Impact of intraguild predation and stage structure on simple communities along a productivity gradient

We analyze the consequences of intraguild predation and stage structure for the possible composition of a three-species community consisting of resour ce, consumer, and predator. Intraguild predation, a special case of omnivor y, induces two major differences with traditional linear food chain models: the potential for the occurrence of two alternative stable equilibria at i ntermediate levels of resource productivity and the extinction of the consu mer at high productivities. At low productivities, the consumer dominates, while at intermediate productivities, the predator and the consumer can coe xist. The qualitative behavior of the model is robust against addition of a n invulnerable size class for the consumer population and against addition of an initial, nonpredatory stage for the predator population, which means that the addition of stage structure does not change the pattern. Unless th e top predator is substantially less efficient on the bottom resource, it t ends to drive the intermediate species extinct over a surprisingly large ra nge of productivities, thus making coexistence generally impossible. These theoretical results indicate that the conditions for stable food chains inv olving intraguild predation cannot involve strong competition for the botto mmost resource.