IMPACT OF HELMINTH PARASITISM ON A SNOWSHOE HARE POPULATION IN CENTRAL WISCONSIN - A FIELD EXPERIMENT

We tested the hypothesis that helminth parasitism is demographically s ignificant to a noncyclic population of snowshoe hares (Lepus american us) near the species' geographic boundary in central Wisconsin (U.S.A. ). During November 1988 to December 1991, we injected 93 individuals ( greater than or equal to 760 g, aged greater than or equal to 2 months ) with anthelmintics: Ivermectin for nematode and Droncit for cestode infections. We injected 98 control hares with propylene glycol, the co mmon vehicle for both drugs. All treated and control hares were radio- collared with mortality-sensing transmitters and monitored daily to we ekly from the ground or air. Prevalence and intensity of lungworms (Pr otostrongylus boughtoni), intestinal worms (Nematodirus triangularis), and stomach worms (Obeliscoides cuniculi) were markedly reduced by Iv ermectin treatment. No other nematodes were found to be present. Treat ment with Droncit to remove intestinal cestodes was apparently unneces sary, as prevalence among necropsied untreated hares and controls was just 10%. We compared body-condition indices (mass changes, response t o trap stress, and bone-marrow fat), reproduction (pregnancy rate and litter size), home-range sizes, and time-specific survival rates of an thelmintic-treated Versus control hares. None of these demographic var iables differed significantly between treated and control cohorts, nor was there any evidence that parasitism increased the risk of death fr om predation, which was the proximate cause of 96% of all natural mort alities. We conclude that helminth parasitism played no detectable rol e in the dynamics of this Wisconsin snowshoe hare population.