Knowledge about predation-risk effects in wildlife is necessary to understand the prevalence of risk effects in nature and the full extent that predators structure natural systems. Studies of small-scale experimental systems suggests that risk effects could represent a large and potentially overlooked portion of a predator’s total effect in nature. However, few wildlife studies have quantified the components of predation-risk effects, including the degree to which risk-induced changes in prey traits affect prey fitness and density (nonconsumptive effects, NCEs) and/or indirectly affect the fitness and density of other species (trait-mediated indirect effects, TMIEs). We address these gaps with the controversial example of wolves, elk, and aspen in northern Yellowstone National Park. Specifically, we examine the evidence in support of the hypotheses that wolf-induced changes in elk habitat selection and vigilance have (i) decreased elk food intake, pregnancy rate, juvenile recruitment, and population growth, and (ii) decreased elk herbivory of young aspen and increased aspen overstory recruitment. We summarize previous research together with the latest findings from our ongoing, long-term studies of Yellowstone wolves, elk, and aspen.
Results/Conclusions
Elk, especially prime-age adults, usually survive their encounters with wolves for reasons that include a substantial size advantage. An adult elk is as much as 5-6 times heavier than an adult wolf. This allows elk to stand their ground and fight off wolves. Elk also aggregate in large groups, and male elk use their antlers as defensive weapons. Simply put, elk are not wolf bait. This helps explain why elk interrupt their foraging only when wolves are an immediate threat, and why elk continue to select for nutritionally-important but risky habitats despite the presence of wolves. Elk home range fidelity and ruminant digestion, which allows elk to simultaneously process their food and scan their surroundings, probably also contributes to the negligible effects of wolves on elk habitat selection and vigilance. This in turn explains why elk maintained high levels of body fat and high rates of pregnancy, and why herbivory of young aspen did not vary as a function of spatial variation in wolf predation risk. In this system, predation-risk effects are less important than consumptive effects for understanding predator-caused changes in prey density, and indirect changes in the resources of the prey.