CBSE Class 10 Science 2019 · Paper 31/1/1/C · Question 15

(a) At each step of a food chain only about $10\%$ of the energy in the food is passed on to the next trophic level; the rest is used in the organisms' life processes and lost as heat. The usable energy therefore falls very steeply (for example $10000\to 1000\to 100\to 10$ J), so after three or four levels too little energy is left to support another level of organisms. Hence a food chain generally has only three or four trophic levels. (b) Biological magnification is the progressive increase in the concentration of harmful non-biodegradable chemicals, such as pesticides, at each successive trophic level of a food chain. Pesticides used on crops are washed into the soil and water, absorbed by plants and passed on to the animals that eat them. Being non-biodegradable, they are not broken down or excreted and accumulate in the body; as each organism eats many organisms of the level below, their concentration rises at every level and is highest in the top consumers, such as human beings.