Interspecific competition is a type of interaction in which two or more species use the same limited resource. For example, both lions and hyenas compete for prey such as zebras. Likewise, many plant species compete for soil or sunlight. Some species of plants prevent other species from growing nearby by releasing toxins into the soil. If two populations compete for a resource, the result may be a reduction in the number of either species or the elimination of one of the two competitors. More often, one species will be able to use a resource more efficiently than the other. As a result, less of the resource will be available to the other species.

Competitive Exclusion

Ecologist George F. Gause was one of the first scientists to study competition in the laboratory. In test tubes stocked with a food supply of bacteria, Gause raised several species of paramecia in various combinations. When grown in separate test tubes, Paramecium caudatum and Paramecium aurelia each thrived. But when the two species were combined, P. caudatum always died out because P. aurelia was a more efficient predator of bacteria. Ecologists use the principle of competitive exclusion to describe situations in which one species is eliminated from a community because of competition for the same limited resource. Competitive exclusion may result when one species uses the limited resource more efficiently than the other species does.

Joseph Connell's study of barnacles along the Scottish coast in the 1960s documented competition in the wild. Connell studied two species, Semibalanus balanoides and Chthamalus stellatus. These barnacles live in the intertidal zone, the portion of the seashore that is exposed during low tide.

figure 20-3

The king snake in (a) may avoid predators because of its mimicry of the color patterns of the coral snake in (b). A closer look reveals the differences: the king snake has a red snout and a black ring separating its red and yellow rings, and the coral snake has a black snout and adjacent red and yellow rings. The ring patterns of other species of coral snakes may differ from the patterns shown in the photo above.

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