Any complete definition of an ecosystem includes the physical environment as well as the biological components and the interaction between the two. The biological or biotic components of an ecosystem include:
i) Organisms, basically green plants, certain bacterial and algae that can synthesize their own food in the presence of sunlight. These are the autotrophy or producers.
ii) All other organisms that do not make their own food but depend on other organisms to obtain their energy for survival. These are called heterotrophs or consumers.
Among consumers, some animals such as goat, cow, deer, rabbit and insects which eat green plants are called primary consumers or herbivores. Organisms which eat a herbivore, like a frog that eats grasshoppers are called secondary consumers. While the primary consumers are herbivores, the secondary and tertiary consumers are carnivores. Animals like lions and vultures which are not killed or eaten by other animals are top carnivores.
Amongst the producers also there is division. Only the green plants and some special types of bacterial, which can trap solar energy and produce food, are called primary producers, the heterotrophs which are food for other animals become secondary producers.
Consequently an ecosystem is considered as a basic unit, where complex natural community obtain their food from plans through one, two, three or four steps and accordingly these steps are known as the first, second, third and fourth trophic levels or food levels such as:
I. Green plants (producers); trophic level I
II. Herbivores (primary consumers); trophic level II
III. Carnivores (secondary consumers); trophic level III
IV. Top carnivores (tertiary consumers); trophic level IV
Both the consumers and producers complete their life cycles and new generations of population develop while the old ones die. You must be wondering what happens to the dead. There is a continuous breaking up or decomposition of the dead organic matter everywhere in the ecosystem. There is a continuous cycling of materials. Certain fungi and bacteria, which are responsible for the decomposition, are called decomposers or reducers. The role of decomposers is very special and important. Certain decomposers are also called scavengers. Water, carbon dioxide, phosphates and a number of organic compounds are largely the by-products of organism activity or death of organisms.
The other important components of the ecosystem are abiotic or nonliving components. These are basically inorganic elements and compounds, such as carbon dioxide and water, nitrogen, phosphates, sulphates etc., and a number of organic compounds largely the by-products of organism activity or death. Other important abiotic components of the ecosystem are the physical factors including temperature, moisture, solar radiation, etc. It is in this abiotic background that biotic organisms i.e. plants, animals and microbes interact.