|More Hard Times for Isle Royale Moose
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|March 15, 2005--It's been another tough year for the moose of Isle Royale National Park, the home of a 47-year study of predators and their prey.|
As they battled summer heat, thick snowcover and a ferocious infestation of ticks, the number of moose on the Lake Superior island has dwindled from 1,100 in the winter of 2002-03 to 740 last year to the current 540.
"Moose numbers have been cut in half in the last three years," said Professor Rolf Peterson, of Michigan Technological University, who has led the study of Isle Royale's wolves and moose for 35 years. "That's pretty remarkable."
Meanwhile, the moose's only predators on the island seem to be doing just fine. Last year, the wolf population jumped nearly 50 percent, from 19 in 2003 to 29 in 2004, and has held relatively steady this year at 30.
The 132,000-acre island has given scientists a rare chance to study the seesawing relationship between wolves and moose in a closed system. Moose emigrated to the island from Canada in the early 1900s, probably swimming the 20 miles or so from the mainland. With no predators to keep them in check, the moose thrived until they ran out of their winter forage, typically balsam fir. Then they starved in vast numbers. This feast-and-famine cycle was broken in the late 1940s, when wolves came to the island.
Since then, wolves have helped keep a lid on the moose numbers. However, both species have grappled with other challenges ranging from disease (parvovirus killed off most of the wolf population between 1980 and 1982) to hot summers to the current plague of ticks.
"Last summer, the average moose had lost over 70 percent of its body hair due to ticks," said Research Assistant Professor John Vucetich, who has been involved in the Isle Royale study for about 10 years. "This is about twice the hair loss of just a few years ago. A moose with 70 percent hair loss could have been carrying in excess of 70,000 ticks during the winter and early spring."
A single moose can host several ticks per square inch, and each tick can suck up about a cubic centimeter of blood. Rather than feed, the moose scratch themselves against trees or bite their hair out trying to remove the parasites. Weight and blood loss may prove such a handicap that the moose don't survive the winter. (This particular species of tick doesn't pester humans; only moose at Isle Royale suffer their effects.)
The weakened moose are made doubly vulnerable in extra-snowy winters such as the last two, since they become mired in the deep snow and are easy prey for wolves.
"And underlying all their other problems is that Isle Royale is on a long, slow decline in terms of food for moose," Peterson said.
So far, the island's three wolf packs have been using the moose's hard times to their advantage. However, they could be riding the caboose of the gravy train.
"There were only 18 moose per wolf this year, the lowest ratio we've ever had," Vucetich said. That proportion is usually between 30 and 70 moose per wolf. On average, wolves may consume about seven moose a year apiece, so they could be eating themselves out of house and home. Or not. Their future has been difficult to predict.
"Can the wolves do it again?" Vucetich wonders. "Or will wolves take a huge hit next year because they're overextended?"
"There's no serious sign yet that wolves are running out of moose," Peterson notes. "The three packs aren't trespassing on each otherís territories when they hunt."
"The last time it seemed like moose were at the end of their rope, parvovirus came in and wiped out the wolves," he added. "We really haven't seen what wolves can do in this situation."
The study is funded by the National Park Service, the National Science Foundation and Earthwatch.
PHOTO INFORMATION: Photos of moose and wolves are available by clicking the links under "Related Files." Moose have black skin, and two photos show moose who have bitten or rubbed off patches of hair due while attempting to remove ticks. "Gang of Four" is of four wolves in winter.
The photos are copyrighted by John and Leah Vucetich, who should be credited if these images are reproduced.