Scholarly Work - Ecology
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Item Neonatal mortality of elk driven by climate, predator phenology and predator community composition(2011-05) Griffin, K.; Hebblewhite, Mark; Robinson, H.; Zager, Peter; Barber-Meyer, S.; Christianson, David A.; Creel, Scott; Harris, N.; Hurley, M.; Jackson, D.Understanding the interaction among predators and between predation and climate is critical to understanding the mechanisms for compensatory mortality. We used data from 1999 radio-marked neonatal elk (Cervus elaphus) calves from 12 populations in the north-western United States to test for effects of predation on neonatal survival, and whether predation interacted with climate to render mortality compensatory. Weibull survival models with a random effect for each population were fit as a function of the number of predator species in a community (3–5), seven indices of climatic variability, sex, birth date, birth weight, and all interactions between climate and predators. Cumulative incidence functions (CIF) were used to test whether the effects of individual species of predators were additive or compensatory. Neonatal elk survival to 3 months declined following hotter previous summers and increased with higher May precipitation, especially in areas with wolves and/or grizzly bears. Mortality hazards were significantly lower in systems with only coyotes (Canis latrans), cougars (Puma concolor) and black bears (Ursus americanus) compared to higher mortality hazards experienced with gray wolves (Canis lupus) and grizzly bears (Ursus horribilis). In systems with wolves and grizzly bears, mortality by cougars decreased, and predation by bears was the dominant cause of neonatal mortality. Only bear predation appeared additive and occurred earlier than other predators, which may render later mortality by other predators compensatory as calves age. Wolf predation was low and most likely a compensatory source of mortality for neonatal elk calves. Functional redundancy and interspecific competition among predators may combine with the effects of climate on vulnerability to predation to drive compensatory mortality of neonatal elk calves. The exception was the evidence for additive bear predation. These results suggest that effects of predation by recovering wolves on neonatal elk survival, a contentious issue for management of elk populations, may be less important than the composition of the predator community. Future studies would benefit by synthesizing overwinter calf and adult-survival data sets, ideally from experimental studies, to test the roles of predation in annual compensatory and additive mortality of elk.Item A survey of the effects of wolf predation risk on pregnancy rates and calf recruitment in elk(2011-12) Creel, Scott; Christianson, David A.; Winnie, John A. Jr.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.