Biogeocenosis

an interrelated complex of living and inert components associated with each other by material and energy exchange; one of the most complex systems in nature. Among the living components of the biogeocenosis are autotrophic organisms (photosynthesizing green plants and chemosynthesizing microorganisms) and heterotrophic organisms (animals, fungi, many bacteria, and viruses), and among the inert components are the atmosphere layer around the earth, with its gas and thermal resources and solar energy; and the soil, with its water and mineral resources and, in part, the weathering crust (water in the case of an aquatic biogeocenosis). Each biogeocenosis maintains both a uniformity (homogeneous, or more often mosaically homogeneous) of the composition and structure of its components and the character of the material and energy exchange between them. The higher and lower green plants, which provide the basic mass of living matter, play a particularly important role in biogeocenoses. They produce the primary organic materials—the matter and energy that are used by the plants themselves and are transmitted along food chains to all heterotrophic organisms. Through the processes of photosynthesis and respiration, green plants maintain the balance of oxygen and carbon dioxide in the air; they participate in the circulation of water through transpiration. The death of organisms or their parts results in a biogenic migration and redistribution of food elements in the soil (N, P, K, Ca, and others). Finally, green plants directly or indirectly determine the composition and spatial location of animals and microorganisms in the biogeocenosis. The role of chemotrophic microorganisms in the biogeocenosis is less significant. In terms of the specific features of their activities, heterotrophs in a biogeocenosis can be divided into consumers, which transform and partially break down the organic matter of living organisms, and decomposers or destroyers (fungi, bacteria), which decompose compound organic substances in dead organisms or their parts to simple mineral compounds. In all conversions the initially accumulated energy is lost and is dispersed in the form of heat in the surrounding space. In the functioning of a biogeocenosis, a great role is played by soil organisms such as saprophages, which feed on the organic remains of dead plants; and also soil microorganisms (fungi and bacteria), which decompose and mineralize these remains. To a significant degree, the structure of the soil, the formation of humus, the content of nitrogen in the soil, the conversion of a number of mineral substances, and many other soil properties depend upon their activity. Without the heterotrophs, the completion of the biological circulation of matter, the existence of autotrophs, and the biogeocenosis itself would not be possible. The inert components of the biogeocenosis serve as a source of energy and primary materials (gases, water, and minerals). The material and energy exchange between the components of the biogeocenosis is shown on the diagram of the biogeocenosis (according to A. A. Molchanov; the influx and consumption of energy are expressed in kilocalories per hectare).

The transition from one biogeocenotic process to another in space or time is accompanied by a change in the states and properties of all its components, and consequently by a change in the nature of biogeocenotic metabolism. The boundaries of a biogeocenosis can be traced from many of its components, but more often they coincide with the boundaries of the plant communities (phytocenoses). The mass of the biogeocenosis is not homogeneous either in terms of composition or the state of its components or in terms of the conditions and results of their biogeocenotic activity. This mass is differentiated into the aboveground, underground, and underwater parts which in turn are divided into elementary vertical structures—biogeohorizons, which are very specific in terms of composition, structure, and the state of the living and inert components. The concept of biogeocenotic parcels has been introduced to designate the horizontal heterogeneity or mosaic quality of a biogeocenosis. Like the biogeocenosis as a whole, this concept is a comprehensive one, since the vegetation, animals, microorganisms, soil, and atmosphere constitute the parcel in the capacity of participants in the exchange of matter and energy.

A biogeocenosis is a dynamic system. Its continuous change and development is the result of the internal contradictory tendencies of its components. The changes in a biogeocenosis can be temporary, caused by easily reversible (daily, weather, and seasonal) reactions of the components in the biogeocenosis, or profound, leading to irreversible changes in the state, structure, and general metabolism of the biogeocenosis and marking a change (succession) from one biogeocenosis to another. The changes can be slow or rapid; the latter often occur under the effect of sudden changes as a result of natural causes or the economic activity of man, who not only transforms and destroys the natural biogeocenoses, but also creates new cultural ones. In addition to dynamic quality, biogeocenoses are also characterized by temporal stability, which is caused by the fact that the modern natural biogeocenoses are the result Of a protracted and profound adaptation of the living components to each other and to the components of the inert environment. For this reason, biogeocenoses which have been removed from a stable state by one or another cause can be restored in a form close to the original after the elimination of this cause. Biogeocenoses similar in composition and structure of the components and in terms of metabolism and direction of development are classified in the same type of biogeocenosis; this is the basic unit of the biogeocenotic classification. The aggregate of biogeocenoses of the entire earth forms the biogeocenotic cover, or biogeosphere. A study of biogeocenoses and the biogeosphere constitutes the subject of the science of biogeocenology.

The concept of a biogeocenosis was introduced by V. N. Sukachev (1940). This was the logical development of the ideas of the Russian scientists V. V. Dokuchaev, G. F. Morozov, G. N. Vysotskii, and others concerning the relationships of living and inert bodies of nature, as well as the ideas of V. I. Vernadskii concerning the planetary role of

Figure 1. Diagram of a biogeocenosis

living organisms. According to V. N. Sukachev, a biogeocenosis is close to the ecosystem of the English phytocenologist A. Tansley but differs by the definition of its content. A biogeocenosis is an elementary unit of the biogeosphere envisaged within the limits of specific plant communities, whereas an ecosystem is a dimensionless concept and can encompass a space of any extent, from a drop of pond water to the biosphere as a whole.

The term “facies” is also used by physical geographers in a sense close to that of a biogeocenosis.