Study Areas and Populations

In 1995-96, wolves were reintroduced into Yellowstone National Park (YNP) after an absence of seven decades (Fritts et al., 1997). Wolves were absent from YNP in 1926 when the National Park Service ended its policy of killing large predators (Smith et al., 2000). The extirpation of wolves was not restricted to the National Park System: wolves were essentially eradicated from the entire American portion of the northern Rocky Mountains. A total of 31 wolves were released in 1995-96, from which the population expanded rapidly both in numbers and distribution. Official annual estimates of the wolf population are reported by NPS at the end of each year. As of January 2001, the Yellowstone ecosystem held 164-169 individuals in 16 groups, including 11-13 breeding packs (Smith et al. 2000).

Concurrent with the Yellowstone reintroduction, 35 wolves were released into central Idaho, and wolves have continued to colonize northwestern Montana, dispersing from southern Alberta (Boyd & Pletchser 1999). Collectively, the wolf population of the US Northern Rocky Mountains has steadily grown, numbering 433 wolves and 28 breeding pairs at the outset of 2001 (USFWS 2001).

The population growth and spatial expansion of wolves in the Northern Rockies over the past six years is now re-establishing a strong predation pressure on ungulate populations that was removed by humans for more than 10 generations (Berger et al. 2000). Working in areas at the edge of the recolonizing wolf population, this study will examine the impact of predator-prey interactions on the behavior, physiology, distribution, demography and population dynamics of both wolves and elk. Relative to the conclusions that could be drawn from a single site, comparisons among three areas will provide good resolution of the factors that affect wolf-elk predation. We have conducted preliminary work in all three areas and have extensive, long-term pre-wolf data on elk numbers and demography for these sites. The sites have been occupied by radio collared wolf packs, with multiple radio transmitters in each pack, for varying lengths of time since the reintroduction. We begin by describing each study site and presenting preliminary data, and conclude this section by describing how comparisons within and among sites will be used to test hypotheses.

Because we have extensive pre-wolf data on the elk population on each site, we can make direct comparisons of herd sizes and compositions before and after wolf colonization. In each herd, we will monitor changes in population size and age-sex composition, and relate these patterns to offtake by wolves. Similar to our comparisons among sites, we will use multiple regression (mixed model GLS) to simultaneously examine the impact of several independent variables when testing the for factors related to kill rates or changes in the size or demographic composition of elk herds.

For all three sites, systematic data on elk numbers and population composition extend back for at least 30 years prior to wolf colonization (as far back as 1927). These data were collected through aerial total counts by biologists working for Montana Fish Wildlife and Parks and Yellowstone National Park. Fixed wing aircraft (Piper PA18 Super Cub) have been used in most counts, but counts in the Gallatin have been conducted by helicopter since the mid-1990's, to allow age-sex classifications during the aerial count. Four trials that used helicopter and fixed wing counts within a span of several days showed a tight correlation (b = 0.97 ± 0.18 SE, r² = 0.94, P = 0.033), so that the time series of population sizes is probably not biased by the shift in methods. These trials were spread over 16 years, in years of low density (808 elk counted) and high density (2145 elk counted).