• Sun is the only source of energy. Producers convert solar energy into chemical energy in the form of food.
• The primary consumers (herbivores) depends on producers for food.
• According to the energy transfer law, only 10% of energy is transferred from one trophic level to the other.
• Therefore, the energy that is captured by the producers does not come back to the Sun and the energy transferred to the herbivores does not come back to the producers.
• It just keeps on moving to the next trophic level in a unidirectional way.
• That is why the flow of energy in the food chain is always unidirectional.