Evolution of plant-mediated interactions among natural enemies

Introduction Indirect evolutionary effects occur when selection by one species on a mediating species influences selection in future generations of a third species. Evolutionary interactions can be pictured in an evolutionary interaction web (Fig. 14.1) just as indirect ecological interactions can be represented in an ecological interaction web. In common with ecological interaction webs these do not involve direct trophic interaction webs. A difference is that ecological interaction webs involve alterations in mediating species that influence other species in the current generation, while in evolutionary interaction webs selection to alter the mediating species affects other species in subsequent generations. The study of the evolution of indirect interactions is not new. For example, character displacement is an indirect interaction where by competition between species for limiting resources mediates the evolution of divergent traits among competitors (Brown and Wilson 1956, Schluter et al. 1985). The intensive study of tritrophic and multitrophic interactions has shown that interactions between species separated by at least one trophic level are widespread and important (Price et al. 1980, Barbosa and Letourneau 1988). An example of this kind of interaction evolutionary effect occurs when plant chemical cues attract parasitoids that attack herbivores feeding on the plant. Many other kinds of indirect evolutionary effects occur in other interactions, such as the mediation of the evolution of two species on the same trophic level by a species on another trophic level, remain relatively unexplored.