Effects of wolf pack size and winter conditions on elk mortality

Abstract

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

How Size and Condition Influence Survival and Cause‐Specific Mortality of Female Elk

“…Our estimates of survival were consistent with estimates of survival for elk in Colorado, Wyoming, and Washington, USA (76–85%; Bender et al 2006, Evans et al 2006, White et al 2006, McCorquodale et al 2011, Webb et al 2011). Survival of elk in Arizona, USA, was estimated at 89% (Ballard et al 2000), higher than our study; however, survival of elk in the region not subjected to harvest was 97%, consistent with our results and similar to other studies of survival when harvest was excluded (91%; Webb et al 2011, Horne et al 2019). Similarly, survival of unharvested adult females ranged from 95–100% in southern New Mexico, USA (Halbritter and Bender 2011), and 99% in Yellowstone when harvest was excluded (White et al 2006).…”

Integrated Carnivore‐Ungulate Management: A Case Study in West‐Central Montana

“…Similarly, in Alaska, wolf predation reduced the abundance of some moose and caribou (Rangifer tarandus) populations (Gasaway et al 1992, Boertje et al 1996. Across Idaho, wolf predation has limited adult and calf elk survival, albeit less so than mountain lion predation (Horne et al 2019). Within the Greater Figure 14.…”

“…Similarly, in Alaska, wolf predation reduced the abundance of some moose and caribou ( Rangifer tarandus ) populations (Gasaway et al 1992, Boertje et al 1996). Across Idaho, wolf predation has limited adult and calf elk survival, albeit less so than mountain lion predation (Horne et al 2019). Within the Greater Yellowstone Ecosystem in Wyoming and Montana, USA, Garrott et al (2005) documented that wolves had important population‐level effects on one elk population with an estimated 20% of the population being killed overwinter, resulting in declining elk population trends, whereas they predicted a nearby elk population incurred only a 4% loss due to wolves, resulting in no change in the elk population trend.…”

“…Yet, also in the Rocky Mountains, an experimental study in a coyote‐mountain lion‐ mule deer ( Odocoileus hemionus ) system found only short‐term, weak responses of survival and abundance following coyote ( Canis latrans ) and mountain lion reductions (Hurley et al 2011). Although public attention is often focused on the effects of wolf recovery on ungulate populations, the effects of recovering wolf populations on ungulate populations vary from relatively minor effects reported in some studies (Vucetich et al 2005, Barber‐Meyer et al 2008, White et al 2010, Eacker et al 2016) to more important effects reported in others (Hebblewhite et al 2002, Garrott et al 2008, Horne et al 2019). These results highlight the need for wildlife managers to better understand the uncertainties associated with carnivore effects on ungulate populations, which ultimately influence the efficacy of integrated carnivore‐ungulate management programs.…”

“…In some areas, bear predation is an important mortality source for elk calves (Raithel 2005, Smith et al 2006, White et al 2010, Lukacs et al 2018), and bear predation on calves has increased over recent decades as bear populations increased (Singer et al 1997, Barber‐Meyer et al 2008). In other systems, wolf predation (Garrott et al 2008) or mountain lion predation is the primary mortality source for calves (Myers et al 1998, Johnson et al 2013, Eacker et al 2016, Horne et al 2019).…”