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The Biodiversity and Management of Aspen Woodlands
Beavers: Aspen heaven or hell?

The Biodiversity and Management of Aspen woodlands: Proceedings of a one-day conference held in Kingussie, Scotland, on 25th May 2001

Dave Batty
SNH Casework Support Officer, Kilmory Industrial Estate, Kilmory, Lochgilphead, Argyll, PA31 8RR. Email:

The paper will provide a background to the ecology of European beavers, discuss their potential impact on woodland in general and on Aspen in Scotland in particular.

European beaver

The European beaver, Castor fiber, is Europe's largest rodent (average weight c 25kg), and is a separate species from the North American beaver Castor canadensis. It has a semi-aquatic lifestyle, and inhabits freshwater, either slow-running rivers or lochs. They generally use water as their main means of travel and are only occasionally found more than c60m from water.

Beavers are social animals and live in family groups consisting of the two parent animals plus the young of the year and sometimes the young of the previous year. The number of animals in a group varies, but an average of 3.8 has been recorded. The group occupy a territory around their lodge, which they defend against other groups. They also use a larger home range, which they might share with other neighbouring groups.

Beavers normally have their young in May-June, and most litters are either two or three young. The young stay with the parents until they are sexually mature and ready to disperse from the home territory, usually at around two years old just before the new young of the year are born. However, the dispersing young may return to the home territory if they fail to find a suitable area elsewhere.

European beavers tend to live in natural holes in banks or excavate burrows with an underwater entrance. Where the banks are not high enough, the Beavers may construct bank lodges consisting of a burrow covered by piles of wood. The lodge contains a vestibule and, typically, one nesting chamber above water level.

Beavers build dams from a variety of materials including wood, mud and sometimes stones. The main reason for dam building is to raise the water level and keep the entrance to their burrow or lodge below the water surface. This reduces the chances of predation and danger to the young. Beavers will also excavate canals to facilitate the transport of material. This can increase their potential feeding range.

Beavers are herbivores, feeding entirely on plant material. They eat an extremely wide range of herbaceous and woody plants, with at least 149 and 80 species recorded respectively. During the late spring and summer they mainly eat herbaceous plants, especially aquatic ones, and take a wide range of grasses, forbs, ferns, shrubs and leaves. The bark and leaves of trees and shrubs forms only a small part of their diet.

Where the preferred herbaceous plants are not available, Beavers will utilise more woody species. This change is most obviously seen in the autumn and winter when woody plants form the vast majority of the diet. Beavers will store branches underwater in the autumn for use in the winter when other food is not available. Trees and shrubs are used for their foliage and their bark, especially in the winter. There is a marked preference for hardwoods, especially Aspen, Birch, Willow, Rowan, Oak, Alder and Ash.

Beavers tend to focus their activities around their burrow or lodge and this influences their foraging behaviour. They travel by water and generally feed up to 100m from the water's edge but most is carried out within 20-60m. Most of their preferred trees and shrubs are harvested near the water's edge. However, observations from Norway indicate that Beavers will travel up to 200m or more to use Aspen, and it is considered that any accessible Aspen within 500m may be at risk of some exploitation. In addition, any Aspen within c 30m of the water would be vulnerable to heavy exploitation. It should be noted that Beavers can only use stands of Aspen that are accessible to them and that they are not as athletic as sheep or deer!

Most trees and shrubs felled are less than 10cm in basal diameter, in Finland the mean being c3cm and in Norway the majority less than 5 cm. The result is that the vast majority of the hardwoods will then coppice naturally, providing other browsing species are not present, to provide another potential 'crop' for the Beavers to harvest in the future. Experience from re-introductions to Brittany and Poland is that the Beavers coppice the woodland rather than clear fell. Although the majority of trees used are small, Beavers are capable of felling trees of greater diameter, up to 1m. Where Beavers have been re-introduced, there has been a range of young regeneration and old mature trees, so the potential situation of just mature Aspen trees has not occurred. Therefore, given Beavers' predilection for Aspen, it would be advisable to have a range of trees of different ages and sizes in an area.

Re-introduction proposal

The European beaver was once widespread across Europe, including Scotland and the rest of Britain, and northern Asia to Siberia. It probably occurred in Scotland until the early 16th century when it was hunted to extinction for its fur. It suffered a similar decline in Europe until by the 19th century there were only a few populations remaining in Norway, Germany and France. However, through legal protection, translocation and re-introduction it has now been successfully returned to much of its former range.

Scottish Natural Heritage (SNH) is now considering the re-introduction of European beaver to Scotland because:

  • It is listed on the EC 'Habitats Directive';

  • It is recognised as being a 'keystone species' in the ecology of woodland and freshwater systems;

  • It would benefit biodiversity; and,

  • Since humans were responsible for its extinction in Scotland, it has been argued that there is a moral responsibility to redress the loss.

Some years ago, SNH began to examine seriously the possibility of returning European beaver back to Scotland. SNH commissioned work on the suitability of habitats in Scotland to support Beaver and also on the effects of Beavers on hydrology, fish and woodland habitats in Europe. A national consultation was carried out in 1998 to ascertain the views of everyone who might have an interest in the subject. Based on that, SNH decided to go ahead, in principle, with a trial re-introduction which would be in a limited area for a specific period of time. Interest from the Forestry Commission led to an assessment of their holdings in Scotland and Knapdale Forest in Argyll was identified as the most suitable area for a trial. A fully planned trial is proposed with monitoring to determine, amongst other things:

  • How the Beavers behave in Scotland;

  • Impact on woodland habitat;

  • Impact on other wildlife interests; and,

  • Impact on water quality.

The trial would last for five years, but if insurmountable problems were encountered during that period the trial would be curtailed prematurely. If the trial proceeds, it is intended that Beavers will be captured in autumn 2002, spend six months in quarantine and then be released in spring 2003. The trial would end in 2008. After the five years, there will be an assessment to help provide information for an informed decision on whether a wider-scale Beaver re-introduction should take place. At this stage there will be wide consultation.

The trial will be taken forward by SNH in partnership with Forest Enterprise, Scottish Wildlife Trust and Argyll and Bute Council.

SNH will make a final decision over the trial in autumn 2002. However, approval will then be needed from the First Minister, as Beavers are not currently part of the native UK fauna and as such a licence is required under the Wildlife and Countryside Act 1981 to release them into the wild.

Potential impact on Aspen

It has already been noted above that Aspen is a favoured food for Beaver and that they will preferentially select it. This could put more pressure on an already restricted resource, both in terms of quantity and quality. However, at present Aspen is under pressure from grazing animals; both domestic (sheep and cattle), and wild (primarily deer, rabbits and hares). A key question is how to reduce or eliminate any extra threat from Beavers?

Direct methods would include the identification of areas of Aspen at high risk from Beavers due to their proximity to water and their accessibility to Beavers. These areas could be easily fenced against Beavers, and, if of high value, might need to be fenced against other animals as well. Relatively low stock-proof fencing should be adequate, avoiding potential significant problems of fence collisions for 'woodland grouse'. Given that Beavers burrow for other purposes, it would probably be sensible to have an apron of netting on the ground to deter burrowing, especially in soft ground.

If there are concentrations of Aspen along particular river systems, an alternative method would be not to re-introduce Beavers to that catchment. Beavers usually travel via water and experience from the continent is that it can take some time for Beavers to move from one catchment to another one.

However, perhaps the best method for dealing with a potential threat to Aspen from a Beaver re-introduction is to increase the quantity and quality of the Aspen resource, be it areas of Aspen woodland or Aspen stands in other woodland types. In this way the aim would be to manage existing Aspen and create new Aspen areas such that if/when Beavers arrived there would be sufficient habitat for both the Beaver and Aspen. The proposed trial re-introduction of European beaver to Scotland could be used as the publicity to raise the profile of Aspen with the public, land owners and managers, and also the bodies which could provide the funds for its management and expansion. European beaver could be used as an Aspen flagship species to heighten awareness.

If the trial went ahead, it would be 2008/2009 before any decision could be made about any wider re-introduction of European beaver to other parts of Scotland. This provides considerable time to begin the active management and expansion of the Aspen resource in Scotland, and to look at ways of improved funding for this work through, for example, Woodland Grant Scheme and agri-environment schemes. If this work took place, and the decision was made not to proceed with a wider re-introduction, then the net result would still be both a raising of awareness of Aspen and a considerable expansion in the Aspen resource.

The return of European beavers to Scotland might be hell for an individual Aspen, but potentially heaven for the Aspen resource as a whole. It is for people with an interest in Aspen to decide how they want to view Beavers, either as a problem or, more sensibly, as an Aspen opportunity.

Further reading

Collen P. 1997. Review of the potential impacts of re-introducing Eurasian beaver Castor fiber L. on the ecology and movement of native fishes, and the likely implications for current angling practices in Scotland. Scottish Natural Heritage Review 86.

Conroy J. and Kitchener A. 1996. The Eurasian beaver (Castor fiber) in Scotland: a review of the literature and historical evidence. SNH Review 49.

Gurnell A. 1997. Analysis of the effects of beaver dam-building activities on local hydrology. SNH Review 85.

Kitchener A. and Lynch J.M. 2000. A morphometric comparison of the skulls of fossil British and extant European beavers, Castor fiber. SNH Review 127.

Macdonald D., Maitland P., Rao S., Rushton S., Strachan R. and Tattersall, F. 1997. Development of a protocol for identifying beaver release sites. SNH Research, Survey and Monitoring Report 93.

Reynolds P. 2000. European beaver and woodland habitats: a review. SNH Review 126.

Scott Porter Research & Marketing Ltd. 1998. Re-introduction of European Beaver to Scotland: results of a public consultation. Scottish Natural Heritage Research, Survey and Monitoring Report 121.

Webb A., French D.D. and Flitsch, A.C.C. 1997. Identification and assessment of possible beaver sites in Scotland. Scottish Natural Heritage Research, Survey and Monitoring Report 94.

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