Famed for their dam-building ability, North America's largest rodents begin by making an underwater foundation of mud and stone. Then they gnaw down trees, leaving characteristic cone-shaped stumps, and drag or float cuttings to the dam site, where they are incorporated into the foundation with more mud. As a pond forms behind the dam, the pair of Beavers build a stick-and-mud lodge with underwater entrances and an inside platform raised above the water. Here they remain much of the day, emerging at dusk to forage for succulent plants or to cut trees and shrubs. In late summer and fall the cuttings are stored in an underwater food pile, to be eaten in winter. The kits are born in spring and stay in the home pond until they are 2 years old.
A beaver's work is easy to spot; a stump or the end of a log that has been gnawed to a point. The telltale tooth marks are one-eighth to a quarter inch wide, and there will be a pile of chips not unlike those left by a woodman's ax. In one year a single beaver may cut down more than 200 trees.
A beaver dam turns a fast-flowing stream into a placid pond&; a place of refuge for these aquatic rodents. But a beaver dam does much more than that. It radically alters the stream and the surrounding forest, setting off a cycle of change that lasts a century or more.
Built of thousands of logs, sticks, and layers of mud, a beaver dam may stretch beyond a quarter mile, but most are less than 100 feet wide. The pond floods the neighboring forest, giving the beavers easy access to such favorite trees as aspen and birch (they eat the juice inner bark, not the wood itself). Their favorites include willow, maple, poplar, beech, birch, alder, and aspen trees. They also eat water vegetation, as well as buds, and roots. Cellulose, which usually can not be digested by mammals, is a major component of their diet. Beavers have microorganisms in their cecum (a sac between the large and small intestine) that digest this material.
At first the pond provides a rich feeding ground for trout, but as the shade trees are cut and the water temperature rises, the pond becomes home to warm-water fish, such as bullheads and sunfish. When enough silt collects behind the dam, marsh plants intrude, attracting nesting waterfowl, rails, and grebes; with time the pond fills completely, creating a wet medow for grazers like deer and elk. Eventually the forest returns; perhaps to attract a new colony of beavers, which starts the cycle once again.
Though gnawing animals called rodents can be formidable foes, they can also be allies. We got our foothold in North America with the help of a 50-pound rodent: the beaver. Besides providing pelts for the fur trade, beavers made the landscape more suitable for human habitation. For thousands of years, silt collected behind the flood-quelling gates of beaver dams, creating some of the fertile farmland that helped settlers survive in the New World.