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Unit2PartTwo—ActivityBOverthehistoryoftheearth,millionsofanimalandplantspecieshavedisappeared.Mostofthesespeciesdisappeared,orbecameextinct,becauseofnaturalcausessuchasclimaticchangesoracatastrophicevent,likeanasteroid(小行星)hittingearth.Whatisdifferenttodayisthatmostspeciesthatareindangerofbecomingextinctarenotendangeredbecauseofnaturalcausesbutbecauseofhumanactivity.Today,wewilllookatsomeofthereasonsthatmanyplantsandanimalsareendangeredandhowthesereasons,orcauses,arerelatedtohumanactivity.Thesinglemostimportantcauseofendangeredspeciestodayisthedestructionand/ordegradationofhabitat.Mostanimalsandplantsareadaptedtoliveandreproduceinaspecificenvironment,orhabitat.Theycannotsurviveiftheylosethehabitat.Therearemanywaysthathumanactivitydestroyshabitat.Forexample,forests,grasslands,anddeserts,whichprovidehabitattomanyplantsandanimals,arcclearedinordertodevelopresidentialareasforpeopletoliveinandindustrialareasforpeopletoworkin.Landisalsoclearedtopreparetogrowcropson.Swampsandmarshes,whichprovidehabitattomanyanimalandplantspecies,areoftendrainedandfilledin,alsotoprovidelandfordevelopmentoragriculture.Inaddition,riversaresometimesdammedinordertoprovidepeoplewithelectricalpower.Allofthesehumanactivities,suchasclearingforests,grasslands,anddeserts;drainingswampsandmarshes,anddammingriversresultinthedestructionofhabitatsthatmanyplantsandanimalsneedtoliveandreproducein.Closelyrelatedtothedestructionofhabitatisthedegradationofhabitat,whichalsoendangersmanyspecies.Someexamplesofmanmadecausesthatdegradehabitatareoilspills,waterpollution,andacidrain.YouprobablyhaveseenpicturesinnewspapersoronTVofdeadordyingmarineanimalsandbirdscoveredwithoilafteranoiltankeraccident.Humanbeingsalsocausewaterpollution,whichendangersthesurvivalofmanyfishandmarineanimals.Acidrain,whichresultsfrompeopleburningfossilfuels,harmsmanyspeciesoffishandmanyspeciesoftrees.Tosumup,someofthethingsrelatedtohumanactivitythatresultinthedegradationoftheenvironmentareoilspills,waterpollution,andacidrain.Illegalwildlifetradeisthenextmajorcauseofendangeredspecies.Althoughmanygovernmentshavepassedlawsprotectingendangeredspecies,manyanimalsarestillillegallyhunted.Sometimespeoplehunttheseanimalsforfood,butmoreoftentheyhuntthemonlyforspecificpartsoftheirbodies.Forexample,somespeciesofanimalssuchastigersareillegallyhuntedfortheirfurs.Elephants,whichareanendangeredspecies,areoftenkilledfortheirhides(兽皮)andtusks.Theseelephanthidesandtusksareusedtomakesouvenirsandworksofarttoselltotouristsandartcollectors.Otheranimalssuchaschimpanzeesaretrappedtobesoldtozoosforpeopletolookatorformedicalexperiments.Somebeautifulbirds,suchassomespeciesofparrots,areindangerofextinctionbecausesomanyarecapturedtobesoldaspetstopeopleallovertheworld.Thethirdmajorcausethatmanyspeciesareendangeredisoverexploitation.Peoplehavealwaysexploited,orused,plantsandanimals,andI’msurethey’llcontinuetodoso.Itisonlywhenpeopleexploitanimalsandplantsinanexcessivemannerthattheybecomeendangered.Someanimalshavealreadybeenhuntedtoextinctionforfoodandsometimesforsport.Letmegiveyouonewell-knownexample-thepassengerpigeonintheUnitedStates.Passengerpigeonswereoncesoplentifulthatpeoplesaidtheydarkenedtheskyforhours,evendays,whentheyflewovertheland.Manypeoplethoughtthatthepassengerpigeoncouldneverdisappear,but,inreality,theybecameextinctatthebeginningofthe20thcentury.Theybecameextinctasaresultofoverhunting,partlyforfoodbutmostlyforsport.Today,somefish,likethecod(鳕鱼),which/sanimportantsourceoffoodforpeopleinmanypartsoftheworld,havebeenoverfished.Asaresult,codareindangerofbecomingextinct.Atonetimecod,likepassengerpigeons,wereveryplentiful,anditseemedtheycouldneverbegone.Andit'snotjustanimalspecieslikethecodfishthatareindangerofbecomingextinct.AccordingtoarecentarticleintheNewScientist,theBrazilnuttree,averyimportantsourceofnutsforbothanimalsandpeople,isendangeredduetooverharvestingofthenuts.Thefourthandfinalreasonthatsomespeciestodayareendangeredhastodowithcompetitionthatisdirectlyrelatedtohumanactivity.Asyouknow,mostanimalandplantspecieshavetocompetewithotherspeciesintheirhabitatforfood,water,andanyotherresourcestheybothneed.Thisisusuallynatural,thatis,notrelatedtohumanactivity.However,someanimalandplantspeciestodayalsofacecompetitionthatisdirectlyrelatedtohumanactivity.Therearetwokindsofcompetitionthatanimalandplantspeciescanfacethatisrelatedtohumanactivity.Onehastodowithdomesticanimalsandtheotherhastodowithwhatiscalledintroducedspecies.Let’sdiscusscompetitionwithdomesticanimalsfirst.I'msureyouareallfamiliarwithmostdomesticanimalssuchascattle,horses,sheep,goats,andsoforth.Butdidyouknowthatthesedomesticanimalscanbeathreattowild,thatis,non-domestic,animals?Thefirstreasonisbecauseforhabitatwithwildanimals.And,thesedomesticanimalsoftenhunt,thesedomesticanimalscompeteinaddition,thepeoplewhoowntrap,andpoisonwildanimalsinordertoprotecttheirlivestock.Thewolfisanexampleofananimalthatiswidelyhuntedtonearextinctiontoprotectdomesticanimals.Anotherseriousthreattosomespeciesiscompetitionwithintroducedspecies,thatis,plantsoranimalstha