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The Natural Control of Animal Populations

M. E. Solomon
Journal of Animal Ecology
Vol. 18, No. 1 (May, 1949), pp. 1-35
DOI: 10.2307/1578
Stable URL: http://www.jstor.org/stable/1578
Page Count: 35
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The Natural Control of Animal Populations
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Abstract

A. This paper attempts to clarify the subject by giving a systematic account of the processes considered to be involved in the natural control of animal populations. B. First, a brief survey is made of the chief types of theories offering an explanation of natural control. The earlier theories looked to natural enemies as the main agents of control. Somewhat later theories gave predominance to physical factors. Control has been attributed exclusively to competition, including competition between natural enemies, or, more broadly, to density-dependent mortality. Among theories of periodic fluctuations, many ascribe control to some aspect of overpopulation of the environment, while mathematical theories envisage control due primarily to natural enemies. Certain `comprehensive' theories lay emphasis on the complexity and essential interconnectedness of the population-environment system, and hold that the particular causes of control vary with the circumstances. C. The elements of natural control are outlined as follows: C1. The numerical variation of populations is often considerable and yet is kept within certain limits. C2. 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 (which includes itself) rather than with the environment only. C3. The distinction between density-dependent and density-independent action is discussed, with examples; it 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 species 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 species, and sometimes that of alternative hosts or prey for the enemies of a population, tends to intensify the action of the control factors involved. C4. 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. C5. The level of abundance depends ultimately on all the major elements of the ecosystem, although the density-related, controlling factors are the immediate determinants. Only in regard to particular types of situations, or of animals, can more precise statements be made. D. Some related matters are also discussed: D1. The possibility that either physical or biotic factors may be primarily responsible for control in general is examined. Decisions can be reached only about particular types of situations. D2. Some of the ways in which the term `balance' or `equilibrium' has been applied to population relationships are checked against its physical meaning. It 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. D3. Although population variation is restricted, the view that density generally fluctuates about a mean value, and the related idea that deviations from this mean set up a tendency to return to it, are over-simplifications. The idea that density tends towards a varying level raises the practical difficulty of distinguishing the two sets of variations, but may be useful. D4. 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 (see C4) might occur. But this is speculative, and perhaps other causes are more important in producing periodic cycles in nature. D5. 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 species, and that the most violent fluctuations tend to occur among the animals in plantations of one species, 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.

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