<P> Beaver dams 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>

How does a beaver's dam changed the environment