The delicate balance between wolves and moose in Michigan's Isle Royale National Park has been a case study in ecology textbooks for years. Now that famous predator-prey relationship may be in jeopardy: Yesterday, biologists announced that half the island's wolves have died since last winter.
Wolves first colonized Lake Superior's Isle Royale in the late 1940s, perhaps after a few had walked to the island across a stretch of the lake that often freezes in midwinter. Wolves have thrived since then, dining primarily on old or sickly moose. The isolated ecosystem may appear simple, but biologists have had a notoriously hard time predicting fluctuations in the number of Isle Royale wolves.
Last winter, the wolf population had swelled to 24, a surprisingly large number considering that 80% of the island's moose had died during the particularly harsh winter the year before. Rolf Peterson, a wildlife biologist at Michigan Technological University in Houghton, says that because only the strongest and healthiest moose survived, his team had expected a decline in wolves last year. Instead, the drop-off happened this year: Peterson's team has spotted only 13 wolves, even though there are actually more moose for them to eat--the moose population jumped from 500 to 700.
"The only time we've seen anything like this was when a new disease arrived on the island," Peterson says. Parvovirus, which inflames the wolf's intestines and causes death by diarrhea within 24 hours, was introduced to the island in 1981, possibly by a visiting dog, and killed nearly half the wolves before disappearing a few years later. Peterson says parvo is unlikely to have killed wolves this time, as survivors of the last outbreak should have acquired immunity. Nevertheless, his team plans to capture a wolf this summer and check its blood for signs of the virus and other diseases.
The vicissitudes of wolf life on Isle Royale are no surprise to biologist Todd Fuller, of the University of Massachusetts, Amherst. "After every 10-year period, they think they've got it figured, then something else happens," he says. He notes that the wolves' extensive inbreeding has made them all genetically similar, which means they could all share the same Achilles' heel.