Until a few decades ago, there were no beavers in Patagonia. That changed when 20 pairs of the tree-chewing creature were introduced with the hopes of creating a fur industry.

Today, their numbers have exploded and they pose a serious threat to the South American area's biodiversity.

Species have always moved. The wind carries seeds; animals swim and fly. But not all are capable of crossing the Atlantic or the Andes.

In ways planned or unforeseen, humans have introduced species, and the newcomers quickly become invaders and threaten to destroy the native flora and fauna of their adopted homes.

"When we lose biodiversity, we are losing a bank of genetic material" that we need for food or to create medicines, said Fernando Baeriswyl, a project coordinator for the Global Environment Fund specializing in invasive species in Chile.

In North America and Europe, beavers live in balance with their natural habitat. But in Patagonia, the native trees don't regenerate fast enough to keep pace with the animals' rampant destructive powers. Plus beavers in Patagonia don't have any natural predators, like bears or wolves.

With the trees they fell, beavers build dams up to three meters tall. These structures can change waterways and lead to flooding or drying of traditional river ways.

Within a few years of their export to the southern tip of Patagonia, the animals had expanded their range around the region.

Their advance has been so swift that today they represent a menace that is proving hard to control. Authorities in Argentina and Chile have authorized the hunting of the animals, but these efforts have not stopped the beaver.

Chile and Argentina are now determined to completely eradicate the tree-chewers, said Adrian Schiavini, a beaver specialist from a regional research center.

- Invasive blackberries -

In the Huilo Huilo reserve in southern Chile, dozens of invasive-species experts got together last month for the country's first national meeting focused on invasive species in protected areas, to try to address the lack of knowledge and rules to tackle the problem.

Invasive species travel in ships, in clothes and shoes, or even in people's stomachs. When they get to a new environment, they can often proliferate thanks to a lack of natural predators.

As they spread, they can gradually alter entire ecosystems, transforming the natural diets of local species or themselves being eaten by other animals. In the worst cases, they can wipe out entire native species.

According to specialists, along with pollution and climate change, invasive species are one of the most damaging challenges for our planet.

"Climate change is causing a major vulnerability for certain species to the effects of invasive species," said Victor Carrion, the administrator of the Galapagos National Park in Ecuador.

The University of Chile carried out a study showing that the Andean country has 119 exotic invasive species, 27 of which are threatening biodiversity, including the European wasp, an invasive slime called "rock snot," red deer and wild boar, among others.

The same can happen with flowers. If we find a flower we like and take it home to plant in the garden, the results can lead to an ecological disaster.

That's what happened with blackberry bushes.

"When that arrives, it's a death sentence," Baeriswyl said.

The shrub invades the ground under trees and stops other plants from photosynthesizing, as well as slurping up their water.

Even rabbits, dogs and goats can cause problems. Many people think they are native to the region, but they would never have reached many places without human intervention.

- Death to the invaders? -

When Europeans first started sailing to South America, they left goats on islands to ensure they had food for future voyages. The animals devoured many plants, causing erosion and altering ecosystems.

Centuries later, in the Galapagos Islands, more than 270,000 goats were eliminated on 10 islands, along with cats, pigeons, donkeys and rodents, Carrion said.

Animal advocates have criticized these approaches.

In the case of beavers, the animals will be caught in traps that ensure a quick death.

But the world's most invasive species, of course, is us. People quickly spread across the globe from Africa, altering ecosystems and impacting endemic species as they went.