Warmer winters hurting moose
ISLE ROYALE:Wolf numbers are up and moose numbers are down, according to the annual survey at the national park.
BY JOHN MYERS
NEWS TRIBUNE STAFF WRITER
Wolf numbers are up and moose are declining on Isle Royale, according to the 46th annual survey in the world’s longest study of predator-prey relationships.
The moose population has slid to 750 on the Lake Superior island, down from 900 last year and 1,100 in 2002.
Moose are faring better than when their numbers bottomed out at 500 after the deep-snow winter of 1995-96, when hundreds perished. But their numbers are in question because of ticks, heat stress and declining habitat.
There are 29 wolves on the island in three healthy packs, the most the national park has seen since 1980 and 11 more than last year. The total includes 12 pups from 2003 that survived this winter.
While the trend of sickly moose has meant easy pickings for wolves, a continued crash in the moose population could cut wolf numbers as well.
Dr. Rolf Peterson, a wildlife biologist with Michigan Tech University who has led the winter survey for the past 33 years, said the island’s moose may be suffering from a period of higher than usual temperatures from 1998 through 2001 and beyond.
“What we think is happening is that wolves are cashing in on moose vulnerability that’s been induced by a warmer climate,” he said. “Moose can’t feed in the summertime if it’s too hot… they have a big fur coat on, and they can’t sweat. They just sit in the shade or in the water.”
When moose don’t eat enough in the summer, they can become weak, sickly and easy prey for wolves during the winter. Moreover, warmer springs and autumns help another wolf nemesis thrive — winter ticks. If there is no snow in the fall and snow melts earlier in the spring, more ticks survive.
A moose can be host to up to 70,000 winter ticks at a time, several per square inch, said Canadian moose researcher Bill Samuels. Moose scratch themselves against trees or bite their hair out trying to remove the parasites. Weight loss and blood loss often are fatal.
Winter ticks are different from wood ticks and only prey on moose.
Warmer weather isn’t all bad for moose: Snow doesn’t get as deep, and there’s more growth of the trees on which they feed. But the negatives outweigh the positives, Peterson said.
The past decade was the warmest on Earth since records have been kept, and regional average temperatures have increased markedly. While researchers can’t prove that increased carbon dioxide and other human-caused gases are warming the Earth’s atmosphere, Peterson said, computer models can account for the higher temperatures only if greenhouse gases are included.
Mark Lenarz, forest wildlife research leader for the Minnesota Department of Natural Resources in Grand Rapids, agreed that winter ticks have been a problem on Minnesota moose as well. Nearly all of the 60 moose captured for an ongoing study in Northeastern Minnesota during the past two years have at least some hair loss from ticks.
Moreover, Lenarz said, moose in the region may be more susceptible to warmer weather patterns.
“This is the southern edge of moose dispersal (range), so any increase in temperature trends may be enough to affect them here,” he said.
The Northeastern Minnesota moose population has been steady, albeit at lower than historical levels, for the past few years. Researchers are trying to determine why a higher than usual number of Minnesota moose are dying, Lenarz said. The mortality rate is about 30 percent, compared with about 10 percent in most moose areas. Brainworms carried by the region’s increasing deer herd, liver flukes, decreasing copper levels in their blood, low pregnancy rates and ticks may be factors, he said.
On Isle Royale, about 20 miles off Minnesota’s North Shore, moose also face a declining number of balsam trees, a staple food in their diets at times.
Moose numbers hit a high of 2,422 in 1995 and bottomed out at 500 in 1996.
Wolves are relatively new to the 45-mile-long, 143,000-acre Michigan island, having crossed Lake Superior ice to get there in 1949. Their numbers have ranged from 11 in 1993 to 50 in 1980.
Peterson tracks moose and wolves to see what effect changes in one species have on the other, all in an environment having little human interference and no competing species such as deer or bear. The animals can’t leave the island, and there are no vehicles, poachers or hunting to affect the population.
If there was an equilibrium between the species on the island, it would be about 25 wolves and 1,000 moose, Peterson has said. But that level is almost never reached — one of the two species is almost always out of balance.