Heterogeneity and the dynamics of host-parasitoid interactions

The dynamical effects of any pattern of distribution of searching parasitoids in such systems can be assessed within a common, simple framework. The authors describe an approximate general rule that the populations of hosts and parasitoids will be regulated if the coefficient of variation squared (CV²) of the distribution of searching parasitoids is >1. CV₂ may be partitioned into a density-dependent component (direct or inverse) caused by the response of parasitoids to host density per patch, and a density independent component. Population regulation can be enhanced as much by density independent as by density-dependent heterogeneity. Thus the dynamical effects of any pattern of distribution of searching parasitoids can be assessed within the same common framework. The greater the density independence, and thus the more scattered the data of percent parasitism against local host density, the more stable the populations are likely to be. Some 65 data sets from field studies have been analysed, in which percentage parasitism in relation to local host density have been recorded. Values of CV² were obtained by using a maximum likelihood procedure. In 18 cases, total heterogeneity was at levels sufficient to stabilize the interacting populations; in 14 of these it is the host-density-independent heterogeneity that contributes most to the total heterogeneity. There is a discussion by G.A. Tingley. -Authors