MECHANISMS OF LOCAL PERSISTENCE IN COUPLED HOST-PARASITOID ASSOCIATIONS - THE CASE MODEL OF MACULINEA-REBELI AND ICHNEUMON-EUMERUS

We examine a spatially explicit 'case model' for the interaction betwe en the lycaenid butterfly, Maculinea rebeli, and its specialist parasi toid, Ichneumon eumerus. This butterfly lives in small, closed populat ions, rarely numbering over a few thousand individuals, and the parasi toid is found at only a small subset of butterfly-harbouring sites. We explore how parasitoid searching intensity and behaviour, and host re fuges from parasitism affect the dynamics of the host-parasitoid coupl e. In the absence of explicit host refuges, the parasitoid persists on ly for a very restricted range of search rates and searching behaviour s. Absolute refuges to parasitism, modelled as a cue-threshold phenome non in the elicitation of intensive search for the host, expand the pe rsistence conditions. We link these results to the more general proble m of what inferences can be drawn concerning the association between p opulation-level variation in the distribution of parasitism and the po pulation dynamics of the system. The parasitoid's persistence depends importantly on heterogeneity in the vulnerability of the host caterpil lars to encounter with parasitoids. Although the host's persistence is also enhanced by such heterogeneity, it is actually intraspecific com petition within ant nests that dominates host dynamics. Deductions of the stabilizing power of parasitoids from measures of spatial heteroge neity in parasitism will be spurious without information about the res pective density-dependent influences of the parasitoid and other limit ation factors affecting the host.