Energy flow

  • Energy is the ability to do work. The main source of energy for an ecosystem is the radiant energy or light energy derived from the sun. 50% of the total solar radiation that falls on earth is photosynthetically active radiation (PAR). The amount of solar radiation reaching the surface of the earth is 2 cals/ It is more or less constant and is called solar constant or solar flux. About 95 to 99% of the energy is lost by reflection. The light energy is converted into chemical energy in the form of sugar by photosynthesis.
  • The rate of energy transfer between elements of an ecological system is called energy flow. The flow of energy is unidirectional in the ecosystem.
  • Plants utilize 2-10% of PAR in photosynthesis.
  • The energy formed by the green plants (producers)then flows through different trophic levels of ecosystem I.e., primary, secondary and tertiary consumers.
  • Producers include green plants which are capable of manufacturing their own food. These are able to fix the energy obtained from the sun. Producers are autotrophic, generally chlorophyll bearing organisms.
  • Consumers (phagotrophs) cannot make their own food but are directly or indirectly dependent on producers for obtaining food. Consumers may be
    1. Primary consumers or herbivores.
    2. Secondary consumers or primary carnivores.
    3. Tertiary consumers or secondary carnivores.


  • Primary consumers obtain their food by directly feeding on producers (plants), secondary consumers from primary consumers (herbivores) and tertiary consumers from secondary consumers.
  • The conversion of radiant energy of sun into chemical energy and its subsequent transfer to other organisms occurs in accordance with the laws of thermodynamics.
  • First laws of thermodynamics: states that energy is one component to another E.g. sunlight energy can be transformed into energy of food & heat.
  • Second law of thermodynamics: At each step of energy transformation, there occurs dissipation of energy and increase in disorderliness.
  • Trophic structures of ecosystem is a type of producer-consumer arrangements in which each food level is called trophic levels.
  • All trophic levels in an ecosystem are connected by transfer of food or energy.
  • Two aspects with respect to energy flow in ecosystem  are important. First, the energy flows unidirectional I.e. from producers through herbivores to carnivores; it cannot be transferred in the reverse direction. Second, the amount of energy flow decreases with successive trophic levels. Producers capture only a small fraction of solar energy (1-5 percent to total solar radiation), and the bulk of initialized energy is dissipated mostly as heat.Part of the energy captured in gross production of producers used and for providing food to herbivores. The unutilized net primary production is ultimately converted to detritus, which serve as energy source to decomposers. Thus, energy actually used by the herbivore trophic level is only a small fraction  of energy captured at the producers levels.
  • A large amount of energy is lost at each trophic level. It is estimated that 90% of the energy is lost when it is transferred from one trophic level to another. Hence, the amount of energy available decrease from step to step. Only about 10% of the biomass is transferred form one trophic level to the next one is a food chain. And only about 10% chemical energy is retained at each trophic may get a large amount of energy. But when the food chain is long, the final consumer may get a lesser amount of energy. But when the food chain is long, the final consumer may get a lesser amount of energy.