阅读理解3篇

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1、Passage1Althoughnumbersofanimalsinagivenregionmayfluctuatefromyeartoyear，thefluctuationsareoftentemporaryand，overlongperiods，trivial.Scientistshaveadvancedthreetheoriesofpopulationcontroltoaccountforthisrelativeconstancy.Thefirsttheoryattributesarelative

2、lyconstantpopulationtoperiodicclimaticcatastrophesthatdecimatepopulationswithsuchfrequencyastopreventthemfromexceedingsomeparticularlimit.Inthecaseofsmallorganismswithshortlifecycles，climaticchangesneednotbecatastrophic：normalseasonalchangesinphotoperiod

3、(dailyamountofsunlight)，forexample，cangovernpopulationgrowth.Thistheory——thedensity-independentview——assertsthatclimaticfactorsexertthesameregulatoryeffectonpopulationregardlessofthenumberofindividualsinaregion.　　Asecondtheoryarguesthatpopulationgrowthis

4、primarilydensity-dependent——thatis，therateofgrowthofapopulationinaregiondecreasesasthenumberofanimalsincreases.Themechanismsthatmanageregulationmayvary.Forexample，asnumbersincrease，thefoodsupplywouldprobablydiminish，whichwouldincreasemortality.Inaddition

5、，asLotkaandVolterrahaveshown，predatorscanfindpreymoreeasilyinhigh-densitypopulations.Otherregulatorsincludephysiologicalcontrolmechanisms：forexample.ChristianandDavishavedemonstratedhowthecrowdingthatresultsfromariseinnumbersmaybringabouthormonalchangesi

6、nthepituitaryandadrenalglandsthatinturnmayregulatepopulationbyloweringsexualactivityandinhibitingsexualmaturation.Thereisevidencethattheseeffectsmaypersistforthreegenerationsintheabsenceoftheoriginalprovocation.Onechallengefordensity-dependenttheoristsis

7、todevelopmodelsthatwouldallowtheprecisepredictionoftheeffectsofcrowding.Athirdtheory，proposedbyWynne-Edwardsandtermed"epideictic，"arguesthatorganismshaveevolveda　"code“intheformofsocialorepideicticbehaviordisplays，suchaswinter-roostingaggregationsorgroup

8、vocalizing;suchcodesprovideorganismswithinformationonpopulationsizeinaregionsothattheycan，ifnecessary，exercisereproductiverestraint.However，Wynne-Edwards'theory，linkinganimalsocialbehaviorandpopulationcontrol，hasbeenchalle