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ECOLOGYBiology 146L

L4: Competition

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BIOTIC INTERACTIONSCompetitionMutualismPredationHerbivoryParasitismDisease

Species 2

Species 1 ‐ / ‐

2

Georgyi F. Gause, 1910 ‐1986

1934

Paramecium;  ciliate protozoans3

K

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This lead to Gause’s COMPETITIVE EXCLUSION PRINCIPLE 

No two species can coexist indefinitely on the same limiting resource 

Many similar species DO coexist; therefore the CEP requires that these species have different NICHES

Monteverde, Costa Rica: 12 species of hummingbirds

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Modelling Competition

ΔN = r1 N1 (  K1 ‐ N1 )               =  Logistic growth in a single speciesΔt K1

But for 2 competing species the K value (resources) is impacted not only by N1 but also by  N2

If the impact of one individual of N1 = one individual of N2, then:

ΔN1 = r1 N1 (  K1 ‐ (N1  + N2 ))     which is  r1 N1 (  K1 ‐ N1  ‐N2 )Δt K1                                                                                                K1

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However the per capita impact of Species 2 on K may differ from the impact of Species 1 :

ΔN1 =  r1 N1 ( K ‐ N1 – α1,2 N2 )Δt                                          K

Where α1,2 is the competition coefficient

Elephant  M ~ 5000 kgGiraffe  M ~ 1000 kg

ΔN2 =  r2 N2 ( K ‐ N2 – α2,1 N1 )Δt                                          K

ele, gir = 0.2 8

Growth with no competition

Competition Isoclines

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In nature, there are three possibilities:

• Competition is prevalent

• Competition is rare, because of COMPETITIVE DISPLACEMENT

• Predation, disease etc hold the population below  levels at which competition would become significant. 

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Balanus excluded by  food requirements

Chthamalus excluded by  competition

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Species sympatric allopatric Sym. ratio

Allop.ratio

Weasels(skull)

Mustelanivalis

39.3 42.9 1.28 1.07

Mustelaerminea

50.4 46.0

Darwinsfinches(beak)

Geospizafortis

12.0 10.5 1.43 1.13

Geospizafuliginosa

8.4 9.3

The Hutchinsonian ratio

Mean separation resulting from competitive displacement is 1.28

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Hutchinsonian ratio

d/w  < 1  = competitive exclusion

d/w >1  < 3  = competitive interaction

d/w > 3  = no competition15