Using the DDT figures for water, algae, fish and a pelican in your NIOS lesson, explain how a pesticide becomes more concentrated at each step of a food chain, and the harm it does at the top.

1. 1 · Define. Biomagnification is the build-up of a non-biodegradable chemical to higher concentrations at each trophic level.
2. 2 · How it enters. DDT sprayed on crops or used against mosquitoes washes into ponds and is taken in by algae.
3. 3 · Why it builds up. DDT is not broken down or excreted, and each animal eats many organisms from the level below.
4. 4 · Example. Water 0.2 ppm → algae 77 ppm → fish 500 to 600 ppm → pelican 1700 ppm.
5. 5 · Effect. Top consumers suffer most: birds lay thin, weak eggshells and their numbers fall.
6. 6 · Real life. Using pesticides only as advised keeps them out of ponds and our food.
7. 7 · Close in one line. So the top of the food chain gets the highest dose.

Scoring tip: Write the food chain with the rising ppm values; the numbers make the answer convincing.