The delicate dance of 2 species, and climate change

Posted: Sunday, April 20, 2008

ISLE ROYALE NATIONAL PARK, Mich. - Ignoring our observation plane circling above the frozen Lake Superior wilderness, the eight gray wolves seemed as harmless as pooches cavorting in the yard. They nipped and pawed each other, pausing occasionally to roll in the snow.

But then the alpha male and female moved away from the shore. They passed through a clearing and plunged into thick woods, the others strung out behind.

They had eaten little for three days. Now they needed to hunt.

A mile northwest, a moose calf lumbered amid evergreen stands, unaware that the pack was closing in.

Overhead, John Vucetich watched intently, though he knew what was coming: the violent climax of a drama at the center of one of the world's longest studies of a predator and its primary prey.

Vucetich, of Michigan Tech University, is co-leader of a team closely monitoring Isle Royale's moose and wolves for five decades. Both species have had ups and downs, but now may be facing their biggest threat.

Declines in pack and herd populations, coming as average temperatures have been rising, make the scientists wonder if global warming may be writing a new story line for the narrative playing out as the plane followed the hungry pack below.

A healthy adult moose can repel enemies with fierce kicks, which is why wolves' preferred targets are the old, the sick and the calves - like the one that was coming into view.

The pack suddenly attacked. One wolf got a solid grip on the snout, another latched onto the hind quarters, and two advanced broadside.

For several days, the wolves would feast on their kill in the blood-soaked snow.

"They'll be fat and happy," Vucetich said.

Until they'd get hungry again.


Neither moose nor wolves are native to Isle Royale, actually a rugged archipelago - one 45-mile-long island and hundreds of smaller ones on the giant lake's northwestern side.

Around 1900, a few moose managed the 15-mile swim from Ontario. They multiplied rapidly, and by mid-century had so overbrowsed the forest that starvation loomed. That's when a wolf pair journeyed to Isle Royale across a rare ice bridge.

The two species formed a bond of interdependence that Purdue University conservation biologist Durwood Allen began studying in 1958. Ecologist Rolf Peterson arrived as a grad student a dozen years later and eventually took over, aided by proteges, including Vucetich.

They have made Isle Royale a gold standard for documenting symbiotic relationships between predator and prey species and their natural surroundings.

"It's the most well-known wolf study in the world," said Douglas Smith, a project alumnus who now directs wolf research at Yellowstone National Park.

Smith credits Peterson with debunking numerous myths about wolves, including their image as indiscriminate thrill killers. They're actually quite choosy, culling weaker members of herd species.

"There are such strong feelings about wolves, so much of it based on fear instead of facts," said Sharee Johnson, a director of the International Wolf Center in Ely, Minn. "One of the biggest fears is that wolves take too many prey animals. The Isle Royale study shows us definitively that that's not the case, that it balances out."

Peterson and Vucetich say the biggest long-term danger to the island's moose is not the wolf.

Climate change is the likely culprit behind a dropoff in moose numbers over the past decade. Isle Royale is on the southern edge of their range, and recent summers have been the hottest since the study began. The moose are showing signs of stress.

No one is predicting their demise for now. But if they decline much further, the wolves - which rely almost entirely on moose for food - could disappear.


Bones and teeth are storehouses of information about the animals and their environment. Increasingly, they are showing elevated levels of the carbon isotope linked with global warming.

Peterson retired a couple of years ago from his Michigan Tech professorship but continues his work on Isle Royale. Even after nearly four decades, it's never grown stale to him.

"Somebody introduced me at a talk and said, 'He's going to die out here,"' he said wryly.

The wiry 58-year-old squirmed through thick stands of alder, cedar and spruce. A few days earlier, walking alone in the rugged terrain, he'd plunged through a weak spot in the ice.

Peterson reached the site after a 90-minute tramp.

Bloodstains and paw prints marked the spot of the kill. Following drag marks down a dry creekbed, he found a thick mat of brownish-gray fur and scattered bones, which he stowed in plastic bags. He did likewise with scat piles and made notes, fingers stiff with cold.

Then back to camp.

The Michigan Tech team has collected bones of more than 4,300 moose over the project's 50 years. They offer hints about why the animals died: low fat levels in marrow indicate malnourishment; arthritic malformations suggest inability to outrun wolves.

Such details are a treasure trove for wildlife biologists with an interest in predator and prey species.

And they're valuable clues for Peterson and Vucetich, who have a mystery to solve. The moose population has nosedived in recent years. They need to know why.


For most of the 50-year study period, scientists have observed a pattern. When wolf numbers were low, moose flourished and the herd aged. That boosted kill opportunities for wolves, whose numbers rose as the moose declined, giving the island's vegetation time to recover from overbrowsing. In time, a shortage of moose would push wolf numbers down again. And so on.

A parvovirus outbreak two decades ago nearly killed off the wolves. The moose population then exploded, only to tumble from 2,400 to 500 during the winter of 1996, when deep snow made food hard to find.

They began to recover, but in recent years have fallen again. Peterson and Vucetich estimated the herd at 385 in 2007, the lowest since the study began. The wolf count fell from 30 the previous year to 21.

And the animals began acting strangely.

Moose need to fatten up in summer, but instead have been wading in the water, lounging in the shade. Wolves, which instinctively shun people, have wandered into campsites.

The biologists figure it must be the heat.

It makes the moose lazy and worsens an onslaught of ticks, which infest the big beasts; they waste feeding time biting their fur and rubbing against trees. Their dropoff has made the wolves more desperate for food and less shy around humans.

Will either species be around in 50 years? No one knows - but don't count them out.

Peterson and Vucetich headed to camp in January expecting to find them down further. Instead, there were 23 wolves - two more than during the previous winter.

Later came another twist: The moose census produced an estimate of 650, up 40 percent. They may have undercounted the moose in 2007, when lack of snow made it harder to see them from the air.

Even so, the herd remains well below its historical average, and the biologists still expect wolf numbers to fall.


The sky was dark gray the next morning. Arctic gusts rolled in from Canada, making it too dangerous to fly.

Sometimes bad weather grounds the scientists for days. But there's always work to be done.

John and Leah Vucetich entered data into a computer spreadsheet. Peterson tinkered with a video camera they'd begun concealing at kill sites to catch images of scavenging wolves.

Always more to learn.

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