We have investigated a stable assemblage of 6 species of Eimeria in the intestine of the Wyoming ground squirrel consisting of three abundant species (E. beecheyi, E. callospermophili and E. morainensis) and three rare species E. larimerensis, E. bilamellata and E. spermophili). To test the hypothesis that no interactions occur among these parasite species, five squirrels were inoculated with 12,500 oocysts consisting of 1 E. larimerensis, 10 E. bilamellata, 22 E. beecheyi and 67 % E. callospermophili. The proportion of each species in the output was quite different: 41 E. larimerensis, 5 E. beecheyi, 0 E. bilamellata and 54% E. callospermophili. When the same squirrels were reinoculated with 10,000 oocysts of 85 E. larimerensis, 7 E. beecheyi and 8% E. callospermophili, the output was 9 E. larimerensis, 13 E. beecheyi and 78% E. callospermophili. In the initial infections, the intrinsic rate of increase (r) for E. larimerensis was considerably higher than that of E. beecheyi or E. callospermophili. During the reinfections, (r) for E. larimerensis was significantly lower and for E. beecheyi and E. callospermophili significantly higher than initial values. Although E. larimerensis appears to have initially compromised the reproductive potential of its congenerics, acquired host immunity may have caused its reduced reproductive potential in the second trial.
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