The biosphere forms a thin crust of living beings over the surface of our planet and consists of an immense variety of organisms out of which only about 1.5 million species have been identified and described.
The existence and well being of living organisms depend on a system of complex interactions within and in between the components of environment – the lithosphere, hydrosphere, atmosphere and the biosphere itself. It is these interactions which satisfy all needs of all living organisms, such as shelter, water, oxygen to respire, mates to reproduce etc. – essential for sustained life and continuation of species.
(1) Ecosystem:
The complex system, in which interactions between the different components of environment occur, is referred to as an ecosystem. To be more precise, any spatial or organizational unit which includes the living and the non-living constituents interacting with each other and producing an exchange of materials between the two is termed as an ecosystem.
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All life on this planet depends on photosynthesis carried out by green plants. The energy of sun is captured and converted to chemical energy which gets locked in chemical bonds between the constituent atoms of organic molecules. The only other process which contributes some organic matter to the living world is chemosynthesis, carried on by some bacteria which are capable of utilizing the chemical energy of inorganic molecules.
A community of plants depends on sun for light energy, on the soil for mineral nutrients and water, while atmosphere provides carbon dioxide. It is on the organic matter produced by green plants that herbivores live, grow and build up animal communities. Carnivores depend for their food supply on the herbivores. Man an omnivorous animal depends both on plants and animals.
(2) Structure of an Ecosystem:
The non-living or abiotic constituents of an ecosystem include mineral nutrients, temperature, light, water, air etc., which surround, influence and shape the living or biotic component. A large number of individuals belonging to different species which adjust, adopt, interact with each other and share the same general environment and resources form a biotic community or the biosphere. Based on the function and the general manner in which organisms obtain their food material, individuals within a biotic community can be grouped into:
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(a) Producers:
The community of green plants, called primary producers, absorbs carbon dioxide, mineral nutrients, and water and built up organic matter with the help of solar energy, releasing oxygen in the process (PP in Fig. 1.9). Without producers life activity in the system shall collapse or else the system shall have to run on organic material (energy) imported from other systems. Minerals nutrients enter the biosphere through green plants.
(b) Consumers:
Producers are consumed by herbivorous animals, the primary consumers, which are in turn consumed by carnivorous animals, the secondary consumers and so or (PC, SC, TC in Fig. 1.9). Thus a chain of organisms based on trophic relationship is established which is known as Food chain. The energy trapped by green plants is released to be used by consumers when organic matter is degraded and oxidized. Oxygen is used and carbon dioxide is released in the process. In a complicated ecosystem where each trophic level consists of a number of species, there may be several interlinked food chains and the trophic structure assumes the shape of a complicated food web.
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(c) Decomposers:
Faecal matter, exudates and excreta of plants, animals and their dead bodies are decomposed by the activity of bacteria, fungi and other small organisms which live on dead and decaying organic matter. These constitute the community of decomposers which bring the constituent elements of the plant and animal bodies back to the surrounding medium or to the soil.
A portion of land, a lake or a river can be visualized as an ecosystem in which plants produce organic matter which is consumed by herbivores. Herbivores are in turn eaten by carnivores while decomposers mineralize the exudates and remains of the living organisms. Abysmal depth of oceans, where no light penetrates, however, represents a different type of ecosystem.
There are no plants and hence there is no primary production. The organic matter or energy to support the living organisms in the system is obtained by dead remains, exudates and debris which sink down from above. Likewise our cities are man-made ecosystems, in which all life activity depends on the food or energy imported from the countryside. Such systems are referred to as incomplete systems.
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(3) Flow of Energy and Bio-Geo-Chemical Cycles :
It is the organic matter produced by green plants or in other words energy trapped by autotrophs which are passed on through a chain of consumer’s right up to the top of trophic structure in an ecosystem. At each step in the food chain respiratory activity and decomposition of dead organic remains dissipate a large amount of solar energy. The successive trophic levels are thus smaller and smaller in terms of energy content, biomass and usually numbers as well. The energy captured by green plants also supports the life activity of decomposers and is finally scattered in the environment. The flow of energy is, therefore, unidirectional.
Apart from energy, synthesis of organic matter requires mineral nutrients also which are the constituent elements of the organic world. These are taken up from the surrounding environment. These elements enter the biosphere at the initial step of trophic structure, through primary producers.
From primary producers they are transferred from one organism to another along the food chain to reach the ultimate consumer. Much is lost in the process as exudates, excreta and faecal matter which return to the surrounding medium.
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Finally when the living organisms die, decay and decomposition releases rest of these elements in the atmosphere from where they were drawn earlier. The entire process can be visualized as a series of reservoirs or pools through which an element moves in a cyclic fashion in an energy driven stream, finally coming to rest in its original pool or reservoir.
All elements which form the basic structural and functional components of a living system circulate in the environment in characteristic channels from one reservoir to another and constitute cycles which are known as bio-geo-chemical cycles.
For example, the carbon of atmospheric pool of carbon dioxide may form a part of green plant at one time, the limb of an herbivore at another and the delicate heart of a child at still another time. Thus the same atom may journey through a series of living organisms or pools but it comes to rest in the vast atmospheric reservoir as carbon dioxide again.
(4) State of Dynamic Equilibrium and System Homeostasis:
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In an undisturbed natural ecosystem all components are in a balanced state. The inputs in each pool are balanced by an equal output and the system remains in a state of dynamic equilibrium. If the reservoir constituted by primary producers grows beyond a certain optimal level, i.e., input increases, the population of herbivores rises which consumes the producers.
Thus, with rise in input, output also increases and the pool size returns to its normal dimensions. Likewise, when herbivores become numerous, predator population rises to regulate the population of herbivores. When the ultimate consumers become numerous their prey declines in number restricting the growth of the predator population.
Thus at each level in the trophic structure any fluctuation in the rate of input is countered by an equal and opposite alteration in the output rates. This tendency of self-regulation which involves resistance to any change and stay in a state of dynamic equilibrium is referred to as System Homeostasis. All ecosystems have the ability to perform homeostatic adjustments within certain limits.
It is important to recognize the limits of tolerance or self-regulatory capacities of an ecosystem. Smaller stresses put on the system could be absorbed at the same level in the trophic structure but larger disturbances could lead to a chain of events involving larger part or the entire trophic structure. Too much strain could cause irreparable damage to an ecosystem.
(5) Diversity, Productivity and Stability in an Ecosystem:
All life has a common bio-chemical basis. There is a certain range of environmental conditions under which life processes are carried on with greatest ease and efficiency. This does not mean that all plants and animals require similar conditions of environment but it does imply that within this range a large number of plants and animal species can live and multiply.
Thus more species are likely to occur between 27°C to 32°C than at freezing temperatures. Of course some organisms may be well adapted to life in extremely cold conditions but the natural selection for this will be rigorous and the number of such species shall be few. The severity or harshness of environment acts as a limiting factor for the variety in species composition.
The Arctic and Antarctic regions, snow-clad mountains, deserts, ocean depths etc. possess little diversity in their species composition. In tropical and sub-tropical regions a large number of species thrive together because of milder conditions (Birch 1957).
An important consequence of increased complexity of trophic structure is the comparative stability of the ecosystem. Simple trophic structures are more vulnerable to catastrophic changes as compared to complicated ones which have several alternative interlinked channels for the flow of energy and materials. Elimination of one or a few species should not be disastrous to the system as some other alternative is always available to the trophic level above to meet its requirements. At the most organisms obligately dependent on the species eliminated, shall suffer but basic functions of the ecosystem, i.e., energy flow and the bio-geo-chemical cycling of materials etc. shall continue although along other channels. The diversity in species composition of the biotic community within an ecosystem enhances its stability. (Elton 1958, May 1973).
However, with an increased diversity and complexity, the energy requirement for the maintenance of the system usually goes up and productivity declines. As ecosystem enlarges, with several trophic levels and greater diversity at each level, the amount of gross production which has to be oxidized or respired to sustain the trophic structure also rises while the part which is added up to bring about further growth diminishes.
A stage is ultimately reached when the energy input in maintenance of the system becomes so high that nothing is left to go into further increment and growth. The system attains a steady state. The amount of biomass which can be sustained under steady state condition is termed as the Optimum carrying capacity. Production efficiency, therefore, is inversely related to system complexity and diversity. Simpler and shorter trophic structures have greater production efficiency.
(6) Circulation of Non-Essential and Toxic Substances :
Several non-essential and toxic materials which have no known biological function may also be taken up by primary producers and passed on to higher trophic levels along with the essential ones. These elements may enter the food chain with essential elements due to their chemical affinity or they are simply carried along and taken up in the general energy driven stream (Odum 1975).
Although many organisms have developed adaptive mechanisms to exclude harmful substances, there is no means available to the biological membrane, through which all substances entering a living system must pass, to function efficiently and yet exclude all unwanted and harmful substances present in the surrounding medium. Toxic heavy metals, poisonous pesticides and radio-active isotopes etc. thus gain entry into the biosphere and are accumulated, their concentration magnified and passed on to higher trophic levels.