Spatial Density Dependence in Parasitoids

Abstract

Ecologists have long been concerned with determining whether populations are stable and, if so, what mechanisms are responsible. Stability in mathema­ tical models implies a tendency to return to equilibrium after a perturbation, and one approach has been to search in real systems for mechanisms shown to be stabilizing in models. One such mechanism that has received a great deal of attention in recent years is aggregation by parasitoids to patches with relatively many hosts. Two somewhat independent schools of thought have converged on the conclusion that direct spatial density dependence (a positive correlation be­ tween parasitism rate and host density in a patch) should be a common feature of host-parasitoid interactions in nature. It has commonly been assumed that many host-parasitoid interactions (and particularly those in biological con­ trol situations) are stable (18, 39, 53, 54; but see 72), and spatially density­ dependent parasitoid attacks have been shown to be stabilizing in some models (44). Furthermore, optimal foraging theory suggests that parasitoids can maximize their oviposition rate by aggregating in high-density patches of hosts (10, 16), and a number of parasitoids have responded in this way in the laboratory (e.g. 52). Despite these expectations, many studies have failed to detect direct density dependence (57, 68). Indeed, limits to either the number of mature eggs or the time available for searching, because of time taken to handle hosts, can result in underexploitation of high density patches, i.e. inverse density depen­ dence. Not only may spatial patterns in nature be influenced by these conflicting