Invertebrate Community Structure and Indirect Effects

Abstract

Ecological theory demonstrates how competition can structure communities of plants and animals, and its importance has been demonstrated experimentally, particularly in plant, bird and rocky-shore communities. But equally many communities are unlikely to be structured by competition, for example herbivorous insects which frequently feed on non-overlapping resources, or which are typically too rare to exhaust food supplies. Here, it may be pressures from higher rather than lower trophic levels that structure communities, and indeed the theory of apparent competition shows that the actions of natural enemies can in many ways be homologous to the effects of resource competition. There is, however, relatively little evidence for apparent competition in the field. I will review the role of apparent competition in insect herbivore communities and describe experiments on two systems. The first involves aphids and the parasitoids, pathogens and predators that attack them (in the UK), the second leaf-miners and their parasitoids (in tropical Central America). I will conclude that the evidence to date suggests apparent competition is widespread, and hence may be very significant in insect community structure.