The European beaver is a species of beaver, which was once widespread in Sweden, but it was hunted to near extinction by the end of the nineteenth century. It was hunted both for fur and for castoreum, a secretion of its scent gland believed to have medicinal properties. Between 1922 and 1939 approximately eighty individuals were reintroduced into Sweden from Norway. Since the reintroduction, the beaver population has increased to an estimated 150 000 beavers and it is now returning to much of its former range.

Beavers are known for their natural trait of building dams on rivers and streams and building their homes (known as "lodges") in the resulting pond. Beavers also build canals to float building materials that are difficult to haul over land. They use powerful front teeth to cut trees and other plants that they use both for building and for food. In the absence of existing ponds, beavers must construct dams before building their lodges. First, they place vertical poles, then fill between the poles with a crisscross of horizontally placed branches. They fill in the gaps between the branches with a combination of weeds and mud until the dam impounds sufficient water to surround the lodge.

The ponds created by these well-maintained dams help isolate the lodges. These are created from severed branches and mud. The beavers cover their lodges late each autumn with fresh mud, which freezes when frosts arrive. The mud becomes almost as hard as stone, thereby preventing wolves and wolverines from penetrating the lodge.

The lodge have underwater entrances, which makes entry nearly impossible for any other animal, although muskrats have been seen living inside beaver lodges with the beavers who made them. Only a small amount of the lodge is actually used as a living area. Beavers dig out their dens with underwater entrances after they finish building the dams and lodge structures. There are typically two dens within the lodge, one for drying off after exiting the water and another, drier one, in which the family lives. Beaver lodges are constructed with the same materials as the dams, with little order or regularity of structure. They seldom house more than four adults and six or eight juveniles. These mammals thrive on land and in water and they tend to equally divide their time between the two habitats. Being great divers and swimmers with strong rear web feet and a paddle shaped tail, beavers are well adapted for aquatic life with an oily waterproof fur.