Multiscale wolf predation risk for elk: does migration reduce risk?

Abstract

While migration is hypothesized to reduce predation risk for ungulates, there have been few direct empirical tests of this hypothesis. Furthermore, few studies examined multiscale predation risk avoidance by migrant ungulates, yet recent research reveals that predator-prey interactions occur at multiple scales. We test the predation risk reduction hypothesis at two spatial scales in a partially migratory elk (Cervus elaphus) population by comparing exposure of migrant and resident elk to wolf (Canis lupus) predation risk. We used GPS and VHF telemetry data collected from 67 migrant and 44 resident elk over the summers of 2002-2004 in and adjacent to Banff National Park (BNP), Canada. We used wolf GPS and VHF telemetry data to estimate predation risk as a function of the relative probability of wolf occurrence weighted by a spatial density model that adjusted for varying pack sizes. We validated the predation risk model using independent data on wolf-killed elk, and showed that combining wolf presence and spatial density best predicted where an elk was likely to be killed. Predation risk on summer ranges of migrant elk was reduced by 70% compared to within resident elk summer ranges. Because wolves avoided areas near high human activity, however, fine-scale selection by resident elk for areas near high human activity reduced their predation risk exposure to only 15% higher than migrants, a difference significant in only one of three summers. Finally, during actual migration, elk were exposed to 1.7 times more predation risk than residents, even though migration was rapid. Our results support the hypothesis that large-scale migrations can reduce predation. However, we also show that where small-scale spatial variation in predation risk exists, nonmigratory elk may equally reduce predation risk as effectively as migrants under some circumstances.