Practical techniques: Dormouse
England and Wales
Common dormice (Muscardinus avellanarius) have undergone a rapid decline in range and numbers over the last 100 years. It is thought that dormice were previously widespread over most of England and Wales. Recent studies suggest that this small mammal is now extinct from at least 7 counties where it was known to be present a century ago. This represents a loss of over half the species range.
Protection and its implications
Dormice and their habitats are fully protected by the Habitats Regulations 2010 with some additional protection against disturbance from the Wildlife and Countryside Act 1981 (as amended). As a result of their full protection under the Habitats Regulations, mitigation on development sites for dormice is governed by a strict licensing procedure administered by Natural England and Natural Resources Wales. A licence will be required whenever disturbance of dormice or damage to their habitat is likely to occur.
The dormouse is listed under Sections 41 and 42 of the NERC Act 2006 (for England and Wales respectively). It is therefore a priority species in England and Wales. Government policy is that local planning authorities should consider dormice when determining planning applications.
In common with most protected species, the presence of dormice can lead to planning permission being refused unless you can clearly demonstrate that the species will be adequately protected during the development process, that disturbance is kept to a minimum, and adequate alternative habitat is provided to sustain at least the existing population. As dormice are a European protected species, it will also be necessary to demonstrate that the tests in the Habitats Regulations can be met at the planning application stage.
Dormice have a restricted range (mainly southern England) and are most commonly found in broadleaved woodland, a habitat which tends to be protected through planning policy and is not often the subject of a development proposal. However, dormice do occur in other habitats including hedgerows, dense scrub, and mixed or even coniferous woodland – habitats which are more frequently present on development sites. Dormice can also be a consideration on linear developments, such as new roads, as these can isolate populations, making them more vulnerable to local extinction.
Mitigation techniques
Protection of individual dormice
Because they will generally be confined to areas of woodland and hedgerows, it may be possible to protect areas holding dormice, and thus the dormice themselves, during the development process.
Where habitat known to support dormice will be lost to development, dormice will need to be removed in advance. If the area of dormouse habitat affected is small and there is sufficient adjoining habitat remaining or created, it may be possible to encourage the dormice to leave the area affected by development by cutting down trees and shrubs and making the habitat unsuitable for them. Cutting down the trees and shrubs to make the habitat unsuitable for dormice can be carried out from November to March, when the dormice are hibernating under the leaf litter on the ground, or in late September or early October when they are active but without dependent young. It will be necessary to wait until dormice have emerged from hibernation in May before ground works can commence. If the area of work is less than 50m2 then works might be permitted to be undertaken when dormice are active. The clearance of vegetation should progress towards areas of suitable habitat that will be retained for dormice after the development is complete.
If large areas of dormouse habitat must be cleared in one season, or there is insufficient adjoining habitat remaining or created, then capturing and removing the dormice is likely to be the only option. It may be possible to relocate the dormice to an unaffected part of the same woodland. However, moving the dormice to an alternative woodland site may be necessary. The translocation of dormice, whether within the same woodland or to an alternative site, is likely to be a very costly and time-consuming process. If the dormice are to be retained within the same woodland, then it is likely that enhancements to the habitat will be required because the dormice may be at a higher density than before. If the dormice are to be removed to an alternative site, it must provide at least 20ha of suitable habitat that is currently unoccupied by dormice.
Capturing dormice is restricted to about 6 weeks of the year when they are not hibernating and are without dependent young. If the dormice are not all removed in the first year, then development programmes may be delayed by up to 12 months or more. The release of dormice is best undertaken in early summer. It involves the use of release cages and needs to be carefully managed in order to have a chance of being successful.
Maintaining habitats for dormice
A Natural England or Natural Resources Wales licence for dormice is likely to require the creation and enhancement of dormouse habitat. Measures that can be undertaken to benefit dormice include:
- Creating new woodland and scrub containing plant species with a known benefit for dormice (such as hazel, oak or honeysuckle)
- Linking isolated areas of existing habitat with new planting (hedgerows or woodland) to provide uninterrupted habitat suitable for dormice
- Installing dormouse nest boxes
- Thinning tall trees in existing woodland to encourage growth of understorey and the production of more fruits, nuts and insects for food
- Reinstating traditional woodland management, particularly coppicing (cutting down to a stump and harvesting the wood) on a 20 year rotation
It would be several years before any new habitats were suitable for dormice and could receive captured animals.
An interesting (but expensive) measure used to mitigate the fragmentation of dormouse habitat through the creation of new roads and railways, is the construction of a habitat bridge. The bridge may have other functions (such as a cycleway) but its main purpose is to link areas of woodland on one side of the road with areas of woodland on the other. The bridge needs to carry sufficient soil to support a continuous, dense and wide strip of shrubs through which the dormice can cross from one side to the other.
Post-development monitoring
Developments proceeding under licence from Natural England and Natural Resources Wales normally include several years of post-project monitoring of dormouse populations and habitats. Dormouse nest boxes can be used for this purpose. as they are used by nesting dormice during the summer months as places of shelter. Monitoring should be undertaken by licensed ecologists who are permitted to disturb dormice. If nest boxes are used for monitoring, data collected should be submitted to the national dormouse monitoring scheme. Box checking may also be a method by which members of the local community may be encouraged to continue to care for the habitat on site once development and monitoring under licence is complete. Dormouse nest tubes can be used to detect presence or absence of dormice.
Timing of works
In areas where dormice are still present, works to above-ground vegetation can be undertaken during the winter months (November to March). Clearance of stumps and roots should be undertaken following the clearance of above-ground vegetation during the months of May to September.
Dormice can be captured for translocation purposes from May to October but released only in the early summer (no later than July).
Further reading
Highways Agency (2007) Design Manual for Roads and Bridges, Volume 10.
Bright, Morris and Mitchell-Jones (2006) The Dormouse Conservation Handbook 2nd Ed English Nature, Peterborough.
Natural England (2015) Hazel or common dormice: surveys and mitigation for development projects. https://www.gov.uk/guidance/hazel-or-common-dormice-surveys-and-mitigation-for-development-projects