Interspecific Competition I.
Possible Outcomes of Interspecific Interactions:
Mutualism: + +Commensalism: + 0
Amensalism: - 0Predation: + -Competition: - -
Interspecific Competition
Two (or more) species cause demonstrablereductions in each other’s growth,survival, or fecundity.
Can range from equal effects to apparentamensalism
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 werecompeting 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 forageddifferently
Where many species present, each ant species’mandible size was less variable
Some general conclusions
2. Competition does not always lead to exclusion of one of the species
3. Coexistence does seem to require differentialuse of resources
4. A species’ morphology or behavior canrespond to competitive pressure
1. Species do not need to be closely relatedin order to compete
How can coexistence work in thepresence of competition?
When can competitors coexist?
David Tilman’s experiments with two speciesof planktonic algae
Each species needed silicate and phosphate,and each species had a different thresholdfor each compound
Resource depletion as a method of competitiveexclusion
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 habitatfundamental vs realized niche
Conceptual model only!
Park’s Flour Beetles
(Begon et al. 1996 p. 93)
Competitive Exclusion Principle
Two species cannot share the identicalniche and coexist
Differentiation can be morphological orbehavioral
Morphological: character displacement
Example: Darwin’s Finches
(Ricklefs and Miller 2002 p. )
This is an exampleof characterdisplacement
Coexistence: Resource Partitioning
Resource partitioning: differentiation of therealized niche
Competitive exclusion often an aspect of sameprocess
Niche complementarity: species not differen-tiated along one niche dimension tend tobe separated along some other nichedimension.
Example: Hermit Crabs
(Vance, 1972. Ecology 53: 1062-1074)
3 species coexisted
Food NOT limiting, butempty shells were!
Two species separated by habitat use, onefrom the other two by shell preference
Shell limitation: interference competition(crab fights!)
Competitive exclusion in the fieldConnell’s Barnacles:
Chthamalus and Balanus
(Connell 1961)
A Tale of Two BumblebeesBombus appositus and B. flavfrons
Delphinium barbeyi and Aconitum columbianum
(Inouye 1978)
Two Kinds of Competition
Interference Competition
Preemtive occupation or taking of aresource (“contest”) e.g., Balanusand Chthamalus
Exploitation Competition
Depleting a resource ahead of thecompetitor (“scramble”) e.g.,B. appositus and B. flavifrons
Competitive Release
Removal of competitor allows expansionof the realized niche
Experimental: Balanus and Chthamalus
Looking for patterns of occurrencein the field...
A field test of competitionExamine 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 specieswould not be excluded by a superior
competitor