Chemicals, pesticides and heavy metals move up the food chain, get into aquatic bodies and are eaten by organisms like fishes which are in turn eaten by hawks/eagles. This process where the chemical enters the food chain and increase in concentration with increasing trophic level is known as biological magnification.

For example in the above food chain, Grass is a producer; it grows by consuming harmful chemicals from the soil. Grass is eaten by grasshopper which is further eaten by frog. Frog is eaten by snake and peacocks feed on snake. Since peacock is at the highest trophic level it will have the maximum concentration of harmful chemicals in its body, according to bio-magnification. The chemicals enter from soil to grass and they increase in concentration with increasing trophic level.