2015年9月19日雅思閱讀考情回顧

treesdestroyedformoresunlightwasbacktothe&13.dust和smoke形成了HowDesertsThedeserts,whichalreadyoccupyapproximaya oftheEarth'slandsurface,haveinrecentdecadesbeenincreasingatanalarmingpace.Theexpansionofdesertlikeconditionsintoareaswheretheydidnotpreviouslyexistiscalleddesertification.Ithasbeenestimatedthatanadditionalone- oftheEarth'slandsurfaceisthreatenedbythisDesertificationis plishedprimarilythroughthelossofstabilizingnaturalvegetationandthesubsequentacceleratederosionofthesoilbywindandwater.Insomecasestheloosesoilisblowncompleyaway,leavingastonysurface.Inothercases,thefinerparticlesmayberemoved,whilethesand-sizedparticlesareaccumulatedtoform hillsorridgesofsand.Evenintheareasthatretainasoilcover,thereductionofvegetationtypicallyresultsinthelossofthesoil'sabilitytoabsorbsubstantialtiesofwater.Theimpactofraindropsontheloosesoiltendstotransferfineclayparticlesintothetiniestsoilspaces,sealingthemandproducingasurfacethatallowsverylittlewaterpenetration.Waterabsorptionisgreatlyreduced;consequentlyrunoffisincreased,resultinginacceleratederosionrates.ThegradualdryingofthesoilcausedbyitsdiminishedabilitytoabsorbwaterresultsinthefurtherlossofsothatacycleofprogressivesurfacedeteriorationisInsomeregions,theincreaseindesertareasisoccurringlargelyasresultofatrendtowarddrierclimaticconditions. globalwarminghasproducedanincreaseinaridityforsomeareasoverthepastfewthousandyears.TheprocessmaybeacceleratedinsubsequentdecadesifglobalwarmingresultingfromairpollutionseriouslyThereislittledoubt,however,thatdesertificationinmostareasresultsprimarilyfromhumanactivitiesratherthannaturalprocesses.Thesemiaridlandsborderingthedesertsexistinadelicateecologicalbalanceandarelimitedintheirpotentialtoadjusttoincreasedenvironmentalpressures.Expandingpopulationsaresubjectingthelandtoincreasingpressurestoprovidethemwithfoodandfuel.Inwetperiods,thelandmaybeabletorespondtothesestresses.Duringthedryperiodsthatarecommonphenomenaalongthedesertmargins,though,thepressureonthelandisoftenfarinexcessofitsdiminishedcapacity,anddesertificationFourspecificactivitieshavebeenidentifiedasmajorcontributorstothedesertificationprocesses:overcultivation,overgrazing,firewoodgathering,andoverirrigation.Thecultivationofcropshasexpandedintoprogressivelydrierregionsaspopulationdensitieshavegrown.Theseregionsareespeciallylikelytohaveperiodsofseveredryness,sothatcropfailuresarecommon.Sincetheraisingofmostcropsnecessitatesthepriorremovalofthenaturalvegetation,cropfailuresleaveextensivetractsoflanddevoidofantcoverandsusceptibletowindandwatererosion.Theraisingoflivestockisamajoreconomicactivityinsemiaridlands,wheregrassesaregenerallythedominanttypeofnaturalvegetation.Theconsequencesofanexcessivenumberoflivestockgrazinginanareaarethereductionofthevegetationcoverandthetramplingandpulverizationofthesoil.ThisisusuallyfollowedbythedryingofthesoilandacceleratedFirewoodisthechieffuelusedforcookingandheatinginmanycountries.Theincreasedpressuresofexpandingpopulationshaveledtotheofwoodyntssothatmanycitiesandtownsaresurroundedbylargeareascompleylackingintreesandshrubs.Theincreasinguseofdriedanimalwasteasasubstitutefuelhasalsohurtthesoilbecausethisvaluablesoilconditionerandsourceofntnutrientsisnolongerbeingreturnedtotheland.Thefinalmajorhumancauseofdesertificationissoilsalinizationresultingtable.Ifnodrainagesystemexists,thewatertablerises,bringingdissolvedsaltstothesurface.Thewaterevaporatesandthesaltsareleftbehind,creatingawhitecrustallayerthatpreventsairandwaterfromreachingtheunderlyingsoil.Theextremeseriousnessofdesertificationresultsfromthevastareasoflandandthetremendousnumbersofpeopleaffected,aswellasfromthegreatdifficultyofreversingorevenslowingtheprocess.Oncethesoilhasbeenremovedbyerosion,onlythepassageofcenturiesormillenniawillenablenewsoiltoform.Inareaswhereconsiderablesoilstillremains,though,arigorouslyen dprogramoflandprotectionandcover-cropntingmaymakeitpossibletoreversethepresentdeteriorationofthe澳大利亞鸚鵡地由于環(huán)境的變遷及現(xiàn)狀題型分布與澳大利亞鸚鵡地由于環(huán)境的變遷及現(xiàn)狀題型分布與14.oneexampleofoneparrotspeciessurvivefromthechange 8.H C.inthecontinentwhichsplit關(guān)于parrotbeaks哪一項是對的 D16thinTheAntandtheIn1476,thefarmersofBerneinSwitzerlanddecidedtherewasonlyonewaytoavoidtheirfieldofthecutwormsattackingtheircrops.Theytookthepesttocourt.Thewormsweretried,foundguiltyand municatedbythearchbishop.In ,farmershadamorepracticalapproachtopestcontrol.andants.andduckswereencouragedtosnapupthepestsinthepaddiesandtheoccasionaloflocusts.Butehnotionofbiologicalcontrolbeganwithanant.Morespecifically,itstartedwiththepredatoryyellowcitrusantOecophyllasmaragdina,whichhasbeenpolishingoffpestintheorangegrovesofsouthern foratleast1700years.Theyellowcitrusantisatypeofweaverant,whichbindsleavesandtwigswithsilktoformaneattent-likenest.Inthebeginning,farmersmadedowiththeoddants’nestshereandthere.Butitwasn’tlongbeforegrowingdemandledtothedevelopmentofathrivingtradeinnestsandanewtypeofagriculture–antfarming.Foraninsectthatbites,theyellowcitrusantisremarkablypopular.Evenbyantstandards,Oecophyllasmaragdinaisafearsomepredator.It’sbig,runsfastandhasapowerfulnip–painfultohumansbutlethaltomanytheinsectsthattheorangegrovesofGuangdongandGuangxiin .Andforatleast17centuries, orangegrowershaveharnessedthesesix-leggedkillingmachinestokeeptheirfruitgroveshealthyandproductive.CitrusfruitsevolvedintheFarEastandthe discoveredthedelightsoftheirfleshearlyon.Astheancestralhomeoforanges,lemonsand alsohasthegreatestdiversityofcitruspest.Andthetreesthatproducethesweetestfruits,themandarins–orkan–attractahostofnt-eatinginsects,fromblackantsandsap-suckingmealybugstoleaf-devouringcaterpillars.Withsomanyenemies,fruitgrowersclearlyhadtohavesomewayofprotectingtheirorchards.TheWestdidnotdiscoverthe orangegrowers’secretweaponuntiltheearly20thcentury.Atthetime,Floridawassufferinganepidemicofcitruscankerandin1915WalterSwingle,antphysiologistworkingfortheUSDepartmentofAgriculture,wassentto insearchofvarietiesoforangethatwereresistanttothedisease.SwinglespentsometimestudyingthecitrusorchardsaroundGuangzhou,andtherehecameacrossthestoryofthecultivatedant.Theseants,hewastold,were‘grown’bythepeopleofthepeopleofasmallvillagenearbywhosoldthemtotheorangegrowersbythenestful.TheearliestreportofcitrusantsatworkamongtheorangetreesappearedinabookontropicalandsubtropicalbotanywrittenbyHisHaninAD304.‘ThepeopleofChiao-Chihsellintheirmarketsantsinbagsofrushmatting.Thenestsarelikesilk.Thebagsareallattachedtotwigsandleaveswhich,withtheantsinsidethenests,areforsale.Theantsarereddish-yellowincolor,biggerthanordinaryants.Inthesouthifthekantreesdonothavethiskindofant,thefruitswillallbedamagedbymanyharmfulinsects,andnotasinglefruitwillbeperfect.’Initially,farmersreliedonnestswhichtheycollectedfromthewildorboughtinthemarket–wheretradeinnestswasbrisk.‘Itissaidthatinthesouthorangetreeswhichareofantswillhavewormyfruits.thepeopleracetobuynestsfortheirorangetrees,’wroteLiuHsuninStrangeThingsNotedintheSouth,writteninabout890.Thebusinessquicklybecamemoresophisticated.Fromthe10thcentury,countrypeoplebegantotrapantsinartificialnestsbaitedwithfat.‘Fruit-growingfamiliesbuytheseantsfromvendorswhomakeabusinessofcollectingandsellingsuchcreatures,’wroteChuangChi-Yuin1130.‘Theytrapthembyfillinghogs’orsheep’sbladderswithfatandcingthemwiththecavitiesopennexttotheants’nests.Theywaituntiltheantshavemigratedintothebladdersandtakethemaway.Thisisknownas‘rearingorangeants’.’Farmersattachedthebladderstotheirtrees,andintimetheantsspreadtoothertreesandbuiltnewnests.Bythe17thcentury,growerswerebuildingbamboowalkwaysbetweentheirtreestospeedthecolonizationoftheirorchards.Theantsranalongthesenarrowbridgesfromonetreetoanotherandestablishednests‘bythehundredsofthousands’.Diditwork?Theorangegrowersclearlythoughtso.Oneauthority,ChhiiTa-Chun,writingin1700,stressedhowimportantitwastokeepthefruittreesofinsectpests,especiallycaterpillars.‘Itisessentialtoeliminatethemsothatthetreesarenotinjured.Buthandlaborisnotnearlyasefficientasantpower…’Swinglewasjustasimpressed.Yetdespitehisreports,manyWesternbiologistswereskeptical.IntheWest,theideaofusingoneinsecttodestroyanotherwasnewandhighlycontroversial.Thefirstbreakthroughhadcomein1888,whentheinfantorangeindustryinCaliforniahadbeensavedfromextinctionbytheAustralianvedaliabeetle.Thisbeetlewastheonlythingthathadmadeanyinroadintotheexplosionofcottonycushionscalethatwasthreateningtodestroythestate’scitruscrops.But,asSwinglenowknew,California’s‘first’wasnothingofthesort.The hadbeenexpertinbiocontrolformanycenturies.Thelongtraditionofantsinthe orchardsonlybegantowaverinthe1950sand1960swiththeintroductionofpowerfulorganicinsecticides.Althoughmostfruitgrowersswitchedtochemicals,anewhungontotheirants.Thosewhoabandonedantsinfavorofchemicalsquicklybecamedisillusioned.Ascostssoaredandpestbegantodevelop tothechemicals,growersbegantorevivetheoldantpatrols.Theyhadgoodreasontohavefaithintheirinsectwork.Researchintheearly1960sshowedthataslongastherewereenoughantsinthetrees,theydidanexcellentjobofdispatchingsomepests–mainlythelargerinsects–andhadmodestsuccessagainstothers.Treeswithyellowantsproducedalmost20percentmorehealthyleavesthanthosewithout.Morerecenttrialshaveshownthatthesetreesyieldjustasbigacropasthoseprotectedbyexpensivechemicalsprays.Oneapparentdrawbackofusingants–andoneofthemainreasonsfortheearlyskepticismbyWesternscientists–wasthatcitrusantsdonothingtocontrolmealybugs,waxy-coatedscaleinsectswhichcandoconsiderabledamagetofruittrees.Infact,theantsprotectmealybugsinexchangeforthesweethoneydewtheysecrete.TheorangegrowersalwaysdeniedthiswasaproblembutWesternscientiststhoughttheyknewbetter.Researchinthe1980ssuggeststhatthegrowerswererightallalong.Wheremealybugsproliferateundertheants’protectiontheyareusuallyheavilyparasitizedandthislimitstheharmtheycando.Orangegrowerswhorelyoncarnivorousantsratherthanpoisonouschemicalsmaintainabetterbalanceofspeciesintheirorchards.Whiletheantsdealwiththebiggerinsectpests,otherpredatoryspecieskeepdownthenumbersofsmallerpestssuchasscaleinsectsandaphids.Inthelongrun,antsdoalotlessdamagethanchemicals–andthey’recertainlymoreeffectivethan 題型分布與題型分布與5題+4題+5 ABattractional change adaptive NotNotMultitaskingTalkingonthephonewhiledrivingisn’ttheonlysituationwherewe’reworseatmultitaskingthanwemightliketothinkweare.Newstudieshaveidentifiedabottleneckinourbrainsthatsomesaymeanswearefundamentallyincapableoftruemultitasking.Ifexperimentalfindingsreflectreal-worldperformance,peoplewhothinktheyaremultitaskingareprobablyjustunderperforminginall–oratbest,allbutone–oftheirparallelpursuits.Practicemightimproveyourperformance,butyouwillneverbeasgoodaswhenfocusingononetaskataTheproblem,accordingtoReneMarois,apsychologistatVanderbiltUniversityinNashville,Tennessee,isthatthere’sastickingpointinthebrain.Todemonstratethis,Moroisdevisedanexperimenttolocateit.Volunteerswatchascreenandwhenaparticularimageappears,aredcircle,say,theyhavetopressakeywiththeirindexfinger.Differentcolouredcirclesrequirepressedfromdifferentfingers.Typicalresponsetimeisabouthalfasecond,andthevolunteersquicklyreachtheirpeakperformance.Thentheylearntolistentodifferentrecordingsandrespondbymakingaspecificsound.Forinstance,whentheyhearabirdchirp,theyhavetosay“ba”;anelectronicsoundshouldelicita“ko”,andsoon.Again,noproblem.Anormal candothatinabouthalfasecond,withalmostnoeffort.ThetroublecomeswhenMariosshowsthevolunteersanimage,thenalmostimmediayysthemasound.Nowthey’reflummoxed.“Ifyoushowanimageandyasoundatthesametime,onetastispostponed,”hesays.Infact,itthesecondtaskisintroducedwithinthehalf-secondorsoittakestoprocessandreacttothefirst,itwillsimplybedelayeduntilthefirstoneisdone.Thelargestdual-taskdelaysoccurwhenthetwotasksarepresentedsimultaneously;delaysprogressivelyshortenastheintervalbetweenpresentingthetaskslengthens.Thereareatleastthreepointswhereweseemtogetstuck,saysMarois.Thefirstisinsimplyidentifyingwhatwe’relookingat.Thiscantakeafewtenthsofasecond,duringwhichtimewearenotabletoseeandrecognizeaseconditem.Thislimitationisknownasthe“attentionalblink”:experimentshaveshownthatifyou’rewatchingoutforaparticulareventandasecondoneshowsupunexpectedlyanytimewithinthiscrucialwindowofconcentration,itmayregisterinyourvisualcortexbutyouwillbeunabletoactuponit.Interestingly,ifyoudon’texpectthefirstevent,youhavenotroublerespondingtothesecond.Whatexactlycausestheattentionalblinkisstillamatterfordebate.Asecondlimitationisinourshort-termvisualmemory.It’sestimatedthatwecankeeptrackofaboutfouritemsatatime,feweriftheyarecomplex.Thiscapacityshortageisthoughttoexin,inpart,ourastonishinginabilitytodetectevenhugechangesinscenesthatareotherwiseidentical,socalled“changeblindness”.Showpeoplepairsofnear-identicalphotos-say,aircraftenginesinonepicturehavedisappearedintheother–andtheywillfailtospotthedifferences.Hereagain,though,thereisdisagreementaboutwhattheessentiallimitingfactorreallyis.Doesitcomedowntoadearthofstoragecapacity,orisitabouthowmuchattentionaviewerisAthirdlimitationisthatchoosingaresponsetoastimulus–brakingwhenyouseeachildintheroad,forinstance,orreplyingwhenyourmotherlsyouoverthephonethatshe’sthinkingofleavingyouredad–alsotakesbrainpower.Selectingaresponsetooneofthesethingswilldelaybysometenthsofasecondyourabilitytorespondtotheother.Thisiscalledthe“responseselectionbottleneck”theory,firstproposedin1952.ButDavidMeyer,apsychologistattheUniversityofMichigan,AnnArbor,doesn’tbuythebottleneckidea.Hethinksdual-taskinterferenceisjustevidenceofastrategyusedbythebraintoprioritisemultipleactivities.Meyerisknownassomethingofanoptimistbyhispeers.Hehaswrittenpaperswithtitleslike“Virtuallyperfecttime-sharingindual-taskperformance:Uncorkingthecentralcognitivebottleneck”.Hisexperimentshaveshownthatwithenoughpractice–atleast2000tries-somepeoplecanexecutetwotaskssimultaneouslyascompetentlyasiftheywerengthemoneaftertheother.Hesuggeststhatthereisacentralcognitiveprocessorthatcoordinatesallthisand,what’smore,hethinksitusesdiscretion:sometimesitchoosestodelayonetaskwhilecompletingMaroisagreesthatpracticecansometimeseraseinterferenceeffects.Hehasfoundthatwithjust1hourofpracticeeachdayfortwoweeks,volunteersshowahugeimprovementatmanagingbothhistasksatonce.WheredisagreeswithMeyerisinthechancetofindlesscongestedcircuit