Chapter 07 COMMUNITY STRUCTURE.docx

Chapter 07 COMMUNITY STRUCTURE.docx

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Chapter07COMMUNITYSTRUCTURE

Chapter07COMMUNITYSTRUCTURE（群落结构）

THECOMMUNITYDEFINED（定义）

Anecologicalcommunityisasystemmadeofspeciespopulationsinteracting;theyareboundbyinteractions.

Plantandanimalpopulationslivingtogetherandinteractingdirectlyorindirectlyformacommunity.

∙Differentspeciespopulations.

∙Inthesamearea.

∙Interactinginspatialandtrophicrelationships.

∙Characterizedbydominantspecies.

Botanistsusethetermassociationsforplantcommunitiespossessingadefinitespeciescomposition.

Communitiesmayberecognizedasautotrophicorheterotrophic.

∙Autotrophiccommunitiesuseenergyfromthesunorthecenteroftheearthtosynthesizecarbohydrates.

∙Heterotrophiccommunitiesconsumemorefoodthantheymake,andaredependentonimportsoffoodfromthesurroundingarea.

PHYSICALSTRUCTURE（物理结构）

Acommunitycanbedescribedbyanalyzingdifferentaspectsofitsstructureororganization:

Speciescomposition:

thespeciesthatformpartofthecommunityandrepresentasubsetofthespeciesoftheregion.Therearepatternsofrelativeabundanceamongthespecies.

Physiognomy:

stratificationandspatialpatternofthespecies.

Temporal:

thedailyandseasonalcyclesofactivity.

Trophic:

thepatternsofenergytransferinvolvingfoodchainsandtrophiclevels.

Guildsandniches:

differentspeciesperformdifferentfunctionsinthesamehabitat;somespeciesutilizethehabitatandresourcesinasimilarmanner.

Whyisthecommunitymadeofthesespeciesandnotothers?

Communitystructureisrelatedtocoactions,thelifehistoryofitsconstituentspeciesandthephysicalenvironment.

Raunkierrecognizedfivelifeformsbasedonthepositionoftheoverwinteringbuds.

Thissystemprovidesastandardmeanofdescribingthestructureofacommunity

VERTICALSTRATIFICATION（垂直结构）

Communitiesusuallyhaveanoticeableverticalstructure.

Onland,plantsdeterminetheverticalstructureofthecommunity.Inthisstructure,otherformsoflifearedistributedandadaptedtolive.

Thisverticaldistributionhasaneffectontheamountoflightthatpenetratestolowerlayers.

Thegradientoflightaffectstheverticaldistributionofplantsandindirectlyofanimals.

Layersorstrata,e.g.treesformthecanopy,smallertreesformtheunderstory,shrubslayer,herblayer,forestfloor,andsubterraneanlayer.

Stratificationisapplicabletoundergroundecosystemsaswellasaquaticecosystems.

Thecanopyisprincipallayerforphotosynthesis.Thestructureofthecanopy（denseoropen）determineshowmuchlightpenetratestothelowerlayers.

∙Thecanopyhasamajorinfluenceontherestofthecommunity.

Understorytreesmostbeabletotolerateshadeincludingtheyoungofcanopytrees.

∙Speciesthatareunabletotolerateshadewilldie;otherswilleventuallygrowandreachthecanopyaftersomeoftheoldertreesdie.

Thenatureoftheherblayerwilldependonthedensityofthecanopyandunderstory,topography,soilconditions,etc.

Theprocessofdecompositiontakesplaceintheforestfloor.Nutrientsarereleasedhereandreusedbyplants.

Aquaticecosystemshavealayeringdeterminedbylightpenetration,andprofilesofoxygenandtemperature.

∙Thereareusuallythreelayersrecognizedinlakesofcertaindepthdependingonlight.

∙Theupperlayer,thephoticzone,iswheremostphotosynthesisoccursandisdominatedbyphytoplankton;thisiscalledtheepilimnion.

∙Themetalimnionischaracterizedbythethermocline,asharpdropintemperature

∙Thelowerlayer,thebenthiczoneorhypolimnion,iswheremostdecompositiontakesplace;itisacoldzoneabout4ºC,usuallypoorinoxygen.

Communities,whetherterrestrialoraquatic,havesimilarbiologicalstructure.

∙Theypossessanautotrophiclayer,whichfixestheenergyofthesun.

∙Thecommunityalsopossesaheterotrophiclayerthatutilizesthefoodstoredbyautotrophs,transfertheenergyandcirculatesthenutrients.

Communitiesalsohaveacharacteristichorizontalpatternofdispersion.

Eachverticallayerinthecommunityisinhabitedbycharacteristicorganisms.Thereisconsiderableinterchangebetweenthelayers.

Ingeneral,thegreatertheverticalstratificationofacommunity,themorediverseitsanimallife.

Ingeneral,themorecomplextheverticalstratificationofthecommunity,themorediversetheanimallife.

HORIZONTALSTRUCTURE（水平结构）

Terrestrialandaquaticcommunitiesmayalsoexhibitahorizontaldistributionofspeciesproducedbyvariationinenvironmentalconditions,resultinginapatchydistributionofspecies.

Walkingacrosstheland,patchesarenoticed:

grassland,forests,oldabandonedfields,etc.

Apatchyenvironmentinturninfluencesthedistributionalpatternofanimallifeacrossthelandscape.

BIOLOGICALSTRUCTURE（生物结构）

Todeterminedominanceplantecologistsuserelativeabundance,relativedominanceandrelativefrequency.

∙Relativeabundanceordensityofspeciescomparesthenumericalabundanceononespecieswiththetotalabundanceofallspecies.

∙Relativedominanceusestheratioofthebasalareaorbiomassofonespeciestototalbasalareaorbiomassforallspecies.

∙Relativefrequencyisbasedonthenumbersamplepointsorplotsinwhichaspeciesisfoundtooccurrelativethetotalnumberofsamplestake.

Thesethreemeasurementsarecombinedintoone,theimportancevalueforeachspecies.

∙Importancevalue=relativedensity+relativedominance+relativefrequency

∙Multiplyingtheimportancevalueby100givesthe%importance.

Speciesthatreachahighlevelofimportancearecalledindexspecies.

Speciesrichnessreferstothenumberofspeciesinacommunity.

SPECIESDIVERSITY（物种多样性）

Shannon-Wienerindex（H'）measuresthelikelihoodthatthenextindividualwillbethesamespeciesasthelast.

∙H'=-（pi）（log2pi）

∙pi=numberofspeciesi/totalnumberofindividualsofallspecies;

theproportionofindividualsofspeciesiinacommunity.

∙Log2C=lnC

ln2

thisisanalternateformulatofindthelog2orofanyotherbase.

∙ThehigherthevalueofH,thegreateristheuncertainty,ortheprobabilitythatthenextindividualchosenatrandomfromacollectionofspeciescontainingNindividualswillNOTbelongtothesamespeciesasthepreviousone.

∙ThelowerthevalueofH,thegreatertheprobabilitythatthenextindividualencounteredwillbethesamespeciesasthepreviousone.Lowindexmeanslittleprobabilityofselectingthesamespeciesinthenextrun,whichmeanshighdiversity.

∙Inastream,H'<1usuallyindicatesheavypollution.Thereislittlediversity.

∙H'>3usuallyindicatescleanwater.Thereisgreatdiversity.

∙Lowestvalue=0;highestvalue=6.64,usinglog2.

Themaximumpossiblespeciesdiversity,Hmax,foracommunityofsspecieswouldbetheconditionwheretheindividualscomposingthecommunitywereevenlydistributedamongallsspecies.Thisistheconditionofmaximumevenness.

Speciesevennesscanbecalculatedbydividingthespeciesdiversityofthecommunity,H,bythemaximumpossiblediversityforthecommunity,Hmax.

∙J=H/Hmax

WhenH=Hmaxthecommunityhasreacheditsmaximumdiversity.

ThevalueofJwillapproach0asthecommunitybecomesdominatedbyasinglespecies.Thismeansthatdiversityisdecreasing.

J=H/Hmax=-（pi）（lnpi）/lns

s=numberofspeciesinthecommunity.

∙Alphadiversity:

whenspeciesdiversityiscomparedindifferentlocalitiesofacommunity,e.g.differentlocalitiesina30haarea.

∙Betadiversity:

whenspeciesdiversitybetweentwoseparatecommunitiesorhabitatsiscompared.

∙Gammadiversity:

whenthecomparisonismadebetweentwogeographicallydifferentregions,e.g.westTennesseebottomlandforestandhighaltitudeforestintheSmokeyMountains.

SPECIESABUNDANCE（种的多度）

Twocommunitieswiththesameindexesdonotnecessarilyhavethesamespeciesrichnessandevenness.

Therank-abundancediagramgivesthepictureofspeciesabundanceinacommunity.Byplottingtherelativeabundanceofaspeciesintheyaxisagainstthespeciessequenceinthexaxis,startingwiththemostabundantspecies.

Abundantspeciespreemptalargerportionofspaceandresourcesthandolessabundantspecies.

Thereareseveralproposedmathematicalmodelsthatattempttosummarizetheabundancerelationshipswithinthecommunity.Noneisperfect.

INFLUENCEOFPOPULATIONINTERACTIONSONCOMMUNITYSTRUCTURE

Thebiologicalstructureofthecommunityisaresultofaricharrayoffactorsrelatingtoboththephysicalandbiologicalenvironment.

COMPETITION

SinceDarwin,ecologistshaveconsideredinterspecificcompetition,especiallycompetitiveexclusion,asthecornerstoneofcommunitystructure.

Toprovethishypothesiswithstudiesinthewildhasproventobeverydifficult.

Thedifficultyarisesbecausepatternsthatappeartobetheconsequenceofcompetitionmayhavealternativeexplanations,likevariationsinenvironmentalfactorsthathaveadirectimpactonpopulationdynamics,suchasclimate,orothertypesofspeciesinteractions.

∙Competitionmayimportantonlyatcertaintimes,e.g.fewerseedintimeofdroughtincreasescompetitionbetweenseedeatingbirds.

∙Parasitismorpredationmaygiveacompetitiveadvantagetoothercompetitors,e.g.oaksdefoliatedbygypsymothlosetoyellowpoplarandsugarmaplecompetitors.

Fieldstudieshaveshownthatcompetitionisoftenimportant-over90%ofthecasesstudied.

Thereseemstobeanabsenceofstrongcompetitioninmanyecosystems.

Onespeciesarisesinthehabitatorimmigratesfromelsewhereanditoverlapswithanotherspeciesintheirhabitatdistribution.

1.Oneofthetwoisveryef