In 1946 the Argentine Navy imported 10 beaver couples from Canada and set them free in Isla Grande, the deep south of Tierra del Fuego, with the intention of “enriching” the native fauna—and the local fur industry.

The consequences of such initiative were disastrous: Protected from hunting for 35 years, and devoid of natural predators, the beavers grew over 5,000 times their initial population, caused irreversible changes in the forest ecosystem, and started advancing over the continent. Now, a study published in Chilean Natural History suggests that the demographic explosion of those beavers could be bigger than suspected because it can take years or even decades for local inhabitants to notice the rodents’ presence and their impact on the surrounding ecosystems. “There could be populations of beavers moving around in the continent and in the islands we don’t know anything about,” biologist Giorgia Graells, of the Institute of Patagonia at Magallanes University and lead author of the study, told Scientific American.

In order to accurately determine whether a beaver has arrived or has recently been to a certain area, Graells and colleagues Derek Corcoran and Juan Carlos Aravena reconstructed the date in which trees from different sites across a continental region in southern Chile were attacked by the rodents. The team worked by counting growth rings in the fallen trunks. “It is the first time the technique, known as dendrochronology, is used to date the presence of invasive species,” Graells says. The felling of trees is one of the beaver’s most damaging habits. The telltale signals from their dirty work are unmistakable: teeth marks around a trunk that has been carved like a pencil tip. You can tell that the attacker was a beaver even if there are no nearby dens.

Next, researchers matched the ages of the attacked trees with the earliest date where beavers were observed across the region. The results were disturbing: The beavers in Tierra del Fuego had already jumped to the continent in 1968, 26 years before they were seen for the first time. “It was quite a surprise,” admits Graells, who now believes there might be a true “invisible” expansion of the species, perhaps more pronounced than previously estimated. The find, she adds, could accelerate the need to put control measures in place.

Nonhuman ecosystem engineers

The beaver is the second-largest rodent on Earth: Adults can weigh up to 30 kilograms and measure more than one meter long. Two species of beaver exist today: Castor fiber, from Eurasia; and Castor canadensis, which evolved in North America, the same species that was introduced in Tierra del Fuego. Both species are great ecosystems engineers, modifying their environments by cutting branches and entire tree trunks in order to build dams, canals and dens.

Since the arrival of the beaver the Patagonian Magellanic forest has never been the same: Many of the fallen trees, even if they have been partially cut, are not able to survive the attack whereas North America’s trees are more resistant to this kind of damage and can rebound. Beaver-built dams can reach around 100 meters in length and cause floods which have serious impact on acres of native vegetation. And even if a dam is destroyed, the accumulated mud buries the seeds that normally allow for forest regeneration. “Studies have shown that after 20 years the forest is not able to go back to its original state, and it is replaced by introduced grass-loving species,” Graells says. The beaver’s action also affects roads, sewers, bridges and fences.

When beavers go out of control, their activities can transform the landscape for decades or centuries, even indefinitely, says Marta Lizarralde, a researcher with the Molecular Ecology Laboratory at the Southern Center for Scientific Research (CADIC) in Ushuaia, Argentina. Out of the original population of 20 individuals, it is estimated that today there are between 100,000 and 150,000 beavers in the Fuegian archipelago alone, even though that number is only an indirect approximation, she says.

Could beavers continue to expand north? Recent evidence seems to support that possibility. In a yet-to-be-published fitness model that takes into account the climatic conditions favoring the species propagation, Graells and colleagues forecast that beavers could colonize South America at least up to 43 degrees south latitude, some 1,300 kilometers north of their latest proved location. The expansion would mainly take place west of the Andes Mountains, on the Chilean side.

Another factor feeding this fear is the beaver’s ability to reproduce in different habitats, a skill that is surprising researchers. A study in the Patagonian region led by Duke University biologist Alejandro Pietrek showed that the size of the beavers’ colonies and the yearly number of offspring are larger in the steppe than in the forest environment. “Beavers are typically associated with forests, but this turns out to be a more plastic species able to populate semiarid regions,” Pietrek told Scientific American.

Scientists today know the effects that introduced animal or plant species can have in an ecosystem. But the beaver expansion in Tierra del Fuego out of so few initial specimens represents an extraordinarily successful and unprecedented biological invasion. For example, seven American beavers were freed in Finland in 1937 and a decade ago it was estimated that their number had “only” reached 12.000.

The invasive capacity of the beavers at the end of the world “is huge,” Lizarralde notes. Yet, she says there is still no evidence they can advance much farther north on their own “imminently or in the midterm.”

But, how can we be sure? According to Graells, the accurate dating of the damaged trees can be useful to assess the rhythm of expansion of these rodents as well as to monitor efforts to contain their numbers. Several specialists think it is impossible to completely eradicate the beaver, except on smaller islands, and that the most practical measure would be to control them with selective traps that can also be used commercially to sell the skins. “We must ameliorate the problem before it is too late,” Graells says.