American Beaver (Castor canadensis)

County List:

Rule:

Forested or Mixed Forested/Nonforested Landscape

      (Lake or Pond or River)
   adjacent to:
      (Aspen (Any Size Class))
      or (Paper Birch (Any Size Class))
      or (Assorted Hardwoods (Any Size Class))
      or (Mixed Upland Hardwoods (Any Size Class))
      or (Balsam Poplar&Swamp Aspen&Swamp Birch (Any Size Class))
      or Lowland Brush

Habitat List:

view size class definitions

Literature:

Today the beaver is common in the northern Great Lakes basin, but less so in southern areas that are heavily urbanized or extensively farmed.

A beaver prefers slow-moving streams or lakes bordered by young forests containing aspen, willow, or alder. Some beavers live in a burrow hollowed out of the bank adjacent to a river or pond. Most often this rodent builds a lodge by piling logs, sticks, and mud on an island or in a shallow pool.

Main food items are the bark, leaves, and twigs of woody plants, particularly aspen, cottonwood, and willow. It supplements the tough diet in the summer with a variety of aquatic and semiaquatic plants, especially the roots, rhizomes, and runners of water lilies. In addition, this burly rodent obtains some of its needed nutrients through coprophagy.

Habitat: Small to large slowly flowing brooks, streams, or rivers that are usually, but not necessarily bordered by woodland.

Special Habitat Requirements: Wetlands that provide an adequate food supply and sufficient water depths.

Habitat Preferences: The semiaquatic habits of the beaver are well-served by the more than 10,000 miles of slow-flowing streams and rivers, and the inland lake shorelines of Michigan. Fast water and streams subject to flash floods, both deterrents to beaver settlement, are rare. Essential aquatic areas must be associated with proper vegetative growth to ensure proper harborage for beaver. Mature waterside forests may attract an occasional beaver colony, but stands of young trees of early successional stages in forest growth, make much more attractive habitat. In fact, beaver populations have prospered where waterside secondary forests of aspen-birch-conifer stands have followed lumbering, clearing, and burning. The replacement of the mature forest by deciduous seral species has produced an overall Michigan environment which is probably more conducive to the beaver's welfare than was the prehistoric. Aspens along with willows, alders and water plants, are needed to make the wetland environment preferable for beaver in Michigan.

Density and Movements: Bradt calculated that the average diameter of aspen (and other trees) cut by beavers was about 2 inches with each colony member needing to cut, on the average, 216 trees per year. If aspen of this diameter grows at the rate of 1,500 to 3,000 trees per acre, a single acre will support one beaver for about seven years or seven beavers for one year.

Since the beaver is essentially adapted to early successional stages of Michigan streamside forest growth, it rarely stays in a given area for long.

Associates: The beaver pond and lodges are rarely permanent additions to the local environment. Beaver logging operations sooner or later deplete the food trees. Removing the seral stage aspen and birch, for example accelerates the plant succession and the development of climax vegetation. The beaver, primarily a seral stage animal, is obliged to depart. The dam then falls into disrepair and the pond drains, leaving the stream trickling through a bare ground environment. This silt-filled area is destined to pass through successional plant-animal stages, the first being a meadow, then a shrub growth, and finally a wooded environment perhaps once again attractive to beaver.

Food Habits: Beavers are almost entirely vegetarians with the ability to eat the soft cambium layer of the bark and the buds, leaves and twigs of certain trees as well as pond and bank-side herbaceous vegetation. There is evidence that non-woody vegetation is the beaver's primary summer food, while woody vegetation is the chief winter fare. The species seems most closely associated with aspens of the genus Populus, with this food tree governing to a large extent the beaver's distribution. In Michigan, when aspen is in short supply, the beaver population will reflect this.

Bradt found that, following aspens, Michigan beaver were fond of maples, then willows. When these favorites are exhausted, Michigan beaver resort to ash, elm, pin cherry, tag alder, white and yellow birch, and sometimes even white cedar, hemlock, and Norway pine. Supposedly, conifers are cut primarily for dam and lodge construction and maintenance. It has not been fully determined if beaver can survive in northern parts of Michigan without some aspen.

The beaver will venture as far from water as 300 to 450 feet in search of food trees.

Usually trees of less than 15 inches in diameter are cut, with beaver apparently making size selections in relation to the distance the cuttings have to be transported. One study showed that there is a decrease in mean size of trees cut with increasing distance from homesites.

Beaver may require as many as 216 trees per beaver per year for food. According to Warren each beaver requires approximately one ton of green aspen per year in order to survive.

Key Components of Habitat: Availability of suitable foods, particularly deciduous woody vegetation within suitable foraging distance (<100 m from water).

Management Actions to Enhance or Maintain Habitat Quality: Managed vegetation (e.g. cutting, burning) to provided early successional stage of vegetation in suitable size-classes.

The benefits to other wildlife species derived from the alteration of aquatic and wetland-associated habitats by beavers have been well documented. The activities of beavers increase habitat diversity by opening the forest canopy, creating standing water, and increasing habitat edge. Habitat potential for river otters is greatly enhanced by the presence of beavers, particularly in stream habitats. Beaver ponds on intermittent streams result in permanent aquatic habitat that is conducive to the river otter's cover and food requirements. The presence of well-established beaver populations within a wetland drainage may account for a partial mitigation of river otter habitat losses. Cover and structural diversity provided by felled trees in and around beaver impoundments, an increased number of potential den sites, and deep water within the impoundment are considered to be the primary attractions of beaver-modified habitats to river otters. Active and abandoned beaver lodges and bank dens are used as resting sites by river otters in Idaho more than any other type of den site. Logs and other debris in and around beaver ponds provide cover for prey populations and foraging cover for otters. Greater amounts of vegetative debris and mud substrates associated with beaver ponds result in a substantial increase in fish biomass compared with that normally present in unaltered stream segments.

Water is the most important feature in the daily lives of beavers. Beavers can live in areas with poor food conditions, but they cannot survive for long in areas where the water supply fluctuates seasonally or is fast-moving. Rocky streams or lakes with rocky shorelines are not preferred. Beavers rarely build houses and feedbeds on large lakes with excessive wave action. Flood prone areas with widely fluctuating water-levels are also poor beaver habitats.

Ideal beaver sites are ponds, small lakes with muddy bottoms, and meandering streams. Granitic bedrock is good because it retains water collected from temporary or seasonal runoffs. In areas such as the Bruce Peninsula, Manitoulin Island, and the Hudson Bay lowlands, which are underlain by porous limestone, beavers have a difficult time maintaining water in their ponds. If food is present, the prairie pothole region is excellent beaver habitat except during periods of drought, when beaver colonies are decimated as water tables drop and many ponds dry up. In the Far North, better beaver habitat occurs in the deltaic complex of the lower Mackenzie River, on thermokarst lakes within the lacustrine basins, and on steams on the gently sloping flanks of large plateaus. Beavers will readily occupy artificial ponds, reservoirs, and drainage ditches if food is available.