Wolves, elk, and bison: reestablishing the "landscape of fear" in Yellowstone National Park, USA

The elk or wapiti (Cervus elaphus) and bison (Bison bison) of Yellowstone N ational 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 int o parts of Yellowstone National Park. Foraging theory predicts that elk and bison would respond to this threat by increasing their vigilance levels. W e 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, mai ntaining the lowest levels of vigilance throughout the study (approximate t o 12 and 7% of the time was spent vigilant, respectively). Female elk and b ison 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 the study. As wolves expanded into non-wolf areas, female elk with and without calves in these areas gradually increased their vigilance levels from initially 2 0.1 +/- 3.5 and 11.5 +/- 0.9% to 43.0 +/- 5.9 and 30.5 +/- 2.8% by the fift h year of the study, respectively. We discuss the possible reasons for the differences seen among the social groups. We suggest that these behavioural responses to the presence of wolves may have more far-reaching consequence s for elk and bison ecology than the actual killing of individuals by wolve s.