托福阅读tpo46R-3-原文+译文+题目+答案+背景知识.pdf

托福tpo46R-3EcosystemDiversityandStability

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原文

EcosystemDiversityandStability

①Conservationbiologistshavelongbeenconcernedthatspecies

extinctioncouldhavesignificantconsequencesforthestabilityof

entireecosystems—groupsofinteractingorganismsandthephysical

environmentthattheyinhabit.Anecosystemcouldsurvivethelossof

somespecies,butifenoughspecieswerelost,theecosystemwouldbe

severelydegraded.Infact,itispossiblethatthelossofasingle

importantspeciescouldstartacascadeofextinctionsthatmight

dramaticallychangeanentireecosystem.Agoodillustrationofthis

occurredafterseaotterswereeliminatedfromsomePacifickelp

(seaweed)bedecosystems:thekelpbedswerepracticallyobliterated

toobecauseintheabsenceofseaotterpredation,seaurchin

populationsexplodedandconsumedmostofthekelpandother

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macroalgae.

②Itisusuallyclaimedthatspecies-richecosystemstendtobemore

stablethanspecies-poorecosystems.Threemechanismsbywhich

higherdiversityincreasesecosystemstabilityhavebeenproposed.First,

iftherearemorespeciesinanecosystem,thenitsfoodwebwillbe

morecomplex,withgreaterredundancyamongspeciesintermsof

theirnutritionalroles.Inotherwords,inarichsystemifaspeciesis

lost,thereisagoodchancethatotherspecieswilltakeoverits

functionasprey,predator,producer,decomposer,orwhateverroleit

played.Second,diverseecosystemsmaybelesslikelytobeinvadedby

newspecies,notablyexotics(foreignspecieslivingoutsidetheirnative

range),thatwoulddisrupttheecosystem’sstructureandfunction.

Third,inaspecies-richecosystem,diseasesmayspreadmoreslowly

becausemostspecieswillberelativelylessabundant,thusincreasing

theaveragedistancebetweenindi