Introduction

Natural enemies include parasites, pathogens, parasitoids, and predators (in the order of how we generally perceive their increasing impact on the survival of their individual victims). It has been increasingly recognized since the 1970s that the ecological dynamics of natural enemies and their victims can be diverse (Begon et al. 1996), and that understanding such dynamics has important implications for applied disciplines such as pest control (Chapter 32) and conservation biology (Dobson and McCallum 1997; Clarke et al. 1998; Hochberg 2000).

It is undeniably the case that natural enemies can be geographically widespread, yet most individuals spend their lives within the limited range of environments suitable for their species. Environmental differences over the geographical range of a natural enemy could, in turn, lead to spatial variation in population and adaptive dynamics. Large-scale environmental variation manifests itself in at least three ways.

First, all species have geographical boundaries, either abiotic barriers such as mountains or lakes, or biotic variables such as the abundance and quality of food, and the presence of competitors or predators (Brown et al. 1996; Holt et al. 1997). For many (but by no means all) species, geographical boundaries approximate those experienced by their resources. However, a more functional view of geographical boundaries of a species would include all of those habitats in which natural selection operates (Holt 1996). Such habitats could include vectors (for some parasites and pathogens), breeding grounds (for migrating predators), and nectar sources (for some species of parasitoid wasp).