PARASITISM AND THE DYNAMICS OF UNGULATE GRAZING SYSTEMS

This study examines the potential impact of directly transmitted macroparasites on the population dynamics of ungulate grazing systems. It combines simple, previously published mathematical models for plant-herbivore and host-parasite interactions in a model that captures the essential features of the plant-herbivore-parasite system. The most ecologically complex and interesting case, in which herbivore numbers are food limited and allowed to vary more or less naturally, is considered in detail. On one hand, this analysis confirms the potential role of parasites as functional "top predators" and in particular the potential of parasite-induced host mortality to increase the plant equilibrium of otherwise overgrazed systems. This result underlines the dangers of indiscriminate anthelmintic application in marginal grazing systems without a consideration of the possible ecological consequences of parasite control. By contrast, the analysis also underlines two unique features of parasitism in the context of grazing system dynamics. First, the effect of the coupling of gut parasite transmission to food consumption simplifies grazing system dynamics by linearizing the herbivore's functional response near the parasite equilibrium. Second, the destabilizing impact of parasite-induced reductions in host reproductive rate can throw the system into chaotic plant-herbivore-parasite cycles. The ecological implications of these results are discussed.