COS 86-7
Selective predation mitigates the impact of wolves on survival of northern Yellowstone elk
Predators often kill subsets of prey defined by traits that hinder escape (e.g., old age, small size, poor health). Although selective predation is a fundamental feature of many predator-prey systems, our understanding of its influence on prey demography is not well developed, particularly in natural vertebrate populations. Presumably, selective predation limits the survival of vulnerable individuals while having little or no effect on the survival of invulnerable ones. We tested this prediction by comparing the age distribution of adult female elk (Cervus elaphus) killed by wolves (Canis lupus) in northern Yellowstone National Park (YNP) to their age-specific survival rates. Our analysis included 640 known-age wolf-killed adult female elk collected mainly in the early (mid-November to mid-December) and late (March) winters of 1995-2011, and survival records of 173 radiocollared adult female elk monitored year-round from 2000-2008 and 2011-2012.
Results/Conclusions
Most wolf-killed adult female elk in YNP were old: 9% were 2-8 years-old, 41% were 9-15 years-old, and 50% were 16-26 years-old. Average age of adult females killed by wolves each year ranged from 13 to 16 years-old. Constraints on wolf hunting ability related to wolf age, size, and social behavior probably favored wolf predation on older females. Annual survival rates for adult female elk were 0.93 (95% CI = 0.89, 0.98) for 2-8 year-olds, 0.82 (95% CI = 0.76, 0.88) for 9-15 year-olds, and 0.55 (95% CI = 0.43, 0.68) for 15-21 year-olds. Increasing numbers of wolves had no effect on the survival rate of 2-8 year-olds (β = -0.01, 95% CI = -0.09, 0.05, P = 0.614), a modest effect on the survival of 9-15 year-olds (β = -0.01, 95% CI = -0.03, 0.00, P = 0.058), and a comparatively strong effect on the survival of 16-21 year-olds (β = -0.02, 95% CI = -0.04, -0.01, P = 0.014). These results are consistent with the prediction that selective predation leads to differential survival of vulnerable and invulnerable prey. This in turn suggests that selective predation plays an important role in mitigating the impact of predators on prey survival.