ENERGY FLOW IN AN ECOSYSTEM
Energy is the capacity to do work. Energy that runs ecosystems ultimately comes from the sun. Green plants trap solar energy through photosynthesis; convert it into chemical energy and store it in the form of chemical energy. The conservation and expenditure of energy is described by the two laws of thermodynamics. The first law states that energy can neither be created nor destroyed but only be converted from one form to another and the other law states that when energy is converted, there is a loss of energy in the form of heat.
In an ecosystem when the energy is transferred from one trophic level to the other, only a part of it is utilized and the rest is wasted or dissipated (in accordance with the second law of thermodynamics).
The energy production in the ecosystem is categorized into primary production and secondary production.
Primary production: Green plants are capable of fixing only 1.5% of the solar energy reaching the earth. This level is called primary trophic level. The plant tissues convert this solar (radiant) energy to chemical energy.
6CO2 + 6H2O hυ C6H12O6 + 6O2
Only a part of this chemical energy is utilized by plants for their metabolic activities and the rest is taken up by heterotrophs or consumers belonging to the next trophic level. As a result the energy is transferred from one trophic level to the second trophic level.
The glucose produced in the plant cell is either stored in the form of starch or combined with other sugar molecules forming specialized carbohydrates like cellulose. It may also combine with Nitrogen, Phosphorus and Sulfur which help in synthesis of complex molecules like proteins, nucleic acids, pigments and hormones. All these are necessary for the normal growth of the plant, maintenance of body tissues and carrying out various physiological activities. Carbohydrates are oxidized to give Carbondioxide, water and chemical energy.
C6H12O6 + 6O2 6CO2 + 6H2O + Energy
The total amount of energy converted into sugar by a plant (by photosynthesis) is called ‘Gross primary production’ or ‘Gross productivity’. Some of this energy is lost through respiration and when it is deducted from gross primary production the remaining energy is known as ‘Net primary production’
Secondary production: In an ecosystem, the potential energy derived from the primary production meets the energy demands of the other trophic levels. Some of the primary production is consumed by herbivores and omnivores in the form of food. These consumers are in-turn eaten by carnivores of higher trophic levels. No animal can digest or assimilate all the food it has eaten. For example, a herbivore can assimilate only 10% of the food it ingests and in the case of carnivores it may go up to 20%. The energy thus assimilated is retained in the body of consumers in different forms. The food that cannot be digested leaves the animal bodies as feces which is an important source of energy for detritus feeders and saprotrophs.
A part of the energy assimilated by the herbivores is utilized in various metabolic activities like respiration, excretion, secretion, locomotion and reproduction. The remaining is stored in their tissues. This energy is called ‘Net secondary production’. The ‘Gross secondary production’ is equivalent to the total plant material ingested by herbivores minus the matter lost as feces.
The energy flow through different stages of an ecosystem occurs as shown below:
Solar energy Producers Primary consumers Secondary consumers
(Green Plants) (Herbivores) (Primary carnivores)