The Natural Control of Animal Populations

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Abstract
The elements of natural control are outlined as follows: The numerical variation of populations is often considerable and yet is kept within certain limits. A population and the items of its environment form a closely interconnected complex or ecosystem. The relationships of a population are with the whole ecosystem rather than with the environment only. The distinction between density-dependent and density-independent action by no means corresponds to the division between biotic and physical factors. Density-dependent action is defined as that which intensifies (per individual) as population density increases, and relaxes as density falls; it is the chief agent of control. Some processes are inversely related to density. Control of a population is a result of the limited capacity of the ecosystem with respect to that sp. or with respect to its enemies or both. The limiting influence begins to operate at densities far below the capacity limit, intensifying as this limit is approached; this is the basis of most kinds of density dependence (the other kinds are modifications of environmental capacity, the degree of modification naturally depending on density). An enemy is density-dependent in its action on a host (or prey) population if it attacks a greater proportion as the host density increases. The enemy will do this if it is capable, and if the supply of the host is a limiting factor in the enemy''s environment. The presence of competitor spp., and sometimes that of alternative hosts or prey for the enemies of a population, tends to intensify the action of the control factors involved. Four phases of control are distinguished: limitation, which sets a variable upper limit; conservation, which tends to prevent extreme reduction; suppression, or a forced decline from high density; and release, a temporary escape from normal control after a severe reduction. Each is brought about by characteristic density relationships. Suppression and release promote fluctuations. The level of abundance depends ultimately on all the major elements of the ecosystem, although the density-related, controlling factors are the immediate determinants. The possibility that either physical or biotic factors may be primarily responsible for control can be reached only about particular types of situations. The term "balance" or "equilibrium" is generally inappropriate to any except nearly constant populations, and this applies equally to "dynamic equilibrium." To prevent confusion, the above terms should not be used in biology without full explanation of the meaning intended. The laboratory demonstration of the "classical" oscillations predicted by the mathematical theories has so far proved very difficult. Instead, "relaxation oscillations," in which the predator population kills all the prey and then dies out, have tended to appear. When "refuges" are available, as in nature, remnants of each population normally survive such crashes, and may increase again. In very regular environments, a succession of these cycles might be maintained; in more general terms, a repeated cycle of suppression and release might occur. There is evidence that animal populations fluctuate less violently in very complex ecosystems, for example, in tropical forests, than in woods poorer in plant and animal spp., and that the most violent fluctuations tend to occur among the animals in plantations of one sp., or in barren places like the far north. Also, irregularities in the physical environment (but not in climate), by increasing the complexity of the ecosystem, probably reduce the numerical variability of populations.