EVIDENCE FOR TWO ALTERNATE STABLE STATES IN AN UNGULATE GRAZING SYSTEM

Simple models describing plant–herbivore interactions predict complex dynamics that depend on both herbivore density and plant abundance. The predictions of such models depend critically on the functional response of herbivores to forage availability, but few field studies have examined these responses or tested the hypothesis that alternate stable states can exist in plant–herbivore systems. We examined interactions between white-tailed deer, Odocoileus virginianus, and a dominant forb species, Laportea canadensis, in the understory of deciduous forests by measuring the functional response of deer to this forb and by conducting exclosure experiments under different deer and forb densities. Deer consumption of Laportea, measured at the scale of a forest stand, showed a Holling Type II functional response where the proportion of stems consumed has a steeply declining monotonic relationship to stem abundance. At high deer density, the deer–forb interaction, as measured by exclosure experiments, produced two alternate stable states that depended on initial forb abundance. Exclosure experiments also identified a lower deer density at which herbivory has no detectable effect on Laportea populations. This potential for a nonmonotonic relationship between plants and herbivores has direct implications for the conservation and restoration of plant species in systems where herbivores can be managed.