Lean Times on Isle Royale
March 8, 2006—
It's shaping up to be a lean year for the wolves and moose of Isle Royale National Park.
The number of moose has sunk to 450, the lowest since researchers began tracking their numbers on this wilderness Lake Superior archipelago. Now in its 48th year, the project is the world's longest-running study of predator-prey relationships.
"The moose are probably hurting because there are so many wolves," said Assistant Professor John Vucetich, who co-leads this ongoing Michigan Technological University investigation with Professor Rolf Peterson. While moose have declined, wolf numbers have been on the rise for the past several years, topping out at 30 in 2004-05 and holding steady this year.
For the last few years, the island's moose have been weakened by a plague of ticks, which distracted them from feeding and caused hair and blood loss. The ticks do not attach to humans, but they have been a bane for the moose, making them easy prey for the wolves, who have had relatively easy pickings since 2002-03, when the moose population stood at 1,100. Now, however, most of the older moose have died, so wolves have had to rely even more than usual on moose calves.
"We had two dozen moose die while we were there this winter," Peterson said. "Moose are really going to go down, unless the wolves back off."
The numbers only tell part of the story. As their primary food source dwindles, Isle Royale's wolf packs are reacting to the strain.
"They're showing signs of turmoil and chaos," Vucetich said.
In addition to a handful of lone animals, Isle Royale is home to three wolf packs: the Middle Pack, the East Pack and the Chippewa Harbor Pack. This year, the East Pack has been trespassing on the Chippewa Harbor Pack's territory, and Vucetich witnessed some of the hostilities while on a routine observation flight over the area Jan. 31.
The Chippewa Harbor Pack had brought down a moose in its own territory, and the alpha male and another wolf were lying on the remains of the carcass, chewing the bones. Eight members of the East Pack were trotting along a snow-covered beach about a mile away when they abruptly changed direction and began crossing rough terrain in the direction of the moose carcass.
"You knew they were up to something," Vucetich recalls. "Then they rallied at an open pond, where they started to howl and sort of pump each other up."
Amazingly, the two Chippewa Harbor Pack wolves, who were well within earshot, took no notice of the commotion. Meanwhile, the East Pack wolves closed in on them and attacked. Both Chippewa Harbor Pack members tried to escape, but the East Pack chased down the alpha male and killed him on the spot.
"They ambushed him," Vucetich said. "He didn't have a chance."
Vucetich watched as the sole survivor fled to join the rest of the pack.
"For 15 or 20 minutes, he was interacting with his pack mates, trying to get them to get out of there," he said. "This was a subordinate trying to tell the alpha female that her mate was gone. Then, finally, they all got up and started to walk away." The pack retreated to the center of their territory, where they spent the next two days sleeping. "During the month that followed, Chippewa Harbor Pack lost about half of its territory to East Pack."
"Within a couple weeks, the alpha female was being courted," Peterson said. "She lost that alpha male just weeks before they would have mated, and a wolf can't lose a year of reproduction; that's totally unheard of."
This inter-pack warfare is not gratuitous, scientists say. It's a struggle for control of a diminishing food supply. "The wolves are short on moose, period, and they are really short on old moose," Peterson said. "Last winter, most moose they killed were 13 years or older, and now they are on the tag end of that baby boom generation, with nothing coming behind them."
"There are about 15 moose for every wolf now, and normally, there'd be about 40 or 50."
There's no reason to think that the moose are going to pop out of their population slump anytime soon, Peterson said. In addition to pressure from the wolves, their habitat has declined over the years, with old spruce and balsam fir slowly replacing the lusher birch and aspen forests.
Another sign of trouble is that moose are eating snow, a signal that they aren't getting enough food to provide them with the water they need. "We didn't see that 20 years ago," Peterson noted. "And they're also eating lichens. A 1,000 pound animal eating lichens! It's like eating dust."
The study is funded by the National Park Service, the National Science Foundation and Earthwatch.
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