Aggregation and the population dynamics of parasitoids and predators.

Aggregation by parasitoids and predators has long been considered an important factor helping to stabilize host-parasitoid and predator-prey interactions. This consensus has recently been challenged by W. W. Murdoch and A. Stewart-Oaten, who conclude, from an analysis of a model formulated in a novel way, that aggregation independent of host density has no effect on stability, whereas aggregation to host or prey patches of high density is normally destabilizing. It is argued here that part of the discrepancy in conclusions results from the use of different concepts of aggregation: behavioral versus statistical. It is also argued that the conclusions of the Murdoch and Stewart-Oaten model should be treated with caution, because unrealistic assumptions have to be made to derive the model, and it is not clear that the conclusions are robust to the relaxation of these assumptions.