Can Sublethal Parasitism Destabilize Predator-Prey Population Dynamics? A Model of Snowshoe Hares, Predators and Parasites

Snowshoe hare (Lepus americanus) populations typically exhibit 8-11 year cycles in abundance over most of their geographic range. Results of a parasite-reduction study suggest that sublethal effects of nematode infections may increase the vulnerability of hares to predation. We explored the potential role of parasite-induced vulnerability to predation in promoting hare population cycles using models of hare, predator and parasite dynamics. Results of long-term studies of hare populations were used to estimate most model parameters. 2. To ensure the generality of our results, we analysed three different models of increasing complexity. The basic model gave a simple description of the dynamics of hares, predators and parasites. The second model added a Type II predator functional response and parasite density-dependent reproduction. The final model added a time delay in hare natality and age structure in the predator population. 3. In all three models, adding parasites to the hare-predator system made the population dynamics less stable. This demonstrates that parasite-induced vulnerability to predation acts in a destabilizing fashion and makes population cycles more likely. 4. To compare with a sublethal parasite that only acts through parasite-induced vulnerability to predation, we constructed a model with a hypothetical parasite that only affects hare natality. Although parasite-induced reductions in natality could in some cases destabilize the population dynamics, the effect was always weaker than the effect of parasite-induced vulnerability to predation. This highlights an important feature of parasite-induced vulnerability to predation. When the dynamics of both predator and parasite are coupled to those of the hare, roughly synchronous cycles occur in predator densities and parasite burdens. Because predators and parasites act multiplicatively on the predation rate, the synchronous cycles in predator densities and parasite burdens create a synergistic effect that destabilizes hare-predator-parasite population dynamics.