In 1930s, Finland brought back beavers but accidentally introduced the wrong species. The mistake still affects wildlife today

Finland's attempt to restore a beaver population in the 1930s led to the accidental introduction of a different species after officials brought North American beavers into the country. The Eurasian beaver had been hunted to extinction in Finland by the late 19th century, with the last recorded animal killed in 1868. Reintroduction efforts began in 1935, initially using beavers from Norway.

When additional Norwegian beavers could not be supplied, Finland turned to North America. Seven North American beavers (Castor canadensis) were brought from New York and released at several locations in 1937. At the time, Eurasian and North American beavers were generally regarded as belonging to the same species.

It was not until 1973 that researchers established that the two were separate species by comparing their chromosomes. A University of Helsinki review of the introduction records the release of the seven North American beavers as part of Finland's effort to restore its lost beaver population.


Also Read: In 1500s, Spanish explorers introduced pigs into North America. Now areas where they roam free have fewer invasive species

Why Finland mistook the two beaver species

The mix-up was partly due to the close similarities between the Eurasian and North American beavers. Both species construct dams and lodges, consume woody vegetation and inhabit areas around rivers and lakes.

For decades, scientists had not reached a clear consensus that the Old World and New World beavers represented separate species. The chromosome research in 1973 established a significant distinction. Eurasian beavers have 48 chromosomes, compared with 40 in North American beavers.

The finding changed the understanding of the animals introduced into Finland. What officials had believed were beavers suitable for restoring the country's native population were recognised as a non-native species.

How the North American beaver spread across Finland

The seven imported animals successfully established themselves. Two pairs reproduced, and their offspring were subsequently moved to other parts of Finland, allowing the North American species to spread beyond the original release locations.

Over the following decades, the introduced beavers expanded across large parts of eastern and central Finland before eventually crossing into neighbouring Russia. A 2021 study of Finnish beaver populations found that North American beavers had spread considerably farther from their original release sites than Eurasian beavers.

The native Eurasian beaver also returned to parts of western Finland, but its population remained smaller and more geographically restricted. Researchers have suggested that the North American species' slightly larger litters may have contributed to its faster expansion, although habitat and dispersal also play a role.

By the late 1990s, Finland's North American beaver population was estimated at around 10,000. Later research continued to find the species across much of eastern and central Finland, along with smaller populations farther north.

Two species competing for similar habitats

The challenge for Finnish conservation was not simply the presence of two closely related beaver species. Their ecological requirements are also very similar, meaning they compete for many of the same resources.

This became a concern as their geographical ranges began to overlap. Finland had originally sought to restore the Eurasian beaver, but the North American species generally expanded more quickly in areas where both had been introduced.

The native species was also reported to have disappeared from some locations where the two species had been introduced together, while stronger Eurasian beaver populations remained in areas without North American beavers.

Why removing the introduced beavers was difficult

By the time scientists identified the species difference in 1973, the North American beaver had already been established in Finland for decades. Its population had expanded beyond the original release sites, and animals had also been deliberately relocated to other areas.

Removing an established population from a country containing thousands of lakes, rivers and wetlands would have been difficult and expensive. Modern genetic testing has made it easier to distinguish between the two species, but that technology was unavailable when the original introduction took place.

A 2021 study estimated that Finland had roughly 3,700 to 5,000 Eurasian beavers and more than 10,000 North American beavers. The latter were concentrated across much of eastern and central Finland and had also reached parts of Lapland. Researchers found that the two species overlapped in several regions, raising concerns about competition for habitat and food.

Also Read: In 1943, a German fighter had a US bomber in his sights, finger on trigger button but what happened left everyone stunned 50 years later

A conservation effort that created a new problem

Finland's beaver reintroduction illustrates how a conservation effort can produce an unexpected outcome when scientific understanding changes. In 1937, officials were attempting to restore a species that had disappeared from the country and selected animals they believed were suitable for that purpose.

The seven beavers from North America survived and reproduced, eventually establishing a population numbering in the thousands. Thirty-six years after their release, chromosome research showed that they were not the same species as Finland's native Eurasian beaver.

The original effort was intended to reverse an ecological loss caused by hunting. Instead, the introduction created a separate conservation challenge, with Finland having to protect the native Eurasian beaver while managing the spread of the North American species introduced by mistake.