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Yellowstone National Park Report

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More and more, evidence indicates that non­lethal interactions between large mammalian ungulates and the predators that feed on them may play significant roles in ungulate population dynamics. Although predators such as wolves and mountain lions directly impact large ungulates like elk (Cervus elaphus) when they kill individuals, the fact that they scare their prey may actually have a greater long term impact on the population (Kotler and Hoyt 1989, Brown 1992, Brown and Alkon 1990 Brown et al. 1999). In response to predation risk, foraging animals are found balancing conflicting demands for food and safety. Research indicates they do this by two principal means: 1) when faced with higher predation risk, prey individuals will reduce feeding effort and/or increase vigilance compared to areas of lower risk (Sih 1980, Lima and Dill 1990), 2) they alter their use of habitat types to help reduce this predation risk. The major result is that reduced feeding efforts or selection of safer but possibly less productive habitat lead to a third prediction of a poorer quality diet as animals seek out safer areas but with likely lower quality forage. The reintroduction of wolves in Yellowstone National Parks offered a unique opportunity to test the impact of wolves on the feeding efficiency of elk and bison. After the wolves were reintroduced into Yellowstone Park in the spring of 1995, they quickly established themselves in specific locations, specifically in the Lamar Valley in the north end of the Park. This allowed us to collect data on areas with and without wolves for the first few years after their release. Additionally, as wolves have expanded their range in the Park, this has also provided an excellent opportunity to compare data on animals from the same areas before and after wolves have arrived. These comparisons then, would provide a critical test of the predictions that large predators can have a major non-lethal impact on their prey. To test these predictions, in 1996 we began a study of the foraging patterns of elk and bison in Yellowstone National Park. Here we report the results of the first four years of this study.