Chapter 13 Competition.docx

Chapter 13 Competition.docx

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Chapter13Competition

Chapter13Competition

AlongacoralreefoffthenorthcoastofJamaica,threespotdamselfishguardsmallterritoriesoflessthan1m2（fig.13.1）.Thesesmallterritoriesareregularlydispersedacrossthereefandcontainmostoftheresourcesuponwhichthedamselfishdepend:

nooksandcranniesforshelteragainstpredators,acarefullytendedpatchoffast-growingalgaeforfood,andintheterritoriesofmales,anareaofcoralrabblekeptcleanforspawning.Thedamselfishconstantlypatrolandsurveythebordersoftheirterritories,vigorouslyattackinganyintruderthatpresentsathreattotheireggsanddevelopinglarvae,ortotheirfoodsupply.Ifyoulookcarefully,however,youmayfindthatnotallmembersofthepopulationhaveaterritory.Damselfishwithoutterritoriesliveinmarginalareasaroundtheterritorialmembers,wanderingfromonepartofthereeftoanother.

FIGURE13.1Territorialreeffish,suchasthisthreespotdamselfish,Eupomacentrusplanifrons,competeintenselyforspace.

Ifyoucreateavacancyonthereefbyremovingoneofthedamselfishholdingaterritory,otherdamselfishappearwithinminutestoclaimthevacantterritory.Someofthenewarrivalsarethreespotdamselfishliketheoriginalresident,andsomearecocoadamselfish,whichgenerallyliveabithigheronthereefface.Thesenewarrivalsfightfiercelyforthevacatedterritory.Thedamselfishchaseeachother,nipeachother’sflanks,andslapeachotherwiththeirtails.Themeleeendswithinminutes,andlifeamongthedamselfishsettlesbackintoakindoftensetranquillity.Thenewresident,whichmayhavedrivenoffahalfdozenrivals,isusuallyanotherthreespotdamselfish.

Thisexampledemonstratesseveralthings.First,individualdamselfishmaintainpossessionoftheirterritoriesthroughongoingcompetitionwithotherdamselfish,andthiscompetitiontakestheformofinterferencecompetition,whichinvolvesdirectaggressiveinteractionbetweenindividuals.Second,thoughitmaynotappearsotothecasualobserver,thereisalimitedsupplyofsuitablespacefordamselfishterritories,aconditionthatecologistscallresourcelimitation.Third,thethreespotdamselfisharesubjecttointraspecificcompetition,competitionwithmembersoftheirownspecies,aswellasnterspecificcompetition,competitionbetweenindividualsoftwospeciesthatreducesthefitnessofboth.Theeffectsofcompetitiononthetwocompetitorsmaynotbeequal,howeverTheindividualsofonespeciesmaysuffergreatlyreducedfitnesswhilethoseofthesecondareaffectedverylittle.Theobservationthatthreespotsgenerallywininaggressiveencounterswithcocoadamselfishsuggeststhissortofcompetitiveasymmetry.

Competitionisnotalwaysasdramaticasfightingdamselfishnorisitalwaysresolvedsoquickly-.InamaturewhitepineforestinNewHampshire,treerootsgrowthroughoutthesoiltakingupnutrientsandwaterastheyprovidesupport.In1931,J.ToumeyandR.Kienholzdesignedanexperimenttodeterminewhethertheactivitiesofthesetreerootssuppresstheactivitiesofotherplants.Theresearcherscutatrench,0.92mdeep,aroundaplot2.74mby2.74minthemiddleoftheforest.Insodoing,theycut825roots,whichremovedpotentialcompetitionbytheserootsforsoilresources.Theyalsoestablishedcontrolplotsoneithersideofthetrenchedplotandthenwatchedastheresultsoftheirexperimentunfolded.Theexperimentcontinuedfor8years,withretrenchingevery2yearsandover100rootscuteachtime.Byretrenching,theresearchersmaintainedtheirexperimentaltreatment,suppressionofpotentialrootcompetition.

Intheend,this8-yearexperimentyieldedresultsasdramaticasthosewiththedamselfish.Vegetativecoveronthesectionofforestfloorthathadbeenreleasedfromrootcompetitionwas10timesthatpresentonthecontrolplots.Apparentlytherootsofwhitepinesexertinterspecificcompetitionforsomecombinationofnutrientsandwaterthatisstrongenoughtosuppressthegrowthofforestfloorvegetation（fig.13.2）.Inaddition,thegrowthofyoungwhitepineswasalsomuchgreaterwithinthetrenchedplotsthaninthecontrolplots.Therefore,considerableintraspecificcompetitionalsooccurredontheforestfloor.

FIGURE13.2CompetitioninaforestcanbeasintenseascompetitiononacoralreefHowever,muchofthecompetitioninaforesttakesplaceundergroundwheretherootsofplantscompeteforwaterandnutrients.

Ecologistshavelongthoughtthatbothinterspecificandintraspecificcompetitionarepervasiveinnature.Forinstance,Darwinthoughtthatinterspecificcompetitionwasanimportantsourceofnaturalselection.Whileecologistshaveshownthatinterspecificcompetitionsubstantiallyinfluencesthedistributionandabundanceofmanyspecies,theyhavealsoquestionedtheassumptionthatcompetitionisanall-importantorganizerofnature.Suchquestioninghasstimulatedmorecarefulresearchandmorerigoroustestingoftheinfluenceofcompetitiononpopulations,andwhilethistestingcontinues,sufficientevidencehasaccumulatedtomakesometentativegeneralizations.

CONCEPTS

●Studiesofintraspecificcompetitionprovideevidenceforresourcelimitation.

●Thenichereflectstheenvironmentalrequirementsofspecies.

●Mathematicalandlaboratorymodelsprovideatheoreticalfoundationforstudyingcompetitiveinteractionsinnature.

●Competitioncanhavesignificantecologicalandevolutionaryinfluencesonthenichesofspecies.

CASEHISTORIES:

resourcecompetition

Studiesofintraspecificcompetitionprovideevidenceforresourcelimitation.

Inchapter11,wesawthatslowingpopulationgrowthathighdensitiesproducesasigmoidal,orS-shaped,patterninwhichpopulationsizelevelsoffatcarryingcapacity.Ourassumptioninthatdiscussionwasthatintraspecificcompetitionforlimitedresourcesplaysakeyroleinslowingpopulationgrowthathigherdensities.Theeffectofintraspecificcompetitionisincludedinthemodeloflogisticpopulationgrowth.Ifcompetitionisanimportantandcommonphenomenoninnature,thenweshouldbeabletoobserveitamongindividualsofthesamespecies,individualswithidenticalorverysimilarresourcerequirements.Thuswebeginourdiscussionofcompetitionwithintraspecificcompetition.

IntraspecificCompetitionAmongHerbaceousPlants

Inchapter6,wereviewedexperimentsbyDavidTilmanandM.Cowan（1989）thatshowedhowplantsalterroot:

shootratiosinresponsetoavailabilityofsoilnitrogen.Theplantsintheseexperimentsreducedtheirallocationtorootsassoilnitrogenconcentrationincreased.Theexperimentsalsoincludedevidenceforintraspecificcompetition.TilmanandCowangrewthegrassSorghastrumnutansatlowdensity（7plantsperpot）andhighdensity（100plantsperpot）.Theresultsshowedthattheroot:

shootratiosarehigherwhentheplantsaregrownathighdensity,suggestingthatcompetitionfornutrientswasmoreintenseunder

theseconditions.

TheresultsofTilmanandCowan'sexperimentsalsoshowthatsoilnitrogenconcentrationandpopulationdensitysubstantiallyinfluencegrowthratesandindividualplantweight.Forexample,theweightofS.nutansincreasedwithincreasedsoilnitrogen（fig.13.3）.Therefore,wecanconcludethatboththeseresponseswerelimitedbynitrogenavailabilityatthelowerconcentrationsintheexperiment.Nowcomparethegrowthratesandplantweightsshownbyplantsgrownatlowandhighdensities.Howaretheydifferent?

Bothgrowthrateandplantweightarehigherinthelow-densitypopulations,andwecanconcludethatcompetitionfornutrients（resources）ismoreintenseatthehigherplantpopulationdensity.Suchcompetitionforlimitedresourcesinnaturalpopulationsusuallyleadstomortalityamongthecompetingplants.

Self-ThinninginPlantPopulations

Thedevelopmentofastandofplantsfromtheseedlingstagetomatureindividualssuggestscompetitionforlimitedresources.Eachspringastheseedsofannualplantsgeminate,theirpopulationdensityoftennumbersinthethousands

FIGURE13.3Populationdensity,soilnitrogen,andthesizeattainedbythegrassSorghastrumnutans（datafromTilmanandCowan1989）.

persquaremeter.However,astheseasonprogressesandindividualplantsgrow,populationdensitydeclines.Thissamepatternoccursinthedevelopmentofastandoftrees.Asthestandoftreesdevelops,moreandmorebiomassiscomposedoffewerandfewerindividuals.Thisprocessiscalledself.thinning.

Self-thinningappearstoresultfromintraspecificcompetitionforlimitedresources.Asalocalpopulationofplantsdevelops,individualplantstakeupincreasingquantitiesofnutrients,water,andspaceforwhichsomeindividualscompetemoresuccessfully.Thelosersinthiscompetitionforresourcesdie,andpopulationdensitydecreases,or"thins,"asaconsequence.Overtimethepopulationiscomposedoffewerandfewerlargeindividuals.

Onewaytorepresenttheself-thinningprocessistoplottotalplantbiomassagainstpopulationdensity.Ifweplotthelogarithmofplantbiomassagainstthelogarithmofplantdensity,theslopeoftheresultinglineaveragesaround–1/2.Inotherwordsthereisanapproximatelyone-unitincreaseintotalplantbiomasswitheachtwo-unitdecreaseinpopulationdensity;plantpopulationdensitymorerapidlythanbiomassincreases（fig.13.4）.

FIGURE13.4Self-thinninginplantpopulations（datafromWestoby1984）.

Anotherwaytorepresenttheself-thinningprocessistoplottheaverageweightofindividualplantsinastandagainstdensity（fig.13.5）.Theslopeofthelineinsuchplotsaveragesaround-3/2.Becauseself-thinningbymanyspeciesofplantscomesclo