Bat surveys and development

Establishing the need for bat surveys when a development is proposed and, if bats are found to be present, ensuring their protection during development, constitute some of the most difficult ecology-related challenges faced by developers in the UK. The nocturnal nature of bats, the broad range of habitat requirements and their complex life cycle, combine to make this group unique among our protected species in terms of the challenges they pose both for those seeking to undertake development projects as well as for the ecological consultants and interest groups trying to ensure the protection of bat species.

All bats are strictly protected in the UK, both by national and EU law. It is crucial that those involved in the development process understand that this protection extends not only to the animals themselves, but also to any place used by bats to roost: namely a breeding place, resting place and place of shelter. Such roosting places vary but can include houses, outhouses, sheds, churches, barns and trees – the location being influenced by their lifecycle, time of year and the species of bat using the roost. The sex and reproductive condition of the bat will influence its roosting location as pregnant females will roost communally and will form maternity colonies in so-called ‘maternity roosts’ during the summer. Bats also need to enter torpor and hibernate and so over the winter months will require sites which are cool, stable and free from disturbance. Such sites include caves, mines, trees and cellars but also stone outbuildings such as ice houses.

It is clear then that different bats will require different habitats in which to roost throughout the year. Therefore, if bats are present on a site, or suspected of being present, they may need to be considered during development works regardless of what time of year the works are scheduled or whether it be buildings, trees or both which are due to be impacted by the works. This frequently leaves developers in need of expert advice about how they can ensure their projects proceed while at the same time having confidence that that they are not contravening environmental law which protects these nocturnal mammals.

A recent project carried out by Thomson Ecology highlighted how development works can proceed smoothly, and with consideration to environmental legislation – provided bats are considered from the outset. This particular project involved the construction of an additional car park adjacent to a busy rural train station. Following the Preliminary Ecological Appraisal (PEA), a number of trees were identified on site as having the potential to support roosting bats. The site was also surrounded by hedgerows which joined adjacent woodlands and so offered foraging and commuting habitat for bats. The construction plans required the removing or altering of a number of these trees in order to facilitate development works as well as the removal of a section of hedgerow. Bats rely heavily on established linear features to orientate themselves when foraging and commuting at night. Such features include hedgerows, woodland edges, riparian corridors, canals etc. and any interruption of these features, for example by bisecting an established hedgerow to put a road through, can lead to bats being separated from their feeding grounds as bats will use historical and long established commuting and foraging routes. Fragmentations to be avoided wherever possible as it can be detrimental to the favourable conservation status of the species.

For this development to proceed and be compliant with environmental legislation, there were two components which needed to be addressed: the severance of the hedgerow which is likely being used as a bat ‘flight-line’, and the removal of trees which may be used as roosts by bats.

The first step was to establish how the bats used the site. This was achieved by carrying out bat activity surveys, in this case through the use of static monitoring devices to “listen” for bats and also through walked-transects by ecologists. These surveys revealed at least five species of bat were using the site, including commuting along established hedgerows due to be removed.

Consultation between Thomson Ecology and the client allowed a solution to be found. This involved altering the location of a new access road which was due to be built (so as to avoid the need to sever the hedgerow) and also by creating a ‘living archway’ which facilitated a pedestrian walkway through the hedgerow while keeping it intact. This meant that bats could continue moving freely from one side of the site to the other, and so would not be cut off either from their roosts, feeding grounds or each other, thus ensuring the population would not suffer from being cut off from their roosting habitat requirements.

As for the trees on site due to removed or altered, the next step was to formally establish the potential of each tree to support roosting bats. Thomson Ecology carried out a Ground Level Tree Assessment (GLTA) to make this formal assessment and to determine what further survey, if any, would be required in each case. Four trees were identified as having potential to support roosting bats. The client was given recommendations from Thomson Ecology about how best to proceed. Standard practice would usually be to carry out dusk emergence and dawn return to roost surveys, or ‘dusk/dawn bat surveys’, of the trees between May and September. The client was keen to remove the trees before the summer in order to make way for the (newly redesigned) access road and it was agreed that a better option would be to investigate the trees at height via Mobile Elevated Work Platform or ‘MEWP’. An inspection via MEWP is not always favoured by developers; they may often see it as an unnecessary expense. The value, however, is that it allows close examination of all potential roosting features (PRF’s) within a tree and so can allow for far more certainty far sooner in the survey (and therefore development) process as to whether bats are/likely to be utilising PRF’s.

The MEWP assessment was carried out by bat-licenced staff from Thomson Ecology and allowed us to confirm initial findings as established during the GLTA, namely that the three trees due to be removed had ‘low’ potential to support roosting bats, while a fourth, a tree due to have its canopy lifted, had ‘moderate’ potential. Further, we were able to confirm that none of the limbs due to be removed on the fourth tree had any potential to support roosting bats.

The newly released third edition of the Bat Conservation Trust’s Bat Surveys for Professional Ecologists Good Practice Guidelines (Collins (ed.), 2016) states that trees with low potential to support roosting bats do not need dusk/dawn surveys and can instead be felled or managed under a Precautionary Method of Works (PMoW). As our MEWP inspection had allowed us to confirm the potential of trees to be removed and altered, arboricultural works were then carried out which involved the sectional felling of the three trees due to be removed and the crown lifting of the fourth. This was all carried out under supervision from a bat-licenced Thomson ecologist according to a PMoW produced by us.

We believe the above example highlights the linear and step-by-step assessment, survey and inspection process which, where appropriate and followed by clients, allows sites with bat potential to be addressed in plenty of time and for any removal or pruning works necessary to be carried out in a safe, considerate and timely matter. In this case, the trees could be removed months before they would have otherwise been if dusk/dawn surveys had been relied upon and all trees felled or worked upon were managed in a much safer manner with respect to the potential presence of bats.

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