The sun is the only source of energy for most ecosystems on this earth. Of the sun’s incident radiation, less than fifty per cent is Photosynthetically Active Radiation, which is the spectral range of solar radiation that can be used by green plants for photosynthesis. However, in reality, plants utilise only about two to ten per cent of PAR and, incredibly, this small amount of energy sustains the whole world. In all ecosystems, the solar energy tapped by plants is converted into chemical energy and is passed on to all other organisms. Hence, there is a unidirectional flow of energy from the sun to producers and then to consumers.
In nature, there are two types of food chains – Grazing Food Chain and Detritus Food Chain. Most ecosystems in nature follow the GFC. In a GFC, the energy flows from producers to grazing herbivores and finally to carnivores.
The second type of food chain, that is the DFC, begins with detritus, which forms the primary source of energy. This detritus is broken down into simple, inorganic materials by digestive enzymes secreted by decomposers or saprophytes and is then absorbed by them to meet their nutrient requirements. Detritivores or decomposers may be consumed by detritivore consumers, which in turn, may be preyed on by small or large carnivores. An interesting fact about GFC and DFC is that they may be connected at certain levels. This happens when an organism belonging to the GFC dies and enters the DFC. Such interconnection of food chains leads to the formation of a food web. Now, all organisms in a food chain occupy a specific place in the ecosystem depending on their source of food and nutrition, which is known as their trophic level. Producers belong to the first trophic level, followed by herbivores and finally carnivores. In addition, each trophic level has a certain mass of living material at a particular time known as standing crop. It is measured as the biomass and is expressed either as fresh or dry weight. However, the number of trophic levels in a GFC is restricted and is anywhere between three and five. This is because the transfer of energy follows a ten-per cent law, that is, on average, only ten per cent of energy is transferred from one trophic level to another while the rest is used up for metabolic activities of individuals or is lost as heat. Moreover, all ecosystems abide by the First and Second Laws of Thermodynamics. This is because energy is being converted from one form to another as it is transferred from the sun to various organisms. In addition, the biosphere is continuously losing usable energy in the form of heat, thereby requiring the continuous supply of energy to organisms for the synthesis of molecules to resist the natural tendency of increasing disorderliness. Hence, we have seen that there is a continuous supply and flow of energy in all ecosystems and this is vital for the sustenance of our biosphere.