Newswise — Wolves are up and moose are down this spring at Isle Royale National Park, the home of a 46-year study of predators and their prey. Researchers suspect that a global warming trend may be behind the shift.
The moose population has slid to 750 on this Lake Superior wilderness island park, down from 900 last year and 1,100 in 2002. In the meantime, the number of wolves has seesawed upward over the past decade and is now up to 29, as many as the park has seen since 1980 and 11 more than last year.
What's bad for moose has been good for the wolves, and moose throughout North America have been hit hard by warmer temperatures that began in 1998 with El Nino and never let up, according to Professor Rolf Peterson of Michigan Technological University, who has lead the study of Isle Royale's wolves and moose for 34 years.
"What we think is happening is that wolves are cashing in on moose vulnerability that's been induced by a warmer climate," Peterson said.
The moose population has been stressed by higher temperatures, particularly the drought of 1998 and then warm fall of 2001. "Moose can't feed in the summertime if it's too hot," Peterson said. "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 summer, they can become weak, sickly and easy prey for wolves during the winter.
And heat precipitates another blight for the big herbivores: ticks.
"Warm weather in spring and fall leads to ticks the following winter, and ticks can kill moose," Peterson said. A single moose can be host to tens of thousands at a time, several per square inch, and each tick can suck up about a cubic centimeter of blood. Rather than browse, the moose scratch themselves against trees or bite their hair out trying to remove the parasites. Weight and blood loss often prove such a handicap that the moose don't survive.
As the moose population struggles against the heat and ticks, the wolves have thrived, largely because it's been easier for them to bring down their biggest prey. "The wolves are killing about twice as many moose as they did last year," Peterson says, which allows them to maintain their peak population.
Initially, researchers didn't know what warmer temperatures would mean for Isle Royale's wolves and moose.
"In this region the change has involved warmer winters, especially in the late 1990s and early years of this century," Peterson said. "We couldn't anticipate the effect for moose, because warmer winters mean less snow and more tree growth, which helps them.
"But it also leads to more ticks, and it impacts their feeding. With two pluses and two minuses, there was no way to forecast how it would come out in the wash. But it looks like it might be to the detriment of moose."
As to what has been causing the warmer temperatures, Peterson cites climate models presented in the journal Science that suggest that greenhouse gases may be the culprit. While researchers do not have proof that increasing levels of carbon dioxide and other gases are warming the Earth's atmosphere, the best computer models of climate can only account for such higher temperatures if greenhouse gases are included.
Peterson's study is funded by the National Park Service, the National Science Foundation, and Earthwatch.