Understanding the behaviorally-mediated indirect effects of predators in ecosystems requires knowledge of predator-prey behavioral interactions, and the risk allocation hypothesis can be used to make predictions about such interactions. In predator-ungulate-plant systems, empirical research quantifying how predators affect ungulate group sizes and distribution, in the context of other influential variables, is particularly needed. We determined non-predation variables that affect elk (Cervus elaphus) group sizes and distribution on a winter range in the Greater Yellowstone Ecosystem (GYE), using regression analyses of 1219 elk groups counted and mapped over 12 years prior to wolf (Canis lupus) recolonization. We next examined the relevance of these non-wolf-predation variables in the presence of wolves using logistic and linear regression on surveys of 513 1-km² areas conducted over 2 years. Using model selection techniques, we evaluated risk allocation and other a priori hypotheses of elk group size and distributional responses to wolf predation risk while accounting for influential non-wolf-predation variables. We found little evidence that wolves affect elk group sizes, which were strongly influenced by habitat type. Following predictions from the risk allocation hypothesis, wolves likely created a more dynamic elk distribution in areas that they frequently hunted, as elk tended to move during the period of safety following wolf encounters in those areas.  We predict that this distributional response to wolf predation risk should decrease the spatial heterogeneity of elk impacts on grasslands in areas that wolves frequently hunt. We also predict that this response should decrease browsing pressure on heavily-browsed woody plant stands in certain areas, which is supported by recent research in the GYE. This research highlights the importance of predator-prey behavioral interactions in large mammal systems.

Figure. Spatial patterns in wolf activity and hunting pressure at the East Madison Site during the 2002 and 2002-03 winters. Darkly shaded regions represent wolf core activity and hunting areas (50% isopleths), lightly shaded areas represent wolf medium activity and hunting areas (95% isopleths), and the unshaded portions of each panel represent infrequent wolf activity and hunting areas (outside of 95% isopleths).