Because ungulates can be dangerous prey, predators generally kill ungulates that are in a vulnerable state (e.g., newborns and juveniles, senescent, poor body condition). Adult female ungulates usually have a high constant survival rate until they become frail at older ages, after which actuarial senescence may be induced by underlying causes of age-specific mortality. We tested the hypothesis that relative to other sources of mortality, selective predation of wolves on old female elk has the dominant influence on actuarial senescence in northern Yellowstone National Park. We monitored VHF and GPS radio-collared elk (n=196) from 2000-2008 and 2011-2016 and determined the cause of mortality for 77 elk. We used a fully-parametric, multi-state survival model in a competing risks framework to estimate age-specific wolf-caused mortality compared to all other known causes of mortality. We tested for the effects of environmental conditions (winter severity, drought severity, green-up rate), competition (elk and bison abundance), and predation pressure (wolf abundance, predation rate, and intraspecific strife) on cause-specific probability of mortality across ages of adult elk.
Results/Conclusions
Elk mortality was low (< 5%) for 2-13 year-olds. After age thirteen, the probability of wolf-caused mortality increased sharply. The probability of wolf predation was 0.10 (95% CI =0.08, 0.14) for 15-year-olds and 0.42 (95% CI = 0.32, 0.54) for 20-year-olds. In contrast, the probability of non-wolf caused mortality was 0.08 (95% CI =0.06, 0.11) for 15-year-olds, and 0.15 (95% CI = 0.11, 0.26) for 20-year-olds. Older elk were more likely to be killed by wolves during years with severe winters (20-year-old mortality = 0.68, 95% CI = 0.41, 0.85) relative to mild winters (0.32, 95% CI = 0.16, 0.60). Increased wolf predation pressure (e.g. wolf abundance) resulted in increased probability of wolf predation for older elk. These results suggest that prey senescence can be shaped by selective predation by cursorial predators. Senescent prey have an increased probability of predation during harsh environmental conditions or when predation pressure is stronger, suggesting that the population-level impact of predation depends on prey age structure and thus additive predation effects are more likely in older populations.