"The wolves are struggling," said Rolf Peterson, a research professor with Michigan Technological University and co-director of the study. "There's definitely a chance they might not last much longer."
Meanwhile, the moose population rose to about 1,050, according to a report by Peterson and co-director John Vucetich obtained by The Associated Press ahead of its scheduled release. That's a sharp jump from the 515 counted in 2011.
The wolf's decline on the island chain comes as its numbers in the nearby states of Minnesota, Michigan and Wisconsin are also falling — but for entirely different reasons. The mainland population had risen so dramatically in recent decades that it exceeded 4,000 in the three states by 2012, leading the U.S. Fish and Wildlife Service to remove Great Lakes wolves from the endangered species list.
Since then, the three states have allowed managed hunts and their combined total has fallen to roughly 2,900. But officials say the populations are healthy and thriving, in contrast to Isle Royale's beleaguered remnant.
Wolves and moose are relatively recent arrivals to the 45-mile-long park, which consists of one large island and hundreds of smaller ones. Moose made their way there in the early 1900s, possibly by swimming 15 miles from Canada. Wolves are believed to have wandered there in the late 1940s over a winter ice bridge.
The moose have given the wolves a reliable food supply, while the wolves have culled weaker members of the moose herd and prevented it from getting too large. Wolf numbers have averaged about 23 over the years. But with the recent slump, their ability to limit moose growth is virtually nonexistent and will remain so for at least several more years, Vucetich said.
Demonstrating the wolf's declining influence, moose numbers rose from 975 last year to 1,050 despite a severe winter that limited the herbivore's access to the balsam fir that is its primary cold-weather food source.
The deep freeze interrupted what had been a warming trend in the region. If that longer-term pattern returns and the next few winters are mild, the moose population may skyrocket, Vucetich said. At some point, they may reach the point where they're severely over-browsing island vegetation, which has happened before.
In the 1990s, following another wolf decline, moose numbers reached about 2,400, leading to mass starvation during the harsh winter of 1996, when about two-thirds of the moose died.
"We know enough right now to know that moose are heading in that direction and it may not be long before they start causing damage to vegetation," Vucetich said.
He and Peterson are lobbying the National Park Service to bring more wolves to the island to refresh the gene pool. They say there is growing evidence that the wolves have lasted because newcomers periodically crossed ice bridges and mated with island wolves. Analyzing decades of field notes, they recently concluded that a pack of seven or eight wolves migrated to Isle Royale in 1967. A lone male immigrant in 1997 had such an effect that by 2008, most of the island's wolves were his descendants.
But ice bridges are forming less frequently, lowering the odds that more wolves will arrive on their own, the scientists said. Although ice provided a pathway for 25 days this winter, no wolves are believed to have traveled to the island and one used the occasion to escape, only to be shot dead on the mainland.
Park Superintendent Phyllis Green announced this month that officials had decided not to intervene as long as a breeding wolf population remains. Meanwhile, the agency will conduct a new environmental impact study on moose and their effects on the forest, as well as their relationship with wolves.
"Bringing wolves to the island remains an option," Green said. "However, the final decision will be based on the best available sound science, accurate fidelity to the law and long-term public interest."