National Park Service
Harlan Kredit, National Park Service
Beavers cut down trees, dig canals to transport branches and logs, and build and maintain dams and lodges. They don’t just live off the land; they modify it to fit their needs. In fact, beavers are second only to humans in their ability to manipulate and change their environment.
A family of beavers can build a 35-foot long dam in a week. They build slides to make getting around easier. And after they’ve used up all the young trees nearby, and it gets too hard for the beavers to drag branches home easily, they forage farther away and dig canals to float the branches home. Canals can be hundreds of feet long.
But the structures beavers create change the environment in other ways, too. Beavers prefer to build their dams across streams in shallow valleys. The flooding kills many of the trees, and aquatic vegetation begins growing. And other trees and shrubs, more tolerant of the new marshy environment, soon grow around the edges of the flooded area, adding to the available food supply. The beaver thus helps create its own habitat. But it’s not only a habitat for beavers: insects lay their eggs in the water, providing food for fish, ducks, frogs, turtles, and birds. And other animals, such as otters and ospreys, feed on the insect-eaters. Plus, beaver-built wetlands soak up floodwaters from upstream, prevent erosion, raise the water table, and create an ecosystem that breaks down toxins and pesticides, cleaning the water.
Eventually, if beavers live in one spot long enough, silt collects behind the dam and the pond fills in, becoming a fertile meadow. Shrubs grow, providing shade for new tree seedlings. When the trees get taller, they shade out the shrubs and, with time, the area again becomes a mature forest.