Shades of Green: Earth's Forests -- Beavers

In ideal habitat, the animals may construct as many as 25 dams along one mile of river.

Beavers and humans are two of the only mammals capable of dramatically changing forest habitats. Humans, of course, do this to a far greater extent, and are far more destructive. Beavers, on the other hand, can improve the habitat for both themselves and other animals. Inhabiting forest streams and rivers (called "riparian zones"), the beavers are not as conspicuous as the structures they build.

A lodge at low-water. Photo by Maya Walters.

It is generally well known that beavers use their powerful, sharp teeth to cut down small trees along river banks. They use these trees to build dams across rivers, which have an enormous impact on water flow. The water is slowed, and pools are formed behind the dams which collect huge amounts of nutrient-rich sediment. These wetlands reduce flooding, and by holding reservoirs of water part way along the stream, they prolong strong water flow into the late summer and fall.

The variety of sizes and types of sticks on a dam. Photo by Maya Walters.

Some of the trees that beavers cut get carried downstream and collect in larger creeks and rivers. Here they may slow the waterflow, improving the habitat for salmon. Even though beavers do not live in these large creeks, their actions have an effect on them. Smaller streams are more suitable habitat than large rivers, which are powerful enough to wash the beaver dams away.

Beavers have an influence over vegetation growth in riparian zones*. They do not arbitrarily choose trees to use in their dams, but have favorite species which they cut first, and ignore other types of trees entirely. As the beavers cut more of their favored trees to use in their dams, the tree species that they don't use begin to dominate the surrounding forest. If the population was extremely high, they could eventually cut almost all of their favorite trees from a small area of the nearby forest, creating open patches where shrubs would grow thickly.

Before Europeans came to North America, beavers were abundant, inhabiting riverbanks in forests throughout the continent. The fur trade was one of the things that brought people to North America, however, and pelts were an extremely valuable commodity. When competition broke out between two rival fur companies, they both overtrapped, in an attempt to keep the other company from getting the beavers.

The European was once common throughout Eurasia, as well as England. But by the early 1900s, only a few remained, mainly in remote areas of Finland and the former USSR. Their populations are now recovering, but some of their remaining habitat has been taken over by the North American species, which was imported in the 1920s.

Two beavers swimming near fallen logs. Photo courtesy Al Walters.

The entrances to dams are usually protected by underwater channels. At low water, however, they can become exposed. Photo by Maya Walters.

It is estimated that when Europeans first came to North America, there were anywhere between 60 million and 400 million beavers, occupying about 6 million square miles of habitat

Although the beavers were almost completely wiped out from most areas, their dams and pools remained for many years. But they did not remain in the same state as they would had the beavers also remained. Without the beavers there to maintain the dams, some dams broke apart, and some ponds gradually changed into marshes, while others filled in completely with sediment and new forest trees grew.

Beavers and streams are intimately linked. Until the beavers were trapped, natural riparian habitats and water flow patterns were shaped to an enormous extent by the dams. Ecologists studying today's forests are studying streams with far fewer beavers than previously inhabited these areas. This means that all our observations of what makes a "natural" stream are really not accurate.

Beavers prefer slower-moving streams and ponds. Photos courtesy Naomi Woods.

For thousands of years, beavers managed and shaped these streams. Humans took thirty years to wipe them out. We can't see streams in their natural state any longer; they were destroyed along with the .

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