A Theoretical Framework for Intraguild Predation

Many important issues in community ecology revolve around the interplay of competition and predation. Species that compete may also be locked in predator-prey interactions, a mixture of competition and predation known as "intraguild predation" (IGP). There is growing evidence for the importance of IGP in many natural communities, yet little formal ecological theory addresses this particular blend of interactions. In this article, we explore the consequences of incorporating IGP into standard models of exploitative competition and food chains (a general resource-consumer model, a Lotka-Volterra food chain model, and Schoener's exploitative competition model). Our theoretical analyses suggest a general criterion for coexistence in IGP systems: the intermediate species (the prey in intraguild predation) should be superior at exploitative competition for the shared resource, whereas the top species (the predator) should gain significantly from its consumption of the intermediate species. Along gradients in environmental productivity, coexistence is most likely at intermediate levels of productivity. Analyses of the models reveal the potential for alternative stable states in systems with IGP; these are particularly likely if the top predator gains little benefit from consuming the intermediate predator. We further show that IGP can lead to unstable population dynamics, even when all pairwise interactions are inherently stable and each species can increase when rare. Persistent, strong IGP raises a puzzle of species coexistence, particularly in productive environments. We conclude by comparing IGP with related community modules (i.e., food chains, exploitative competition, apparent competition) and discussing mechanisms that should foster coexistence in systems with strong IGP.