The dark side of light: how artificial lighting is ... - Nature

Although many studies have documented how artificial light harms individual species, the impacts on whole ecosystems and the services they ... Skiptomaincontent Thankyouforvisitingnature.com.YouareusingabrowserversionwithlimitedsupportforCSS.Toobtain thebestexperience,werecommendyouuseamoreuptodatebrowser(orturnoffcompatibilitymodein InternetExplorer).Inthemeantime,toensurecontinuedsupport,wearedisplayingthesitewithoutstyles andJavaScript. Advertisement nature newsfeature article Inmini-ecosystemsintheNetherlands,researcherstesttheeffectsofartificiallight.Credit:KamielSpoelstra/NIOO-KNAW DownloadPDF It’sasummernightnearaforestlakeinGermanyandsomethingunnaturalisgoingon.Beyondthedarkwaterslappingattheshores,afaintglowemanatesfromringsoflighthoveringabovethesurface.Nearby,bobbingredtorchlights—theleast-disruptivepartofthevisiblespectrum—betraythepresenceofscientistsontheshoreline.Theyaretestingwhathappenswhentheyrobthelakecreaturesoftheirnight.ThisexperimentnearBerlinisthemostambitiousofseveralprojectsgoingonindarkpatchesofcountrysidearoundEurope,setupinthepastfewyearstoprobewhatlightpollutionisdoingtoecosystems.Researchersaregrowingincreasinglyconcernedabouttheproblem.Althoughmanystudieshavedocumentedhowartificiallightharmsindividualspecies,theimpactsonwholeecosystemsandtheservicestheyprovide,suchascroppollination,islessclear.Severalfieldstudieshopetoprovideanswers,bymonitoringhowplantandanimalcommunitiesrespondtobothdirectlightandthemorediffuseunnaturalluminanceofthenightsky,knownasskyglow.Ecologistsfacechallengessuchasmeasuringlightaccuratelyandassessinghowmultiplespeciesbehaveinresponse.Butearlyresultssuggestthatlightatnightisexertingpervasive,long-termstressonecosystems,fromcoaststofarmlandtourbanwaterways,manyofwhicharealreadysufferingfromother,morewell-knownformsofpollution.It’sanimportantblindspot,saysSteveLong,aplantbiologistattheUniversityofIllinoisatUrbana–ChampaignandeditorofthejournalGlobalChangeBiology.“WeknowagreatdealnowabouttheimpactsofrisingCO2,”hesays.“Buthowextensivearetheimpactsoflightpollution?We’regamblingwithourfutureinwhatwe’redoingtotheenvironment.”Inthe1950s,DutchphysiologistFransVerheijenbegantostudyhowlightsattractanimalsandinterferewiththeirbehaviour.Andduringthe1970s,morebiologicalobservationsoftheimpactsoflightstartedpoppingupintheliterature.Butittooktwolateral-thinkingbiogeographers—CatherineRich,presidentoftheUrbanWildlandsGroupinLosAngeles,California,andTravisLongcore,nowattheUniversityofSouthernCaliforniainLosAngeles—toseethelinksbetweenthemandorganizeaconferencein2002,followedbyabook,EcologicalConsequencesofArtificialNightLighting(Island,2006),pointingouthowfarthetendrilsoftheilluminatednightextend.Forthevastmajorityoforganisms—whetherhuman,cockroachorwispofplankton—thecycleoflightanddarkisaninfluentialregulatorofbehaviour.Itmediatescourtship,reproduction,migrationandmore.“Sincelifeevolved,Earthhaschangeddramatically,buttherehavealwaysbeenlightdaysanddarknights,”saysChristopherKyba,aphysicistattheGermanResearchCentreforGeosciencesinPotsdam.“Whenyouchangeit,youhavetheworrythatitcouldscrewupalotofthings”.Thepaceofthatchangeisincreasing.Strikingimagesfromspaceoverthepasttwodecadesrevealtheextenttowhichthenightisdisappearing.Estimatessuggestthatmorethanone-tenthoftheplanet’slandareaexperiencesartificiallightatnight1—andthatrisesto23%ifskyglowisincluded2.Theextentofartificiallylitoutdoorareasspread3by2%everyyearfrom2012to2016.Anunexpecteddriverofthetrendisthewidespreadinstallationoflightemittingdiodes(LEDs),whicharegrowinginpopularitybecausetheyaremoreenergyefficientthanotherbulbs.Theytendtoemitabroad-spectrumwhitelightthatincludesmostofthefrequenciesimportanttothenaturalworld.Thetrendhashadprofoundimpactsonsomespecies;lightsarewellknowntodisorientmigratingbirdsandseaturtles,forexample.Scientistshavealsofoundthatdisappearingdarknessdisturbsthebehaviourofcrickets,mothsandbats,andevenincreasesdiseasetransmissioninbirds.Themostlethaleffectsareperhapsoninsects—vitalfoodsourcesandpollinatorsinmanyecosystems.AnestimateoftheeffectsofstreetlampsinGermanysuggestedthatthelightcouldwipeoutmorethan60billioninsectsoverasinglesummer4.Someinsectsflystraightintolampsandsizzle;somecollapseaftercirclingthemforhours.Fewerstudieshaveexaminedplants,butthosethathavesuggestthatlightisdisruptingthem,too.InastudyintheUnitedKingdom5,scientiststooka13-yearrecordofthetimingofbudopeningintrees,andmatcheditupwithsatelliteimageryofnight-timelighting.Aftercontrollingforurbanheat,theyfoundthatartificiallightingwaslinkedwithtreesburstingtheirbudsmorethanaweekearlier—amagnitudesimilartothatpredictedfor2°Cofglobalwarming.Astudyofsoya-beanfarmsinIllinois6foundthatthelightfromadjacentroadsandpassingcarscouldbedelayingthematurationofcropsbyuptosevenweeks,aswellasreducingyield.EcosystemeffectsNow,theresultsofsomeambitiousexperimentsarecomingin.OneofthelargestisafieldexperimentintheNetherlands,whereeightlocationsinnaturereservesanddarkplaceshostseveralrowsofstreetlamps.Therowsaredifferentcolours—green,red,whiteandacontrolrowturnedoff—andrunfromagrasslandorheathfieldintoaforest7.Forsixyearsnow,scientistsandvolunteershaveusedcameratrapstomonitortheactivityofsmallmammals;automaticbatdetectorstorecordecholocationcalls;mistnetsfortrappingbirds;andnestboxestoassessthetimingandsuccessofbreeding.Botanistsarestudyingthevegetationunderneaththelamps. Amapofnight-skybrightnessoverEurope,whereblackispristineskyandredareasare5-10timesbrighter.Credit:Ref.2 Theteamhasfoundphysiologicalevidenceofthedetrimentaleffectsoflightpollutiononthehealthofwildanimals.Songbirdsroostingaroundthewhitelightwererestlessthroughthenight,sleptlessandhadmetabolicchangesthatcouldindicatepoorerhealth8.Theprojectalsolookedathowlightaffectsbats,whichhavehadmixedfortunesundertheexplosionofartificialillumination.Somespecies,suchasthecommonpipistrelle(Pipistrelluspipistrellus),feastonthebuffetofinsectsthattheyfindcirclinglamps.Other,light-shy,batshavelosthabitatandhavedisappearedfromsomeplaces.IntheNetherlandsstudy,redlighthadnoeffectonanyofthebatspecies9,whichmeansitcouldbedeployedinsteadofwhite.Buttheexperimenthasyieldedsomepuzzlingfindings.Severalurbanstudieshadfoundthatartificiallightatnighttriggerssongbirdstosingearlierintheday.Becausefemalestendtoselectearly-singingmales,theshifteddawnchorusmightbeaffectingwhichbirdsgettoreproduce.ButtheteamintheNetherlandsfoundnoeffectonanyof14songbirdspecies10.It’spossiblethatthelightingwastooweaktoelicitaneffect—itiscalibratedtoreflecttheleveloncountryroadsandcyclepaths,ratherthantheglareofanurbanpark.Bothkindsofresultareusefulforlocalgovernments,saysKamielSpoelstra,abiologistattheNetherlandsInstituteofEcology(NIOO-KNAW)inWageningen,wholeadstheproject.Histeam’sfindingsarebeingincorporatedintoDutchregulationsonoutdoorlighting.Forinstance,hesays,someareasseekingtosupportlocalbatpopulationshaveswitchedtoredlight,atrendthatheexpectstoincrease.ColouredlightalsogracesgrasslandsinsouthwestEngland,whereaprojectknownasEcolightislookingforevidenceof‘cascadeeffects’‚inwhichtheinfluencesoflightononespecieshaveknock-oneffectsontheecosystem.TheglowingcubesusedbyEcolightmightbemistakenforanartinstallation.ScientistsledbyKevinGaston,abiodiversityandconservationspecialistattheUniversityofExeter,UK,havejustfinishedresearching54artificialcommunitiesofgrassland.Insomeofthecubes,beetles,slugs,peaaphidsand18speciesofplantmuddledalongfor5years,isolatedfromtheoutsideworld.Otherboxesweresimpler—containingjustplantsandherbivores,orplantsalone.Atnight,somewereilluminatedwithwhitelight,somewithamber,andsomejustsawtherawsky.Theeffectsoflightongrasslandsareimportant,partlybecauseroadsidegrassprovidesrefugesandcorridorsforwildlifeinbuilt-upareas.Thescientistsdiscoveredthatamberlightand,toalesserextent,white,suppressedfloweringinthetrefoil(Lotuspedunculatus)11.Andtherewasacascadeeffectintheamber-litboxes.DuringAugust,whenpeaaphidsswitchfromeatingshootstofeastingonflowerheads,theirnumbersfell,presumablybecausetheirfoodwaslessabundant.“Ithinkthisisthefirstexperimentalevidenceofastrong,bottom-upeffectofexposuretoartificiallight,”saysGaston.Initslatest,unpublished,work,theteamrevealsfurthereffects,cascadingontothepredatorsinthesystems.Anotherelaborateexperiment,inadark-skiesreserveinWesthavellandNatureParkinGermany,hasshownthatthesecascadeeffectscanspilloverintoneighbouringecosystems.Streetlampserectednearwater-filledditcheslureaquaticinsectsoutofthewater12,saysFranzHölker,anecohydrologistattheLeibnizInstituteofFreshwaterEcologyandInlandFisheriesinBerlin.Theinsectsflocktothelamps,exhaustthemselvesandbecomefoodfornearbypredators.Meanwhile,thehinterland,whichmightotherwisehavereceivedinsectvisits,isdeprivedofanimportantsourceoffood,hesays.Studiessuchasthese,whichlaysuchrelationshipsbareinwell-controlled,small-scalestudies,meanthat“thoseimpactsaremorelikelytobetakenseriouslyinthefieldandbyregulatorsconsideringimpactsfromlighting”,saysLongcore.Artificiallightcanalsohaveimpactsonecosystemservices—thebenefitsthatecosystemsprovidetohumans.AstudypublishedinNaturelastyearfoundthatilluminatingasetofSwissmeadowsstoppednocturnalinsectspollinatingplants13.AteamledbyEvaKnopoftheInstituteofEcologyandEvolutionattheUniversityofBerne,foundthatinsectvisitstotheplantsdroppedbynearlytwo-thirdsunderartificiallightandthatdaytimepollinationcouldn’tcompensate:theplantsproduced13%lessfruit.Knop’steamforecastthatthesechangeshadthepotentialtocascadetothedaytimepollinatorcommunitybyreducingtheamountoffoodavailable.“Thisisaveryimportantstudy,whichclearlydemonstratesthatartificiallightatnightisathreattopollination,”saysHölker.LightskiesMuchofEarthremainsfreeofdirectartificiallight,butskyglow—lightthatisscatteredbacktoEarthbyaerosolsandclouds—ismorewidespread.Itcanbesofaintthathumanscan’tseeit,butresearcherssayitcouldstillthreatenthe30%ofvertebratesand60%ofinvertebratesthatarenocturnalandexquisitelysensitivetolight.Skyglow“almostcertainly”hasanimpactonbiodiversity,Gastonsays,becausetheleveliswellabovethethresholdsfortriggeringmanybiologicalresponses.Andyet,hesays,“it’sactuallyquitehardtodothedefinitivestudy”. Thisgrasslandexperimentsupportstheideathatredlightisrelativelybenigntowildlife.Credit:KamielSpoelstra/NIOO-KNAW That’swheretheforest-lakeexperimentcomesin.GlowingcirclesoflighthoverabovecylinderssunkintoLakeStechlin,recreatingskyglow.TheyaretheworkofLeibnizphysicistAndreasJechow,whohadtofindawaytoproducelow-level,evenilluminationwithoutblockingdaylightorimpedingaccessforscientists.Heandhisteamachievedthisusingstate-of-the-artphotonicstoolssuchasanadvancedray-tracingmodel.“Weweretooignorantasbiologistsaboutthecomplexityoflightasaphysicalphenomenon,”saysMarkGessner,directoroftheproject,knownasTheLakeLab,andco-leaderofitsartificial-lightproject,calledILES(IlluminatingLakeEcosystems).Inthepast,someexperimentshaveevenfailedtoaccountforthefactthattheMoonmovesacrossthesky,headds.TheideaforILESwastoextendfindingsfromawell-knownstudyofzooplankton,whichliveindeep,darkwaterduringthedayandmigrateupintoshallowerwatersatnighttograzeonalgae.Thismovementisthoughttobethebiggestmigrationofbiomassintheworld.Astudy14inlakesnearBoston,Massachusetts,inthelate1990ssuggestedthatskyglowreducesthezooplankton’sascentby2metres,andthenumberoforganismsthatascendby10–20%.Thisbehaviouralchangemaybeanunacknowledgeddriveroffundamentallakeprocessessuchasalgalblooms.AtILES,the24cylinders—each9metresindiameter—lookfromthesurfacelikeafishfarm.Lightingthemwithdifferentlevelsof‘skyglow’andmeasuringthedistributionofthetinyplanktonusingvideocameras,thescientistsfoundthatskyglowhadnomassiveeffectonthemovementofzooplankton.“WemayhaveachangedmigrationpatternbutI’mnotyetcertainaboutthis,”saysGessner.“Ifthereisaneffect,though,itlookslikeit’snottheprofoundonewewereexpecting.”Thesurpriseresultistypicalofthesedifficultstudies.Gessnerpointsoutthattheirexperimenthasonlycompleteditsfirstseason.“Maybewedon’tneedtobeworriedormaybeweneedtobelessworried—wedon’tknow,atleastasfarastheeffectsofskyglowonlakesisconcerned,”hesays.BrightfutureIt’sslow,meticulouswork,butthefieldiscoalescingasevidenceaccumulates,saysGaston.“Thelasttwoorthreeyearshasseenadramaticimprovementinthelevelofourunderstanding,”hesays.Nonetheless,thereareimprovementstomake.Evenmeasuringexposureishard.Inthefield,thelightanorganismreceivescanbedifficulttomeasure;abirdcouldretreattotheshadowofanearbytreetoavoidillumination,forexample.Sosomescientistshavetriedstrappinglightmeterstobirdstogetabetterideaofdosage.Astheresultsseepout,onethingthatbothfrustratesandinspiresecologistsisthattheremedyisathand.Longcoreisnowgatheringpublisheddataonhowdifferentspecies,suchasshearwatersandseaturtles,respondtodifferentpartsofthespectrum,andmatchingtheresultstothespectraemittedbydifferenttypesoflighting.Hewantstoinformdecisionsaboutlighting—forexample,whichtypeoflamptouseonabridgeandwhichataseasideresort.Engineersandecologistsknowthatwell-consideredlightingcanperformitstaskwithout“sprayinglightintothesky”,asKybaputsit.LEDscanbetweakedtoshineincertainpartsofthespectrum,todimandtoswitchoffremotely.“Myvision,”saysKyba,“isthatin30years’time,thestreetswillbenicelylit—betterthantoday—butwe’lluseone-tenthofthelight.”Thatwouldbegreatnewsforecologicalsystems,saysHölker,becausedarknessisoneofthemostprofoundforcestoshapenature.“Halfoftheglobeisalwaysdark,”hesays.“Thenightishalfthestory.” Nature553,268-270(2018) doi:https://doi.org/10.1038/d41586-018-00665-7 Updates&Corrections Correction29January2018:Anearlierversionofthisstoryerroneouslystatedthatdifferinglevelsofskyglowhadnoeffectonalgae.Italsocitedthewrongjournalinreference9. 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