This story was originally Appear in highland news and is climate desk cooperate.

One day in early January, Cyrus Harris hopped on a snowmobile and sped around a peninsula near Kotzebue, Alaska, clearing the way for his sled dogs. “The first beaver dam I encountered was about three miles from town,” he said. “There’s another near that, another about five miles away, that’s just a small area.” Born in 1957, Harris (Inupiaq) spent his childhood in the Kotzebue Strait in West Sulik. “Beavers are really just unheard of,” he said. “The number of beavers coming in is crazy, they’re just hitting the whole area.”

Beavers—once rarely seen in northwest Alaska—became more frequent in the 1980s and 1990s. Pastor Lance Kramer (Inupiaq) traps beavers today, mostly to make fur hats. He recently asked an elder about the first sighting in the area. “They saw this thing on the tundra and it looked like a Wolverine, but it was a very long beaver,” Cramer said. “[It] It had come this far on the tundra to get up, and the bottom of its tail had worn away. ”

Now animals – and their ponds, dams and huts – are everywhere. Using satellite imagery of the Kotzeb region, scientists found that the number of beaver dams surged from two in 2002 to 98 in 2019, an increase of 5,000 percent. It’s not just Kotzebue: Beaver ponds have doubled regionally since 2000, and there are now 12,000 in northwest Alaska. Beavers are known as “ecosystem engineers” because of how they flood their surroundings, changing the tundra.

Part of the reason North America’s largest rodent is moving north is climate change: As the tundra gets warmer and greener, it also attracts beavers, which need shrubs for food, dams and huts. Their spread has also been linked to a rebound in population: beaver trapping, which has been popular for centuries, has dwindled, and the animals are thriving.

Beavers were recently named “new disturbances” in NOAA’s 2021 Arctic Report Card, an annual report that tracks changes in the region. That’s because they dam rivers, creating deeper, warmer ponds and opening up new types of aquatic habitats. “Wherever you’re standing in the Arctic, the key question to ask is, ‘How long does it take for beavers to get there?'” says Ken Tape, an ecologist at the University of Alaska Fairbanks who studies beaver expansion. “Because when they get there, it’s never going to be the same again.”

Harris worries that beavers swimming in the reservoir that supplies Kotzebue with drinking water could overwhelm the community water treatment plant. Beavers (and other animals) carry the Giardia parasite, which they excrete into the environment, and water contaminated with their feces can lead to intestinal infections. Harris and others used to drink straight from the river on hunting and fishing trips, but today they’re reconsidering. “If our water quality is compromised, where do we go?” Harris said.