STABLE PARASITE GUILDS - COCCIDIA IN SPERMOPHILINE RODENTS

A guild of six species of intestinal protozoan (Apicomplexa: Eimeriida e) was monitored in 496 live-trapped Wyoming ground squirrels (Spermop hilus elegans) in two populations in 1989 and 1990. Three species were present at high levels (>40% of all squirrels infected at any time) a nd three at low levels (<20% infected) in both populations within and across years. Results of logistic regression indicated that age of ind ividual was a key parameter in predicting infection within each of the species. For the common species, adults had a higher probability of i nfection than juveniles and the observed number of reinfections in rec aptured squirrels was slightly greater than predicted. For the three r are species, juveniles had a higher probability of infection than adul ts and there was a slight decrease in probability of reinfection. Over the two years, time of sampling had no effect on probability of infec tion. There was no evidence for negative interactions (competition) be tween guild members but several positive interactions were noted. This eimerian guild appears stable within these populations and we hypothe size that the three prevalent species are commensals (little or no eff ect on host fitness) and the three low prevalence species have a great er effect on host fitness (''parasite''). Further, some species of thi s assemblage also infect two other species of sciurids sympatric with the Wyoming ground squirrel. The processes important in structuring th is assemblage may include the differential immune response of the host animals to the different eimerian species, the non-limiting resource base (the intestinal epithelia), the high reproductive capacities of t he eimerian species, the varying ability of infective oocysts for each species to persist under ambient environmental conditions, the high c olonization rates of host populations, and the ability of the symbiont s to infect sympatric host species.