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   Castor canadensis (mammal)
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      Castor canadensis adult (Photo: Terry Spivey, USDA Forest Service, - Click for full size   Damage caused by Castor canadensis (Photo: Charles T. Bryson, USDA ARS, - Click for full size   Damage caused by Castor canadensis (Photo: Paul Bolstad, University of Minnesota, - Click for full size
    Taxonomic name: Castor canadensis (Kuhl 1820)
    Common names: American beaver (English), beaver (English), Canadian beaver, castor (French), castor americano (Spanish), North American beaver (English)
    Organism type: mammal
    Castor canadensis (beaver) is native to North America, and has been introduced to Tierra del Fuego in southern South America, Finland, France, Poland and Russia in recent times. In its introduced range, the damming activity of the beaver can cause flooding which can damage forests. They also have the ability to quickly cut down large numbers of trees. In Finland, they compete with native beaver populations. In their native range, they cause flooding on major highways by plugging highway culverts.
    Castor canadensis (beaver) is a large hervbivorous rodent typically found near water. Adults may be up to 1200mm long and weigh between 18-47kg. Colour ranges from yellowish-brown to black with reddish-brown most common. Guard hairs are long and coarse and the under fur is dense and lead grey in colour. The tail is broad, scaly and dorsoventrally flattened. It is black in young animals but becomes lighter with age. Adaptations for aquatic life include nictitating membranes on the eyes, valvular ears and nose, lips closing behind incisors and webbed hind feet (Jenkins and Busher, 1979; Nummi, 2006).
    Similar Species
    Castor fiber

    Occurs in:
    lakes, natural forests, range/grasslands, riparian zones, scrub/shrublands, tundra, water courses, wetlands
    Habitat description
    Castor canadensis (beavers) are always found close to water and they require forest to provide food and building material (Nummi, 2006). Beavers have a unique ability to cut trees and this allows them to build mud and wood lodges in which they live, nest and store food. Lodges may be completely surrounded by water or built on the banks of ponds, lakes or streams.They are also able to build watertight dams which create ponds behind them where the beavers are then able to build lodges (Jenkins and Busher, 1979). This behaviour alters large areas of habitat and is the reason why beavers are termed “ecosystem engineers” (Nummi, 2006).
    General impacts
    Castor canadensis (beavers) are known as "ecosystem engineers" for their ability to alter the physical and chemical nature of water bodies and their adjacent terrestrial systems in both their native and introduced range (Nummi, 2006). Two recent studies have investigated the impacts of beavers on ecosystems in their introduced range in southern South America. Beavers have been found to cause significant reduction in forest cover up to 30m from water effectively removing riparian forest. In their introduced range of South America Beavers modify the original ecosystem from closed Nothofagus forest to a grass- and sedge-dominated meadow. Nothofagus forest and seedlings are suppressed by beavers but herbaceous plants have been shown to increase in number and diversity. Unfortunately most of the increase in herbaceous plant diversity is due to invasion of the areas by non-native species (Anderson et al. 2006). Deforestation caused by C. canadensis also has the immediate effect of increased erosion due to exposed slopes (Lizzaralde et al, 2004). Forests may not completely regenerate in meadows for more than 20 years after removal of beavers due to flooding and sediments completely covering the forest floor which impedes seedling germination and establishment (Martinez Pastur et al, 2006). Anderson and Rosemand (2006) investigated the effect of beavers on the aquatic ecosystem and found that ponds created by beavers had increased productivity but at the expense of significantly reduced macroinvertebrate diversity. Via physical, chemical and geomorphological alterations, beavers modify the structure and function of entire biotic communities and ecosystems. Lizarralde et al (2004) found that beaver colonized sites in the Tierra del Fuego Archipelago, Argentina had submerged vegetation and algae indicative of high nitrogen concentrations. Wood debris from fallen trees causes an accumulation of organic material that modifies the biochemical composition of waters, sediments, soils and adjacent riparian areas. These alterations make beaver-altered sites more suitable for introduced fish species (Salmo trusttafario, Salvelinus fontinalis and Onchorrychus mybis) and sustained invertebrate communities typidcal of slow-water habitats (Lizzaralde et al, 2004). Beavers dam the river in which their lodge occurs, and sometimes the dam breaks causing extensive flooding. Dams act as barriers to migration in the stream and also form areas of impounded water behind them, increasing water temperature (Alexander, 1998). Beavers are also known for their ability to rapidly clear a forested area, and also cause flooding to roads by plugging highway culverts (Jensen et al. 2001).
    Castor canadensis (beavers) are trapped and used primarily for their pelt (Langan, 1991). Beavers are being reintroduced to areas where they have been made extinct to improve wetland ecosystems.
    Castor canadensis (beavers) can swim up to 8 km an hour. They secrete waterproofiing oil from glands at the base of their tail.
    Geographical range
    Native range: North America from northern Mexico to northern Canada.
    Known introduced range: Finland, France, Poland, Russia, Argentine and Chilean Tierra del Fuego.
    Introduction pathways to new locations
    Landscape/fauna "improvement": Castor canadensis (beaver) was introduced to southern South American during an Argentine government program to establish furbearers in Tierra del Fuego.
    Natural dispersal: Castor canadensis (beavers) have colonized adjacent islands dispersing themselves only by their own means.
    Other: Castor canadensis (beaver) was introduced to Finland as part of a programme to reintroduce the European beaver (C. fiber). (Nummi, 2006). They were introduced to Poland and farmed (Nummi, 2006)

    Local dispersal methods
    Escape from confinement: Castor canadensis (beavers) have escaped from fur farms etc.
    Management information
    Most Castor canadensis (beaver) management is through various forms of trapping for pelts. Demand for pelts has decreased so now there is little incentive for trappers to hunt beavers. Beaver colonies have been moved to other areas but in most cases other beavers move into the area and replace the beavers that were removed. Similar problems occur with trapping – removing the resident population simply allows other beavers to replace them. Dams in Canada have been blown up but it is a costly process and frequently new dams are created in the same place. Jensen et al (2001) suggest installing oversized culverts as a way of discouraging beaver plugging activity. McKinstry and Anderson (1998) state that Hancock and Bailey traps are typically used for live trapping beavers, but are bulky and expensive, and suggest steel cable snares as an alternative.
    Castor canadensis (beavers) are "choosy generalist" herbivores. They eat leaves, twigs and bark of most species of woody plants growing near water and also herbaceous plants, particularly aquatics. Whilst they have a wide ranging diet they show a large preference for certain plant species such as aspen (Populus spp.) and willow (Sailx spp.). Roots and rhizomes of water lilies are a particularly important source of winter food (Jenkins and Busher, 1979).
    Castor canadensis are monogamous. They usually become sexually mature during their second winter at the age of 1.5 years, although it can be delayed until 2.5 years or later (Nummi, 2006). Beavers mate once a year during winter. Gestation lasts about 105 days and the sole litter is born in spring. Litter size is usually between three and four, but can vary from one to nine (Jenkins and Busher 1979; Hill 1982 in Nummi, 2006). Kits weigh about 500g at birth.
    Lifecycle stages
    The offspring are born fully furred and eyes wide open. They can swim within 24 hours and after several days they are also able to dive out of the lodge without any accompaniment. They leave the dam at two years of age (Anderson, 2002).
    Principal sources: Christopher Anderson and Brett Maley, Institute of Ecology, University of Georgia, Athens GA 30605 and Omora Foundation, Puerto Williams, XII Region, Chile.
    Nummi, P. 2006. NOBANIS – Invasive Alien Species Fact Sheet – Castor canadensis.
    Jenkins, S.H. and Busher, P.E. 1979. Castor canadensis. Mammalian Species
    Jenkins and Busher, 1979 and Nummi, 2006
    Compiled by: Viki Aldridge, University of Washington, Tacoma, Supervised by Deborah Rudnick and IUCN/SSC Invasive Species Specialist Group (ISSG)
    Last Modified: Sunday, 13 December 2009

ISSG Landcare Research NBII IUCN University of Auckland