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

What are some possible elements of this organism’s niche?

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

Resources

• Include such things as:• Food or nutrients• Shelter• Space to grow• Water• Light (plants)

Resources

• Resources are usually limited• Individuals compete for limited resources

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

Competition

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

Intraspecific Competition

• Leads to the process of self-thinning in plants

Intraspecific Competition

• Intraspecific competition regulates population growth in a density-dependent manner: • as individuals deplete resources, population

growth slows until the population size = K

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

K

Intraspecific Competition

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

K1 K2

Reso

urce

ava

ilabi

lity

Population density

Competition

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

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

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 -21N1and

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)

Modeling Interspecific Competition: Lotka-Volterra Model

dN1/dt = rmax1N1(K1-N1-12N2/K1) = 0

dN2/dt = rmax2N2(K2-N2-21N1/K2) = 0

When population growth has stopped…

This can be rearranged algebraically…

Modeling Interspecific Competition: Lotka-Volterra Model

N1 = K1-12N2

N2 = K2-21N1

To predict when population growth in each species will stop:

and

Modeling Interspecific Competition: Lotka-Volterra Model

Modeling Interspecific Competition: Lotka-Volterra Model

K1> K2/21

Species can only coexist when:

K2> K1/12and

…that is, when intraspecific competitionis greater than interspecific competition

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)

K of P. aurelia alone = 195

K of P. caudatum alone = 137

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

Interspecific competition & the niche

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

Interspecific competition - Gallium

G. saxatile G. pumilumA. Tansley (1917)

Interspecific competition

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)

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)

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)

Interspecific competition - barnacles

Joseph Connell (1961)

REVIEW QUESTION

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

REVIEW QUESTION

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

Disruptive selection