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1、LatitudeandBiodiversityWhenwelookatthewayinwhichbiodiversity(biologicaldiversity)isdistributedoverthelandsurfaceoftheplanet,wefindthatitisfarfromeven.Thetropicscontainmanymorespeciesoverallthananequivalentareaatthehigherlatitudes.Thisseemstobetrueformanydifferen
2、tgroupsofanimalsandplants.Whyisitthathigherlatitudeshavelowerdiversitiesthanthetropics?Perhapsitissimplyamatteroflandarea.Thetropicscontainalargersurfaceareaoflandthanhigherlatitudes—afactthatisnotalwaysevidentwhenweexaminecommonlyusedprojectionsofEarth’scurveds
3、urface,sincethistendstoexaggeratetheareasoflandinthehigherlatitudes—andsomebiogeographersregardthedifferencesindiversityasareflectionofthiseffect.ButananalysisofthedatabybiologistKlausRohdedoesnotsupportthisexplanation.Althoughareamaycontributetobiodiversity,iti
4、scertainlynotthewholestory;otherwise,largelandmasseswouldalwaysbericherinspecies.Productivityseemstobeinvolvedinstead,thoughperhapsitsinfluenceisindirect.Whereconditionsaremostsuitableforplantgrowth—thatis,wheretemperaturesarerelativelyhighanduniformandwherether
5、eisanamplesupplyofwater—oneusuallyfindslargemassesofvegetation.ThisleadstoacomplexstructureinthelayersofplantmaterialInatropicalrainforest,forexample,averylargequantityofplantmaterialbuildsupabovethesurfaceoftheground.Thereisalsoalargemassofmaterial,developedbel
6、owgroundasroottissues,butthisislessapparent.Carefulanalysisoftheabovegroundmaterialrevealsthatitisarrangedinaseriesoflayers,theprecisenumberoflayersvaryingwithageandthenatureoftheforest.Thearrangementofthebiologicalmass("biomass")ofthevegetationintolayeredformsi
7、stermedits“structure”(asopposedtoits“composition,”whichreferstothespeciesoforganismsformingthecommunity)Structureisessentiallythearchitectureofvegetation,andasinthecaseoftropicalforests,itcanbeextremelycomplicated.InamaturefloodplaintropicalforestintheAmazonRive
8、rbasin,thecanopy(theuppermostlayersofaforest,formedbythecrownsoftrees)takesonastratifiedstructure.Therearethreeclearpeaksinleafcoveratheightsofapproximately3,6,and30m
