Intraguild predation in the presence of a shared natural enemy.

Intraguild predation (IGP) is a widespread phenomenon in nature, and yet the simplest theoretical models of IGP predict that coexistence of intraguild predator and prey is only possible under restrictive assumptions. Here I examine how a specialist or generalist natural enemy of these species affects their long-term persistence and abundance, as functions of the natural enemy's relative attack severity and fecundity on each species. Notably, I show that failure to include the effects of a higher trophic level in models of IGP can lead to incorrect predictions about the coexistence or exclusion of guild members. I then consider how an interaction between native species and a natural enemy is perturbed by the arrival of an invasive intraguild predator. I outline the conditions under which the native species and/or its natural enemy are threatened by the arrival of the intraguild predator, and also when the natural enemy is beneficial in preventing the initial invasion or eventual dominance of the invader. This work provides new insights on the influence of omnivory on food web stability, and also generates testable hypotheses for predicting the impact of a novel intraguild predator on the recipient community at multiple trophic levels.