Lecture 22: Competition Models

Reading:  Economy of Nature, pp. 483-485.

Reminder:  Examination II (on lectures 12-22)

Fluctuating environments may permit species coexistence where we would otherwise predict competitive exclusion. This appears to be the explanation for the "paradox of the plankton" (Hutchinson, 1961). Among planktonic species in both freshwater and maine environments, many species coexist in a simple environment without apparent niche differentiation. However, environmental conditions are constantly changing, diurnally and seasonally.

Ephemeral habitats represent another kind of fluctuating environment. Brown (1982) studied two species of freshwater snails, Physa gyrina and Lymneae elodes in ponds in northeast Indiana. Removal experiments showed that fecundity of Physa gyrina was reduced by the presence of Lymneae elodes, but competitive inhibition was not reciprocal. Physa gyrina reproduces at a smaller body size and more rapidly than does Lymneae elodes. When ponds dry-up in early July, Physa gyrina may be the only species to have produced desiccation resistant eggs in time. Consequently, Physa gyrina is the only surviving species in ponds that dry-up early, and Physa gyrina persists despite its competitive disadvantage if ponds do not dry-up.

Character Displacement

Phenotypes of two species are different in sympatry (species living together) and in allopatry (living alone).

The three spine stickleback, Gasterosteus aculeatus in lakes in British Columbia occur in two varieties (Schluter and McPhail, 1993).

In lakes containing both varieties (sympatry), the morphology is distinct. When either variety is allopatric (alone) in a lake, morphology is intermediate, and the fishes feed on both planktonic and benthic food organisms.

Experiment attempting to induce character displacement (Schluter, 1994):

Treatment: Introduce Limnetic variety to lake containing Benthic variety with intermediate phenotype

Prediction: Character displacement from the intermediate form of the Benthic variety when in the presence of the Limnetic variety

Among colonies of the seed eating harvester ant, Veromessor pergandi in desert environments of the southwestern United States, Davidson (1978) found that variation in mandible length decreases with increased diversity of competing ant species. Mandible size frequency distributions vary depending on the mandible size of the competing species (after Begon, Harper and Townsend, 1996, p 296, Fig. 7.18).

Competitive Release
In the absence of a competing species, niche expansion occurs.  Diamond (1975) studied ground doves on islands in the New Guinea Archipelago. Three species are found on these islands, Chalcophaps indica (CI), Chalcophaps stephani (CS), and Gallicollumba rufigula (GR).

When all three of these species are found in sympatry, they are found in different habitats, but on other islands where one or two species are absent, habitat utilization expands because competition is reduced or absent.

Copyright 1999 Northern Arizona University ALL RIGHTS RESERVED