焦点教育出品——托福阅读考题答案及解析生态系统长期稳定.docx

焦点教育出品——托福阅读考题答案及解析：生态系统长期稳定

TPO托福阅读真题答案及解析：托福阅读文段的结构和出题点都有一定的规律，比如说托福阅读文段一般是总分总的格式，考生重点关注首段，尾段，转折句，开头句，基本上就能找到主题句，从这个方面来说，练习托福阅读真题，从中总结托福阅读考试经验是十分必要的，下面时代焦点教育为大家整理一份“TPO托福阅读真题答案及解析”，希望对大家有所帮助。
托福阅读真题：
TheLong-TermStabilityofEcosystems
Plantcommunitiesassemblethemselvesflexibly,andtheirparticularstructuredependsonthespecifichistoryofthearea.Ecologistsusetheterm“succession”torefertothechangesthathappeninplantcommunitiesandecosystemsovertime.Thefirstcommunityinasuccessioniscalledapioneercommunity,whilethelong-livedcommunityattheendofsuccessioniscalledaclimaxcommunity.Pioneerandsuccessionalplantcommunitiesaresaidtochangeoverperiodsfrom1to500years.Thesechanges—inplantnumbersandthemixofspecies—arecumulative.Climaxcommunitiesthemselveschangebutoverperiodsoftimegreaterthanabout500years.
Anecologistwhostudiesapondtodaymaywellfinditrelativelyunchangedinayear’stime.Individualfishmaybereplaced,butthenumberoffishwilltendtobethesamefromoneyeartothenext.Wecansaythatthepropertiesofanecosystemaremorestablethantheindividualorganismsthatcomposetheecosystem.
Atonetime,ecologistsbelievedthatspeciesdiversitymadeecosystemsstable.Theybelievedthatthegreaterthediversitythemorestabletheecosystem.Supportforthisideacamefromtheobservationthatlong-lastingclimaxcommunitiesusuallyhavemorecomplexfoodwebsandmorespeciesdiversitythanpioneercommunities.Ecologistsconcludedthattheapparentstabilityofclimaxecosystemsdependedontheircomplexity.Totakeanextremeexample,farmlandsdominatedbyasinglecroparesounstablethatoneyearofbadweatherortheinvasionofasinglepestcandestroytheentirecrop.Incontrast,acomplexclimaxcommunity,suchasatemperateforest,willtolerateconsiderabledamagefromweathertopests.
Thequestionofecosystemstabilityiscomplicated,however.Thefirstproblemisthatecologistsdonotallagreewhat“stability”means.Stabilitycanbedefinedassimplylackofchange.Inthatcase,theclimaxcommunitywouldbeconsideredthemoststable,since,bydefinition,itchangestheleastovertime.Alternatively,stabilitycanbedefinedasthespeedwithwhichanecosy