North American beaver

The North American beaver (Castor canadensis) is one of two extant beaver species (the other being the Eurasian beaver, Castor fiber).[3] It is native to North America and introduced in South America (Patagonia) and Europe (primarily Finland and Karelia). 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[3] and Canadian beaver,[27] distinguish this species from the other extant beaver species, Castor fiber, which is native to Eurasia. The North American beaver is an official animal symbol of Canada and is the official state mammal of Oregon and New York.[28]

North American beaver
A male North American beaver
Conservation status
Least Concern  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Rodentia
Family:	Castoridae
Genus:	Castor
Species:	C. canadensis
Binomial name
Castor canadensis Kuhl, 1820[2]
Subspecies[3][4][5][6][7]
List C. c. acadicus Bailey and Doutt, 1942[8] C. c. baileyi Nelson, 1927[9] C. c. belugae Taylor, 1916[10]^:429–433 Cook Inlet beaver C. c. caecator Bangs (1913:513)[11] Newfoundland beaver C. c. canadensis Kuhl 1820[2] Canadian beaver C. c. carolinensis Rhoads 1898[12]^:420–421 Carolina beaver C. c. concisor Warren and Hall, 1939[13] C. c. duchesnei Durrant and Crane, 1948[14] C. c. frondator Mearns, 1897[15] Sonora beaver C. c. idoneus Jewett and Hall, 1940[16] C. c. labradorensis Bailey and Doutt, 1942[8] C. c. leucodonta Gray, 1869[17] Pacific beaver C. c. mexicanus Bailey, 1913[18] Rio Grande beaver C. c. michiganensis Bailey 1913[18] Woods beaver C. c. missouriensis Bailey 1919[19] Missouri River beaver C. c. pacificus Rhoads 1892[12]^:422–423 Washington beaver C. c. pallidus Durrant and Crane, 1948[14] C. c. phaeus Heller, 1909[20] Admiralty beaver C. c. repentinus Goldman, 1932[21] C. c. rostralis Durrant and Crane, 1948[14] C. c. sagittatus Benson, 1933[22] C. c. shastensis (Taylor, 1916)[10]^:433–436 Shasta beaver C. c. subauratus (Taylor, 1912)[23][24] California golden beaver C. c. taylori Davis, 1939[25] C. c. texensis Bailey, 1905[26] Texas beaver
Distribution of the North American beaver (dark green – native, light green – introduced)

Description

North American beaver skeleton (Museum of Osteology)

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. The European species is slightly larger on average but the American has a larger known maximum size. Adults usually weigh from 11 to 32 kg (24 to 71 lb), with 20 kg (44 lb) being typical. In New York, the average weight of adult male beavers was 18.9 kg (42 lb), while non-native females in Finland averaged 18.1 kg (40 lb). However, adults of both sexes averaged 16.8 kg (37 lb) in Ohio.[29][30][31] The species seems to conform to Bergmann's rule, as northern animals appear to be larger. In the Northwest Territory, adults weighed a median of 20.5 kg (45 lb).[32] The American beaver is slightly smaller in average body mass than the Eurasian species.[30] The head-and-body length of adult North American beavers 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) (higher than the maximum known for the Eurasian beaver).[33][34][35][36]

Like the capybara, the beaver is semiaquatic. The beaver has many traits suited to this lifestyle. It has a large, flat, paddle-shaped tail and large, webbed hind feet. The unwebbed front paws are smaller, with claws. The forepaws are highly dextrous, and are used both for digging, and to fold individual leaves into their mouth and to rotate small, pencil-sized stems as they gnaw off bark.[37] The eyes are covered by a nictitating membrane which allows the beaver to see under water. The nostrils and ears are sealed while submerged. Their lips can be closed behind their front teeth so that they can continue to gnaw underwater.[38] A thick layer of fat under its skin insulates the beaver from its coldwater 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 colors, 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. There is also another set of oil glands producing unique chemical identifiers in the form of waxy esters and fatty acids.[37] The lush, workable fur was made into a number of products, most notably hats. Demand for furs for hats drove beavers nearly to the point of extinction, and the North American species was saved principally by a sudden change in style. Before their near-extirpation by trapping in North America, beavers were practically ubiquitous and lived from the arctic tundra to the deserts of northern Mexico, and from the Atlantic to the Pacific Oceans.[39][40][41] Physician naturalist Edgar Alexander Mearns' 1907 report of beaver on the Sonora River may be the earliest report on the southernmost range of this North American aquatic mammal.[42] However, beavers have also been reported both historically and contemporarily in Mexico on the Colorado River, Bavispe River, and San Bernardino River in the Mexican states of Sonora and Chihuahua.[43][44][45]

The beaver possesses continuously (or endlessly) growing incisors, and is a hindgut fermenter whose cecum, populated by symbiotic bacteria, helps to digest plant-based material. These traits are not unique to beavers, and are in fact present among all rodents.[46] Nonetheless, the beaver is remarkably specialized for the efficient digestion of its lignocellulose-heavy diet.[47]

Brain anatomy of the beaver is not particularly specialized for its semiaquatic life history. The brain masses of a beaver weighing 11.7 and 17 kg are 41 and 45 g respectively. C. canadensis has an encephalization quotient of 0.9 compared to other rodents; this is intermediate between similar terrestrial rodents and arboreal squirrels, and higher than similar aquatic terrestrial rodents, the muskrats and nutria. The cerebrum is well developed, and the neocortex comparatively large. Larger areas of the beaver's somatosensory cortex are dedicated to the processing of stimuli from the lips and the hands, more so than the tail and whiskers, which play a relatively minor role. The visual area of the brain is smaller than the gray squirrel.[38]

Skull of a North American Beaver found on San Francisco Bay shore

Behavior

Beaver lodge, Ontario, Canada

Beaver dam, northern California, USA

Beavers use rocks for their dams when mud and branches are less available as seen on Bear Creek, a tributary to the Truckee River, in Alpine Meadows, California.

Beavers are active mainly at night. They are excellent swimmers and may remain submerged up to 15 minutes. More vulnerable on land, they tend to remain in the water as much as possible.[48] 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.[49] 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 lodges in the artificial ponds which form. 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.

Dam-building

The purpose of the dam is to create deepwater 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.[50] The trees are cut down using their strong incisor teeth. Their front paws are used for digging and carrying and placing materials. 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 do not repair breaches in the dam made by otters, and sometimes breach the dam themselves and lower the water level in the pond 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.[51]

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. Although it was on dry land, the beaver covered the tape player with branches and mud.[52] 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.[53] 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).

C. c. canadensis, feeding in winter

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. In colder climates where their pond freezes over, beavers also build a food cache from this food resource[37] To form the cache, beavers collect food in late fall in the form of tree branches, storing them under water (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.[54] The frozen combination of branches and ice is known as a cap, sealing the food cache. Beavers often maintain an underwater entrance to their dam, and they can access their food cache from their lodge by swimming under the ice. In warmer climes, a winter food store is less common.[37]

Muskrats have been thought to steal food from beaver lodges, but seemingly cooperative relationships exist, with beavers allowing muskrats to reside in their lodge if they gather fresh reeds.[55]

Canals

Another component to the beaver's habitat is the canal. Canals are used to float logs to a pond, and dams may also be used to maintain the water levels in these canals. Several land trails can extend from the canals.[56] Despite being widespread in some beaver-inhabited areas, beaver canals and their environmental effects are much less studied than beaver dams. Beaver primarily develop canals to increase accessibility of river resources, facilitate transport of acquired resources, and to decrease the risk of predation. Beaver canals can be over 0.5 km in length.[57] Beavers build canals by pushing through soil and vegetation using their forelimbs.

It has been hypothesized that beavers' canals are not only transportation routes to extend foraging, but also an extension of their "central place" around the lodge and/or food cache. A 2012 study of beavers' mark on the landscape found that cut stumps were negatively related to distance from beaver canals, but not to the central body of water. This finding suggested that beavers may consider the canals to be part of their "central place" as far as foraging activity is concerned.[58]

Social behavior

Communication is highly developed in beaver, including scent marking, vocalization, and tail slapping. Beaver deposit castoreum on piles of debris and mud called scent mounds, which are usually placed on or near lodges, dams, and trails less than a meter from water. Over 100 of such mounds can be constructed within one territory.[37] Beaver colonies with close neighbors constructed more "scent mounds" than did isolated colonies, and the number of scent mounds at each active lodge is correlated with the distance to the nearest occupied lodge.[59]

Although seven vocal sounds have been described for beaver, most zoologists recognize only three: a whine, hiss, and growl. Vocalizations and tail slapping may be used to beg for food, signal to family members to warn of predators, or to drive away or elicit a response from predators.

Beavers usually mate for life, forming familial colonies. Beaver "kits" are born precocious and with a developed coat. The young beaver "kits" typically remain with their parents up to two years. Kits express some adult behaviors, but require a long period in the family to develop their dam construction skills, and other abilities required for independent life.[37]

Diet

Beaver are herbivorous generalists with sophisticated foraging preferences. Beavers consume a mix of herbaceous and woody plants, which varies considerably in both composition and species diversity by region and season.[37] They prefer aspen and poplar, but also take birch, maple, willow, alder, black cherry, red oak, beech, ash, hornbeam, and occasionally pine and spruce.[54] They also eat cattails, water lilies, and other aquatic vegetation, especially in the early spring. Contrary to widespread belief, they do not eat fish.[60]

Beavers select food based on taste, coarse physical shape, and odor. Beavers feed on wood, bark, branches, twigs, leaves, stems, sprouts, and in some cases, the sap and storax of pine and sweetgum.[37]

When herbaceous plants are actively growing, they make up much of the beaver's diet. In the winter, beavers switch to woody plants and the food they have stored over the winter. The protein to calorie ratio of a beaver's diet is 40 mg/calorie in summer and 8 mg/calorie for the rest of the year. In northern latitudes, the water lilies Nymphaea and Nuphar are the most important herbaceous component. The rhizomes are stored in the food cache and remain actively growing.[37]

Willow is an important protein source and is likely to be available for the longest period of time in a beaver's habitat especially in the far north. When available, aspen and poplar are preferred over willow. Conifers are also cut or gnawed by beavers, and used for food and/or building material.

Beavers do not necessarily use the same trees as construction material and as food. Inedible material is more likely to be used as the cap of a beaver family's food cache, the upper part which is frozen in the ice, while the cache itself is composed of edible, high quality branches, which remain unfrozen and accessible.[37]

Beavers avoid red maple, which can be the only tree left standing at the edges of some beaver ponds.[37][61]

The beaver's gut microbiome is complex and specialized for a wood-heavy diet, sharing a number of similarities with other mammalian herbivores. However the microbial community in the beaver shows less taxonomic diversity than the "typical" mammalian gut. The major OTUs are Bacteroidetes and Firmicutes.[47]

Predators

Brooklyn Museum – American Beaver – John J. Audubon

Common natural predators include coyotes, wolves, and mountain lions.[62] American black bears may also prey on beavers if the opportunity arises, often by smashing their paws into the beavers' lodges.[63][64][65] Perhaps due to differing habitat preferences, brown bears were not known to hunt beavers in Denali National Park.[66] Less significant predators include wolverines, which may attack a rare beaver of up to adult size, and Canadian lynx, bobcats, and foxes, predators of kits or very sick or injured animals rather than full-grown beavers due to their increasingly smaller size. American alligators, which only minimally co-exist in the wild with beavers, also seldomly threaten them. Both golden eagles (Aquila chrysaetos) and bald eagles (Haliaeetus leucocephalus) may on occasion prey on a beaver, most likely only small kits.[67] Despite repeated claims, no evidence shows that river otters are typically predators of beavers but anecdotally may take a rare beaver kit.[68]

Reproduction

North American beavers 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 have a range of three to six kits per litter (usually 4-5).[69] Most beaver do not reproduce until they are three years of age, but about 20% of two-year-old females reproduce.[70]

Subspecies

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 beavers in North America are of two beaver teeth near Dayville, Oregon, and are 7 million years old.[71]

At one time, 25 subspecies of beavers 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 substantial genetic mixing of populations has occurred 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).[72] The Canadian beaver originally inhabited almost all of the forested area of Canada,[73] 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

Skulls of a European and Canadian beaver.

Although North American beavers are superficially similar to the European beaver (Castor fiber), several important differences exist 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 relatives, 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 color is also different. Overall, 50% of North American beavers have pale brown fur, 25% are reddish brown, 20% are brown, and 6% are blackish, while in European beavers, 66% have pale brown or beige fur, 20% are reddish brown, nearly 8% are brown, and only 4% have blackish coats.[74]

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.[74]

Ecology

The beaver was trapped out and almost extirpated in North America because its fur and castoreum were highly sought after.[40] 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.[75] From 1630 to 1640, around 80,000 beavers were taken annually from the Hudson River and western New York.[76] 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. Archaeological and historical evidence suggests that beaver ponds created "moth-hole like" habitats in the deciduous forest that dominated eastern North America. This nonforest habitat attracted both Native American and early colonial hunters to the abundant fish, waterfowl, and large game attracted to the riparian clearings created by these aquatic mammals. The first colonial farmers were also attracted to the fertile, flat bottomlands created by the accumulated silt and organic matter in beaver ponds.[77]

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, an earlier, 19th-century California Fur Rush drove the earliest American settlement in that state. During the roughly 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 beavers before the days of the fur trade.[78][79]

A wire mesh fence installed around a tree trunk in Toronto. Wire mesh fences are used in an attempt to prevent beavers damage.[80]

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.[81] Because they are persistent in repairing damage to the dam, they were historically relocated or exterminated. Nonlethal methods of containing beaver-related flooding have been developed.[82] One such flow device has been used by both the Canadian and U.S. governments, called "beaver deceivers" or levelers, invented and pioneered by wildlife biologist Skip Lisle.[83]

The beaver is a keystone species, increasing biodiversity in its territory through creation of ponds and wetlands.[84] As wetlands are formed and riparian habitats enlarged, aquatic plants colonize newly available watery habitat. Insect, invertebrate, fish, mammal, and bird diversities are also expanded.[85] Effects of beaver recolonization on native and non-native species in streams where they have been historically absent, particularly dryland streams, is not well-researched.[86]

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 beavers on openwater area in east-central Alberta, Canada, found that the mammals were associated with a 9-fold increase in openwater area. Beavers 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 beavers were present, 60% more open water was available than those same areas during previous drought periods when beavers were absent. The authors concluded that beavers have a dramatic influence on the creation and maintenance of wetlands even during extreme drought.[87][88]

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, thus improving stream water quality.[89][90] 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.[91]

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.[92][93][94] 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.[95] New Zealand has giardia outbreaks, but no beavers, whereas Norway has plenty of beavers, but had no giardia outbreaks until recently (in a southern part of Norway densely populated by humans but no beaver).[96]

Effects on bird abundance and diversity

Canada goose nest on beaver lodge

Beavers help waterfowl by creating increased areas of water, and in northerly latitudes, they thaw areas of open water, allowing an earlier nesting season.[97] In a study of Wyoming streams and rivers, watercourses with beavers had 75-fold more ducks than those without.[98]

Trumpeter swans (Cygnus buccinator) and Canada geese (Branta canadensis) often depend on beaver lodges as nesting sites.[85][99][100] 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.[101][102]

Beavers may benefit birds frequenting their ponds in several additional ways. Removal of some pondside trees by beavers increases the density and height of the grass–forb–shrub layer, which enhances waterfowl nesting cover adjacent to ponds.[85] Both forest gaps where trees had been felled by beavers 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.[103] Coppicing of waterside willows and cottonwoods by beavers leads to dense shoot production which provides important cover for birds and the insects on which they feed.[104] 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 semiarid climates.[105]

As trees are drowned by rising beaver impoundments, they become ideal nesting sites for woodpeckers, which 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).[85] 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), use 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.[106]

By perennializing streams in arid deserts, beavers can create habitat which increases abundance and diversity of riparian-dependent species. For example, such as the upper San Pedro River in southeastern Arizona, reintroduced beavers have created willow and pool habitat which has extended the range of the endangered Southwestern willow flycatcher (Empidonax trailii extimus) with the southernmost verifiable nest recorded in 2005.[107]

Effects on trout and salmon

Salmon (Oncorhynchus nerka) jumping beaver dam

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 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.[108] This study also found that beaver ponds increased smolt salmon production 80 times more than the placement of large woody debris.[108] 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.[109] Beaver-impounded tidal pools on the Pacific Northwest's Elwha River delta support three times as many juvenile Chinook salmon (Oncorhynchus tshawytscha) as pools without beaver.[110]

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 (1,800 m) in the northern Sierra Nevada.[111] 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.[112] Similarly, brook trout, coho salmon, and sockeye salmon (Oncorhynchus nerka) were significantly larger in beaver ponds than those in unimpounded stream sections in Colorado and Alaska.[113][114] In a recent study on a headwater Appalachian stream, brook trout were also larger in beaver ponds.[115]

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.[116] 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.[117] 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.[118] Rainbow, brown, and brook trout have been shown to cross as many as 14 consecutive beaver dams.[111] 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.[116] 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.[119] In Oregon coastal streams, beaver dams are ephemeral and almost all wash out in high winter flows only to be rebuilt every summer.[120] 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.[121] Downstream migration of Atlantic salmon smolts was similarly unaffected by beaver dams, even in periods of low flows.[121] 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.[122]

The importance of winter habitat to salmonids afforded by beaver ponds may be especially important 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."[123] Cutthroat trout and bull trout 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.[121] 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.[117]

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.[124] 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.[125] 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 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.[108]

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.[49][126]

Effects on insects

The fallen trees and stripped bark produced by beaver activity provides popular sites for oviposition of the virilis group of Drosophila, including the fruit fly Drosophila montana. Capture of these species of Drosophila for research is significantly more successful near beaver residences. The preference of beavers for birch, willow, and alder corresponds with oviposition site preferences of the Drosophila virilis species group, leading to commensalism between beavers and these species.[127]

Effects on riparian trees and vegetation

Tree felled by beaver (C. c. canadensis), diameter 20 cm

Conventional wisdom has held that beavers 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 beavers 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 regrowing vigorously in most reaches. He further speculated that without control of beaver populations, aspen and cottonwood could go extinct on the Truckee River.[128] Not only have aspen and cottonwood survived ongoing beaver colonization, but a recent study of ten Sierra Nevada streams in the Lake Tahoe basin using aerial multispectral videography has also shown that deciduous, thick herbaceous, and thin herbaceous vegetation are more highly concentrated near beaver dams, whereas coniferous trees are decreased.[129] These findings are consistent with those of Pollock, who reported that in Bridge Creek, a stream in semiarid 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.[130] In a second study of riparian vegetation based on observations of Bridge Creek over a 17-year 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 resprout after beaver cutting, they frequently grew from seeds landing on freshly exposed alluvial deposits subsequent to beaver activity.[131] 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. Beavers play an important role in seed dispersal for the water lily populations that they consume.[132]

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.[133]

Beavers and stream restoration

In the 1930s, the U.S. government put 600 beavers to work alongside the Civilian Conservation Corps in projects to stop soil erosion by streams in Oregon, Washington, Wyoming, and Utah. At the time, each beaver, whose initial cost was about $5, completed work worth an estimated $300.[134] In 2014, a review of beaver dams as stream restoration tools proposed that an ecosystem approach using riparian plants and beaver dams could accelerate repair of incised, degraded streams versus physical manipulation of streams.[135]

The province of Alberta published a booklet providing information on using beaver for stream restoration.[136]

Utah published a Beaver Management Plan which includes reestablishing beavers in ten streams per year for the purpose of watershed restoration each year from 2010 through 2020.[137]

In a pilot study in Washington, the Lands Council is reintroducing beavers to the upper Methow River Valley in the eastern Cascades 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.[138] Beavers were nearly exterminated in the Methow watershed by the early 1900s by fur trappers. 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.[139] As of January, 2016, 240 beavers released into the upper Methow River at 51 sites had built 176 beaver ponds, storing millions of gallons of water in this semiarid east region.[140] One beaver that was passive integrated transponder tagged and released in the upper part of the Methow Valley, swam to the mouth of the Methow River, then up the Okanogan River almost to the Canada–US border, a journey of 120 miles (190 km).[141][142]

In efforts to 'rebeaver' areas of declined beaver populations, artificial logjams have been placed. Beavers may be encouraged to build dams by the creation of a "beaver dam analog (BDA)". Initially, these were made by felling fir logs, pounding them upright into the stream bed, and weaving a lattice of willow sticks through the posts, which beavers would then expand.[143] However, to minimize labor further, newer postless designs have been used, which in smaller streams, beavers can still expand into sequential dams.[144]

Urban beavers

Canada

A tree felled by a beaver at Prince's Island Park in Calgary.

Several Canadian cities have seen a resurgence in its beaver population in recent decades.[145][146][147] The beaver population in Calgary was approximately 200 in 2016, with the majority of the population located near the Bow, and Elbow River.[148] When required, the City of Calgary will use a combination of methods to prevent beaver damage to trees and river parks. Methods of damage prevention includes the placement of a mesh wire fence around the tree trunk, planting trees less palatable to beavers near shorelines, placing under-dam drainage systems to control water levels; and placing traps designed to kill instantly, as Alberta Environment and Parks does not allow the relocation of caught beavers to other areas.[149]

Beavers have occasionally wandered into Downtown Ottawa, including Parliament Hill, Major's Hill Park, and Sparks Street.[150] Beavers caught in the urban core of Ottawa by the National Capital Commission's conservation team are typically brought to a wildlife centre, and later released near the Ottawa River, close to the Greenbelt.[150] In 2011, the City of Ottawa began to trap beavers taking up residence in the stormwater pond in the Stittsville neighbourhood.[151] Ottawa is situated 4 kilometres (2.5 mi) south of the southern entrance to Gatineau Park.[152] Located on the Quebec side of the Ottawa River, the park is home to one of North America's densest populations of beavers, with more than 1,100 beavers in 272 beaver colonies according to a 2011 aerial inventory of the park.[152] The beaver population at Gatineau Park is monitored by the National Capital Commission in an effort to protect local infrastructure, and maintain public safety.[152]

A beaver swimming in Humber Bay, Toronto. Beavers make their way through the city by traversing its waterways.

The City of Toronto government, and the Toronto and Region Conservation Authority (TRCA) do not keep track of the number of beavers residing in Toronto,[148] although an estimate from 2001 places the local beaver population at several hundred.[147] Beavers are commonly found along the shoreline of Lake Ontario, and make their way throughout the waterway corridors of the city,[153] most notably the Don, Humber, and Rouge River; the ravine system adjacent to the waterways, and the Toronto Islands.[147] The City of Toronto government does not have any plans to either control the spread, or contain the number of beavers in the city.[147] However, the city's urban forestry department will occasionally install heavy mesh wire fences around the trunks of trees to prevent them from being damaged by beavers.[80] In 2013, flow devices were installed along the Rouge River, in order to prevent beaver dams from flooding the river. Prior to their installation, beavers whose dams caused the river to flood were trapped.[154] In a 2017 TRCA report on local occurrences of fauna in Greater Toronto, beavers were given a score of L4. The score was given to species whose populations were secure in the rural portions of Greater Toronto, but whose populations in the urban areas of Greater Toronto remained vulnerable to potential long-term decline of its habitats.[155]

Several dozen beavers were estimated to inhabit Vancouver in 2016.[148] Beavers have inhabited Jericho Beach as early as 2000, although they did not move into the other areas of Vancouver until later in that decade. After an 80-year absence, a beaver was spotted in Stanley Park's Beaver Lake in 2008.[156] In 2016, five beavers inhabited Beaver Lake.[157] In the same year, a pair of beavers built a dam in Hinge Park.[156] The Vancouver Park Board approved a strategy that included plans to promote the growth of the beaver population near the Olympic Village in 2016.[148]

Beavers in Winnipeg numbered around 100 in 2019, and live along the city's rivers and streams.[158] After receiving complaints for beaver-related damages in 2012, the City of Winnipeg has placed mesh wire fence around tree trunks along the shore of the Assiniboine River during the winter; as well as laid down traps designed to kill the beavers. Like Alberta, provincial guidelines in Manitoba do not allow for the live capture and relocation of beavers.[159] The City employs one contractor 10 times a year to manage the beaver population in Winnipeg, who is authorized to remove beavers with a firearm under Manitoba's Wildlife Act.[158]

United States

A beaver lodge in Lincoln Park, Chicago, in the fall of 2009

Several cities in the United States have also seen the reintroduction of beavers within their city limits. 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.[160] 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 beaver recolonizing North Pond in Lincoln Park has been recorded in 1994, 2003, 2004, 2008, 2009, 2014, and 2018[160][161][162][163]

In downtown Martinez, California, a male and female beaver arrived in Alhambra Creek in 2006.[164] 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".[165] 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 beavers 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.[166][167] The Martinez beavers probably originated from the Sacramento-San Joaquin River Delta, which once held the largest concentration of beavers in North America.[168]

A beaver grooming itself near the Alhambra Creek, in Martinez, California.

After 200 years, a lone beaver returned to New York City in 2007, making its home along the Bronx River, having spent time living at the Bronx Zoo and the Botanical Gardens.[169] Though beaver pelts were once important to the city's economy and a pair of beavers appears on the city's official seal and flag, beavers had not lived in New York City since the early 19th century, when trappers extirpated them completely from the city.[170] The return of "José", named after Representative José Serrano from the Bronx, has been seen as evidence that efforts to restore the river have been successful.[171][172][173] In the summer of 2010, a second beaver named "Justin" joined José, doubling the beaver population in New York City.[174] In February 2013, what appears to be both José and Justin were caught on motion-sensitive cameras at the New York Botanical Garden.[175]

In 1999, Washington, D.C.'s annual Cherry Blossom Festival was interrupted by a family of beavers that 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.[176][177]

Relationship with humans

As introduced non-native species

See also: Beaver eradication in Tierra del Fuego

Beaver damage on the north shore of Robalo Lake, Navarino Island, Chile

In the 1940s, beavers were brought to 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 main island, and to other islands in the archipelago, 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.[178] 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.[178] In contrast, areas with introduced beaver were associated with increased populations of the native catadromous puye fish (Galaxias maculatus).[179][180] 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.[180] They have also been found to cross saltwater to islands northward; and reached the Chilean mainland in the 1990s.[181] 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.[182]

North American beavers were released in Finland in 1937, before it was realized that they formed a separate species; following this, 7 beavers expanded to a population of 12,000 within 64 years.[183] Eurasian beavers had earlier been extirpated from the region, so the release was intended as a reintroduction project.[184] By 1999, it was estimated that 90% of beavers in Finland were the American species. However, the species is not always considered invasive, as in Europe it has a similar keystone effect to European beavers, which have not recolonized the area. The beaver population has been controlled by issuing hunting licenses.[185] A report in 2010 concluded that while the current population of American beavers 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.[186]

In Europe, significant invasive populations of Canadian beaver are only present in Finland and Karelia, as the boundary between species has somewhat stabilized, but smaller occurrences have been detected elsewhere.[184] Ephemeral populations of C. canadensis in Germany and Poland were found from the 1950s to 1970s. Zoo escapes in 2006 created a small population of invasive C. canadensis in Luxembourg, Rhineland-Palatinate and Belgium.[183] American beavers have not been detected in Sweden, Norway, or Denmark.[186]

As food

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.[187]

Symbolism

Beaver sculpture over entrance to Canadian Parliament Building

As one of the national symbols of Canada,[188] the North American beaver is depicted on the Canadian nickel.[188] This beaver was also featured on the first Canadian postage stamp, the Three Penny Beaver, which is considered the first postage stamp to show an animal instead of a head of state.[189] It is also the state animal of Oregon and New York of the United States, and a common school emblem for engineering schools, including the California Institute of Technology, the Massachusetts Institute of Technology, and the University of Alberta as well as the mascot for Oregon State University, Babson College, and the City College of New York. A beaver is featured prominently on the stamp and seal issued to Professional Engineers and Geoscientists by APEGA. It also appears on the back on the state flag of Oregon. The beaver also appears in the coats of arms of the Hudson's Bay Company,[190] 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.

See also

• Beaver in the Sierra Nevada

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Further reading

• "Castor canadensis". Integrated Taxonomic Information System. Retrieved March 18, 2006.
• Müller-Schwarze, Dietland & 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 978-1-55566-251-6.
• 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)

External links

• Leave It To Beavers, PBS video documentary online
• Ecology of the Beaver
• "Worth a Dam" (beaver information and educational site)
• The Beaver A Keystone Species, a short video by Mike Foster
• Video Eager Beavers Take on Climate Change: Restoring Nature's Engineers in Utah by Grand Canyon Trust