Beaver Natural History Narrative

Beavers are members of Rodentia, the largest mammal order. Rodents are gnawing animals and have two pairs of prominent, chisel-shaped incisor teeth. These teeth grow continuously and maintain their sharp edges; they must be used frequently for gnawing or they will become too long.

The beaver, weighing up to 60 pounds, is the largest North American rodent. Beavers are excellent swimmers and can be easily identified by their scaly, flat tails. The tails are used for steering while swimming, and bracing them on land as they gnaw trees. Their back feet are webbed and used like paddles when swimming. Beavers move slowly on land, where they are prey for coyotes and mountain lions. Being excellent swimmers, they are safe from predators while in the water.

Beavers often dam streams to create deep ponds. They then build a lodge in the pond near the bank. On large streams and rivers, however, dams are not constructed.

To build a dam and make a pond, beavers cut down trees and bushes with their sharp teeth. They take branches in their mouths and pull them into the water. Holding the branches with their teeth and front paws, they push the branches into the mud at the bottom of the creek. They dig up mud from the creek bottom and pile it on top of the branches to fill up the holes. This continues until the beavers have constructed a strong dam. Water builds behind the dam creating a pond.

The beaver lodge is similar in construction to the dam. A large pile of branches and mud is piled until it is higher than the surface of the water. The beavers swim to the bottom of the pond and gnaw up through the pile until they have made a tunnel that reaches above the water line. There they make a living chamber lined with leaves and grass. Beavers are monogamous and work together to choose the spot to build a dam and lodge.

Beavers eat the inner bark or cambium layer of branches of deciduous trees. Preparing for winter, beavers cache branches in the water. When the pond freezes, the store of food is easily available to them.

When beavers dam a stream, they set in motion a form of succession. The resulting backwater floods lowland near the creek. Trees are soon killed, creating an opening in the forest canopy. Water-associated plants and shrubs quickly invade the pond and shoreline, creating favorable habitat for waterfowl, moose, blackbirds, amphibians, fish, insects, muskrats, wading birds, warblers, marsh hawks, and a score of other animals. After many years the water becomes shallow, filling in with silt and plant debris.

Stimulated by the nutrient-rich mud, grasses, sedges, and shrubs begin to choke the water with their accumulating debris. The ground begins to firm as more silt is trapped.

As years pass, the trees near the lodge are cut down by the beavers for use as food and shelter. The beavers must move on and find a new spot to support themselves. Without the beavers to keep it strong, the old dam collapses, draining the pond. The area becomes meadow, supporting grasses, sedges, and other flowering plants. Trees begin to re-invade the drier ground and eventually the meadow reverts to forest. Centuries may be required to see this process completed.

At each stage, many of the animal inhabitants change because the habitat has changed. The robin and the red squirrel in the original, pre-beaver forest give way to the heron; the heron is replaced by the insect and berry eating cedar waxwing; the waxwing is followed by the tree-dwelling robin and red squirrel once again.