Wisconsin to Study Predator Impacts on Deer

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Scientists with the Department of Natural Resources and the University of Wisconsin will launch an ambitious, multi-year field research effort to better understand the impacts predators such as wolves, bears, coyotes and bobcats have on white-tailed deer in Wisconsin.

A review of existing literature on predation and a mathematical analysis using Wisconsin's extensive harvest and population data suggest a low level impact, but just how much, at what times, by what predators and under what conditions are all relatively unknown at this time, scientists said.

"The literature review and data analysis are first steps that identified the need for specific field research on deer predation and how that research might be designed," said DNR research scientist, Christopher Jacques.

A particular concern being raised by hunters is the fact of expanding wolf and bear populations in Wisconsin. Both are large predators that feed on deer either primarily as in the case of wolves or occasionally as in the case of bears.

Some hunters argue an expanded wolf population, in particular, has resulted in fewer deer in the northern and central forests.

Review of published research and preliminary data analysis suggests bears are having a small but measurable impact on fawn mortality and that wolves have a small impact on the mortality rate of adult does.

One study in northern Wisconsin will use radio telemetry to track fawns and determine how many are killed by predators and by which predators. Almost nothing is known, for example, about the impact of coyote and bobcat predation on deer in Wisconsin. Similar research is underway in Michigan's Upper Peninsula, and the two states are sharing results.

Another study, set to run for five years, will use a combination of field research methods, including radio telemetry, to study buck mortality. This research, on the DNR wish list for more than a decade, was recommended by independent auditors in 2006 to fine tune Wisconsin's procedures for estimating herd size.

The review of published research on predation and the early analysis of Wisconsin data were done by researchers Jacques of the DNR Bureau of Science Services and Tim Van Deelen of the University of Wisconsin- Madison. They unearthed 90 studies from the U.S. and Canada, most conducted in recent years with a few dating back to the 1960s and 1970s. Their complete review is available on the Wisconsin wildlife survey page of the DNR Web site (look under deer).

Many deer hunters tend to believe that every deer killed by a predator results in one less deer available for a human hunter to harvest. Research has found the truth to be more complicated. Predators do not, in fact, always reduce the population growth rates of prey.

Biologists use the terms "compensatory" and "additive" to describe the impact of predation on any given wildlife population. If predation is "compensatory," it means the total number of prey to die in any given year does not change as a result of predation. It means the predators remove the number of animals that would have been lost anyway to other causes.

If predation is "additive," then the predator is killing prey animals that otherwise would have survived the cycle of seasons. In these cases, the predator is slowing the growth of the prey population, or in some instances, causing that population to decline.

Not surprisingly, the truth in any given predator-prey system often lies somewhere between with some predation being compensatory and some being additive. The trick is to determine degree. If predation has an "additive," or negative, effect on prey population growth, is the effect small or large?

Jacques and Van Deelen applied statistical analysis to ten years of field and registration data (1998-2008) supplied by hunters from 57 deer management units, or DMUs, in the northern and central forests.

This 10 year period coincides with a steady increase in wolf populations and a probable increase in bear populations in the northern and central forests.

The Wisconsin researchers were looking for relationships, or correlations, within this large mass of data. For instance, do the data show a relationship between increasing wolf numbers in a given area and the growth rate of the deer herd? If such a relationship is revealed, researchers call this a "signal." The signal can be positive or negative, weak or strong.

This statistical analysis does not show cause and effect, Van Deelen cautions. Nevertheless, the numerical "signals" it produces are valuable markers, pointing to areas where further research is needed to explain the numbers.

The statistical model created by Jacques and Van Deelen suggests black bear presence is associated with a reduced population growth rate for deer of less than one percent. Under this model deer herds would still grow in the presence of a large bear population, just at a slightly reduced rate. The model also suggests that the presence of wolves is related to a slightly increased rate of doe mortality.

"This early analysis appears to say that impacts on deer populations associated with bears and wolves are clearly minor relative to impacts associated with hunting by humans," said Van Deelen.

These statistical relationships reflect what could be happening across large regions according to biologists. The numbers have nothing to say when it comes to an individual hunter and the 40 or 80 acres he or she hunts.

"If you happen to be hunting in an area of wolf activity," Jacques said, "it could affect deer behavior or movement patterns and the number of deer you see."

DNR wildlife officials are committed to continually improving the quality of the state's deer management program. Hunters play a critical role in this process, and as the agency moves forward with its ambitious research program, hunter involvement will be more important than ever.