A survey of the effects of wolf predation risk on pregnancy rates and calf recruitment in elk

We have previously found that the behavioral responses of elk to the risk of predation by wolves are correlated with changes in habitat selection, altered diets, reduced food intake, decreased fecal progesterone concentrations, and decreased calf recruitment. P. J. White et al. recently questioned these results, concluding that "multiple lines of evidence and/or strong validation should be brought to bear before relying on indirect measures of how predators affect pregnancy rates." Here, we systematically surveyed available data and found that five studies (with data from 10 widely distributed populations) have directly detected decreases of 24-43% in elk pregnancy rates in response to increased predation risk. This survey includes data not discussed by White et al. from their own research, which reveal a 32% decrease in pregnancy rates following wolf recolonization in central Yellowstone. Following the survey of available data, we discuss several methodological and statistical problems in White et al.'s study that would be expected to mask the effects of predation risk. While other factors also clearly affect elk recruitment, multiple lines of evidence using a broad array of methods have detected an association between predation risk and reduced reproduction in elk.