As the Arctic climate warms and woody shrubs spread northward over the tundra, several animal species have followed.
Among the latest: wood-chomping beavers have apparently established a beachhead near the Arctic coastline.
A Canadian team has documented the first beaver dam, lodge and winter food cache on the Beaufort Sea coastal plain, a discovery described in a study recently published in the journal Canadian Field-Naturalist.
The beaver settlement was found in July 2015 — by accident. Researchers spotted the beaver structures when they were doing a raptor survey on the Canadian side of the Beaufort coastal plain.
"We just happened to be flying over the Babbage River and I noticed a beaver dam out the window. I thought, 'That's sort of odd,'" said lead author Thomas Jung, a senior wildlife biologist with the Yukon territorial government.
When they returned to the site in September, they spotted the cache of vegetation that would soon be under the ice, providing winter food for the beaver family living in the lodge.
Jung and his colleagues made wider inquiries and learned local Inuvialuit hunters had spotted beavers in the region in 2008 and 2009. Those sightings, along with the dam-lodge-food cache complex, are the first documented signs of North American beaver occupancy on the Beaufort coastal plain.
The site, in Yukon's Ivvavik National Park, is about 16 miles from the Beaufort coast and roughly 60 miles east of the Alaska border. Jung believes, based on the abundant food cache discovered in the fall of 2015, that the beavers are still living on their tundra frontier.
"They certainly need to eat. I think the structures up there are probably providing good enough food for them that they can make a go of it," he said.
Beavers are very adaptable, he added, and in Yukon they have been known to use rocks to build dams when not enough wood is available.
Is beaver colonization of new tundra terrain a good or bad thing?
"I guess it depends on who you ask," Jung said.
The beaver is considered a keystone species, with implications for the rest of the ecosystem, he said. "They create habitat for some species and they also destroy habitat for some species."
Among those affected by beavers are humans.
In some sites in Northwest Alaska, where beaver numbers are increasing, there are complaints that the multitudes of dams are impeding fish movement and making boat travel more difficult, according to surveys by the National Park Service.
There are also concerns about diseases that might be spread to new places by the beaver newcomers, specifically the intestinal disease known as giardia. The U.S. Centers for Disease Control's Arctic Investigations Program has cited beaver expansion as an issue of concern for drinking-water quality.
On the plus side for humans, subsistence hunters and trappers who use beaver meat and fur might reap some benefits of an expanded beaver range.
Beavers are not the only boreal animals that have followed shrubs into the far-north tundra regions. Moose and snowshoe hares have also established themselves there in response to the relatively new woody growth, according to recent studies.
Just how the beavers found their way to the Babbage River is yet unknown.
Beavers could have leapfrogged over time and numerous generations from the Mackenzie River delta, an area to the east where the animals were previously established, Jung said. However, there is a puzzling absence of beavers between the Mackenzie delta and the Babbage River, he said.
Another potential pathway, Jung said, is the ocean. "Beavers aren't supposed to swim in the sea, without a doubt, but wildlife often surprises us," he said.
Ken Tape, a University of Alaska Fairbanks biologist who has been studying Arctic shrub expansion and its impacts, leans toward the former explanation.
Boreal forest habitat extends along the Mackenzie, and so do boreal animals like moose, he said. The expansion of shrubs and beavers in Northwest Alaska has been along rivers, he noted.
But for the most part, the Brooks Range creates a formidable obstacle to beaver colonization of the Arctic Alaska coastal plain, Tape said. "There's a pretty significant barrier to dispersal," he said.
North is not the only direction where Yukon beavers appear to be following new shrub growth. They are also moving up in elevation, Jung said.
In mountainous Tombstone Territorial Park, on the Dempster Highway north of Dawson, he is finding more signs of beaver in the waters dotting the broad, high-altitude alpine plateaus.