VOYAGEURS NATIONAL PARK— Recently published research here suggests that beavers may make up a significantly larger percentage of the diet of gray wolves than has previously been documented, at …
VOYAGEURS NATIONAL PARK— Recently published research here suggests that beavers may make up a significantly larger percentage of the diet of gray wolves than has previously been documented, at least in regions with high beaver populations.
The study, by biologists Tom Gable and Steve Windells, tracked for several months the kills of a single radio-collared wolf from the Ash River pack, whose territory straddles the southern boundary of the park just south of Lake Kabetogama. Their research concludes that the wolf pack, comprised of four adult wolves, likely removed between 80-88 beavers from their territory between pond ice-out in April to late November, when ice cover returned. If so, that represented roughly 40 percent of the beaver population, a rate of mortality that is well above previous known levels, and would likely reduce the beaver population without in-migration from surrounding areas.
Previous studies of beaver predation by wolves elsewhere in North America and Europe have suggested that wolves may take less than 15 percent of the local beaver population, but Gable and Windells note that those studies have relied on analysis of wolf scat, which they argue can be unreliable.
The VNP study used the radio-collaring to directly link the studied wolf with verified or extrapolated beaver kills. During the course of the roughly seven-month study period, the researchers attributed 22 beaver kills to this single wolf. During the spring and fall, when beavers tend to be most vulnerable to wolf predation, beavers may comprise a majority of a wolf’s diet. In May, the researchers estimated that beavers comprised 60 percent of the wolf’s diet, and it made up just over 50 percent of the animal’s diet in September and October.
Wolf reliance on beavers dropped significantly in the summer months, comprising ten percent or less of the wolf’s diet from June through August.
Beavers appear to be most vulnerable to wolf predation in the spring, when they spend considerable time repairing and expanding their dams, as well as in the fall when they spend a lot of time on shore gathering their winter forage, which they stockpile near their lodges.
The researchers were able to document that the wolf spent a considerable amount of time in the vicinity of beaver ponds during the spring and fall. That finding was confirmed by radio-tracking of five other wolves from three other packs located in or adjacent to the park, which showed clusters of activity near beaver ponds during certain portions of the year.
While biologists have long recognized that beaver comprise a portion of the diet of wolves in many locations, little was known about how they hunt beaver. But the VNP study suggests wolves use three primary methods to hunt beaver. The wolves appear to lie in wait for extended periods in the vicinity of beaver ponds and will frequently ambush beavers. In the spring, they catch them most often while working on the back side of their dams. In the fall, they capture them most frequently on land while the beavers are gathering winter forage. At other times, it appears they jump in the water when beavers are near shore and drag them onto land.
The researchers acknowledge that the study is limited by its sample size. The researchers determined the total beaver consumption of the Ash River pack by assuming that the other three adult wolves consumed similar numbers of beavers.
The findings likely also reflect the high beaver population in and around Voyageurs National Park, which makes a high level of predation possible. According to the researchers, beaver ponds are abundant in their study area and the park has maintained a high beaver density for over 40 years, despite predation from wolves during that entire period.
The researchers speculate that the beaver population in their study area is replenished both through beaver births locally, as well as in-migration of juvenile beaver from neighboring areas, which may allow the population to remain stable and at a high density despite significant losses to wolves.