Interactive Effects of Sublethal Nematodes and Nutritional Status on Snowshoe Hare Vulnerability to Predation

Abstract

1. We studied the effect of parasitism on snowshoe hare (Lepus americanus) survival through a field experiment that reduced natural burdens of sublethal nematodes in a free-ranging hare population. We treated half the hares on each of six study areas year-round with an anthelminthic drug, and the other half with a placebo. Potential interactive effects of parasitism and nutrition were examined by supplementing the natural food supply on three of the six study areas during two winters. Survival was measured by radio-collaring a total of 612 hares with mortality-sensitive transmitters and monitoring survival daily between April 1991 and June 1993. 2. Overall, nematode burdens in hares were highest between March and October, and Obeliscoides cuniculi was the most abundant of the five species present. The proximate cause of 95% of mortalities (n = 318) during the study was predation. Predators killed hares with heavy burdens of O. cuniculi disporportionately during May-June, but burdens of the four other species (Nematodirus triangularis, Trichuris leporis, Dirofilaria scapiceps, Protostrongylus boughtoni) were similar between predator-killed hares and the live population. 3. During May-October 1991, the anthelminthic treatment did not affect hare survival, but during May-October 1992 survival of parasite-reduced hares was 2.4-times higher than that of controls. During November 1991-April 1992 survival of food supplemented hares was 21% higher than food-normal animals, but survival was similar during November 1992-April 1993. 4. The effect of parasite-reduction on hare survival was apparently contingent on overwinter food supplies between November 1991 and April 1992, with survival being highest in hares subjected to both treatments, intermediate in those receiving only food supplementation, and lowest in unfed hares. We conclude that the effect of sublethal nematode parasitism on hare survival can be important if interactive with predation, and that synergistic effects of parasitism and nutrition may affect hares if food availability is limited.