DENSITY DEPENDENCE OF UNGULATES AND FUNCTIONAL RESPONSES OF WOLVES: EFFECTS ON PREDATOR-PREY RATIOS
Predator-prey ratios have been used widely to interpret effects of wolf (Canis lupis) predation on their ungulate prey. Use of those simple indices may be misleading unless they are interpreted with regard to species composition of available prey, rate of growth of the prey population, proximity of the prey to carrying capacity (K), and the functional response in the rate of predation by wolves to changing density of prey. We incorporate theoretical models for predator-prey dynamics with data from field studies on ungulated and wolves to test for differences between species of ungulates in their ability to support wolves per unit of biomass. We conduct sensitivity analyses of those models, which integrate density-dependent growth of ungulate populations and functional responses of wolves, to evaluate the relative importance of those factors to the relation between ungulate and wolf densities. Finally, we develop a stochastic model that predicts ungulate-wolf populations at equilibrium, which indicates why interpreting those ratios in wild, may be problematic. Our models and analysis indicate that some ungulates, such as white-tailed deer (Odocoileus virginianus), likely will support a higher density of wolves than will other species with lower intrinsic rates of increase. Thus, we question the use of comparisons of total ungulate biomass and wolf density unless species composition of prey is considered. We hypothesize that growth rates of prey populations, their response to density dependence, and their proximity to K have important influences on the relation between ungulate and wolf density, whereas the functional response exhibited by wolves is relatively unimportant, especially at high density of prey.
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