Interspecific Competition I. Possible Outcomes of Interspecific Interactions: Mutualism: + + Commensalism: + 0 Amensalism: - 0 Predation: + - Competition: - -
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 differen- tiated 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