Applying predator-prey theory to evaluate large mammal dynamics : wolf predation in a newly-established multiple-prey system

Abstract

I studied wolf prey selection and kill rates during 1996-97 through 2006-07 winters in a newly established two-prey system in central Yellowstone National Park. Prey differed substantially in their vulnerability to wolf (Canis lupus) predation and wolves preyed primarily on elk (Cervus elaphus) but also used bison (Bison bison) to varying degrees within and among winters and packs. Winter severity, wolf abundance, distribution, and prey selection varied during the study, concurrent with variations in the demography, distribution, and behavior of elk and bison. A total of 759 wolf-killed ungulates were detected and prey selection by wolves was influenced by the absolute and relative abundance of prey types, the abundance of predators, and the duration of snow pack. Wolves strongly preferred elk calves relative to all other prey types, and elk calf abundance was inversely related to the occurrence of bison in wolf diets. Increasing wolf numbers also broadened prey selection from elk calves, and predation on bison and adult elk increased with increasing snow pack accumulation and duration, likely due to its long-term debilitating influence. Elk abundance and wolf pack size best explained variation in kill rates for elk while bison calf abundance and snow pack duration best explained kill rates of bison. The functional response of wolves for elk was best described by a Type II ratio-dependent model, indicating significant predator dependence. Prey-switching evaluations indicated increasing selection of bison with increasing bison:elk ratios, however no concurrent decrease in elk predation occurred. Increased bison predation is not solely dependent on relative abundance of the two prey species; therefore it is unlikely at this time that wolf prey-switching will stabilize the system.