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Winter Severity and Wolf Predation on
a Formerly Wolf-free Elk Herd

Introduction


The reintroduction of wolves into Yellowstone National Park (YNP) has provided new opportunities to study several aspects of wolf predation, such as effects of winter severity on predation patterns (Mech and Frenzel 1971, Mech and Karns 1977, Peterson 1977, DelGiudice 1998, Mech et al. 1998). However, except in the Glacier National Park area (where colonizing wolves preyed primarily on white-tailed deer [Odocoileus virginianus]; Boyd et al. 1994), previous studies have involved long-extant wolf-prey systems. Conceivably, winter severity could be so overwhelming a factor that regardless of the high prey:wolf ratio or number of unculled prey in such a herd, winter severity still might strongly influence wolf predation.

The reintroduction of wolves in YNP provided an opportunity to pose the following 5 questions: (1) How would winter severity affect wolf predation on a previously wolf-free elk herd? (2) Would introduced wolves tend to take older, malnourished, or otherwise vulnerable elk as they do with other prey (summarized by Mech 1970 and Mech et al. 1998)? (3) What is the kill rate and amount of prey consumed? Usually the degree to which wolves consume each kill depends on prey vulnerability at the time. During rare periods when prey are especially vulnerable and abundant, wolves kill often and may not completely consume each carcass (Pimlott et al. 1969, Mech and Frenzel 1971, Peterson and Allen 1974, Carbyn 1983, Miller et al. 1985, Boyd et al. 1994, DelGiudice 1998, Mech et al. 1998). This pattern also is common in other carnivores (Kruuk 1972). (4) Would reintroduced Yellowstone wolves find predation so easy that they would eat small amounts from each kill? (5) Finally, how would these relationships be affected by winter severity?

We sought to answer these questions by studying wolf predation on elk in YNP 2-3 years after the first wolves were reintroduced. Except for possible loners passing through the area, wolves were extinct in YNP after 1930 (Weaver 1978) but were reintroduced in 1995 and 1996 (Bangs and Fritts 1996, Bangs et al. 1998). Until wolf reintroduction, the mortality of most Yellowstone ungulates in winter, especially elk, was due to malnutrition (Houston 1982, Singer et al. 1997). Furthermore, the YNP elk herd was near or at ecological carrying capacity (Singer et al. 1997) and undoubtedly contained at least as many old and vulnerable individuals (Mech 1970:248-261, Mech et al. 1998:121-137) as other elk herds.


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