Effects of wolf pack size and winter conditions on elk mortality

Elk (Cervus canadensis) are high-profile game animals for many states in the western United States, yet over the past several decades some populations have experienced a persistent and broad-scale decline in recruitment. Over this same period, gray wolves (Canis lupus) have become an integral component of many western landscapes and agencies are increasingly challenged to maximize hunting opportunities of ungulates via predator management while simultaneously ensuring wolf conservation. To better understand the implications of predator management on elk populations, we monitored survival of 1,244 adult female elk and 806 6-month-old calves from 29 populations distributed throughout Idaho, USA, from 2004 to 2016. We developed predictive models of mortality that related mortality risk to wolf pack size, winter conditions, and individual-level characteristics. Annual mortality rates (excluding harvest) for adult females and calves were 0.09 and 0.40, respectively. Calf mortality was predicted best with a model that included additive effects of chest girth at time of capture, mean size of surrounding wolf packs, and snow depth. Adult female mortality was predicted best with a model that included female age, mean size of surrounding wolf packs, and snow depth. Based on a sensitivity analysis, chest girth had the largest effect on risk of mortality for calves followed by pack size and snow depth. Other than the effect of senescence in the oldest (>15 yr) individuals, pack size and snow depth had the largest effect on risk of mortality for adult females. We estimated cause-specific mortality and predation was the dominant cause of known-fate mortalities for adult females (35% mountain lion Puma concolor] and 32% wolf) and calves (45% mountain lion and 28% wolf), whereas malnutrition accounted for 9% and 10% of adult female and calf mortalities, respectively. Wolves preferentially selected smaller calves and older adult females, whereas mountain lions showed little preference for calf size or age class of adult females. Our study indicates managers can increase elk survival by reducing wolf pack sizes on surrounding winter ranges, especially in areas where, or during years when, snow is deep. Additionally, managers interested in improving over-winter calf survival can implement actions to increase the size of calves entering winter by increasing the nutritional quality of summer and early fall forage resources. Although our study was prompted by management questions related to wolves, mountain lions killed more elk than wolves and differences in selection of individual elk indicate mountain lions may have comparably more of an effect on elk population dynamics. Although we were unable to relate changes in mountain lion populations to elk survival in our study, future research should seek a better understanding of multi-predator systems, including how management of one predator affect others and ultimately how these interactions affect elk survival. © 2019 The Wildlife Society