CBSE Class 10 Science 2022 · Paper 31/4/1 · Question 9

Trophic levels are the steps of a food chain at which food, and with it energy, is transferred; each step or level of a food chain forms one trophic level, with the producers first, herbivores second, small carnivores third and large carnivores fourth. Autotrophs are placed at the first trophic level because they make their own food from carbon dioxide and water using sunlight (photosynthesis). They fix the Sun's energy in food and make it available to all the consumers, which depend on them directly or indirectly, so a food chain begins with them. The number of trophic levels is limited because the flow of energy is one-way and only about $10\%$ of the energy at one trophic level reaches the next; the rest is used in life processes and lost as heat. After three or four steps so little usable energy is left (for example, $10,000$ J in the producers leaves only $10$ J at the fourth level) that it cannot support a further level, so food chains usually have only three or four trophic levels.