Evaluating the contribution of individual variation in parasite-mediated anorexia to trophic cascades

Recent evidence suggests that parasite-mediated reductions in food intake (i.e., anorexia) in herbivores can trigger trophic cascades that increase producer biomass. This outcome assumes homogeneous host responses to parasite infection; however, individual variation in parasite-mediated anorexia is common. To understand the potential consequences of such variation, we quantified individual variation in host feeding responses to parasitism empirically using a wild herbivore-helminth system. We then evaluated the impact of ecologically relevant levels of variation in anorexia on producers using stochastic individual-based models composed of parasites, herbivores, and plants. Our empirical data showed that although higher helminth burdens were associated with lower population-level feeding rates, there was considerable individual variation in the presence and magnitude of anorexia. Our models revealed a pronounced effect of variation in anorexia prevalence but not magnitude on plants. Plant biomass increased as anorexia became prevalent in the herbivore population, and there was a strong dampening effect of anorexia prevalence on plant biomass variance, suggesting that parasite-mediated anorexia in herbivores can stabilize producer population dynamics. Interestingly, the association between higher anorexia prevalence and lower variance in plant biomass was due, in part, to negative feedback between herbivore feeding rates and helminth ingestion, suggesting that negative feedback between host behavior and parasitism, a phenomenon that can help stabilize certain host-parasite interactions, may have stabilizing effects that extend to other members of the ecological community via trophic cascades.