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.

BBC Wildlife Magazine - - Contents - Re­port by Alex Morss

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.

Mi­gra­tory bats

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.”

Ex­plor­ing al­ter­na­tives

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.

Joined-up think­ing

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; alex­


Learn more about Bri­tain’s bats – and how you can watch, sur­vey and help them – at

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.

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