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Should beavers be damned?

By Peter Collen

If Europe's largest rodent is re-introduced to Scotland will it prove to be an asset or a liability? Scientist, Peter Collen weighs up the evidence

The beaver is Europe's largest rodent. On a world scale they are exceeded in size only by the South American capybara.
The beaver is Europe's largest rodent. On a world scale they are exceeded in size only by the South American capybara.
Although most Eurasian beaver dams are reported to be small, they do possess the skills to create larger structures.
Although most Eurasian beaver dams are reported to be small, they do possess the skills to create larger structures.

The reintroduction of once native animals is an important conservation management technique. It is not a new idea in Scotland with perhaps one of the earliest and best known examples being the reintroduction of capercaillie on Lord Breadalbane's estate at Taymouth Castle during the 1830's.

The osprey, by migration, and the sea eagle by Man's intervention are already back in Scotland. In both cases their successful return was due not to some dramatic improvement in the environment, but to a change in Man's attitude to the creatures themselves. However, it is perhaps worth noting that even though the sea eagle numbers are still very low there have already been questions asked about their effects at fish farms. For species like the wolf and lynx, again there already exists suitable habitat for them in Scotland, but wolves, lynx and brown bear would need an enormous public relations exercise to make them acceptable. This might not be so for the beaver which is strictly vegetarian.

Scottish Natural Heritage (SNH) selected the beaver from a list of animals which, according to the Habitats Directive, the British government should be considering as potential reintroduction candidates. In March last year [1998], SNH launched a consultation process on whether beavers should be reintroduced to Scotland. The outcome of this consultation was published recently and while the reintroduction proposal received much public support, the strongest opposition came from the fishing and angling sectors of the community.

Although exploitation for its fur was the main reason for the disappearance of beaver in Scotland, the change in attitude to the fur trade has largely eliminated this threat. Loss of habitat was probably not an important factor in the elimination of beaver from Scotland. However, during its absence, there have been some dramatic changes in the quantity of suitable habitat. Beavers rarely travel more than a hundred metres from water to feed and they use a wide range of plants from algae right up to large trees.

Normally their distribution is linked to sites with a good supply of riparian deciduous trees, especially species such as aspen, willow, and birch. While sites do exist in Scotland where beavers could be released immediately, the well-recorded loss of native deciduous woodlands would probably require the creation of additional beaver habitat to enable a reintroduction programme to sustain itself.

During periods of vegetative growth, beavers feed extensively on water plants, a wide range of herbaceous vegetation, and the leaves and bark from twigs of woody plants. They may however be dependent on trees, especially the bark, during the winter period. Beavers can store branches in a discrete winter food cache and the use of this food supply will be determined by the severity of the winter climate. Although aspen is the preferred species, beavers will make use of most tree species, even some conifers, depending upon availability and population density. Trees of 3-8cm diameter are most often used, but there are exceptional records of trees exceeding 1m diameter being felled by Eurasian beavers.

The beaver is Europe's largest rodent with adults normally weighing between 15 and 35kg. They create their own living accommodation which can range from a simple chamber excavated into a river bank to an elaborate structure built of branches and mud, the classic beaver lodge. They are sociable and territorial animals which normally live in a family group, referred to as a colony. Generally, just before the parturition, the two-year-old animals leave the parental colony to establish their own territories.

An essential feature required by beavers is access to water which has sufficient depth to cover the entrance to their living accommodation and allows them to construct the winter food cache. Such conditions are often present in large streams and rivers of lochs but may be missing from smaller streams. The beaver can often remedy this, and alter the environment to suit its own needs, by constructing a dam. It is this industrious skill which often brings the beaver into direct conflict with Man, especially when it occurs in man-made landscapes.

Some of the consequences of their damming activities are without question damaging. For example, the blocking of culverts and subsequent flooding of roads would certainly rate them as a nuisance animal in the eyes of the motoring public. The response to other effects of their constructional abilities would vary depending upon the respondent's particular interests. The conservationist would welcome the increase in biodiversity found in beaver impounded areas. The beaver ponds provide excellent habitat for a wide range of animals including dragon flies, frogs waterfowl, and otters. They would also argue that by reducing silt loads, increasing the production of aquatic invertebrates, improving water quality, reducing the impact of hydrological extremes, and providing shelter the presence of beaver ponds is beneficial to fish. Beaver ponds can also provide angling opportunities in small streams where none previously existed and, despite possible problems of difficult access and numerous snags for fishing tackle, such angling is reported to be enjoyed by both game and coarse anglers. The angler and fishery manager however, would be impeded by dams and spawning areas could be flooded or silted up. Also a variety of piscivorous species, such as otter, mink, heron, merganser and pike, might benefit from beaver activity.

Often, in relation to fish, there is some confusion over the interpretation of beaver facts. It is frequently stated that salmon in North America benefit from beaver activity, but it should be made clear that this information relates to Pacific salmon rather than Atlantic salmon. LArge woody debris and pools, common features of beaver-modified streams, are particularly important for Pacific salmon, while the importance of such habitats for Atlantic salmon remains debatable. Although Atlantic salmon parr have been reported to yes beaver ponds in Canada, these salmon generally coexist with brook charr rather than our more aggressive brown trout. Another commonly encountered argument is that Atlantic salmon and beavers coexist in some of the major salmon rivers in Norway, and there are no reports of any deleterious effects of beaver activity on the salmon. However, the differences between salmon rivers in Scotland and Norway may lessen the importance of this observation to the Scottish situation. Generally in Norway, beavers and salmon occur together in the larger rivers where beavers have minimum impact on fish. It is only in the smaller damnable streams that beaver activity will be capable of influencing salmon movements. While such streams may have limited value for salmon in Norway, they can be particularly important in Scotland. Clearly these examples demonstrate the necessity of considering all the information carefully before making general statements.

Foresters might be grateful for the availability of potentially useful quantities of water for fire fighting and console themselves with the fact that, when the beaver can be selective, it will fell species of tree which presently have little economic value. However, in lowland areas the extent of tree loss through flooding may occasionally be a cause for concern. It is also important to be aware that the Eurasian beaver is well able, and often does, fell large trees. Incidentally, the conservationist might be interested to know that beavers are very selective for aspen and can be responsible for the local elimination of this species. Beavers might also be instrumental in destroying areas of riparian woodland in Scotland where grazing pressure from other animals is high. Normally, if beavers overexploit their food supply they move to another area which should then provide an opportunity for the original site to regenerate. Unfortunately, the subsequent grazing of these abandoned sites, by deer, sheep, cattle or rabbits, could potentially prevent future regeneration unless excessive grazing was controlled. Such loss of riparian woodland would be of particular concern to the many groups, including fishery organisations and various conservation bodies, who are actively trying to develop these woodland areas.

Members of the general public would undoubtedly be captivated by this fascinating animal. Although beavers are mainly active at night, it is relatively easy for quiet and patient observers to watch them. Also, the activities of beavers may produce a great deal of obvious signs which can be quite dramatic, and the diligence and skill involved are often admired. For example, large trees may occasionally be felled, complicated wetland areas can be created by their dams, and in low gradient site the beavers might even develop an elaborate system of canals to extend their feeding area. As a result of such activities, beavers could provide much scope for educational activities and may even have some potential as a tourist attraction.

Although the two species of beaver are practically identical in appearance and have many similarities, it has been argued that the Eurasian beaver will cause fewer problems than its North American cousin. This is based on reports that have demonstrated various differences between them. The American species apparently has larger litters, possibly matures earlier, and may indulge in greater building activity than the Eurasian beaver. While there may be some truth to this, there is no doubt that the Eurasian beaver can bulled large, durable dams and in some situations may exhibit yearling dispersal and early attainment of sexual maturity. As the population of Eurasian beavers increases so does our knowledge of its biology. Possibly some of the differences will not be as great as was first thought but, regardless of that, already reports are appearing of Eurasian beaver-related problems. Consequently, it is not constructive to argue that there will be no problems from beavers in Scotland. Rather, emphasis should be placed on how problems would be controlled and the benefits encouraged.

The extent of any harmful or beneficial effects is largely determined by the population density of the beavers and local site characteristics. For this reason it would be useful to know if the intention is to have a token beaver presence in restricted areas or whether the beaver is to once again become a fully integrated member of the Scottish fauna. If a reintroduction were successful in Scotland it might well take several decades before problems required serious action. There would be no excuse at that time for anyone to say that they had no idea of what the beaver's abilities were. Initially, problem animals could be live trapped and moved to less sensitive areas. Other management procedures might include the use of various deterrents to keep them away from certain areas, trees could be protected and if necessary dams could be removed or modified. The management of beaver dams would be of particular concern to the various fishery organisations who already practice the removal of natural obstructions in streams when they are considered to impede the free passage of exploitable fish. However, as adult beavers would have no major predator in Scotland, unless we also bring back the wolf, Man would eventually be responsible for controlling their populations directly. Fortunately, there are many tried and tested management techniques both for controlling beaver populations and for preventing or reducing beaver-related problems. The fact that, at one time, beavers were practically exterminated throughout Eurasia, testifies to the ease of regulating their populations should it become necessary.

In North America, the careful management of beavers is often necessary to maintain their beneficial effects. It is now generally recognised that the formulation of a suitable, ecologically sound management programme for beaver in Scotland is an essential prerequisite before any reintroduction is attempted. Anyone who doubts the importance of sound management has only to consider the present controversy which rages over the management of some of our existing wildlife such as red deer, seals and cormorants. Perhaps the beaver could act as a catalyst in persuading us to rethink some of attitudes to wildlife management in this country.

This article was published in Fly Fishing & Fly Tying on March 1999


Peter Collen is a biologist at the Pitlochry Fisheries Laboratory who has been investigating the potential effect of beavers on Scottish freshwater fish.

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