4. Assessment of Environmental Effects
4.1 Overview of beaver ecology
This section sets the context for the assessment of the impacts of the policy on the other SEA environmental receptors. It is based on the findings in the Beavers in Scotland Report 2015, ( BiS), provided in Annex 1 of this Environmental Report ( ER) for further reference if required.
4.1.1 Beaver ecology
126.96.36.199 Beaver colonies and territories
Beavers are semi-aquatic rodents. Beavers form lifetime pairs, with a pair defending a strict territory against unrelated intruders. Beaver colonies are made of family groups, typically consisting of an adult pair, and a number of kits (young under one year of age) and sub- adults. The size of territories is often measured by the length of water bank utilised and is quite variable . Territories are rarely permanent. Beavers are strict herbivores, and their preferred food sources slowly deplete over time. Therefore beavers may leave a territory for a number of years, and will not recolonise the area until enough suitable food has regenerated.
188.8.131.52 Feeding and habitat
Beavers are strict herbivores and feed on a wide variety of plant species, including aquatic and terrestrial herbaceous and woody vegetation (see sections 4.2 and 4.4). Smaller stems, less than 0.1 m in diameter, are often preferred. However, larger stems (up to 0.2 m) may still be commonly utilised, and the use of trees of more than 1 m diameter has been recorded.
Beavers are semi-aquatic and are reliant on water to escape from any potential predators. Because of this they feed only in close proximity to watercourses.
184.108.40.206 Beaver structures
Beavers live in lodges and/or burrows. Lodges are often highly visible structures made from cut branches, logs and mud. Burrows are often inconspicuous with underwater entrances. The two may be combined in a bank lodge, which is a burrow with further reinforcement and insulation provided above with a structure of logs and branches.
Beaver dams are built from a variety of logs, branches, grass, mud and stones. The majority are less than 1.5 m in height, ranging from 0.2 m in height and 0.3 m in length, up to 3 m in height and more than 100 m in length, although the latter are exceptional cases. They are built to retain water, create feeding areas, provide safe refuge (and keep the lodge entrance under water) and facilitate travel and movement of logs and branches. Dams may have a range of effects on the surrounding environment and nature of the watercourse (see section 4.4).
Owing to either siltation or dam failure, beaver ponds are often temporary. After a beaver pond has returned to a terrestrial state, a beaver meadow may be created, which can persist for many decades. However, a pond may also develop into other states such as emergent wetland, bogs or forested wetland, which may remain stable for centuries.
4.1.2 Distribution of suitable beaver habitat in Scotland
It is useful to predict where potential habitat exists for beavers in Scotland, and to use this to estimate potential future beaver distribution. Work has therefore been done, using Geographic Information System ( GIS) tools, to provide this information. This will help to identify where beavers may have effects on particular ecological and socio-economic factors
Beavers may utilise particular habitats, in particular riparian, broadleaf woodland, which provides a key source of food and materials for building structures (see section 4.2). GIS tools were used to create datasets of suitable beaver woodland across Scotland. The datasets were then used in a variety of overlapping analyses, described in later sections of the BiS report ( Annex 1), to predict where beavers may potentially interact with certain species or land use issues.
Potential beaver woodland can be identified by the following characteristics, described in detail in Annex 1 section 3.2
- Broadleaf woodland and shrub - the main predictor of the presence or absence of beavers is the availability of food, in particular the abundance of suitable woodland
- Within 50 m of freshwater edge - beavers prefer to feed in close proximity to water.
- Streams with less than 15% gradient - higher gradient streams are known to be sub-optimal habitat for beavers.
- Not in tidal sections - beavers are only rarely seen in salt/tidal water and do not establish territories in such habitats .
Using these parameters, a dataset of 'potential beaver woodland' was created, which identified all woodland that could potentially be used by beavers in Scotland. This resulted in the identification of 120,390 ha of potential woodland on the mainland.
220.127.116.11 Potential core beaver woodland
The 'potential beaver woodland' dataset was further refined. Beavers require a certain area of suitable woodland to set up a territory. The potential beaver woodland dataset contains all woodland that could be utilised by beavers, but many of these are small, isolated patches. The minimum amount of woodland needed for a beaver to establish a long-term territory was estimated based on the literature. The potential core beaver woodland map consists of 57,309 polygons, covering 105,586 ha of suitable woodland. It is anticipated that beavers would be more likely to set up long-term territories in proximity to these areas of potential core beaver woodland. Section 3.2 provides further detail and map 4 which illustrates core beaver woodland within the SEA beaver policy areas. The Knapdale beaver policy area comprises 64,978 ha in size, with Tayside comprising 1,140,075 ha. Of this, the potential core woodland in the policy areas extends to 970 hectares (ha) in Knapdale (less than 1.5% of the total beaver policy area) and 14,717 ha in Tayside (less than 1.3%).
18.104.22.168 Catchment mapping
A previous mapping exercise identified four catchments as key woodland areas for beavers: Lomond, Tay, Spey and Ness. Analysis showed that the catchments with the most core beaver woodland were the Tay and Spey. Analysing which catchments have the most core woodland is useful, but is biased by the size of the catchment. For the purpose of this report, the River Tay falls within the SEA boundary, with only a small part of the upper section of the Spey. The River Tay and its riparian woodland comprises some 47% of the total potential core beaver woodland found within the Tayside SEA boundary.
22.214.171.124 Areas where dam-building is less likely
It would be useful to predict where beavers may build dams in Scotland, assuming any reintroduction. However, key ecological measures which might help predict dam sites (e.g. stream depth) are not currently available in national geospatial datasets. Therefore, it was decided that a reliable dataset could not be produced at the present time, and, instead, a dataset was created to predict where beavers are unlikely to dam. Areas not identified by this dataset contain watercourses where the potential for dam-building is unknown.
Building dams is a high-cost activity for beavers. For this exercise it was assumed that beavers would justify the investment in building and maintaining a dam only where resources exist to sustain a beaver territory. Hence, watercourses not adjacent to potential core beaver woodland were identified as being less likely dam sites.
Beavers cannot build dams where the flow rate of a stream is too great. The larger a watercourse, the more likely a dam will get washed away during flooding. This is why the great majority of beaver dams are found on smaller watercourses less than 6 m in width. Hence, all watercourses greater than 6 m in width were also identified as being unlikely dam sites.
Using these parameters, it was estimated that a minimum of 87% of watercourse length on mainland Scotland is less likely to be a dam site for beavers.