BIOCENOSI. — The term b., or biological association, introduced by Moebius in 1877, denotes the entire complex of living beings that coexist within the same environment and therefore have reciprocal relationships, arising both from the simple cohabitation of space and from the establishment of exchanges of nutrients or of another kind. In a b., plants provide nourishment to herbivorous animals, which in turn fall prey to carnivorous animals; autotrophic plants, capable of producing organic matter from mineral substances, thus function as producer organisms, and animals as consumers. For a complete circulation of matter to take place, however, the coexistence of the so-called reducing organisms is necessary; these consist essentially of bacteria that degrade the organic substances of dead animals and plants, transforming them back into mineral substances that can again be used by plants.
A b. in which all the aforementioned components are represented and functioning is called autarchic, and therefore subsists in complete independence from the surrounding world. Less self-sufficient b. are frequent, however, and are consequently dependent on other nearby biological associations: examples are found in the depths of the sea and in caves, where producer plants are lacking. In general, the totality of living beings inhabiting the earth may be defined as a gigantic b., more or less distinctly divided into smaller biological associations. Among the components of a b., in addition to the aforementioned relationship of mutual domination between prey and predators, there is always competition in the search for nourishment and space; as a result of both these factors, each animal or plant species is limited by the others in its multiplication and expansion, until a state of equilibrium is reached, known as biocenotic equilibrium. This also depends on the physical influences of the environment and is subject to frequent fluctuations as the factors regulating it vary. A violent disruption of a biocenotic equilibrium may be caused by the introduction of a new species, which multiplies excessively if it finds no effective competitors in the b.; the subsequent introduction of a natural enemy, however, reestablishes a state of equilibrium. This is the basis of the so-called biological control, widely used against insects harmful to agriculture.
A mathematical theory for the study of biocenotic equilibria was developed by Lotka (1925) and, above all, by Volterra (1927).