Deer-Dispersal Finding Has Implications for CWD Containment

Even if chronic wasting disease (CWD)—lethal to white-tailed deer with no known cure—had not shown up last year in New York and West Virginia less than 100 miles from the Pennsylvania border, the ramifications of Eric Long’s discovery would be exciting to wildlife managers in the Northeast. But because the finding, which was published in a recent issue of the Journal of Mammalogy, will provide scientists with a tool to predict how far and how fast CWD could spread in deer inhabiting the broken forested tracts of the region, it is being considered a breakthrough.

Long, a Huntingdon, Pa., resident and Penn State graduate student in ecology, has been studying the dispersal of juvenile male deer. His research is part of a joint study by the Pennsylvania Game Commission and the U.S. Geological Survey’s Pennsylvania Cooperative Fish and Wildlife Research Unit at Penn State.

“By scouring literature from previous research and combining that knowledge with results from our deer-dispersal study, Eric was able to establish a solid relationship between the percentage of forest cover and how far young male deer disperse,” says Duane Diefenbach, adjunct assistant professor of wildlife and Long’s adviser in the School of Forest Resources. “I can tell you that if CWD shows up in Pennsylvania, this information will be crucial in determining the size of the containment zone.

“To use the New York situation as an example, it is likely that the disease was introduced by a captive deer propagator,” Diefenbach adds. “So we know where ground zero is. If we can determine when the disease was introduced into the wild—and with Eric’s discovery that we can predict how far juvenile male deer will disperse based on forest cover—we can look at the landscape and make accurate estimates about how far the disease could spread in a year.”

By blending the results of previous smaller studies done in the eastern United States over the last few decades and the Penn State deer-dispersal research still in progress, Long found that in an area where the percentage of forest cover is lower, deer disperse longer distances. In more heavily forested cover, they don’t travel as far.

“So for example, in an area with 60 percent forest cover—Pennsylvania’s average—the average dispersal distance will be less than six miles,” says Long. “However, in an area with less than 30 percent forest cover, the dispersal distance would be 12 to 25 miles. We already have data about forest cover for any region of the state, so we could easily calculate how much forest exists and average deer-dispersal distance in those regions.

“It’s all about habitat,” Long continues. “That part is not surprising. But what did surprise us is that we didn’t find a relationship between deer density and dispersal. We expected that more deer would disperse in populations with greater density, which doesn’t seem to be the case.”

Game Commission biologist Chris Rosenberry, who conducted his doctoral research on deer dispersal in Maryland, says it is widely accepted that young male deer disperse for “sociobiological reasons,” such as maternal aggression and breeding competition among males, neither of which may be affected by population density. Scientists know that dispersal of young male deer is critical to the species because the phenomenon prevents inbreeding and protects a population’s gene pool.

“Although white-tailed deer are considered habitat generalists, forest cover is an important component of deer habitat because forest stands provide thermal cover, escape shelter, and food resources,” Long’s paper states. “In less-forested landscapes, then, deer may need to travel farther to find suitable habitat patches.”

Most studies on deer dispersal have had small sample sizes, according to Diefenbach. “The only large study that we know of was done more than a decade ago,” he says. “Our Penn State study is much larger. We have followed 442 yearling bucks over the course of three years from 2002 to 2004. Pennsylvania has a huge diversity of forest landscapes. So depending on where you are and the percentage of forest cover, the dispersal rate of juvenile male deer could range from just a couple miles to more than 15 miles.”

“These research findings and their application to CWD response plans demonstrate the importance of research in deer management,” says Robert Boyd, chief of the Game Commission’s research division. “Through the collaborative research program between the Game Commission and the Cooperative Fish and Wildlife Research Unit at Penn State, we continue to learn more about deer biology and its application to deer management.”