11 Jan Do solar farms affect foraging & commuting bats?
A recent study published by a team led by the University of Bristol (Tinsley et al, 2023[1]) claims to show a strong negative effect of solar farm developments on foraging and commuting bats. Our recent experience is that this has led to nature conservation stakeholders requesting additional survey to support solar planning applications.
This viewpoint article sets out our thoughts on the the extent to which the study changes the landscape in terms of baseline bat activity survey for solar farms, and the circumstances in which a change in survey methods or effort might be considered.
Background and methods of the study
The authors aimed to test the hypothesis that bat foraging and commuting activity and species richness are lower in solar farms than in similar habitats in the wider landscape.
The study is a comparison of bat activity levels and species richness between 19 operational solar farms and 19 control sites in agricultural landscapes in south-west England, using simultaneous deployments of static bat detectors to assess activity. It is spatially paired (i.e. each solar site had a control site in the same landscape) and also assesses how landscape location (open field vs boundary) affects bat activity and how these two factors interact (the statistical models include an ”interaction” to test, for instance, whether a solar versus control difference in bat activity occurs uniquely at boundaries or in the middle of fields)
Static detectors were deployed for one seven-night period per site (simultaneously in control and treatment plots) in either 2019 or 2020, with microphones being placed at a standard height and distance from boundary features. The activity levels (number of recorded calls or “passes” per hour by calling bats) for individual bat species were compared using statistical modelling, to determine whether there was a difference between solar and control sites, between centre-field and boundary locations and to test for any interaction between the two (i.e. are differences between bat activity in solar versus non-solar fields stronger in the middle of fields, or at the boundaries, or equally strong in both locations?). The authors also ran the same models for overall species richness.
What does the study show?
Over 50,000 call sequences were recorded. Of the eight remaining species / groups with enough records for statistical modelling, six had significantly lower activity levels in the presence of solar arrays (common pipistrelle, soprano pipistrelle, Plecotus, Myotis sp., Nyctalus sp. and serotine). There were interactions between the solar/control and the boundary/open field effects for four species/species groups:
- Activity of Myotis and serotine was only reduced at boundaries.
- Activity of Plecotus and soprano pipistrelle was only reduced in field centres.
- Activity of Nyctalus and common pipistrelle was reduced at both landscape locations (i.e. no interaction in the model).
Mean activity for these species at control sites vs solar was up to 7.3 x higher (common pipistrelle in open habitats). Activity of barbastelle and greater horseshoe did not differ between solar and control sites. There was also no significant difference in bat species richness between solar sites and control sites.
Potential caveats
At face value the study suggests that ecological assessments might underestimate strong adverse impacts of solar farms on bats, and that a more detailed approach to assessment and mitigation is always required. However, there are several caveats which suggest the picture may be more complex.
Many details of the study sites’ management history and the design of the studied solar farms are not clear and may vary between and within sites. Compared with a typical season of data collection for a development project, seven sampling nights per site additionally represents a short snapshot of bat activity, which often varies widely through a season. These factors are generally likely to cause random noise not systematic bias, and the fact that trends were observed despite them supports the authors’ conclusions. However, several factors do potentially limit the applicability and/or validity of the conclusions:
- The study aimed to pair agricultural management between control and solar sites, but only 10 of 19 paired sites featured the same management (all of the paired sites were grazed grassland). Differences in habitat value of different types of agricultural land management are likely to be important for bats.. The generalised conclusions drawn by the authors may well be valid in the grazed-grazed comparisons but the effects of solar conversion on bats in other management regimes are much less clear. Matched pairing is not necessarily informative: consultants often encounter the situation where arable fields are converted to grazed grassland with solar arrays, but only two site pairings represented this scenario – too few to draw any conclusions.
- The history of the control and solar plots used in the study is unclear. As mentioned above, many existing grassland solar plots were formerly arable land. If the study’s solar plots were recently converted to grassland, the grazed-grazed pairs in the study may involve control and solar grassland habitats of very different ages. If new grassland takes time to develop a rich invertebrate community, this may translate into different value for bats. It is also unclear whether any biodiversity mitigation features (such as rough grassland, ponds or scrub planting) were present in the solar sites, and if so, how established these were. Without clarification on whether or not the age and management of habitats was controlled for, it is hard to determine how much weight to attach to the conclusions.
- The standardised height of the detector microphones (1.27 m) is about the mid-height of a typical solar panel. We might expect solar arrays to reflect and scatter sound (including bat calls) so a microphone at this height surrounded by panels should detect fewer calls than one in an open spaces even if bat activity is similar. It might also fail to pick up bat calls in ‘acoustic blind spots’ below the height of the panels (on field edges or along tracks through the arrays[2]). The observed effect could at least partly (or even fully) be an artefact of this effect and without further clarity on the robustness of the results to microphone placement, it is hard to determine how much weight to attach to the conclusion.
To understand more fully the effects of solar farms on bats, we really need temporal studies following the same sites from baseline farmland to established solar site (rather than comparing separate control and solar sites of unknown histories); as well as mechanistic studies to determine if and how bat behaviour is actually affected by the presence of solar infrastructure.
What are the implications for solar developments?
This paper is likely to be invoked in discussions with nature conservation stakeholders regarding bat survey effort on many future solar developments. We believe that there are a number of situations where applying its conclusions to justify increased survey effort are unlikely to be proportionate or justifiable, but conversely, it indicates that adverse effects on bats could be more common than previously recognised and that a circumspect, site-by-site approach is sensible.
Some factors which may indicate the need for survey and mitigation for bats on solar projects include:
- Sites where the baseline habitat is long-established pasture under low-intensity management.
- The presence of potential for significant roost sites, designated sites for bats or high-quality foraging habitat in the landscape.
- Projects proposing battery storage or other significant noise-and light-producing infrastructure.
Early consultation with planning authority ecologists, together with thorough desk studies and habitat surveys as part of Preliminary Ecological Appraisal are likely to be essential.
Some typical mitigation features for bat-sensitive landscape design on solar farms include:
- Landscape buffers around important foraging and commuting features such as hedgerows, woodland and waterbodies.
- Incorporating habitat enhancements within retained areas, designed to enhance commuting connectivity and provide new foraging habitat.
- Sensitive siting of infrastructure such as inverters, maintenance compounds and battery storage systems away from valuable bat foraging habitat and/or known roosts
If you have any questions about BSG Ecology can help you with a solar energy project, please contact us.
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[1] Tinsley, E.; Froideveaux, J.S.P; ZsebőK, S.; Szabadi, K.L.; and Jones, G. (2023). Renewable energies and biodiversity: Impact of ground- mounted solar photovoltaic sites on bat activity. Journal of Applied Ecology 60, 1752–1762.
[2] The distance to which bat detectors pick up echolocation calls in field situations is unknown.