We recently attended the Bats and Artificial Lighting Symposium 2019 which was developed by the Bat Conservation Trust with the Institute of Lighting Professionals, and co-sponsored by Arup and Kingfisher Lighting. The event bought together experts to share their knowledge and experience in terms of artificial lighting and its impact on bats. The following is a very brief version of what was discussed.
As nocturnal animals’ bats have evolved for a life in darkness, partly as a means of avoiding predation by birds of prey during daylight hours. Therefore, artificial lighting can be extremely problematic to bats, in some cases leading to mortality or starvation. Light disturbance may cause delays or prevention of bats being able to emerge from their roosts, leading to reduced foraging opportunities which may have a severe impact on the survival of adult bats and their young. In more severe cases it may cause bats to abandon their roosts or become entombed.
The commuting and foraging routes of bats are also affected. Slower flying species tend to avoid lighting, even though their prey is attracted to the surrounding lit areas. This causes a so called ‘vacuum effect’ that causes darker areas, which are preferred by slower flying bat species, to become poor quality foraging sites. Although some faster flying species will opportunistically feed on the denser prey that accumulate under certain types of lighting, they are left in danger of predation. Peregrine falcons for instance have been recorded successfully hunting bats in our illuminated cities.
For such reasons it is important to consider bats when planning to use artificial lighting and possible mitigation measures within a development.
The need for lighting should be questioned, as well as its placement, the type of lighting and the length of time it will be used for. Lighting is commonly used to help prevent crime. However, studies on Gravesend and other locations have shown that whilst a lack of light may increase vehicle break ins, no light significantly reduced house break ins. This indicates that lighting may not provide effective prevention. A very simple method of altering lighting for bats on future and current developments would be to switch off lighting where possible at around 10-11pm, being selective about where lighting is placed.
The colour of lighting may also have an effect. Philips Lighting, now known as Signify, have been running tests for six years and have found that lights using red tones have fewer impacts on bats and have successfully provided a new development in Holland with lighting in this way. Due to red light connotations this does require residents to fully understand the reasons behind this colour of lighting for it to be popular!
Baseline light surveys, good lighting plans and modelling schemes in the vertical plane are fundamental to assessing the impacts of a development on bats. The Bat Conservation Trust provide some useful guidance about bats and lighting on their website, as well as information for those planning work that might affect bats. If you need an ecological consultant e-mail us at firstname.lastname@example.org.