<P> Beaver dams or beaver impoundments are dams built by beavers to provide ponds as protection against predators such as coyotes, wolves, and bears, and to provide easy access to food during winter . These structures modify the natural environment in such a way that the overall ecosystem builds upon the change, making beavers a keystone species . Beavers work at night and are prolific builders, carrying mud and stones with their fore - paws and timber between their teeth . </P> <P> A minimum water level of 0.6 to 0.9 metres (2.0 to 3.0 ft) is required to keep the underwater entrance to beaver lodges from being blocked by ice during the winter . In lakes, rivers and large streams with deep enough water, beavers may not build dams and instead live in bank burrows and lodges . If the water is not deep enough to keep beavers safe from predators and their lodge entrances ice - free, beavers build dams . </P>

Why does building dams make beavers a keystone species
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