Beavers are semi-aquatic rodents native to North America and Europe. They are the only living members of the family Castoridae, which contains a single genus, Castor. Genetic research has shown the European and North American beaver populations to be distinct species and that hybridization is unlikely.
Beavers are best known for their natural trait of building
dams in rivers and streams, and building their homes (known as
beaver lodges) in the eventual pond. They are the second-largest
rodent in the world (after the capybara).
Beavers continue to grow throughout life. Adult specimens weighing over 25 kg (55 lb) are not uncommon. Females are as large as or larger than males of the same age, which is uncommon among mammals.
The European Beaver (Castor fiber) was hunted almost to extinction in Europe, both for fur and for castoreum, a secretion of its scent gland believed to have medicinal properties. However, the beaver is now being re-introduced throughout Europe. Several thousand live on the Elbe, the Rhone and in parts of Scandinavia. A thriving community lives in north east Poland, and the European Beaver also returned to the Morava River banks in Slovakia and the Czech Republic. They have been reintroduced in Bavaria, The Netherlands and Serbia (Zasavica bog) and are spreading to new locations.
The beaver finally 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 first hand. In October 2005, six European beavers were re-introduced to Britain in Lower Mill Estate in Gloucestershire, and there are plans for re-introductions in Scotland and Wales.
The North American Giant beaver (Castoroides ohioensis) was one of largest rodents that ever evolved. It disappeared, with other large mammals in the Holocene extinction event, which began about 13,000 years ago.
The habitat of the beaver is the riparian zone inclusive of stream bed. The habit of the beaver for hundreds of thousands of years in the Northern Hemisphere has been to keep these watery systems healthy and in good repair, although to a human observer, seeing all of the downed trees, it might sometimes seem that the beavers are doing just the opposite. The beaver works as a keystone species in an ecosystem by creating wetlands that are used by many other species. Next to humans, no other extant animal does more to shape its landscape. Introduced to an area without its natural predators, as in Tierra del Fuego, beavers have flooded thousands of acres of land and are considered an unstoppable plague. One notable difference in Tierra del Fuego from most of North America is that the trees found in Tierra del Fuego do not coppice as do willows, poplars, aspens, and other North American trees. Thus the "damage" by the beavers seems more severe.
Beaver dams are created both as a protection against predators, such as coyotes, wolves and bears, and to provide easy access to food during winter. Destroying a beaver dam without removing the beavers takes a lot of effort, especially if the dam is downstream of an active lodge. Beavers can rebuild such primary dams overnight, though they may not defend secondary dams as vigorously.
Beavers have been known to build very large dams. The largest known was discovered near Three Forks, Montana, and was 2,140 feet long, 14 feet high, and 23 feet thick at the base. When objectionable beaver flooding occurs, modern water level control devices can be installed for a cost-effective and environmentally sound solution. Unwanted damage to trees can be prevented by wrapping chicken wire or sheet metal around the base of trees.
It is primarily the sound of water in motion that stimulates the beavers to build. However, studies involving beaver habitual activities have indicated that beavers may respond to an array of stimuli, not just the sound of running water. In two experiments Wilson (1971) and Richard (1967, 1980) demonstrate that, although beavers will pile material close to a loudspeaker emitting sounds of water running, they only do so after a considerable period of time. Additionally the beavers, when faced with a pipe allowing water to pass through their dam, eventually stopped the flow of water by plugging the pipe with mud and sticks. The beavers were observed to do this even when the pipe extended several meters upstream and near the bottom of the stream and thus produced no sound of running water. Beavers normally repair damage to the dam and build it higher as long as the sound continues. However, in times of high water, they often allow spillways in the dam to flow freely.
Beaver dams can be disruptive; the flooding can cause extensive property damage, and when the flooding occurs next to a railroad roadbed, it can cause derailments by washing-out under the tracks, or when a beaver dam bursts and the resulting flash flood overwhelms a culvert. This disruption is not limited to human geography; beavers can destroy nesting habitat for endangered species, and often destroy mature trees for which they have no use.
On the other hand, dam building is extremely beneficial in restoring wetlands, the land's most beneficial ecosystem. Such wetland benefits include flood control downstream, biodiversity (by providing habitat for many rare as well as common species), and water cleansing, both by the breakdown of toxins such as pesticides and the retention of silt by beaver dams. Over the eons, this collection of silt produces the rich bottom land so sought after by farmers. Beaver dams also reduce erosion as well as decrease the turbidity that is a limiting factor for much aquatic life. While beavers can create damage, part of the problem is one of perception and time scale. Such damage as the undermining of a roadway or the drowning of some trees is very visible shortly after the beginning of beavers' activity in an area. The benefits, mentioned below, are long-term and not easily seen except by someone who is monitoring a catchments and realizes the beneficial effects of beaver dams.
A beaver dam has a certain amount of freeboard above the water
level. When heavy rains occur, the pond fills up and the dam
gradually releases the extra stored water. Often this is all
that is necessary to reduce the height of the flood wave moving
down the river, and will reduce or eliminate damage to human
structures. Flood control is achieved in other ways as well. The
surface of any stream intersects the surrounding water table. By
raising the stream level, the gradient of the surface of the
water table above the beaver dam is reduced, and water near the
beaver dam flows more slowly into the stream. This helps in
reducing flood waves, and increases water flow when there is no
rain. Beaver dams also smooth out water flow by increasing the
area wetted by the stream. This allows more water to seep into
the ground where its flow is slowed. This water eventually finds
its way back to the stream. Rivers with beaver dams in their
head waters have lower high water and higher low water levels.
If a beaver pond becomes too shallow due to the settling of sediment, or if the tree supply is depleted, beavers will abandon the site. Eventually the dam will be breached and the water will drain out. The rich thick layer of silt, branches, and dead leaves behind the old dam is the ideal habitat for wetland species. Many of them will have been on the fringes of the pond. Wetlands have significant environmental benefits.
As the wetland fills and dries out, pasture species colonize it and it becomes a meadow suitable for grazing. In an area with nothing but forest down to the stream edge, this provides a valuable niche for many animals which otherwise would be excluded.
Finally the meadow will be colonized by riverine trees, typically aspens, willows and such species which are favored by the beaver. Beavers are then likely to re-colonize the area, and the cycle begins again.
As related above, each time this process repeats itself another layer of rich organic soil is added to the bottom of the valley. The valley slowly fills and the flat area at the bottom gets wider. Research is sparse on this topic, but it seems likely that much of the fabled bottom land in North America was created, or at least added to, by the efforts of the generations of beavers that lived there.
The removal of nutrients from the stream flow by beaver ponds is an interesting and very valuable process. Farming along the banks of rivers often increases the loads of phosphates, nitrates and other nutrients, causing problems downstream when this water is extracted for drinking. Besides silt, the beaver dam collects twigs and branches from the beavers' activity and leaves, notably in the fall. The main component of this material is cellulose, a polymer of β-glucose monomers (This creates a more crystalline structure than is found in starch, which is composed of α-glucose monomers. Cellulose is a type of polysaccharide.) Many bacteria produce cellulose which can split off the glucose and use it for energy. Just as algae get their energy from sunlight, these bacteria get their energy from cellulose, and they form the base of a very similar food chain. However, a source of energy is not enough for growth. These bacterial populations face serious shortages of nitrous and phosphorous compounds, and will absorb these nutrients as they pass by in the water stream. In this way, these and other nutrients are fixed into the beaver pond and the surrounding ecology, and are removed from the stream.
Agriculture also introduces herbicides and pesticides into our streams. Bacteria are an extremely variable lot and some of these toxicants are metabolized and decomposed by the bacteria in the cellulose-rich bottom of a beaver dam.
Some scientists believe that the nitrate cascade, the production of far more fixed nitrogen than the natural cycles can turn back into nitrogen gas, may be as much of a problem to our ecology as carbon dioxide production. It is likely, but not proven, that beaver dams along a stream may contribute to denitrification (the removal of nitrogen). In sewage plants, denitrification is achieved by passing the water through successive aerobic and anaerobic stages. Under a beaver dam, as the water seeps down into the soil, the oxygen is used up by the fauna in the rich organic layer. At some point all the oxygen is used up and the soil becomes anaerobic. This water eventually finds its way into the stream and into another beaver dam. This aerobic, anaerobic cycle continues all the way down the stream and denitrification is a likely result.
The ponds created by well-maintained dams help isolate the beavers' home, their lodge, which is also created from severed branches and mud. The lodge has underwater entrances to make entry nearly impossible for any other animal (however, muskrats have been seen living inside beaver lodges with the beavers who made it). A very small amount of the lodge is actually used as a living area. Contrary to popular belief, beavers actually dig out their den with an underwater entrance after they finish building the dam and lodge structure. There are typically two dens within the lodge, one for drying off after exiting the water, and another, drier one where the family actually lives.
When startled or frightened, a swimming beaver will rapidly dive while forcefully slapping the water with its broad tail. This creates a loud 'slap', audible over large distances above and below water. This noise serves as a warning to other beavers in the area. Once a beaver has made this danger signal, all nearby beavers will dive and may not reemerge for some time.
This Beaver Page is Copyright The Animal Web Guide © 2004 - 2009 Chuck Ayoub