HOUGHTON, Mich.--After several years of concern about whether wolves would survive the ravages of canine parvovirus on Isle Royale National Park, park officials and researchers have now turned the focus of their attention to the island's moose herd.
Park Superintendent Douglas Barnard says that a survey of moose and wolf populations completed this winter shows only about 500 moose on the island, compared to an estimated 2,400 in 1995. "We knew that we lost a lot of moose during the especially long, harsh winter of '95 - '96," says Barnard, "but no one anticipated that losses would be this dramatic. After completing field work on the island in March of 1996, biologists estimated that there were 1,000 to 1,200 moose within the park. By then, surviving moose were severely stressed from lack of food, deep snows, and a heavy infestation of winter ticks. The moose population was dealt yet another blow, the late arrival of spring greenup, which resulted in additional moose mortality from starvation.
Dr. Rolf Peterson, a wildlife biologist at Michigan Technological University, conducts the annual survey for the National Park Service. Peterson notes that the park's moose population had grown dramatically over the past 15 years. This was due to a population crash in '81 - '82 of their only predator, wolves. That wolf decline coincided with the arrival of canine parvovirus in the region. The dynamic relationship between moose, wolves, their environment, and available food sometimes results in extreme population fluctuations.
"The carrying capacity of the moose range on Isle Royale has been greatly exceeded, resulting in serious overbrowsing that grew worse each year as moose continued to increase," says Peterson. "The length of last winter proved especially costly for calves and older, weaker moose, but it also killed a surprising number of young adults."
Peterson says the only thing that saved most of the moose that survived was an abundance of balsam fir browse on the east end of the island. Interestingly, he says, those trees grew up when wolves had reduced the moose herd to 700-800 animals back in the 1970s. Under the best conditions, mountain ash, aspen, and willow are the favored browse species of moose, but when those species are not available, moose will turn to balsam fir and other less nutritious species.
Peterson says the reduction in moose is good from the standpoint that it gives the island's vegetation a chance to recover from years of overbrowsing. He expects moose numbers to increase slightly if this year's calf production is good, but says wolves should play a significant role in determining the size of the moose herd in the foreseeable future.
"Everything depends on the wolves now," says Peterson. "There are so few moose that if wolves are able to continue increasing, they should be able to keep moose at a relatively low level.
While the Isle Royale moose have had their problems in the past couple of years, the park's wolves have been gradually building their numbers. Peterson says there are 24 wolves on Isle Royale now, and that's two more than last year and 12 more than the all time low of 12 recorded in 1988-89 and again in 1991-92.
"Seven pups survived from last year's litters, so despite five deaths during the year, the total number of wolves increased by two," he says. "The East Pack produced two young, the Middle Pack successfully raised three pups, and a lone pair of wolves had two pups; only the West Pack failed to produce any young." He says the Middle Pack now has nine members, making it the largest pack on the island.
Despite their success, wolves had to work hard for every morsel of food this winter, according to the MTU biologist. "Some even dug up carcasses of moose that had died last year and ate their dried up hides. I'd never seen that before."
The annual wolf-moose study on Isle Royale is funded by the National Park Service, the National Science Foundation, and Earthwatch.