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Wolf-Bison Interactions in Yellowstone National Park

Materials and Methods

Yellowstone National Park is a 891,000-ha protected area primarily in northwestern Wyoming, with a variety of habitats from high alpine to lower elevation sagebrush grasslands (Despain 1990). YNP and the surrounding area (Greater Yellowstone Area) support an estimated 120,000 elk, 87,000 mule deer (Odocoileus hemionus), an unknown but low number of white-tailed deer, 5,800 moose, 3,900 bighorn sheep (Ovis canadensis), 2,000-4,000 bison, 800-1,000 mountain goats (Oreamnus americanus), and 400 pronghorn (Antilocapra americanus—Bangs and Fritts 1996; Varley and Brewster 1992).

Ungulate behavior and migration in winter are variable depending on species and winter severity (Houston 1982; Meagher 1973; Merrill and Boyce 1991). Distribution and abundance by season for any species are too complex to report individually for any given year, but for areas in question, wintering populations of ungulates do exist each year.

In March 1995, 14 wolves in 3 packs and in April 1996, 17 wolves in 4 packs were reintroduced into YNP (Bangs and Fritts 1996; Fritts et al. 1997; Phillips and Smith 1996). Wolves released in 1995 were captured just east of Jasper National Park, Alberta, Canada, where elk, deer, and moose were their main prey. The nearest bison were >600 km away. Because wolves sometimes disperse distances of >886 km (Fritts 1983), some donor wolves might have had some experience killing bison. Wolves reintroduced in 1996 came from an area where they preyed on bison (Weaver and Haas 1998). In addition to the 31 Canadian wolves, 10 bison-naive wolves were brought to YNP in September 1996 from northwestern Montana. Those 5-month-old pups were placed in a pen with other adult wolves for acclimation over the winter and later released as yearlings in March or April 1997.

All wolves released into YNP were radiocollared (Bangs and Fritts 1996), and subsequently 30-50% of the pups born were collared. Wolves were acclimated for 10 weeks inside pens, which allowed us to identify individuals from the air by their markings. They were located aerially at least weekly for 10 months of the year and daily (weather permitting) from mid-November through mid-December and in March. We also located and observed wolves from the ground throughout the year, except for 3 remotely located packs of 6-33 wolves. Wolves in the Madison-Firehole area were radio tracked and then back-tracked from the ground from December 1997 through March 1999. In summer, wolf observations were made opportunistically and less frequently.

When wolves were sighted aerially, we noted time of day, location, habitat type, group size, age, and activity, and sex (for collared wolves). We circled for several minutes if wolves were involved in an activity such as attacking prey; otherwise, we left the area. Any interactions with prey observed from the ground were monitored with spotting scopes and recorded onto data forms in the field or into a dictaphone and later transferred to data forms. Ground observers recorded the same data as did aerial observers but also recorded sequence of events: which species approached (wolf or prey), numbers of wolves or prey involved, response of wolves and prey (stand, walk away, flee, or attack), duration of encounter, outcome, and distance over which the encounter took place. Observations reported here are limited to wolf-bison interactions. Bison numbers and distribution were recorded during aerial surveys as part of long-term research (Meagher 1998).

Wolf-killed prey were defined as prey observed being killed or prey determined as killed by wolves by observing wolves feeding on a carcass and then finding other sign that indicated that the prey had been killed by wolves (Peterson 1977). For example, we looked for blood in the snow along with signs of a struggle or searched for tracks indicative of a chase (Carbyn 1983; Peterson 1977). Condition of wolf-killed bison was assessed either by marrow fat from a leg bone (Cheatum 1949), or observation of the bison before it was killed by wolves.

Wolf selectivity and success rate in killing prey (elk or bison) were analyzed using a chi-square with Yates' correction for continuity (Sokal and Rohlf 1981). Results were considered significant at P < 0.05.

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