THE PERSISTENCE OF HOST-PARASITOID ASSOCIATIONS IN PATCHY ENVIRONMENTS .2. EVALUATION OF FIELD DATA

We introduce a method for estimating the parameters of simple host-parasitoid models from field data relating percent parasitism to local host density per patch. By using these parameters, the coefficient of variation (CV2) of the density of searching parasitoids can be calculated. This quantity is important because a general criterion for heterogeneity to regulate discrete-generation host-parasitoid models is that CV2 > 1 (Hassell et al. 1991). The method further allows one to partition CV2 into two components: a host-density-dependent component caused by nonzero covariance between the spatial distributions of the host and parasitoid (HDD), and a component that is independent of host density (HDI). By using this method, we have analyzed 34 sets of published data. Heterogeneity is sufficient to regulate the populations (CV2 > 1) in approximately one-third of the published cases. In approximately 90% of the cases in which CV2 > 1, the density-independent component of CV2 is considerably more important for population regulation than the direct or inverse density-dependent components.