Over the last few years the controversy about competition as a major determinant of community structure has made a variable contribution to the ecological literature. Empirical work in natural communities which provides unambiguous data to support either of the arguments is uncommon. The experiments, field and lab, which are proposed will demonstrate the existence or absence of competition within a guild of parasites. Results will show if it is possible for several species to occupy the "same niche" at the same time. In addition, sufficient data will be gathered on host parasite interactions to determine if the relationship is parasitic or commensal. This proposal will test the hypothesis that: (1) the Wyoming ground squirrel (Spermophilus elegans elegans)-Coccidia (Eimeria) association is the product of a long coevolutionary history and that the "parasites" are really commensals with animal effects on host fitness, (2) the parasites elicit no immune response in the host, and (3) there is no niche partitioning among parasite species with the host. It is proposed that these parasites have evolved to use ephermeral resources, the quickly regenerating cells of the intestinal epithelium, and they are the epitome of fugitive species. The proposal will test the lack of community-level controls on "fugitive" species using a model system of ground squirrels and their parasites. This is the first long-term experimental study on host-parasite interactions of this duration or with such a large sample size in a natural population.
