GENERALIST AND SPECIALIST PREDATORS THAT MEDIATE PERMANENCE IN ECOLOGICAL COMMUNITIES

General dynamic models of systems with two prey and one or two predato rs are considered. After rescaling the equations so that both prey hav e the same intrinsic rate of growth, it is shown that there exists a g eneralist predator that can mediate permanence if and only if there is a population density of a prey at which its per-capita growth rate is positive yet less than its competitor's invasion rate. In particular, this result implies that if the outcome of competition between the pr ey is independent of initial conditions, then there exists a generalis t predator that mediates permanence. On the other hand, if the outcome of competition is contingent upon initial conditions (i.e., the prey are bistable), then there may not exist a suitable generalist predator . For example, bistable prey modeled by the Ayala-Gilpin (theta-Logist ic) equations can be stabilized if and only if theta < 1 for one of th e prey. It is also shown that two specialist predators always can medi ate permanence between bistable prey by creating a repelling heterocli nic cycle consisting of fixed points and limit cycles.