Chapter 07 COMMUNITY STRUCTUREWord文件下载.docx
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Chapter 07 COMMUNITY STRUCTUREWord文件下载.docx
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∙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;
itisacoldzoneabout4º
C,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.
>
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
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