POSITIVE INDIRECT EFFECTS BETWEEN PREY SPECIES THAT SHARE PREDATORS

Mathematical models are used to explore the interaction between two pr ey species that share a common predator. The models assume that the pr edator experiences density dependence via some mechanism other than pr ey depletion. The models also assume that the predator's functional re sponse to each prey decreases as the density of the other prey species increases. This can occur either because of predator satiation or pre dator switching. The results suggest that positive indirect effects of one prey on the equilibrium density of others should occur frequently , especially when there is predator switching. Decreasing the mortalit y rate of one prey or adding a prey species may make it easier for add itional prey species to invade the system and coexist. This occurs bec ause the resulting decrease in the predator's functional response is g reater than its positive numerical response. In many cases, different magnitudes of perturbation to one prey species will have opposite effe cts on the population density of the other prey species.