DENSITY OF DALL'S SHEEP IN ALASKA - EFFECTS OF PREDATOR HARVEST, WEATHER, AND CARRYING CAPACITY
By Carl D. Mitchell (Wildlife Biologist, retired) and R. Terry Bowyer (Professional Member, Boone and Crockett Club)
Managing game populations subject to predation has long been a topic of research, discussion, and dissension. Predation and predator-prey relationships are complex ecological phenomena involving a multitude of factors, which makes comprehensive conclusions elusive. Dall's sheep, for example, are well-studied, but effects of predation on their population ecology (abundance, density, survival and population growth rates, age and sex ratios) are not clear. We took advantage of a fortunate coincidence to cooperate with private hunters to measure the effects of wolf and coyote harvest on density of Dall's sheep in Wrangell-St. Elias National Park and Preserve (WRST), in eastern Alaska (see http://www.nps.gov/wrst/index.htm and map).
At 13.2 million acres, WRST is the largest National Park in the U.S. Park Service system. Surveys in the early 1990s indicated as many as 25,000 Dall's sheep inhabit the Park. That is roughly one-half of the Dall's sheep estimated to live in Alaska, and perhaps a quarter of all Dall's sheep in North America. Because WRST was created by the Alaska National Interest Lands Conservation Act (ANILCA), subsistence and sport hunting and trapping occur in some areas of the Park and Preserve. WRST is also renowned for the large number of Boone and Crockett trophy rams taken over the years. (Harry Swank Jr. killed the current B&C World's Record Dall's sheep in the Wrangell Mountains in 1961.) So hunters, the Alaska Department of Fish and Game, and the National Park Service all have an interest in Dall's sheep populations there. WRST is also home to healthy populations of sheep predators, including wolves and coyotes.
While counting Dall's sheep in the area around Ptarmigan Lake, we discovered that some private hunters were legally harvesting wolves and coyotes out of a lodge there. They agreed to share data on their wolf and coyote harvest. We used this canid harvest as a proxy for predation pressure; more canids harvested meant less potential predation on Dall's sheep. We had one study area where no canid harvest occurred, and the adjacent area where the individuals were hunting canids from October through March. Twenty-six wolves and 153 coyotes were taken between 1998 and 2000, primarily by shooting, in this 325 mi2 area. Dall's sheep hunting seasons and harvest occurred in both areas.
Our surveys indicated that sheep in that area where wolves and coyotes were killed increased by 0.28 sheep/mi2 between 1998 and 1999, from about 1.07 sheep/mi2 to almost 1.35 sheep/mi2. Dall's sheep density in the area without canid harvest showed no significant change in density, remaining at about 0.38 to 0.48 sheep/mi2. During 1999-2000 Dall's sheep density where canids were hunted declined by (0.82 sheep/mi2) to less than 0.58/mi2 while densities in the area with no canid harvest again showed no significant change. The difference? The winter of 1999-2000 had significantly more snow and significantly warmer temperatures, resulting in crusted snow and reduced forage for Dall's sheep. In addition, wolf and coyote hunting apparently allowed the population of Dall's sheep in that area to increase to near or beyond their carrying capacity. Thus there were more sheep and less forage per capita to sustain them.
What does this mean?
With appropriate seasons and bag limits, skilled and dedicated private hunters can take enough wolves and coyotes to result in big game increases at least in areas with habitats similar to those in WRST. We also documented that effects of a severe winter (1999–2000) were sufficient to counteract any increase in Dall’s sheep in the treatment (predator harvest) site resulting from reduced densities of wolves and coyotes. We believe that this effect was more severe in the hunted area because canid reduction allowed an already reasonably dense population of Dall’s sheep to approach or possibly exceed carrying capacity. The sheep in the area where no wolf and coyote harvest occurred (and hence where sheep experienced heavier predation) did not decline during that hard winter.
Therefore, while predator control may result in an increase in prey populations, wildlife managers and predator control advocates have to consider not only the actual effects of predation on ungulate populations, but also habitat conditions, especially in relation to carrying capacity, and weather patterns, to avoid unintended consequences. This is especially true in areas where variable and harsh seasonal weather is common.
This edition of Trophy Points was based on information from the following studies:
Mitchell, C.D., R. Chaney, K. Aho, J. Kie and R.T. Bowyer. 2015. Population density of Dall's sheep on Alaska: effects of predator harvest? Mammal Research 60(1):21-28.
Strickland, D., L.L. McDonald, D. Taylor, K. Jenkins and J. Kern. 1992. Estimation of Dall sheep numbers in the Wrangell-St. Elias National Park and Preserve. Biennial Symposium Northern Wild Sheep and Goat Council 8:237-255.