In Britain, all 17 species of bat are legally protected meaning that it is an offence to, amongst other things, disturb a bat in its roost or deliberately disturb a group of bats, and damage or destroy a bat roosting place. Therefore, approval of a development proposal lies partially upon the presence of bats. The local planning authority and licensing agencies require information on the effects of the development on bats, to identify which, if any, species are present, and to identify if mitigation is required. Bat mitigation strategies come in a variety of forms such as modified roosts, compensatory roosts and bat boxes. Whilst bat mitigation strategies are widespread there has been limited research on their effectiveness, until now.
A recent study has been carried out by the Universities of Exeter and Bristol, and the Chartered Institute of Ecology and Environmental Management (CIEEM) titled “Reviewing the evidence on mitigation strategies for bats in buildings: informing best-practice for policy makers and practitioners.”. The aims of this study were to determine the effectiveness of mitigation strategies for bats within buildings and to identify which characteristics of mitigation features are associated with the likelihood of successful retention of bat populations, post-development. As mentioned, the reasoning behind the study is due to the lack of research, and therefore understanding, around which mitigation strategies have been most successful and are most effective at protecting and preserving bat populations.
A variety of methods were used in this study including, but not limited to:
- Licence reports;
- Case studies of Myotis, common pipistrelles, soprano pipistrelles and brown long-eared bats as they are commonly encountered during developments and have UK and Ireland wide distribution;
- Comparison of roost presence and roost sizes pre- and post-mitigation to determine whether bat roosts are being retained and whether extent of disturbance or type of mitigation feature could predict success;
- Features of bat lofts, bat boxes and reroofing projects were identified that were linked with greater probability of bat occurrence and higher population post-development.
Types of Mitigation
The most successful type of mitigation, with regards to bats returning and retaining population size, was modified roosts. These are roofs that have been modified due to reroofing work yet remain in the same building. Modified roosts retained 67% of bats whereas newly created bat lofts attracted bats at 52% of sites. Additionally, the least effective mitigation strategy was the provision of bat boxes. However, only 31% of sites were successful at retaining bats.
The success of mitigation strategies for bats also depends on the species present and roosting at the development site. Brown long-eared bats were found to use new bat lofts most frequently, followed by common pipistrelles, then soprano pipistrelles. The probability of pipistrelles occupying a new bat loft was strongly dependent on the number of roost entrances provided. However, it is important to consider that there is probably an upper limit whereby additional roost entrances will create a draught and increase light exposure to the loft.
The overall conclusion is that evidence-based mitigation is key yet more evidence is required to establish the most effective mitigation strategies.
Where Do Acer Ecology Fit In?
Acer Ecology is home to a number of licensed bat specialists experienced in a range of survey types, European Protected Species licensing and mitigation design and implementation. Expert advice will be provided at all stages of your project where required. The great depth of knowledge and extensive experience of ecologists at Acer Ecology means that there have been many successful bat mitigation projects, a range of which can be found in other blog articles on this website. A great example is at St Tewdrics which can be found here.
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