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All living beings require a constant supply of energy for the various functions they perform. The energy is obtained from the sun. This energy is introduced into the biosphere by green plants by a process known as photosynthesis. Plants are able to trap this light energy and by the process of photosynthesis convert it into chemical energy. Thus the initial point of entry of energy in the ecosystem is through the green plants (autotrophs or producers). It has been estimated that on an average, about one per cent of total sun's energy reaching the earth is trapped by green plants during the process of food manufacture (i.e., photosynthesis). The energy thus trapped by plants is stored as carbohydrates. Some of the energy trapped by plants is used by them in performing their metabolic activities like respiration, growth, etc., and some energy is released in the atmosphere as heat. When the plants are eaten by herbivorous animals, the energy stored in plants gets transferred to the consumer animals. These animals utilize this energy for their own metabolic activities. Here again some of the energy is released as heat. When primary consumers are eaten by secondary consumers, the same process is repeated at every step of the food chain. Here it must be remembered, that energy released as heat is considered as energy lost.
We can thus conclude the following:
- Conversion of energy takes place from one form to another. The light energy gets converted into chemical energy by plants during photosynthesis.
- Plants act as converters of energy. They do not produce energy.
- The energy lost as heat is quite substantial, if taken together from various trophic levels
- During the transfer of energy through successive trophic levels in an ecosystem, there is a lost of energy at each trophic level. In other words, the level following the previous one always receives less amount of energy. After studying various food chains and energy transfers in different ecosystems, Lindemann in 1942 gave the generalization called "The 10 per cent law". It shows that about 90% of the energy is used up at each level and only 10% of it is transferred to the next trophic level. As a result, at the last trophic level (decomposer), no energy is left for recycling. This is how the flow of energy is unidirectional. Fast decreasing energy level at each step sets the limit of trophic levels only to 4-5 at the maximum. Therefore, the plants (producers) receive the maximum energy and as you go further down the trophic levels, the energy in the food goes on decreasing.
- The flow of energy is unidirectional. It enters into the living system from non-living environment
- The energy lost at heat cannot be utilised
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