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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
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
K
dN/dt = rmaxN(K-N/K)
Intraspecific Competition
Resource availability
• When genetic factors influence the efficiency
of resource use, evolution tends to increase
the competitive ability within a population
K1
Population density
K2
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
and
-21N1
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
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…
Modeling Interspecific Competition:
Lotka-Volterra Model
To predict when population growth in each species will stop:
N1 = K1-12N2
and
N2 = K2-21N1
Modeling Interspecific Competition:
Lotka-Volterra Model
Modeling Interspecific Competition:
Lotka-Volterra Model
Species can only coexist when:
K1> K2/21
and
K2> K1/12
…that is, when intraspecific competition
is 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. pumilum
A. 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