Influence of wolf predation risk on elk movement in northern Yellowstone National Park.

The loss and restoration of large apex predators can have substantial effects on community structure and dynamics. For example, predator-caused changes in prey movement and habitat use may instigate a behaviorally-mediated trophic cascade (BMTC). In a popularly cited example, wolf (Canis lupus) reintroduction to Yellowstone National Park (YNP) has been hypothesized as a driving factor in the recovery of riparian vegetation via behavioral modification of elk (Cervus elaphus); however the evidence for a BTMC is inconclusive due in part to a lack of knowledge regarding the extent that elk actually avoid areas of high wolf predation risk. We used an empirically based (i.e., turning-angle, step-length) conditional logistic regression movement analysis of GPS collared female elk (n = 27) during winter (2000-2004) on the Northern Range of YNP to evaluate elk response to a commonly used kernel density estimates and fine-scale integrated measures of wolf predation risk (e.g., elk distribution, wolf distribution, wolf-killed elk locations, environmental factors). We found a non-linear response to predation risk at the population level with elk increasing their use of riskier sites followed by either no relationship or slight avoidance after a predation risk threshold. Furthermore, individuals varied widely in their response to predation risk demonstrating that individual-specific attributes may be highly important in explaining BMTC. Lastly, elk response to predation risk varied across the 24 hour solar cycle as expected when prey are responding to crepuscular wolf hunting activities. This research may explain contradictory findings reported in the literature related to a wolf-induced BMTC in YNP, and thus has implications for the conservation and management of predator and prey; particularly as large predator restoration is increasingly viewed as a tool for ecological recovery.