Overview:

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Artificial lighting is an integral part of modern society; street lights, for example are commonly found in towns and cities across the world and as a result have the potential for widespread effects on the environment, biodiversity and human health. Generally, bats show species-specific responses to artificial lighting: some feed on the insects that are attracted to street lights (light-opportunistic), whereas others avoid light (light-averse).

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The number of street lights is not only increasing, but their spectral signatures are also changing; currently there is a trend to use broad-spectrum lights, such as light-emitting diode (LED), metal halide (MH) and fluorescent (FL) lights. I used a before-after-control-impact paired design to examine the effects of the switch-over from low-pressure sodium (LPS) to LED street lights on bat activity across southern England. I found no significant differences in either bat activity or feeding behaviour around LPS and LED lights. These findings are important given that many existing street lights are being, or have been, switched to LED lights before their ecological consequences had been assessed. However, it is important to mention that most bats recorded were light-opportunistic species, where artificial lighting seems to have less effect than for slow-flying species.

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As a follow-up study, I explored how light intensity affected bat activity. In addition to the installation of LED lights, many local authorities are also implementing schemes such as part-night lighting and dimming. Dimming not only reduces the intensity of the street light, but also the amount of light distributed from the light source. Reducing the spread of light may be less detrimental to light-averse bats, by creating dark refuges and corridors which light-averse bats may use. Reducing the light intensity by 25% of its original output did not significantly affect the number of passes of light averse (Myotis spp.) or light-opportunistic species (Pipistrellus pipistrellus). This is a particularly encouraging result for light-averse species, many of which are often described as threatened. My results suggest that dimming may be an effective strategy in mitigating some of the ecological impacts of artificial lighting at night (ALAN), possibly realigning the balance between light-opportunistic and light-averse species, whilst also offering cost benefits and reducing carbon footprints.

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In addition to affecting local insect populations, the spectral emissions of street lights may also vary in relation to the spectral sensitivity of bats eyes, thereby having implications for street lighting guidelines. British bats have UV-transmissive lenses, so are sensitive to UV wavelengths that are emitted from FL and MH, but not LED, lights. The results from all three experiments, in addition to research in the current literature suggest that where street lighting is necessary, LED lights should be installed as they can be adapted to mitigate some of the negative ecological impacts of ALAN.