A tale of wolves, moose and missing ice

moose in Isle Royale

With few wolves left on Isle Royale, the moose population has been growing.

Ray Dumas/Flickr (CC BY-SA 2.0)

Isle Royale, 200 square miles of forested island, sits in the middle of Lake Superior, just south of the line that divides the United States from Canada. No one lives there — it’s a national park — but from mid-April through October, visitors can hike, canoe and otherwise enjoy the remote location. During the winter, though, the island is left to the moose (which number in the hundreds), the wolves that prey on them and a small band of scientists conducting the longest continuous predator-prey study in the world.

The fate of that study is in question not because of funding or lack of interest, but because the wolf population has grown tiny and is suffering from inbreeding, and the wolves’ survival is not guaranteed. Their disappearance would have consequences for the island’s moose, trees and the rest of the ecosystem.

What happens may depend on ice. Or on us.

The wolves and moose are relatively new additions to Isle Royale. The moose arrived first, in the early 20th century, and “their numbers fluctuated with weather conditions and food abundance,” notes the Isle Royale project website. The wolves showed up in the late 1940s, crossing an ice bridge that connected the island with Canada one winter. A predator-prey cycle began, with wolf and moose numbers fluctuating in response to one another. The scientists began watching all this in 1958.

Wolves were once far more numerous on Isle Royale, and they could easily gang up on the much larger moose. L. David Mech/Wikimedia Commons
The wolves have never numbered more than about 50, but their small numbers were able to overcome problems of inbreeding through irregular additions of new wolves from the mainland. Like the founders, those wolves made their way over to the island by crossing an ice bridge.

For example, a family tree based on DNA analysis of fecal samples from the wolves told researchers that the wolves got a DNA boost from an interloper, named Old Grey Guy, who moved over from Canada in 1997. That “genetic rescue” helped the wolves limp along for two to three generations, but inbreeding is a problem again, project researchers reported last month in Conservation Genetics.

In the last decade, the wolf population has crashed. As of the most recent count this past winter [pdf], there are only nine wolves on the island, split into two packs. Six belong to the West Pack. Another three — an elderly female wolf and her male and female offspring — belong to the Chippewa Harbor Group. The alpha male of that group drowned in a flooded copper mine shaft in late 2011, and there have been no pups since. But if the younger male and female reproduce, their offspring will be “particularly inbred,” the scientists write in their latest annual report.

The wolves need new blood. Without it, they’ll die out, and the moose will proliferate. The moose eat balsam fir, and with nothing to keep their numbers in check, they could destroy all the island’s fir trees, the researchers warn.

Climate change has reduced the frequency of winters cold enough to produce ice bridges for wolves to cross onto the island. And even when there is a bridge, new wolves aren’t guaranteed to arrive. During this past winter, for instance, one wolf crossed from Isle Royale to the mainland; it was found dead in Minnesota in January.

The wolf project researchers argue in Conservation Genetics that “human-assisted gene flow is necessary” to conserve the island — in other words, humans should bring in some wolves from off-island to boost the packs already there.

This is a dilemma that conservationists will face more and more — what is to be done when natural populations are threatened by climate change? We can’t save everything. When do we intervene, and when do we let nature take its course?

Editor’s note: This article was updated on May 17 to correct the area of Isle Royale.

Sarah Zielinski

Sarah Zielinski is managing editor of Science News for Students. She has a B.A. in biology from Cornell University and an M.A. in journalism from New York University. She writes about ecology, plants and animals.

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