Population dynamics and species interactions

Abstract

Agricultural monocultures are often thought to be more prone to herbivore outbreaks than natural systems, and early agroecologists posited that the lack of biodiversity in agricultural systems contributes to their instability (Pimentel, 1961; van Emden & Williams, 1974). In contrast, some detailed reviews have concluded that perhaps one or two particularly effective natural enemies are all that is needed for effective pest control (Hawkins et al ., 1999). Such issues come to the fore when a decision must be made in classical biological control about whether to introduce one or several natural enemy species in an effort to control exotic pests (Myers et al ., 1989; Denoth et al ., 2002), and when designing schemes to conserve indigenous natural enemies by modifying cultural practices (Landis et al ., 2000; Tscharntke et al ., 2005). Here, we first review the major classes of natural enemies – specialists and generalists – and the traits of each that are likely to contribute to (or detract from) their effectiveness as biological control agents. We then discuss interactions within diverse communities of natural enemies that are likely to affect biological control. Specialist natural enemies: the best biological control agents? Biological control practitioners have long debated the question: what are the traits of an effective biological control agent? General consensus seems to focus around a few traits that a successful agent will possess (see Chapter 9).