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