Indirect Effects, Apparent Competition and Biological Control.

In biological control in its simplest form only direct interactions between the control agent and the pest and potential non-targets are considered. Ecologists are however amassing an ever increasing body of evidence for the importance of indirect effects in ecological communities. Indirect effects are the effects of one species on another mediated by at least one intermediate species. An example is so-called apparent competition which is the negative indirect effect that prey species have on each other when they share natural enemies. This effect is thought to play a particularly significant role in phytophagous insect communities where the scope for resource competition is limited. We show that there is experimental evidence for apparent competition amongst phytophagous insects. We describe a community of aphids and their parasitoids, predators and pathogens that we have been studying for over 10 years. We discuss how this species-rich community in a relatively natural environment may be structured by indirect effects. Returning to biological control we discuss how these ideas from community ecology can be applied to enhance pest control and to assess the ecological risks of the introduction of control agents. Introducing or encouraging species that share natural enemies with a target pest may lead to increased pest control through an apparent competition effect by boosting the natural enemy population. We conclude that although occasional attempts are made, such techniques are currently still much underutilised. Equally, we show how indirect effects may cause or increase the impact of introduced control agents on native flora and fauna but that these possible effects are rarely taken into account. [ABSTRACT FROM AUTHOR]