TRAVERSE CITY, Mich. (AP) -- Mercilessly hounded by blood-sucking ticks, the Isle Royale moose herd is on a downward spiral -- and the wolf packs that roam the national park in Lake Superior are taking advantage.
The moose population fell to about 540 this winter, down from 740 last year and 1,100 during the winter of 2002-03, wildlife biologist Rolf Peterson of Michigan Tech University said Tuesday. Meanwhile, wolf numbers jumped from 19 to 29 last season and reached 30 this year.
Although at their lowest ebb in nearly a decade, the moose have not reached a crisis point, Peterson said. They bounced back in the mid-1990s after plummeting from 2,500 to 500 within a couple of years.
Still, scientists say the animals might be feeling the effects of long-term challenges such as global warming and a shortage of balsam fir, their primary food source in winter.
"The moose are not in danger of total annihilation, but we could expect lower density in coming years,'' said John Vucetich, another Michigan Tech researcher. "The interesting question is how low they'll go, and whether it will get low enough to affect the wolves.''
Scientists have spent decades studying the predator-prey relationship on Isle Royale, a 45-mile-long archipelago. Moose are believed to have migrated to the island from Canada in the early 1900s, possibly by swimming. Wolves likely crossed an ice bridge nearly a half-century later.
Both species have gone up and down since then, influenced by factors such as illness, weather, food availability and parasites.
A recent warming trend has borne a bumper crop of ticks. Tens of thousands can attach themselves to a single moose, each sucking a cubic centimeter of blood.
The agonized moose spend so much time rubbing against trees and biting their hair, they neglect to fatten up for winter. Weak from hunger and blood loss, they are less capable of fighting off wolves -- particularly when the snow is thick and crusty because of midwinter thaws, as it was this year.
"Moose just hate crusty snow,'' Peterson said. "It makes their footing uncertain, it cuts their shins. The wolves just dance around on top of it.''
Meanwhile, the balsam fir is slowly dying out on the western two-thirds of the island, Vucetich said. Moose keep munching the tops, preventing them from reaching heights needed to produce seeds for new generations of trees.
If the trend continues, moose either will find a new food source or become scarce on the park's western side, he said.
In the short term, the moose's troubles are an advantage for the wolves. Although 10 died in the past year, 11 pups were born, pushing the total population to 30.
That's a big comeback from the early 1990s, when their numbers fell to 12 and biologists feared extinction.
But if the moose dropoff continues, the wolves may begin running short of prey and invading each other's territory, leading to their own decline. The island had only 18 moose for every wolf this year; the ratio usually is between 30 and 70 moose per wolf.
"This situation may help us find out just how many moose it takes to support a wolf pack,'' Vucetich said.
