TOEFL TPO EX2 ReadingWord文件下载.docx
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TOEFL TPO EX2 ReadingWord文件下载.docx
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♦ThereareeightchipmunkspeciesintheSierraNevadamountainrange,andmostofthemlookprettymuchalike.Buteightdifferentspeciesofchipmunksscurryingaroundapicnicareawillnotbefound.NowhereintheSierradoalleightspeciesoccurtogether.Eachspeciestendsstronglytooccupyaspecifichabitattype,withinanelevationalrange,andtheoverlapamongthemisminimal.
♦TheeightchipmunkspeciesoftheSierraNevadarepresentbutafewofthe15speciesfoundinwesternNorthAmerica,yetthewholeofeasternNorthAmericamakesdowithbutonespecies:
theEasternchipmunk.WhyaretheresomanyverysimilarchipmunksintheWest?
ThepresenceoftallmountainsinterspersedwithvastareasofariddesertandgrasslandmakestheWestecologicallyfardifferentfromtheEast.TheWestaffordsmuchmoreopportunityforchipmunkpopulationstobecomegeographicallyisolatedfromoneanother,aconditionofspeciesformation.Also,therearemoreextremesinwesternhabitats.IntheSierraNevada,highelevationsareclosetolowelevations,atleastintermsofmileage,butecologicallytheyareverydifferent.
♦Mostecologistsbelievethatancientpopulationsofchipmunksdivergedgeneticallywhenisolatedfromoneanotherbymountainsandunfavorableecologicalhabitat.Thesescatteredpopulationsfirstevolvedintoraces—adaptedtothelocalecologicalconditions—andthenintospecies,reproductivelyisolatedfromoneanother.Thisperiodofevolutionwasrelativelyrecent,asevidencedbythesimilarappearanceofallthewesternchipmunkspecies.
EcologistshavestudiedthefourchipmunkspeciesthatoccurontheeasternslopeoftheSierraandhavelearnedjusthowthesespeciesinteractwhileremainingseparate,eachoccupyingitsownelevationalzone.Thesagebrushchipmunkisfoundatthelowestelevation,amongthesagebrush.Theyellowpinechipmunkiscommoninlowtomid-elevationsandopenconiferforests,includingpinonandponderosaandJeffreypineforests.Thelodgepolechipmunkisfoundathigherelevations,amongthelodgepoles,firs,andhigh-elevationpines.Thealpinechipmunkishigherstill,venturingamongthetalusslopes,alpinemeadows,andhigh-elevationpinesandjunipers.■Obviously,therangesofeachspeciesoverlap.■Whydon'
tsagebrushchipmunksmoveintothepinezones?
■Whydon'
talpinechipmunksmovetolowerelevationsandsharetheconiferforestswithlodgepolechipmunks?
■
♦Theanswer,inoneword,isaggression.Chipmunkspeciesactivelydefendtheirecologicalzonesfromencroachmentbyneighboringspecies.Theyellowpinechipmunkismoreaggressivethanthesagebrushchipmunk,possiblybecauseitisabitlarger.Itsuccessfullybulliesitssmallerevolutionarycousin,excludingitfromthepineforests.ExperimentshaveshownthatthesagebrushchipmunkisphysiologicallyabletoliveanywhereintheSierraNevada,fromhighalpinezonestothedesert.Thelittlecreatureisapparentlyrestrictedtothedesertnotbecauseitisspecializedtoliveonlytherebutbecausethatistheonlyhabitatwherenoneoftheotherchipmunkspeciescanlive.Thefactthatsagebrushchipmunkstolerateverywarmtemperaturesmakesthem,andonlythem,abletolivewheretheydo.Thesagebrushchipmunkessentiallyoccupiesitshabitatbydefault.Inonestudy,ecologistsestablishedthatyellowpinechipmunksactivelyexcludesagebrushchipmunksfrompineforests;
theecologistssimplytrappedalltheyellowpinechipmunksinasectionofforestandmovedthemout.Sagebrushchipmunksimmediatelymovedin,butyellowpinechipmunksdidnotentersagebrushdesertwhensagebrushchipmunkswereremoved.
♦Themostaggressiveofthefoureastern-slopespeciesisthelodgepolechipmunk,afeistyrodentindeed.Itactivelypreventsalpinechipmunksfrommovingdownslope,andyellowpinechipmunksfrommovingupslope.Thereislogicbehindthelodge-pole'
saggressivedemeanor.Itlivesinthecool,shadedconiferforests,andofthefourspecies,itistheleastabletotolerateheatstress.Itis,inotherwords,thespeciesofthestrictesthabitatneeds:
itsimplymustbeinthoseshadedforests.However,ifitshareditshabitatwithalpineandyellowpinechipmunks,eitherorbothofthesespeciesmightoutcompeteit,takingmostoftheavailablefood.Suchacompetitioncouldeffectivelyeliminatelodgepolechipmunksfromthehabitat.Lodgepolessurviveonlybyvirtueoftheiraggression.
1.Whydoestheauthormentiona"
picnicarea"
inparagraph1?
0Toidentifyasitewhereavarietyofdifferentspeciesofchipmunkscanbeseen
0Tosupportthepointthateachspeciesofchipmunkinhabitsadistinctlocation
0Toemphasizetheideathatallspeciesofchipmunkshaveasimilarappearance
0Toprovideanexampleofalocationtowhichchipmunksarelikelytoscurryforfood
2.
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