Biotic factors of ecosystem linked together for food and form a chain called food chain. The various steps in food chain are called trophic levels. According to pyramid of energy-the energy flows from one trophic level to next in one direction only. According to law of thermodynamics, when energy transformed from one step to next step then some energy is liberated in the form of heat. As the autotrophs (green plants) form the base of food chain, therefore, they have highest amount of energy.