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1、Unit5AdelicateworldStep1诵读经典·渗透文化意识Ⅰ.细品味经典文章(2019·天津高考)Howdoesanecosystem(生态系统)work?Whatmakesthepopulationsofdifferentspeciesthewaytheyare?Whyaretheresomanyfliesandsofewwolves?Tofindananswer,scientistshavebuiltmathematicalmodelsoffoodwebs,notingwhoeatswhomandh
2、owmucheachoneeats.Withsuchmodels,scientistshavefoundoutsomekeyprinciplesoperatinginfoodwebs.Mostfoodwebs,forinstance,consistofmanyweaklinksratherthanafewstrongones.Whenapredator(掠食动物)alwayseatshugenumbersofasingleprey(猎物),thetwospeciesarestronglylinked;whenapr
3、edatorlivesonvariousspecies,theyareweaklylinked.Foodwebsmaybedominatedbymanyweaklinksbecausethatarrangementismorestableoverthelongterm.Ifapredatorcaneatseveralspecies,itcansurvivetheextinction(灭绝)ofoneofthem.Andifapredatorcanmoveontoanotherspeciesthatiseasiert
4、ofindwhenapreyspeciesbecomesrare,theswitchallowstheoriginalpreytorecover.Theweaklinksmaythuskeepspeciesfromdrivingoneanothertoextinction.Mathematicalmodelshavealsorevealedthatfoodwebsmaybeunstable,wheresmallchangesoftoppredatorscanleadtobigeffectsthroughoutent
5、ireecosystems.Inthe1960s,scientistsproposedthatpredatorsatthetopofafoodwebhadasurprisingamountofcontroloverthesizeofpopulationsofotherspecies—includingspeciestheydidnotdirectlyattack.Andunplannedhumanactivitieshaveprovedtheideaoftop-downcontrolbytoppredatorsto
6、betrue.Intheocean,wefishedfortoppredatorssuchascodonanindustrialscale,whileonland,wekilledofflargepredatorssuchaswolves.Theseactionshavegreatlyaffectedtheecologicalbalance.Scientistshavebuiltanearlywarningsystembasedonmathematicalmodels.Ideally,thesystemwouldt
7、elluswhentoadapthumanactivitiesthatarepushinganecosystemtowardabreakdownorwouldevenallowustopullanecosystembackfromtheborderline.Preventioniskey,scientistssay,becauseonceecosystemspasstheirtippingpoint(临界点),itisremarkablydifficultforthemtoreturn.Ⅱ.按要求完成任务1.写出文
8、章中关于生态系统的单词和短语。_____________________________________________________________________________________________________________________________________________________________________