Document: PET-3-92-17

Effects of mortality on competitive coexistence and coevolution.

ABRAMS, P.A.*

University of Toronto, Toronto, ON M5S 3G5 Canada ¹

Abstract:
Past ecological theory suggests that mortality imposed uniformly upon consumptive competitors will usually hinder coexistence and have little effect on character displacement. These conclusions are in part a consequence of some simplifying assumptions in previous models. Differential equation models of two consumers competing for two or more resources in the presence and absence of a predator species were analyzed to determine the range of relative efficiencies of the consumer species that will allow coexistence. These models suggest that mortality imposed by predators can widen the range of relative efficiencies that allow consumer coexistence. Conditions required for this result are that the competing consumers utilize self-reproducing resources and that they do so very efficiently. In this case, the mortality imposed by a predator often prevents the exclusion of the consumers' resources that would otherwise occur via apparent competition. The resulting broader resource base widens the range of conditions allowing consumer species to coexist. Similar models of two consumers and two resources with evolutionarily flexible consumption rates were also analyzed to determine the effects of predation or mortality on character displacement of the consumer species. The results show that predation is likely to reduce selection for divergent resource use patterns, resulting in higher competition coefficients between the coexisting species. Thus, the ecological and evolutionary effects of predators will often have opposite effects on the ease of coexistence of their prey.

Keywords: competition, predation, coexistence, exclusion

This abstract is being presented at: 11:45 AM in session:
Oral Session #39: Theoretical Ecology.