Wolves, elk, and bison: reestablishing the "landscape of fear" in Yellowstone National Park, U.S.A.

The elk or wapiti (Cervus elaphus) and bison (Bison bison) of Yellowstone National Park have lived in an environment free of wolves (Canis lupus) for the last 50 years. In the winter of 1994-1995, wolves were reintroduced into parts of Yellowstone National Park. Foraging theory predicts that elk and bison would respond to this threat by increasing their vigilance levels. We tested this prediction by comparing vigilance levels of elk and bison in areas with wolves with those of elk still in "wolf-free" zones of the Park. Male elk and bison showed no response to the reintroduction of wolves, maintaining the lowest levels of vigilance throughout the study (12 and 7% of the time was spent vigilant, respectively). Female elk and bison showed significantly higher vigilance levels in areas with wolves than in areas without wolves. The highest vigilance level (47.5 ± 4.1%; mean ± SE) was seen by the second year for female elk with calves in the areas with wolves and was maintained during the subsequent 3 years of...