In 1995, wolves Canis lupus were reintroduced into Yellowstone National Park, USA, where they began to prey on ungulate species. In response to this new predation risk by wolves, we predicted that the two main ungulate species, elk Cervus elaphus and bison Bison bison, should compensate by reducing their use of riskier open meadows and increasing their use of safer forest. Additionally we predicted that this shift in habitat use would result in reduced diet quality. We tested the first prediction by regressing the number of faecal groups in 10-m(2) sampling plots against distance from forest edges. To test the second prediction, we compared percent faecal nitrogen in elk and bison faeces between areas with and without wolves. We found a significant negative relationship between number of elk faecal groups and distance from forest edge in areas with wolves (r(2) = 0.65, P = 0.001), but we did not find a relationship between these two factors in areas without wolves. Mean percent faecal nitrogen in elk was significantly lower (F-(1,F-116) = 13.9, P < 0.001) in areas with wolves (1.7%, SE = 0.09, N = 40) than in wolf-free areas (2.1%, SE = 0.08, N = 80). For bison, we did not find any significant relationship between numbers of faeces and distance from forest edge nor in dietary nitrogen between wolf and wolf-free areas. We concluded that predation pressure from the reintroduced wolves was consistent with our prediction that elk shifted habitat use, thus lowering the quality of their diet. However, a similar change in use pattern and dietary quality of bison in response to wolf presence was not found.
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