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A food chain is a series of steps by which energy is obtained, used, and transformed by living things. For example; sunlight helps plants to grow, the plants are eaten by cattle, and lions eat the cattle.
All living organisms of the biosphere are related to each other by a common factor, i.e., food, which contains not only energy but also materials in usable forms that are needed by the organisms.
We know that green plants manufacture their own food with the help of sun's energy and from common elements derived from air, water and soil. These green plants are, therefore, called the autotrophs. The autotrophs are the chief source of potential energy for the living world. Hence they are called the producers. When the producers are eaten by some animal, the energy of the producers is passed on to the animal which is called consumer. The primary consumer is eaten by another animal which is called the secondary consumer which may be eaten by a tertiary consumer and so on. This pathway of energy transfer from one organism to another constitutes a food chain. For example, in a forest community, grass is eaten by a deer which, in turn, is eaten by a lion. This flow of energy from grass (producer) to deer (primary consumer) and then to lion (secondary consumer) is called a food chain.
The shorter the food chain, the greater is the available energy. A simple generalized food chain may be represented as shown below.
A food chain always begins with the producers, i.e., green plants. Next in the chain is always the plant eater or the herbivore which is called the primary consumer. The primary consumers are eaten by flesh eaters - the secondary and tertiary consumers. Certain food chains may be very long and may extend to fourth, fifth or even higher order consumers.
Food Chains in Aquatic Ecosystems
In aquatic ecosystems of the biosphere, like fresh water ponds, lakes or sea, the food chain starts with microscopic free floating plants (phytoplankton).
Here, again there may be many trophic levels depending upon the type of organisms present.
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