Interspecific%20Competition%20I.

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Interspecific Competition I.
Possible Outcomes of Interspecific Interactions
Mutualism
Commensalism 0
Amensalism - 0
Predation -
Competition - -
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Interspecific Competition
Two (or more) species cause demonstrable reductio
ns in each others growth, survival, or
fecundity.
Can range from equal effects to
apparent amensalism
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A Field Example Ants and Rodents
Brown and Davidson studied desert rodents, ants,
and seeds
Observed that seeds appeared to be limiting
Tested hypothesis that rodents and ants
were competing for seeds
(Brown and Davidson 1977, Brown 1977)
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Ants and Rodents
(Begon et al. 1996, p. 79)
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Davidson then looked at ants more closely
Ant species varied widely in size and had two
different foraging behaviors
If similar-sized ants coexisted, they
foraged differently
Where many species present, each ant
species mandible size was less variable
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Some general conclusions
1. Species do not need to be closely related in
order to compete
2. Competition does not always lead to
exclusion of one of the species
3. Coexistence does seem to require
differential use of resources
4. A species morphology or behavior can respond
to competitive pressure
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How can coexistence work in the presence of
competition?
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When can competitors coexist?
David Tilmans experiments with two species of
planktonic algae
Each species needed silicate and phosphate, and
each species had a different threshold for each
compound
Resource depletion as a method of
competitive exclusion
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Tilmans experiments
(Tilman 1976, 1982)
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The Ecological Niche
The n-dimensional hypervolume
Fundamental versus realized niche
1950s G. E. Hutchinson
Niche is from the species point of view
Niche restricted by available habitat fundamental
vs realized niche
Conceptual model only!
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Parks Flour Beetles
(Begon et al. 1996 p. 93)
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Competitive Exclusion Principle
Two species cannot share the identical niche and
coexist
Differentiation can be morphological
or behavioral
Morphological character displacement
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Example Darwins Finches
This is an example of character displacement
(Ricklefs and Miller 2002 p. )
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Coexistence Resource Partitioning
Resource partitioning differentiation of
the realized niche
Competitive exclusion often an aspect of
same process
Niche complementarity species not
differen- tiated along one niche dimension tend
to be separated along some other
niche dimension.
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Example Hermit Crabs
3 species coexisted
Food NOT limiting, but empty shells were!
Two species separated by habitat use, one from
the other two by shell preference
Shell limitation interference competition (crab
fights!)
(Vance, 1972. Ecology 53 1062-1074)
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Competitive exclusion in the field
Connells Barnacles Chthamalus and Balanus
(Connell 1961)
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A Tale of Two Bumblebees
Bombus appositus and B. flavfrons
Delphinium barbeyi and Aconitum columbianum
(Inouye 1978)
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Two Kinds of Competition
Interference Competition
Preemtive occupation or taking of a resource
(contest) e.g., Balanus and Chthamalus
Exploitation Competition
Depleting a resource ahead of the competitor
(scramble) e.g., B. appositus and B. flavifrons
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Competitive Release
Removal of competitor allows expansion of the
realized niche
Experimental Balanus and Chthamalus
Looking for patterns of occurrence in the
field...
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A field test of competition
Examine species distributions where are
they found in the absence of competitors?
(Begon et al. 1996, p. 96)
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Competition in a Patchy World
Assume two competing species
One always loses
Can coexistence occur?
YES
Under what conditions?
There are at least three...
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Ungraded writing assignment
List three scenarios in which a species would not
be excluded by a superior competitor