CHAPTER 4 Communities and ecosystems.docx
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CHAPTER 4 Communities and ecosystems.docx
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CHAPTER4Communitiesandecosystems
CHAPTER4:
Communitiesandecosystems
Noorganismlivesintotalisolationformallothers.Wehaveseeninthepreviouschapterthatdifferentorganismsinteractwithoneanotherinboththelongandtheshortterm,competingforresourcesandsometimesexcludingoneanotherfromcertainareas.Overevolutionarytime,thiscanleadtopopulationsspecializingincertainways,perhapsinthewaytheyobtainfood,orthetypeoffoodtheyeat,orthetypeofmicroclimateinwhichtheyperformbest.Thiscanleadtothedevelopmentoftwoormorespeciesfromasingleoriginalspecies,asinthecaseofthegiantgroundsels(seep.38).Thisprocesswillbeexaminedingreaterdetailinthenextchapter.However,thereareotherwaysinwhichspeciesinteract.Oneanimalmayfeedexclusivelyuponanother,sothattheconsumerisassociatedwithitsfoodspeciesinitsdistribution.Thisisparticularlytrueofsomebutterflyspecies,whosecaterpillarsmayhaveaveryspecificdemandforacertainfoodplant,likethemonarchbutterflyandthemilkweedinNorthAmerica.Thesameistrueofparasitesandtheirhosts;theparasiteistotallylimitedinitsowndistributionbyitshost’srange.Someplantsmaybedependentonspecificpollinatorsorseed-dispersalagents.Evenhumanbeingsareconstrainedintheirdistributiontoacertainextentbytheclimaticdemandsofourdomesticplantsandanimals.
Theseareexamplesofdependencies,butthereareothersituationsinwhichtheorganismsmaynotactuallyneedoneanother,buthavesimilarrequirementsformtheirenvironment.Manyplantspecies,forexample,demandmoistconditionsbutcannottolerateconstantwaterlogging,andtheyarefrequentlyfoundtogetherinspecifichabitatswheretheserequirementsaresatisfied.Otherspeciesmayrequirecertainelementsintheirmineralsupply,suchascalcium,andtherewillbefoundtogetheroncalcareoussoils.Aslongascompetitiveinteractionsdonotleadtotheeliminationofonespeciesoranother,thesemaycoexistinadistinctassemblage.Aparticulargroupofspeciesmaybesoregularinitsoccurrencethatonecanpredictformthepresenceofoneortwoofthespeciesthatcertainotherswillalsobefound[1].Thisisespeciallytrueofstenotopic(narrowecologicalrange)organismsthatareconfinedtocertainverydistincttypesofhabitat.Plantecologistsandvegetationscientiststakeadvantageoftheseassociationsbetweenspeciestodividevegetationintoconvenientunits,variouslytermed‘associations’or‘communities’.
Thecommunity
Theuseoftheterm‘plantcommunity’bybotanistsisobjectionabletosomeecologistsbecauseitisbasedonjusttheplantcomponentofthebiotaofanarea.‘Community’shouldreallyrefertothetotalassemblageoflivingspeciesfoundinasite,interactinginawholerangeofdifferentwaysandformingacomplexgroupingofplant,animalandmicrobialcomponents.Someanimalspecies,suchasthoseherbivoreswithveryspecificfoodrequirements,mayformclose,dependentunionswithcertainplants,butothersmayhaverequirementsforcertainspatialarchitecturalconditionsthatarevestsuppliedbyparticularassemblagesofplants.Again,theoutcomeofsuchassociationsistheexistenceofcommunitiesofplantsandanimalsinnaturewhich,withinageographicalarea,mayberepeatedinsimilartopographicandenvironmentallycomparablesites,andwhichmaybeverypredictableintheirspeciescomposition.
Thestudyofplantcommunitieshasdevelopedasanindependentareaofecology(sometimestermed‘vegetationscience’)mainlybecausetheplantcomponentsofcommunitiesareoftenthemostevidentfeatures.Interrestrialsites,theplantsgenerallyoccupythemostbiomass,arestaticsocaneasilybesampledandcounted,andarerelativelyeasytoidentifywhencomparedwithsuchgroupsastheinvertebrateanimals,Thegreenplantsarealsoallinvolvedinthesamebasicexercise—trappingsolarenergyandfixingatmosphericcarbondioxide-andsocanbetreatedasadistinctivesetoforganismsinthegeneralcommunity.
Theideaofdiscreteplantcommunitiesthatcanbedescribed,andevennamedandclassified,isveryattractivetotheneatnessofthehumanmindandiscertainlyvaluableintheprocessofmappingareasandassessingtheirvalueforconservationandindeterminingplansformanagement.Whetherplantcommunitieshaveanobjectiverealityasdiscreteentitieshasbeenenergeticallydebatedthroughoutthetwentiethcentury,oftenturningupontheviewsofthetwoAmericanecologists,FredericClementsandHenryGleason,whobeganthediscussioninthe1910sand1920s.Essentially,Clementsregardedtheplantcommunityasanorganicentityinwhichthepositiveinteractionsandinterdependenciesbetweenplantspeciesledtotheirbeingfoundindistinctassociationsthatwerefrequentlyrepeatedinnature.Theviewprovedbothattractiveandpragmaticallyuseful,formingthebasisforearlyattemptsatdescribingandclassifyingvegetationbysuchecologistsasBraunBlanquetinFranceandArthurTansleyinBrintain.
Gleason’sargumentemphasizedtheindividualecologicalrequirementsofplantspecies,pointingoutthatnotwospecieshavequitethesameneeds.Veryrarelydothedistributionalorecologicalrangesofanytwospeciescoincideprecisely,andthedegreeofassociationbetweengroundfloraandcanopyisoftenweakerthanonmightassumeformcasualobservation.Theapplicationofstatisticaltechniquestotheproblemsoondemonstratedthat,althoughspeciesoftenoverlapingrequentlyoccurringassemblages,thecompositionofthesegroupsvariesgeographicallyasthephysicallimitsofspeciesareencountered.Studiesofthepasthistoryofplantandanimalspeciesoverthelast10,000yearsorsohavealsodemonstratedthattheycomeintocontactatcertaintimesintheirhistory,buthavealsobeenperiodicallyseparatedastheclimatechanged.Oftentheassociationswenowobserveareofrelativelyrecentoriginandshouldberegardedastransitory,amomentinhistorywhencertainspecieshappentohavecoincided.Theconceptofthecommunity,accordingtothisschoolofthought,mustbelookeduponasusefulbutsomewhatartificial;vegetationisactuallyacontinuumbothinspaceandintime.
ThesetwoapproachestothedescriptionofplantcommunitiescanbeexpressedgraphicallyintheformshowninFig.4.1,wherethedistributionofindividualspeciesaredepictedalonganenvironmentalgradient.Thismayrepresentanyenvironmentalfactorsuchassoilmoisture,acidity,altitude,andsoon.Allspecieshavetheiroptimumforagivenfactor(asshownpreciouslyinFig.3.17)andhavetheirecologicallimitswithinwhichsurvivalandgrowthispossible;somemayhaveanarrowrangeoftoleranceandothersmayhaveawiderangeoftoleranceandhence,potentially,awiderdistribution.IftheClementsmodeliscorrectthenwewouldexpectthatanumberofspeciesmightcoincideintheiroptimaandlimits,asshowninFig.4.1(a).ButiftheindividualisticconceptofGleasoninpreferred,thepatternshouldbeasshowninFig.4.1(b),wherethereislittlecoincidencebetweenspecies.MoststudieshavedemonstratedthattheGleasonmodelisclosertothetruth,butthetwoconceptsarenottotallymutuallyexclusive.Itispossiblethatspeciesmayshowadegreeofclusteringwithoutattainingacompleteseparationofdiscreteunits.Muchdependsuponwhethercertainsetsofenvironmentalvariablesareencounteredmorefrequentlythanothers;also,theexistenceofsharpboundariesbetweenhabitatsinnature(asattheedgeofasteep-sidedlike)mayleadto‘communities’beingapparentlymorediscrete.Manymodernsystemsofvegetationclassification,suchasthatdevisedbyJohnRodwellfordescriptionofthevegetationtypesofBritain[2]arebasedontheideaoffrequentlyrepeatedcombinationsbeingselectedanddescribedwhichcanthenbeusedasreferencepoints.Thistypeofschemeallowsthepossibilityofawiderangeofintermediatetypes,whichiswhatonewouldexpectifGleason’sideasarevalid.
Theexistenceofdiscretecommunitiesdependsuponpositiveinteractionsbetweenspecies,andthisisnotalwayseasytoaccountfor,especiallyinthecaseofplantsthatarebasicallyallseekingthesameresourcesofsun,space,waterandsoilnutrients.Itismucheasiertovisualizecompetitive,negativeinteractionsbetweenspeciesthatmutuallybeneficialones.TheDawinianapproachtoecologyhasalsoledustobesuspiciousofanyhintofaltruisminahypothesisiinvolvingtheinteractionofspecies.Butpositiverelationshipscanoccur[3].Oftenwefind,forexample,thattreespeciessurvivebestasseedlingswhenunderthecoverofspecificshrubplants,asinthecaseofthecoastliveoak(Quercusagrifolia)fromCalifornia[4].Ithasbeenfoundthat80%oftheseedlingsofthistreearelocatedbeneaththecoverofjusttwospeciesofshrub.This‘nurse’effectofonespeciesuponanotherisnotuncommoninnatureandisanexampleof‘facilitation’,whichisanimportantelementintheprocessofsuccession(seep.27).Inallvegetationstudies,thepossibilityoffacilitationinteractionsthatleadtopositiveassociationsbetweenspeciesmustbebalancedwiththecompetitiveinteractionsthatleadtonegativeassociations.
Theimportanceofscaleinthisargumentisveryevident.Asweincreasethesizeoftheareaunderstudy,increasingnumbersofspeciesarerecordedtogetherandthusappearpositivelyassociated,whilereducingthesamplingareamustinevitablyleadtonegativeassociations.Inaddition,vegetationitselfcanoftenbevisualizedasamosaic[5].Patchesofvegetationmaybeindifferentstagesofrecoveryfromdisturb
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