Abstract

Gray wolves (Canis lupus) were extirpated from Yellowstone National Park (YNP) by the 1920s through predator control actions (Murie 1940,Young and Goldman 1944, Weaver 1978), then reintroduced into the park from 1995 to 1996 to restore ecological integrity and adhere to legal mandates (Bangs and Fritts 1996, Phillips and Smith 1996, Smith et al. 2000). Prior to reintroduction, the potential effects of wolves on the region’s threatened grizzly bear (Ursus arctos) population were evaluated (Servheen and Knight 1993). In areas where wolves and grizzly bears are sympatric, interspecific killing by both species occasionally occurs (Ballard 1980, 1982; Hayes and Baer 1992). Most agonistic interactions between wolves and grizzly bears involve defense of young or competition for carcasses (Murie 1944, 1981; Ballard 1982; Hornbeck and Horejsi 1986; Hayes and Mossop 1987; Kehoe 1995; McNulty et al. 2001). Servheen and Knight (1993) predicted that reintroduced wolves could reduce the frequency of winter-killed and disease-killed ungulates available for bears to scavenge, and that grizzly bears would occasionally usurp wolf-killed ungulate carcasses. Servheen and Knight (1993) hypothesized that interspecific killing and competition for carcasses would have little or no population level effect on either species. As a component of post-reintroduction wolf and grizzly bear monitoring programs, interspecific interactions between the species were recorded. We expected reintroduced wolves to occasionally kill grizzly bears, especially cubs-of-the-year (cubs). We also predicted that adult males, solitary adult females, and female grizzly bears accompanied by yearling or 2-year-old offspring would occasionally usurp wolf-killed ungulates and scavenge the remains. We hypothesized that these cohorts of grizzly bears would be more successful than subadults at usurping wolf-kills. We further hypothesized that due to potential danger to cubs, females with cubs would not attempt to displace wolf packs from their kills. Our monitoring of interspecific interactions between wolves and grizzly bears is ongoing. From wolf reintroduction in 1995 until January of 2003, 96 wolf– grizzly bear interactions have been recorded (Ballard et al. 2003; D. Smith, National Park Service, Yellowstone National Park, Wyoming, USA, unpublished data). Here we report observations of interactions between wolves and female grizzly bears with cubs and evidence of wolf packs killing grizzly bear cubs near carcasses. Due to grizzly bears’ low reproductive rate (Schwartz et al. 2003) and status as a threatened species (USFWS 1993), the effects of wolves on carcass availability and cub survival is an important consideration for wolf reintroduction and grizzly bear conservation efforts. YNP encompasses 891,000 ha in the states of Wyoming, Montana, and Idaho, USA. The park contains a variety of habitats from high elevation alpine to low elevation sagebrush grasslands (Despain 1990). YNP and the surrounding area (Greater Yellowstone Ecosystem, GYE) support an estimated 56,100 elk (Cervus elaphus), 29,500 mule deer (Odocoileus hemionus), 5,800 moose (Alces alces), 3,900 bighorn sheep (Ovis canadensis), 3,600 bison (Bison bison), and smaller numbers of whitetail deer (Odocoileus virginianus), mountain goat (Oreamnos americanus), and pronghorn antelope (Antilocapra americana) (U.S. Fish and Wildlife Service 1994). Large carnivores in the GYE include grizzly bears, American black bears (U. americanus), wolves, and mountain lions (Felis concolor). In 2002, the reintroduced wolf population in the GYE was estimated at 273 wolves in 31 packs (Smith et al. 2003a). More than 90% of the prey killed by wolves in the GYE is elk (Smith et al. 2003b). Other prey species killed by wolves include deer, bison, and moose, but individually none of these prey comprise .2% of GYE wolves’ diet. The GYE grizzly bear population is estimated at 280–610 bears (Eberhardt and Knight 1996). The GYE is unique among areas inhabited by grizzly bears in North America because kerry_gunther@nps.gov doug_smith@nps.gov

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