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