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study of the relationships of organisms to their physical environment and to one another. The study of an individual organism or a single species is termed autecology; the study of groups of organisms is called synecology.
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A functional system that includes an ecological community of organisms together with the physical environment, interacting as a unit. Ecosystems are characterized by flow of energy through food webs, production and degradation of organic matter, and transformation and cycling of nutrient elements. This production of organic molecules serves as the energy base for all biological activity within ecosystems. The consumption of plants by herbivores (organisms that consume living plants or algae) and detritivores (organisms that consume dead organic matter) serves to transfer energy stored in photosynthetically produced organic molecules to other organisms. Coupled to the production of organic matter and flow of energy is the cycling of elements. See Ecological communities, Environment
All biological activity within ecosystems is supported by the production of organic matter by autotrophs (organisms that can produce organic molecules such as glucose from inorganic carbon dioxide; see illustration). More than 99% of autotrophic production on Earth is through photosynthesis by plants, algae, and certain types of bacteria. Collectively these organisms are termed photoautotrophs (autotrophs that use energy from light to produce organic molecules). In addition to photosynthesis, some production is conducted by chemoautotrophic bacteria (autotrophs that use energy stored in the chemical bonds of inorganic molecules such as hydrogen sulfide to produce organic molecules). The organic molecules produced by autotrophs are used to support the organism's metabolism and reproduction, and to build new tissue. This new tissue is consumed by herbivores or detritivores, which in turn are ultimately consumed by predators or other detritivores.
Terrestrial ecosystems, which cover 30% of the Earth's surface, contribute a little over one-half of the total global photosynthetic production of organic matter—approximately 60 × 1015 grams of carbon per year. Oceans, which cover 70% of the Earth's surface, produce approximately 51 × 1015 g C y-1 of organic matter. See Biomass
Organisms are classified based upon the number of energy transfers through a food web (see illustration). Photoautotrophic production of organic matter represents the first energy transfer in ecosystems and is classified as primary production. Consumption of a plant by a herbivore is the second energy transfer, and thus herbivores occupy the second trophic level, also known as secondary production. Consumer organisms that are one, two, or three transfers from photoautotrophs are classified as primary, secondary, and tertiary consumers. Moving through a food web, energy is lost during each transfer as heat, as described by the second law of thermodynamics. Consequently, the total number of energy transfers rarely exceeds four or five; with energy loss during each transfer, little energy is available to support organisms at the highest levels of a food web. See Ecological energetics, Food web
In contrast to energy, which is lost from ecosystems as heat, chemical elements (or nutrients) that compose molecules within organisms are not altered and may repeatedly cycle between organisms and their environment. Approximately 40 elements compose the bodies of organisms, with carbon, oxygen, hydrogen, nitrogen, and phosphorus being the most abundant. If one of these elements is in short supply in the environment, the growth of organisms can be limited, even if sufficient energy is available. In particular, nitrogen and phosphorus are the elements most commonly limiting organism growth. This limitation is illustrated by the widespread use of fertilizers, which are applied to agricultural fields to alleviate nutrient limitation. See Biogeochemistry, Nitrogen cycle
Carbon cycles between the atmosphere and terrestrial and oceanic ecosystems. This cycling results, in part, from primary production and decomposition of organic matter. Rates of primary production and decomposition, in turn, are regulated by the supply of nitrogen, phosphorus, and iron. The combustion of fossil fuels is a recent change in the global cycle that releases carbon that has long been buried within the Earth's crust to the atmosphere. Carbon dioxide in the atmosphere traps heat on the Earth's surface and is a major factor regulating the climate. This alteration of the global carbon cycle along with the resulting impact on the climate is a major issue under investigation by ecosystem ecologists. See Conservation of resources, Human ecology
a natural complex (biologically integral system) formed by living organisms (biocenosis) and their environment (abiotic, such as the atmosphere, or biotic, such as the soil and bodies of water), which are interrelated by the exchange of matter and energy. One of the basic concepts of ecology, it is applicable to objects of varying complexity and size.
One example of an ecosystem is a pond, with its plants, fish, invertebrates, microorganisms and bottom deposits and with the characteristic temperature fluctuations, quantity of oxygen dissolved in the water, the substances present in the water, and other factors with a specific biological productivity. Another example is a forest, with its litter, soil, microorganisms, birds, and herbivorous and predatory mammals and with the characteristic range of temperature, atmospheric humidity, light conditions, groundwaters, and other environmental factors and the characteristic exchange of matter and energy. A decaying stump in a forest, with the organisms living on and in it, together with their living conditions, may also be considered an ecosystem.
Ideally, an ecosystem in which the vital activities of autotrophic and heterotrophic organisms is in balance can approach the conditions of a closed system, which only exchanges energy with its surroundings. Under natural conditions, however, the continued existence of an ecosystem is possible only when there is a flow of both energy and a certain amount of matter from its surroundings. All existing ecosystems, which together make up the earth’s biosphere, are of the open type, exchanging both matter and energy with their surroundings.
The term “ecosystem” is applicable to both natural and man-made ecosystems, such as agricultural land, gardens, and parks.
In the comprehensive study of natural complexes of interacting plants, animals, and microorganisms, scientists have assigned many different names to these supraorganismic units. Many of the terms have not gained wide usage, and some are used only in particular circumstances; for example, the term “biome” is used in the USA to designate such major ecosystems as the coniferous forest zone and the steppe zone.
The term “ecosystem,” which has displaced many other terms of similar meaning, was proposed in 1935 by the British botanist A. Tansley. In 1944, V. N. Sukachev introduced the term biogeocenosis for terrestrial living systems, but he did not consider it to be synonymous with the term “ecosystem.” Indeed, even aquariums and beehives are certainly examples of ecosystems, but they cannot be called biogeocenoses. Moreover, a general feature of a biogeocenosis is a smaller total animal biomass in comparison with the plant biomass, while in an aquatic ecosystem the opposite is true.
An ecosystem is distinguished by its species composition, the number of individuals of the various species, and the biomass, distribution, and seasonal dynamics of the species. Beginning in the 1940’s and 1950’s, research has been carried out that makes it possible to characterize quantitatively the functional features of an ecosystem (especially the food chains) by way of which the biological transformation of matter and energy proceed. The quantitative expression of the rate and efficiency of these processes by means of modern methods, and particularly by means of mathematical modeling of ecosystems, is an essential starting point for dealing with the urgent questions of the rational use of natural resources and the preservation of man’s environment.
REFERENCESOsnovy lesnoi biogeotsenologii. Edited by V. N. Sukachev and N. V. Dylis. Moscow, 1964.
Duvigneaud, P., and M. Tanghe. Biosfera i mesto v nei cheloveka. (Ekologicheskie sistemy i biosfera), [2nd ed.]. Moscow, 1973. (Translated from French.)
Menshutkin, V. V. Matematicheskoe modelirovanie populiatsii i soobshchestv vodnykh zhivotnykh. Leningrad, 1971.
Odum, E. Osnovy ekologii. Moscow, 1975. (Translated from English.)
G. G. VINBERG