Nature Guides

Beavers: The Boreal Engineers

Mid-June 2012

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 speciesFlooded 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 WatershieldCommon 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 BullfrogsGreen FrogsMink FrogsEastern 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.

Other Happenings:

  • Loons are incubating their eggs. They build their nest near shorelines and islands with a pile of aquatic vegetation. Usually the same nest site is used year after year. Loons are easily scared off their nests, which can cause egg chill and increase the likelihood of predation. The low-lying nests are also vulnerable to swamping from boaters. Boaters can help by slowing down near shorelines and avoiding nesting areas. Loons will also nest on loon nest platforms built to attract loons to lakes without suitable habitat. If you consistently paddle a particular lake, consider joining the Canadian Lakes Loon Survey to help monitor population health.
  • Male Mallards  start moulting. Males only stay with females for about the first 10 days of incubation. They then leave breeding territories, and hide out in vegetation while they moult all their feathers, which renders them flightless. Their new “eclipse moult” resembles the female plumage, not their brilliant breeding plumage. Males can still be distinguished by their more yellow bill.
  • Giant silk moths are mating. In fact mating is about all they do - the adults don’t even eat! The females attract males by releasing airborne sex pheromones in extremely small quantities. The large, feathery-looking antennae of males are able to detect these chemical molecules as far as 5km away! Luna and Cecropia Moths are two well-known giant silk moths. Others include the Polyphemus, the Promethea, and the Io Moth. They are attracted to bright, white lights particularly near water.   Note the eyespots on many of the wings.  These may serve to startle predators, or make them think that the moth is actually a predator itself.
  • Brook Sticklebacks are spawning and like members of the Sunfish Family it is the male that does most of the work. The male builds a nest with vegetation, small sticks, and a sticky secretion produced by his kidneys. He entices one or more females to lay eggs in it. The male also fans and defends the eggs and young.
  • Yellow pollen from Eastern White and Red Pines will dust lakes and flat surfaces for a week or more. The pollen grains  have two air bubbles to make it light so that it can be carried by the wind to facilitate pollination.
  • The summer solstice occurs on June 20th this year (more information coming in Late June).  People have been celebrating this event for thousands of years, so consider planning your own celebration of the Earth’s longest day.
  • Mars and Saturn continue to grace the night sky, while Venus, after transiting the sun for the last time in the 21st century June 5-6, pops up with Jupiter in the morning sky.