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1、江苏省吁盼中学2017届高三实验班限时训练材料（25）（建议用20分钟完成A、B、C三篇文章，鼓励同学们挑战D篇）ThereisahugedemandforsharkfinsinAsiancountries・Asaresult,manysharksarecaughtandkilled・InPalau,wheremorethanhalfoftouristsaredrawnbydivingexcursions,eachreefsharkbringsinabout$179,000intourismrevenueannua
2、lly,oraboutSI.9millionduringitslifetime.Duetooverexploitationandlackofpropermanagement,manysharkspeciesareunderconsiderableriskofunrecoverabledeclinewithsomespecieshavingdeclinedtonearextinctioninrecentyears・Accordingtoreports,sharksarebeingkilledatanalarmingr
3、ateofupto273millionworldwideperyear.Someexpertspredictthatifthekillingcontinuesatthecurrentratemanysharkspecieswillbelostforever.Itcanbeverydangerous・Sharksareveryimportantandtheyshouldbeprotected・InastudyoftheeastcoastoftheUnitedStates,11speciesofsharkswereel
4、iminatedfromtheirrange・Ofthe14speciesofoceanlifethatthosesharksusedtoeat,thepopulationsof12explodedandcausedgreatdamagetotheecosystem・Forexample,thecownoseray（鳞）populationwasnolongerkeptundercontrolbysharksandsogrewoutofcontrol.Asaresult,theraysdestroyedthepop
5、ulationofbayscallops（扇贝）,theirfavoritefood.Thescallopfishery,whichhadbeeningoodconditionforover100years,waswipedout.Also,theremovalofthescallopsmostlikelyhadaneffectonwaterqualityastheywerenolongertheretoperformtheirfunctionofcleaningthewater.Sharksplayavitalr
6、oleintheoceansinawaythattheaveragefishdoesnot.Mostsharksserveastoppredatorsatthepinnacleofthemarinefoodpyramid,andsoplayacriticalroleinoceanecosystems.Directlyorindirectlytheyregulatethenaturalbalanceoftheseecosystems・Sharkstendtoeatveryefficiently,goingaftert
7、heold,sick,orslowerfishinapopulation,keepingthatpopulationhealthy.Sharkshelpkeepmanypopulationsofoceanlifetotherightsizesothatthoseprey（3昔4勿）speciesdon'tcauseharmtotheecosystembybecomingtoopopulated.Theoceanecosystemismadeupofverycomplexfoodwebs・Forthemostpart
8、,sharksareatthetopofthesewebsandareconsideredbyscientiststobeukcystonc,rspecies,meaningthatremovingthemmaycausethewholestructuretofallapart,includingkeyfisheriessuchastunaandotheri
