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Interspecific Competition
Chapter 6
Interspecific Competition Individuals of one
species suffer reduction in fecundity, survivorship, or growth as a result of exploitation of resources or interference by individuals from another species
Interspecific Competition May exclude one
another from particular habitats (no coexistence)
Or may coexist (utilize slightly different habitats)
Interspecific Competition Exploitation or
interference
Exploitation - diatoms OK when alone, but
one disappears when together
More effective exploiters exclude less effective ones
Depresses resources so low other species can’t survive
Interference - barnacles
One species pushes other off rock surfaces
Highly Assymetric Consequences not
same for both species One harmed greatly,
other less so or not much at all
More Effects Competition for one
resource affects competition for other resources
E.g., plants competing for light have ability to compete for nutrients affected
Competition Models Lotka-Volterra model Examines changes in
abundance of one species while in competition with another species
Competition Models
Competition Models4 possibleoutcomes
Competition Models Species 1 is stronger competitor, so 1 wins Species 2 is stronger competitor, so 2 wins Both species are stronger competitors on
each other than on themselves, so may exist in unstable coexistence
Both species are weak competitors, so exist in stable coexistence
Coexistence Partitioning of resources, differentiation of
realized niches Species deprived of realized niche by a
competitor is driven to extinction
Competitive Exclusion Principle If two competing species coexist in stable
environment, then they do so as a result of niche differentiation.
If there is no such differentiation, one competing species will eliminate the other.
Niche Differentiation How much niche
differentiation is necessary for species to coexist?
Niche Differentiation Interspecific
competition tends to increase separation along gradient (reduce overlap) to reduce the effects of the competition
Niche Differentiation Intraspecific
competition tends to decrease separation along gradient by expanding each species’ use of the gradient
Niche Differentiation Niche differentiation
results from interaction of the effects of intra- and interspecific competition
End result? Environment may not
be constant enough for effects of competition to reach final conclusion
Environment may change before one competitor wins
Evidence for interspecific comp. Laboratory
experiments Controlled conditions Simple environment,
unlike natural
Evidence for interspecific comp. Manipulative field
experiments Reduce density of
one, see how other responds
Best for sessile organisms
Evidence for interspecific comp. Natural experiments Competitive release -
expansion of niche in absence of competitor compared to size in presence of competitor
Evidence for interspecific comp. Natural experiments Character
displacement - morphological difference between allopatric (species alone) and sympatric (species together) populations