Density-Dependent Processes and Key Factors in Insect Populations

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Abstract
(1) I examined the frequency of different population processes acting either as key factors or as density-dependent sources of regulation on insect populations in the field, as reported in the literature. That is, processes were scored as to whether they were most important in perturbing populations away from equilibrium levels (as key factors) or in acting as negative-feedback mechanisms to return populations to equilibria. (2) Factors affecting population density in this study covered a wide range, from parasitism and predation to starvation, reduced fecundity, and dispersal. For fifty-eight species of insects, no single process could be regarded as a key factor of overriding importance; different sources of mortality were important in different systems. The same was true for regulatory or density-dependent mechanisms. (3) A striking feature was that, in about half of the studies, there were no obvious density-dependent factors operating. This finding lends weight to the idea that many population densities may commonly fluctuate between an upper ceiling and a lower floor rather than be maintained around a more fixed equilibrium. (4) In cases where density dependence was detected, it was more likely to be due to a source of mortality acting from the trophic level below (i.e. the plant) than from the trophic level above (predators and parasites). (5) Any given type of mortality or population process was more likely to perturb densities away from a mean level than to be density-dependent and act to return populations to equilibrium.

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