CBSE Class 10 Science 2022 · Paper 31(B)/6 · Question 7

The flow of energy in an ecosystem is said to be unidirectional because energy moves in one direction only - from the Sun to the producers, and then from one trophic level to the next - and never flows back. The energy captured by the autotrophs (producers) does not revert to the solar input, and the energy that passes to the herbivores does not come back to the autotrophs; as it moves progressively through the trophic levels it is no longer available to the previous level. At each level a large part of the energy is also used in life processes or lost as heat, which cannot be used again by the producers. Food chain with three trophic levels: grass $\to$ deer $\to$ lion. The grass (producer, first trophic level) captures about $1\%$ of the sunlight falling on its leaves; in this chain the deer (herbivore, second trophic level) gets energy only by eating grass, and the lion (carnivore, third trophic level) only by eating deer. Only about $10\%$ of the energy of each level passes to the next - for example $10,000$ J in the grass, about $1,000$ J in the deer and about $100$ J in the lion - and the rest is used up or lost as heat. None of this energy flows back from the lion to the deer, from the deer to the grass, or from the grass to the Sun.