Winter Prey Selection and Estimation of Wolf Kill Rates in Yellowstone National Park, 1995–2000

Abstract

Wolf (Canis lupus) impacts on prey are a central post‐wolf‐reintroduction issue in the greater Yellowstone ecosystem (GYE) of the western United States. Further, estimates of wolf kill rates, used to understand these impacts, can be biased due to unrecovered kills. In Yellowstone National Park (YNP), visibility of wolves allowed us to combine independent aerial and ground observations and use a double‐count method to estimate the probability of recovering kills. We consequently used these data to adjust estimates of wolf kill rates. We conducted monitoring annually from 1995 to 2000 during 2 30‐day periods in early (15 Nov–14 Dec) and late winter (Mar). Estimated recovery rates of wolf kills for ground and aerial crews were 50% and 45%, respectively, although we determined that this varied by location (distance from road) and possibly age (calf or adult) of the kill. The estimated combined recovery rate was 73%. Estimated wolf kill rates were higher in late winter (2.2 kills/wolf/month) compared to early winter (1.6 kills/wolf/month), with an overall estimated rate of 1.9 kills/wolf/month. The primary prey of wolves in winter was elk (Cervus elaphus; 90%). During our study, 43% of the elk killed were calves, 28% were adult females (cows), 21% were adult males (bulls), and 9% were of unknown age/sex. Comparing prey selection to prey availability, wolf packs residing on the northern range (NR) of the GYE selected for calves, against cows, and approximately proportional to availability for bulls. Prey use was different for wolf packs occupying the NR compared to packs residing in other areas (non‐northern range [NNR]) and varied seasonally for NR packs. Variation in wolf kill rates by season, and the relative stability of the northern Yellowstone elk herd during a series of mild winters despite increases in wolf density, suggest that kill rates and ultimately elk population size are influenced by winter weather. Management of ungulates should reflect the addition of wolves combined with the unpredictability of winter weather in the mountainous terrain of the western United States.

Prey selection and dietary response by wolves in a high-density multi-species ungulate community

“…In reviewing these studies, Mech and Peterson (2003) argued that kill rates of a specific prey species could depend more on pack size and prey vulnerability than prey density. In addition, in Yellowstone National Park, Smith et al (2004) reported no functional response to variations in elk density, thus suggesting that wolves had adopted a minimum kill rate strategy so as to reduce the risk of injury during predation.…”

“…Foraging behaviour is a driving factor of predator-prey dynamics, and its understanding is fundamental for proper management and conservation of large carnivore and wild ungulate communities (Huggard 1993;Kunkel et al 2004;Smith et al 2004). Opportunistic predators tend to select the most abundant prey (apostatic selection, Yearsley 2003), and their patterns of selection are influenced by changes in prey abundance.…”

Survey of Selected Pathogens and Blood Parameters of Northern Yellowstone Elk: Wolf Sanitation Effect Implications

“…Yellowstone supports an intact large-predator complex, including black bears (Ursus americanus), coyotes (Canis latrans), grizzly bears (U. arctos) and cougars (Puma concolor); and wolves (C. lupus), which were reintroduced in 1995-96. Wolves increased rapidly and during 2000-2005 reached one of the highest densities recorded worldwide (Smith et al, 2003). Elk comprised 89% of wolf kills during the winters of 1995-2005(Smith et al, 2004 and elk in the northern part of the park decreased by approximately 50% (White and Garrott, 2005).…”

“…Wolves increased rapidly and during 2000-2005 reached one of the highest densities recorded worldwide (Smith et al, 2003). Elk comprised 89% of wolf kills during the winters of 1995-2005(Smith et al, 2004 and elk in the northern part of the park decreased by approximately 50% (White and Garrott, 2005). Wolves and hunters each took 7-8% of adult cow elk annually (Evans et al, 2006).…”

Three approaches to estimate wolf Canis lupus predation rates on moose Alces alces populations

“…One possibility is that predation pressure varies through the year (e.g., Smith et al 2004). If predation rates are higher during winter than during summer, we would expect that predation rates estimated by approaches 2 and 3 (summer and winter) to be higher than those estimated by approach 1 (only summer).…”