Predation and Resource Partitioning in One Predator -- Two Prey Model Communities

Models of the dynamics of a predator and two competing prey are examined for conditions under which the predator's presence makes competitive coexistence possible. Despite the fact that the prey in the absence of the predator exploit identical resources, predation-induced coexistence can occur; but its occurrence depends on the prey being different in at least one of several ways. Suitable differences include differences in spatial refuges from the predator, differences in appearance and/or location which induce frequency-dependent predation, and a difference in energy allocation between competitive and predatory defense. Thus these models predict, in keeping with the results of field studies on predators, that predation can play a central role in determining community species composition. However, prey species coexistence still depends on their being somehow different; i.e., Gause's Law is just as true when predators are common and important as when they are absent. Most of those prey in nature whose coexistence is known to depend on predation differ in resource use; i.e., these prey appear to partition environmental resources just as is expected of coexisting species in predator-free systems. A large proportion of cases of coexistence of similar species in nature probably results not from resource partitioning alone or from predation alone but from both mechanisms operating simultaneously.