Bats and Artificial Light

Flight at night with artificial light – friend or fiend?

By Ashley Dale

Artificial lighting
Artificial Lighting

Artificial light at night has been beneficial to the human population by extending our days over the winter months, however, it has negatively impacted our wildlife. Aesthetic illuminations have been on the increase across Europe, especially on old buildings such as churches, monasteries and towers; buildings perfect for many species of bat. How has this been impacting our bat species and is there anything that we can do?

Bats and light

Bats and Lighting
Bats and Lighting

Bats are nocturnal and most of the UK species of bats are averse to any form of lighting. However, some species of bat that are far more opportunistic than others. Common and soprano pipistrelle bats are less affected by light and are amongst the first bat species to emerge at night (often straight after sunset) and are usually the last ones to return to their roosts in the morning. However, even these relatively light tolerant species can be affected by too much artificial light at night.

Positive impacts

One positive impact is the attraction of insects around artificial lights, depending on the make of the bulb. With the insects in one place, it may make it easier for a bat to forage, compared to insects dispersed within foliage. The colour of the light and its closeness to surrounding foliage and trees can benefit some bat species. This means that they have suitable places to hide if there is a predator nearby. If the light is of a low intensity then it will not affect the bats as negatively as the brighter, whiter LED lights do.

Negative impacts

Predation risk is far higher for bats foraging within well-lit areas. This danger can disrupt their normal activity, preventing them from foraging or mating. Streetlamps along roads may also be bat death traps by causing bats to collide with vehicles, increasing mortality rates. However, more studies need to be conducted to study this impact.

Moreover, lights on old buildings can prevent bats returning to old roosts or stop them from roosting altogether despite suitable habitats nearby. This could cause them to roost in more dangerous or less suitable buildings. The overall numbers of churches with bat roosts within them has decreased by a shocking 38% in 2016, compared to the 1980s. All of the churches surveyed had aesthetic floodlights installed on them. This suggests that the main reason for the rapid desertion of the bat colonies are these illuminations.

Bat roosts often have multiple exits in churches and other buildings to confuse predators. If there is excessive lighting over a building, this will limit the areas that bats will be able to exit from, enabling predators to predict where their prey may emerge.

Illuminated Church
Illuminated Chirch at Night

Research has indicated that the reproduction rates of insects has dramatically reduced due to continuous contact with artificial light, resulting in declining insect populations, inevitably impacting the bat population. From studying the foraging habits of bats situated near areas of high light exposure, it was recorded that feeding buzzes were reduced by more than 60%, despite the increase in the presence insects.

Lighting installed on waterways, canals, streams and rivers have some of the most adverse impact of a group of bat species, called the Myotis species: Daubenton’s bat (M. daubentoniid), pond bat (M. dasycneme)and long-fingered bat (M. capaccinii). As these are often important flyways and feeding sites, this could be having highly negative consequences.

Mitigation

Illuminated Bridge
Illuminated Bridge

The most obvious way of making sure light doesn’t affect bats when completing a development is not installing artificial light; however, developers must consider human health and safety. Areas allocated for dark corridors are an easy way to mitigate for this; areas such as parks and gardens should be used for this purpose, as they offer bats feeding opportunities. A network of dark corridors can allow the bats to commute between their roosts and feeding areas, preventing the fragmentation of their habitat due to highly exposed areas of land. Where foliage is scarce, particularly in urban areas, it is important to have a ‘light exclusion network’ in order to allow bats to get to the nearest suitable feeding area. The dark corridors should provide protective vegetation cover and a closed canopy. This will enable bats to be protected from skyglow and to follow the path of the corridor to their feeding site. Examples of light mitigation that Acer Ecology has been involved with can be found here.

Trees and Hedgerows
Bat Foraging and Commuting Routes

Paving material for roads and trails can be constructed so that they reflect moonlight, reducing the need for street lighting.

Motion sensor-based lighting could also be a suitable alternative, or using a minimal number of lighting points or using luminaires on low positions (e.g. under eaves height, hooded to prevent upward light spill or light spill into adjacent habitat, etc.) in order to prevent light shining onto suitable bat habitats.

Creating screens such as hedgerows, trees or walls so that the illuminated roads don’t shine onto the surrounding habitat can also reduce the impacts on bats. Roosts near illuminated roads should be given suitable protection to protect the circadian rhythm of the bats roosting inside. If artificial lighting is not required for safety reasons, then it is important that this is either not present or kept to the bare minimum for the sake of any nocturnal wildlife.

Light Spectrum
Light Spectrum

Depending on the species, the light spectrum can affect bats in different ways, including emergence behaviour. A study on the behaviour of bats when exposed to different light spectra discovered that red light had the least impact on the number of bats emerging from their roost. Two roosts of soprano pipistrelle bats were also studied and emergence numbers dropped dramatically when the roost exits were exposed with blue and white lights. Red light has been suggested to surveyors to use when doing roost checks due to their minimal impact and have been used for a variety of wildlife in captivity so as to keep them warm but not disrupt their sleeping habits. Reducing the blue part of the spectrum and heightening the red part of the spectrum of a light source has been found to be a solution when mitigating for slow-flying Myotis species and long-eared bats during their foraging activities. A negative effect of this action was that it actually reduced the number of insects and in turn, the attention of opportunistic species such as pipistrelles. This also suggests that changing the light spectrum of artificial lights will not only attract certain species but also the tolerance of certain light spectrums may change according to the time of year.  

Compensation for the loss of roosts is very hard to enforce, whereas mitigation for current roosts or nearby roosts to a development is easier. This is because bats very rarely accept alternative roosts within their areas easily. Alternative dark roosts could be offered but due to this factor this would need strict monitoring of its success.

What can we do?

Be aware!!

Bats are everywhere, and often you don’t even notice them. Your gardens and homes are perfect places for bats to forage and roost. When you are considering developing your property, just take the time to consider that bats may increase biodiversity in your area, making your home a far nicer place to live!

Bat Survey
Brown Long-eared Bat

Reference: http://www.eurobats.org/publications/eurobats_publication_series