HMW 6 - Family text: Family Castoridae (Beavers)

Family text: 

Class Mammalia

Order Rodentia

Suborder Castorimorpha

Family CASTORIDAE (BEAVERS)

 

  • Large, robust, semi-aquatic rodents with dorsal-ventrally flattened tails about a quarter of the total body length, small forelimbs, and webbed hindfeet.
  • 100–120 cm.
  • Palearctic and Nearctic Regions.
  • All types of freshwater wetlands from streams and ponds to large rivers and lakes.
  • 1 genus, 2 species, 2 taxa.
  • No species threatened; none Extinct since 1600.

Systematics

Rodent systematics is complex, and this is also true for the family Castoridae. There is one extant genus, Castor, and two extant species, the Eurasian Beaver (Castor fiber) and the North American Beaver (Castor canadensis). Here beavers are placed in the suborder Castorimorpha based on the structure of the lower jaw, but they are sometimes placed in the suborder Sciurimorpha based on the structure of the infraorbital canal. Recent molecular, multi-gene, and mitochondrial DNA studies suggest relationships of Castoridae with Geomyidae (pocket gophers) and Anomaluridae (scaly-tailed squirrels).

Castoridae has a rich fossil history, with extensive material coming from North America, Europe, and Asia, but it is not closely related to any other recent rodent group. Castorids originated in the late Eocene, about 40 million years ago, most probably in North America and moved back and forth between Eurasia and North America. Closest fossil relatives to the castorids are from the extinct family Eutypomyidae. There are four generally accepted fossil subfamilies based on morphology: Agnotocastorinae, Palaeocastorinae, Castoroidinae, and the still extant Castorinae. The Agnotocastorinae had the most primitive characteristics and were closely related to the Eutypomyidae based on dental complexity and cranial features. The Agnotocastorinae occurred from the very late Eocene or early Miocene to the late Miocene, although timing varied by continent. The Palaeocastorinae were a relatively short-lived group found only in North America from the late Oligocene to the early Miocene. The Palaeocastorinae had clearly defined fossorial adaptations and were the only castorids with a distinctly flattened anterior surface of the incisors. The Castoroidinae represented the most diversified group of castorids and originated in North America during the Miocene. Two distinct tribes separated in the late Tertiary: the Castoroidini in North America and the Nothodipoidini in Europe and Asia. Castoroidinae included the giant beavers, Castoroides in North America and Trogontherium in Europe. Individuals in these groups were larger than other beavers, up to 2·2 m long and massive at about 100 kg. The giant beavers persisted into the Pleistocene. The Castorinae is the subfamily that leads directly to modern beavers with distinct aquatic adaptations. The first castorine appeared in the late Miocene in North America. The one extant genus, Castor, arose in the late Miocene to early Pliocene 5–9 million years ago; many authors use eight million years as the age of this genus.

Recent molecular studies of rodents suggest there are five monophyletic suborders with one being Castorimorpha, which includes Castoridae, Heteromyidae, and Geomyidae. Mitochondrial DNA relationships link Castoridae in a clade with Anomaluridae, Pedetidae, Dipodidae, Muridae, Heteromyidae, and Geomyidae . Mitochondrial DNA analysis suggests that all rodents shared a common ancestor 57–76 million years ago and the castorids diverged from anomalurids about 54 million years ago. Additional research is needed because multi-gene studies place Castoridae closest to Geomyidae and Heteromyidae, while mitochondrial DNA studies indicate a relationship with Anomaluridae. What is evident is that beavers have a long and diverse evolutionary history going back at least 40 million years and possibly 65 million years.

The extant genus Castor might have evolved in Asia as a close relative to the Eurasian Castorinae Steneofiber and dispersed to North America across the Bering Land Bridge until it flooded 5·5 million years ago. The oldest fossil Castor in North America is from 7·5 million years ago, which is close to the best estimate for the divergence of Castor (about eight million years ago). Fossil and molecular evidence on this is not precise, and it is possible that Castor might have had a North American origin and moved to Asia before the Bering Land Bridge flooded. The two extant species, the Eurasian Beaver and the North American Beaver, evolved because of continental isolation, and although they are physically and ecologically similar, they have different chromosome numbers (48 in the Eurasian Beaver versus 40 in the North American Beaver). The chromosome number of the Eurasian Beaver is considered primitive, and fusion of eight chromosomes has occurred in the North American Beaver. Mitochondrial DNA evidence from the Eurasian Beaver suggests geographic isolation of subpopulations occurred approximately 200,000 years ago, which might tie any present geographic variations observed in this species to climatic events, not simply exploitation by humans. Eight subspecies of the Eurasian Beaver and 24 subspecies of the North American Beaver have been traditionally recognized; however, the exploitation of both species and subsequent reintroduction programs in Europe and North America, which led to mixing of subspecies, have made these geographical distinctions less significant, if not meaningless. Recent genetic analysis recognizes only two phylogroups (western and eastern) in Europe, with neither being true subspecies. A similar situation is most likely true in North America.

Morphological Aspects

The two extant species of Castoridae, the Eurasian Beaver and the North American Beaver, are similar in form and the second largest rodents in the world behind the Capybara (Hydrochoerus hydrochaeris). Beavers are the largest rodents in the Northern Hemisphere (North America, Europe, and Asia). Adult beavers range from 1000 mm to 1200 mm in total body length (including the tail) and generally weigh 18–20 kg. Weights of 30–40 kg have been documented in both species, but beavers of this size are rare. A maximum weight of 39 kg has been recorded for the North American Beaver. Although both species are similar in size and form, geographical variation does exist. The two species are virtually impossible to tell apart in the field but have small anatomical differences in their skulls and tails. There is little if any sexual dimorphism, but there are clear separations in size among young age classes and adults.

The two species of beavers have a number of morphological features that set them apart from other rodent groups, which are related to their semi-aquatic lifestyle. The most distinguishing morphological feature is the dorsal-ventrally flattened tail of both species. The tail has a scaly appearance, and few hairs are present, with tail hairs being more prominent on young individuals. The tails of some geographical groups of Eurasian Beavers are more linear and less oval (Norway), while others are more oval (Germany, Russia), but generally the tails of the two species are similar in shape and size. Lengths and widths of the tail are variable. Mean tail lengths and widths of beavers from populations in California (281 mm and 128 mm), Massachusetts (274 mm and 131 mm), Norway (273 mm and 103 mm), and German beavers raised in Scotland (296 mm and 136 mm) illustrate the variability in tail morphology. The tail is black in young individuals and more pliable, but it becomes browner and stiffer with age. The tail consists of fibrous tissue in an adipose matrix, with attachments to the caudal vertebrae that support locomotion. The tail undulates side-to-side when a beaver is swimming, which produces thrust. As a further aquatic adaptation, beavers have webbed hindfeet, with five digits that provide forward thrust. The webbed hindfeet are the primary means of propulsion, although the tail is also used when sculling and diving. The addition of tail undulation and hindfoot stroking are considered an evolutionary stage for adaptation to an aquatic habitat. Other aquatic adaptations of beavers include membranes that cover the eyes allowing them to remain open underwater and small external ears and nostrils that close like valves when individuals submerge. Beavers can close their mouth (lips) behind their incisors allowing the cutting and carrying of woody vegetation underwater. Beavers lack external genitalia, and pectoral nipples are prominent only on lactating females. They have a pouch or cloaca into which their urogenital canal, rectum, castor sacs, and anal glands open and, in females, the vagina. Internally, beavers have an intranarial epiglottis allowing the trachea to open only into the nares. The posterior of the tongue fits against the palate, blocking the pharynx except when swallowing. They have a split nail on the second toe of each hindfoot, which is used along with the innermost nails on the hindfoot for grooming. Grooming is necessary for removal of ectoparasites and to support water repellency of the fur. Coat color ranges from pale or light brown (almost blonde) to reddish-brown to blackish. Brown coats are more common in both species, and they have are long and coarse guard hairs and fine underfur. Some Eurasian Beavers in the small, geographically isolated southern Siberian population in Central Asia have distinctive white spots on ventral sides of their bodies. The similarity in form between the Eurasian Beaver and the North American Beaver is striking considering that they have been geographically separated for 5–8 million years.

Beavers have streamlined or teardrop-shaped, robust bodies, and they are more agile in water than on land. They are capable of short bursts of speed when startled on land. The skull is robust with no zygomatic process and a distinct pit in the subocciptial. The cheekteeth are considered hypsodont. In the water, beavers might be mistaken for Muskrats (Ondatra zibethicus), although they are much larger and their tails do not undulate in the same manner. Otters (Lutrinae) also resemble beavers while swimming, but they tend to hold their heads high out of the water and swim erratically.

Habitat

The Eurasian Beaver and the North American Beaver are distributed throughout the Nearctic and Palearctic regions (except for introduced populations in Tierra del Fuego). Their success and distributions are tied to the presence of primarily deciduous trees and shrubs associated with riparian habitats. Beavers are always associated with water and are found living in almost every type of wetland habitat. They occur in many types of natural wetlands, ranging from small tributary streams, large rivers, large lakes, and small ponds. They use freshwater systems but do occur in ecotones between fresh and saltwater (estuaries) and have been observed swimming near the shore in salt water. They are considered freshwater, semi-aquatic species and only use saltwater habitats sporadically when living on freshwater systems that connect to the open ocean. They also use human-constructed wetlands including reservoirs, fishponds, and agricultural irrigation canals and drainage ditches. They will colonize wetlands close to humans and persist in ponds and rivers in large cities (e.g. Vienna, Chicago, and Washington, DC).

Beavers are selective but often opportunistically colonize wetlands. When occupying lotic systems such as small-order streams, stream gradient is important because steep gradients create the potential for disrupting their dams during high-flow events. Over much of their distributions in Eurasia and North America, beavers are closely associated with woody plants in the family Salicaceae (e.g. aspens and poplars, both Populus spp., and willows, Salix spp.), although they successfully live in areas where no species of Salicaceae are present. During the most recent ice age, beavers were forced to move to southerly latitudes, but they moved back as climatic conditions again allowed growth of woody vegetation. The general habitat of both species of beavers is a wetland with moderate gradient and sufficient woody vegetation for food and construction of dams. Although they have evolved to use trees for food and to construct their dams and lodges, they are found in areas that do not have trees, such as extreme northern parts of their distributions.

General Habits

The Eurasian Beaver and the North American Beaver are generally nocturnal, with peak activity near dusk and dawn. They are occasionally active during the day, but this is not common. When active, beavers are observed swimming or feeding on the shoreline. They spend their inactive time in lodges constructed of woody sticks, mud, and other vegetation or in burrows dug into a bank. When beavers first emerge from their rest site they tend to patrol the area checking their dams, if present. They then spend the remainder of their active time feeding and grooming. Limited social interaction is observed outside the lodge or burrow, although this might be age-dependent. Beavers become active between 18:00 h and 20:00 h and remain active throughout the night, with activity periods ending between 06:00 h and 08:00 h. They return to their lodge or burrow multiple times during their active periods. When alarmed or startled, beavers slap their tails on water surfaces, which might be a warning to other beavers or meant to illicit a response from the stimulus.

Because Castoridae is a family of the Northern Hemisphere (except for the introduced population in Tierra del Fuego of Argentina and Chile), beavers tend to shift their behavior and activity depending on the season. In spring as the ice melts in northerly latitudes, they emerge and use more evergreen trees than deciduous trees and shrubs (for example, in Norway, Massachusetts, and mountainous areas of California). This is also a time of active scent-mound building to mark boundaries of territories, or for a dispersing individual to advertise for a mate. Dispersing two-year-old beavers tend to begin the process in spring, making longer and longer forays from their natal ­areas. In summer, dispersing beavers settle into their territories and, at times, move between, or at least through, other family territories—this is habitat-dependent. In autumn, beavers increase construction and maintenance activities by adding sticks and mud to their lodges, and especially in northern latitudes, they begin to establish food caches. A food cache consists of branches of woody species placed in the water in front of the site where a beaver family will spend the winter. There might be more variation in the construction of food caches between Eurasian Beavers and North American Beavers, although this has not been thoroughly investigated. For example, in the Sierra Nevada of California and Massachusetts, all beaver families constructed their food caches in autumn, but only 50% did so in Sweden. There might also be more variation in food-cache construction in populations at low latitudes and low elevations. More complete data are needed on this important evolutionarily behavior. While North American Beavers are thought to exhibit more construction behavior than Eurasian Beavers, this might be habitat-dependent and not a clear differentiation between species. Given the broad geographical distributions of the two species of beavers (about 40–45 degrees latitude, ranging from 30° N to 70–75° N), their use of woody vegetation is variable.

For both species, the family group is the social unit. They are all generally nocturnal, can be shy around people, and slap their tails when startled or alarmed. Nevertheless, some beavers tolerate human activities and live in rivers, lakes, and ponds in urban areas and near industrial sites along larger rivers. The Gray Wolf (Canis lupus) is the most significant predator on both beaver species. Coyotes (Canis latrans), American Black Bears (Ursus americanus), Brown Bears (Ursus arctos), Wolverines (Gulo gulo), American Minks (Neovison vison), Eurasian Otters (Lutra lutra), and American alligators (Alligator mississippiensis) also prey on beavers. While predation, especially by wolves, can be significant in specific populations, it is not an important mortality factor across the distributions of either the Eurasian Beaver or the North American Beaver.

Communication

The Eurasian Beaver and the North American Beaver communicate with vocalizations, postures, and tail slapping and chemically by creating scent mounds. Up to seven types of sounds from beavers have been documented in the laboratory, but these are rarely heard in natural settings. The most common vocalizations are a mewing whine, most often heard from young, and a forced hiss by adults. Young beavers when approaching adults tend to whine or mew, which has been heard while begging for food. The adult hiss is often associated with aggression, as is teeth sharpening. The most visible communication exhibited by beavers is the tail slap when an individual slaps the surface of the water with its flat tail, accompanied by a forceful rearward thrust of the hindfeet and followed by a shallow dive under the water. The tail slap is thought to warn other beavers of disturbance in the area or the presence of potential predators. There might be differential responses to tail slaps by age groups in a family, but tail slapping appears to be very individualized. For example, some young-of-the-year, yearlings, and adults tail-slap more frequently than others. Tail slapping is also observed more often in some families, while it is rare in others. Tail slapping can be a startle response by a beaver and might also function to elicit movement from an unknown object on the shoreline in an attempt to determine threat. There might also be a seasonal component to tail slapping and aggression. Aggressive beavers will lunge at a potential predator on land, but they rarely continue this behavior and quickly retreat to water. Beavers have anal glands and castor sacs that are used for chemical communication. The anal glands are holocrine secretory glands, and the anal gland secretion (AGS) is composed of compounds with high molecular weights that are mainly fatty acids, free alcohols, sterol esters, waxy esters, and cholesterols. The castor sacs are pockets in the cloaca region that are lined with non-secretory cells. The castoreum is urine that flushes through the castor sacs and contains compounds stored in the sacs. Compounds in the castoreum are phenolics and terpenes with lower molecular weights than the compounds in the AGS. Beavers build mounds of mud and sticks on edges of shorelines—often at the margins of their territories or near the active lodges. Beavers will excrete fluid by passing urine through the castor sacs onto the scent mound and by rubbing the anal glands on the mound. There is some evidence that castoreum is most often deposited on the scent mounds, but beavers also rub their anal glands on scent mounds. Many recent studies have examined similarities and differences in communication with scent mounds by both beaver species. Castoreum is composed of hundreds of compounds, mostly secondary metabolites that are derived from the diet, and the specific composition of castoreum might change seasonally. Castoreum has been associated with territorial defense and can signal occupation of an area and potentially food availability to other beavers. Compounds in castoreum with low molecular weights allow them to serve as airborne pheromones. The AGS is sexually dimorphic in both species. In the Eurasian Beaver, the female’s AGS is thicker and grayish, and the male’s AGS is oily, thinner, and lighter in color. In the North American Beaver, the female’s AGS is lighter in color and runny, and the male’s AGS is brownish and viscous. Secretions of the anal gland might signal more individual information to other beavers, such as age and sex, than castoreum.

Food and Feeding

The Eurasian Beaver and the North American Beaver have evolved to live with and use woody vegetation. One unique aspect is their ability to obtain and manipulate food and construction items much larger than themselves in the form of large trees. They are considered choosy generalist herbivores, using a variety of herbaceous and woody vegetation, but in most cases, they show a preference for a small number of species. They prefer deciduous species to evergreen species but do take conifers such as pine (Pinus spp.), hemlock (Tsuga spp.), fir (Abies spp., all Pinaceae), and other evergreen trees seasonally. In the eastern USA, long-term occupation of sites is closely associated with the presence of eastern white pine (Pinus strobus), although this might be related to changes in the plant community caused by beavers rather than habitat preference. There is a direct correlation between latitude, species availability, and species use. In habitats with higher diversity of woody species, beavers use more species but still rely on only a few plant species for the majority of their diets. When aspen is available, it is preferred by both beaver species. Willow species also are considered a staple in beaver diets when they are present. Other major deciduous species used by beavers include maple (Acer spp., Aceraceae), alder (Alnus spp.) and birch (Betula spp., both Betulaceae), oak (Quercus spp., Fagaceae), and cherry (Prunus spp., Rosaceae). Beavers have micro-biota that allows them to digest some of the cellulose in their diets, but the majority of their energy requirements come from nutrients in the cambium layer of the trees and shrubs they consume. They eat leaves of young plants and strip bark off small branches, ingesting the nutrient layer directly under the bark. They also take herbaceous species seasonally, especially in spring. Aquatic plants (e.g. water lilies, Nuphar spp. and Nymphaea spp., Nymphaeaceae) are important in many locations, and the rhizomes are even included in winter food caches. It has been suggested that some plant species (e.g. red maple, Acer rubrum) might be more palatable if they are left in water before consumption. The general pattern is for beavers to cut branches of trees or shrubs and sit in a spot and feed by holding the small branch in the front hands and using the incisors to strip off the bark, much like humans might eat an ear of corn (maize). Feeding occurs outside and inside resting sites. Resting metabolism has been measured as 86·2 kj/h for a 14 kg beaver with active metabolism being 3–4 times higher. Intake of food is inversely related to retention time in the gut. Retention time is dependent on the plant species ingested and can range from ten to 80 hours; a reasonable average range of retention time of a mixed diet is 20–40 hours.

Breeding

Beavers reproduce only once per year. They mate in winter, have a relatively long gestation period of 100–110 days, give birth in spring, and have a fairly long maturation period for the young that stay with their natal families for two years. Females come into estrus for a relatively short period of time (12–24 hours), and if not fertilized, they can come into estrus again. Copulation has been observed in captive beavers; it most often occurs in water with individuals facing each other but can also occur in the lodge or burrow. For rodents, beavers have relatively few young. Litter sizes are 1–6 young, but 2–4 young are most common. There is amazing consistency in the number of young observed in a family, with a mean of 2·5 (2–3) offspring/litter most often reported for both species.

There might be differences in reproductive success between the two beaver species: average number of young surviving outside the natal lodge or burrow is 1–2 young for the Eurasian Beaver and 2–4 young for the North American Beaver. Nevertheless, this is variable, and habitat quality influences reproductive success. For example, mean autumn litter sizes in northern Wisconsin and Minnesota in the USA were 0·8–3·5 offspring and were associated with specific habitat factors. In captive Eurasian Beavers in Poland, average litter size was 2·7 offspring. As the overall population of Eurasian Beavers continues to grow after almost complete extirpation by the end of the 19th century, and more studies of expanding populations are conducted, it would not be surprising to find that effective litter sizes (young surviving to become part of the family group) are similar for both species in habitats of similar quality. Breeding occurs in December–May in both species, but some reports documented breeding as early as November. Conception peaks in January–February. Births occur about three months later, and young are weaned 6–8 weeks after birth. Females can be reproductively active at 1·5 years of age, but this is rare. Sexual maturity most often occurs in two-year-old beavers of both sexes, and there is evidence that reproductive success is correlated with age of the mother.

Beavers have been reported to live over 20 years in captivity, while between eight and ten years may be more common in wild populations.

Movements, Home range and Social organization

The Eurasian Beaver and the North American Beaver are territorial and thus relatively sedentary. Beavers construct elaborate dams to impound water in their territories (except those in large rivers and large lakes) and lodges made of mud and branches of woody vegetation for resting and protection. This investment makes it energetically costly to colonize and then abandon an area; however, beavers in some geographical areas move around year-to-year or over a series of years. These periodic movements are connected to food availability or changing water conditions. For example, on a 50-km2 peninsula in Massachusetts where beaver activities have been documented for 62 years, 82% of potential wetland sites have been used by beavers, but only 31% of those have been active for 30 or more years. The longest period of occupation at any site was 42 years. Within wetland areas, beavers also might make shorter (0·5–1 km) annual shifts in areas of use. The longest movements occur during dispersal of young adults from their natal family areas, most often in spring and summer of their second year. Dispersal distances are variable and depend on density of family groups in the area. In a population in New York, mean dispersal distances were 8·9 km for females and 4 km for males. Other North American studies of dispersal found similar distances although there are reports of long-distance movement by individual beavers. Home range estimates for beavers are problematic because they use vegetation along shorelines of rivers, streams, and ponds. Most beaver activity is within 30 m of the shore. Linear home range estimates are 0·5–1·5 km of stream or shoreline. Because vegetation composition varies along aquatic sites, linear home ranges of beavers might expand, and studies of a reintroduced population in France documented home range lengths up to 5 km. Some authors have calculated home range areas of 20–30 ha. Beavers appear to expand or contract their home ranges depending on food availability. Because beaver families tend to use the most preferred food more rapidly than it can be replenished, they continually make adjustments to the areas they use or move up or down streams or along shorelines to more favorable areas. These movements are usually not large but might be 1–2 km.

Beavers are monogamous (at least socially, if not genetically), and instances of non-monogamous mating are rare and habitat- or density-dependent. Monogamy most likely evolved through mate-guarding behavior because females only come into estrus once a year, and it is during winter when, in northern latitudes, beaver movements are limited by ice. After a pair bond has formed, it is vital for the male to be present when the female goes into estrus. Other behaviors like territorial defense and investments in construction of dams and lodges support paternal fidelity. Having two adults present to defend and maintain a territory is essential for survival of offspring and increases genetic fitness. Beavers live in family groups, called colonies by G. W. Bradt in 1938, although they are not colonies in the traditional sense. A family group has a mated pair of adults, young-of-the-year, and yearlings from the previous year. Two-year-old beavers might stay with their parental families, especially if an area is saturated with other family groups that inhibit dispersal; however, normally, two-year-old beavers disperse.

Paleontological discoveries suggest that group living by beavers has existed for millions of years. Age-class dominance exists within a family, with older individuals dominant to younger individuals, and there is no clear pattern of one sex being dominant over the other within an age class. All individuals in a family contribute to construction and maintenance of dams and lodges in their territory, although there is some evidence that adult males are more active in patrolling their territorial boundaries. Young beavers often beg for food from older individuals, especially adults, and adults, two year olds, and yearlings bring food back to the lodge, or other rest site, that is shared with young. When beavers are out of their lodge, they most often swim, cut and consume vegetation, groom, and, depending on the season, repair or construct dams and lodges. Activities in the rest site consist of sleeping, feeding, grooming, and, to a limited extent, social interactions.

Relationship with Humans

Beavers and humans have a long and complicated history, extending to the present. Beavers have been harvested for their pelts to produce clothes and hats and for their castoreum that was used medicinally and in perfumes. North American Beavers hold a special relationship with many Native Americans, who have given ceremonial names, included them in totem poles, and used them as hunting fetishes. Neolithic people in England used beaver lodges to live in and moved through wetland areas on their dams. The importance of beavers and the relationship between beavers and humans are illustrated by the number of place names referring to beavers in North America and Europe.

Eurasian Beavers were hunted to almost complete extinction by the 19th century; their populations were in decline by the 17th century in Europe. Exploration of the North American continent was fueled by the discovery and development of the extensive fur resource in North America. Dutch settlers and traders in New York actively traded with Native Americans and sent thousands of beaver pelts to Europe in the early 1600s. The English Hudson Bay Colony, founded in 1670, had a Royal Charter and, along with the competing Canadian North West Company, controlled the fur market in North America in the late 1700s. In the early 1800s, the American Fur Company, founded by John Jacob Astor, provided direct American competition to the English and Canadians and solidified American claims in the Pacific Northwest. The quest for beaver pelts by these companies encouraged European exploration of North America and, in many ways, insured the sovereignty of Canada from the USA. Therefore, beavers, or at least the desire for beaver fur, were important in the geographical and political development of North America.

Historically, Native Americans probably harvested beavers in a sustainable manner for food and clothing; however, the arrival of Europeans with a different view of resource use and value encouraged Native Americans to help overexploit the seemingly inexhaustible populations of beavers. The demand for beaver fur hats in 18th- and 19th-century Europe exacerbated depletion of beaver populations in North America. The continued search for beavers across the North American continent encouraged exploration and every account of early 18th- and 19th-century expeditions include exploitation of beavers. By the early 1900s, beaver populations were severely depleted in Eurasia and North America. Wetland loss caused by expanding human populations and laws enacted to drain wetlands in North America and use them for agriculture contributed to the declining populations of beavers. Essentially, beavers were viewed as a valuable natural resource, and the wetland areas they occupied were equally valuable land for human development. In the USA, more than 50% of all wetlands were drained by the 20th century.

Beavers and humans continue to have a complicated relationship in contemporary times. With recovery of both beaver species during the 20th century, conflicts involving land use have increased. As beavers were removed from wetland areas, humans moved in and filled the land using it for agriculture and human structures. Beavers construct dams creating ponds and raising the water level. This can lead to flooding of agricultural lands, railroad lines, highways, and buildings. Even in Europe where by the early 20th century beaver populations were restricted to a few small refugia, increasing populations have led to increases in nuisance complaints about property loss and flooding. Currently, humans are learning to live with beavers where possible and a number of non-invasive techniques have been developed (water-flow devices) to alleviate the impact of beaver flooding. Beavers have significant impacts on wetland ecosystems. They influence sedimentation rates, stream-flow dynamics, and nutrient load, and their dams can alter water table levels and mitigate flooding. They also influence the spatial heterogeneity and biological diversity of riparian areas. The value of wetlands is also receiving renewed interest, such as their role in providing ecosystem services. Beavers have evolved as a component of wetland habitats in Eurasia and North America and play a vital role in maintaining wetland habitats and the other species than depend on them.

Status and Conservation

Populations of the Eurasian Beaver and the North American Beaver were severely depleted during the 17th, 18th, and 19th centuries. By the early 20th century, only about 1200 Eurasian Beavers remained in a handful of refugia in France, Germany, Norway, Poland, and Russia. In the USA, North American Beavers were also reduced or completely extirpated in many areas. Canada maintained more viable populations, but even those were impacted by human behavior and exploitation. Beginning in the 1920s, efforts to reintroduce beavers began in their former distributions on both continents. Reintroduction efforts in Europe have been a remarkable success story in conservation, and viable expanding populations of Eurasian Beavers are found throughout the continent. In many countries in Europe, the numbers have expanded to such a degree that hunting seasons have been instituted. The small populations of Eurasian Beavers in Asia might be of concern in the future. In North America, the story is much the same and reintroduction efforts have directly led to population increases and range expansion. Trapping rather than hunting is more of a cultural tradition in North America, and trapping seasons are in effect in most US states and Canadian provinces.

Because the two beaver species appear similar in form and are similar in ecological activity, North American Beavers were introduced into Finland in the 1930s and other European countries (either purposefully or accidentally) and in Kamchatka, Russian Far East. This led to the long-standing conflict in dealing with North American Beaver as an invasive species, especially in areas of potential overlap in distributions between it and Eurasian Beavers. While small pockets of North American Beavers might remain in Luxembourg and potentially Germany and France, the major areas of conflict are in northern Finland and the Karelian Peninsula in Russia. There is a strong movement to remove all free-living North American Beavers from Eurasia, although the success of such action is questionable. In the 1940s, North American Beavers were also introduced to Isla Grande, Tierra del Fuego, Argentina, and they have expanded and modified riparian environments and southern beech forests (Nothofagus spp., Nothofagaceae) in the region. They have expanded their distribution and are now found on other islands in Argentina and Chile. With no evolutionary history in the Southern Hemisphere, the impact of beavers on this sensitive habitat is a concern, and there is an ongoing effort in Argentina and Chile to eradicate this exotic species from the region.

A further conservation issue might be the maintenance of geographical forms, sometimes referred to as subspecies, which is more commonly done in Europe. For example, the recent government-controlled introduction of beavers in Scotland used beavers from Norway because they were considered the closest “subspecies” relative to the once native beaver found in England and Scotland. A compounding factor is the accidental or purposeful release of beavers from other geographical regions in Europe along the River Tay in Scotland and free-living beavers in southern England. A recent genetic analysis suggests that there are only western and eastern clades in Europe so the subspecies question might not be significant in the future. Regardless, the human-influenced restorations of Eurasian Beavers and the North American Beavers are conservation success stories, vitally important to both species and ecosystem functions.

 

General Bibliography

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