A biotic community can not exist by itself. It lives in an environment which supplies it material and energy. There is always a reaction between the biotic community and its environment. A biotic community together with its abiotic environment forms an ecosystem. Ecosystem therefore includes both the living organisms and the non-living environment. A pond, a meadow and a forest are examples of ecosystem. The term ‘ecosystem’ was coined by the British ecologist A.G. Tansley in the year 1935. An ecosystem may be defined as “a dynamic system where the biotic and abiotic components are constantly acting and reacting upon each other bringing forth structural and functional changes”.
Structural Components of an ecosystem:
Each ecosystem normally consists of four components namely: abiotic substances, the producers, the consumers and the decomposers.
Abiotic substances: The non-living or abiotic substances are inorganic materials such as oxygen, carbon dioxide, nitrogen, calcium, phosphorus and water. These are taken up by the plants. The latter produce food from these basic nutrients and sunlight.
Producers: The green plants and other organisms like green algae and diatoms are unique in the world. They are the only living beings which take up energy from the non-living world and make it available to other living organisms. They are known as producers. They convert light energy of the sun to potential chemical energy in the form of organic compounds. In the presence of sun light they produce carbohydrates from simple inorganic substances like carbon dioxide and water. The process is known as photosynthesis. The producers and sunlight together constitute the essential components of an ecosystem. Producers prepare their own food and are therefore known as autotrophs.
Consumers: They consume the food directly or indirectly made by the producers. They are unable to prepare their own food. As they depend upon others for food, they are also known as heterotrophs. The consumers are of the following types:
1) Primary consumers or herbivores – Herbivorous animals are the primary consumers of the first order. Protozoans, small crustaceans and molluscs are the primary consumers of the pond and feed upon floating algae, insects, rodents and ruminants and the primary consumers in the terrestrial environments. A deer or a rabbit is a primary consumer in the forest and so also rat in a garden.
2) Primary carnivores or secondary consumers - These feed on primary consumers. The examples are wolves, dogs, cats and foxes. In a pond the smaller fishes are primary carnivores feeding on smaller crustaceans.
3) Secondary carnivores or tertiary consumers – These feed upon primary carnivores as well as herbivores. These are lions, tiger and hawks. For example, the lion may prey upon a wolf, a deer. In a pond smaller fishes are eaten by larger predator fishes which are secondary carnivores.
4) Decomposers or Reducers – The microorganisms (bacteria, fungi and moulds) are the decomposers of the ecosystem. They feed on dead and decaying matters and break them into simpler inorganic substances, which pass into the environment. These substances are utilized by the producers for preparation of food.
Thus there is a cyclic exchange of materials between the biotic community and abiotic environment.
Functional aspect of Ecosystem:
The function of the ecosystem is very broad which is specific, interrelated and interdependent. The principal functional aspects of ecosystem are: Food chain and food web and trophic levels, Energy flow, Ecological pyramids and biogeochemical cycles.
The living world subsists on food and therefore food is the primary necessity of a living organism. Food not only contains energy but also materials needed for the organism. In an ecosystem there is flow of energy and circulation of materials among its member. Therefore, the different animals and plants in an ecosystem are linked to each other for food. Individuals thus related form food chain. A food chain consists of a group of organism in which there is transfer of food energy through a series of repeated eating and being eaten up. The different groups in a food chain form a link called food link.
The primary source of energy is sun. The solar energy is trapped by the producers who in turn are fed upon by the primary carnivores or secondary consumers and they in turn are consumed by the secondary carnivores or tertiary consumers.
A simple food chain in the forest will comprise: grass -> deer -> tiger. In a pond chain is phytoplankton -> zooplankton -> small fish -> large fish. In the grass land the food chain is: grass -> grasshopper -> frog -> snake -> hawk.
The number of steps in a food chain is limited to four or five. At each step, a large portion of potential energy is lost as heat. Hence the food chain with fewer numbers of links or steps will prove to be more efficient.
In nature each food level has usually more than one food organism. Numerous food chains can therefore exist in a biotic community. Many of these chains are interconnected by species which occur in more than one chain. For example, man and many other organisms are herbivores as well as carnivores. A herbivore may consume many types of plants and plant products. For example a rat feeds on various kinds of stems, roots, fruits and grains. There may be several carnivores with over lapping type of prey. These interconnected food chain forming a network is known as food web.
Various communities showing similar mechanism of food getting are said to belong to same trophic level. In a food chain, each link is known as a trophic level. The plants are the producers and constitute the first trophic level. The herbivores or the primary consumers constitute the second trophic level. Similarly the primary carnivores constitute third trophic level whereas secondary carnivores constitute the fourth trophic level in an ecosystem. In a community, there are rarely more than four or five trophic levels in a food chain. However it should be remembered that classification of organisms by trophic levels is one of the functions and not of species as such. A given species may occupy more than one trophic level. All organisms which occupy a trophic level belong to that trophic level regardless to their taxonomic position. Thus a grasshopper, deer and grain eating bird belong to second trophic level as all of them derive their energy directly from plants.
All the energy available from sun is not fixed by the plants. Only a small portion of solar energy is absorbed by the green plants for photosynthesis and is converted into potential chemical or food energy. This energy flows in one direction through various trophic levels. Some of the energy is lost in the form of heat energy during respiration in the successive trophic levels. i.e. energy is lost to the environment at each food transfer. So the production of biomass plus respiration of all organisms of any one trophic level is the energy flow through that level. In this way a continuous flow of energy in the ecosystem takes place and it works according to laws of thermodynamics. The first law states that the energy is neither created nor destroyed and may be converted from one form to another. According to second law, there can be no transformation of energy unless the energy change is from a concentrated to a dispersed form.
The energy flow at each trophic level is less than that at the preceding level. It has been estimated that 80% to 90% of potential energy is lost as heat with each transformation. In other words, much of the energy loss at each level is through respiration and all the potential energy is not assimilated. The conversion of one kind of energy into another is never 100% efficient. Therefore, if the food chain is shorter, then the availability of food energy to the organisms will be greater.
Lindeman suggested a 10% transfer of energy from one trophical level the succeeding trophic level, which is termed as 10% rule.
The trophic structure and trophic function of an ecosystem can be shown graphically by means of ecological pyramids. In this pyramid, the first trophic level forms the base and the successive levels forms different tiers, giving the shape of a pyramid. In such a pyramid, the producers are individually more numerous than the consumers and these in turn are associated with fewer secondary and tertiary consumers.
Ecological pyramids are of three general types: Pyramid of numbers, Pyramid of biomass and Pyramid of energy.
1. Pyramid of numbers – They show the relationship between producers, herbivores and carnivores in successive trophic levels in terms of number. The pyramid of numbers is found in animal communities of the world.
It indicates the numerical relationship between the different trophic levels in a food chain. This kind of pyramid can well be illustrated by taking a lake as an example. In this ecosystem the lowest trophic level is occupied by producer (phytoplankton). The second trophic level is represented by zooplanktons which are primary consumers and are less abundant. The third trophic level is occupied by a still smaller number of small fish and the apex is formed by large fishes which are least in number. Thus in a lake ecosystem the pyramid is upright. In pyramid of numbers, there is a progressive decrease in numbers and a corresponding increase in the size of the body.
However, in a parasitic food chain the pyramids are always inverted. This is due to the fact that a single tree may support a large number of herbivorous birds which in turn support a greater number of parasites like bugs and lice.
2. Pyramids of biomass - Biomass may be defined as the total weight of living matter present at any time in the ecosystem. This pyramid indicates gradual decrease or reduction in the biomass of organism at each trophic level from the base to the apex. The biomass of the producer is the greatest.
3. Pyramid of energy – The pyramids of energy give the best picture of the overall nature of the ecosystem. This pyramid indicates the amount of energy flow at each trophic level in the ecosystem and also the role of different organisms in the transfer of energy. At each trophic level there is loss of energy and material as assimilation and growth are not 100 percent efficient. Greater amount of energy is available at the producer level than at the primary consumer level. It gradually diminishes becoming less in the secondary consumer level and reduced to minimum in the tertiary consumer level.
The chemical elements are circulated in a cyclic order from the environment to the organisms and again back to the environment. This is known as biogeochemical cycle. This is also termed as nutrient cycle. Some of the elements like carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus are required by the living organisms in large quantities but others like iron, copper, zinc, cobalt and sodium are necessary in minute quantities. As most of these elements are essential for protoplasm, they are known as nutrients. The former required in large quantities are known as macronutrients and the latter in minute quantities are known as micronutrients. These elements either remain in the free state or in the form of compounds in the protoplasm. A sort of homeostasis is maintained in part within the ecosystem, because the elements are constantly being recycled. Homeostasis means a state of equilibrium maintained by the factors of the environment which resists change in the system as a whole.
The biogeochemical cycle may be either gaseous cycle like oxygen, carbon dioxide and nitrogen or sedimentary cycle like sulphur, phosphorus etc. Water is also circulated in a cyclic order in the nature. Presently three cycles such as carbon cycle, nitrogen cycle and oxygen cycles are described showing the general pattern of recycling of elements in the ecosystems.
1. The carbon cycle – The carbon is an essential constitute of all living materials. The carbon may be present in various forms such as carbon dioxide in the air or incorporated in the organic compounds in plants or animals. Chiefly carbon is present as atmospheric carbon dioxide which is produced due to respiration, combustion of wood, coal and its products like coke and coal gas.
The atmospheric carbon dioxide is used by green plants during photosynthesis and carbon gets incorporated in organic compounds of protoplasm. When plants are eaten by the animals, the organic matter is digested and built into organic compounds making the animal tissues. Again after the death of the animals, the CO2 is released into the atmosphere by the action of decaying bacteria. Thus the carbon returns to the environment and the cycle is repeated.
2. The nitrogen cycle – Nitrogen is also an essential element and takes part in the formation of protein, nucleic acid and other inorganic compounds. The nitrogen is chemically inert and is not directly utilized by the organism as such. But it is utilized in the form of nitrite or nitrate. Nitrogen in the form of compounds is present in the soil. For example, ammonia which is water soluble is produced by the decomposition of tissue of dead plants or animals.
This ammonia undergoes nitrification by nitrifying bacteria which are of two types: nitrite bacteria and nitrate bacteria. The nitrite bacteria get energy by oxidizing the ammonium compounds to nitrites. But the nitrate bacteria further oxidize nitrites to nitrates. These nitrogenous compounds are absorbed by the plants and take part in the formation of proteins. The plants are eaten by the animals and are converted into animal proteins finally.
There are also some denitrifying bacteria in the soil which get energy by breaking down nitrogenous compounds to gaseous nitrogen which pass into the atmosphere.
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