Resources for extending the learning
Featured Species: Can’t Live Without the Dam Beavers
Beavers that have been entombed in their lodges all winter are out and active. They are busy repairing damages to their dams, and in case of dispersed young, building new ones. Beavers are nocturnal and meticulously maintain their dams, so don’t be surprised if any damage to a dam is repaired by the next morning!
Beavers are good swimmers, but fairly slow and vulnerable on land. They build dams to raise the water level upstream high enough that it doesn’t freeze in the winter, creating suitable and safe habitat for their lodge and winter food cache, and easier access to stands of Trembling Aspen (a favoured food). The lodge is built so that it has above ground chambers, but only aquatic entrances/exits that are inaccessible to land predators. Their winter food cache is placed under water so that in the winter they never have to go out on land.
Beavers prefer waterways with a firm, mud bottom that aren’t too deep or fast flowing. Trees, sticks, rocks, mud, and grass are used as construction materials. Trees up to about 40 cm in diameter can be felled, and if needed two Beavers will work together. Dams are built at narrow points in the waterway where the current is fastest. To begin a dam, Beavers jam in branches with the butt ends facing upstream secured with mud and stones, using the current to spread and securely embed them. Layer upon layer, branches and sticks are weaved in, and stones, roots, and mud are packed in. A pair of beavers needs just a few days to build a basic dam . Large dams can be as high as 5m, and as wide as 33m (note how these ‘engineers’ curved the dam for greater strength). Dams are maintained throughout the year, but the most material is added during periods of high water, typically in the spring.
These Beaver-created wetlands results in the loss of habitat for some species, and creation of habitat for others. Because of this role, the Beaver is considered a keystone species. Flooded trees die and attract woodpeckers. Sediments and organic materials accumulate upstream, which increase bottom-dwelling invertebrates that feed on the debris. By felling trees more sun reaches the area, causing increases in plankton, which in turn increases aquatic invertebrates. Habitat is created for aquatic plants like the Watershield, Common Bladderwort, White Water Lily , and Bullhead Lily. Dragonfly and damselfly larvae, whirligig beetles, and water striders are common residents that are preyed upon by American Bullfrogs, Green Frogs, Mink Frogs, Eastern Kingbirds, and Tree Swallows. Beaver ponds also attract waterfowl such as Wood Ducks and Black Ducks. The threatened Blanding’s Turtle may use the lodge and dam to sun on. Moose feed on the water lilies and new shoots. Downstream habitat is likely important for the very rare Hungerford’s Crawling Water Beetle.
Natural succession occurs when Beavers leave the area once their food supply, such as Trembling Aspen, has been depleted, or when a colony is killed. The dam breaks, the area drains, and nutrient rich muck that was previously the pond bottom is colonized by sedges and grasses, and turns into a lush meadows for species such as the Swamp Sparrow, and Meadow Jumping Mouse. Eventually, sun-loving Trembling Aspen return, attracting Beavers yet again.
A great video on Beavers in action can be viewed here.