Unit 7 The Sixth Extinction.docx
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Unit 7 The Sixth Extinction.docx
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Unit7TheSixthExtinction
Unit7Ecology
Questions:
1.Whatdoyouthinkofyesterday’s“fog”?
2.Whoorevenwhatmadethefollowingsharpcontrastpresentedinthepictures?
Backgroundknowledge:
Ecologyisthescientificstudyofthedistributions,abundance,shareaffects,andrelationsoforganismsandtheirinteractionswitheachotherinacommonenvironment.ThedefinitionhereappliestothestudyofNature.Ecologyisthestudyoftheinteractionsbetweenlifeanditsphysicalenvironment;therelationshipbetweenanimalsandplantsandhowonespeciesaffectanother.Acomponentinecologicalstudyusuallyfocusesontheecosystemofanarea.
Anecosystemistheuniquenetworkofanimalandplantspeciesthatdependsoneachothertosustainlife.Theinteractionsbetweenandamongorganismsateverystageoflifeanddeathcanimpactthesystem.Anecosystemcanbeasmallareaorbigastheocean.Infact,onecansaythewholeworldisonebigecosystem.
Soanecologistcouldbestudyingandresearchingeverythingfromthetiniestformsoflifelikebacteriatoeverychainoforganismsitaffectsandhowthoseorganismscanimpactthetropicalrainforests,thedeserts,theoceans,theatmosphere,etc.
TheSixthExtinction
ThereislittledoubtleftinthemindsofprofessionalbiologiststhatEarthiscurrentlyfacedwithamountinglossofspeciesthatthreatenstorivalthefivegreatmassextinctionsofthegeologicalpast(geologicalhistory).Aslongagoas1993,HarvardbiologistE.O.WilsonestimatedthatEarthiscurrentlylosingsomethingontheorderof30,000speciesperyear—whichbreaksdowntotheevenmoredaunting(fearsome,terrible)statisticofsomethreespeciesperhour.Somebiologistshavebeguntofeelthatthisbiodiversitycrisis—this“SixthExtinction”—isevenmoresevere,andmoreimminent,thanWilsonhadsupposed.About30,000speciesgoextinctannually.
Extinctioninthepast
Themajorglobalbioticturnoverswereallcausedbyphysicalevents.
·Firstmajorextinction(c.440mya):
Climatechange(relativelysevereandsuddenglobalcooling)seemstohavebeenatworkatthefirstofthese—themassextinctionthatcausedsuchpronounced(salient)changeinmarinelife(littleornolifeexistedonlandatthattime).25%offamilieslost(afamilymayconsistofafewtothousandsofspecies).
·Secondmajorextinction(c.370mya):
Thenextsuchevent,mayormaynothavebeentheresultofglobalclimatechange.19%offamilieslost.
·ThirdmajorExtinction(c.245mya):
Thegreatestmassextinctioneventofthemallmayhavebeencausedbycomplexclimatechange.Veryrecently,however,evidencesuggeststhatabolide(火流星)mayhavebeenthecause.54%offamilieslost.
·Fourthmajorextinction(c.210mya):
Theeventattheendoftheperiodshortlyafterdinosaursandmammalshadfirstevolved,alsoremainsdifficulttopindown(decide,definedclearly)intermsofprecisecauses.23%offamilieslost.
·Fifthmajorextinction(c.65mya):
Itwipedouttheremainingdinosaursandmanyotherspeciesinallhabitats(placeswhereanimalslive)sampledfromthefossilrecord.ThiseventwascausedbycollisionsbetweenEarthandanextraterrestrial(outsidetheearth)bolide(probablycometary).Somegeologists,however,pointtothegreatvolcaniceventthatproducedtheDeccantraps(德干地盾,itisoneofthegreatestvolcanicterrainontheearthsurface.ItisontheDeccanPlateauinthesouthofIndia.)ofIndiaaspartofthechainofphysicaleventsthatdisruptedecosystemssoseverelythatmanyspeciesonlandandsearapidlysuccumbedtoextinction.17%offamilieslost.Thepreviousmassextinctionswereduetonaturalcauses.
HowistheSixthExtinctiondifferentfrompreviousevents?
Atfirstglance,thephysicallycausedextinctioneventsofthepastmightseemtohavelittleornothingtotellusaboutthecurrentSixthExtinction,whichisahuman-causedevent.Forthereislittledoubtthathumansarethedirectcauseofecosystemstressandspeciesdestructioninthemodernworldthroughsuchactivitiesas:
∙transformationofthelandscape
∙overexploitationofspecies
∙pollution
∙theintroductionofalienspecies
AndbecauseHomosapiensisclearlyaspeciesofanimal,theSixthExtinctionwouldseemtobethefirstrecordedglobalextinctioneventthathasabiotic,ratherthanaphysical,cause.Yet,uponfurtherreflection,humanimpactontheplanetisadirectanalogueofthecometarycollision:
humansarecausingvastphysicalchangesontheplanet.Thecurrentmassextinctioniscausedbyhumans.Wearebringingaboutmassivechangesintheenvironment.
WhatistheSixthExtinction?
WecandividetheSixthExtinctionintotwophases:
ThefirstphasebeganshortlyafterHomosapiensevolvedinAfricaandthemodernhumansbeganmigratingoutofAfricaandspreadingthroughouttheworld.HumansreachedtheMiddleEast90,000yearsago.TheywereinEuropestartingaround40,000yearsago.Neanderthals(homoneanderthalensis,ahumanspeciesoncelivinginEuropeandWestAsiabefore120,000to30,000yearsago.),whohadlonglivedinEurope,survivedourarrivalforlessthan10,000years,butthenabruptlydisappeared.
Everywhere,shortlyaftermodernhumansarrived,manynativespeciestypicallybecameextinct.HumanswerelikebullsinaChinashop:
∙Theydisruptedecosystemsbyoverhuntinggamespecies,whichneverexperiencedcontactwithhumansbefore.
∙Andperhapstheyspreadmicrobialdisease------causingorganismsaswell.Thefossilrecordatteststohumandestructionofecosystems:
∙HumansarrivedinlargenumbersinNorthAmericaroughly12,500yearsago-andsitesrevealingthebutcheringofmammoths(akindofelephant)andbuffaloarewelldocumentedthroughoutthecontinent.
∙TheCaribbeanlostseveralofitslargerspecieswhenhumansarrivedsome8000yearsago.
∙ExtinctionstruckelementsoftheAustralianfauna(animalgroups)muchearlier-------whenhumansarrivedsome40,000yearsago.Madagascarwherehumansonlyarrivedtheretwothousandyearsago,alsofitsthepatternwell:
thelargerspecies(elephantbirds,aspeciesofhippo,pluslargerlemurs(akindofmonkey))rapidlydisappearedsoonafterhumansarrived.
Indeedonlyinplaceswhereearlierhominidspecieshadlived(Africa,ofcourse,butalsomostofEuropeandAsia)didthefauna,alreadyadaptedtohominidpresence,survivethefirstwaveoftheSixthExtinctionHumansbegandisruptingtheenvironmentassoonastheyappearedonEarth.Whereverearlyhumansmigrated,otherspeciesbecameextinct.
WhydoestheSixthExtinctioncontinue?
PhasetwooftheSixthExtinctionbeganaround10,000yearsagowiththeinventionofagriculture.Agriculturerepresentsthesinglemostprofoundecologicalchangeintheentire3.5billion-yearhistoryoflife.Withitsinvention:
∙humansdidnothavetointeractwithotherspeciesforsurvival,andsocouldmanipulateotherspeciesfortheirownuse
∙humansdidnothavetoadheretotheecosystem’scarryingcapacity,andsocouldoverpopulate
Homosapiensbecamethefirstspeciestostoplivinginsidelocalecosystems.Allotherspecies,includingourancestralhominidancestors,allpre-agriculturalhumans,andremnanthunter-gatherersocietiesstillextantexistassemi-isolatedpopulationsplayingspecificrolesinlocalecosystems.Thisisnotsowithpost-agriculturalrevolutionhumans,whoineffecthavesteppedoutsidelocalecosystems.Indeed,todevelopagricultureisessentiallytodeclarewaronecosystems--------convertinglandtoproduceoneortwofoodcrops,withallothernativeplantspeciesallnowclassifiedasunwanted“weeds”—andallbutafewdomesticatedspeciesofanimalsnowconsideredaspests.
Thetotalnumberoforganismswithinaspeciesislimitedbymanyfactors------mostcrucialofwhichisthe“carryingcapacity”ofthelocalecosystem:
given(考虑到)theenergeticneedsandenergy-procuringadaptationsofagivenspecies,thereareonlysomanysquirrels,oaktreesandhawks(eagle)thatcaninhabitagivenstretchofhabitat.Agriculturehadtheeffectofremovingthenaturallocal-ecosystemupperlimitofthesizeofhumanpopulations.Thoughcropsstillfailregularly,andfamineanddiseasestillstalk(spread)theland,thereisnodoubtthatagricultureinthemain(forthemostpart)hashadanenormousimpactonhumanpopulationsize:
∙Estimatesvary,butrangebetween1and10millionpeopleonearth10,000yearsago.
∙Therearenowover6billionpeople.
∙Thenumberscontinuetoincrease—therewillbe8billionby2020.
∙Thereispresumablyanupperlimittothecarryingcapacityofhumansonearth—ofthenumbersthatagriculturecansupport—andthatnumberisusuallyestimatedatbetween13-15billion
Thisexplosionofhumanpopulation,especiallyinthepost-IndustrialRevolutionyearsofthepasttwocenturies,coupledwith(togetherwith)theunequaldistributionandconsumptionofwealthontheplanet,istheunderlyingcauseoftheSixthExtinction.Thereisaviciouscycle:
∙Morelandsareclearedandmoreefficientproductiontechniquestofeedthegrowingnumberofhumans—andinresponse,thehumanpopulationcontinuestoexpand.
∙Higherfossilenergyuseishelpingagriculturespread,furthermodifyingtheenvironment.
∙Humanscontinuetofishandharvesttimberforbuildingmaterialsandfuel.Pollution,andsoilerosionfromagriculturecreatesdeadzonesinfisheries(asintheGulfofMexico).
∙WhilethehumanDiaspora(grandmigration)hasmeantthespread,aswell,ofalienspeciesthatmoreoftenthannotthrivetothedetriment(damage)ofnativespecies.Forexample,invasivespecieshavecontributedto42%ofallthreatenedanden
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