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Competition. Individual Interactions, part 1. Niche. A concept that encompasses all of the individual environmental requirements of a species This is definitely an abstract concept, but it helps us to organize and explain ecological phenomena.

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Competition

Individual Interactions, part 1


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Niche

  • A concept that encompasses all of the individual environmental requirements of a species

    • This is definitely an abstract concept, but it helps us to organize and explain ecological phenomena



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Niche

  • A species’ niche is composed of both physical (e.g., average temperature) and biotic (e.g., food sources) components

  • Resources comprise a critical subset of these niche elements


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Resources

  • Include such things as:

    • Food or nutrients

    • Shelter

    • Space to grow

    • Water

    • Light (plants)


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Resources

  • Resources are usually limited

  • Individuals compete for limited resources


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Competition

  • Competition can be separated into two broad categories of interactions:

  • Interference Competition – direct antagonistic behavior towards other individuals (e.g., defending territory)

  • Resource Competition – individuals compete indirectly through the exploitation of a shared resource


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Competition

  • Competition for resources between individuals of the same species = Intraspecific competition


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Intraspecific Competition

  • Leads to the process of self-thinning in plants


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Intraspecific Competition

  • Intraspecific competition regulates population growth in a density-dependent manner:

    • as individuals deplete resources, population growth slows until the population size = K

K

dN/dt = rmaxN(K-N/K)


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Intraspecific Competition

  • When genetic factors influence the efficiency of resource use, evolution tends to increase the competitive ability within a population

Resource availability

K1

K2

Population density


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Competition

  • Competition for resources between individuals of different species = Interspecific competition


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Interspecific Competition

  • Played a prominent role in Darwin’s theory of natural selection

  • “struggle for existence” based on competition for limited resources

  • Competition should be most intense between closely related species


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Modeling Interspecific Competition: Lotka-Volterra Model

Population growth of species 1: dN1/dt = rmax1N1(K1-N1/K1)

Population growth of species 2: dN2/dt = rmax2N2(K2-N2/K2)

We can modify these logistic growth equations to account for interspecific competition by adding competition coefficients…

-12N2

-21N1

and


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Modeling Interspecific Competition: Lotka-Volterra Model

Population growth of species 1: dN1/dt = rmax1N1(K1-N1-12N2/K1)

Population growth of species 2: dN2/dt = rmax2N2(K2-N2-21N1/K2)


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Modeling Interspecific Competition: Lotka-Volterra Model

When population growth has stopped…

dN1/dt = rmax1N1(K1-N1-12N2/K1) = 0

dN2/dt = rmax2N2(K2-N2-21N1/K2) = 0

This can be rearranged algebraically…


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Modeling Interspecific Competition: Lotka-Volterra Model

To predict when population growth in each species will stop:

N1 = K1-12N2

and

N2 = K2-21N1



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Modeling Interspecific Competition: Lotka-Volterra Model

Species can only coexist when:

K1> K2/21

K2> K1/12

and

…that is, when intraspecific competition

is greater than interspecific competition


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Interspecific Competition

  • G.F. Gause (1934) integrated the idea of the niche and interspecific competition:

  • Competitive Exclusion Principle –

    • Two species with identical niches cannot coexist indefinitely (one will out-compete the other for limited resources)


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K of P. aurelia alone = 195

K of P. caudatum alone = 137


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Competitive Exclusion

  • Grown separately, P. aurelia had a higher K than P. caudatum

  • Grown together, P. aurelia out-competed P. caudatum for resources (growth medium), and P. caudatum was eliminated


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Interspecific competition & the niche

  • Hutchinson: fundamental nichedefines the environmental conditions in which a species might live, in the absence of interactions with other species; realized nicheis the actual niche of a species, which is limited by biotic interactions (competition, predation, etc.)


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Interspecific competition - Gallium

G. saxatile

G. pumilum

A. Tansley (1917)



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Interspecific competition - Gallium

On limestone (basic) soils, G. pumilum overgrew and eliminated G. saxatile by the end of the first growing season

On acidic soils, G. saxatile was completely dominant, but G. pumilum was not completely eliminated by the 6th year. Growth of both species was much slower on the acidic soils.

A. Tansley (1917)


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Interspecific competition - Gallium

  • The fundamental niches of both Gallium species include a wider variety of habitats (soil types) than those they actually inhabit in nature

  • Interspecific competition restricts the realized niche of each species to a narrower range of soil types

  • Asymmetric competition – each species is able to specialize in its realized niche because each is better at doing a different thing (e.g., exploiting resources vs. tolerating stress)


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Interspecific competition - Gallium

  • Asymmetric competition – each species is able to specialize in its realized niche because each is better at doing a different thing (e.g., exploiting resources vs. tolerating stress)



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REVIEW QUESTION

  • What type of selection pattern (stabilizing, directional, disruptive) would you expect to observe in a population undergoing intense intraspecific competition for resources?


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REVIEW QUESTION

  • How might the realized niches of two competing species evolve?



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