Chapter 13Competition

Modes of CompetitionModes of Competition

Interference vs. exploitation:Interference vs. exploitation: – Direct aggressive interaction between Direct aggressive interaction between

individualsindividuals– Using up resourceUsing up resource

Intraspecific:Intraspecific: – Competition with members of their own speciesCompetition with members of their own species

Interspecific:Interspecific: – Competition between individuals of two species – Competition between individuals of two species –

reduces fitness of bothreduces fitness of both

Studies of Intra-Specific Studies of Intra-Specific CompetitionCompetition

Resource CompetitionResource CompetitionIntraspecific Competition - Herbaceous PlantsIntraspecific Competition - Herbaceous Plants– Tilman + Cowan (1989): Tilman + Cowan (1989): – Indiangrass grown at two densitiesIndiangrass grown at two densities

7 per pot; 100 per pot7 per pot; 100 per pot

Different NitrogenDifferent Nitrogen

– Plant growth rates and weights increase in low Plant growth rates and weights increase in low density populationsdensity populations

Resource competitionResource competition

Competition for resources is more intense Competition for resources is more intense at higher population densitiesat higher population densities

– Usually leads to mortality among competing Usually leads to mortality among competing plantsplants

Intraspecific Competition Among Intraspecific Competition Among PlanthoppersPlanthoppers

Denno and Roderick (1992)Denno and Roderick (1992)

Experiment: 3,11,40 leafhoppers per cage Experiment: 3,11,40 leafhoppers per cage with salt marsh grasswith salt marsh grass

– Attributed prevalence of competition among Attributed prevalence of competition among leafhoppers to habit of aggregating, rapid leafhoppers to habit of aggregating, rapid growth, and mobile nature of food supplygrowth, and mobile nature of food supply

Fig 13.6

NichesNichesNiche:Niche: – summarizes environmental factors that influence summarizes environmental factors that influence

growth, survival, and reproduction of a speciesgrowth, survival, and reproduction of a species

GauseGause: : Principle of Competitive Principle of Competitive

ExclusionExclusion– Two species with identical niches cannot Two species with identical niches cannot

coexist indefinitelycoexist indefinitelyOne will be a better competitor and thus have One will be a better competitor and thus have higher fitness and eventually exclude the otherhigher fitness and eventually exclude the other

NichesNichesHutchinson:Hutchinson: – defined niche as:defined niche as:– n-dimensional hyper-volumen-dimensional hyper-volume

n is number of environmental factors important to n is number of environmental factors important to survival and reproduction of a speciessurvival and reproduction of a species

NicheNiche

– Fundamental nicheFundamental niche – hypervolume – hypervolumeAll of these environmental factorsAll of these environmental factors

– Realized niche:Realized niche: includes interactions such as includes interactions such as competition that may restrict environments competition that may restrict environments where a species may livewhere a species may live

Feeding Niches of Galapagos Feeding Niches of Galapagos FinchesFinches

– GrantGrant (1986): differences in beak size among (1986): differences in beak size among ground finches - translated directly into dietground finches - translated directly into diet

Fig 13.8

Size of seeds eaten estimated by measuring beak Size of seeds eaten estimated by measuring beak depthsdepths

– Individuals with deepest beaks fed on hardest seedsIndividuals with deepest beaks fed on hardest seeds

Fig 13.9

After 1977 drought, remaining seeds were very hard After 1977 drought, remaining seeds were very hard – thus mortality was most heavy in birds with smaller – thus mortality was most heavy in birds with smaller beaksbeaks

Fig 13.10

Fig 13.11

Mathematical and Laboratory Mathematical and Laboratory ModelsModels

Models:Models:– Abstractions and simplifications, not Abstractions and simplifications, not

facsimiles of naturefacsimiles of nature– Man-made construct; partly empirical and Man-made construct; partly empirical and

partly deductivepartly deductive– Used to provide insights into natural Used to provide insights into natural

phenomenaphenomena

Lotka VolterraLotka VolterraEffect of interspecific competition on pop. Effect of interspecific competition on pop. growth of each species:growth of each species:– dNdN11 / dt = r / dt = rm1m1NN11 ((K ((K11-N-N11--1212NN22) / K) / K11))

– dNdN22 / dt = r / dt = rm2m2NN22 ((K ((K22-N-N22- - 2121NN11) / K) / K22))

12: 12: effect of individual of species 2 on rate of pop. effect of individual of species 2 on rate of pop.

growth of species 1growth of species 1

21: 21: effect of individual of species 1 on rate of pop. effect of individual of species 1 on rate of pop.

growth of species 2growth of species 2

Laboratory Experiments for Laboratory Experiments for Inter-specific CompetitionInter-specific Competition

Paramecia Lab ExperimentsParamecia Lab ExperimentsGauseGause tested Lotka-Volterra predictions tested Lotka-Volterra predictions

He demonstrated resource limitation with He demonstrated resource limitation with Paramecium caudatumParamecium caudatum and and Paramecium Paramecium aureliaaurelia in presence of two different in presence of two different concentrations of concentrations of Bacillus pyocyaneusBacillus pyocyaneus

– When grown alone, carrying capacity When grown alone, carrying capacity determined by intraspecific competitiondetermined by intraspecific competition

– When grown together, When grown together, P. caudatumP. caudatum quickly quickly declineddeclined

Fig 13.15

Competition and NichesCompetition and Niches

Competition can restrict species to fewer Competition can restrict species to fewer environmental conditionsenvironmental conditions– But if competitive interactions are strong and But if competitive interactions are strong and

pervasive, may produce evolutionary pervasive, may produce evolutionary response in competitor populationresponse in competitor population

Changes fundamental nicheChanges fundamental niche

Competition ExamplesCompetition Examples

Tansley (1971):Tansley (1971):

2 species of bedstraw 2 species of bedstraw that grow in different that grow in different soils. soils.

Mutual competitive Mutual competitive exclusion?exclusion?

ExperimentsExperiments

Tansley’s results suggested interspecific Tansley’s results suggested interspecific competition restricts realized niche of each competition restricts realized niche of each of two species of bedstraw (of two species of bedstraw (Galium spp.)Galium spp.) to to a narrower range of soil typesa narrower range of soil types

Fig 13.18

Field Experiments for Inter-Field Experiments for Inter-specific Competitionspecific Competition

Removal Studies

Connell (1961)Connell (1961)

Found interspecific competition in barnaclesFound interspecific competition in barnacles

BalanusBalanus play role in determining lower limit of play role in determining lower limit of ChthamalusChthamalus within intertidal zone within intertidal zone– Exposure to air did not account for all observed Exposure to air did not account for all observed

patternspatterns

Fig 13.20

Competition ExamplesCompetition Examples

Brown and Munger (1985)Brown and Munger (1985) studied studied competition among rodents in Chihuahuan competition among rodents in Chihuahuan DesertDesert

Largescale experiments: 20 ha siteLargescale experiments: 20 ha site– 24 plots that were 50 x 50 m24 plots that were 50 x 50 m

Definitions of mammal groups:Definitions of mammal groups:

Granivores: seed eatersGranivores: seed eaters– Large = Large = DipodomysDipodomys - 100 g kangaroo rats - 100 g kangaroo rats– Small = Small = PerognathusPerognathus - 15 g pocket mice - 15 g pocket mice

Insectivores: eat insectsInsectivores: eat insects– OnychomysOnychomys - 30 g grasshopper mice - 30 g grasshopper mice

Experiments in enclosures:Experiments in enclosures:

Wire mesh buried, and too small for Wire mesh buried, and too small for rodentsrodents

Holes in fence initially to allow all inHoles in fence initially to allow all in

Then exclude large kangaroo ratsThen exclude large kangaroo rats– Little guys should increase if they were Little guys should increase if they were

competing for foodcompeting for food

Fig 13.23

Character DisplacementCharacter Displacement

Because degree of competition is Because degree of competition is assumed to depend upon degree of niche assumed to depend upon degree of niche overlap, overlap, – interspecific competition is predicted to lead to interspecific competition is predicted to lead to

directional selection for reduced niche overlapdirectional selection for reduced niche overlap

Character displacement:Character displacement:Fig 13.25

allopatry

sympatry

Character DisplacementCharacter DisplacementTaper and Case:Taper and Case: Necessary criteria: Necessary criteria:

– Morphological differences between sympatric Morphological differences between sympatric species greater than differences between species greater than differences between allopatric populationsallopatric populations

– Differences between sympatric and allopatric Differences between sympatric and allopatric populations have genetic basispopulations have genetic basis

– Differences between sympatric and allopatric Differences between sympatric and allopatric pops. evolved in place: pops. evolved in place:

not derived from different founder pops already not derived from different founder pops already differing in the characterdiffering in the character

Taper and Case: CharacteristicsTaper and Case: Characteristics– Variation in character must have known effect on Variation in character must have known effect on

use of resourcesuse of resources

– Must be demonstrated competition for the Must be demonstrated competition for the resource and competition must be directly resource and competition must be directly correlated with character similaritycorrelated with character similarity

– Differences in character cannot be explained by Differences in character cannot be explained by differences in resources available to each of the differences in resources available to each of the populationspopulations

SummarySummaryStudies of intraspecific competition provide Studies of intraspecific competition provide evidence for resource limitationevidence for resource limitation

A niche reflects the environmental A niche reflects the environmental requirements of a speciesrequirements of a species

Mathematical and laboratory models provide Mathematical and laboratory models provide theoretical foundation for studying theoretical foundation for studying competitive interactions in naturecompetitive interactions in nature

Competition can have significant ecological Competition can have significant ecological and evolutionary influences on the niches of and evolutionary influences on the niches of speciesspecies