Density-dependent dispersal in host-parasitoid assemblages

Most spatial population models assume constant rates of dispersal. However, in a given community, dispersal may not only depend on the density of cons pecifics. i.e. density-dependent dispersal, but also on the density of othe r species, a phenomenon we term 'community-dependent dispersal'. We co-vary the densities of both the beetle host Callosobruchus chinensis and its par asitoid wasp. Anisopteromalus calandrae, in a laboratory study and record t he proportions of each species that disperse within a two-hour period. The parasitoid in these systems exhibits community-dependent dispersal - disper sing more frequently when parasitoid density is high and larval host densit y is low. This supported our prediction that individuals should disperse ac cording to competition for available resources. However, in this study the host's dispersal was independent of density. We suggest that this may be du e to less intense selection acting on host dispersal strategies than in the parasitoid. We consider some possible consequences of community-dependent dispersal for a number of spatial population processes. A well-known host-p arasitoid metapopulation model is expanded so that it includes a greater ra nge of dispersal functions. When the model is parameterised with the parasi toid community-dependent dispersal function observed in the empirical study , similar population dynamics are obtained as when fixed-rate dispersal fun ctions are applied, The importance of dispersal functions for invasions of both competitive and host-parasitoid systems is also considered. The model results demonstrate that understanding how individuals disperse in response to different species' population densities is important in determining the rate of spread of an invasion. We suggest that more empirical studies are needed to establish what determines dispersal rate and distance in a range of species, combined with theoretical studies investigating the role of the dispersal function in determining spatial population processes.