Studies on the Organization of Animal Communities

Animal communities may be considered organized if any property of a natural assemblage of species can be predicted. Inasmuch as some prediction is always possible, the occurrence of organization will scarcely be debated under this definition, but the degree to which it exists and the relationships involved are more open to question. It will be agreed, however, that the degree of organization will determine the accuracy and generality of the predictions that are made. The relationships that force order upon natural communities may be placed into two broad categories: energy transfer and competition. In its general form, the transfer of energy through communities is reasonably well understood, although much remains to be learned. This organizing force will not be considered directly in the present paper. Competition is less obvious, and its importance and even its existence have been the subject of intense debate (Nicholson 1933; Andrewartha & Birch 1954; Birch 1957; various papers and discussions in Population Studies: Animal Ecology and Demography, Cold Spring Harbor Symposium on Quantitative Biology, Vol. 22, 1957). There are a few generally accepted observations which appear to lead to a strong conclusion that competition for resources is widespread in natural communities (Hairston, Smith & Slobodkin 1960). These authors use three principal observations: bound energy is fossilized at an ecologically insignificant rate; in terrestrial communities, most of the organic matter produced by green plants falls to the ground uneaten; and when herbivores are protected either by the removal of predators or by being introduced into regions which their predators and parasites have not reached, they frequently become numerous enough to deplete the vegetation. From these observations, it is deduced that as whole groups, the terrestrial decomposers, producers and predators are limited by their own depletion of their respective resources. Terrestrial herbivores appear to be limited in abundance by predation and parasitism. Now, it can be regarded as an axiom that when the populations of two or more species are limited by the supply of a common resource, those species are in competition. Inasmuch as the numerous species of decomposers are not completely segregated in their needs, it follows that competition must exist among these species. The same reasoning applies to terrestrial plants and to predators. Hairston, Smith & Slobodkin repeatedly point out that individual exceptions for particular species by no means invalidate their conclusions. Thus, many carnivore species are at least partly regulated by territoriality, but predators and parasites must in general control the populations of herbivores, and in general be limited by the supply of these. Otherwise, the herbivore populations would expand to the point where the vegetation is depleted. It is concluded that competition for resources exists among decomposers, among producers, and among carnivores in terrestrial communities. Therefore, it seems worth while to examine those approaches to the organization of