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Supplement: the Balance of Animal Populations

A. J. Nicholson
Journal of Animal Ecology
Vol. 2, No. 1 (May, 1933), pp. 131-178
DOI: 10.2307/954
Stable URL: http://www.jstor.org/stable/954
Page Count: 48
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Supplement: the Balance of Animal Populations
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Abstract

1. The whole of this communication is essentially the summary of a much larger work, so it has been impossible here to do more than give an outline of its main features. 2. The densities of animal populations are known to bear a relation to the environmental conditions to which they are subject, and the existence of this relation shows that populations must be in a state of balance with their environments. 3. For balance, it is essential that the action of a controlling factor should be governed by the density of the population controlled, and competition seems to be the only factor that can be governed in this way. 4. Examination of the competition to which animals are subject shows that it is generally competition between animals when seeking the things they require for existence, or competition between natural enemies that hunt for them. 5. Detailed investigation of the question of competition between searching animals shows that, whether the animals are controlled by competition for food, or for suitable places in which to live, or by the competition of their natural enemies, there is for each species a particular density, referred to as the ``steady density,'' at which balance exists. The value of this steady density depends upon the properties of the species, the properties of the species with which it interacts, the nature of this interaction, and the properties of the environment. 6. Competition always tends to cause animals to reach, and to maintain, their steady densities. Factors, such as climate and most kinds of animal behaviour, whose action is uninfluenced by the densities of animals, cannot themselves determine population densities, but they may have an important influence on the values at which competition maintains these densities. 7. When the densities of animals are controlled by natural enemies, this interaction itself produces oscillation, even in a constant environment. With very simple types of interaction, such as that of a specific entomophagous parasite and its host, the oscillation of density increased in amplitude with time. 8. According to the circumstances, the final result of such oscillation is either the perpetual maintenance of the oscillation with constant amplitude under constant conditions, or the population is broken into widely separated small groups of individuals, the positions of which continually change with time. In the first situation the densities of the animals fluctuate about their steady values, but in the second the average densities are permanently maintained much below their steady values. 9. With more complex types of interaction the oscillation decreases in amplitude with time, so that in a constant environment the densities are caused eventually to reach, and to remain at, their steady values. 10. However, the environment does not remain constant, and it is found that periodic environmental changes, such as those of the seasons, tend to impress their period upon interspecific oscillation. Consequently the oscillation that is produced by the interaction of animals should generally be found to correspond in time with seasonal and other periodic environmental changes, but its violence is greater than that of the oscillation which these environmental changes could themselves produce. 11. Also, environmental oscillation applies an external force which prevents interspecific oscillation of the decreasing type from dying out. Irregular environmental fluctuations tend to cause corresponding irregular fluctuations in the densities of animals, but, owing to the delayed effects produced by interspecific oscillation, further irregularities in the densities of animals are subsequently produced that have no evident relation to environmental conditions.

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