Interspecific Competition I.
Possible Outcomes of Interspecific Interactions:
Mutualism:
Commensalism:
Amensalism:
Predation:
Competition:
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Interspecific Competition
Two (or more) species cause demonstrable
reductions in each other’s growth,
survival, or fecundity.
Can range from equal effects to apparent
amensalism
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)
Ants and Rodents
(Begon et al. 1996, p. 79)
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
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
How can coexistence work in the
presence of competition?
When can competitors coexist?
David Tilman’s 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
Tilman’s
experiments
(Tilman 1976, 1982)
The Ecological Niche
The n-dimensional hypervolume
Fundamental versus realized niche
1950’s : G. E. Hutchinson
Niche is from the species’ point of view
Niche restricted by available habitat
fundamental vs realized niche
Conceptual model only!
Park’s Flour Beetles
(Begon et al. 1996 p. 93)
Competitive Exclusion Principle
Two species cannot share the identical
niche and coexist
Differentiation can be morphological or
behavioral
Morphological: character displacement
Example: Darwin’s Finches
This is an example
of character
displacement
(Ricklefs and Miller 2002 p. )
Coexistence: Resource Partitioning
Resource partitioning: differentiation of the
realized niche
Competitive exclusion often an aspect of same
process
Niche complementarity: species not differentiated along one niche dimension tend to
be separated along some other niche
dimension.
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)
Competitive exclusion in the field
Connell’s Barnacles:
Chthamalus and Balanus
(Connell 1961)
A Tale of Two Bumblebees
Bombus appositus and B. flavfrons
Delphinium barbeyi and Aconitum columbianum
(Inouye 1978)
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
Competitive Release
Removal of competitor allows expansion
of the realized niche
Experimental: Balanus and Chthamalus
Looking for patterns of occurrence
in the field...
A field test of competition
Examine species distributions: where are they
found in the absence of competitors?
(Begon et al. 1996, p. 96)
Competition in a Patchy World
Assume two competing species
One always loses
Can coexistence occur?
YES
Under what conditions?
There are at least three...
Ungraded writing assignment
List three scenarios in which a species
would not be excluded by a superior
competitor