A fur trade scheme goes wrong
In 1946, the government of Argentina imported 20 North American beavers (Castor canadensis) from Canada and released them near Lake Fagnano in Tierra del Fuego. The goal was to establish a commercial fur industry in the remote region. This plan failed, as the market for beaver pelts was already in decline. But for the beavers, Tierra del Fuego was a paradise. The archipelago offered abundant food and water, with one feature missing: natural predators. In North America, beaver populations are controlled by wolves and bears, but in their new South American home, the beavers had no such threats.
Their numbers exploded. From the initial 20 animals, the population grew to an estimated 70,000 to 110,000 individuals today. They spread from the initial release site across the main island and, by the 1990s, had crossed the Strait of Magellan to the South American mainland. The beavers now cover an area of at least 7 million hectares, impacting thousands of kilometers of waterways. The ecological alteration caused by these beavers is considered the largest large-scale change in the subantarctic forests since the last ice age.
Engineers of destruction
The beavers' impact comes from their signature behavior: building dams. In doing so, they have re-engineered the entire hydrological system of the region. Fast-flowing, narrow streams have become wide, stagnant ponds and marshes. A 2020 study identified over 206,000 beaver dams across the archipelago. These dams flood vast areas of forest, killing the trees and creating what locals call "ghost forests".
Native southern beech trees, such as lenga (Nothofagus pumilio) and ñire (Nothofagus antarctica), are vulnerable. Unlike North American trees like willows and aspens, which co-evolved with beavers and can resprout after being felled, the Nothofagus species die permanently. Damage is extensive, with beavers having destroyed more than 31,000 hectares (over 119 square miles) of native forest. This destruction has economic consequences, with one study estimating the annual cost of damages to infrastructure like roads and pastures at over $73 million. In 2008, Argentina and Chile signed a binational agreement to move from control to complete eradication, though the process is slow and complex.