BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Castor canadensis | Beaver 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Breeding season - Breeding occurs between January and March
[18,30,31,35].  Beavers are generally monogamous, although males will
mate with other females [22,31].  Only the colony's dominant female
breeds, producing one litter a year [30].

Gestation/litter - Gestation period lasts 4 months.  Average litter size
varies between 2.3 and 4.1 [27,30,31,35].  Kits are weaned at 2 to 3
months and can swim by 1 week of age [31,35].

Age at sexual maturity - Beavers become sexually mature between age 2
and 3 [18,36].

Colony/dispersal - The colony consists of three age classes of beavers:
the adults, the kits, and the yearlings born the previous spring
(average 5.1 beavers per colony) [18].  After young beavers reach their
second or third year, they are forced to leave the family group
[18,22,35].  Dispersal may be delayed in areas with high beaver
densities.  Subadults generally leave the natal colony in the late
winter or early spring [30].  Subadult 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) [2].

Life span - Up to 11 years in the wild, 15 to 21 years in
captivity [22,27].

The species is active throughout the year and is usually nocturnal.
Adult beavers are nonmigratory [2].


PREFERRED HABITAT : 
Suitable habitat for 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 [2].
Beavers can usually control water depth and stability on small streams,
ponds, and lakes.  Large lakes or reservoirs (20 acres [8 ha] 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 beavers [2,28].
Intermittent streams or streams that have major fluctuations in
discharge will have little year-round value as beaver habitat [2].

Stream characteristics such as gradient, depth, and width are
determining factors in habitat use by beaver [2,11].  Steep topography
prevents the establishment of a food transportation system [2].
Additionally, narrow valley bottoms cannot support the large amounts of
vegetation needed by beavers.  Consequently beaver populations in narrow
valley bottoms are more cyclic than are populations in wider valley
bottoms [24].  Valleys less than 150 feet (46 m) wide are occupied less
frequently [2,24].  One study found that 68 percent of the beaver
colonies recorded in Colorado were in valleys with a stream gradient of
less than 6 percent.  No 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 beaver habitat, while valleys wider than
the stream channel were frequently occupied by beavers [24].

Food availablity is another factor determining suitable habitat fpr
beavers [11].  Marshes, ponds, and lakes are often occupied by beavers
when an adequate supply of food is available.  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 [2].


COVER REQUIREMENTS : 
The lodge is the major source of escape, resting, thermal, and
reproductive cover for beavers.  Lodges may be surrounded by water or
constructed against a bank.  Water protects the lodge from predators and
provides concealment for beavers when traveling to and from food
gathering areas and caches [2].  On lakes and ponds, lodges are
frequently situated in areas that provide shelter from wind, waves, and
ice [2].  Damming large streams with swift, turbulant waters creates
calm pools for feeding and resting [11].


FOOD HABITS : 
Beavers are herbivores.  During late spring and summer their diet
consists mainly of fresh herbaceous matter [2,18].  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 [2].

Winter is a critical period, especially for colonies on streams because
they must subsist solely on their winter food caches.  In contrast with
stream 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 [18].

Aquatic vegetation such as duck-potato (Sagittaria spp.), duckweed
(Lemma spp.), pondweed (Potamogeton spp.), and water weed (Elodea spp.)
are preferred foods when available [2].  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 [2,18].
Other important winter foods of beavers living on lakes include the
rhizomes of sedges and the rootstocks of mat-forming shrubs [18].

Important woody foods of 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) [2,18,22].  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)
[18,21].  Aspen and willows are considered preferred beaver foods;
however, these are generally riparian tree species and may be more
available for beaver foraging but not necessarily preferred over all
other deciduous tree species.  Beavers have been reported to subsist in
some areas by feeding on conifer trees; however, these trees are a poor
quality source of food [2].

Woody stems cut by 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 [2].


PREDATORS : 
Beavers have few natural predators.  However, in certain areas, 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.).  Alligators, minks (Mustela
vison), otters (Lutra canadensis), hawks, and owls periodically prey on
kits [19,22,27].  Humans kill beavers for their fur [18,22].


MANAGEMENT CONSIDERATIONS : 
Beavers will live in close proximity to humans if all habitat
requirements are met [27].  However, railways, roads, and land clearing
adjacent to waterways may affect beaver habitat suitablity.  Transplants
of 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 beaver if
suitable foods are present [2].

Beaver activity can have a significant influence on stream and riparian
habitats [3,14,24,30].  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
[2].  Additionally, beaver ponds conserve spring runoff, thus ensuring
more constant stream flow, diminishing floods, conserving soil, and
helping maintain the water table [12].

Through tree harvesting activity, beavers can have an effect on natural
succession.  According to Barnes and Dibble [3] tree cutting by beavers
on the lower Chippewa River in west-central Wisconsin will alter the
course of succession on the riverbottom site studied.  Beavers were
selective in their choice of woody plants, preferring ash (Fraxinus
spp.) and hickory (Carya spp.) over all other woody plants.  These
authors predict a major reduction in density for future populations of
ash, hickory, and hackberry (Celtis spp.) in areas of beaver activity
and an increase in the density of basswood (Tilia spp.) and elm (Ulmus
spp.) [3].

Beaver activity can be beneficial to some wildlife species [13,30].
Waterfowl often benefit from the increased edge, diversity, and
invertebrate communities created by beaver activity [30].  Occupied
beaver-influenced sites produce more waterfowl because of improved water
stability and increased brood-rearing cover; the production declines
with beaver abandonment.  Great-blue herons (Ardea herodias), ospreys
(Pandion halietus), eagles (Haleaeitus leucocephalus), kingfishers
(Ceryle alcyon), and many species of songbirds benefit from beaver
activity as well.  Otters, raccoons (Procyon lotor), mink, and muskrat
(Ondatra zibithica) thrive on the increased foraging areas produced by
beaver activity.  Berry-producing shrubs and brush in areas cut over by
beavers attract white-tailed deer (Odocoileus virginianus) and black
bear (Ursus americanus) [30].
 
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 beaver ponds [8].  Smallmouth bass (Micropterus dolomieuis) and
northern pike (Esok lucius) also benefit from beaver impoundments [30].
In some instances beaver ponds have provided up to six times the total
weight of salmonids per acre than that in adjacent stream habitat
without beaver ponds [24].  In areas of marginal trout habitat, however,
beaver activity can reduce trout production.  Beaver-caused loss of
streamside shade and diminished water velocity can result in lethal
water temperatures [30].

The amount of influence that cattle have on riparian environment can be
reduced by beaver activity in many valley bottoms.  If 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 [24].

Beaver activity can also have detrimental effects.  Beaver-caused
flooding often kills valuable lowland timber [30].  Human/beaver
conflicts occur when beavers flood roadways and agricultural lands, and
dam culverts and irrigation systems.  The economic cost of nuisance
beaver activities often exceeds the value of their pelts and has been
estimated at $75 to $100 million annually in the United States.
Additionally, beavers have potential to increase water-borne pathogens
(including Giardia lamblia) downstream from their activity [30].
 
Beavers are harvested for their pelts.  In most states with substantial
beaver populations, the species is now managed to provide a reliable
annual harvest and a relatively stable population [12].


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Castor canadensis | Beaver