Aggregation of Risk: Relationships Among Host-Parasitoid Models

Abstract

We explore the relationships among host-parasitoid models involving aggregation, with particular emphasis on May's negative binomial model. Models in which parasitoid density in a patch is strictly a function of host density in the patch, with no error about this function, are pure-regression models. Those in which there is random variation in the number of parasitoids per patch, with no relationship between local parasitoid density and local host density, are pure-error models. The key factor in these models is not the distribution of parasitoids per se, but the distribution of the relative risk of parasitism, p, which in the present formulation can result from variation in the number of parasitoids in a patch, Pj, or from variation in host vulnerability, a. We show that May's model, and the Bailey et al. (1962) model of which it is a special case, arises naturally from pure-error models. By contrast, it is very difficult to obtain May's model from biologically plausible pure-regression models. There a...