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1、UnitlUnderstandingExtinctionAspeciesisconsideredextinctwhencarefulscientistsurveysshowthatthelastindividualofthatspecieshasdied.Thelossofmanyspeciesofplantsandanimalhasbeenattributedtochangesinclimate,geneticweaknesses,andhumanactivitiessuchashuntingandenvironmentaldestruction.Stud
2、ieshaveshownthatalmostalllivingthingswilleventuallybecomeextinct.Infact,morethan99percentofthespeciesthathaveeverlivedonEarthhavenowdisappearedpermanently.Althoughthissoundslikeaterriblesituation,itisimportanttounderstandthattheearthseemstorecoverfromthelossofgreatnumbersofspecies.
3、Inmanycases,replacementspeciesemergeovertime;however,extinctiondoesdecreasethediversityoflifeontheplanetintheshortterm.Thesereplacementspeciesoftenrequirerecoveryperiodsofmillionsofyears.Humanbeingshavehadahugeimpactontheextinctionratesofvariousspecies.Therefore,scientistshaverecen
4、tlybeguntoexaminehumankindseffectsonthenumberofspeciesthathavebecomeextinct.Throughtheuseofgeographicalandbiologicalresearch,a"backgroundextinctionrate”hasbeenestablished.Thesefiguresshowthenumbersofspecieswhichhavedisappearedduringspecifictimeperiods.Bycomparingrateofextinction,be
5、foreandafterhumanexistence,researcherswereabletodiscovertheexacteffectsofhumankindonnature・Studiesnowcitethattheextinctionratesafterhumanlifebeganare100to1,000timesgreaterthanbeforehumaninfluenceonEarth.Futureratesareforecastedtobeevenhigher.Thisdemonstratesthattremendousnegativeim
6、pactthathumanactivityishavingonextinctionrates.Scientistsarecurrentlyresearchingmethodsofdealingwiththisastonishingtrend・Asindividuals,educators,andgovernmentstrytomanagespeciessurvival,onekeyfactortheyfindisthatwithouthelp,someoftheworld'sspeciescannotsurvive.Inaddition,scientists
7、mustlearntomakedifficultdecisionsbasedonincompleteinformation.Theymustalsodealwithgroupsthatmayhaveavestedinterestinnotprotectingcertainspecies.Forexample,peopleinterestedindevelopingoilfieldsintheArcticmaynotthinktheprotectionofthewildlifeintheregionisofprimaryimportance.Unit2Clim
8、ateChange:NotOnlyaHumanWor
