Conservation Northwest

Predators and prey In a recent comprehensive 15-year study of white-tailed deer and wolves, the Minnesota Department of Natural Resources (MDNR) monitored the movements, survival, and mortality causes of 450 radio-collared does in four study areas. Simultaneously, department biologists monitored 55 radio-collared wolves from eight packs whose territories overlapped the deer study areas. The research showed that doe mortality from wolves ranged from 4% to 22% per year but most typically was between 5% to 10% per year with the highest rate observed in the very severe winter of 1995-96. Despite the fact that deer outnumber wolves in Minnesota by 150 to 1, wolves are not particularly effective hunters of white-tails. According to the MDNR, "Wolves end up surviving primarily on the most vulnerable individuals in the deer population, such as very young, old, sick, injured, or nutritionally compromised deer, because those are the ones they can catch. The result being, that under certain conditions…many of the deer that wolves kill likely would have died from other causes, such as starvation or disease." Biologists refer to such predation impacts as "compensatory" as opposed to the "additive" effects of human hunters, who kill most prey in the prime of their reproductive lives. Where have all the mule deer gone? Researchers from Washington State University wanted to understand the reasons for long-term mule deer declines in the intermountain West. Hunters had long been blaming cougars. They were right...sort of. Cougars do kill mule deer. So do wolves, coyotes, bobcats, black bear, and grizzly bears. But as with all natural systems, nothing's that simple. It turns out that the open, mixed forest habitat preferred by mule deer has been so dramatically altered in the West through irrigated agriculture that it's provided wonderful white-tailed deer habitat. White-tails, historically rare in Washington, now outnumber mule deer in eastern Washington. And as white-tailed deer numbers grow, mule deer decline. It appears as though landscape level habitat changes have created the white-tailed equivalent of tenements for cockroaches. It also appears that cougars have responded in kind. But while there may be a slight uptick in cougar numbers as a result of increased ungulate numbers, cougar numbers have not exploded as some people seem to think. "It's particularly striking how little difference there is in resident cougar densities across Yellowstone cougar range in wolf—and elk. western North © David Moskowitz America," says Gary Koehler, carnivore biologist, Washington Department of Fish and Wildlife. AcKeeping the Northwest wild Ewes in Umptaneum Canyon. Bighorn use precipitous terrain to protect themselves from predators such as cougars. © David Moskowitz cording to Dr. Koehler and his colleagues, "North American cougars exist in densities of about 1 to 2 adult animals per 100 sq. km everywhere they live—almost without fail. Female cougars are limited by prey availability, but males are limited by the availability of females in their territories, which they defend vigorously." However, the WSU researchers have found that cougar predation is having a greater impact on mule deer than on white-tails and occurs in the summer when white-tails move into higher elevation mule deer habitat. Mule deer are the "secondary" prey, but as they're already in decline, predation is having a greater effect on them. A similar dynamic has happened with mountain caribou in British Columbia's inland rainforest. As the caribou's historically extensive old-growth forest habitat has been increasingly fragmented, it's opened more niches for deer, elk, and moose. Cougars and wolves follow and opportunistically prey on caribou which cannot withstand the "new normal." For centuries the mountain caribou old forest and high elevation niche was at the heart of their predator avoidance strategy. Predators simply weren't able to get to them enough to make a difference in caribou numbers. Like steelworker jobs in Pittsburgh, jobs for mountain caribou have diminished. Now the wolves are literally at the door and it's forced some tough choices for managers and conservationists alike until the habitat and historic prey species numbers are restored. The Mule Deer Working Group of the Western Association of Fish and Wildlife Agencies has been studying mule deer dynamics, particularly mortalities and predation. According to their findings, many factors confound the question about mule deer declines. Most deer mortality occurs in young animals soon after birth or in winter of their first year. Some biologists believe that the question of whether mortality is compensatory or additive is density dependent— it has to do with how many of the mule deer jobs are filled— also referred to as "carrying capacity" or the ability of the habitat to support the herd. Carrying capacity can be measured by the overall condition of the animals and their range. When the herd numbers are consistent with what the habitat can support—at carrying Continued next page Spring/Summer 2011 7

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