turtle Straw-colored minnow. Johnny darter Mussel Planarian Mussel Mussel Polyzoan Leech Brook silverside Snail Snail Midges Amphipod May-fly nymph '' Dragon-fly njTnph . Polyzoan Shrimp Cricket-frog Top minnow Snail Snail Snail , Damsel-fly n3rmph . . Dragon-fly nymph . . Dragon-fly nymph . . Back -swimmer Back-swimmer Back-swimmer Back -swimmer Leech May-fly *. . . Isopod Bug Beetle Beetle Goera sp Molanna sp Polycentropidae Pleurocera subulare Lea Goniohasis livescens Mke Camhariis virilis Hag Aromochelys odorata Lat Graptemys gcographicus LeS . . Notropis blennius Gir Boleosoma nigrum Raf Lampsilis luteola Lam Planar ia maculata Leidy. .... Anodonta grandis Say A nodonla marginata Say Plumatella polymorpha Kraep Placohdella parasitica Say. ... Labidesthes siccidus Cope .... Ancylus fuscus Adams Segmentina armigera Say. . . . Chironomus sp Hyalella knickerbockeri Bate. . Callibaetis sp Ischnura verticalis Say Pectinatella magnifica Leidy. . Palaemonetes paludosus Gib . . Acris grylliis Lee Fundidus dispar Ag Physa gyrina Say Planorbis campanulatus Say. . Planorbis parvus Say Enallagma sp Tetragoneiiria cynosura Say . . Anax Junius Dru Buenoa platycnemis Fieb Notonecta variabilis Fieb Plea slriola Fab Notonecta itndulata Say Macrobdella decora Say Ephemerella excrucians Walsh Mancasellus danielsi Rich. . . . Zaitha fluminea Say Coptotomus inlerrogalus Fab. . Donacia sp S S S S S S ; S s s s S,SH,B S,SH s- s S,SH S,SH S,SH,B B B B B B Lake Where Recorded vo vo vo vo vo VO,VB VO,VB VO,VB VO,VB VO.VB VB VB VB VB VB VB VB VB VB VB VB VB VB VB VB VB W F W W W W W W W,F W,F W,F W W W,F W,F W,F W W W W W W F W F W F F F W W F W CHAPTER VIII ANIMAL COMMUNITIES OF PONDS I. Introduction Ponds are fascinating to all, and do not lack interest from the scien- tific point of view. They are of especial interest to those familiar with the laboratory study of zoology. The common animals of the laboratory are pond animals, because pond animals are forms that will live in stagnant water. The common aquarium fishes are all pond fishes, as the brook forms die quickly if they are not supplied with running water. The frog, so much studied, is a pond form. The conditions in ponds are different from those in lakes and streams, because currents are not strong nor particularly important. The water doubtless piles up at one side or end of a pond during strong winds, and a complete circulation is effected, but this is not important. All of the conditions of lakes are duplicated in ponds, but on a smaller scale. One of the chief differences between ponds and lakes is the vegetation. Ponds are usually very largely captured by vegetation which is very much like that in the bays of lakes. Succession of plants in ponds is similar to that in lakes; the age of a pond is therefore a matter of first importance. The bottom materials are of most importance at the beginning (6, 112). The bottom materials in the ponds of the Chicago area are rock, clay, and sand. Rock-bottomed ponds have been but little studied, though there are a number of ponds in abandoned quarries of different ages which would make a good series for investigation. Clay bottom occurs in the moraine area. Nearly all the natural clay-bottomed ponds have reached a stage at which the bottom is not important, but one could no doubt find a good series if he were to make a special study. Sand-bottomed ponds are the commonest of all, and for the purpose of studying the effect of age upon ponds, a series of sandy-bottomed ponds, which differed chiefly in the matter of age, was selected. II. Area of Special Study The ponds that have been made the subject of special study lie in the sand area at the south end of Lake Michigan, within the corporate limits of the city of Gary, Ind. They may be reached from the stations known as Pine, Clark Junction, and Buffington (Fig. 84). The locality 136 ORIGIN OF PONDS 137 c ^ ^^ -5 _ _C c/3 D XI «i ^- "rt :^ ■^ (U 0 ^ Os .2 H Ui 0 -^ -d <^ ^^ tn T3 d ^ =««5 ^•^ » '/3 -ts '^ K 2 S^r o -5 6 H ■i: Z ^ -G c -^ Q S c 'I § J i3 rt fe cr g -^ o o =^ ^ little Little c Much 7 Much 14. All 20 AH C2 All ORIGIN OF PONDS 139 I40 POND COMMUNITIES A decrease in depth, due to the accumulation of humus and the lowering of the ground-water level, is to be noted in the older ponds. The series is, then, an ecological age-series, and throughout our discussion we refer to earlier and later phases of the various associations concerned. III. Communities of Ponds I. THE PELAGIC FORMATION We have in the ponds a pelagic formation. Though it is limited in number of species, many of which breed on the bottom, it is similar to that of larger lakes. We have found little difiference in the pelagic species inhabiting younger and older permanent ponds. Diaptomus reighardi has not been taken from ponds filled with the vegetation which reaches the surface. Other species are about the same in the different permanent ponds. The pelagic formation is poorly developed. 2. PIONEER FORMATION (TERRIGENOUS BOTTOM) (Ponds, I, 5, 7) (113) (Stations 9 and 32; Tables XXVII and XXXIV) The youngest ponds of the Chicago area are near Waukegan. The outer end of the Dead River receives the force of the winter waves from the lake and the bottom is bare, with a few scattered aquatic plants. Here animals are few. We have taken only a few invertebrates. The fish present probably get their food from the older parts farther back from the lake. The fish are: the pike {Esox lucius) which prefers clear, clean, cool water (79); the red-horse {Moxostoma aureolum) which dies in the aquarium if the water is the least bit impure, and which also suc- cumbs to any impurities in its natural environment (79); Notropis cayuga, which prefers clear waters; the common shiner {Notropis cornutus) which breeds on bare bottom (105), and the white crappie {Pomoxis annularis) which lives in streams. On the bottom at such a period one is likely to find the larvae of caddis-flies {Goera sp.), snails, mussels, etc., but we have found none in the Dead River. Vegetation quickly captures parts of such a pond. Chara is the first plant to cover parts of the bottom. After this has happened, the pioneer formation may still continue. In Pond i of the series of special study (Fig. 85) we have a considerable area of bare sand, and the forms present are the caddis- worm {Goera sp.) and the mussels {Anodonta marginata and grandis, and Lampsilis luteola). These are preyed upon by muskrats (Fig. 86). There are a number of fish that belong to this PIONEER COMMUNITIES 141 formation because of their breeding relations. The large-mouthed black bass, the bluegill, the pumpkin-seed, and the speckled bullhead all make nests on the sand, the male fish guarding the nests and driving off other fish that approach. These species are the same as those of the bare-bottom formations of a lake. In their feeding the fish belong in part to another formation in the pond, namely, that of the chara. Character of the formation : The formation may be designated as the bare-bottom formation, the forms present being those that are dependent Fig. 85. — Shows Pond i at the extreme low water of the drought of 1908. In the -spring the old boat is usually covered with water. In the foreground a large area of bare sand bottom is shown; to the right a few rushes and sedges. The absence of shrubs near the water's edge should be noted. upon bare bottom in their most important activities— the fish in breeding, the caddis-worms in making their cases, the mussels in their general activities. It is necessary for the mussels to be on bare bottom in order to maintain themselves in an upright position. Tendencies in the formation: This formation is similar to that of the bare bottom of lakes. The vegetation comes in, as has been indicated in the protected situations, and the bare bottom disappears, its place being taken by the chara. The chara gives rise to humus, upon which chara 142 POND COMMUNITIES will grow for a long time, so the bottom becomes a humus- and chara- covered bottom. - 3. THE SUBMERGED VEGETATION ASSOCIATION (Ponds I, 5, and 7; Stations 32, 33, and 34) The Chara community is entirely different from that of the bare bottom, and differs also from that of other vegetations. Chara is highly siliceous. It is probably eaten only accidentally by animals or at least forms no important part of their food. It should be considered simply as a covering for the bottom and a resting- and living-place for animals. Some fish culturists (113) have said that it is very rich in life. This may be true under certain artificial pond conditions; but the chara ponds are poorer than any others of our series. Chara differs from some other plants in not reaching to the surface of the water. Many aquatic insects that carry air beneath the surface must cling to objects which reach the surface when obtaining a fresh supply, and others must crawl to the surface on some object in order to emerge from the nymphal skin (96). Associated with chara are often growths of bulrushes near the sides of the ponds and on the sterile bottom. In the sparse chara the most characteristic animal forms are Anodonta grandis footiana (Fig. 86), and the musk turtle {Aromochelys odorata), which is abundant on these bottoms but is not found elsewhere. There are often nests of a few un- identified fishes that clear off the bottom in building. The burrowing dragon-fly nymph (Fig. 87) lives on the bottom among sparse chara, in the presence of but little oxygen. It lies half buried in the mud, with its abdomen protruding a little at the end. The mud minnow {Umbra limi) (Fig. 88), the golden shiner {Abramis crysoleucas) (Fig. 88), the chub- sucker {Erimyzon sucetta), bullheads, the little pickerel (Esox vermicula- lus), the tadpole cat {Schilbeodes gyrinus), and occasionally the warmouth bass {Chaenobryttus gulosus) spend their time in the denser chara. The shiner and mud minnow place their eggs on the chara or other plants. Among the most abundant forms in the association are the midge larvae (Chironomus); these (Figs. 89, 90, 91) are present sticking to the vegetation in their small silken cases in great numbers (81). They are important articles in the food of the fishes. Aquatic insects are not numerous except for the midge larvae and a little May-fly. Others are occasional horseflies (Fig. 92), damsel-fly nymphs, May- fly nymphs {Siphlurus sp.), and occasional dragon-fly nymphs {Tramea, Anax, Leucorhinia) . There are also a number of dytiscid beetles, many of which are common in all shallow waters, even rain pools, because of their powers of flight. PIONEER COMMUNITIES 143 Ecologically one of the most interesting insects is a caddis-worm {Leptoceridae), which creeps over the Char a and submerged wood. It (Fig. 93) has a case made of the minutest sand grains and pieces of humus, such as are stirred up by the waves and which are to be found Representatives of a Youxg Pond Community Fig. 86. — The shell of a mussel (Anodonla grandis footiana) that has been broken open by a muskrat; slightly enlarged. Fig. 87.— The burrowing dragon-fly nymph {Gomphus spicatiis), with the mask extended. Fig. 88. — Some fishes of the pond. The dark fish which rests near the bottom is the mud minnow {Umbra limi). The fish swimming about is the golden shiner {Abramis crysoleucas) ; 1/5 natural size. among the chara. This species is the successor of the bottom species (Goera). It belongs to a different group and has structural characters which distinguish it from Goera, but which probably have no relation to its habitat or habits. On the other hand, the mores as indicated by case-building is also different but is related to the environment. The 144 POND COMMUNITIES crustaceans constitute an important element in this association. The smaller amphipod {Hyalella knickerhockeri) is abundant among the chara. The crayfish (Cambarus immunis) occurs here sparingly. In ponds there is an important element of small crustaceans that belong to the vegetation and the bottom; this element is composed chiefly of Representatives of the Submerged Vegetation Association Figs. 89, 90, 91. — Larva of a midge (89), pupa of the same (90), the adult. Midges are inhabitants of the chara-covered bottom; enlarged about 4 times (after Johannsen, Bull. N.Y. State Museum). Fig. 92. — ^The eggs of the common large black horsefly on the tip of the bulrush stalk. Fig. 93. — The chara-inhabiting caddis-worm (Leptocerinae); enlarged as indi- cated. Fig. 94. — Ostracod {Notodromas monacha Miill.); 30 times natural size (after Sharp) . Ostracoda (Fig. 94), which are small bivalved forms resembling the bivalved MoUusca. They form food for fishes to a small degree. Especially abundant just under the chara are the red water-mites {Limnochares aquaticus) (Fig. 80, p. 130). One sees numbers of these PIONEER COMMUNITIES 145 when he stirs the bottom. Creeping over the plants are the small snails (Amnicola limosa) (Fig. 100, p. 146). These respire by means of gills. Other snails are also occasionally present. Physa and Lym- naea, etc., are always small or juvenile. We have never taken an adult specimen of these from the young ponds and in all only a few specimens have been taken. These animals get into the ponds that are formed by the removal of sand. We are not at all sure but that the few forms found in Pond i are the result of such entrance, rather than the regular establishment of the species. Among the bulrushes are a few aquatic insects that belong to the vegetation that comes above the surface. One of the most characteristic forms is the neuropterous larva {Chauliodes rastricornis) (Figs, no, III, p. 150), which is a marsh form and will drown in water. Characters of the association: This association differs from the preceding and from the others generally in being distinctly aquatic and also essentially independent of the bare bottom and of the surface. The animals of this association are, however, strictly dependent upon the vegetation for nesting-places, shelter, etc. The mud minnow has been studied experimentally and shows avoidance of direct light. Tendencies in the association: This association, like all the others, is destined not to last; changes are taking place all the time. The chara is filling the pond at the rate of one inch a year (58) and is making a fine soil for roots of other plants. As soon as the dense chara stage has existed for a time we find other plants, such as Myriophyllum, Pota- mogeton, and water-lilies. As soon as these have become established we have the commencement of the next association. These plants usually appear in spots, and in many cases the zones are much less important than in the lakes because of the small areas of the plants. We can, however, recognize a zone of water-lilies, and zones or patches of other plants. Just as we noted that the formations of the bare-bottom type existed in the small ponds with the Chara, we see also that the surface-reaching vegetation occurs with the Chara association and often all three occur together. Pond 5 contains a poorly developed phase of all three, the bare bottom being of minor importance. Pond 7 contains the chara association and the surface-reaching association. Ponds 14 and 30 are the best expressions of the surface- reaching type, and Pond 52 is the last stage of it. This will be discussed more fully, and we will pass directly to the association of the vegetation which reaches the surface. 146 POND COMMUNITIES 3. THE ASSOCIATION OF EMERGING VEGETATION (Stations 34-37. 39; Ponds 5, 7, and 14) (Fig. loi) (30 and 52) With the incoming of the water-lilies and the fine-leafed plants, we have the inauguration of a new state of affairs. Among the new animals _L T n Representatives of the Dense Bulrush Association (Pond 5) (All about natural size) Fig. 95. — The common diving spider (Dolomedes sexpunctatus). The individual from which this drawing was made was taken with a nymph of the dragonfly shown, in its jaws. Figs. 96, 97, 98. — Various stages of a dragon-fly {Leucorhinia inlacta) \ 96, nymph ; 97, about to shed its outer covering; 98, the adult. (Modified from Needham.) Fig. 99. — The larva of a caddis- worm (Phryganeidae), which makes its case from bits of grass blades, etc. Fig. too. — Small gill-breathing snail {Amnicola limosa). that come in, the bivalved moUusks deserve special mention. The Unionidae must have bare bottom for their activities; they are too large and heavy to climb on such small vegetation, and the development of such a habit has not taken place. They disappear with the sparse MATURE COMMUNITIES 147 Char a. Their place is taken by other bivalves, viz., the Sphaeridae, such as Musculium partumeium, which lives in the humus of the bottom, and Musculium secure and truncatum, which live in the vegetation and are able to climb on the vegetation and on the side of aquarium jars. In the early phases, shrubs and young trees have begun to grow by the sides of the ponds and these from time to time fall into the water, thus forming a resting-place for many forms that are not found in the other situations. Diving spiders (Fig. 95) are common on the bulrushes which Fig. ioi. — Showing Pond 14 at moderate low water. In contrast with Pond i we see that it is choked with emerging vegetation and the margin occupied by shrubs and bulrushes, etc. are here growing on a bottom of humus outside leaf-bearing plants (Fig. ioi), inside the shrubs. These spiders dive for the immature aquatic insects which are here at their maximum. We find numerous damsel-fly nymphs and dragon-fly nymphs, both the creeping form {Leu- corhinia intacta) (Figs. 96, 97, 98) and the climbing form. The burrow- ing dragon-fly nymph has gone, or is present in small numbers only, and there are but few May-fly nymphs. Those that persist creep about on submerged sticks in company with Amnicola and are especially likely to occur in the earlier phases of this community. With these occur the 148 POND COMMUNITIES caddis- worms {Phryganeidae: Neuronia) (Fig. 99), which are also abun- dant in the later stages of dense vegetation. This worm's case is some- what similar in form to that of Leptoceridae, being a circular tube, but it is made of pieces of grass blades or other pieces of plant fragments instead of sand grains. The pieces are fastened together with silk. The worm is found creeping among the vegetation, drawing its case after it. Amnicola (Fig. 100), the river-dwelling snail, is common, especially on twigs and logs. In the mature stage represented by Pond 14 (Fig. loi) the com- mon newt (Fig. 102) probably reaches its maximum abundance. The snails which are at best advantage in these ponds are the lung breathers. They can here come to the surface for air, and food is abundant, as the surfaces of the plants are covered with algae and these form the food of the snails. Those snails which come to the surface for air are common. Planorhis campanulatus (Fig. 103) is characteristic of the mature stage and Lymnaea reflexa (Fig. 104) in the older stages. The individuals in this case are larger than those of the temporary marshes (cf. Figs. 104 and 125, pp. 149, 175). Planorhis parvus (Fig. 105) is commonest in the earliest phases and Planorhis hirsutus (Fig. 106) in the later. Diving beetles (Fig. 107), which are common throughout, are most numerous in the denser vegetation. The soldier-fly larvae (Fig. 108) are often common in the dense filamentous algae of the mature phases of the asso- ciation; here the number of all dipterous larvae is greater than at any other point. Midge larvae occur in great numbers, having their cases among the algae. Horseflies (Fig. 92), also Tony pus, Ceratopogon, and some mosquitoes are present. Specific identification, however, is not possible, and whether or not the species differ in modes of life or reactions from those inhabiting the earlier stages in the pond series has not been determined. Adult aquatic insects have increased with the increase in vegetation, in a remarkable fashion. The prominent forms are the larger bugs, such as the electric-light bugs (Zaitha fluminea and Belostoma americana Leidy, with Benacus griseus Say). The water-boatmen are also common. The species of these are not well known, and we cannot say whether or not they are the same in the older and younger ponds. Back-swimmers are also abundant {Nolonecta variabilis and undulata, Biienoa platycnemis, and the small form. Plea striola, occur here). They are few in number or absent from the younger ponds. Some animals particularly abundant in the older stage are the common leech (Placohdella parasitica) (Fig. 109), the larvae of a netted- winged insect (Chauliodes rastricornis) (Figs, no, in), the large flat MATURE COMMUNITIES 149 snail (Planorbis tr holms) (Fig. 112), and the amphipod {Eucrangonyx gracilis) (Fig. 113). All these occur in the senescent stage, where in dry years the pond goes almost dry. The vertebrates of the mature and later stages are not numerous. The fish are limited to mud- and muck-preferring species, the black bullhead {Ameiurus melas) and the mud minnow {Umbra limi) (106). The grass pickerel and the dogfish are found in such \egetation-choked ponds. Representatives of the Emerging Vegetation Association (Pond 14) Fig. 102. — The common newt {Diemictylus viridescens); natural size (after Hay). Fig. 103. — A flat pond snail {Planorbis campanulatus); natural size. Fig. 104. — The common pond snail (Lymnaea reflexa); natural size. Fig. 105. — Small flat snail {Planorbis parvus); 3 times natural size. Fig. 106. — A snail {Planorbis hirsutus); 3 times natural size. Fig. 107. — A predaceous diving beetle (Cybister fimbriolatus Say); natural size. Fig. 108. — A soldier-fly larva — unidentified; twice natural size. The amphibia are the frogs which occur in all stages of the associa- tion, and the common salamander {Amblysloma tigrinutn), which burrows in the soft mud where it remains during the greater part of the year. It comes out in spring (February or March) and deposits eggs in the pond, where the young are found later. Of the turtles the common I50 POND COMMUNITIES painted turtle {Chrysemys marginata) is abundant, basking on the fallen trees. The geographic turtle and the snapping turtle are found also in the younger phases. Garter-snakes pick up their food along the ponds (Fig. 114), while muskrats, occasional minks, and various acjuatic birds (108) feed in the ponds. Senescent Pond Inhabitants Fig. 109. — A leech with young attached to the ventral side {Placobdelhi para- sitica) ; natural size. Fig. no. — The larva of a netted-winged insect (Cliauliodes raslricornis). Fig. III. — -Pupa of the same (slightly enlarged). Fig. T12. — A smiil {Planorbis trivolvis); natural size. Fig. 113. — Common amphipod {Eucrangonyx gracilis); twice natural size. Fig. 114. — Pond 58 in a dry season, showing dead fish (mud minnows) both on bottom and out of water and in the water. A garter-snake {Tharnnophis sp.) feeding on the fish. Consocies of logs. — This is the chief place to find the sponge and the polyzoa. Their numbers vary from year to year but they are usually SUCCESSION OF COMMUNITIES 151 present. With them are often found leeches, especially Macrohdella decora, which is a brilliant red-and-green form. The only character- istic insect is the dytiscid beetle {Agabus semipunctatus Kirby) (99c) a slender reddish-brown form. The other forms found here are inci- dental in the vegetation. Hollow logs are probably used for breeding- places by the fishes, such as the bullheads (105), while the eggs of Physa and of water-mites, and some of the aquatic insects, are also placed here. The mammals of these ponds are the muskrat, which occurs in all the stages, and the mink, which is now rare. Tendencies of the association: This association is unstable. Its fate is heralded by the incoming of different amphibious plants at the sides. This is the form Proserpinaca, with the divided leaves above water and the entire ones below. This is often associated with Equisetum and plants that have the growth form of grasses. Following these are the shrubs, such as the buttonbush (6). Before these have captured the entire pond it becomes dry during the dry season and the end of the aquatic community is come. The formation which follows is the tempo- rary pond, swamp, or marsh type. Characters of the formation: The formation composed of the two associations mentioned may be characterized as made up of forms which require but little oxygen, and no bare bottom. The reproduction is one of two types: either the young are carried or the eggs are attached to plants. Some of those carrying the young are the Sphaeridae, the amphipods, and the isopods. Those sticking the eggs onto or into the vegetation are the snails (all), the Dytiscldae, all the species recorded, the Hydro philidae, the Notonectidae, the Belostomidae, the Ranatras,\ht caddis-flies, the Donacias, and in fact most of the forms of the formation. IV. Succession The first formation to take possession of a pond when it is first separated from a lake like Lake Michigan is the bare-bottom formation; chara soon makes its appearance in the deeper parts and we have the beginning of the chara association. The chara association so acts upon the bottom by covering it with humus and vegetation that it renders the continued existence of the bare-bottom formation impossible (6, 1 1 2, 1 14, 1 14a). At the same time it prepares a way for the vegetatiori which reaches to and above the surface. This, in turn, fills the pond still further, and the strictly marsh vegetation takes possession. The history of the true pond is then at an end and the story of the marsh begins. Our series of 95 ponds illustrates the series of stages. The 152 POND COMMUNITIES vegetation which comes to the surface of the water and the later marsh and swamp vegetation encroach from the sides toward the center. Entornostraca do not ordinarily show so clear a succession of species as do other groups and our collections are very incomplete. The follow- ing have been noted: Cladocerans: Ceriodaphnia reticulata Jurine, C. pulchella Sars, and C. qiiadrangula Muel. from Ponds 52 to 93. Copepods: Cyclops albidus Jurine appears more common throughout the series and C. viridis Jurine is common in the older ones. Diaptomus reighardi Marsh is in the younger ponds and its place is taken by D. leptopus Forbes beginning with Pond 30. Of the ostracods, Cypria exsculpta Fisch. is common throughout the series. Cypridopsis vidua MiiU. is common in the semi-temporary ponds. I. FATE OF THE PONDS In the late stages the pond dries during extreme droughts and passes rapidly from the stage at which it dries occasionally during a dry season to the stage when it dries every season. It is then known as a marsh or swamp, or often vernal marsh or swamp, or summer dry pond. At such a stage it is a land habitat in summer and a water habitat in spring. As the pond bottom is built up higher by the accumulation of peat, and the surrounding ground- water level is lowered by the forces of erosion, the question of w^hat is to become of the pond brings us to a question of great importance in connection with climatic formations. It will become what- ever the surrounding climatic formation may be. If it is forest, directly or indirectly, the pond becomes forest, and if it is steppe the pond be- comes steppe, while if prairie or savanna the pond becomes savanna. We have already noticed that the area of study is on the border of the forest and prairie (steppe formations). A pond in the area of study may therefore become prairie or forest. Ponds with sloping sides usually become prairie, and those with steep abrupt banks or shores turn into forest. There is no marked difference between the animal life of the two. Collections made in a series of three prairie ponds which are situated near Wolf Lake, Ind., and which in ecological age may be compared with Ponds I, 7, and 14 of the Lake Michigan series, are almost parallel with the collections from the Lake Michigan ponds. The differences to be noted are that the snail Planorhis trivolvis, which usually occurs in old ponds only, is found in the earliest pond of the prairie pond series, while the snail Vivipara contectoides and the shrimp Palaemonetes paludosus, which usually occur only in streams and small lakes, also occur in the prairie pond series. The presence of the latter two may be explained, how- ever, by the fact that the ponds were once connected with Wolf Lake. POND ANIMALS 153 In the pond formation proper, the fate of the pond early becomes evident along the margin. This will be discussed in connection with swamps and marshes. The discussion of the areas properly called marshes and swamps is the most complex of all our discussions, and will be taken up in the chapter on swamps, marshes, and temporary ponds. Tables XXVII-XXXIII show animals recorded from the series of ponds at the head of Lake Michigan (Stations 32-37). TABLE XXVII Sponges Pond Numbers I S 7 14 30 52 93 95 Meyenia{?) craterijonnis Pot.. . . Meyenia flimatllis Auct Heteromeyenia argyrosperma Pot . Spongilla fragilis Leidy * * * * He * TABLE XXVIII Leeches Pond Numbers I SC 7