Bats and light pollution
Light pollution is a growing problem. Now new evidence suggests that there may not be any such thing as truly ‘ bat-friendly’ lighting.
Conservationists question whether there can be any such thing as truly ‘bat-friendly’ lighting
Plastic and air pollution have been making headlines, but there’s mounting concern about another insidious pollutant – light. Globally, light pollution is growing on average by six per cent each year, with a rising proportion coming from LEDs. Now, a new study suggests we should reconsider how we illuminate the night if we are to protect bats and other sensitive creatures active after dark.
But just how bad is light pollution for nocturnal animals? The Bat Conservation Trust (BCT) has warned there are still big gaps in current knowledge and, following new research, is recommending more care is taken over how we use artificial lighting.
Bats are among the world’s most sensitive nocturnal mammals and have for several decades found themselves increasingly in the spotlight due to light pollution. A world map of artificial ‘sky glow’ revealed in 2016 that 80 per cent of the world’s people, including 99 per cent of the US and European populations, live under light-polluted skies.
A report last year in the journal Global Change Biology warned that the impacts of sky glow on wildlife and ecosystems were poorly understood. It was already known that white, blue and green light were bad for most bats – but it had also been widely assumed that a switch to red light was a better solution, because many bats were, supposedly, not significantly affected by it. Now it appears that’s not the case.
New research published in the journal Ecology and Evolution reports a major issue which has been largely overlooked – bat migration – and raises concerns about the wider impacts of red lighting, too.
The authors found that some migratory British and mainland European bat species changed their behaviour due to red light, and were adversely affected by green and white light. Their warning suggests that red lighting may have serious unexplored
consequences, too. Red safety lighting is used on aircraft, ships, tall buildings and shorelines, and also on wind turbines, where it may worsen the problem of thousands of bats and birds dying each year due to collisions. A study published by The Mammal Society in 2016 suggested that wind turbines are among the largest causes of mass bat deaths around the world.
The drive to switch to ‘green’ and ‘eco-friendly’ forms of renewable energy is of course hugely important, but not necessarily ‘bat-friendly’ in itself. As Dr Voigt, lead author of the new research, points out: “The replacement of conventional lighting with energysaving LEDs is a worldwide trend, yet consequences for animals and ecosystems are poorly understood.”
Voigt’s team tested the responses of migrating bats to red, white and green light wavelengths. They found different light colours triggered changes in behaviour, especially with the soprano pipistrelle and Nathusius’ pipistrelle. The light did not prompt increased hunting by bats, suggesting the attraction was not down to opportunistic feeding on insects drawn to the light, but something else, perhaps navigational.
“Migratory bats may be more susceptible to light sources of specific wavelength spectra, because vision may play a more dominant role than echolocation during migration,” Voigt suggests. “Our findings call for caution in the application of red aviation-safety lighting, particularly on wind turbines, as this light colour might attract bats, leading to an increased collision risk.”
Finding a better solution is complex. Dr Voigt’s team has proposed exploring alternative lighting such as infrared, as is used by pilots. “Birds are also attracted to red light, and less so to green light,” he says, “therefore green lighting has been used as a measure to avoid bird collisions on and offshore, such as on oil-drilling platforms. It looks like the effect of green light, however, is also quite strong on bats compared to birds. We lack comprehensive studies. It is dangerous to say any lighting is batfriendly. Bats are not neutral to it.”
This news will trouble conservationists who are already worried about the lethal attraction of wind turbines to bats and birds. They die after becoming confused into colliding with the blades, or following barotrauma – lung damage caused by sudden air-pressure changes near turbines. The animals’ natural migration routes often involve navigating across large offshore wind farms which have been lit up by red light to alert aircraft and shipping pilots.
It has been difficult to determine precisely how many bats are affected, with many dead animals thought to fall at sea, while bodies are also hard to find on land. One innovative study led by Fiona Mathews, Chair of The Mammal Society, used sniffer dogs in a bid to calculate how many bats were killed by wind turbines. Previously, the BCT had reported that at some sites in Europe and the USA fatalities are already high
enough to spark “serious concern for the conservation of the species concerned”.
Bats face worsening light pollution in urban areas, too. In a 2014 report for the BCT, Emma Stone of Bristol University advised that lighting reduces the quantity, quality and connectivity of habitats bats use for foraging and roosting, including river corridors, woodland edges and hedgerows, creating barriers and delaying the emergence of light-shy species who miss peak feeding opportunities at dusk when insect prey is most abundant. Stone estimates that almost a quarter of all bat species are threatened worldwide, with the biggest threats to Europe’s bat populations being disturbance, urbanisation (including lighting), habitat change, wind turbines and loss of roosts.
In recent years, warm-spectrum white LED lighting has been recommended by ecologists, architects and town planners, instead of the blue-rich or UV lighting that can be particularly harmful to bats. Zuidhoek-Nieuwkoop, a town in Holland, is trialling the replacement of white street lights with red ones. The Netherlands Institute of Ecology (NIOO-KNAW) has also tested the impact of these lights at eight dark locations in forest edge habitat.
The BCT says there’s still much to learn about light and bat behaviour. In September, the BCT and Institute of Lighting Professionals published ‘Bats and Artificial Lighting in the UK’, offering 25 pages of guidance for lighting professionals, designers, planning officers, developers, bat ecologists and anyone making decisions about lighting.
Jo Ferguson, Built Environment Officer at the BCT, says the new guidance has a strong focus on addressing LEDs. “This is an important first step in raising awareness of the problems with artificial lighting and how to avoid or reduce them.” She points out that although people often see bats hunting around street lights, this behaviour is limited to a few of Britain’s fastest flying species. “Bats risk predation by feeding this way,” she warns.
“Artificial lighting can interfere with where and how bats roost, commute and feed, which is why the Bat Conservation Trust prefers the term ‘reduced-impact’ rather than ‘bat-friendly’ lighting.” Bat ecologists also want higher standards of sympathetic design and planning, as well as tougher enforcement, says Lisa Kerslake, England Vice President of the Chartered Institute of Ecology and Environmental Management. “There are concerns and gaps at all levels,” Kerslake cautions. “Even where better lighting is specified and conditioned, this is not always enforced post-development. Many schemes go ahead without any constraint on lighting whatsoever.”
“Our consultants and local authority ecologist members encounter frequent problems in relation to lighting and its impact on bats. I know of a scheme where lesser horseshoe bats – a particularly light-sensitive species – were having to fly across a road to a roost site under street lights that were not installed to the ecologist’s specifications. This was rectified, but in many cases it simply wouldn’t be noticed.”
Many councils currently favour variable lighting designs, which are dimmed or turned off at critical times for bats, or triggered to light up temporarily by passing pedestrians, or are opting for less bat-reactive colour wavelengths. Such choices rely on rigorous monitoring by ecologists before and after installation. Light barriers can also be used, such as planting trees to reduce light ‘spill’. Creating and maintaining dark corridors through towns and cities is critical to support some light-shy species, such as lesser and greater horseshoe bats, grey and brown long-eared bats and some Myotis bats, some of which are now rare in the British Isles.
ALEX MORSS is an ecologist, bat surveyor and science writer; alexmorss.co.uk.
Learn more about Britain’s bats – and how you can watch, survey and help them – at bats.org.uk.
Right: a Dutch town is trialling the replacement of white street lights with red ones. Above: The Netherlands Institute of Ecology is testing red street lights to see if they better protect light-shy bats, such as the Natterer’s.
Globally, light pollution is growing on average by six per cent each year. Sky glow: around 80 per cent of the world’s people live under lightpolluted skies.
Fatal attraction: migratory bats are among the species at highest collision risk from wind turbines.