If you’re a boreal toad — or a wood duck, or a brook trout, or a moose — you might owe your life to a beaver. (Kudos, also, on learning to read.)
Castor canadensis, the North American beaver, is the ultimate keystone species, that rare creature that supports an entire ecosystem. By building dams and forming ponds, beavers serve as bucktoothed housing developers, creating watery habitat for a menagerie of tenants. Songbirds nest in pondside willows, frogs breed in shallow canals, and trout shelter in cold pools. There’s even a beaver beetle that eats the skin of you-know-what.
Modern beavers have been wandering North America for 7.5 million years, giving flora and fauna plenty of time to adapt. Willow, a favorite snack, resprouts multiple stems when it’s gnawed down, like a hydra regrowing heads. Cottonwoods produce distasteful tannins to deter chewing. America’s rarest butterfly, the St. Francis Satyr, eats little but sedges that grow in beaver wetlands. The evolutionary connection runs so deep it’s often boiled down to a pithy bumper-sticker: “Beavers taught salmon to jump.”
Before European traders set about turning their furs into fancy hats, beavers roamed most of the continent, stopping up streams from the Arctic tundra-line to the Mojave Desert. But the mammals never ventured beyond northern Mexico, leaving Central and South America historically beaverless.
Until, that is, an ill-conceived scheme unleashed nature’s architects on a landscape that had never known their teeth — and forever rearranged ecosystems at the bottom of the world.
Tierra del Fuego, Argentina.
The bizarre experiment was launched in 1946, when Argentina relocated 20 Canadian beavers to Tierra del Fuego, the windswept archipelago at South America’s tip, to “enrich” local wildlife and foster a fur trade. The pelt industry never took off, but the beavers, unchecked by North American predators like wolves and bears, flourished. They swam glacier-scoured fjords between islands, dispersing throughout both the Argentine and Chilean sides of Tierra del Fuego. Some decades after their arrival, a beaver clambered from an icy strait and established a beachhead on the Patagonian mainland. These days, their population numbers about 200,000.
And as beavers spread, they did what beavers are wont to do: They transformed their surroundings.
Just as New Zealand’s flightless birds had no recourse against invasive rats, Tierra del Fuego’s trees were ill-equipped to withstand “los castores.” The region’s forests are dominated by beeches that never evolved beaver coexistence strategies: They don’t resprout after cutting, produce unsavory chemicals or tolerate flooded soils. As beavers chewed down beeches and expanded free-flowing streams into broad ponds, forests opened into stump-dotted meadows. In 2009, Chris Anderson, an ecologist at Chile’s Universidad de Magallanes, found that beavers had reshaped up to 15 per cent of Tierra del Fuego’s total land area and half its streams — “the largest alteration to the forested portion of this landscape since the recession of the last ice age.”
“Basically, everything that’s cuttable has been clear-cut,” Anderson said. Drowned trees and gnawed logs, freeze-dried by icy winds, litter the landscape like the ghosts of forests past. “You just …read more