Stable Parasite Guilds: Coccidia in Spermophiline Rodents

Abstract

A guild of six species of intestinal protozoan (Apicomplexa: Eimeriidae) was monitored in 496 live-trapped Wyoming ground squirrels (Spermophilus elegans) in two populations in 1989 and 1990. Three species were present at high levels (>40% of all squirrels infected at any time) and three at low levels (<20% infected) in both populations within and across years. Results of logistic regression indicated that age of individual was a key parameter in predicting infection within each of the species. For the common species, adults had a higher probability of infection than juveniles and the observed number of reinfections in recaptured squirrels was slightly greater than predicted. For the three rare species, juveniles had a higher probability of infection than adults and there was a slight decrease in probability of reinfection. Over the two years, time of sampling had no effect on probability of infection. There was no evidence for negative interactions (competition) between guild members but several positive interactions were noted. This eimerian guild appears stable within these populations and we hypothesize that the three prevalent species are commensals (little or no effect on host fitness) and the three low prevalence species have a greater effect on host fitness (parasite). Further, some species of this assemblage also infect two other species of sciurids sympatric with the Wyoming ground squirrel. The processes important in structuring this 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 the eimerian species, the varying ability of infective oocysts for each species to persist under ambient environmental conditions, the high colonization rates of host populations, and the ability of the symbionts to infect sympatric host species.