GENERAL COMPETITION

Two kinds of organisms that are each using a resource that is in short supply cm with one another. Interspecific competition is often greatest between organisms obtain their food in similar ways; thus green plants compete mainly with other green plants, herbivores, with other herbivores, and carnivores with carnivores. In addi competition is more acute between closely similar kinds of organisms than bet1 ones that are less similar. Interspecific competition is to be distinguished from i petition between individuals of a single species, which is called intrasperific coi tition. More than 50 years ago, the Soviet ecologist G.F. Gause formulated what is called the principle of competitive exclusion. This principle states that if two species are competing with one another for the same limited resource, then one of the species will be able to use that resource more efficiently than the other, and the former will therefore eventually eliminate the latter locally.
In many experiments, the results of growing individuals of two species together in the laboratory have not always been easily predictable. For example, when Thomas Park and his colleagues at the University of Chicago grew two species of flour beetle, Tribolium, together in the same container of flour, one always became extinct. The species that survived, however, varied. Tribolium castaneum usually won under relatively hot and damp conditions, whereas Tribolium confusum won under cooler, drier conditions. Subsequent experiments with these species demonstrated that a genetic component was also involved in the unpredictability of the outcome. Some strains of one species would win over some-but not all-strains of the other, under a given set of conditions.
Similarly, John Harper and his colleagues at the University College of North Wales grew two species of clover together, Trifolium repens (white clover) and Trifo-lium fragiferum (strawberry clover). Each was sown at two densities, 36 and 64 plants per square foot (930 square centimeters), using all the possible combinations. Al-ihough white clover initially formed a dense canopy of leaves, the slower-growing strawberry clover, whose leaf stalks are longer, eventually produced enough leaves lhat overtopped the lower while clover and overcame it. Strawberry clover did this by mmpeting more effectively for light; the outcome was the same regardless of the ini-lial densities at which the plants were sown.
Among plants generally, competition between root systems for the nutrients that ihe plants require and which they obtain from soil is of central importance. The roots of one species not only may deplete essential minerals and thus outcompete another, but may also excrete toxic substances that depress the growth of the other species. Such interactions are poorly understood, but experimental studies are beginning to resolve their complexity.