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2. Ecosystem

The term ecosystem was first proposed by Tansely in 1935, but similar concepts have been known earlier e.g. “Microcosm” of Forbes (1887). According to E.P. Odum (1959), an ecosystem is “any area of nature that includes living organisms and non-living substances interacting to produce an exchange of materials between the living and non-living parts.” Functionally four components can be recognized in the ecosystem, namely : (i) abiotic substances - these are the basic inorganic and organic compounds in the environment (ii) producers, autotrophic organism; mainly green plants which produce food from inorganic substances (iii) donsumers (also called macroconsumers), heterotrophic organisms mainly animals which ingest other organisms or depend on ‘producers’ for food, and (iv) decomposers (also known as micro-consumers or saprophytes), heterotrophic organisms, mainly bacteria and fungi, which break down complex compounds of dead protoplasm, absorb some of the decomposition products and release substances usable by producers.

Ecosystem is the basic functional unit in ecology. A lake or a fish pond is a good example of an ecosystem, which includes biotic communities interacting with the abiotic environment. In the fish pond the major biotic communities of our interest are the fishes and those in the food web (see later discussion) of the fishes under culture and their abiotic environment is consciously regulated by man making the best use of the natural principle of biological production.

Homeostasis of the ecosystem: This can be roughly equated to the “Balance of Nature”, remniscent of course of the homeostasis at the organismal level, a concept in physiology first explained by Cannon (Wisdom of the body, 1932).

Homeostasis is the process of maintaining an equilibrium between organisms in the community and the environment by “factors which resist change as a whole”. The homeostatic mechanisms include those which regulate the storage and release of nutrients as well as those which regulate growth of organisms and the production and decomposition of organic substances. The role of “ectocrine” substances (exudates/excretions of organisms) in co-ordinating units of ecosystem may also be important. The “constancy of the ecosystem” is achieved only by means of maintainance of dynamic equilibrium and not static.

For example an environmental factor such as pH or DO, or again one unit such as the population of one species in the ecosystem, would fluctuate widely, but by means of feed-backs the condition would be often brought back to the equilibrium (‘normal’) level, except in case when the system mechanics are grossly violated say by entry of pollutants or other interferences. The homeostasis of a water body for purifying itself can be clearly known from the events which follow the entry of treated sewage in a stream or a pond. The resilience of the system is evident from the fact that the stream returns to normal condition a few miles downstream. In the case of pond, time and volume relations are important.

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