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A whole host of different mammals appear to benefit from having beavers in the area. In forests where beavers have been introduced in Finland, their presence is linked to increased activity of several species, including moose, otters, and weasels.

Beavers are described as 鈥渆cosystem engineers鈥� because their dam-building work has such a huge effect on habitats. Both the Eurasian beaver and the American beaver were almost driven extinct by hunting in the early 20th聽century, but they have since recovered in North America.

More recently, beavers have begun to re-establish themselves in Europe, thanks in part to at least 157 reintroduction projects that have taken place in 24 European countries. The touted benefits they bring to the environment include drought prevention, carbon sequestration, flood management and keeping streams cool.

They are also thought to boost biodiversity. To learn more about their impact on other mammals, Petri Nummi at the University of Helsinki, Finland, and colleagues set up camera traps and surveyed snow tracks at forest sites in southern Finland where the American beaver was introduced in the 1950s.

Moose, red foxes and raccoon dogs visited beaver ponds more than control ponds where beavers were not present. Snow tracks showed that moose, otters, weasels and pine martens were more active in beaver patches than other sites.

鈥淭he otter is a species of some concern in Europe, so this may be important from that point of view,鈥� says Nummi.

Beavers change the environments they occupy in several ways. Their dams flood large areas, creating shallow ponds that harbour lots of invertebrates. The trees they fell create open spaces in the forest where young saplings can grow. When beavers leave a pond and their dams break, the previously flooded area is rich in nutrients and can become a meadow.

Felled trees, saplings and aquatic plants can all provide food for moose. Weasels and pine martens feed on smaller mammals, which may benefit from dead trees they can shelter under. Red foxes and raccoon dogs eat frogs, which are plentiful in beaver ponds. Besides feasting on frogs and fish in the ponds, otters make use of abandoned beaver lodges and ice holes during the winter.

Previous studies in the same area have found that beavers are associated with a greater abundance of bats, frogs and waterbirds.