The Long View
Isle Royale National Park draws visitors who like nature rugged and remote. It's also the site of the longest-running study ever of a predator and its primary prey--even as global warming shows signs of upsetting the natural balance.
It's standing room only tonight in the Rock Harbor hall, unlike on my first visit in 1962. Rolf's aerial shots of wolf-moose encounters in midwinter, taken from an altitude of 500 feet with a long, stabilized telephoto lens--are breathtaking if, well, pretty vivid. Red on white. A wide-angle close-up of a coal-black raven guarding a moose carcass that's mostly skeleton elicits some laughs. Rolf's laptop computer--the Kodak is obsolete--projects a graph on the screen showing that there were 20 wolves and 570 moose on Isle Royale when the study began and that the score was 20 to 1,250 when he arrived. And as the biologist explains, predator and prey numbers cycled in tandem, with the wolf population peaking about a decade after the moose herd when there were a lot of aged animals to bring down on the snowy killing fields. (That's not as easy as it sounds. The front legs of a 900-pound moose are formidable weapons, and Rolf once watched an old, blind bull stand its ground against a pack of wolves for three days until they gave up and decided to look elsewhere for a meal.)
By the winter of 1980, he relates, a record high of 50 wolves in five packs prowled the island, while the moose herd had slowly declined to 750 animals. But researchers were astounded to find only 14 wolves when they arrived at their winter camp, at Windigo on the island's southwest end, in January 1982. Canine parvovirus, a mutant disease that would ravage domestic dogs on the mainland, had swept Isle Royale. National Park Service policy explicitly bans pets from the park. But as Rolf later learned, a clueless Chicago man had brought his sick dog along on a fishing trip over the previous Fourth of July holiday. For a decade, the wolf population staggered along at a dozen or so animals and grew top-heavy with old wolves that had little success at breeding. At times Rolf wondered if they were on a fast track to extinction. And the impact of the tragedy is still being felt.
During a lifetime of 10 to 12 years, an Isle Royale wolf will feed on 200 or more moose kills. But now that the packs were decimated by disease, the herd exploded as it had in the 1920s, peaking at 2,400 animals in 1995. The next winter was especially severe, with blizzards bearing down in December. Moose, infested with tens of thousands of blood-sucking winter ticks that can weaken even a big bull, struggled through three feet of snow to find scarce food. From his plane, Rolf watched the starving animals leaning over steep cliffs for a mouthful of browse--and falling into Lake Superior. "The real clincher," he tells me over coffee the next morning, "was that snowmelt occurred six to eight weeks later than usual. Moose simply ran out of fat and died before any new forage was available." Only 500 moose were left by the winter of 1997, most of the old animals had perished, calves were few, and the handful of surviving wolves were digging up old carcasses and eating the sun-dried hides.
The next few years proved less tumultuous. The island's battered forest was recovering, and the moose herd rebounded to an estimated 1,100 by the winter of 2003. The wolf population likewise surged, to 30 animals in 2005. By then, however, moose numbers were again heading downhill and wolves followed their slide due to a lack of prey. Last winter's survey tallied just 385 moose and 21 lean wolves. And John Vucetich, a population biologist at Michigan Tech who has been Rolf's research partner since 2000, blames climate change. Five of the past six summers, he notes, were the warmest since the Isle Royale study began in 1958. "Hot summers are particularly hard on moose," Vucetich explains. "They tend to rest more and don't eat enough forage to get through a long, bitter winter." A warm spring or fall, he adds, can also trigger a devastating tick infestation. It's not unusual for a hiker to encounter moose that have lost most of their hair because of ticks. Another sign of global warming: No snow fell on Isle Royale for a month last winter when researchers were at the Windigo camp.
Playing devil's advocate, I ask Rolf if there's anything more to be learned by continuing the study into another decade or beyond. "We're not simply watching moose and wolf populations go up and down," he answers. "The dynamics of the predator-prey relationship on Isle Royale are extremely complex, involving subtleties of weather, parasites, snow conditions, the food base. That's just for starters. Then pivotal events occur with effects that are felt for decades. Finally, each 10-year period of this study has been unique, bearing little resemblance to other decades. So we don't put much stock in predictions anymore. That's especially true in a world where accelerating climate change will influence virtually everything."