Parasitoid-mediated effects: apparent competition and the persistence of host-parasitoid assemblages

Indirect effects such as apparent competition (in which two hosts that do n ot compete for resources interact via a shared natural enemy) are increasin gly being shown to be prevalent in the structure and function of ecological assemblages. Here, we review the empirical and theoretical evidence for th ese enemy-mediated effects in host-parasitoid assemblages. We first address questions about the design of experiments to test for apparent competition . Second, we consider factors likely to affect the coexistence of host spec ies that share a parasitoid and are involved in apparent competition. We sh ow that parasitoid aggregation, and the switching effect that this can gene rate when hosts occur in separate patches, not only promotes persistence bu t is also strongly stabilizing. The broader consequences of these effects a re discussed.