Predation and Competition: The Interaction of Two Types of Species Interactions

Abstract

A perennial endeavor in community ecology is to unravel the relative importance of competition and predation in determining the structure of natural communities. The comparison stems in part from the suggestion that predation affects communities largely through its effects on densities of competing prey species (e.g., Jeffries and Lawton 1984) and that competition is only when predation is weak (e.g., Connell 1975). We suggest that this comparison is misdirected. As we discuss here, predation and competition together can affect prey communities in a multitude of ways that often interact; appreciating the manifold complexities and consequences of this interaction of interactions may prove to be more fruitful than searching for which factor is most important in structuring ecological communities. In this essay, we propose a simple classification of the effects of predation upon the outcome of competition. The basic idea is to distinguish between those effects that are mediated directly or indirectly through changes in prey population densities brought about by predatorinflicted mortality (trophic link indirect effect, sensu Miller and Kerfoot 1987), and those mediated through changes in prey behavior which affect the intensity of competition among the prey species and are brought about in response to changes in the risk of predation (behavioral indirect effect, sensu Miller and Kerfoot 1987). We briefly discuss some of the diverse outcomes to be expected in systems with a mixture of predation and competition, and for illustration, relate theoretical models to a few examples from natural communities. We present the following models as a convenient way for clearly catagorizing the interaction of predation and competition. We present no analysis here, since much of that has already been done elsewhere. One can represent the interaction between a pair of competing species with the following pair of differential equations: