Species Competition

Interspecific competition occurs when two or more species seek the same limited resource. In the 1930s, Russian biologist G. F. Gause devised a set of elegant laboratory experiments that provide the basis for our formal understanding of competition. Gause grew two different species of the single-celled Paramecium —P.aurelia and P. caudatum — separately and together. Populations of both species always increased more rapidly when they were grown alone. When grown together, populations of both species grew more slowly. Eventually, P. aurelia totally displaced P. caudatum . The results of his experiments with Paramecium species, along with similar experiments he performed on other organisms, led Gause to form this postulate: two species that directly compete for essential resources cannot coexist; one species will eventually displace the other. This postulate has come to be known as the competitive exclusion principle.

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An acre of tropical forest may include over 100 species of trees, all of which depend on the same soil, water, and nutrients. Freshwater lakes may have dozens of species of fish, all of which feed on the planktonic algae and animals suspended in the water. Indeed, two or more species of Paramecium may be found in the same lake. These and many other examples from ecological communities in nature seem to contradict Gause's principle. If two competing species cannot coexist in the laboratory, how are they able to coexist in natural settings? This question has been the basis for hundreds of ecological studies.

Ecologist G. Evelyn Hutchinson provided one of the most important explanations for the coexistence of competing organisms. He proposed that each species has a fundamental niche , the complete range of environmental conditions, such as requirements for temperature, food, and water, over which the species might possibly exist. Hutchinson noted, however, that few species actually grow and reproduce in all parts of this theoretical range. Rather, species usually exist only where they are able to compete effectively against other species. Hutchinson used the term realized niche to describe the range of conditions where a species actually occurs given the constraints of competition. Species whose fundamental niches overlap significantly are potential competitors. Hutchinson suggested that these potential competitors are able to coexist because they divide up the fundamental niche. Hutchinson called this division of resources niche differentiation.

Niche differentiation occurs among many different kinds of organisms. For example, five different species of warblers, small insect-eating birds, occur together in the evergreen forests of the United States. During nesting season, the primary food of all the warblers is caterpillars. Careful studies of the birds' feeding behavior reveal that each species competes most effectively in a different part of the forest's highest layer, and that is where each species can be found. The diverse grasses and herbs that grow in native prairies provide another example of niche differentiation. Above ground, these plants appear to be vying for the same space and resources. However, a careful mapping of root systems shows that different species are adapted to exploiting different portions of the soil. In addition, some species compete most effectively when growing in bright light, whereas others compete effectively when growing in the shade of taller plants.

Some of Gause's experiments support Hutchinson's niche differentiation hypothesis. Under any specific set of conditions-the same temperature, water availability, food source, etc. Gause's principle holds true. But if conditions change, competition among species may produce different winners and losers. Indeed, if waste products are periodically removed, the outcome of the competition between P. aurelia and P. caudatum is reversed and P. caudatum wins. Thus, in a complex environment where waste materials are collected in some places and not in others, these two species could coexist.

Time is required for one species to competitively displace another, and the competitive exclusion principle presumes that environmental conditions remain constant during that time. In nature, however, environments change from season to season and from year to year, so conditions that are favorable to a particular species may not persist and environments that are constantly changing may allow competing species to coexist.