Flow Of Energy

Each organism needs energy to carry on vital activities, and for building up and repairing the body tissues.

•  The ultimate source of entire energy, used by living organisms, is the sun.
•  Of the total solar radiations falling on the earth, only about 1% are captured by green plants in a terrestrial ecosystem and converted into food energy by photosynthesis.
• When green plants are eaten by herbivores (primary consumers), a great deal of energy is lost as heat to the environment in accordance with the second law of thermodynamics. Some amount of energy is used in doing work and the remaining goes towards growth and reproduction. On an average, 10% of the food eaten is turned into body of herbivores and is made available for the next level of consumers. In other words, on an average, 10% of the amount of organic matter that is present at each step reaches the next consumer level (Ten percent law).

• Since, amount of available energy goes on decreasing at each trophic level, food chains usually consist of only 3 or 4 steps and rarely maximum of 5 steps.
•  In an ecosystem, generally, the producers are maximum in number. As we move along the chain, number of individuals at each trophic level decreases.

From the energy flow diagram two things become clear. Firstly, the flow of energy is unidirectional. The energy that is captured by the autotrophs does not revert back to the solar input and the energy which passes to the herbivores does not come back to autotrophs. As it moves progressively through the various trophic levels it is no longer available to the previous level.

Another interesting aspect of food chain is how some harmful chemicals enter our bodies through the food chain. In water pollution, one of the reasons is the use of several pesticides and other chemicals to protect our crops from diseases and pests. These chemicals are either washed down into the soil or into the water bodies. From the soil, these are absorbed by the plants along with water and minerals, and from the water bodies these are taken up by aquatic plants and animals. This is one of the ways in which they enter the food chain. As these chemicals are not bio-degradable, these get accumulated progressively at each tropic level. As human beings occupy the top level in any food chain, the maximum concentrations of these chemicals get accumulated in our bodies. This phenomenon is known asbiological magnification. This is the reason why our food grains such as wheat and rice, vegetables and fruits, and even meat, contain varying amounts of pesticide residues. They cannot always be removed by washing or by other means.

Bio-magnification