A THEORETICAL FRAMEWORK FOR INTRAGUILD PREDATION

Many important issues in community ecology revolve around the interpla y of competition and predation. Species that compete may also be locke d in predator-prey interactions, a mixture of competition and predatio n known as ''intraguild predation'' (IGP). There is growing evidence f or the importance of IGP in many natural communities, yet little forma l ecological theory addresses this particular blend of interactions. I n 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 Schoe ner's exploitative competition model). Our theoretical analyses sugges t a general criterion for coexistence in IGP systems: the intermediate species (the prey in intraguild predation) should be superior at expl oitative competition for the shared resource, whereas the top species (the predator) should gain significantly from its consumption of the i ntermediate species. Along gradients in environmental productivity, co existence is most likely at intermediate levels of productivity. Analy ses of the models reveal the potential for alternative stable states i n systems with IGP; these are particularly likely if the top predator gains little benefit from consuming the intermediate predator. We furt her show that IGP can lead to unstable population dynamics, even when all pairwise interactions are inherently stable and each species can i ncrease when rare. Persistent, strong IGP raises a puzzle of species c oexistence, particularly in productive environments. We conclude by co mparing IGP with related community modules (i.e., food chains, exploit ative competition, apparent competition) and discussing mechanisms tha t should foster coexistence in systems with strong IGP.