CBSE Class 10 Science 2022 · Paper 31(B)/6 · Question 8(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. These chemicals are washed into the soil and water, are absorbed by plants and aquatic organisms, and so enter the food chain. As they are not degradable, they are not broken down and are stored in the body, and each organism eats many organisms of the level below. The concentration therefore rises at every trophic level: the producers have the least, and each higher level of consumers has more. The tertiary consumers, at the top of a four-level chain, accumulate the highest concentration and are affected the most. In the DDT example printed in NCERT Class 12 Biology, large fish (tertiary consumers) carry $2$ ppm, $50$ times the $0.04$ ppm in zooplankton, and the fish-eating birds one level above them reach $25$ ppm; in birds, high DDT concentrations disturb calcium metabolism, making eggshells thin so that they break early and bird populations fall.