Flow of energy in an ecosystem is one way process. The sequence of organism through which the energy flows, is known as food chain.
Plants are eaten by insects, insects are eaten by frogs, the frogs are eaten by fish and fishes are eaten by humans. The pattern of eating and being eaten forms a linear chain called food chain. Such a food chain can always be traced back to the producers.
The primary producers trap radiant energy of the sun and transfers to chemical or potential energy of organic compounds like proteins, fats and carbohydrates. The formation of ATP during photosynthesis is the first nutritional level. ATP is stored in food matter which is utilized by herbivores, the plant eaters. This process constitutes the second tropic level. When a herbivores animal eats the plant, the organic compounds re oxidized and the energy is liberated. S >me of the energy is produced as heat which is not a useful energy. Flow of energy is greatly reduced at each successive level of nutrition because of the energy utilization by the organisms and heat loses at each step in transformation of energy. This accounts for the decrease in biomass at each successive level.
It should be noted that the number of steps in a food chain are always restricted to maximum four or five steps. Humans are at the end of a number of food chains.
There are three types of food chain in the nature.
The consumers (which starts the food chain) utilizing the plant as their food, constitute grazing food chain. This food chain begins from green plants and the primary consumer is herbivore. Most of the ecosystem in nature follows this type of food chain. From energy view point such chains are very important. For example:
Grass -> Grasshopper -> Birds -> Falcon.
The organic wastes exudates, and dead matter derived from grazing food chain are termed as detritus. This type of food chain starts from dead organic matter of decaying animals and plant bodies to the micro-organisms and then to detritus feeding organism and to other predators.
The food chain depends mainly on the influx of organic matter produced in another system. It represents an important component in the energy flow of an ecosystem. The organism of the food chain includes algae, bacteria, fungi, protozoa, insects, nematodes etc.
In a community of organisms in a shallow sea, about 30% of the total energy flows via detritus food chain. However, in a forest with large biomass of plants and a relatively small biomass of animals even larger portion of energy flow may be obtained via detritus pathways.
The knowledge of food chain helps in understanding the feeding relationship as well as the interaction between organism and ecosystem. Besides it's also help in understanding the mechanism of energy flow and circulation of matter in ecosystem. In addition to these, it also helps to understand the movement of toxic substance and the problem associated with biological magnification in the ecosystem.
Food chains are not always as simple as it looks. Quite often several different species of food organism.
Food Web :
A food chain represents only one part of energy flow through an ecosystem, whereas the ecosystem may consist of several interrelated food chains. But for simplicity, in general, a food chain implies a simple isolated relationship which rarely occurs in ecosystems. The same food resource may be a part of more than one chain. This is possible when the resource is at the lower tropic level. Accordingly the inter connected networks of feeding relationships is known as food webs. Most animals in nature utilize more than one species for their food. Therefore, food chains in an ecosystem become interconnected with each other which are summarized in Fig.6.
Several different trophic levels are recognised in any complex food webs. This is mentioned as under:
An example of food web may consist of as many as food chain.
(i) Grass -> Grasshopper -> Hawk
(ii) Grass -> Grasshopper-> Lizard -> Hawk
(iii) Grass -> Rabbit ->Hawk
(iv) Grass -> Mouse -> Hawk
(v) Grass -> Mouse -> Snake -> Hawk