Why Do Beavers Build Dams? - New Scientist

Beavers have a rare ability to transform the landscapes they live in, which is why biologists describe them as ecosystem engineers. They do this by chewing through tree trunks with their huge teeth and building dams in streams, flooding large areas to create beaver ponds.

If you have read The Lion, the Witch and the Wardrobe, you might be forgiven for thinking that beavers create these ponds to fish in: the beavers in C.S. Lewis’s book are partial to a fish supper. But in fact, beavers are entirely herbivorous, feeding on aquatic plants, grasses, tree bark and leaves.

The reason they build dams is to create deep water that gives them protection from predators. They live in dome-shaped constructions called beaver lodges within the ponds that can only be accessed by underwater entrances. Inside, they are safe from threats such as bears and wolves. They can also store food inside and keep warm in winter.

Beavers build dams up to 5 metres high, and the largest one, in Alberta, Canada, is 850 metres long. Most dams are built from pieces of wood, with stones at the base and a sealing layer of mud and plants on the upstream side. The dams are thicker at the bottom with a shallow slope on the upstream side, meaning that the weight of the water pushes down on the dam, keeping it in place and allowing it to hold back large volumes of water. Their engineering work is so effective that beaver dams can last for decades.

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Wild beavers are called keystone species because of their importance for biodiversity. Their impact on the environment brings benefits for a whole range of other species. The wetlands they create are homes for many frogs, fish and invertebrates, and these animals provide food for birds and bats. In forests where beavers have been introduced in Finland, their presence is linked to increased activity of several species, including moose, otters, and weasels.

Beaver dams also contribute to carbon storage, flood management and drought prevention, and they help to keep streams cool.

In the early 20th century, beavers were almost driven extinct by hunting in Europe and North America, but thanks to more than 100 successful reintroduction projects, they are making a comeback. Although there are no predators in some of the places where they have been reintroduced, such as the UK, beavers will instinctively build their dams and lodges to make homes where they feel safe.

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