Wolves Come Home.
Winters of deep snow weren't the only deer-killers in DelGiudice's study. Seven wolf packs lived within the study area and preyed on some of the radio-collared deer. When DelGiudice began the study in 1991, wolves had just recently returned to the area, after having been extirpated from the woods around Grand Rapids in the 1960s. DelGiudice's team trapped and radio-collared wolves to determine their pack territorial boundaries, so they could ascertain which packs were killing which deer.
The ensuing data toppled several myths about how wolves affect deer populations. One myth held that wolves hurt deer populations by engaging in surplus-killing (taking more prey than they can eat, a behavior documented among carnivores worldwide). During the severe winter of 1995-'96, DelGiudice indeed documented incidences of surplus-killing by wolves, with some wolves taking down four deer within spans of 100 yards. But as he examined the fat reserves on the deer carcasses -- and evaluated their nutritional fitness from urea nitrogen in urine samples -- DelGiudice found that most of the wolf-killed deer were seriously undernourished and about to die anyway. The wolves were just finishing off deer on their deathbeds. Relatively little surplus-killing by wolves was observed during the following winter of 1996-'97, and none during any of the other 13 winters.
"Surplus-killing by wolves of healthy deer rarely occurred in our study," DelGiudice says.
His study also clearly demonstrated that wolves do not depress the female segment of the white-tailed deer population that hunters harvest in fall. While wolves and hunters each accounted for 24 percent of collared female deer mortality over the entire study, the two mostly killed different types of does at different times. The average age of a doe killed by hunters was less than 5 years old, whereas the average age of does taken by wolves was 8 years old. Age-specific analyses of the study clearly showed that compared with wolf predation, hunting had the greatest impact on younger does. Hunter-kills occurred during the fall hunting season, while wolves tended to kill deer in winter -- when deer are naturally undernourished, struggle in deeper snow, and are easier prey for wolves and their pack hunting strategy.
"Hunters tended to kill young deer, while wolves served the role of culling the herd of older deer or those that were weaker," says DelGiudice. "Hunters and wolves help to keep deer close to the carrying capacity of their habitat. And by ensuring that only the fittest deer survive, wolves help the deer herd to be stronger over the long term."
In the presence of seven wolf packs, the deer numbers in the region of DelGiudice's study rebounded. Between 1997 and 2003, hunters quadrupled their harvest in that area. By 2003 the annual harvest greatly exceeded pre-crash levels of the mid-1990s.
"These study results will help inform the DNR [when] Minnesota assumes responsibility for wolf management," says DelGiudice. Wolves in the western Great Lakes region are currently listed as a federal endangered species, though wolf populations are recovered and the U.S. Fish and Wildlife Service has been working to transfer management back to state jurisdictions. "[The study results] will also help Wisconsin and Michigan as they contemplate deer management in an expanding wolf range. And for any states considering the reintroduction of wolves, this study tells them how deer might respond."
