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.
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, then an ecologist at Chile’s Universidad de Magallanes, found that beavers had reshaped up to 15 percent 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 see acres and acres of white trees.”
Conservationists, aghast at the loss of old-growth forest, put their faith in natural barriers. Patagonia has two primary habitats, the forest and the steppe — the latter a wind-blasted, arid grassland whose paucity of trees seemed likely to limit beavers’ growth. In 2017, however, an Argentine biologist named Alejandro Pietrek found that, contrary to predictions, beavers were actually producing more offspring on the steppe. Unbothered by the lack of trees, the colonists were happily weaving dams from a shrub called mata negra.
“As long as they have water, they can expand,” Pietrek said. “They can colonize all of Patagonia if they want.”
Over the years, Chile and Argentina have made halfhearted attempts at curtailing the invasion. A bounty program failed to motivate trappers, while proposed markets for beaver meat never materialized. Recently, though, the two nations have gotten more serious: In 2016, they announced a plan to cull 100,000 — one of the largest invasive-species-control projects ever attempted.
Although the massive trapping program, in a pilot phase, should help contain the spread, most scientists say the toothy loggers are in South America to stay. “I think eradication is not possible,” said Chilean biologist Giorgia Graells. On many islands, Graells said, dense forests and scarce roads will thwart trappers. If beavers persist on even a single island, she pointed out, the survivors could repopulate the rest of the archipelago — Sisyphus’s boulder in furry form.
In some respects, the South American beaver narrative is a familiar one: Humans introduce nonnative species; nonnative species wreak havoc; humans futilely attempt to erase their error. Yet the beaver story is more interesting — for, befitting a keystone species, the rodent takeover has produced winners as well as losers. Research suggests that beavers have benefited native Magellanic woodpeckers, perhaps by making trees more susceptible to the wood-boring insects upon which the birds feast. The slackwaters behind dams also support native fish called puye, which are four times more abundant around beaver impoundments than elsewhere in southern Chile.
“Before you determine whether a change is good or bad, you always have to ask, ‘For whom?’ ” Anderson said. “If you’re a duck and you want ponds with lots of little crustaceans to eat, well, beaver ponds are full of them.”
The biggest beneficiaries, however, have been the beaver’s fellow North Americans: the muskrat and the mink, two other lusciously furred mammals the Chilean government naively plopped down in Tierra del Fuego in the 1940s. On their own, the imports might have perished; beavers, however, ensured their survival. When researchers scoured one invaded island, they found a whopping 97 percent of muskrat tracks, scats and burrows around beaver ponds and wetlands, suggesting that one rodent was supporting the other. Mink, a weasel-like carnivore, have in turn feasted on the muskrats — as well as native birds and mammals.
The scientists who studied that ecological chain reaction called it an “invasional meltdown.” A less ominous phrasing might be “novel ecosystem,” a natural community that’s been altered by human activity but has since escaped our control. Like it or not, novel ecosystems are all around us — scrubby pine forests in Puerto Rico, urban wetlands in New Jersey, replanted grasslands in an old Colorado mining pit. Assuming utter eradication fails, some corners of Patagonia may be forced to surrender to the awesome power of an indomitable rodent.
The whole saga, ultimately, is a sort of Bizarro Beaver story: The very same tree-gnawing, dam-building, pond-creating talents that normally make them such miracle-workers have mostly produced disaster below the equator. South America’s beavers are both charismatic and catastrophic, life-sustaining and forest-leveling, an invasive scourge and a popular tourist attraction. As the compassionate conservation movement dawns, beavers pose, too, an ethical dilemma: How do we balance ecological health with animal welfare? Is the only solution really mass slaughter?
The paradoxes can’t help but affect the scientists studying Patagonia’s beavers, who admire the architects even as they desire to see them wiped out.
“We have to focus on the big picture, on the ecosystem level,” Pietrek said. “But it’s hard to think about beavers being eradicated. You relate to them in a way.”
Ben Goldfarb is an environmental journalist and the author of “Eager: The Surprising, Secret Life of Beavers and Why They Matter.”