All the organisms (plants and animals) depend on the sun for their constant need of energy, and upon earth for the materials which enter into their body. We will now describe how energy received from the sun flows in the various trophic levels of an ecosystem in the form of chemical energy of food.
The food chain in a community actually represents a stepwise transfer of food and the energy contained in food. The food and energy enter the living components of the ecosystem through the process of photosynthesis.
This is because photosynthesis is a process which combines the substances like carbon dioxide, water and sunlight energy to form food like carbohydrates and converts light energy of the sun into chemical energy of carbohydrates. This food and energy is then transferred from the producer organisms to herbivores and from herbivores to carnivores, through the food chain. Let us discuss this flow of energy in detail.
The green plants (or producers) have a mechanism for trapping solar energy (sun’s energy) with the help of their green pigment called chlorophyll. The green plants after trapping the solar energy, convert it into chemical energy which is stored as carbohydrates in the plants.
Thus, the initial point where energy from the environment enters into the living components of ecosystem (like plants and animals) is the process of preparation of food by green plants through photosynthesis.
On an average, about 1% of the sun’s energy falling on the leaves is used by the plants in the process of photosynthesis and stored as chemical energy of food. The plants utilise the energy stored in them for their metabolic activities like respiration and growth (tissue building). Some of the energy is, however, not utilized and it is released as unusable heat into the community environment.
The plants (or producers) are eaten up by herbivores. The chemical energy stored in plant food is transferred with food to herbivores. The herbivores utilize this energy for their various metabolic activities like respiration and also for their growth. Some of the energy, however, remains unutilized which is released by the herbivores as heat energy to the environment.
The herbivores are eaten up or consumed by carnivores. The chemical energy stored in the flesh of herbivores is transferred with food (or flesh) to the carnivores. The carnivores utilize this energy for their various metabolic activities like respiration and also for their growth.
Some of the energy, however, remains unutilized by the carnivores and it is released as heat energy into the environment. This process of the transfer of energy is repeated with large carnivores or top carnivores (who eat small carnivores), and so on.
It should be noted that some of the energy from producers and consumers (like plants, herbivores and carnivores) is also utilized for the life processes of micro-organisms called decomposers.
The decomposers, in turn, release the unutilized energy as heat into the environment. It is obvious from the above discussion that the energy which remains unutilized by producers, consumers (herbivores and carnivores) and decomposers is lost into the environment as heat. It is called community heat.