Community Stability and the Strategy of Biological Control

Abstract

Community organization is defined as the mean number of trophic links connecting species of different trophic levels in a community. For the special purposes of this paper, competition is assumed to occur whenever two species are known to eat the same food species. Community stability is defined as the reciprocal of the mean, for all species, of the standard error of logarithms of annual collection sizes. It is thus a measure of stability over time of the species populations in a community. Several authors writing about the relationship between community organization and community stability have insisted that the stability of a complex ecological system increases with the number of avenues by which energy can flow through it. This theory does not seem consistent with the observation that some insect pest species of notorious instability are attacked by a great number of entomophagous species. In this paper we seek additional evidence bearing on the relationship between community organization and stability, using computer analysis of data collected by the Canadian Forest Insect Survey on forest Macrolepidoptera and their food plants. After considering our results, and those of other workers, we postulate the following hypothesis. Stability at any herbivore or carnivore trophic level increases with the number of competitor species at that level, decreases with the number of competitor species that feed upon it, and decreases with the proportion of the environment containing useful food. If this hypothesis is valid, too much competition in the entomophagous trophic level will not allow the species in that level to be instable enough to control an unstable pest. Therefore, the best type of biological control agent is one that has no direct competitor species. Also, the most unstable biological control agents, and hence those capable of controlling an unstable pest, will be polyphagous.