Populations and Communities

In nature, any particular habitat contains animal and plant species which exist together in both time and space. Many of these species will interact with each other, for example when one species feeds on another or when two species compete for the same food or other resource. A group of species having a high degree of spatial and temporal concordance, and in which member species mutually interact to a greater or lesser extent, constitute a community (Askew and Shaw, 1986). The size and complexity of a community will depend upon how broadly that community is defined. For example, we could consider as a community the organisms which interact with each other within a particular area of woodland, the herbivore species which compete for a particular food plant or the complex of natural enemies associated with a particular prey or host species. Here, we are especially interested in communities of natural enemies, which are often surprisingly species-rich (Carroll and Risch, 1990; Hoffmeister and Vidal, 1994; Settle et al., 1996; Sunderland et al., 1997; Memmott et al., 2000).