Scientists who study wolves at Isle Royale National Park say no new wolf pups were born last year, and the population is dwindling.
While wolves are thriving on the mainland, just eight remain on the island, biologist John Vucetich of Michigan Technological University said.
"In the 40 years that we've been able to monitor reproduction carefully, this is the first year that it appears no pups have been born," Vucetich said.
He said the Isle Royale wolves stopped reproducing because they're inbred. The last time fresh wolf DNA was introduced to the island was 16 years ago, when a lone male crossed the ice from Ontario.
However, the moose population is growing on the island, and without a natural predator Vucetich says moose could devour the island's vegetation.
The National Park Service is expected to decide later this year whether to introduce more wolves, or let nature run its course.
"Most scientists would say wherever there are moose, wherever there are these large herbivores like on Isle Royale, there should also be a top predator," Vucetich said.