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1、短文阅读练习题及参考答案GRE考试 Manytheorieshavebeenformulatedtoexplaintheroleofgrazerssuchaszooplanktonincontrollingtheamountofplanktonicalgae(phytoplankton)inlakes.Thefirsttheoriesofsuchgrazercontrolweremerelybasedonobservationsofnegativecorrelationsbetweenalgalandzoop
2、lanktonnumbers.Alownumberofalgalcellsinthepresenceofahighnumberofgrazerssuggested,butdidnotprove,thatthegrazershadremovedmostofthealgae.Theconverseobservation,oftheabsenceofgrazersinareasofhighphytoplanktonconcentration,ledHardytoproposehisprincipleofanimale
3、xclusion,whichhypothesizedthatphytoplanktonproducedarepellentthatexcludedgrazersfromregionsofhighphytoplanktonconcentration.Thiswasthefirstsuggestionofalgaldefensesagainstgrazing. Perhapsthefactthatmanyofthesefirststudiesconsideredonlyalgaeofasizethatcouldb
4、ecollectedinanet(netphytoplankton),apracticethatoverlookedthesmallerphytoplankton(nannoplankton)thatwenowknowgrazersaremostlikelytofeedon,ledtoade-emphasisoftheroleofgrazersinsubsequentresearch.Increasingly,asintheindividualstudiesofLund,Round,andReynolds,re
5、searchersbegantostresstheimportanceofenvironmentalfactorssuchastemperature,light,andwatermovementsincontrollingalgalnumbers.Theseenvironmentalfactorswereamenabletofieldmonitoringandtosimulationinthelaboratory.Grazingwasbelievedtohavesomeeffectonalgalnumbers,
6、especiallyafterphytoplanktongrowthratesdeclinedattheendofbloomperiods,butgrazingwasconsideredaminorcomponentofmodelsthatpredictedalgalpopulationdynamics. Thepotentialmagnitudeofgrazingpressureonfreshwaterphytoplanktonhasonlyrecentlybeendeterminedempirically
7、.StudiesbyHargraveandGeenestimatednaturalcommunitygrazingratesbymeasuringfeedingratesofindividualzooplanktonspeciesinthelaboratoryandthencomputingcommunitygrazingratesforfieldconditionsusingtheknownpopulationdensityofgrazers.Thehighestimatesofgrazingpressure
8、postulatedbytheseresearcherswerenotfullyaccepted,however,untilthegrazingratesofzooplanktonweredetermineddirectlyinthefield,bymeansofnewexperimentaltechniques.Usingaspeciallypreparedfeedingchambe
