Lecture 21: Competition Experiments

Reading:  None.

Experimental Methods for Studying Interspecific Competition

Removals and Enclosures

Starfishes on Island Reefs (Menge, 1972)

Pisaster and Leptasterias are two starfish species (Echinodermata) that have very similar diets and both occur on three tropical island reefs. There is little migration by adults between reefs.

If these two species are competing, we would predict that remove of one species would have a positive effect on the remaining species, and addition of more individuals of one species would have a negative effect on the other species.

Wild Oats and Flax (Bell and Nalewaja, 1968)

Flax is an agricultural seed crop and wild oats is a weed species in the northern Great Plains of the United States.

If these two species are competing, we would predict that as the density of the wild oats increases, the yield of flaxseed would decrease.

Flaxseed yields were evaluated on fields with varying densities of wild oats. The application of fertilizer was also a variable. Comparisons were made to plots containing no wild oats.

Flaxseed yield reductions occur with increased wild oat densities (after Krebs, 1994, p 253).

Flour Beetles (Park, 1962)

Two species of flour beetle, Tribolium confusum and T. castaneum have very similar habitats. Competition occurs along with cannibalism and reciprocal predation by adults and larvae on eggs and pupae.

Climate and chance influence the outcomes of competition between these two species, but one species always excludes the other in culture experiments. This is very suggestive of a Lotka-Volterra unstable equilibrium with ultimately leads to competitive exclusion (after Begon, Harper, and Townsend, 1996, p 281, Table 7.2).

Complete Competitors Cannot Coexist
        Organisms with similar requirements in nature compete most severely.

Exceptions:

1. Environmental patchiness, heterogeneity

2. Non-equilibrium conditions, both species below carrying capacity

3. Species not resource limited, shared resources not limiting, abiotic factors or predation dominate

4. Constant immigration

5. Fluctuating conditions, direction of competition changes before one species is excluded (for example food harvest vs. food digestion)

6. Shifts in resource requirements by one or both species

Just because resources are shared does not mean competition is occurring.

Gause’s Principle or The Competitive Exclusion Principle

If two competing species coexist in a stable environment, then this must result from niche differentiation (differentiation of realized niches). If there is no niche differentiation, or if it is precluded, then one species will eliminate (exclude) the other.

Finding niche differentiation is often difficult even though competition with coexistence can be clearly documented.

The competitive exclusion principle is widely accepted because:

1. there is much evidence to support it

2. this concept makes intuitive good sense

3. theoretical models (Lotka-Volterra) point to this principle

Examples of Exceptions to Competitive Exclusion

Heterogeneity, Patchiness, and Ephemeral Environments (spatial, temporal, and individual)

Gaps of unoccupied space may be most rapidly colonized by a species that is a poor competitor, but if gaps regularly appear, coexistence is possible.

Paine (1979) studied an intertidal brown algae called sea palm, Postelia palmaeformis and the bivalve mussel, Mytilus californianus in the Pacific Northwest. Sea palms are annuals that are only able to attach to bare rock, and must do so each year. Bare rock gaps are gradually filled by mussels. However, the two species coexist where gap formation is 7% of rock surface area per year or greater.

Temporal variation in the colonization of gaps can also make coexistence possible. The timing of a competitively superior species entering a site can influence the outcome of competition.

Harper (1961) studied two annual grass species, Bromus madritensis and Bromus rigidus. When grown together from seeds started at the same time and in equal densities, Bromus rigidus represents more than 70% of the total plant biomass after 126 days of growth. The contribution to total biomass by Bromus rigidus declines as the introduction of Bromus rigidus seeds is delayed (after Begon, Harper and Townsend, 1996, p 283, Fig. 7.10).

Copyright 1999 Northern Arizona University
ALL RIGHTS RESERVED