Chapter 13 Competition.docx
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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
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