ANIMAL NUMBERS Although the intensive study of the structure of animal corn munities has not so far been carried beyond the very earliest stages, yet it is obvious that farther research in this branch of ecology affords a good prospect of throwing light upon the mode in which the numbers of animals are regulated during normal times, and the reasons for the breaking down of this system of regulation—a breakdown which is quite frequent in nature, and which manifests itself in the form of plagues of animals (e.g., mice, locusts or aphids), or, in the case of parasites, as epidemic diseases. Even in the present state of our knowledge of the social structure of the animal world, we are in a position to make certain suggestions along these lines. For instance, there is the fact that every animal community, whether it be living under the bark of a dead pine tree, on the shores of a mountain lake, or in the wainscot of an old house, seems to be constructed fundamentally in the same way. This fact enables us to conclude that the means by which animal numbers are regulated may be similarly uniform in their action in all habitats. It seems very probable that if a suitable team of ecologists settled down like locusts on one very small, simple animal community, and studied it intensively from every point of view for a good many years, they might discover almost all the important laws about animal numbers—laws which may elude one if the community studied is very complicated (e.g., tropical rain-forest or marine plankton) or if observations are not carried out consecutively for a number of years.
Up to this point we have dealt chiefly with the relative num bers of different species in a community. Before proceeding farther, we must say something about the absolute density of numbers; in other words, it is necessary to inquire whether any censuses have been made of animal numbers. To carry out a census in practice is not at all simple, for a variety of reasons; but there have been a number of beginnings made in this direction during the last zo years. The most successful and accurate census results have been obtained with marine or freshwater plankton animals, since these are the easiest to deal with. On the Continent and in America a number of quantitative surveys have been car ried out, and these give an interesting and clear-cut picture of the density of various Crustacea, Rotifera, etc., in the water. On land a certain amount of census work has been done on soil ani mals, since these lend themselves to much the same sampling methods as are employed with aquatic animals. Otherwise, little progress has as yet been made with accurate censuses of land animals, except in the case of birds (Nicholson, Cook, Grinnell and Storer) and certain mammals; the work on the latter is of a sporadic nature, and chiefly applies to species which have some special economic or sporting interest (e.g., an annual census is taken of the baboons on the Rock of Gibraltar, and a similar one of the fur seals on the Pribiloff islands of Alaska). In addition, census methods have frequently been applied, though usually in rather a rough way, to various insects which are of economic significance. In the present state of our knowledge, it is not possible to draw any very far-reaching conclusions from this side of ecology, except that it appears that most species are a great deal more numerous than one would expect. We must therefore confine ourselves to pointing out the importance of accurate figures for density of numbers of animals, and the fact that it is rapidly coming into prominence in ecological work.
In spite of this general lack of precise quantitative data about the actual numbers of animals in a given area, there are several important principles which have been discovered about numbers of animals. We shall conclude this article by giving an account of them. We have already noted that the number of individuals of a species is often extremely great in any given area—for in stance, one acre of arable soil has been estimated to contain over two million springtails and about 800,000 earthworms. Now coupled with this is the fact that any species of animal is, in nine cases out of ten, endowed with powers of multiplication which are enormously greater than it can actually realize in practice, at any rate during normal times. If a species over-increases, it runs a definite danger of over-eating its food supply, but at the same time, if it does not produce a very large number of extra individuals in each generation it will be in danger of extinction through the operation of checks other than starvation, of which the most important are enemies. The position of nearly all animals is therefore this; they are always tending to increase enormously, but are prevented from doing so by various checks—mainly by carnivorous or parasitoid enemies ("parasitoids" meaning those Hymenoptera and other insects which have parasitic larvae but free-living adults). In this way the population is prevented from eating out its food supply; but the system also involves a danger ous tendency for the numbers to oscillate a great deal about their average density. If any species allows its numbers to fall below a certain density it is in danger of being blotted out by any un usually severe check such as occasional droughts or bad winters, or an unusually large number of enemies. We find, therefore, that each species tends to approach an optimum density for its population ; both below this optimum and above it the conditions become disadvantageous to the species. The limits of this optimum may be rather wide, thus differing to some extent from the rather precise optimum density which is found in a human population. Nevertheless the optimum exists for each species, and unless the latter is able to maintain its numbers somewhere near this, it is liable to become locally extinct. In fact, it is very probable that the limits to the range of any animal are often fixed by factors acting on its numbers, rather than directly upon the animal itself.
We have next to inquire whether animals do succeed in keeping their numbers at the density which suits them best. Anyone who has kept a watch on the animals in his own district continuously for a stretch of years must have noticed that there is a great variation in the numbers of most animals from year to year; often it is impossible to predict whether a species (a butterfly or a wasp or a slug or a bird) will be abundant or not. Notes of this sort are continually being made in a scattered way, such as that "this year we had very few long-tailed tits, but jays were unusually abundant" or that "this was a good year for Vanessidae, but not for other butterflies." A survey of a portion of the vast sea of literature bearing on this question leaves no doubt that the num bers of most species vary considerably from year to year, but that most of them do not usually become either extraordinarily numerous or extremely scarce. It is clear, then, that the con ception of an optimum density as applied to animals, like that of niches in animal communities, must be an elastic one, and that in practice the desirable density is not a point, but ranges over a fairly good distance either way. Even then, it is not always realized or even approached. One of the reasons for this is that what we call the optimum is itself varying, since the food supply varies, and the enemies and the climate and other factors also vary. The impression of anyone who has studied animal numbers in the field is that the "balance of nature" hardly exists, except in the minds of scientists ; it seems that animal numbers are always tending towards some ideal stability and communities are always tending to settle down into a smooth and harmonious working mechanism, but that something always happens before this happy state is reached. This liability to upsets of the "balance of nature" is as much a property of animal communities as is that of a boiling pot of water to overflow if left unattended. It is not merely a phenomenon characteristic of regions or habitats which have been interfered with by man. Even in places completely untouched by civilization, e.g., in the barren grounds of arctic Canada, or in a tropical forest, the occasional unbalance of animal communities is a normal thing, and the numbers of animals are found to vary to a greater or less extent—witness the periodic flights of sand grouse from the Gobi, or of painted lady butterflies from the Sahara, plagues of mice and lemmings in the barren grounds, or of aphids in the northern coniferous forests of Europe. Perhaps the most striking account of this kind of thing is that given by Cabot in the appendix to his book on Labrador, where he de scribes the extraordinary cycle of events accompanying a year of mouse abundance in the interior of that country; of ter reading that account no one can remain in doubt about the natural variability of animal communities in places almost untouched by man except as a rare native) animal. Are these fluctuations in numbers, these failures of the regulating mechanism of animal increase, caused by internal changes, after the manner of an alarm clock which suddenly goes off, or the boilers of an engine blowing up, or are they caused by some factors in the outer en vironment—weather, vegetation, or something like that? It appears that they are due to both, but that the latter is the more important of the two, and usually plays the leading part.
The environment of animals is so exceedingly unstable itself that it would be surprising if some of its irregularities were not reflected in animal communities, which form such delicate indices of any changes in the outer world. We cannot describe the vari ous rhythms which are going on in the weather, in the tides, or the various accidental circumstances which play a part in causing upsets in nature. But it may be worth while to examine a little more closely what actually happens when a plague of animals occurs, since this is one of the most important practical ecological problems met with in the field. The trouble usually starts with an excessive increase of some herbivorous animal, which is favoured in a particular year by unusual conditions of food or climate or some other factor. Now, the carnivore, which is nearly always larger than its prey, is in normal times able to destroy all the extra herbivores which are produced ; that is the ordinary balance of numbers in a community. But we have to remember all the time that the smaller animal is increasing at a faster rate than the large carnivore, so that if the herbivore once gets out of control of its enemies, the latter are never able to catch up by increasing themselves, since their powers of increase are relatively so much less. For instance, if there is a plague of field mice in progress we should find that while the mice were having about four or five litters in a year, the foxes which prey on them could not have more than one or two, and the young foxes would not be in a fit condition to breed until the following spring, whereas the mice, which had been born in the previous spring would already be producing young by the autumn, and adding to the total num bers. So, if unusually favourable conditions have enabled the herbivore (mouse, sandgrouse, aphid, crossbill, or oakmoth) once to escape from the control of its carnivorous enemies, increase goes on at a rapidly faster and faster rate, and the conditions of a "plague" are reached. In the summer of 1924 enough aphids were blown from the spruce forests of northern Europe, over Boo miles of sea, to cover in a broad belt the whole ice-cap of North East Land, an arctic island about as big as Wales. With them were found the syrphid flies which are among their most important enemies, but these were very much less abundant. This gives some idea of the scale attained by a really big plague of animals. It is obvious that, as enemies are no longer a serious check to numbers, starvation will follow, unless something else happens to avert this disaster.
Examination of the cases of plagues about which we know any thing, shows that the situation is met by animals in several differ ent ways, some animals doing one thing, and some another. In mammals and game birds, and also in certain birds and insects, the increasing density of population favours the spread of para sites—bacteria (as in the case of the marmot plague described earlier), or Protozoa (as in the willow grouse), or tapeworms (as in certain fresh-water fish). When the numbers of the host reach a certain point, the rate of circulation of the parasites increases and speeds up very suddenly, and the in festation of the host animal becomes serious, and usually leads to death. Just as the herbivore can increase faster than its enemies, so can the parasite, owing to its smaller size, increase faster than its host. The parasite in this case acts as a sort of emergency check on numbers, and we see therefore that it is absurd to consider the enemies and parasites of an animal as hostile, for the whole association stands or falls together as one unit. If the mouse is wiped out, the foxes and weasels, the tape worms and the protozoa and bacteria will be wiped out also. It is difficult to imagine exactly how natural selection acts on such a unit ; if the argument be carried to its logical conclusion, it would seem that natural selection either acted on everything from bacteria and mice to thunderstorms and sunspots or upon nothing. This case of mammal fluctuations in numbers illustrates very well how ecology throws light on, or at any rate raises problems in connection with, evolution and adaptation. The increase in num bers resulting from a breakaway from control by enemies is sometimes partly relieved by migration, but this does not neces sarily do any more than transfer the problem elsewhere. How ever, migration usually implies concentration of the animals into swarms or flocks, and so favours the generation of epidemics, and also makes it more likely that enemies will kill more of them. During "lemming-years" in Norway, the migrating animals are eaten by all manner of unusual enemies including reindeer, trout in lakes, and cod out at sea. Migration may also lead to other kinds of destruction, as in the aphids which were blown on to the ice-cap of North East Land but perished in a blizzard three days later. Migration and epidemics seem to be the usual check when enemies fail ; where these in turn cannot control the num bers, starvation occurs, but so numerous are the ways in which animal communities react to variations in their environment that starvation seems to occur curiously seldom, although plagues of animals on a greater or lesser scale are quite common in all com munities.
It is impossible to treat the various problems of animal num bers in any detail here, since apart from the general ideas outlined above, our knowledge is mostly unorganized and empirical, and the literature on the subject immense. It may be noted however, that the question of numbers has an intimate connection with migration and with the spread of species, since at the limits of their range animals are always fluctuating in numbers in a very marked way, and problems of distribution often turn out to be really problems in numbers. Dispersal is another subject which cannot be treated fully here, and we shall merely point out that the psychology of animals comes in and assumes great importance in connection with dispersal, and that the manner in which ani mals actually find the habitat suited to their needs is a subject which has not been adequately studied. It would seem that this is the most natural field for people working upon the instincts and tropisms of animals.