TRAVERSE CITY, Mich. — Federal officials announced a tentative plan Friday to relocate 20-30 gray wolves to Isle Royale National Park in Michigan over three years to replenish a population that has nearly died out because of inbreeding and disease.
The National Park Service said it would make a final decision after giving the public a month to react to its proposal for rescuing the predator species that has roamed the Lake Superior wilderness park for about 70 years. The wolves have helped to maintain the ecosystem by culling a moose herd that otherwise would overeat the island's vegetation, while delighting tourists with their eerie howls and occasional appearances on backwoods hiking trails.
But their numbers have plummeted in recent years as a warming climate formed fewer ice bridges for mainland wolves to reach the park 15 miles (25 kilometers) offshore and refresh the gene pool. Only two remained this winter — a father and daughter that apparently have not bred.
"It's been heartbreaking to witness the harm that a lack of genetic diversity has caused on Isle Royale," said Nicole Paquette, vice president of wildlife with The Humane Society of the United States, which supports the park service plan.
Wolves are believed to have crossed the frozen lake surface from Minnesota or the Canadian province of Ontario in the late 1940s, about half a century after moose had arrived. They are the subject of one of the world's longest-running studies of the relationship between a predator and prey in a closed ecosystem, led by scientists at Michigan Technological University.
Wolf numbers averaged in the low- to mid-20s, divided into several packs, before going into a deep and apparently irreversible slump.
As it became evident that the wolves wouldn't recover on their own, scientists and advocates have debated whether people should intervene or let nature take its course, the latter being standard policy in federal wilderness areas.
"The question is: If you want to keep the ecosystem intact, what is the best way to help it retain its natural processes and at what cost?" said Phyllis Green, the park superintendent. Rounding up 20-30 mainland wolves and transporting them to Isle Royale "is definitely the alternative that all of our experts we convened felt was preferred," she said.
The environmental impact statement released Friday estimated the wolf restoration price at nearly $2 million over 20 years.
The park service received nearly 5,000 comments on its draft plan released in December 2016. It offered several options that also included taking a smaller number of wolves — perhaps 15 — to the park in the short term and adding more over a 20-year period, or doing nothing immediately while keeping the door open to adding wolves later. Or simply letting them die off.
But the park service opted for taking a larger number of wolves to the island sooner, which Green said should ensure the population's survival for the foreseeable future, although their long-term prospects remain uncertain.
"Things are changing on the island in ways that none of us will be able to stop," Green said. "I think we'll make it through the next 20 years or so and see how the weather is warming up and what the moose population is."
For now, moose are flourishing on Isle Royale, their numbers estimated at 1,600 last year. But some scientists have raised concerns about how climate change could affect moose in the northern U.S.
"Restoring wolves to their pivotal role in Isle Royale National Park is the right thing to do," said Michael Robinson, conservation advocate with the Center for Biological Diversity.
Franz Camenzind, a wildlife biologist and board vice president with Wilderness Watch, argued for a hands-off approach, saying the park is too small to host a genetically healthy wolf population indefinitely and that another rescue would probably be needed in the future. And as climate change alters the island's plant life, the moose may need a bailout too.
"Allow Isle Royale to be a wilderness park, let its future be shaped as it was during its not so distant past — by nature's forces," Camenzind said .