Wetland Curriculum Resource
Unit 6. Keystone Species - The Beaver
Keystone: The uppermost and last stone put into an arch. It locks all the other stones in place making the structure stable and strong.
In recent years biologists have started to recognise certain animals as keystone species, a species creates or maintains the habitat for many other plants and animals. In Ontario the beaver is just such a species. Unlike most other animals the beaver actually changes to its environment in an active way. In a very short period of time it can alter a fast running forest stream into a beaver pond and wetland meadow, providing a home and breeding place for a whole new range of wild life species. Without the beaver's actions this process would normally take decades or even centuries. The insatiable drive of the beaver to instinctively build dams whenever and wherever they hear the sound of running water has dramatically changed, and is continually changing, Ontario's landscape.
When beavers dam the moving water they create a better environment for themselves. The new pond expands the area of available food. As the water backs up, it floods into the nearby forest edges. The beaver can now reach trees that had been too far back from the stream edge to cut before (beavers are clumsy on land and it is difficult to haul trees any great distance). Sometimes, other food sources, water lilies and other aquatic plants can establish themselves in the slower moving water. The pond decreases the chance for predation by wolves and other predators. In the summer, the beaver can quickly escape into the water. In the winter, while the hungry wolves search for food, the beavers spend their winter in their pond safe under a protective layer of ice.
A single beaver can cut down more than 200 trees every year!
In creating a better environment for itself, the beaver dramatically changes the surrounding ecosystem and changes the lives of many other wild life species. Dams built across streams or lake outlets affect the shoreline vegetation by raising the water table of the pond and shoreline soil. New aquatic plants may move into the area. Trees may be drowned and stand decaying; woodpeckers nest in, or feed on insects in the dead trees, while ospreys or herons build stick nests on the tree-towers.
These water level changes are not simple local events. By disrupting the flow of water, the beaver may indirectly affect the habitats upstream and downstream from its home territory. A small stream in a forest glade may dry up, changing the types of plants and, perhaps, animals that can live there. A seasonal pond, used annually by amphibians and other creatures to lay their eggs may dry up or be flooded. The changes are many and because of the complexity of the ecosystem they are difficult to forecast.
Around the pond itself, the resident wild life species may change. New species of ducks will find beaver ponds perfect for raising their young or for feeding and resting when they migrate. Muskrat, mink and otters occupy beaver ponds and make use of their abandoned lodges and bank dens. Frogs and insects will lay there eggs along the new nooks and crannies created along the waters edge. Great Blue Herons move in to feed on the new frogs.
If kept in repair, dams may stay in place for decades. When beavers abandon their creation, a whole new dynamic is created. Often there has been a significant build up of silt and decaying matter in the beaver pond. As the pond drains due to natural seepage or a break in the dam, a sedge or grass-covered field, a beaver meadow, is created. This provides a whole new habitat for wild life such as Blanding's Turtles, swamp sparrows and meadow jumping mice. Beaver meadows usually have rich fertile soils, and have been farmed by many Ontario settlers and farmers because of this.
A typical beaver environment is illustrated above. Normally a colony made up of 6 to 10 beavers builds a dam or series of dams along a short section of stream In turn, this creates the pond that provides the beaver with food and a place to live. The perfect beaver pond is deep and surrounded by lots of trees. Beaver need deep water to keep the pond from freezing solid in the winter.
The lodge, where the beavers sleep (and eat in winter) is typically divided into two areas, one for sleeping and another for eating. It is partially above the surface and partially underwater. The lodge begins as a mound of sticks, branches and mud piled up be the beavers. When the pile reaches about one metre above the water, the beavers cut a tunnel from underwater up through the pile, to the water line. At this point, the beavers round out a feeding platform a few centimetres above the water line and they form a higher sleeping platform where the floor is covered with shredded wood for comfortable sleeping. The beavers insulate the lodge by plastering the entire outer surface with mud, except for an airhole left at the top.
Located nearby the lodge is a food pile of branches that is submerged in the pond. The beavers cache food here during the summer so that they have enough food to eat during the winter months when they will be contained under the ice and unable to forage for food. Over the winter the beavers will strip the food pile branches of their bark, buds, and leaves, but during the summer they feast on the aquatic plants that flourish in the environment that they created. One of their favourites is water lily. The water lilies are of particular importance because they allow the beavers to spend the summer eating nutritious, easily obtained food without the risk of predation that there would be if they were on land. Also there is no danger of them being squished by a fallen lily pad that was just too big!
The group of beavers that occupy a single pond is called a beaver colony. In summer a typical colony consists of a mated pair of adults, three or four young beavers or "kits" born in May, and one or two yearlings which were born the year before. All capable members of the colony cooperate in lodge building, dam repair, and food gathering.
The beaver is amazingly well-adapted for its lifestyle. View this pop-up table (available here if you cannot display pop-up windows) for details on some of the beaver's remarkable adaptations. The following is a sample of the many books and articles available about the beaver:
Beaver, Hinterland Who's Who. Canadian Wildlife Service (1973)