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The6thMassExtinction ThehistoryoflifeontheEarthwitnessedfivemassextinctionsofspeciesasaresultofnaturalevents.Currently, biologistsaretalkingmoreandmoreoftenabouta6thmassextinction. Asoftoday,taxonomistshavealreadydescribednearly2millionspecies.Estimatestakethetotalnumberof speciesonEarthanywherefrom5millionto100million.But90to99percentofspecieseverexistingonthe planethavealreadybecomeextinct.Theoverwhelmingmajorityvanishedasaresultoftheso-callednormalor backgroundextinction.Thebackgroundextinctionisduetothelimitedperiodoftimespeciesexistencebefore dyingout.Theaveragemammalspeciesinthefossilrecordexistsfor1millionyearsandforsomegroupsof marineinvertebratestheaveragejumpsto11millionyears. Besidesthebackgroundextinction,lifeonEarthhasexperiencedfivemassextinctions,asaresultofwhich50to 95percentofthespeciesexistingatthattimedisappearedwithinashortgeologicperiodoftime.Thefirsttwo massextinctioneventsoccurredduringthemiddleofthePaleozoicEra.Bothextinctionsarethoughttobe causedbydecreasingglobaltemperatureswhichcausedsealevelstodrop.Thethirdmassextinctionevent occurredattheendofthePaleozoicEraapproximately250millionyearsago.Thisextinctionkilledoff approximately95%ofallmarinespeciesandnearly70%ofthelandanimals.Thecatastrophewasprobably causedbymajortectonicchangesintheEarth’scrust,includingtheformationofPangaeawhichalsocaused majorclimaticchangesduetomountainbuildingandchangestooceancurrents.Thefourthmassextinction happenedinthemiddleoftheMesozoicEra,whichopenedthedoorforthediversityofdinosaurstodominate therestoftheera.Thefifthmassextinction,whichisthemostfamousextinctionevent,happened65million yearsago.ThemostlikelycauseofthismassextinctionistheimpactofalargemeteorthathittheEarthin Mexico.Asaresult,approximately50%oftheplantandanimalgroupsonourplanetdiedout.Onlyabout12%of speciesonlandsurvivedthecatastrophe. Atpresent,accordingtomanyscientists,thesixthmassextinctionisoccurring,whichisprimarilybeingcausedby us.Giventhecurrentaverageextinctionrateof40speciesaday,itwouldtakeonly16thousandyearsforthe extinctionof96%ofthespeciesonEarthtoday–whichisequaltothepercentageofspecieslostduringthe greatestmassextinctionattheendofthePaleozoicEra.Themajorreasonforthisnewmassextinctionisthe destructionofplantandanimalhabitat.About1%oftropicalrainforestsdisappearsannually.Estimatesproject thatupto70plantandanimalspeciesbecomeextincteveryday,whichmakesabout3speciesperhour.At present,aboutonetenthoftheplanet’scoralreefshavebeendestroyed.Coralreefsarehometothehighest amountofbiodiversityfoundintheworld’sshallowseas.Coralsdieoutmainlyduetoglobalenvironmentand climatechange.Perhapstheonlyspeciesnotimpactedbyhumansaretheextremeorganismsofdeep-water hydrothermalvents.Theseorganismswilllikelybeabletoescapeconsequencesoftheplanet’sglobal environmentandclimatechanges. Accordingtoarecentpoll,sevenoutoftenbiologiststhinkwearecurrentlyinthemidstofasixthmass extinction.Somesayitcouldwipeoutasmanyas90%ofallspecieslivingtoday.Yetotherscientistsdisputesuch direprojections.Whatdoyouthink? ReflectionQuestions: *****(Answerthequestionsonthebackofyourpaper) 1. Whyarewethepossiblecauseofthe6thmassextinction? 2. Inyouropinion,areweenteringa6thmassextinction?Supportyouanswer.
