CBSE Class 10 Science 2022 · Paper 31/3/2 · Question 13

(a) At each trophic level, most of the energy in the food eaten is used by the organisms for their own life processes and is lost to the surroundings, mostly as heat. On average only about 10% of the energy is passed on to the next level (the ten per cent law): of 10 000 J held by the producers, about 1 000 J reaches the herbivores, 100 J the next consumers and 10 J the fourth level. With less and less energy available at each higher level to support the organisms there, the number of individuals generally decreases from the lower to the higher trophic levels. (b) The energy captured by the producers does not go back to the Sun, and the energy passed to the herbivores does not come back to the producers; as energy moves up through the trophic levels it is no longer available to the previous level. The energy lost at each level, mostly as heat, cannot be used by plants to make food. So energy flows in one direction only, from the Sun to the producers, herbivores and carnivores, and is not recycled.