As the Arctic warms, beavers are moving in

It started decades ago, with a few hardy pioneers slogging north across the tundra. It is said that a person walked so far to get there that he rubbed the skin off the underside of his long, flat tail. Today, its species has homes and colonies scattered across the tundra in Alaska and Canada – and their numbers are increasing. Beavers have found their way to the far north.

It is not yet clear what these new inhabitants mean for the Arctic ecosystem, but concern is mounting and locals and scientists are paying close attention. Researchers have observed the dam beavers building accelerated changes already in play due to a warming climate. Indigenous peoples fear the dams could threaten the migration of fish species they depend on.

“Beavers really do change ecosystems,” said Thomas Jung, senior wildlife biologist for the Canadian government of Yukon. In fact, their ability to transform landscapes is perhaps second only to humans: Before they were nearly wiped out by trappers, millions of beavers used to power North American waters. In temperate regions, beaver dams affect everything from the height of the water table to the types of shrubs and trees that grow.

Until a few decades ago, the northern edge of beaver territory was defined by boreal forest, as beavers depend on woody plants for food and materials to build their dams and huts. But rapid warming in the Arctic has made the tundra more hospitable to the big rodents: Earlier melting snow, thawing permafrost and a longer growing season have led to a boom in shrubby plants like alders and willows that beavers need.

Aerial photos from the 1950s showed no beaver ponds in Arctic Alaska at all. But in a recent study, Ken Tape, an ecologist at the University of Alaska Fairbanks, scanned satellite images of nearly every stream, river and lake in the Alaskan tundra and found 11,377 beaver ponds.

Further expansion may be unavoidable.

Beaver hotspots

All these new dams could do much more than change the flow of streams. “We know that beaver dams create warm areas,” Tape explains, “because the water in the ponds they create is deeper and doesn’t freeze all the way to the bottom in the winter.” The warm pond water melts the surrounding permafrost; the thawed soil in turn releases long-stored carbon in the form of the greenhouse gases carbon dioxide and methane, contributing to further warming of the atmosphere.

While changes in the Arctic due to warming will occur with or without beavers, the fragility of the far north’s ecosystems makes them particularly vulnerable to the kinds of disturbances that beavers can cause. According to paleobotanist Jennifer McElwain of Trinity College Dublin, author of an article on plant responses to ancient warming episodes in the Annual Review of Plant Biology, the tundra may be the environment most threatened by climate change on the planet.

McElwain and her colleagues examine fossil leaves and use the number and size of the pores, or stomata, on the leaves to infer the carbon dioxide level in the atmosphere that plants breathed. “When there’s a very high carbon dioxide atmosphere, you see plants with bigger and fewer stomata,” she explains. At times when atmospheric CO₂ was higher than about 500 ppm, forests grew in the high Arctic.

“During greenhouse intervals in Earth’s deep past, we have forested ecosystems up to 85.86 degrees latitude north and south,” says McElwain. There were no places on Earth where the climate was too cold for trees to grow during these times. And where there are trees, the animals that depend on them, such as beavers, can thrive. There is even evidence that a forested Arctic is where the beaver’s dam-building skills first developed millions of years ago (see sidebar).

In the past, as now, the polar regions warmed faster than the rest of the planet because heat was transported to the poles by the global circulation patterns of the oceans and atmosphere. And as human burning of fossil fuels has now pushed atmospheric CO₂ levels up to 415 ppm and climbing, the proliferation of shrubs and trees on today’s warming tundra seems inevitable — as does the proliferation of animals that those plants need to survive.

Tape has tracked beavers as well as other creatures that have moved north on the tundra in the wake of climate change, including moose feasting on tall, dense shrubbery that didn’t exist there 70 years ago. But the impact of beavers on the landscape is unique.

“It’s best to think of beavers as a disturbance,” says Tape. “Their closest analogue is not a moose. It’s wildfire.”