Energy Flow in the Ecosystem

The principal source of  energy for any ecosystem is sunlight. All food materials that we or other animals consume are manufactured directly or indirectly by plants. Think of your breakfast, bread  is made of a cereal that  is produced from plant material, egg from hen which has fed on plant products; and milk from cow which has consumed grass or fodder derived from plants. In a nutshell, the energy that we obtain from plants either by  burning wood or by  eating them, represents the solar energy trapped by  the plants. We are dependent on the stored resources of solar energy. When we eat meat, we obtain energy that had been stored by plants several years before and then  taken up by an  animal like a goat through grazing. When we cut firewood for fuel, we obtain energy accumulated and stored by trees for perhaps a century or more. When we bum coal or petroleum, we obtain solar energy stored by  plant life, millions of years ago. Now  let us  trace the energy flow through an ecosystem. This is represented diagramatically in Fig.  

Notice that the energy from the producer to the last level has decreased considerably. The energy passed on to the next trophic level is represented by  letter E of  three different sizes, each by  its size indicates the comparative  amount of  energy. The arrows in the upper half of the diagram indicate  the loss of energy in the form of heat given out by different organisms. The arrows in the lower half  show the energy loss via the wastes secreted and energy trapped in dead bodies of  the organisms. Decomposers use these wastes and dead bodies as sources of  food and derive energy from them. They also give out some unused heat energy. Energy flowing through an ecosystem is thus first captured by  the producers. From the producers the energy passes to various consumers, via food. At  the end of the chain we  find that very  little energy is left for the last trophic level, because some energy is always lost in going from one level to the next. This loss of energy limits the number of trophic levelsin the ecosystem,  and so they are seldom more than five. This also clearly shows  that why  the steps  in a food chain are limited to four or five.