North American beaver
|A male North American beaver|
|Distribution of the North American beaver (dark green – native, light green – introduced)|
The North American beaver (Castor canadensis) is one of two extant beaver species. It is native to North America and introduced to Patagonia in South America and some European countries (e.g., Norway). In the United States and Canada, the species is often referred to simply as "beaver", though this causes some confusion because another distantly related rodent, Aplodontia rufa, is often called the "mountain beaver". Other vernacular names, including American beaver and Canadian beaver, distinguish this species from the other extant beaver species, Castor fiber, which is native to Eurasia.
- 1 Description
- 2 Behaviour
- 3 Reproduction
- 4 Subspecies
- 5 Differences from European beaver
- 6 Ecology
- 6.1 Effects on stream flows and water quality
- 6.2 Effects on bird abundance and diversity
- 6.3 Effects on trout and salmon
- 6.4 Effects on riparian trees and vegetation
- 6.5 Beavers and stream restoration
- 7 Urban beavers
- 8 As introduced non-native species
- 9 As food
- 10 Symbolism
- 11 See also
- 12 References
- 13 Further reading
- 14 External links
This beaver is the largest rodent in North America and competes with its Eurasian counterpart, the European beaver, for being the second-largest in the world, both following the South American capybara. Adults usually weigh from 11 to 32 kg (24 to 71 lb), with 20 kg (44 lb) being typical. The head-and-body length is 74–90 cm (29–35 in), with the tail adding a further 20–35 cm (7.9–13.8 in). Very old individuals can exceptionally exceed normal sizes, weighing more than 40 kg (88 lb) or even as much as 50 kg (110 lb).
Like the capybara, the beaver is semi-aquatic. The beaver has many traits suited to this lifestyle. It has a large flat paddle-shaped tail and large, webbed hind feet reminiscent of a human diver's swimfins. The unwebbed front paws are smaller, with claws. The eyes are covered by a nictitating membrane which allows the beaver to see underwater. The nostrils and ears are sealed while submerged. A thick layer of fat under its skin insulates the beaver from its cold water environment.
The beaver's fur consists of long, coarse outer hairs and short, fine inner hairs (see Double coat). The fur has a range of colours but usually is dark brown. Scent glands near the genitals secrete an oily substance known as castoreum, which the beaver uses to waterproof its fur.
Before their near extirpation by trapping in North America, beaver were practically ubiquitous and lived from the arctic tundra to the deserts of northern Mexico, and from the Atlantic to the Pacific Oceans. Physician naturalist Edgar Alexander Mearns' 1907 report of beaver on the Sonora River may be the southernmost extent of the range of this North American aquatic mammal. However, beaver have also been reported both historically and contemporaneously in Mexico on the Colorado River, Bavispe River and San Bernardino River.
Beavers are active mainly at night. They are excellent swimmers and may remain submerged for up to 15 minutes. More vulnerable on land, they tend to remain in the water as much as possible. They use their flat, scaly tail both to signal danger by slapping the surface of the water and as a location for fat storage.
They construct their homes, or "lodges," out of sticks, twigs, rocks and mud in lakes, streams, and tidal river deltas. These lodges may be surrounded by water, or touching land, including burrows dug into river banks. Beavers are well known for building dams across streams and constructing their lodge in the artificial pond which forms. When building in a pond, the beavers first make a pile of sticks and then eat out one or more underwater entrances and two platforms above the water surface inside the pile. The first is used for drying off. Towards winter, the lodge is often plastered with mud which when it freezes has the consistency of concrete. A small air hole is left in the top of the lodge.
The purpose of the dam is to create deep water refugia enabling the beaver to escape from predators. When deep water is already present in lakes, rivers or larger streams, the beaver may dwell in a bank burrow and bank lodge with an underwater entrance. The beaver dam is constructed using branches from trees the beavers cut down, as well as rocks, grass, and mud. The inner bark, twigs, shoots, and leaves of such trees are also an important part of the beaver's diet. The trees are cut down using their strong incisor teeth. Their front paws are used for digging and carrying and placing materials. Some researchers have shown that the sound of running water dictates when and where a beaver builds its dam. Besides providing a safe home for the beaver, beaver ponds also provide habitat for waterfowl, fish, and other aquatic animals. Their dams help reduce soil erosion and can help reduce flooding. However, beaver dams are not permanent and depend on the beavers' continued presence for their maintenance. Beavers generally concentrate on building and repairing dams in the fall in preparation for the coming winter. In northern areas, they often don't repair breaches in the dam made by otters, and sometimes breach the dam themselves and lower the water level in the pond in order to create more breathing space under the ice or get easier access to trees below the dam. In a 1988 study in Alberta, Canada, no beavers repaired "sites of water loss" during the winter. Of 178 sites of water loss, beavers repaired 78 when water was opened, and did not repair 68. The rest were partially repaired.
Beavers are best known for their dam-building. They maintain their pond-habitat by reacting quickly to the sound of running water, and damming it up with tree branches and mud. Early ecologists believed that this dam-building was an amazing feat of architectural planning, indicative of the beaver's high intellect. This theory was tested when a recording of running water was played in a field near a beaver pond. Despite the fact that it was on dry land, the beaver covered the tape player with branches and mud. The largest beaver dam is 2,790 ft (850 m) in length—more than half a mile long—and was discovered via satellite imagery in 2007. It is located on the southern edge of Wood Buffalo National Park in northern Alberta and is twice the width of the Hoover Dam which spans 1,244 ft (379 m).
Normally, the purpose of the dam is to provide water around their lodges that is deep enough that it does not freeze solid in winter. The dams also flood areas of surrounding forest, giving the beaver safe access to an important food supply, which is the leaves, buds, and inner bark of growing trees. They prefer aspen and poplar, but will also take birch, maple, willow, alder, black cherry, red oak, beech, ash, hornbeam and occasionally pine and spruce. They will also eat cattails, water lilies and other aquatic vegetation, especially in the early spring (and contrary to widespread belief, they do not eat fish). In areas where their pond freezes over, beavers collect food in late fall in the form of tree branches, storing them underwater (usually by sticking the sharp chewed base of the branches into the mud on the pond bottom), where they can be accessed through the winter. Often the pile of food branches projects above the pond and collects snow. This insulates the water below it and keeps the pond open at that location.
Beavers usually mate for life. The young beaver "kits" typically remain with their parents for up to two years.
Common natural predators include gray wolves, coyotes, and mountain lions. Beaver can be particularly important food for lone wolves. American black bears may also prey on beavers if the opportunity arises, often by smashing their paws into the beaver's lodges. Perhaps due to differing habitat preferences, Brown bears were not known to hunt beavers in Denali National Park. Less significant predators include wolverines, Canadian lynx, bobcats, and fox, which are increasingly unlikely to take full-grown beavers due to their smaller size; and American alligators, which only minimally co-exist with beavers. Both golden eagles (Aquila chrysaetos) and bald eagles (Haliaeetus leucocephalus) may on occasion predate a beaver, most likely only small kits. Despite repeated claims, there is no evidence that river otters are predators of beavers.
North American beaver have one litter per year, coming into estrus for only 12 to 24 hours, between late December and May but peaking in January. Unlike most other rodents, beaver pairs are monogamous, staying together for multiple breeding seasons. Gestation averages 128 days and they average two to three kits per litter with a range of two to six kits. Most beaver do not reproduce until they are three years of age, but about 20% of two-year-old females reproduce.
The first fossil records of beaver are 10 to 12 million years old in Germany, and they are thought to have migrated to North America across the Bering Strait. The oldest fossil record of beaver in North America are of two beaver teeth near Dayville, Oregon, and are 7 million years old.
At one time, 25 subspecies of beaver were identified in North America, with distinctions based primarily on slight morphological differences and geographical isolation at the time of discovery. However, modern techniques generally use genetics rather than morphology to distinguish between subspecies, and currently the Integrated Taxonomic Information System, (which provides authoritative taxonomic information on plants, animals, fungi, and microbes of North America and the world), does not recognize any subspecies of C. canadensis, though a definitive genetic analysis has not been performed. Such an analysis would be complicated by the fact that there has been substantial genetic mixing of populations because of the numerous reintroduction efforts intended to help the species recover following extirpation from many regions.
The most widespread (formerly recognized) subspecies, which perhaps are now best thought of as populations with some distinct physical characteristics, are C. c. acadicus (New England beaver), C. c. canadensis (Canadian beaver), C. c. carolinensis (Carolina beaver), and C. c. missouriensis (Missouri River beaver). The Canadian beaver originally inhabited almost all of the forested area of Canada, and because of its more valued fur, was often selected for reintroductions elsewhere. The Carolina beaver is found in the southeastern United States; the Missouri River beaver, as its name suggests, is found in the Missouri River and its tributaries; and C. c. acadicus is found throughout the New England area in the northeastern United States.
Differences from European beaver
Although North American beavers are superficially similar to the European beaver (Castor fiber), there are several important differences between the two species. North American beavers tend to be slightly smaller, with smaller, more rounded heads; shorter, wider muzzles; thicker, longer and darker underfur; wider, more oval-shaped tails and longer shin bones, allowing them a greater range of bipedal locomotion than the European species. North American beavers have shorter nasal bones than their European cousins, with the widest point being at the middle of the snout for the former, and in the tip for the latter. The nasal opening for the North American species is square, unlike that of the European race which is triangular. The foramen magnum is triangular in the North American beaver, and rounded in the European. The anal glands of the North American beaver are smaller and thick-walled with a small internal volume compared to that of the European species. Finally, the guard hairs of the North American beaver have a shorter hollow medulla at their tips. Fur colour is also different. Overall, 50% of North American beavers have pale brown fur, 25% are reddish brown, one fifth are brown and 6% are blackish, while in European beavers 66% have pale brown or beige fur, 20% have reddish brown, nearly 8% are brown and only 4% have blackish coats.
The two species are not genetically compatible. North American beavers have 40 chromosomes, while European beavers have 48. Also, more than 27 attempts were made in Russia to hybridize the two species, with one breeding between a male North American beaver and a female European resulting in one stillborn kit. These factors make interspecific breeding unlikely in areas where the two species' ranges overlap.
The beaver was trapped out and almost extirpated in North America because its fur and castoreum were highly sought after. The beaver furs were used to make clothing and beaver hats. In the United States extensive trapping began in the early 17th century with more than 10,000 beaver per year taken for the fur trade in Connecticut and Massachusetts between 1620 and 1630. From 1630 to 1640, approximately 80,000 beaver were taken annually from the Hudson River and western New York. From 1670 onwards, the Hudson's Bay Company sent two or three trading ships into the bay every year to take furs to England from Canada. As eastern beaver populations were depleted, English, French and American trappers pushed west. Much of the westward expansion and exploration of North America was driven by the quest for this animal's fur. Before the 1849 California Gold Rush, there was an earlier 19th century California Fur Rush which drove the earliest American settlement in that State. During the approximately 30 years (1806–1838) of the era of the Mountain Man, the West from Missouri to California and from Canada to Mexico was thoroughly explored and the beaver was brought to the brink of extinction.
With protection in the late 19th and early 20th centuries, the current beaver population has rebounded to an estimated 10 to 15 million; this is a fraction of the originally estimated 100 to 200 million North American beaver before the days of the fur trade.
These animals are considered pests in parts of their range because their dams can cause flooding, or because their habit of felling trees can pose danger to people, as in Charlotte, North Carolina's Park Road Park. Because they are persistent in repairing damage to the dam, they were historically relocated or exterminated. Non-lethal methods of containing beaver-related flooding have been developed. One such flow device has been utilized by both the Canadian and U.S. governments, called "Beaver Deceivers," or levelers, invented and pioneered by wildlife biologist, Skip Lisle.
The beaver is a keystone species, increasing biodiversity in its territory through creation of beaver ponds and wetlands. As wetlands are formed and riparian habitats enlarged, aquatic plants colonize newly available watery habitat. Insect, invertebrate, fish, mammal, and bird diversity are also expanded.
Effects on stream flows and water quality
Beaver ponds increase stream flows in seasonally dry streams by storing run-off in the rainy season, which raises groundwater tables via percolation from beaver ponds. In a recent study using 12 serial aerial photo mosaics from 1948 to 2002, the impact of the return of beaver on open water area in east-central Alberta, Canada found that the mammals were associated with a 9-fold increase in open water area. Beaver returned to the area in 1954 after a long absence since their extirpation by the fur trade in the 19th century. During drought years, where beaver were present, there was 60% more open water than those same areas during previous drought periods when beaver were absent. The authors concluded that beaver have a dramatic influence on the creation and maintenance of wetlands even during extreme drought.
From streams in the Maryland coastal plain to Lake Tahoe, beaver ponds have been shown to remove sediment and pollutants including total suspended solids, total nitrogen, phosphates, carbon and silicates, improving stream water quality. In addition, fecal coliform and streptococci bacteria excreted into streams by grazing cattle are reduced by beaver ponds, where slowing currents lead to settling of the bacteria in bottom sediments.
The term "beaver fever" is a misnomer coined by the American press in the 1970s, following findings that the parasite Giardia lamblia, which causes Giardiasis, was putatively carried by beavers. Further research has shown that many animals and birds carry this parasite, and the major source of water contamination is by humans. Recent concerns point to domestic animals as a significant vector of giardia, with young calves in dairy herds testing as high as 100% positive for giardia. In addition, fecal coliform and streptococci bacteria excreted into streams by grazing cattle have been shown to be reduced by beaver ponds, where the bacteria are trapped in bottom sediments. New Zealand has giardia outbreaks but no beaver, whereas Norway has plenty of beaver but had no giardia outbreaks until recently (in a southern part of Norway densely populated by humans but no beaver).
Effects on bird abundance and diversity
Beaver help waterfowl by creating increased areas of water, and in northerly latitudes they thaw areas of open water, allowing an earlier nesting season. In a study of Wyoming streams and rivers, watercourses with beaver had 75-fold more ducks than those without.
Trumpeter swans (Cygnus buccinator) and Canada geese (Branta canadensis) often depend on beaver lodges as nesting sites. Canada's small trumpeter swan population was observed not to nest on large lakes, preferring instead to nest on the smaller lakes and ponds associated with beaver activity.
Beaver may benefit birds frequenting their ponds in several additional ways. Removal of some pondside trees by beavers would increase the density and height of the grass–forb–shrub layer, which enhances waterfowl nesting cover adjacent to ponds. Both forest gaps where trees had been felled by beaver and a "gradual edge" described as a complex transition from pond to forest with intermixed grasses, forbs, saplings, and shrubs are strongly associated with greater migratory bird species richness and abundance. Coppicing of waterside willows and cottonwoods by beavers leads to dense shoot production which provides important cover for birds and the insects they feed on. Widening of the riparian terrace alongside streams is associated with beaver dams and has been shown to increase riparian bird abundance and diversity, an impact that may be especially important in semi-arid climates.
As trees are drowned by rising beaver impoundments, they become ideal nesting sites for woodpeckers, who carve cavities that attract many other bird species including flycatchers (Empidonax spp.), tree swallows (Tachycineta bicolor), tits (Paridae spp.), wood ducks (Aix sponsa), goldeneyes (Bucephala spp.), mergansers (Mergus spp.), owls (Tytonidae, Strigidae) and American kestrels (Falco sparverius). Piscivores, including herons (Ardea spp.), grebes (Podicipedidae), cormorants (Phalacrocorax ssp.), American bitterns (Botaurus lentiginosa), great egret (Ardea alba), snowy egret (Egretta thula), mergansers and belted kingfishers (Megaceryle alcyon), utilize beaver ponds for fishing. Hooded mergansers (Lophodytes cucullatus), green heron (Butorides virescens), great blue heron (Ardea herodias) and belted kingfisher appeared more frequently in New York wetlands where beaver were active than at sites with no beaver activity.
Effects on trout and salmon
Beaver ponds as rearing habitat for salmonids
Beaver ponds have been shown to have a beneficial effect on trout and salmon populations. Many authors believe that the decline of salmonid fishes is related to the decline in beaver populations. Research in the Stillaguamish River basin in Washington state, found that extensive loss of beaver ponds resulted in an 89% reduction in coho salmon (Oncorhynchus kisutch) smolt summer production and an almost equally detrimental 86% reduction in critical winter habitat carrying capacity. This study also found that beaver ponds increased smolt salmon production 80 times more than the placement of large woody debris. Swales and Leving had previously shown on the Coldwater River in British Columbia that off-channel beaver ponds were preferentially populated by coho salmon over other salmonids and provided overwintering protection, protection from high summer snowmelt flows and summer coho rearing habitat. The presence of beaver dams has also been shown to increase either the number of fish, their size, or both, in a study of brook trout (Salvelinus fontinalis), rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) in Sagehen Creek, which flows into the Little Truckee River at an altitude of 5,800 feet in the northern Sierra Nevada. These findings are consistent with a study of small streams in Sweden, that found that brown trout were larger in beaver ponds compared with those in riffle sections, and that beaver ponds provide habitat for larger trout in small streams during periods of drought. Similarly, brook trout, coho salmon and sockeye salmon (Oncorhynchus nerka) were significantly larger in beaver ponds than those in un-impounded stream sections in Colorado and Alaska. In a recent study on a headwater Appalachian stream, brook trout were also larger in beaver ponds.
Beaver ponds and salmon passage
Contrary to popular myth, most beaver dams do not pose barriers to trout and salmon migration, although they may be restricted seasonally during periods of low stream flows. In a meta-review of studies claiming that beaver dams act as fish passage barriers Kemp et al. found that 78% of these claims were not supported by any data. In a 2013 study of radiotelemetry-tagged Bonneville cutthroat trout (Oncorhynchus clarki utah) and brook trout (Salvelinus fontinalis) in Utah, both of these fish species crossed beaver dams in both directions, including dams up to 2 metres (6.6 ft) high. Rainbow, brown and brook trout have been shown to cross as many as 14 consecutive beaver dams. Both adults and juveniles of coho salmon, steelhead trout, sea run cutthroat (Oncorhyncus clarki clarki), Dolly Varden trout (Salvelinus malma malma), and sockeye salmon are able to cross beaver dams. In southeast Alaska, coho jumped dams as high as two meters, were found above all beaver dams and had their highest densities in streams with beaver. In Oregon coastal streams, beaver dams are ephemeral and almost all wash out in high winter flows only to be re-built every summer. Migration of adult Atlantic salmon (Salmo salar) may be limited by beaver dams but the presence of juveniles upstream from the dams suggests that the dams are penetrated by parr. Downstream migration of Atlantic salmon smolts was similarly unaffected by beaver dams, even in periods of low flows. Two year old Atlantic salmon parr in beaver ponds in eastern Canada showed faster summer growth in length and mass and were in better condition than parr upstream or downstream from the pond.
Beaver ponds role in winter survival of salmonids
The importance of winter habitat to salmonids afforded by beaver ponds may be especially important (and underappreciated) in streams without deep pools or where ice cover makes contact with the bottom of shallow streams. Enos Mills wrote in 1913, "One dry winter the stream...ran low and froze to the bottom, and the only trout in it that survived were those in the deep holes of beaver ponds." Cutthroat trout (Oncorhynchus clarki) and bull trout (Salvelinus confluentus) were noted to overwinter in Montana beaver ponds, brook trout congregated in winter in New Brunswick and Wyoming beaver ponds, and coho salmon in Oregon beaver ponds. In 2011, a meta-analysis of studies of beaver impacts on salmonids found that beaver were a net benefit to salmon and trout populations primarily by improving habitat (building ponds) both for rearing and overwintering and that this conclusion was based over half the time on scientific data. In contrast, the most often cited negative impact of beavers on fishes were barriers to migration although that conclusion was based on scientific data only 22% of the time. They also found that when beaver dams do present barriers, these are generally short-lived, as the dams are overtopped, blown out, or circumvented by storm surges.
Beavers importance to salmonid survival
By creating additional channel network complexity, including ponds and marshes laterally separated from the main channel, beavers may play a role in the creation and maintenance of fish biodiversity. In off-mainstem channels restored by beaver on the middle section of Utah's Provo River, native fish species persist even when they have been extirpated in the mainstem channel by competition from introduced non-native fish. Efforts to restore salmonid habitat in the western United States have focused primarily on establishing large woody debris in streams to slow flows and create pools for young salmonids. Research in Washington state found that the average summer smolt production per beaver dam ranges from 527 to 1,174 fish, whereas the summer smolt production from a pool formed by instream large woody debris is about 6–15 individuals, suggesting that re-establishment of beaver populations would be 80 times more effective.
Beavers impact on fish in estuarine systems
Recently, beaver have been discovered living in brackish water in estuarine tidal marshes where Chinook salmon (Oncorhynchus tshawytscha) densities were five times higher in beaver ponds than in neighboring areas.
Effects on riparian trees and vegetation
Conventional wisdom has held that beaver girdle and fell trees and that they diminish riparian trees and vegetation, but the opposite appears to be true when studies are conducted longer-term. In 1987, Beier reported that beaver had caused local extinction of Quaking aspen (Populus tremuloides) and Black cottonwood (Populus trichocarpa) on 4–5% of stream reaches on the lower Truckee River in the Sierra Nevada mountains; however Willow (Salix spp.) responded by re-growing vigorously in most reaches. He further speculated that without control of beaver populations, aspen and cottonwood could go extinct on the Truckee River. Not only have aspen and cottonwood survived ongoing beaver colonization, but a recent study of ten Sierra Nevada streams in the Lake Tahoe basin utilizing aerial multispectral videography has shown that deciduous, thick herbaceous, and thin herbaceous vegetation are more highly concentrated near beaver dams, whereas coniferous trees are decreased. These findings are consistent with those of Pollock, who reported that in Bridge Creek, a stream in semi-arid eastern Oregon, the width of riparian vegetation on stream banks was increased several-fold as beaver dams watered previously dry terraces adjacent to the stream. In a second study of riparian vegetation based on observations of Bridge Creek over a 17-year time period, although portions of the study reach were periodically abandoned by beaver following heavy utilization of streamside vegetation, within a few years, dense stands of woody plants of greater diversity occupied a larger portion of the floodplain. Although black cottonwood and thinleaf alder did not generally re-sprout after beaver cutting, they frequently grew from seeds landing on freshly exposed alluvial deposits subsequent to beaver activity. Therefore, beaver appear to increase riparian vegetation given enough years to aggrade sediments and pond heights sufficiently to create widened, well-watered riparian zones, especially in areas of low summer rainfall.
The surface of beaver ponds is typically at or near bank-full, so even small increases in stream flows cause the pond to overflow its banks. Thus, high stream flows spread water and nutrients beyond the stream banks to wide riparian zones when beaver dams are present.
Finally, beaver ponds may serve as critical firebreaks in fire-prone areas.
Beavers and stream restoration
In the 1930s, the U.S. Government put 600 beaver to work alongside the Civilian Conservation Corps in projects to stop soil erosion by streams in Oregon, Washington, Wyoming, and Utah. At the time it was estimated that each beaver, whose initial cost was about $5, completed work worth $300.
In a pilot study in Washington state, the Lands Council is reintroducing beavers to evaluate its projections that if 10,000 miles of suitable habitat were repopulated, then 650 trillion gallons of spring runoff would be held back for release in the arid Autumn season. This project was developed in response to a 2003 Washington Department of Ecology proposal to spend as much as $10 billion on construction of several dams on Columbia River tributaries to retain storm-season runoff. The State of Utah published a Beaver Management Plan which includes re-establishing beavers in ten streams per year for the purpose of watershed restoration each year from 2010 through 2020.
After 200 years, a beaver returned to New York City in 2007, making its home along the Bronx River, having spent time living at the Bronx Zoo as well as the Botanical Gardens. Beavers have not lived in New York City since the early 19th century when trappers extirpated them completely from the state. The return of "José", named after Representative Jose Serrano from the Bronx, has been seen as evidence that efforts to restore the river have been successful. In the summer of 2010, a second beaver named "Justin" joined José, doubling the beaver population in New York City. In February 2013, what appears to be both José and Justin were caught on motion-sensitive cameras at the New York Botanical Garden. Beaver were once important to the city's economy and a pair of beavers appears on the city's official seal and flag.
In Chicago, several beavers have returned and made a home near the Lincoln Park's North Pond. The "Lincoln Park Beaver" has not been as well received by the Chicago Park District and the Lincoln Park Conservancy, which was concerned over damage to trees in the area. In March 2009, they hired an exterminator to remove a beaver family using live traps, and accidentally killed the mother when she got caught in a snare and drowned. Relocation costs $4,000–$4,500 per animal. Scott Garrow, District Wildlife Biologist with the Illinois Department of Natural Resources, opined that relocating the beavers may be "a waste of time", as there are records of beaver recolonizing North Pond in Lincoln Park in 1994, 2003, 2004, 2008 and 2009. As of fall 2009, a new beaver lodge has appeared on North Pond's northwest bank.
Outside San Francisco, in downtown Martinez, California, a male and female beaver arrived in Alhambra Creek in 2006. The Martinez beavers built a dam 30 feet wide and at one time 6 feet high, and chewed through half the willows and other creekside landscaping the city planted as part of its $9.7 million 1999 flood-improvement project. When the City Council wanted to remove the beavers because of fears of flooding, local residents organized to protect them, forming an organization called "Worth a Dam". Resolution included installation of a flow device through the beaver dam so that the pond's water level could not become excessive. Now protected, the beaver have transformed Alhambra Creek from a trickle into multiple dams and beaver ponds, which in turn, led to the return of steelhead trout and river otter in 2008, and mink in 2009. The Martinez beavers probably originated from the Sacramento-San Joaquin River Delta which once held the largest concentration of beaver in North America.
In 1999, Washington, D.C.'s annual Cherry Blossom Festival was plagued by a family of beavers who lived in the Tidal Basin. The offenders were caught and removed, but not before damaging 14 cherry trees, including some of the largest and oldest trees.
As introduced non-native species
In the 1940s, beavers were brought to the island of Tierra Del Fuego in southern Chile and Argentina for commercial fur production and introduced near Fagnano Lake. Although the fur enterprise failed, 25 mating pairs of beavers were released into the wild. Having no natural predators in their new environment, they quickly spread throughout the island, and to other islands in the region, reaching a number of 100,000 individuals within just 50 years. Although they have been considered an invasive species, it has been more recently shown that the beaver have some beneficial ecological effects on native fish and should not be considered wholly detrimental. Although the dominant Lenga beech (Nothofagus pumilio) forest can regenerate from stumps, most of the newly created beaver wetlands are being colonized by the rarer native Antarctic beech (Nothofagus antarctica). It is not known whether the shrubbier Antarctic beech will be succeeded by the originally dominant and larger Lengo beech, however, and the beaver wetlands are readily colonized by non-native plant species. In contrast, areas with introduced beaver were associated with increased populations of the native catadromous puye fish (Galaxias maculatus). Furthermore, the beavers did not seem to have a highly beneficial impact on the exotic brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss) which have negative impacts on native stream fishes in the Cape Horn Biosphere Reserve, Chile. They have also been found to cross saltwater to islands northward; and reached the Chilean mainland in the 1990s. On balance, because of their landscape-wide modifications to the Fuegian environment and because biologists want to preserve the unique biota of the region, most favor their removal.
North American beavers were also released in Finland in 1937, before it was realised that they formed a separate species. (Eurasian beavers had been extirpated from the region, so the release was intended as a reintroduction project.) By 1999, it was estimated that 90% of beavers in Finland were the American species. The species is not considered invasive, as in Europe it has a similar keystone effect to European beavers, which have not recolonised the area. The beaver population has been controlled by issuing hunting licences. A report in 2010 concluded that while the current population was not problematic, as the species has larger litters than European beavers and builds somewhat larger dams, it could become a problem if its range continues expanding into Russia, but this does not seem to be taking place.
Beaver meat is similar tasting to lean beef, but care must be taken to prevent contamination from the animal's strong castor (musk) gland. It is usually slow-cooked in a broth, and was a valuable food source to Native Americans. Early French Canadian Catholics considered beaver to be "four-legged fish" that could be eaten at Lent.
Despite their name, the fried pastries found in parts of Canada called beaver tails contain no beaver.
As the national animal and one of the national symbols of Canada, the beaver is depicted on the Canadian five-cent piece and was on the first Canadian postage stamp, the Three Penny Beaver. It is also the state animal of Oregon and New York, and a common school emblem for engineering schools, including the California Institute of Technology and the Massachusetts Institute of Technology as well as the mascot for Oregon State University, Babson College, and the City College of New York. The beaver also appears in the coats of arms of the Hudson's Bay Company, University of Toronto, Wilfrid Laurier University, and the London School of Economics.
Much of the early economy of New Netherland was based on the beaver fur trade. As such, the seal of New Netherland featured the beaver; likewise, the coats of arms of Albany, New York and New York City included the beaver.
- Linzey, A. V. & NatureServe (Hammerson, G. & Cannings, S.) (2011). "Castor canadensis". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. Retrieved January 18, 2012.
- Helgen, K. M. (2005). "Family Castoridae". In Wilson, D. E.; Reeder, D. M. Mammal Species of the World (3rd ed.). Johns Hopkins University Press. p. 842. ISBN 978-0-8018-8221-0. OCLC 62265494.
- Warner, Richard E. and Hendrix, Kathleen M. (eds.). California Riparian Systems: Ecology, Conservation, and Productive Management University of California Press, 1984, p. 952. Retrieved on August 4, 2007.
- Browse Genus equals Castor by Scientific Name for All Museums. Berkeley Natural History Museums. Retrieved on August 4, 2007.
- Tesky, Julie L. Wildlife Species: Castor canadensis Fire Effects Information System (Online), U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory. 1993. Retrieved on August 4, 2007.
- Invasive Species Specialist Group (ISSG) (2006). "Castor canadensis (mammal)". Global Invasive Species Database (GISD). IUCN Species Survival Commission. Retrieved July 16, 2009: "Common names: American beaver (English), beaver (English), Canadian beaver, castor (French), castor americano (Spanish), North American beaver (English)"
- "Home Page, Alaska Department of Fish and Game". Adfg.state.ak.us. Retrieved March 16, 2013.
- The Beaver – Life Tracks. Timberwolfinformation.org
- Burnie D and Wilson DE (Eds.), Animal: The Definitive Visual Guide to the World's Wildlife. DK Adult (2005), ISBN 0789477645
- Boyle, Steve and Owens, Stephanie (February 6, 2007) North American Beaver (Castor canadensis): A Technical Conservation Assessment. USDA Forest Service, Rocky Mountain Region
- Morgan, Lewis H. (1868). The American Beaver and his Works. J. B. Lippincott & Co. p. 32.
- Naiman, Robert J.; Johnston, Carol A. and Kelley, James C. (Dec 1988). "Alteration of North American Streams by Beaver". BioScience: 753–762. Retrieved Feb 28, 2010.
- Mearns, Edgar Alexander (1907). Mammals of the Mexican boundary of the United States: A descriptive catalogue of the species of mammals occurring in that region; with a general summary of the natural history, and a list of trees. Government Printing Office. p. 359.
- Gallo-Reynoso, Juan-Pablo; Suarez-Gracida, Gabriela; Cabrera-Santiago, Horacia; Coria-Galindo, Else; Egido-Villarreal, Janitzio and Ortiz, Leo C. (Sep 2002). "Status of Beavers (Castor Canadensis Frontador) in Rio Bavispe, Sonora, Mexico". The Southwestern Naturalist. Retrieved November 25, 2011.
- Mapes, Lynda V. (May 18, 2009). "Scientist discovers beavers building prime salmon habitat in Skagit Delta". The Seattle Times. Retrieved June 22, 2010.
- Gallant, D., Bérubé, C.H.; Tremblay, E. and Vasseur, L. (2004). "An extensive study of the foraging ecology of beavers (Castor canadensis) in relation to habitat quality". Canadian Journal of Zoology 82 (6): 922–933. doi:10.1139/z04-067. Retrieved May 4, 2010.
- Donald G. Reid, Stephen M. Herrero and Thomas E. Code, "River Otters as Agents of Water Loss from Beaver Ponds," Journal of Mammalogy, February 1988.
- Richard P.B. (1983). "Mechanisms and adaptation in the constructive behaviour of the beaver (C. fiber L.)". Acta Zoologica Fennica 174: 105–108.
- Thie, Jean. "National Geographic photos". Geostrategis.com. Retrieved March 16, 2013.
- Soodin, Vince. "Beaver Dam Seen from Space". The Sun. Retrieved May 5, 2010.
- Müller-Schwarze, Dietland and Sun, Lixing (2003). The Beaver: Natural History of a Wetlands Engineer. Cornell University Press. pp. 67–75. ISBN 978-0-8014-4098-4.
- Young, Mary Taylor (August 13, 2007). Colorado Division of Wildlife: Do Beavers Eat Fish? at the Wayback Machine (archived September 10, 2010) wildlife.state.co.us
- Thurber, J. M., & Peterson, R. O. (1993). Effects of population density and pack size on the foraging ecology of gray wolves. Journal of Mammalogy, 879-889.
- "The American Bear Association Home Page (Web Pages2/index)". The American Bear Association.
- "Adirondack Black Bears". Environmental Information Series.
- Smith, D. W., Trauba, D. R., Anderson, R. K., & Peterson, R. O. (1994). Black bear predation on beavers on an island in Lake Superior. American Midland Naturalist, 248-255.
- Engelhart, A., & Müller-Schwarze, D. (1995). Responses of beaver (Castor canadensis Kuhl) to predator chemicals. Journal of Chemical Ecology, 21(9), 1349-1364.
- "Beaver". Study of Northern Virginia Ecology, Fairfax County Public School. Retrieved 2013-01-03.
- Müller-Schwarze, Dietland and Sun, Lixing (2003). The Beaver: Natural History of a Wetlands Engineer. Cornell University Press. pp. 113–114. ISBN 978-0-8014-4098-4.
- "Beaver Biology". Beaver Solutions. Retrieved 22 November 2013.
- Müller-Schwarze, Dietland and Sun, Lixing (2003). The Beaver: Natural History of a Wetlands Engineer. Cornell University Press. p. 80. ISBN 978-0-8014-4098-4.
- "N. America's Earliest Beaver Found Near Dayville – Discovered Teeth "A Dam Important Find,' Scientists Say". KVTZ. September 19, 2011. Retrieved September 20, 2011.
- Baker, B. W., and E. P. Hill. Beaver (Castor canadensis). G. A. Feldhamer, B. C. Thompson, and J. A. Chapman, editors. Wild Mammals of North America: Biology, Management, and Conservation. Second Edition. The Johns Hopkins University Press, Baltimore, Maryland, USA, pp. 288–310. 2003. Retrieved on August 4, 2007.
- Kieffer, Michael Meadows in Mist Bull Run Mountains Conservancy, Inc. Retrieved on August 4, 2007.
- Kitchener, Andrew (2001). Beavers. p. 144. ISBN 1-873580-55-X.
- Moloney, F. X. (1967). The Fur Trade in New England, 1620–1676. Hamden, Connecticut: Archon Books. p. 150.
- Hays, W. J. (Sep 1871). "Notes on the range of some of the animals in America at the time of arrival of the whitemen". The American Naturalist 5 (7): 25–30. doi:10.1086/270797. JSTOR 2447602.
- Seton-Thompson, cited in Sun, Lixing; Müller-Schwarze, Dietland (2003). The Beaver: Natural History of a Wetlands Engineer. Ithaca, NY: Cornell University Press. pp. 97–98. ISBN 0-8014-4098-X.; but note that to arrive at this figure he assumed a population density throughout the range equivalent to that in Algonquin Park
- Outwater, Alice (1997). Water:A Natural History. New York, NY: Basic Books. p. 89. ISBN 0-465-03780-1.
- Parks, Mike (March 1, 2012). "South Charlotte Weekly: Beavers damaging Park Road Park; must go". Thecharlotteweekly.com.
- "Beavers: Wetlands & Wildlife: Solutions to Beaver/Human Conflicts". Beaversww.org. Retrieved March 16, 2013.
- Agar, Charles (March 24, 2008). "Beaver Deceiver expert coming to Pitkin County". The Aspen Times (Aspen, Colorado). Retrieved Nov 22, 2009.
- Wright, J.P.; Jones, C.G.; Flecker, A.S. (2002). "An ecosystem engineer, the beaver, increases species richness at the landscape scale". Oecologia 132 (1): 96–101. doi:10.1007/s00442-002-0929-1. Retrieved March 1, 2010.
- Rosell F, Bozser O, Collen P, Parker H (2005). "Ecological impact of beavers Castor fiber and Castor canadensis and their ability to modify ecosystems". Mammal Review 35 (3–4): 248–276. doi:10.1111/j.1365-2907.2005.00067.x. Retrieved March 1, 2010.
- Hood, Glynnis A. and Bayley, Suzanne E. (2008). "Beaver (Castor canadensis) mitigate the effects of climate on the area of open water in boreal wetlands in western Canada". Biological Conservation: 556–567. doi:10.1016/j.biocon.2007.12.003.
- "Busy Beavers Can Help Ease Drought". Science Daily. February 26, 2008. Retrieved February 23, 2011.
- Correll, David L.; Jordan, Thomas E. and Weller, Donald E. (June 2000). "Beaver pond biogeochemical effects in the Maryland Coastal Plain". Biogeochemistry 49 (3): 217–239. doi:10.1023/A:1006330501887. JSTOR 1469618.
- Muskopf, Sarah (October 2007). The Effect of Beaver (Castor canadensis) Dam Removal on Total Phosphorus Concentration in Taylor Creek and Wetland, South Lake Tahoe, California (Thesis). Humboldt State University, Natural Resources. Retrieved February 27, 2011.
- Skinner, Quentin D.; Speck, John E.; Smith, Michael and Adams, John C. (March 1984). "Stream Water Quality as Influenced by Beaver within Grazing Systems in Wyoming". Journal of Range Management 37 (2): 142–146. doi:10.2307/3898902. JSTOR 3898902.
- Gaywood, Martin; Batty, Dave and Galbraith, Colin (2008). "Reintroducing the European Beaver in Britain". British Wildlife. Retrieved March 26, 2011.
- Erlandsen, S. L., and Bemrick, W. J. (1988). Waterborne giardiasis: sources of Giardia cysts and evidence pertaining to their implication in human infection in P. M. Wallis and B. R. Hammond (ed.), Advances in Giardia research. Calgary, Alberta, Canada: University of Calgary Press. pp. 227–236.
- Erlandsen SL, Sherlock LA, Bemrick WJ, Ghobrial H, Jakubowski W (January 1990). "Prevalence of Giardia spp. in Beaver and Muskrat Populations in Northeastern States and Minnesota: Detection of Intestinal Trophozoites at Necropsy Provides Greater Sensitivity than Detection of Cysts in Fecal Samples". Applied and Environmental Microbiology 56 (1): 31–36. PMC 183246. PMID 2178552. Retrieved March 26, 2011.
- Thompson, R. C. A. (November 2000). "Giardiasis as a re-emerging infectious disease and its zoonotic potential". International Journal of Parasitology 30 (12–13): 1259–1267. doi:10.1016/S0020-7519(00)00127-2. PMID 11113253.
- Nygård, Karin; Schimmer, Barbara; Søbstad, Øystein; Walde, Anna; Tveit, Ingvar; Langeland, Nina; Hausken, Trygve and Aavitsland, Preben (2006). "A large community outbreak of waterborne giardiasis-delayed detection in a non-endemic urban area". BMC Public Health. Retrieved November 26, 2011.
- Bromley, Chantal K. and Hood, Glynnis A. (2013). "Beavers (Castor canadensis) facilitate early access by Canada geese (Branta canadensis) to nesting habitat and areas of open water in Canada's boreal wetlands". Mammalian Biology 78 (1): 73–77. doi:10.1016/j.mambio.2012.02.009. Retrieved February 14, 2013.
- McKinstry, M. C.; Caffrey, P. and Anderson, S. H. (2001). "The Importance of Beaver to Waterfowl and Wetlands Habitats in Wyoming". Journal of the American Water Resources Association 37 (6): 1571. doi:10.1111/j.1752-1688.2001.tb03660.x.
- Brenner, F.J. (1960). "Canada geese nesting on a beaver lodge". The Auk 77 (4): 476–477. doi:10.2307/4082428. Retrieved Mar 1, 2010.
- Mitchell, C.D. (1994). A. Poole and F. Gill, ed. Trumpeter Swan (Cygnus buccinator). In The Birds of North America No. 105. Philadelphia: The Academy of Natural Sciences. p. 10.
- McKelvey RW, Denningtonz MC, Mossop (March 1983). "The Status and Distribution of Trumpeter Swans (Cygnus buccinator) in the Yukon". Arctic: 76–81. Retrieved Mar 1, 2010.
- Hilfiker, E.L. (1991). Beavers, Water, Wildlife and History. Interlaken, New York: Windswept Press. p. 198. ISBN 978-1-55787-067-4.
- Rosell, Frank; Bozser, Orsolya; Collen, Peter and Parker, Howard (2005). "Ecological impact of beavers Castor fibre and Castor canadensis and their ability to modify ecosystems". Mammal Review 35 (3–4): 248. doi:10.1111/j.1365-2907.2005.00067.x.
- Bulluck, Jason F. and Rowe, Matthew P. (2006). "The Use of Southern Appalachian Wetlands by Breeding Birds, with a Focus on Neotropical Migratory Species". The Wilson Journal of Ornithology 118 (3): 399. doi:10.1676/04-116.1.
- Longcore, Travis; Rich, Catherine and Muller-Schwarze, Dietland (February 2007). "Management by Assertion: Beavers and Songbirds at Lake Skinner (Riverside County, California)". Environmental Management 39 (4): 460–471. doi:10.1007/s00267-005-0204-4. PMID 17318698.
- Cooke, Hilary A. and Zack, Steve (2008). "Influence of Beaver Dam Density on Riparian Areas and Riparian Birds in Shrubsteepe of Wyoming". Western North American Naturalist 68 (3): 365–373. doi:10.3398/1527-0904(2008)68[365:IOBDDO]2.0.CO;2. Retrieved February 16, 2013.
- Grover, A.M. & Baldassarre, G.A. (1995). "Bird species richness within beaver ponds in south-central New York". Wetlands 15 (2): 108–118. doi:10.1007/BF03160664.
- Pollock, M. M.; Pess, G. R. and Beechie, T. J. (2004). "The Importance of Beaver Ponds to Coho Salmon Production in the Stillaguamish River Basin, Washington, USA". North American Journal of Fisheries Management 24 (3): 749–760. doi:10.1577/M03-156.1. Retrieved Feb 28, 2010.
- Swales, S. and Levings, C. D. (1989). "Role of Off-Channel Ponds in the life Cycle of Coho Salmon (Oncorhynchus kisutch) and Other Juvenile Salmonids in the Coldwater River, British Columbia". Canadian Journal Fisheries Aquatic Sciences 46 (2): 232–242. doi:10.1139/f89-032.
- Gard R (1961). "Effects of beaver on trout in Sagehen Creek, California". Journal of Wildlife Management 25 (3): 221–242. doi:10.2307/3797848. JSTOR 3797848.
- Hägglund, Å. & Sjöberg, G. (1999). "Effects of beaver dams on the fish fauna of forest streams". Forest Ecology and Management 115 (2–3): 259–266. doi:10.1016/S0378-1127(98)00404-6.
- Rutherford, W.H. (1955). "Wildlife and environmental relationships of beavers in Colorado forests". Journal of Forestry: 803–806. Retrieved Feb 28, 2010.
- Murphy, M.L., Heifetz, J., Thedinga, J.F., Johnson, S.W. & Koski, K.V. (1989). "Habitat utilisation by juvenile Pacific salmon (Onchorynchus) in the glacial Taku River, southeast Alaska". Canadian Journal of Fisheries and Aquatic Science 46 (10): 1677–1685. doi:10.1139/f89-213.
- J. M. Niles, K. J. Hartman, and P. Keyser (2013). "Short-Term Effects of Beaver Dam Removal on Brook Trout in an Appalachian Headwater Stream". Northeastern Naturalist 20 (3): 540–551. doi:10.1656/045.020.0317. Retrieved 2013-11-17.
- Pollock, Michael M.; Heim, Morgan and Werner, Danielle (2003). "Hydrologic and Geomorphic Effects of Beaver Dams and Their Influence on Fishes". American Fisheries Society Symposium 37. Retrieved Jan 17, 2010.
- Kemp, Paul S.; Worthington, Tom A.; Langford, Terence E. L.; Tree, Angus R. J. and Gaywood, Martin J. (June 2011). "Qualitative and quantitative effects of reintroduced beavers on stream fish". Fish and Fisheries 13 (2): 158. doi:10.1111/j.1467-2979.2011.00421.x.
- Ryan L. Lokteff, Brett B. Roper, Joseph M. Wheaton (2013). "Do Beaver Dams Impede the Movement of Trout?". Transactions of the American Fisheries Society. Retrieved 2013-12-23.
- Bryant, M. D. (1984). Walton, J.M. and Houston, D.B., ed. "The Role of Beaver Dams as Coho Salmon Habitat in southeast Alaska Streams". Proceeding, Olympic Wild Fish Conferences (Port Angeles, Washington: Peninsula College, Fisheries Technology program): 183–192.
- Leidholt-Bruner, Karen; Hibbs, David E. and McComb, William C. (1992). "Beaver Dam Locations and Their Effects on Distribution and Abundance of Coho Salmon Fry in Two Coastal Oregon Streams". Northwest Science. Retrieved April 16, 2011.
- Collen P, Gibson RJ (2001). "The general ecology of beavers (Castor spp.), as related to their influence on stream ecosystems and riparian habitats, and the subsequent effects on fish – a review". Reviews in Fish Biology and Fisheries: 439–461. Retrieved Mar 2, 2010.
- Sigourney, D. B.; Letcher, B. H. and Cunjak, R. A. (2006). "Influence of Beaver Activity on Summer Growth and Condition of Age-2 Atlantic Salmon Parr". Transactions of the American Fisheries Society 135 (4): 1068–1075. doi:10.1577/T05-159.1.
- Mills, Enos A. (1913). In Beaver World. Kessinger Publishing. p. 280. ISBN 978-0-7661-9387-1.
- Burchsted, Denise; Daniels, Melinda; Thorson, Robert and Vokoun, Jason (2010). "The River Discontinuum: Applying Beaver Modifications to Baseline Conditions for Restoration of Forested Headwaters". BioScience 60 (11): 908–922. doi:10.1525/bio.2010.60.11.7.
- Eric J. Billman, Joshua D. Kreitzer, J. Curtis Creighton, Evelyn Habit, Brock McMillan, and Mark C. Belk (2013). "Habitat enhancement and native fish conservation: can enhancement of channel complexity promote the coexistence of native and introduced fishes?". Environmental Biology of Fishes 96: 555–566. doi:10.1007/s10641-012-0041-2. Retrieved 2013-06-13.
- Hood, W. Gregory (2009). "An Overlooked Ecological Web: Sweetgale, Beaver, Salmon, and Large Woody Debris in the Skagit River Tidal Marshes". Skagit River Cooperative. Retrieved June 22, 2010.
- Beier, Paul and Barrett, Reginald H. (1987). "Beaver Habitat Use and Impact in Truckee River Basin, California". Journal of Wildlife Management 51 (4): 794–799. doi:10.2307/3801743. JSTOR 3801743.
- Benson Ayers, Michael (1997). Aerial Multispectral Videography for Vegetation Mapping and Assessment of Beaver Distribution within Selected Riparian Areas of the Lake Tahoe Basin (Thesis). University of Nevada at Reno. p. 71. Retrieved August 26, 2010.
- Pollock, Michael M.; Beechie, Timothy J. and Jordan, Chris E. (2007). "Geomorphic changes upstream of beaver dams in Bridge Creek, an incised stream channel in the interior Columbia River basin, eastern Oregon". Earth Surface Processes and Landforms. doi:10.1002/esp.1553.
- Demmer, Rick and Beschta, Robert L. (September 2008). "Recent History (1988–2004) of Beaver Dams along Bridge Creek in Central Oregon". Northwest Science 82 (4): 309–318. doi:10.3955/0029-344X-82.4.309.
- Eric Collier (1959). Three Against the Wilderness. Victoria, British Columbia: Touchwood. p. 288. ISBN 1-894898-54-0.
- Ruedemann, Rudolf and Schoonmaker, W. J. (December 2, 1938). "Beaver-Dams as Geologic Agents". Science 88 (2292): 523–525. Bibcode:1938Sci....88..523R. doi:10.1126/science.88.2292.523. PMID 17840531.
- Groc, Isabelle (April 19, 2010). "Beavers Sign up to Fight Effects of Climate Change". Discover. Retrieved July 27, 2010.
- "The Beaver Solution: Solving our Water Storage Dilemma in Eastern Washington". The Lands Council. March 2010. Retrieved July 27, 2010.
- Utah Beaver Management Plan (Report). Utah Division of Wildlife Resources. January 6, 2010. pp. 25. http://wildlife.utah.gov/furbearer/pdf/beaver_plan_2010-2020.pdf. Retrieved August 29, 2010.
- "New York City Beaver Returns". Science Daily. December 20, 2008.
- Miller, Peter (September 2009). "Manhattan Before New York: When Henry Hudson first looked on Manhattan in 1609, what did he see?". National Geographic.
- O'Connor, Anahad (February 23, 2007). "After 200 Years, a Beaver Is Back in New York City". New York Times. Retrieved Dec 4, 2009.
- Trotta, Daniel. "Beaver Returns to New York City After 200 Years." World Environment News. Dec 26, 2007.
- Design Trust for Public Space (June 17, 2009). "Bronx River Crossing". Retrieved Dec 4, 2009.
- Paddock, Barry (September 19, 2010). "Another beaver makes Bronx River home – doubles total beaver population". New York Daily News. Retrieved September 19, 2010.
- Newman, Andy (February 7, 2013). "Beaver Gets Busy at Botanical Garden". New York Times. Retrieved February 13, 2013.
- Boehm, Kiersten (Nov 14, 2008). "Lincoln Park Beaver Relocated". Inside at Your News Chicago, IL Edition. Retrieved Dec 4, 2009.
- Holingue, Scott (Jan 1, 1994). Tales from an Urban Wilderness: Wildlife's Struggle for Survival in a Park Where City & Wilderness Meet. Chicago, IL: Chicago Historical Bookworks. p. 140. ISBN 0-924772-25-5.
- "Park District Kills Beaver in Lincoln Park". MyFoxChicago. April 2009. Retrieved December 4, 2009.
- Greenfield, John (May 7–13, 2009). "Why are there signs that claim the Park District murdered a beaver?". Time Out Chicago. Retrieved Dec 4, 2009.
- Jones, Carolyn (April 16, 2008). "Moment of truth for Martinez beavers". San Francisco Chronicle.
- "Worth a Dam website".
- George, Aleta (2008). "Martinez Beavers". Bay Nature (Bay Nature Institute). Retrieved Nov 6, 2009.
- DeRobertis-Theye, Nicola. "Beavers and More in Martinez:New Habitat Thanks to Beavers". Bay Nature (Bay Nature Institute). Retrieved Nov 6, 2009.
- Farnham, Thomas Jefferson (1857). Life, adventures, and travels in California. Blakeman & Co. p. 383.
- Aiken, Jonathan (April 7, 1999). "Beaver is bad guy at cherry blossom time". CNN.com. Retrieved Nov 22, 2009.
- Wheeler, Linda (April 7, 1999). "Beaver Chomps Into Cherry Blossom Season". Washington Post. Retrieved Nov 22, 2009.
- Anderson, Christopher B.; Pastur, Guillermo Martinez; Lencinas, Maria Vanessa; Wallem, Petra K.; Moorman, Michelle C. and Rosemond, Amy D. (2009). "Do introduced North American beavers Castor canadensis engineer differently in southern South America? An overview with implications for restoration". Mammalian Review. Retrieved March 17, 2012.
- Vila, I., Fuentes, L.S. & Saavedra, M. (1999). "Ictiofauna en los sistema límnicos de la Isla Grande, Tierra del Fuego, Chile". Revista Chilena de Historia Natural: 273–284.
- Moorman, Michelle C.; Eggleston, David B.; Anderson, Christopher B.; Mansilla, Andres and Szejner, Paul (2009). "Implications of Beaver Castor canadensis and Trout Introductions on Native Fish in the Cape Horn Biosphere Reserve, Chile". Transactions of the American Fisheries Society 138 (2): 306–313. doi:10.1577/T08-081.1.
- Wallem, P.K., Jones, C.G., Marquet, P.A. & Jaksic, F.M. (2007). "Identificación de los mecanismo subyacentes a la invasión de Castor canadensis (Kuhl 1820, Rodentia) en el archipiélago de Tierra del Fuego, Chile". Revista Chilena de Historia Natural: 309–325.
- "Argentina eager to rid island of beavers". CNN. Retrieved May 20, 2010.
- Hartmann, Goran (1999). Beaver Protection, Management, and Utilization in Europe and North America. New York: Kluwer. pp. 1–14.
- Nummi, Petri. "NOBANIS – Invasive Alien Species Fact Sheet – Castor canadensis". Online Database of the European Network on Invasive Alien Species – NOBANIS. NOBANIS. Retrieved 2 October 2014.
- Poirier, Nelson (November 27, 2010). "Our symbolic beaver overcomes challenges of past". Moncton, New Brunswick: Times & Transcript. Retrieved November 18, 2010.
- The Beaver Heritage Canada
- The HBC Coat of Arms, Hbc Heritage
- "Castor canadensis". Integrated Taxonomic Information System. Retrieved March 18, 2006.
- Müller-Schwarze, Dietland and Sun, Lixing (2003). The beaver: natural history of a wetlands engineer. Cornell University Press. p. 190. ISBN 978-0-8014-4098-4.
- Mills, Enos (1913). In Beaver World. Kessinger Publishing. p. 255. ISBN 978-0-7661-9387-1.
- Collier, Eric (2007). Three Against the Wilderness. Touchwood Editions. p. 288. ISBN 978-1-894898-54-6.
- Long, Kim (2000). Beavers: A Wildlife Handbook. Boulder: Johnson Books. p. 37. ISBN 1-55566-251-X.
- Dugmore, A. Radclyffe (1914). The Romance of the Beaver; being the history of the beaver in the western hemisphere. Illustrated with photographs from life and drawings by the author. Publisher: Philadelphia, J.B. Lippincott company; London, W. Heinemann (a searchable facsimile at the University of Georgia Libraries)
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