The die-off is most dire in Minnesota, where ecologists say moose could be gone within a decade. But it extends across the southern edge of the animal's global range: Populations are falling as far away as Sweden.
No single cause seems to be responsible. In Minnesota, many moose seem to be dying of parasitic worms called liver flukes; in Wyoming, some researchers are pointing to a worm that blocks the moose's carotid arteries; in New Hampshire, massive tick infections seem to be the culprit. This diversity of reasons makes some experts think they need to dig deeper.
"The fact that you've got different proximate causes killing off the moose suggests there's an underlying ultimate cause," says Dennis Murray, a population ecologist at Trent University in Canada.
Murray suspects that the underlying cause is climate change. Moose are adapted to the bitter cold of northern climates, and those living farther north in Canada and in northern Scandinavia appear healthy for the most part. But moose in southerly habitats can become heat-stressed when the weather gets warm. This prevents them from building body-fat reserves to help them survive the winter. Heat stress may also weaken their immune systems and make them more susceptible to parasites, a link that is well established for cattle in Africa.
Indeed, Murray and his colleagues have found that moose populations in Minnesota decline more quickly in years with warmer summers. Parasites - and their main hosts, white-tailed deer - are also more likely to survive the milder winters of recent years, says Ron Moen of the University of Minnesota at Duluth.
Researchers have yet to prove a link to climate change. But Murray notes that lynx and snowshoe hares also are declining in the southern parts of their ranges. "We're in the process of seeing a pretty dramatic change in the distribution of the boreal forest ecosystem," he says.
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