Theoretical dynamics of competitors under predation.

Continuous population models of two prey species and a predator were explored by isocline analysis. When predator satiation and substitution between prey (with or without switching) are introduced in the models, many qualitatively different kinds of dynamic behaviour become possible. These depend in a complex but predictable way on competitive relations between prey and on predator feeding behaviour and efficiency. Under constant predation many cases of threshold responses between two or more alternate stable states are possibly; the numerical response of the predator population reduces some of the possibilities.Apparently contradictory community phenomena previously proposed, e.g. prey coexistence versus exclusion by addition of predator, exclusion versus stabilization by addition of alternate prey, are all possible as special cases. A prey which is relatively tolerant to predation can act as a keystone species, on which the existence of other prey species in the community depends, in either a positive or a negative sense. In certain conditions predator-induced obligatory mutualism between two prey species is theoretically possible.