North American Beaver in Detail.

The American beaver is found throughout most of North America except in the Arctic tundra, peninsular Florida, and the Southwestern deserts.

Breeding season - Breeding occurs between January and March. American beavers are generally monogamous, although males will mate with other females. Only the colony's dominant female breeds, producing one litter a year.

Gestation/litter - Gestation period lasts 4 months. Average litter size varies between 2.3 and 4.1 .Kits are weaned at 2 to 3 months and can swim by 1 week of age.

Age at sexual maturity - American beavers become sexually mature between age 2 and 3.

Colony/dispersal - The colony consists of three age classes of American beavers: the adults, the kits, and the yearlings born the previous spring (average 5.1 American beavers per colony). After young American beavers reach their second or third year, they are forced to leave the family group. Dispersal may be delayed in areas with high American beaver densities. Subadults generally leave the colony in the late winter or early spring. Subadult American beavers have been reported to migrate as far as 147 miles (236 km), although average migration distances range from 5 to 10 miles (8-16 km).

Life span - Up to 11 years in the wild, 15 to 21 years in captivity. The largest threat to American beavers is being crushed by falling trees. The species is active throughout the year and is usually nocturnal.

Diet American beavers are herbivores. During late spring and summer their diet consists mainly of fresh herbaceous matter. American beavers appear to prefer herbaceous vegetation over woody vegetation during all seasons if it is available. Woody vegetation may be consumed during any season, although its highest utilization occurs from late fall through early spring when herbaceous vegetation is not available. The majority of the branches and stems of woody vegetation are cached for later use during the winter. Winter is a critical period, especially for colonies on streams because they must subsist solely on their winter food caches. In contrast with stream American beavers, colonies on lakes are not solely dependent on their stores of woody vegetation; they can augment their winter diet of bark with aquatic plants. Aquatic vegetation such as duck-potato (Sagittaria spp.), duckweed (Lemma spp.), pondweed (Potamogeton spp.), and water weed (Elodea spp.)are preferred foods when available. The thick, fleshy rhizomes of water lilies (Nymphaea spp. and Nuphar spp.) may be used as a food source throughout the year. If present in sufficient amounts, water lily rhizomes may provide an adequate winter food source, resulting in little or no tree cutting or food caching of woody materials. Other important winter foods of American beavers living on lakes include the rhizomes of sedges and the rootstocks of mat-forming shrubs. Important woody foods of American beavers include quaking aspen, willow,cottonwood, alder, red maple (Acer rubrum), serviceberry (Amelanchier spp.), mountain maple (Acer glabrum), red-osier dogwood, and green ash (Fraxinus pennsylvanica). Other woody species occasionally utilized for food include sugar maple (Acer saccharum), black ash (Fraxinus nigra), yellow birch (Betula alleghaniensis), hazels (Corylus spp.), hemlocks (Tsuga spp.), and Oregon crab apple (Malus fusca). Aspen and willows are considered preferred American beaver foods; however, these are generally riparian tree species and may be more available for American beaver foraging but not necessarily preferred over all other deciduous tree species. American beavers have been reported to subsist in some areas by feeding on conifer trees; however, these trees are a poor quality source of food.

Woody stems cut by American beavers are usually less than 3 to 4 inches (7.6-10.1 cm) in d.b.h. One study reported that trees of all size classes were felled close to the water's edge, while only smaller diameter trees were felled farther from the shore. Trees and shrubs closest to the water's edge are generally utilized first.

Predators: American beavers have few natural predators. However, in certain areas, American beavers may face predation pressure from wolves (Canis lupus), coyotes (Canis latrans), lynx (Felis lynx), fishers (Martes pennanti), wolverines (Gulo gulo), and occasionally bears (Ursus spp.). Minks (Mustela vison), otters (Lutra canadensis), hawks, and owls periodically prey on kits. Humans kill American beavers for their fur.

Management: American beavers will live in close proximity to humans if all habitat requirements are met. However, railways, roads, and land clearing adjacent to waterways may affect American beaver habitat suitability. Transplants of American beaver may be successful on strip mined land or in new impoundments where water conditions are relatively stable. Highly acidic waters, which often occur in strip-mined areas, are acceptable for American beaver if suitable foods are present.

American beaver activity can have a significant influence on stream and riparian habitats. American beavers are the only mammals in North America other than humans that can fell mature trees; therefore, their ability to decrease forest biomass is much greater than that of other herbivores. Additionally, American beaver ponds conserve spring runoff, thus ensuring more constant stream flow, diminishing floods, conserving soil, and helping maintain the water table.

Through tree harvesting activity, American beavers can have an effect on natural succession. American beaver activity can be beneficial to some wildlife species. Waterfowl often benefit from the increased edge, diversity, and invertebrate communities created by American beaver activity. Occupied American beaver-influenced sites produce more waterfowl because of improved water stability and increased brood-rearing cover; the production declines with American beaver abandonment. Great-blue herons (Ardea herodias), ospreys (Pandion halietus), eagles (Haleaeitus leucocephalus), kingfishers (Ceryle alcyon), and many species of songbirds benefit from American beaver activity as well. Otters, mink, and muskrat (Ondatra zibithica) thrive on the increased foraging areas produced by American beaver activity. Berry-producing shrubs and brush in areas cut over by American beavers attract black bear (Ursus americanus).

American beaver activity can also improve fish habitat. Production of three trout species (Salomo spp. and Salvelines fontinalis) in a stream in the Sierra Nevada increased due to a higher standing crop of invertebrates in American beaver ponds. Smallmouth bass (Micropterus dolomieuis) and northern pike (Esok lucius) also benefit from American beaver impoundments. In some instances American beaver ponds have provided up to six times the total weight of salmonids per acre than that in adjacent stream habitat without American beaver ponds. In areas of marginal trout habitat, however, American beaver activity can reduce trout production. American beaver-caused loss of streamside shade and diminished water velocity can result in lethal water temperatures.

The amount of influence that cattle have on riparian environment can be reduced by American beaver activity in many valley bottoms. If American beavers are thoroughly established in wide valley willow habitats prior to the introduction of cattle, the immediate effect of cattle on the stream is often minor. American beaver activity can also have detrimental effects. American beaver-caused flooding often kills valuable lowland timber. Human/American beaver conflicts occur when American beavers flood roadways and agricultural lands, and dam culverts and irrigation systems. The economic cost of nuisance American beaver activities often exceeds the value of their pelts and has been estimated at $75 to $100 million annually in the United States. Additionally, American beavers have potential to increase water-borne pathogens (including Giardia lamblia) downstream from their activity.

American beavers are harvested for their pelts. In most states with substantial American beaver populations, the species is now managed to provide a reliable annual harvest and a relatively stable population.

Gray Wolf

Pacific beaver (Castor canadensis leucodenta) - are found along the Coast Ranges from California to Alaska.

Suitable habitat for American beavers must contain all of the following: stable aquatic habitat providing adequate water; channel gradient of less than 15 percent; and quality food species present in sufficient quantity.

Adult American beavers are non-migratory.

American beavers can usually control water depth and stability on small streams, ponds, and lakes. Large lakes or reservoirs (20 acres in surface area) with irregular shorelines provide optimum habitat for the species.Lakes and reservoirs that have extreme annual or seasonal fluctuations in the water level are generally unsuitable habitat for American beavers. Intermittent streams or streams that have major fluctuations in discharge will have little year-round value as American beaver habitat.

Stream characteristics such as gradient, depth, and width are determining factors in habitat use by American beaver. Steep topography prevents the establishment of a food transportation system. Additionally, narrow valley bottoms cannot support the large amounts of vegetation needed by American beavers. Consequently American beaver populations in narrow valley bottoms are more cyclic than are populations in wider valley bottoms. Valleys less than 150 feet (46 m) wide are occupied less frequently. One study found that 68 percent of the American beaver colonies recorded in Colorado were in valleys with a stream gradient of less than 6 percent. No American beaver colonies were recorded in streams with a gradient of 15 percent or more. Valleys that were only as wide as the stream channel were unsuitable American beaver habitat, while valleys wider than the stream channel were frequently occupied by American beavers.

Food availability is another factor determining suitable habitat for American beavers. Marshes, ponds, and lakes are often occupied by American beavers when an adequate supply of food is available. American beavers generally forage no more than about 300 feet (90 m) from water; however, foraging distances of up to 656 feet (200 m) have been reported. The lodge is the major source of escape, resting, thermal, and reproductive cover for American beavers. Lodges may be surrounded by water or constructed against a bank. Water protects the lodge from predators and provides concealment for American beavers when traveling to and from food gathering areas and caches. On lakes and ponds, lodges are frequently situated in areas that provide shelter from wind, waves, and ice. Damming large streams with swift, turbulent waters creates calm pools for feeding and resting.