Fresh water fishes and their ecology

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ILLINOIS STATE LABORATORY

OF

NATURAL HISTORY

URBANA, ILLINOIS, U. S. A.

STEPHEN A. FORBES, PH. D., LL.D.

DIRECTOR

FRESH WATER FISHES AND THEIR ECOLOGY

BY
STEPHEN A. FORBES

1914

FRESH WATER FISHES AND THEIR ECOLOGY*
BY STEPHEN A. FORBES

When we watch a summer thunder storm, which covers the earth
with a sudden flood and makes rivulets by the road-side, each carrying
down to the smaller streams its load of leaves and other organic debris,
together with the lighter parts of the soil, and when we see these silt-
laden streams unite in rivers turbid with the rich spoil of the land, we
are inclined to lament the enormous and oft-repeated waste, seeing no
way in which it can be recovered in any considerable measure to the use
of man ; but if we follow it to the lake bottom and the river bed we shall
see much of it arrested there, to become an aquatic soil, partly muddy
water and partly wet mud, more fertile even than the richest fields, and
sustaining a new population of plants and animals, of many grades and
classes, one climbing upward, as we may say, upon the shoulders of
another, to reach a level which makes them accessible again to our use.

Since the waters which wash the surface of the earth fall virtually
lifeless and sterile from the sky, whatever population they eventually
contain must evidently be supplied from the contributions made to them
by the earth, including, of course, the organic and inorganic substances
dissolved out of the earth by surface wash and underground nitration.
The aquatic population of a lake or stream is thus sustained by the wastes
of the land — materials which would otherwise be carried down prac-
tically unaltered to the sea ; and our rivers and lakes may be looked upon
as a huge apparatus for the arrest, appropriation, digestion, and assimila-
tion of certain raw materials about to pass from our control, valueless
and sometimes deleterious as they leave us, but capable of being worked
over, renovated, and returned to us in new and valuable forms, mainly
as fishes available for food.

The raw materials thus contributed by the land vary according to
their origin. In uncivilized nature they were mainly the washings and
sweepings of the primitive prairie and forest, rich in carbon but with a
minimum amount of nitrogen. With the occupation of the country, the
cultivation of its lands, and the building of towns and cities, the animal
wastes are increased, with their larger increment of nitrogen, and larger
quantities of the soil itself are swept into the streams — all alike available

* Read at the University of Chicago, August 20, 1913.

in the end as containing food for aquatic plants and animals, and so
finally for the support of man. Even the sewage of great cities is to be
classed with the rest as an available resource, capable of being arrested,
redeemed, and returned to us in acceptable form, provided that certain
conditions are observed necessary to the protection of these organs of
digestion against chemical poisoning and against a mechanical overload-
ing with more food stuffs than they can continuously assimilate. From
this point of view we may say that as the land loses fertility, the waters
should gain; and if they do not, it is because of faulty management.

We may be helped to an analysis and understanding of the con-
struction and operation of this aquatic apparatus of appropriation and
assimilation if we so arrange the principal organisms of our Illinois
waters in the form of a table of feeders and their food, of eaters and
things eaten, that we may see at a glance for each group of fresh water
animals both what it feeds upon and what feeds upon it in turn. (Fig. 1.)

This table, T need hardly say, might be indefinitely complicated and
enlarged; but I am intending it only to show the main features of the
relationship. If I had made it to include details and exceptions and
organisms of secondary importance, or even internal parasites, it would
have been too complex for our present purpose.

Notice especially the evident predominance of fishes in this scheme
of vital relationship, shown by the fact that they feed upon everything
in the bill of fare from terrestrial wastes to frogs, while they are, on
the other hand, their own worst enemies, more fishes falling a prey to
other fishes than to all other aquatic enemies combined. Further, if we
take account not only of the food of fishes, but also of the food of their
food, we shall- see that it covers every item on our table excepting a few
at the lower right-hand corner relating to turtles, serpents, birds, and
mammals, including, of course, man ; and that even these exceptional
groups themselves all feed on fishes. It is thus graphically evident to us
that to understand the ecology of fishes completely we must study also
the ecology of every class of living things in the midst of which they
live. We must even go outside the aquatic environment and analyze the
relations of fishes to their terrestrial enemies, and to many terrestrial
sources of their food. To handle anything so complex, we must have
the aid of such groupings and classifications as our materials will per-

mil; and for even so brief a discussion of the topic as is possible for
us today, a rough classification will be useful.

Ecological classification of animals may take either one of two princi-
pal directions, or indeed both of them in turn. Ecology being the rela-
tions of interaction between organisms and their environment, if we take
it up from the side of the organism we shall naturally prefer a classifica-
tion based on differences of animal reactions to the same environmental
conditions — differences of behavior, that is — and our classification will
be a habit classification; if it is the environment, on the other hand,
which interests us primarily, we shall assemble our animals in groups
according to their environmental preferences, and our classification will
be a habitat classification. The habit classification is fundamentally
physiological and the habitat classification is primarily physical or spatial,
and the two cut across each other, often at right angles, each habitat con-
taining associated animals of various habit, and each habit group being
distributed, as a rule, over various habitats. A mill pond, for instance,
is a very definite habitat, but it may contain fishes of every sort of habit ;
strictly piscivorous fishes form a very definite habit group, one or more
of which may be found in almost any kind of aquatic habitat.

I do not myself favor the attempt to reduce the facts and materials
of animal ecology to one hard and fast, all-including classification, such
as biologists attempt to establish, in the face of almost infinite difficul-
ties, for descriptive botany and zoology ; but I believe that our ecological
classifications should be as various as the objects we have in view, being
made sometimes on one basis and sometimes on another, as best serves
our purpose at the time. It will serve my present purpose to classify
fishes first in general terms according to the principal elements of their
food, thus forming habit groups, without present reference to their
habitats.

I must first acknowledge, however, that fishes can not be completely
and clearly divided into mutually exclusive groups upon this basis, for
their choices of food and their capacities for its appropriation are not
sufficiently fixed and definite in the different species to make this prac-
ticable. I can best describe the actual situation by saying that fishes
have a common body of food resources of miscellaneous character upon
which many of them draw almost indiscriminately according to the cir-
cumstances at the time, but that from this common mass of resources,
habits, and capacities there is a tendency to specialize in various direc-
tions, which tendency goes to its limit in some species, halts at various

4

intermediate stages in others, and in still others is hardly discernible at
all. Moreover, the food choices of scarcely any fishes are so definite and
unchangeable as to be unmixed and identical under all conditions, in all
parts of their habitat, and at all times of the year. We shall find, indeed,
the same state of affairs when we come to deal with a habitat classifica-
tion; and if we look the whole field of ecology over we shall see it rather
characteristic of an ecological classification generally, especially on the
animal side. We can nevertheless profit greatly by such groupings of
our heterogeneous data as are still possible, if we admit the limitations
of the scheme and understand their significance.

One of the most peculiar of our food habitat groups contains the
gizzard-shad and the stone-roller as its most notable representatives, to-
gether with a few minnows less strictly limited to it, all of them char-
acterized by unusually long, convoluted intestines, the gizzard-shad hav-
ing also the digestive surface still further increased by the development
of a very large number of finger-like caeca on its anterior section. These
fishes all discard intermediary agents, and help themselves to the raw ma-
terials of their food in the form of the mere mud and slime of the bot-
tom, which contains, of course, a considerable quantity of organic debris,
mostly of vegetable origin. They form the group of the mud-eaters.
(Figs. 2, 3, and 4.)

The gizzard-shad, although but little eaten, is one of our most val-
uable fishes, since it is enormously abundant in our large waters; both
rivers and lakes, competes with no other species for food, and is itself
the principal food of our game or predaceous fishes — the most highly
valued products of our fisheries. It affords, also, a remarkable instance
of a transformation or development in the food habits and resources of
fishes, coincident with increase in size. From the time this fish hatches
from the egg until it comes to an inch or so in length it is as slender as a
minnow, with the alimentary canal a simple straight tube. Still more
remarkable, although the mouth of the adult is perfectly toothless, the
young have, at this stage, a row of conical, pointed teeth upon the upper
jaw. (Fig. 5.) Teeth would evidently be useless to it in sucking up
mud or straining out plankton from the water; but to the larva— if such
it may be called — they must be very useful, for instead of being a mud-
eater the fish is predaceous in this stage, its prey being the minute animals
of the plankton, especially the Entomostraca, which it pursues and cap-
tures one by one as a pike might capture minnows. With its growth and
transformation it changes its habits slowly, its food becoming more mixed

with mud; but it develops effective gill-rakers also, and we sometimes
find the stomach of the adult stuffed with a fairly clean plankton.

The stone-roller, although a mud-eater and especially equipped for
that function by its long intestine, wound in a close coil around the air-
bladder, nevertheless avoids muddy waters as a rule, preferring quick
currents over rocky streams, from the stones of which it nibbles and sucks
the sediment and slime.

The most remarkable in many ways of our American fresh-water
fishes is the Polyodon or paddle-fish (Fig. 6), and in nothing is it more
peculiar than in the fact that, although it is one of our largest fishes,
reaching a maximum length of six feet and a weight of a hundred and
sixty pounds, it is essentially a plankton-eater, feeding largely, and some-
times almost wholly, on the smallest aquatic animals and plants, for the
appropriation of which it has, in its gill- rakers, a straining apparatus
scarcely less effective than that of the whalebone whale. To strain out
the plankton, it holds its enormous but weak-jawed mouth wide open
as it swims about, permitting the water to flow through its very wide
gill slits, getting thus not only the smallest animals and plants, but many
insect larvae also of kinds abundant on the open bottom in comparatively
shallow water. It is, indeed, a living, fine-meshed, water-net. This fish is
our only proper member of the special class of plankton-eaters, although
plankton is taken in quantities at times, especially in spring, by a con-
siderable number of other fishes of various sizes, all with long and fine
gill-rakers — structures which have, in fact, no other use than to strain
from the water food particles too small to be taken in any other way.
The crappies and certain other sunfishes will often so gorge themselves
with plankton by this means that the bulging of their stuffed stomachs
can be seen from the outside. Lake herring and white-fish are other
examples of this class, not dependent, however, upon plankton as their
most important food.

While adult plankton-eaters are thus relatively few, it is an interest-
ing and peculiarly important fact that plankton is almost the sole infant
food of nearly all our fresh-water fishes, of whatever kind or adult food
habit. The hatching season of most of our fresh-water species is, in fact,
the prime season of the year for plankton production in the shallows and
back-waters where most fishes spawn ; and the minute mouths and gill
slits of the very young are perfectly fitted, without special adaptation,
for the capture of this microscopic prey.

6

I have already given you, in the gizzard-shad, one example of a
young fish especially armed with teeth for this sort of hunting, and I
may mention, in passing, another case of the kind which is even more
remarkable. The common whitefish of the Great Lakes is, as you all
know, quite toothless, and, as an adult, is what the Germans call a Klein-
tierfresser — a convenient word for which we can hardly substitute its
literal translation — a small-animal-devourer. The recently hatched white-
fish, however, is a pure plankton-eater, and it must snatch its minute
prey, one at a time, from the sparsely inhabited waters of the open lake.
It is very important to it, consequently, that it should not miss its catch
or lose its hold, and we find it specially equipped against this accident
with four acute, curved, raptatorial teeth on its lower jaw (Fig. 7), as
effective against a Cyclops or a Diaptomus as the fangs of a tiger against
an antelope.

Such transformations in food habit with increasing size are, indeed,
the rule among fresh-water fishes. Starting together as plankton-eaters,
they presently diverge in habit, reaching their adult food stage through
two or three degrees of change. The sheepshead (Fig. 8), for example,
begins, like the rest, with plankton, becomes insectivorous when it is a
few inches long, living almost wholly on the insect larvae of the bottom,
and as it reaches adult size its habits change again to those of a mollusk-
eater, in adaptation to which it develops in its throat a powerful crush-
ing apparatus, with pharyngeal jaws capable of smashing the thickest
shells of our water snails, and even those of clams or mussels of con-
siderable size.

Other mollusk-eaters are the Great Lakes sturgeon (Fig. 9) and
certain species of the catfish (Fig. 10), sucker (Fig. 11), and sunfish
families (Fig. 12), several of them especially equipped for crushing
shells — the suckers and sunfish by stout, blunt teeth set in their strong
pharyngeal jaws (Fig. 13), and the catfish by pads of sharp conical
teeth in their premaxillaries (Fig. 14) and mandibles. By the use of
these they seem able to crack a snail as a boy cracks a hazelnut, reject-
ing the broken shells to swallow the juicy meats. Among the suckers
there is a curious inverse correlation in the development of certain of their
feeding structures, gill-rakers and pharyngeal jaws growing, one may
say, each at the expense of the other. That is, where gill-rakers are
long and numerous, the pharyngeal jaws are weak and their teeth are
numerous and small, and the species feeds largely on Entomostraca ;
while if the pharyngeal jaws are thick and strong, with strong crush-

ing teeth, the gill-rakers are short and thick, and relatively ineffective
as a straining apparatus. This is so generally true that one may even
tell whether or not a sucker is a mollusk-eater by looking at its gill-
rakers, although these have nothing to do directly with the collection
or mastication of molluscan food.

Another terminus to the series of changes in food choice through
which most fishes pass is in the piscivorous habit, characteristic of what
we commonly call our game fishes; especially the pike, the pike-perch,
and the Great Lakes trout; but the largest number of our fresh-water
species linger in the intermediate, insectivorous stage. Indeed, taking
our adult fresh-water fishes as they come, we find that insects are by
far the most important general element of the food of the class, eaten
more or less by nearly every kind of fish and the main dependence of a
great many species which, by mere increase in size and the consequent
coarser structure of their gill apparatus, have lost the original capacity
of the young to strain out the plankton, without attaining to a size and
strength sufficient for the capture of a prey larger and stronger than
aquatic insect larvae.

Some kinds of insects occur in such abundance in situations diffi-
cult of access, that certain groups of fishes have become especially adapted
to their search and capture there. Darters, for example (Fig. 15),
live mainly on insect larvae which hide under stones in swift water, and
they are enabled to get at this food by virtue of their large pectoral and
anal fins, by which they can support themselves on the bottom in a swift
current or make their way among the ripples of a rocky stream, and by
their small heads and pointed noses which enable them to pry about
under stones where worm-like Chironomus larvae and larvae of small May
flies abound. A little cyprinoid fish — the sucker-mouthed minnow (Fig.
16) — is very similarly equipped and to a like advantage. Access to
the same kind of food under the heavier stones of larger streams is given
to a sucker known as the hammerhead (Fig. 17). It has a similar develop-
ment of the paired and anal fins, and a large square head with which it
can push and roll about the stones under which day-fly and stone-fly
larvae may be found in great abundance ; and these are its principal food.

Besides the six food classes which I have already mentioned, namely,
the mud-eaters, the plant-eaters, the plankton-eaters, the mollusk-eaters,
the insect-eaters, and the fish-eaters, we may doubtfully distinguish two*
more — the garbage-eaters and the omnivora. There is, indeed, but one
of our fresh-water fishes — the common eel — which seems to live by pref-

erence on dead food or decayed flesh; but the class of garbage-eaters
may be made to include three or four of the catfishes also, which resort
to such food willingly when it is convenient to them. Certainly fishes
in whose stomachs we have found, from time to time, distillery slops,
ham bones, dead rats, dead cats, and heads and entrails of fish thrown
out from fish boats, need not complain if they are provisionally assigned
to the humble class of scavengers.

These same catfishes might perhaps be better classed as omnivorous,
for they eat, in fact, very nearly every kind of food which the water
contains, including insects, mollusks, fishes, crawfishes, and sometimes
unusual quantities of algae and other aquatic vegetation. In this omnivo-
rous class we may also place the common European carp, except that this
fish does not eat carrion.

If, now, we review the generalities and the peculiarities of food
and feeding habits which I have imperfectly sketched, seeking to under-
stand their differentiation and succession, we may best interpret the
facts by attempting to realize the food resources of an average, typical,
undifferentiated fish, which should reach adult condition without acquir-
ing any special adaptations of structure or of preference in respect to the
choice, appropriation, and assimilation of its food. Such an undiffer-
entiated fish would have a subcylindrical body with only the ordinary
equipment for locomotion; it would be toothless both as to its jaws and
its pharyngeal bones ; its mouth would be neither suctorial nor especially
protractile; and its gill-arches would be without specialized gill-rakers.
In other words, it would be a simple product of growth, without progress
or differentiation, from the state of the recently hatched fry. Such a
fish would necessarily begin, as all our fishes now do, with a mixed
plankton for its earliest food, taking the smaller organisms first and the
larger ones later. As it gradually becomes too large for the pursuit of so
minute a prey, and its gill structures too coarse to serve longer as a plank-
ton strainer, it would draw next upon the insects, and mainly on the
insect larvae of the bottom and the shores — creatures especially avail-
able to it because their soft and poorly protected bodies make them fit
for digestion without mastication or other special preparation ; and with
these it might mingle also amphipod crustaceans, and the smaller thin-
shelled mollusks, especially those which could be picked from an aquatic
vegetation. Next would come such young fishes as it could seize and
swallow without a special armature of jaws and throat; and at this stage
of growth and progress it would apparently stop. To go farther as a

0

predaceous fish it would need the swimming capacity and the raptatorial
teeth of a pike-perch or a pike; to get effective access to the abundant
stores of molluscan life, gastropod and bivalve, in our streams and
lakes, it would need either a suctorial mouth or strong pharyngeal jaws
with crushing teeth, or both of these, and thus it might become the
equivalent of a sheepshead, a sunfish, or a sucker, as other conditions
should determine. To continue as a plankton-eater, it would need the
numerous, long, and slender gill-rakers of a paddle-fish or a lake herring,
and with these, especially if it had a suctorial mouth and a very long in-
testine, it might be able to sift and strain from the silt of the bottom
the finer organic particles derived from the debris of aquatic vegetation
and from the wash of the land. Other specialties of differentiation in
structure or in habit might open to it less usual food resources, as with
the darters and the top-minnows ; and a mere deviation or degradation
of taste might add the carrion of the stream to its menu.

Evolution of food habits must thus have taken the course of struc-
tural evolution — an advantageous specialization in various degrees and
in various directions from a generalized, undifferentiated original. Some-
times added specialties of advantageous equipment have brought in their
train limitations or prohibitions in other directions, which have shut a
species out from certain- food resources in making others more available.
The same set of gill-arches, for example, can not serve at once as a
plankton-net and a shell-crusher; but, generally speaking, the structural
differentiations mentioned have enlarged the resources of the fish in some
directions without reducing them in others. Even so definitely predaceous
a fish as the Great Lakes trout, which lives habitually on the abundant
herring of the lakes, has been known to devour salt pork, ham bones,
chicken bones, raw potatoes, corn cobs, rags, spoons, tin cans, silver
dollars, and in single instances, a watch and chain, an open jack knife
seven inches long, and a two-foot piece of tarred rope.

From this it would appear that these structural differentiations
have not necessarily followed upon differentiations of preference, fitting
the fish to get more easily and abundantly the kind of food which it had
already come to prefer; they seem to have arisen independently of any
peculiarities of choice, and may, indeed, have forced the species, in a
sense, into directions which it would not otherwise have been inclined
to follow.

10

If we turn now from these examples of habit groups to a classi-
fication by habitats — from physiological to spatial ecology — from a dis-
cussion of the food of fishes to the subject of their local distribution and
their assemblage in what are called animal associations, we shall find a
similar state of affairs to that just noticed. Some of our fresh-water fishes
are so widely and thoroughly distributed over a large variety of situa-
tions that they may be likened to the omnivorous class in the classifica-
tion by habits, while others are as narrowly limited in habitat as is the
carnivorous pike in respect to its food. For my detailed data of local
distribution I shall have to draw almost wholly on our Illinois observa-
tions, for the reason that we have made in Illinois much larger and more
intensive collections of our native fishes than have been made in any
other state — larger, indeed, as I believe, than in any other area of like
size anywhere in the world.

The blunt-nosed minnow (Fig. 18) is an example of what we may
call omnilocal distribution, the map of its local occurrences in Illinois
(Map XXVII), being a fair abstract of the map of localities for all our
Illinois fish collections. It is relatively rare only in our larger rivers,
the frequency of its occurrence there being, by our data, as 5 to 34 for
the smaller rivers, and to 43 for creeks. That is to say, if we were to
take equal numbers of fish collections from each of these classes of
waters throughout the state until we found this species five times in
larger rivers, we might expect to find it about thirty- four times in small
rivers, and about forty-three times in creeks. It is not limited in its range
or habitat by its choice of food, for it feeds mainly on mud, and that it
could easily find almost anywhere in Illinois. It prefers streams with
a rocky bottom, it is true, its occurrence in such waters having a fre-
quency of 46 as compared with 27 in other places ; and the kinds of vegeta-
tion mixed with the mud of its intestinal contents give us reason to think
that it nibbles and sucks the slime from stones and other submerged
objects.

Contrast with this, now, the distribution map of the spot-tailed min-
now (Map XXXVIII), and another minnow species, Notropis heterodon
(Map XXXIV), not well enough known to have received an English
name. The spot-tailed minnow, very common in lakes and ponds and
especially in the Great Lakes, occurs elsewhere mainly in the larger
rivers, its average frequency in our collections in these two situations
being as 33 to 3.5 in the smaller streams. That is, we have found it nearly
ten times as common in the larger waters as in the smaller ones ; and its

11

food, consisting mainly of insects, crustaceans, algae, and fragments of
aquatic plants, is as different from that of the preceding species as is its
distribution. The little heterodon, on the other hand — about two inches
long — is essentially a lake and pond species, and its abundance, not only
in the lake region of northeastern Illinois but also along the larger rivers,
is explained by the fact that it is mainly in the lowlands of the river bot-
toms that lakes and ponds are to be found in Illinois. It is in such sit-
uations that it finds the bottoms of mud and sand which seem to attract
it, its frequency ratio there being 71 as compared with 22 over rock and
sand, and 7 over mud. The food of this species is consistent with this
preference of location, being mainly Entomostraca and small larvae of
gnats. It is, indeed, essentially a plankton-eater, being of the size to
make the plankton of its favorite resorts its most convenient and abundant
food.

I know well that these specific details are hardly fit for a general
lecture, but they are the materials of my generalizations, and I must ask
you to indulge me to the extent of two more examples, chosen from an-
other family of fishes — that most interesting division of the perches
commonly known as the darters. These are the johnny darter (Fig.
19; Map XC), and another species, Cottogaster shumardi (Fig. 20; Map
LXXXVIII), which has no English name. They are particularly inter-
esting, because the johnny darter, although very abundant all over the
state, seems to avoid the larger streams, having a frequency there of
only 3 as against 53 for creeks, while the Cottogaster, although compara-
tively rare, occurs almost wholly in the larger rivers and in the bottom-
land lakes in their immediate neighborhood. Whether there are differ-
ences in food corresponding to their distribution we cannot tell, be-
cause the food of the rarer species has not been studied.

Many other instances might be given of the fact that fishes can be
separated into groups according to their habitats as well as by differences
in their food, but that the groups so formed are of very unequal scope.
It is as if, in classifying fishes structurally, we should find that there
were some families which combined the characteristics of nearly all the
others ; that other kinds present many such common characters, but a
smaller number; and that only a few had differentiated so far from the
common mass as to have fixed distinguishing characters of their own.
An ecological classification, while quite possible, and indispensable also,
ought not to be framed in imitation of the classifications of the taxonomist,
but must have objects and methods of its own.

12

You are all familiar, no doubt, with the idea of animal associa-
tions— groups of species habitually associated in the same environments ;
and these are as recognizable among fishes as among the animals of the
land ; and here again, substantially as in the other cases, we find animals
which, taken by themselves, may be seen to form associate groups
•with a large extension, covering many distinguishable kinds of situations,
and others which are rather narrowly limited to a single sort of habitat,
characterized by quite special conditions. There is a group of creek
fishes, for example, made up of species which may be found almost any-
where in a creek and in almost any kind of a creek, and another group,
like certain of the darters and the stone-roller, which are thoroughly
at home only in rocky streams with a relatively rapid flow of at least
fairly clear water. If we analyze the aquatic environment into all the
situations clearly distinguishable, we shall find, perhaps, a group of cer-
tain species distinctive for each situation, but for other species our
analysis will be seen to have gone too much into detail ; it will distinguish
differences of condition to which they are indifferent. Even the distinc-
tion between small river and creek, and between river and lake, is too
narrow for some fishes, which are found with almost equal frequency in
both. The grass pickerel (Fig. 21), which we have taken in one hundred
and eleven Illinois collections, is almost equally abundant in creeks and
in ponds; and the river-chub (Fig. 22) has been found about equally
common in creeks and in small rivers, and virtually absent from the larger
rivers and from lakes. Nevertheless, the distinction of animal associa-
tions is very helpful to our grasp and understanding of the system of
living nature, and will become much more so as our knowledge becomes
both more comprehensive and more precise.

An organization of the animal population of a region into associa-
tions may be approached either from the side of the environment or from
that of the animal inhabitants; either by an analysis of the environment
into habitats and situations, and a critical survey of the inhabitants of
each, or by an analysis of the animal population into groups of most fre-
quent associates, and a study of the local and ecological distribution of
each such group. By the first method, spatial units — so-called units of
environment — are first distinguished and delimited, and the animals con-
tained in each such unit are then identified, listed, and enumerated. By
the second method, the associate groups are first distinguished, defined,
and analyzed, and the territory under examination is mapped in a way to
mark the area of distribution of each such group. The first method is

13

primarily geographical, and the second is essentially biological. Both
methods are useful and the product of both is necessary to a full knowl-
edge of our complex subject; but of the two, the biological method seems
to me much the more useful and significant.

The most general division of the environment of fresh-water fishes
into habitats distinguishes large rivers, small rivers, creeks, upland lakes,
lowland lakes, marshes, and stagnant ponds. Streams are still further
divisible into those with rocky bottoms, bottoms of sand, and bottoms of
mud; into those with a swift or with a sluggish current; and into those
with clear or with turbid waters. Even parts of streams are distinguish-
able into different habitats — the rippled reaches of rocky streams differ ma-
terially from the deep, still pools between. In the larger rivers, like the
Illinois, we may sometimes distinguish between the opposite margins,
where, as at Havana, one has a muddy bank and the other a sandy one.
In lakes there are notable differences between the marginal shoals and
the deep interior waters, between sandy bars and mud flats; between
open waters and those filled with weeds ; and in the weedy parts, be-
tween those in which reeds, rushes, and other coarse, rooting plants are
present and those in which the plants are mostly submerged. These pre-
sent their characteristic differences in the fishes which resort to them —
differences clearly discernible, however, only by the use of quantitative
methods, which give us the relative numbers of each species found in
each situation over a sufficient length of time and variety of external
conditions to make us sure that we are getting fair and stable averages.

The first thoroughly practical work of this kind that I know of was
done at Havana, Illinois, under my direction, in 1898-99, by Wallace
Craig, later a doctor of philosophy of the University of Chicago, but at
that time a temporary assistant on the Illinois Natural History Survey
and also a graduate student in the University of Illinois. He began in
August, 1898, a detailed study of the local distribution and the move-
ments of fishes, with a view to making out preferences of situation or
choices of environment of the various species of fish under varying con-
ditions and at different times of the year. Using identical apparatus by
uniform methods at regular intervals in the waters of the locality, it was
possible to get totals and averages by a comparison of which the strik-
ing features of the different situations were made manifest when the
statistical tests were compared with each other. By this method it was
shown that the Illinois River gave us different data of frequency for
the different species on the two sides of the stream, one of which was

14

muddy and the other clear; but the most interesting conclusion was a
notable difference in degree of specialization in the fishes inhabiting the
different sections of a stream system. Those from the larger river were,
as a rule, not only the largest, but the most primitive, or the least special-
ized; those preferring the bottomland lakes were, on the whole, more
highly differentiated; and those from the creeks were smallest and the
most highly specialized of all.

It is perhaps what we ought to expect, that the creek species should
be more diverse and highly organized than any other fishes, for they
must have had longer experience of fresh-water life. As the continent
first began to rise from the sea, all its streams were necessarily small,
and its first fishes were consequently those adjusted to life in creeks.
As the process continued, as the surface of the land became more di-
versified, as the small stream systems of the coast ate their way back,
with many lateral branches, and united to form large rivers, and these
again to make rivers of the largest size, new habitats would be formed,
both in the uplands and along the coast, and new adaptations of fishes
to them would naturally lead to about the kind of classifiable diversity
which we actually find.

That ecological differentiations and divisions among fresh-water
fishes are, as a rule, of no very compelling force is shown in a remark-
able manner by the fact that the whole system of such distinctions breaks
down almost completely at least once a year, when a great migration
movement up-stream and into shallow water seizes all species alike, under
the overpowering impulse of the breeding instinct. At this time fishes
of the most varied habit and habitat seem temporarily to desert or forget
their favorite places of resort, and throng together, indifferent to their
individual welfare, in search of places for the deposit of their eggs,
and, with many species, for the subsequent care and protection of their
young. Even under less extraordinary circumstances I have found, in
fact, that a large river like the Illinois becomes a sort of metropolis of
the fish population of its drainage basin, in which representatives of the
various groups or associations, separate and distinct in its headwaters
and smaller tributaries, may be found indiscriminately commingled, just
as in this great city we see people from scores of smaller cities and
hundreds of smaller towns and thousands of rural communities. Sunfish
species, for example, which rarely occur in each other's company in
collections from the smaller tributaries, were found together twice as
often in collections from ihe larger streams.

15

Such are some of the products of a study of the populations of
predetermined habitats. It remains to be seen to what extent these
habitat populations coincide with real ecological groups of most fre-
quent associates — to what extent our habitat characters are the real de-
terminers of the actual associative distribution of our fishes. It may
be that the effective sensibilities of fishes are not altogether what we
have supposed them to be, a priori; that things we have not thought of
in that connection have much to do with their assemblage in more or
less definite and in more or less permanent societies. This is especially
possible with fishes, because we can see so little of them as a rule, and
because their power of free and rapid locomotion enables them to as-
semble and to disperse so readily and so rapidly. To get at the funda-
mental facts we must find a means of learning what and where definite
associations among fishes really exist; what is the local center and what
are the optimum conditions of each such associate assemblage ; and what
are its most typical and constant components — what species, that is to
say, are the most clearly and constantly characteristic of it. This means
that we must study the details of the distribution, and hence of the
associate grouping of fishes, with reference at first to their location only ;
and then, when our associations have thus been determined, located, and
described,. we must see how they compare with the habitat system ar-
rived at by our preliminary analysis of the environment.

This sort of critical study of the essential details of ecological dis-
tribution has almost never been made, at least for animals, and even
the methods of it are scarcely agreed upon. Those which I shall briefly
describe to you were devised for the purpose of utilizing, for ecological
description and inference, the product of extensive collections of Illi-
nois fishes, made in the long course of the natural history survey of
the state. They are based upon the obvious fact that a biological asso-
ciation is made up of species which are associated with one another more
frequently than they are with other species ; from which it follows that
to find an association one must find a group of such most frequent asso-
ciates ; and to determine the center of its location and the extent of its
range we must find where this associative frequency — this frequency of
joint occurrence — of the several species of the group is greatest, and
how far in each direction each species of it continues to be more fre-
quently associated with the other members of the group than with any
other species. If, for example, we make a hundred collections over a
given area of complex ecological composition, and find that we have

16

fifty species of fish represented in these collections, it is easy for us to
tell, by a simple examination of our numerical data, which of these fifty
species have come out in our nets in each other's company the most fre-
quently, and in what situation or habitat each of these most frequent
associates has been found most abundant. In so far as frequency of
habitat occurrences and frequency of associate occurrences coincide, we
have evidently a local association distinguished, together with its char-
acteristic and accustomed habitat.

I tested the utility of these simple ideas during the summer vaca-
tion of 1905, by an application of them to our Illinois collections of the
so-called darters — species of the subfamily Etheostominse — in a way to
prove, what, indeed, we already knew as a matter of common observation,
that, taken as a whole, these darters are an associate group, and that
their characteristic habitat is what also we already knew it to be — the
rocky rapids of small streams. The essential correctness of the method
was thus verified ; and I was also able to distinguish six species of darters
peculiarly typical of the group, to be regarded as especially characteristic
of it because they were found more than two and a fourth times as fre-
quently associated with each other as they were with the seven remain-
ing species; and likewise because they were about two and a half times
as frequently associated with each other as were the seven remaining
species among themselves.

It was easy to show, on the other hand, by similar methods, that
the sunfishes (see Fig. 23 and 24), although as much alike to a general
observation as the darters, are not a homogeneous ecological group, but
that they are so variously related to different habitats — to different fea-
tures of their environment — that several of the sunfish species are much
more frequent associates of fishes widely different from themselves than
they are of each other; that the various sunfish species often belong, in
fact, to different zoological associations. Indeed, it was found by re-
peated use of this method of analysis that it was a rather common thing
for closely related species of fishes — near neighbors in the taxonomic
system — to be in some sense averse to each other's company — to avoid
each other, seemingly, and to find their closest and most familiar asso-
ciates in fishes far removed from them in taxonomic relationship.

I have supposed this to be an expression of the disadvantages of
close competition between closely similar species, and of the advantage,
consequently, of such differentiations in habit and such separations in
ecological preference as would carry these natural competitors into non-

competing ecological groups. Such an apparent evasion of competition
by near relatives is well illustrated by our observations on the top-
minnows (Fig. 25) — very small fishes of the killifish family, of which
we have three species in Illinois. Two of these species are distributed
throughout the state, but the third is southern in its distribution, lapping
over on the Illinois area of the other two only in the southern part of
the state. Now it was to me an extremely interesting fact that we found
this southern species much more frequently in company with the two
more widely distributed top-minnows than these two were with each
other. It was as if those which occupy the same area conjointly had found
themselves compelled to an ecological division of it, such as would keep
them largely out of each other's way, while those of an essentially un-
like geographical distribution had found no such mutual avoidance nec-
essary. It seems quite possible that this intra-local separation of com-
petitive groups, this effective isolation of forms inhabiting the same ter-
ritory, may be one of the early steps — sometimes the very first step,
perhaps — in the differentiation and fixation of species. A difference in
respect to choice of breeding grounds especially, by preventing the inter-
breeding of two diverging groups, would separate them as effectually
as an impassable mountain chain running through the area of their
original distribution.

This principle of an evasion of competition seems to apply to asso-
ciations also, as well as to species, and to explain in part the composi-
tion of neighboring associations. Similarly endowed species, similarly
disposed towards their environment, would profit mutually by a geo-
graphical separation, which should give to each a range not entered by
the other; and adjacent associations might thus be formed, alike in their
ecological make-up but different in their species. It is in some such
way that we may perhaps explain a few otherwise unexplained limita-
tions of the distribution of our Illinois fishes. Six of our one hundred
and fifty Illinois species are so definitely limited to the Wabash drainage
as to suggest that there must be some ecological barrier against their
spread, since there is certainly no geographical one, namely: brindled
stonecat, Schilbcodes miurus (Fig. 26 and Map LIX) ; green-sided darter,
Diplesion blennioides (Fig. 27 and Map LXXXIX) ; Notropis ille'ce-
brosus (Fig. 28 and Map XXXVII) ; silver-mouthed minnow, Ericymba
buccata (Map XLVI) ; long-eared sunfish, Lepomis megalotis (Fig. 29
and Map LXXVI) ; and Boleichthys fusiformis (Fig. 30 and Map
XCVIII).

18

These are all species whose general distribution throughout the
country would lead us to expect to find them anywhere in Illinois.

Even more interesting is another series of limitations upon the local
distribution of our Illinois fishes, because it seems to be clearly explain-
able as due to an ecological factor of geological origin — to the physical
character of the surface soils of a large part of southern Illinois cor-
responding to the area known as the lower Illinoisan glaciation. This
area is notable for the extremely fine division of its soil particles, due
to its geological history, and for the consequent persistent and even
permanent muddiness of its waters, such that the suspended particles
cannot be completely separated by repeated filtering with the finest filter
paper, and do not subside even after long intervals of stagnation. This
persistent turbidity of the waters might well be expected to have an
effect to repel or exclude certain kinds of fishes, particularly those having
a special preference for the clean water and hard bottom of the lakes or
streams which they inhabit. Other species, on the other hand, which are
found in muddy situations elsewhere, might be expected to tolerate the
persistently muddy waters of this southern Illinois district. An analysis
of our data bears out this assumption in a remarkable way, a fact most
clearly shown by examples of the distribution of species selected from
our lists of those tolerant, and those intolerant, of muddy waters gen-
erally. Compare, for example, our Illinois distribution maps of the stone-
cat (Map LVII), the common sucker (XVIII), the hogsucker (XIX),
the stone-roller (XXIII), the common shiner (XLI), and the river chub
(LI), all rare or wanting in the lower Illinois glaciation, with the fol-
lowing six other species freely distributed there, namely : the black bull-
head (LV), the tadpole cat (LVIII), the chub-sucker (XVI), the blunt-
nosed minnow (XXVIII), the golden shiner (XXXI), and the long-
eared sunfish (LXXVI) ; and also the fact that our statistics of ecological
distribution, crude as they are, serve to distinguish these two groups
strongly with respect to their relation to muddy situations. The fishes of
the first group, for example, have occurred over muddy bottom only once
to nearly four times over a bottom of mud and sand, while those of the
second group have occurred with about equal frequency in the two situa-
tions.

With these merely miscellaneous illustrations of method and prod-
uct, I must leave this subject, much too large and too complex for any
fairly comprehensive treatment, at least by me, within an hour's lecture.
1 am the less disturbed by the fragmentary character of this discussion
because I know that you have in charge of your ecological studies a

19

leader in this line of progress, abundantly able to make good its defi-
ciencies, and especially competent to describe to you the aims, methods,
and results of an intensive experimental study of separate problems in
animal ecology, quite in contrast to the broad reconnaissance and general
orientation work which naturally falls to the director of a biological
survey of so large an area as the state of Illinois.

PLATE I

ut?j/v

X

X

X

X

spuig

X

X

X

x

x

X

x

sjuadjag

x

x

x

X

S9/j^nj_

x

X

X

x

X

X

sdfodpej. -'sfaojj

x

X

X

x

X

X

x

S914SIJ

X

X

X

X

x

X

x

X

X

x

x

x

X

sysn/iow

X

x

x

x

X

S-439S-U/

X

x

X

X

x

x

x

x

sdysijAtej^

x

X

x

x

X

x

SUJJO/l/l

X

X

x

x

x

x

x

x

X

&3&J}SOUJOJU3

X

X

X

x

X

X

x

sj9j.lj.oy

x

x

X

x

x

eozojoud

X

X

X

x

x

siueid JayblH

x

X

X

9T2&IV

x

eudjoeg

x

PRINCIPAL

FOOD RELATIONS

OF

AQUATIC ORGANISMS
(ILLINOIS)

j Terrestrial Wastes

j Bacteria

ft

^

Or

^

_£>

c
g

o:

fc

•c

Qr

^

Protozoa

| Rotifers

| Entomostraca

e>

I

j Crawfishes

! Insects

| Mollusks

Fishes

CO

x>
o

t

Turtles

j Serpents

j Birds

PLATE II

Fisr. 4. Keel-bellied Dace (f7iro,s-on;i<s erythrogaster) . X2

!

PLATE III

Fig. 5. Upper jaw of young Gizzard-shad Fig. 7. Lower jaw of young Whitefish (Core-

(Uorosoma cepedianum), showing minute gonus clupeiformis), showing rapta-

teeth. X30 torial teeth. X30

Fig. t>. Paddle-fish (Poloydon spatlmlti) . Xr/r«

Fig. 8. Sheepshead (Aplodinotus grunniens). X

Fig. 9. Lake Sturgeon (Acipemer rubicundus) . XVr,,

PLATE IV

Fig. 10. Common Bullhead (Amfiunix ?te7iM?oxMs).

Fig. 11. Common Sucker (Catostomus commersonii) . X !4

Fig. 12. Pumpkinseed (Eupomotis gibbosus).

PLATE V

Fig, 13. Lowe

left pharrnffeal jaw of Pumpklnseed (Eupomotte glbbagus):

(a), from above; (b), from outside

-maxillary Teeth of Catflshes: (a). Noturus flavus: (b). Leptop:
(c). Schilheodes gyrinus: (d'. Ameiurus melas

PLATE VI

Fig. 15. Sand Darter (Aitnuocrypta pellucidti). X2

:. Hi. Sucker-mouth

Fig. IS. Blunt-nosed Minnow (.Pintrphalrs notatus).

PLATE VII

\

Fig-. 19. Johnny Darter (liiilenxumn nigrum). X2

Fig. 23. Cattog-a.iter shumardi X2

sr. 21. Grass Pickei-Pl (Exo.r FcrmicuMuK\ X%

PLATE VIII

Fig. 22. River Chub (Hybopsis kcntuckiensis).

Fig. 23. Bluegill (Leponiis pallidus).

Hlue-siK)tterl Sunfish (Lciiomi* cuanelli,

PLATE IX

Fijr. 25. Top Minnow (Fnndiilu* dfepar) male. X2

Fig. 2ii. Brindled Stonecat ^ScJiilbeodes miunt*).

Fig. 27. Green-sided Darter (Dit>lexi»n Jile.tniinidex).

PLATE X

Fitr. :>S. \ittr»i>i* iUn-'hi-itxnx. X1H

Kig. 29. Lcng- cared Sunfish (Lcpomis megalotis). X3A

Fig. 30. Bolrichthys fusiformis X2

..Drainage Canal
. County Sea

XVIII

Distribution

of

Catostomus

commersonii

..111. and Mich. Canal
..111. and Miss. Canal
..Drainage Canal
.County Seat

XIX

Distribution

of

Catostomus
nigricans

J1L and Mich. Canal

.......111. and Miss. Canal

Drainage Canal

.County Seat

XXIII

Distribution

of

Campostoma
anomalum

.III. and Mich. Canal
.III. and Miss. Canal
.Drainage Canal
, County Seat

XXV

Distribution

. III and Mich. Canal

Ill and Miss. Canal

Drainage Canal

. County Seat

XXXI

Distribution

I1L and Miss. Canal

Drainage Canal

. County Seat

XXXIV

Distribution

of

Notropis
heterodon .

.. Jll. and Mich. Canal
....111. and Miss. Canal
...Drainage Canal
.County Seat

XXXVII \Y
Distribution

rf

Notrop
illecebrosus

Ul. and Mich. Canal

III. and Miss. Canal

Drainage Canal

.County Seat

XXXVIII

Distribution

of

Notropis
hudsonius

III. and Mich. Canal

III. and Miss. Canal

Drainage Canal

.County Seat

III. and Mich. On*

...111. and Miss. Canal

Drainage Canal

.County Seat

XLVI

Distribution

of

Ericymba
buccata

..111. and Miss. Canal

Drainage Canal

.County Seat

LI

Distribution

LV

Distribution

of

Ameiurus
melas

111. and Mich. Canal

111. and Miss. Canal

.....Drainage Canal
.County Seat

..111. and Mich. Canal
..III. and Miss. Canal
.Drainage Canal
.County Seat

III. and Mich. Canal

III. and Miss. Canal

Drainage Canal

. County Seat

LIX

Distribution

of
Schilbeodes

III. and Mich. Canal

III. and Miss. Canal

Drainage Canal

.County Seat

LXXVI

Distribution

of
Lepomis

megalotis

. HI. and Mich. Canal

111. and Miss. Canal

Drainage Canal

.County Seat

LXXXVIII

Distribution

of

Cottogaster
shumardi

Jll. and Mich. Canal

...... III. and Miss. Canal

Drainage Canal

.County Seat

LXXXIX

Distribution

of

Diplesion
blennioides

111. and Mich. Canal

III. and Hiss. Canal

Drainage Canal

.County Seat

xc

Distribution

of

Boleosoma
nigrum

III. and Mich. Canal

III. and Miss. Canal

Drainage Canal

. County Seat

XCVIII

Distribution

Boleichthys
fusiformis

...111. and Mich. Canal
....111. and Miss. Canal
...Drainage Canal
.County Seat