Behavioral responses of elk to winter wolf predation risk in the Madison Headwaters area, Yellowstone National Park

Abstract

Over the past few decades a large body of literature has provided evidence that predators can influence the ways in which prey behave. This in turn may influence prey demography and predator-prey dynamics and therefore predators may influence the structure and function of populations and communities, independent of direct killing. Using data collected from 1991 to 2007, I evaluated the behavior of elk (Cervus elaphus) in the Madison headwaters area of Yellowstone National Park in response to the colonization and establishment of wolves (Canis lupus). Changes in home range size, fidelity, group size, foraging behavior, and large-scale spatial responses were evaluated. After wolf colonization, elk movements were more dynamic as elk moved more over the landscape as they were increasingly encountered, attacked, and displaced by wolves. Home range sizes were larger, with slight decreases in fidelity. These results show that elk made modest adjustments in space use presumably to reduce their vulnerability from predators at a fine-scale within their range. More dramatic larger scale spatial shifts were also documented as radio-collared elk adopted long-distance dispersal and migratory movements away from high-density wolf areas. These apparent predator-avoidance movements were never observed prior to wolf colonization or from areas where the risk of predation was lower. Prior to wolf colonization, the grouping behavior of elk was relatively stable and predictable as elk attempted to conserve energy and decrease starvation risk in the absence of wolves. Following wolf reintroduction group size and group size variation increased. This more dynamic behavior likely reflects a strategy to minimize predation risk and maximize food acquisition. The decision to forage was heavily influenced by local snow, habitat type, and time of day but remained relatively stable with and without the presence of wolves. The lack of any substantial change in the foraging behavior of elk in the presence of wolves illustrates that elk can maintain the same level of foraging time and retain a relatively constant level of nutrition. Together these results suggest that in a harsh winter environment such as the Madison headwaters, elk can adaptively manage their behavior to cope with environmental constraints both in the presence and absence of wolves. Landscape variation such as snow pack severity and habitat types, complexity, and patch size also influences predation risk and may dictate the way in which prey behave.