Argentina released just 20 Canadian beavers into Patagonia in 1946 to start a fur industry, but with no natural predators, they multiplied into tens of thousands, building thousands of dams that flooded entire valleys and killed millions of native trees

In 1946, Argentine authorities introduced 20 North American beavers from northern Manitoba, Canada, to the forests of Tierra del Fuego in southern Patagonia, intending to establish a thriving fur industry that could support the local economy.

However, instead of a thriving fur economy, the introduction of beavers sparked one of the most dramatic biological invasions ever documented in South America. The beavers, with abundant food, suitable habitat, and no natural predators, multiplied rapidly over the following decades. Notably, scientists now estimate the population numbers in tens of thousands, spreading across both Argentina and its neighbour, Chile.

Furthermore, researchers claim that animals have built thousands of dams, flooded native forests, and transformed entire river valleys, thus creating ecological changes that are difficult and, in some cases, impossible to reverse. A small release caused enormous consequencesFor years, the accounts of how beaver introduction differed on how many animals were originally released. However, in 2014, historical analysis by researchers Alejandro G. Pietrek and Laura Fasola in their article Origin and history of the beaver introduction in South America examined the archival records and concluded that the introduction occurred in a single release event involving 20 beavers imported from northern Manitoba.

Notably, the findings corrected earlier claims that suggested 25 pairs or even 50 beavers were introduced. Interestingly, the entire Patagonian population descended from this small founding group is now considered a remarkable example of how even a limited number of non native animals can establish a thriving population under favourable conditions.

In their native North America, beaver populations are naturally regulated by wolves, bears, and cougars, while native forests have evolved alongside the animals for thousands of years.

Patagonia offered a very different environment for the beavers, as the southern forests contained abundant streams and vast stands of native Nothofagus trees but very few predators who could control the beaver population. Furthermore, the native trees in Patagonia had never evolved alongside dam-building beavers and were very poorly adapted to repeated floods and tree falling.

With beavers expanding across the landscape, they constructed thousands of dams that slowed the river flow and created ponds.

While such wetlands can be beneficial in North America, the prolonged flooding in Tierra del Fuego often kills native trees rather than allowing them to regenerate. Researchers have estimated that millions of trees have been affected over decades of beaver activity, fundamentally altering the forest structure and biodiversity. Dams transformed entire landscapesBeaver engineering has reshaped large areas of Patagonia with each dam backing up water, flooding surrounding forests, and creating wetlands where even mature trees could die.

Once abandoned, several of these ponds do not return to their previous condition and instead become meadows or permanently altered wetlands, changing river flow, soil conditions and habitats for other species. The research showed that there are thousands of beaver dams across Tierra del Fuego and nearby islands.

This cumulative effect has been extensive in modifying the landscapes, with altered waterways visible in satellite imagery and aerial surveys. After recognising the growing ecological damage, Argentina and Chile signed a binational agreement to coordinate beaver management and ultimately pursue eradication across Tierra del Fuego. Conservationists have pointed out that removing the invasive population offers the best chance of protecting the region’s unique temperate forests. The task is, however, expected to take many years due to the animal’s widespread distribution and continuous breeding.