Print PagePredator-prey interactions have long been a focus of ecological study. Early theory emphasized the direct killing and consumption of prey, but more recently, ecologists have recognized nonconsumptive effects (NCEs) of predators. NCEs occur when predators alter the stress physiology or behavior of their prey strongly enough to induce nutritional or reproductive costs. Although accumulating evidence has led some to suggest such NCEs are strong and widespread, most studies have been conducted in small-scale experimental systems, and relatively little is known about the occurrence and strength of NCEs in large mammal systems. In the Greater Yellowstone Ecosystem (GYE), there is currently conflicting evidence as to whether the risk of wolf predation influences elk body fat levels and pregnancy rates, thus it remains unclear whether NCEs of wolves might be contributing to recent declines in elk productivity and abundance. We report on a three-year study of the foraging behavior, body fat dynamics, and pregnancy rates of a partially-migratory elk herd under variable wolf predation risk on the northeastern boundary of the GYE.
Results/Conclusions
The hypothesized mechanism for an NCE of wolves is that the risk of wolf predation depresses elk pregnancy by causing female elk to forage sub-optimally in winter, resulting in accelerated fat loss and, ultimately, intrauterine mortality. In our study, migratory elk were exposed to four times higher wolf densities than resident elk during winter, but spent twice as much time feeding and no additional time vigilant. Migratory elk also tended to be fatter in winter than resident elk, probably because high rates of direct predation on their calves during the prior growing season relieved most migratory females from the high costs of lactation. Though we observed a lower rate of pregnancy among migratory (68%) vs. resident elk (89%), the pregnancy rate of migrants was low in early winter, suggesting a link to summer habitat conditions. Together, these results are at odds with the hypothesized mechanism for NCEs in the wolf-elk system. Meanwhile, NDVI and climate analyses suggest that the reduced pregnancy of migratory elk has been caused primarily by drought-induced changes in habitat quality. Our findings suggest bottom-up constraints on the expression of elk antipredator behavior and associated NCEs, and are consistent with a large body of behavioral evidence for the state-dependency of antipredator behavior. Further research is necessary to improve our understanding of NCEs in large mammals, especially because predator-prey interactions in these systems are often highly managed.
