The Eurasian beaver or European beaver (Castor fiber) is a species of beaver which was once widespread in Eurasia. It was hunted to near-extinction for both its fur and castoreum, and by 1900 only 1,200 beavers survived in eight relict populations in Europe and Asia. Re-introduced through much of its former range, it now occurs from Great Britain to China and Mongolia, although it is absent from Italy, Portugal and the southern Balkans.
The fur colour of Eurasian beavers varies geographically. Light, chestnut-rust is the dominant colour in Belarus. In Russia, the beavers of the Sozh River basin are predominantly blackish brown, while beavers in the Voronezh Reserve are equally distributed between brown and blackish-brown.
Eurasian beavers are one of the largest living species of rodent and are the largest rodent native to Eurasia. They weigh around 11–30 kg (24–66 lb), with an average of 18 kg (40 lb). While the largest specimen confirmed on record weighed 31.7 kg (70 lb), the Smithsonian has reported that this species can exceptionally exceed 40 kg (88 lb). Typically, the head-and-body length is 80–100 cm (31–39 in) and the tail length is 25–50 cm (9.8–20 in).
Differences from North American beaver
Although the Eurasian beaver appears superficially similar to the North American beaver, there are several important differences between the two species.
Aside from being the larger of the two species, the Eurasian beaver has the following anatomical differences to North American beaver: it has a larger, less rounded head; a longer, narrower muzzle; a narrower, less oval-shaped tail; shorter shin bones, making it less capable of bipedal locomotion than the North American species. The Eurasian beaver also has longer nasal bones, with the widest point being at the end of the snout; in the case of the North American beaver, the widest point is at the middle of the snout. The Eurasian beaver has a triangular nasal opening, unlike those of the North American beavers, which are square. Furthermore, the foramen magnum is rounded in the Eurasian beaver, but triangular in the North American beaver. The anal glands of the Eurasian beaver are larger, and thin-walled, with a large internal volume, relative to that of the North American beaver. The guard hairs of the Eurasian beaver have longer hollow medullas at their tips. There is also a difference in fur colour: overall, 66% of Eurasian beavers have beige or pale brown fur, 20% have reddish brown, nearly 8% are brown, and only 4% have blackish coats; in North American beavers, 50% have pale brown fur, 25% are reddish brown, 20% are brown, and 6% are blackish.
The two species are not genetically compatible. The North American beaver has 40 chromosomes, while the Eurasian beaver has 48. After more than 27 attempts, made in Russia, to hybridize the two species, the result was one stillborn kit that was bred from the pairing of a male North American beaver and a female Eurasian beaver. The aforementioned factor makes interspecific breeding unlikely in areas where the two species' ranges overlap.
Subspecies of Eurasian beaver
Historically, eight subspecies of Castor fiber were described, one for each of the eight 19th–20th century refugia where the species never became extinct. The basis of the differentiation was morphological, largely based on very small differences in cranial morphology, but has been recently refuted based on genetic studies. In 2005, Durka et al. showed that only two evolutionarily significant units (ESUs) exist based on mitochondrial DNA studies, a western phylogroup (C. f. gallicus, C. f. albicus and C. f. fiber) and an eastern phylogroup (C.f. ssp., C. f. tuvinicus, C. f. pohlei, C. f. birulai). In addition, Ducroz et al. found that even in the more genetically diverse eastern phylogroup, the degree of genetic divergence was below thresholds considered sufficient for subspecies differentiation.
Eurasian beaver have one litter per year, coming into estrus for only 12 to 24 hours, between late December and May but peaking in January. Unlike most other rodents, beaver pairs are monogamous, staying together for multiple breeding seasons. Gestation averages 107 days and they average three kits per litter with a range of two to six kits. Most beaver do not reproduce until they are three years of age, but about 20% of two-year-old females reproduce.
The Eurasian beaver is recovering from near extinction, after depredation by humans for its fur and for castoreum, a secretion of its scent gland believed to have medicinal properties. The estimated population was only 1,200 by the early 20th century. In many European nations, the beaver went extinct but reintroduction and protection has led to gradual recovery to approximately 639,000 individuals by 2003. Milishnikov found in genetic studies that beaver likely survived east of the Urals from a nineteenth-century population as low of 300 animals, and that factors contributing to their survival include their ability to maintain sufficient genetic diversity to recover from a population as low as 3 individuals, and that beavers are monogamous and select mates that are genetically different from themselves. 83% of Eurasian beavers live in the former Soviet Union thanks to reintroductions, however the result is that beaver in Mongolia or Siberia do not appear significantly genetically different from samples from the European part of Russia, despite the great geographical distance.
In France, the Eurasian beaver was almost wiped out, but a small population survived on the Rhône, near Lyon, from where it has been reintroduced to other parts of the country. The French population of beavers is estimated to be 10,000-12,000 individuals in 2009.
Beavers have been reintroduced in the Netherlands in 1988 after being completely exterminated in the 19th century. After its reintroduction in the Biesbosch, the Dutch population has spread considerably (supported by additional reintroductions), and can now be found in the Biesbosch and surrounding areas, along the Meuse in Limburg, and in the Gelderse Poort and Oostvaardersplassen. In 2012 the population is estimated about 600 animals and could easily grow to 7000 in twenty years time. According to the Mammal Society and the Dutch Water board, this will cause a threat to the river dikes. The main problem is that beavers excavate corridors and caves in dikes, thereby undermining the stability of the dike, just as the muskrat and the coypu do. If problems become unmanageable, as local administrators in Limburg fear, the beaver will be captured again. 
Non-government sanctioned re-introduction in Spain around 2003 has resulted in tell-tale beaver sign documented on a 60 km stretch on the lower course of the Aragon River and the area adjoining the Ebro River in Aragon, Spain.
In the former Soviet Union, almost 17,000 beavers were translocated from 1927 to 2004, of which 12,000 were to Russia, and the remainder to the Ukraine, Belarus, the Baltic States and Kazakhstan. They are now common in Estonia and Latvia.
The recently resurgent beaver population in Eurasia has resulted in increases in human-beaver encounters. Indeed, in May 2013, a Belorussian fisherman died after being bitten several times by a beaver, severing an artery in his leg and causing him to bleed to death.
In Sweden, the beaver had been hunted to extinction by around 1870. Between 1922 and 1939, approximately 80 individuals were imported from Norway and introduced to 19 separate sites within the country.
Norwegian beavers also played an important role in reintroducing the locally extinct animal to Finland, but there the population also includes a substantial number of C. canadensis of Canadian origin. (The North American beavers were imported to Finland in 1937, when it was not yet known that C. canadensis was a different species from the Eurasian beaver.) Beaver re-introduced to central Norway's Ingdalselva River watershed on the Agdenes peninsula, Sør-Trøndelag County in 1968-1969, were recently studied and shown to cross multiple mountain divides and to swim across 25 km of sheltered salt water in order to recolonize outlying suitable habitat.
In Denmark, the beaver was reintroduced to the wild in western Jutland in 1999 and in Arresø, northern Zealand, in 2009 after it was hunted to extinction c. 1000 CE. The reintroduced beavers were caught in the river Elben in Germany. As of 2009[update], the Danish population of beavers was estimated to be 100—120 individuals.
The beaver became extinct in Great Britain in the sixteenth century: Giraldus Cambrensis reported in 1188 (Itinerarium ii.iii) that it was to be found only in the Teifi in Wales and in one river in Scotland, though his observations are clearly secondhand. The last reference to beavers in England dates to 1526. About the same time, Hector Boece wrote that they were still common in parts of Scotland, especially around Loch Ness.
In 2001, the Kent Wildlife Trust with the Wildwood Trust and Natural England imported two families of Eurasian beaver from Norway to manage a wetland nature reserve. This project pioneered the use of beaver as a wildlife conservation tool in the UK. The success of this project has provided the inspiration behind other projects in Gloucestershire and Argyll. The Kent beaver colony lives in a 130-acre (0.53 km2) fenced enclosure at the wetland of Ham Fen. Subsequently, the population of beaver has been supplemented in 2005 and 2008. The beaver continue to help restore the wetland by rehydrating the soils. Six Eurasian beavers were released in 2005 into a fenced lakeside area in Gloucestershire. In 2007, a specially-selected group of four Bavarian beavers were released into a fenced enclosure in the Martin Mere nature reserve in Lancashire. It is hoped that the beavers will form a permanent colony, and the younger pair will be transferred to another location when the adults begin breeding again. The progress of the group will be followed as part of the BBC's Autumnwatch television series. A colony of beavers is established in a large enclosure at Bamff, Perthshire. On November 19, 2011, a pair of beaver sisters were released into a 2.5 acre enclosure at Blaeneinion, Furnace, Mid Wales.
The first sustained and significant population of wild-living beavers in the United Kingdom became established on the Tay catchment in Scotland as early as 2001 and has spread widely in the catchment, numbering from 20 to 100 individuals. Because these are likely escapees from any of several nearby sites with captive beavers, or were illegally released, they were targeted for removal by Scottish Natural Heritage in late 2010. Proponents of the beavers argue that they have not been proven to be of "wrong" genetic stock and there is scientific evidence to support that they may represent a rather ideal mix of western European populations, since any single relict population in western Europe is relatively genetically depauperate. The first of the wild Tayside beavers was trapped by Scottish Natural Heritage on the River Ericht in Blairgowrie, Perthshire in early December 2010 and is being held in captivity in the Edinburgh Zoo. More recently a Beaver has taken up residence on the Loch of the Lowes. The Beaver is thought to be of relation to the Tay beavers whose existence is most likely due to escapees. However this is exciting news as the Beaver was once part of Scotland's ecology.
In 2005, the Scottish Government turned down a licence application for unfenced reintroduction. However, in late 2007, a further application was made for a release project in Knapdale, Argyll. This application was accepted, and the first beavers were released on 29 May 2009. This initial release into the wild of 11 animals received a setback during the first year with the disappearance of two animals and the alleged illegal shooting of a third. This allegation was later refuted by Simon Jones of the Scottish Beaver Trial as there was no evidence to support the allegation and all three missing beavers were sighted after they had left the release loch. However, the remaining population was increased by further releases in 2010. In August 2010, at least two kits, estimated to be eight weeks old and belonging to different family groups, were seen in Knapdale Forest in Argyll.
With the exception of the Knapdale and Tayside animals, all the beavers in the United Kingdom today are in semi-enclosed sites and not fully released into the wild. A 2009 report by Natural England, the Government’s conservation body, and the People's Trust for Endangered Species recommended that beaver be reintroduced to the wild in England.
A study has been undertaken on the feasibility and desirability of a reintroduction of beavers to Wales by a partnership including the Wildlife Trusts, Countryside Council for Wales, Peoples Trust for Endangered Species, Environment Agency Wales, Wild Europe, Forestry Commission Wales, with additional funding from Welsh Power Ltd. The resulting reports were published in 2012 with the launch of the Welsh Beaver Project, which is a partnership led by the Wildlife in Wales, and are downloadable from www.welshbeaverproject.org.[dated info]
Beaver are a keystone species helping support the ecosystem of which they are a part. They create wetlands, which increase biodiversity and provide habitat for many rare species such as water voles, otters and water shrews. They coppice waterside trees and shrubs so that they re-grow as dense shrubs which provide cover for birds and other animals. Beaver dams trap sediment and improve water quality; recharge groundwater tables and increase cover and forage for trout and salmon. A recent study in Poland found that beavers increased the abundance and diversity of bats, apparently because they create gaps in forest cover making it easier for bats to navigate in.
Effects on fish
Beaver ponds have been shown to have a beneficial effect on trout and salmon populations, in fact many authors believe that the decline of salmonid fishes is related to the decline in beaver populations. A study of small streams in Sweden found that brown trout in beaver ponds were larger than those in riffle sections, and that beaver ponds provide habitat for larger trout in small streams during periods of drought. These findings are similar to several studies of beaver effects on fish in North America. Brook trout, coho and sockeye salmon were significantly larger in beaver ponds than those in un-impounded stream sections in Colorado and Alaska. In addition, research in the Stillaguamish River basin in Washington state, found that extensive loss of beaver ponds resulted in an 89% reduction in coho salmon smolt summer production and an almost equally detrimental 86% reduction in critical winter habitat carrying capacity. Migration of adult Atlantic salmon (Salmo salar) may be limited by beaver dams during periods of low stream flows, but the presence of juveniles upstream from the dams suggests that the dams are penetrated by parr. Downstream migration of Atlantic salmon smolts was similarly unaffected by beaver dams, even in periods of low flows. Two year old Atlantic salmon parr in beaver ponds in eastern Canada showed faster summer growth in length and mass and were in better condition than parr upstream or downstream from the pond. The importance of winter habitat to salmonids afforded by beaver ponds may be especially important (and underappreciated) in streams without deep pools or where ice cover makes contact with the bottom of shallow streams. A 2003 study showed that Atlantic salmon and Sea trout (S. trutta morpha trutta) spawning in the Numedalslågen River and 51 of its tributaries in southeastern Norway were unhindered by beaver. In Norway, beaver dams are considered beneficial for Brown and Sea Trout populations (these are potamodromous and anadromous forms of the same species). There, beaver ponds produce increased food for young fish and provide refugia for large adults heading upstream to spawn.
Effect on water quality
The misnomer ‘beaver fever’ was invented by the American press in the 1970s after an outbreak of Giardia lamblia, which causes Giardiasis, was blamed on beavers. However, the outbreak area was also frequented by humans, who are generally the primary source of contamination of waters. In addition, many animals and birds carry this parasite. Giardiasis affects humans in southeastern Norway, but a recent study found no Giardia in the beavers there. 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. New Zealand has Giardia but no beavers. In a 1995 paper recommending re-introduction of beaver to Great Britain, MacDonald stated that the only new diseases that beaver might convey to that country's birds and mammals, are rabies and tularaemia - both diseases that should be preventable by statutory quarantine procedures and prophylactic treatment for tularaemia.
In addition, fecal coliform and streptococci bacteria excreted into streams by grazing cattle have been shown to be reduced by beaver ponds, where the bacteria are trapped in bottom sediments.
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