Bats Evolved for a Darker Environment than 21st-Century Britain

Our suburban neighbourhood in south-east England has lots of bat-friendly habitats. There are trees, hedges, old buildings with accessible roof spaces, ponds, and waterways. Perfect for the 18 species of bat who live in England, Wales and Scotland. British bats are small, nocturnal, and eat insects. As the primary predators of night-flying insects they are important for a balanced ecosystem.

We see the bats at dusk. British bats are divided into two groups: slow and fast flyers. Our local bats are fast; blink and you’ll miss them. Dashing through the air, they could be mistaken for little birds. Recently we saw two chasing each other at high speed. It’s fairly rare to observe them interacting because they’re so quick.

Los Angeles basin night sky.

Los Angeles basin night sky.

Conditions in the suburb look great for bats but there is a serious problem. At night the sky glows because of artificial lighting. Only a few stars of the 4,000 we should see are actually visible. When the moon is full and bright it makes no difference here. Its beams are hidden by light pollution. At night our neighbourhood has much more light than the bats evolved to live with.

Light pollution is widespread in Britain. It’s estimated that 55% of the population cannot see the stars of the Milky Way. England is the worst affected country. Only 21.7% of its skies have pristine darkness. Wales does better, with 57% of its skies pristine. In Scotland the situation is much better at 77%. Light pollution is concentrated in towns, cities, and also on the road network. In southern England the M25 and M21 motorways are visibly lit up in satellite images of nighttime lighting.

Light is measured in a unit called “lux.” Bats here evolved to live in the levels of lux found at twilight and under the stars and the moon. They emerge at dusk in direct response to the dimming light. Some bats species fly in and out of the roost to check the light level (PDF) before properly emerging.

A cloudy, overcast day is 5,000 lux. At sunset it goes down to 10 lux and in twilight it is 1 lux.

Artificial lights can prevent bats from seeing this change. Good main road lighting is between 5 and 20 lux. Typical side road lighting is 5 lux and security lighting is 2 lux. In all cases brighter than twilight and in some cases brighter than sunset.

We can see how artificial light outshines the moon and the stars when we compare lux measurements. The light of a clear full moon, which seems so strong in dark, rural areas is only 0.25 to 1 lux. Typical starlight is 0.001 lux. (Source of lux measurements. PDF)

In our neighbourhood and elsewhere, the lights create a complicated landscape for bats. A natural night is a blanket of darkness and artificial lighting interrupts this with rivers and pools of illumination. Highly-lit roads cut across darker areas for miles, leaving bats with no choice but to cross or avoid. Even the darker streets are patched with light from the street lamps and from buildings with unscreened windows. A well-lit office is 500 times brighter than twilight.

In very highly-lit areas there is also an effect called sky glow, in which the light reflects off particles in the air, brightening the whole sky. When I look up at the sky at night it often appears purplish blue or whitish brown because of the light bouncing off the clouds.

Barbestelle Bat

Barbestelle Bat

Light pollution disadvantages bats in numerous ways

Slow-flying bats tend to avoid light. In Britain, slow-flying bats include long-eared bats, Myotis species, barbastelle, and greater and lesser horseshoe bats. Light intolerance means slower bats face disadvantages (PDF) when trying to access to food and territory. The ultraviolet (UV) in many artificial lights draws insects away from darker areas and out of the slower bats’ reach. Fast flying, light tolerant bats, such as pipistrelles, gain the advantage because they can successfully hunt around lights.

The lights may give some fast-flying bats an evolutionary advantage over others within their own species. In Britain the common pipistrelle frequently hunts around street lights. Research shows (PDF) that in Italy its southern cousin, Kuhl's pipistrelle, may have physically adapted to artificial lights. Since 1945 their skull sizes have increased, perhaps to better enable them to eat the larger insects found under the lights. These larger insects include moths, which are also affected by light pollution. Research indicates that moths may lose the ability to hear bats’ echolocation signals under mercury vapour and LED lights.

Hunting under artificial lights is not all good news for fast-flying bats, as it makes them prey for cats and for large birds such as hawks and seagulls. Flying low near roads also increases the risk of collisions with vehicles.

Bats follow commuting paths through their habitat as they forage for insects. Studies show that while some bats avoid light completely, others have varying reactions to light depending on the time of year, and the presence of trees and insect-rich pasture or buildings. Where light interferes with the bats’ commuting paths (PDF) it can restrict their available foraging territory or force them to use more energy by flying a farther for less food.

Artificial light near bat roosts causes significant problems because it delays the bats’ evening exit, causing them to miss the period of intense insect activity at dusk. One study of Geoffroy’s bat and the lesser mouse eared bat found that pups in illuminated maternity roosts (PDF) had shorter forearms, lower body mass and delayed birth dates when compared to pups in non-illuminated maternity roosts. Maternity roosts attract bats from a wide surrounding area (PDF) year after year. Light pollution or indeed any other damage to a roost can significantly disrupt a local bat population. Bats only produce one pup a year and prefer to use the same roosts throughout their lives.

In the worst-case scenario artificial lights near roosts cause bats to stay entombed inside and eventually starve to death.

Bechstein’s Bat

Bechstein’s Bat

British laws and government policies protect bats from some light pollution

In Scotland, England, and Wales it is a criminal offence to interfere with a bat or a bat roost, with punishments including fines and imprionment. Bats are protected by multiple laws, including the Wildlife and Countryside Act of 1981 and The Conservation of Habitats and Species Regulations of 2010.

The 2010 rules state it is an offence to “Disturb a bat deliberately (disturbance includes any disturbance which is likely to impair a bat’s ability to: survive, to breed or reproduce, or to rear or nurture their young), hibernate or migrate (in the case of animals of a hibernating or migratory species).” It is also an offence to “Affect significantly the local distribution or abundance of the species to which they belong.”

In guidance issued on its website, the government reminds the public that placing an artificial light outside or inside a roost harms bats.

Developers seeking planning permission for construction projects are responsible for ensuring that work does not harm bats.

An ecological survey conducted before the conversion of a hotel into a care home resulted in the following recommendations for developers:

  • cease work and consult a licensed bat ecologist if a bat is found

  • carry out demolition work when bats are not hibernating or summer roosting

  • direct artificial light away from trees and scrub.

In southwestern England a local government issued bat-friendly lighting guidelines (PDF) for developments along a river and a canal, both of which are known bat commuting routes. Recommendations included:

  • prevent artificial light from spilling onto the river, the banks and the water’s edge

  • plant along the banks to encourage foraging bats

  • zoning changes to allow unlimited lux in the development zone and decreasing levels in the three zones closest to the water (3 lux, 0.5 lux, 0.1 lux).

Bats and other nocturnal creatures receive further help under planning regulations concerning light pollution. Paragraph 180 of the National Planning Policy Framework for England (PDF) states:

“Planning policies and decisions should also ensure that new development is appropriate for its location taking into account the likely effects (including cumulative effects) of pollution on health, living conditions and the natural environment…”

Paragraph 180 states that new developments should:

“limit the impact of light pollution from artificial light on local amenity, intrinsically dark landscapes and nature conservation.”

Natterer’s Bat

Natterer’s Bat

Laws have helped bats but observation suggests more legislation is required

Bats received legal protection in Britain because of the major drop in their numbers in the 20th century. The decline coincided with the mass adoption of outdoor and indoor electric lighting in the 20th century.

These updated laws and the hard work of conservationists have combined to stabilise the population compared to the figures known for 1999. Only two of Britain’s 18 species have near threatened status on IUCN’s red list.

Although bat protection policies help mitigate the effects of artificial light, the population’s stabilisation might be due to a decline in the physical destruction of roosts. Each year the National Bat Helpline receives 14,000 queries “from building and planning professionals, householders with bat roost questions, and members of the public who have found injured and grounded bats. As a result, thousands of bats and their roosts are conserved.”

Light pollution still exists at a high level so the risk to bats remains. In 2016 the Campaign for Rural England urged the authorities to take action. It called on the national government to ensure developers were following planning guidelines, on local governments to protect dark areas and to prevent developers from increasing light pollution, on highway authorities to tackle light pollution on roads, and on businesses and large facilities to dim or switch off lights.

Our knowledge of bat behaviour tells us that the present high levels of light pollution must still be disturbing bats.

Current laws and policies protect bats in a wide range of situations including human discovery of roosts and new construction projects. What the law doesn’t do is treat all of Britain as a potential bat habitat. If we accept that bats could choose to roost, commute, forage, or mate in almost any area, the current light pollution problem means that bats are not completely protected, despite it being a criminal offence to hurt them.

Is it time for an Earth Law that regulates all existing and future artificial lighting to avoid damage to bats, regardless of whether we have knowledge of their presence? This law would acknowledge that this island belongs to the bats, just as much as it belongs to us humans.

What would an Earth Law to prevent light pollution from disturbing bats look like?

Complete natural darkness would be ideal for bats, but would not work for humans. We need light for safety. Earth Law practitioners seek to balance the needs of the entire Earth community, which includes people. An Earth Law would have to find a compromise that is acceptable to humans and bats.

Some compromises are common sense. We should wave goodbye to decorative architectural lighting schemes outside buildings. We should avoid directing lights where we do not have a functional use for them. We should also encourage people to use thicker curtains and screens to stop light leaving buildings at night.

Scientific study of bats shows other compromises are possible. We could switch to red outdoor lighting. A pioneering study in the Netherlands found that red street lights do not disturb light avoidant or light tolerant bats. The study said:

“Our findings show that bat activity in red light, which has less light of short wavelength and more light of long-wavelength, most resembles dark. This holds up for both light-shy species and more agile non-light shy species. Therefore, this finding opens the possibility for the mitigation of adverse consequences of artificial lighting for bats in situations where natural habitat has to be exposed to illumination.”

(A problem with red light at night is that it disturbs some light sensitive trees. This would have to be considered in an Earth Law that balances the needs of the whole community.)

Another study found that dimming lights, although not as effective as switching off, lessens the impact on bats. This compromise could be introduced in areas where “just enough” lighting is needed.

An Earth Law would affect everyone. Householders would need to screen windows and not place unnecessary lights in gardens. Stores would have to avoid bright lights in windows. Universities, airports and hospitals would need to consider hundreds of light fittings, windows and outdoor areas.

A new law, introduced too quickly, would incur a lot of resistance. The best solution would be to set a date for compulsory compliance several years into the future. This would give everyone the chance to prepare for the change.

In the meantime, everyone can help by voluntarily cutting down on light polluting activities and by spreading the word about why it matters. The good thing about light pollution is that it goes away when we switch off the lights.

Here’s more you can do: