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A pesticide sprayed on crops can end up most concentrated not in the crop but in fish and birds that never touched the field. That is biological magnification, a short paragraph in Class 10 science that often turns up as a three-mark question. This post explains why the concentration rises, shows a real example with numbers, and gives you a model answer.
Where is it in the Class 10 syllabus?
It is in NCERT Class 10 Science Chapter 13, Our Environment (reprint 2026-27), right after food chains and the flow of energy. Exercise question 6 asks: "What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?"
The CBSE Science curriculum for 2026-27 lists "Our environment: Eco-system, Environmental problems... Biodegradable and non-biodegradable substances" in Unit V, so the chapter is in the assessed syllabus this year. Check your school's latest circular too.
What does NCERT say happens?
NCERT's chain of events goes like this:
Pesticides and other chemicals used to protect crops are washed into the soil or into water bodies.
Plants absorb them from the soil along with water and minerals. In water, aquatic plants and animals take them up.
Because these chemicals are not degradable, they accumulate progressively at each trophic level.
Humans occupy the top level of many food chains, so the maximum concentration builds up in our bodies.
NCERT adds that this is why wheat, rice, vegetables, fruits and even meat carry varying amounts of pesticide residue, which cannot always be removed by washing.
Why does the concentration rise at each level?
The same chapter gives the key fact. When an animal eats, much of the energy is lost as heat or used for work, and on average only about 10% of the food eaten is turned into its own body.
Now picture a pesticide the body cannot break down or easily get rid of. Here is a simple model with sample numbers:
A frog eats 100 g of insects over some weeks. Each gram of insect holds 0.1 µg of pesticide, so the frog takes in 100 × 0.1 = 10 µg.
By the 10% figure, the frog builds about 10 g of new body mass from that food.
If it keeps all 10 µg, its concentration is 10 µg ÷ 10 g = 1 µg per gram.
That is ten times the concentration in its food. Repeat for the snake that eats frogs and the bird that eats snakes, and the multiplier compounds. Real animals get rid of some of the chemical, so the factor varies, but the direction is the same.
Worked example: DDT in a salt marsh
In 1967, Woodwell, Wurster and Isaacson sampled a salt marsh on Long Island in the United States, where the soil held large amounts of DDT, a persistent insecticide. Their abstract reports DDT concentrations increasing with trophic level "through more than three orders of magnitude", from 0.04 parts per million (ppm) in plankton to 75 ppm in a ring-billed gull.
Work out the magnification:
75 ÷ 0.04 = 1,875.
"Three orders of magnitude" means a factor of about 1,000, so 1,875 is "more than three orders" as the authors say.
They found the highest levels in scavenging and carnivorous fish and birds, with birds holding 10 to 100 times more than fish. Our profile of Rachel Carson and Silent Spring covers the wider story of DDT.
A model answer to NCERT's question
*Biological magnification is the progressive increase in the concentration of non-biodegradable chemicals, such as pesticides, at each higher trophic level of a food chain. Yes, the levels differ: they are lowest in producers and highest in top consumers. These chemicals are not broken down, so they pass up with food and accumulate. Since each consumer eats much more food than it builds into its body, the chemical becomes more concentrated at every step. Humans, at the top of many food chains, collect the most.*
Which mistakes cost marks?
Saying biodegradable substances magnify. Only non-degradable ones build up.
Saying energy increases up the chain. Energy decreases; concentration increases.
Saying washing removes all residue. NCERT says it cannot always.
Thinking it only happens in water. NCERT describes both soil and water routes.
Leaving out "at each trophic level" from the definition.
If one idea in the chapter still feels vague, Fermi's AI tutor can work out what you already know and build a plan for the session, so you start with the gap.
A practice routine
Write the four-step NCERT chain from memory.
Redo the frog model with new sample numbers, such as 200 g of food at 0.05 µg per gram.
Learn the 0.04 ppm and 75 ppm example and the 1,875 factor.
Do NCERT Activity 13.3 with your family: list foods that may carry residues and ways to reduce intake.
Revise food chains, ozone and waste with our guide to Our Environment in Class 10.
This week, answer exercise question 6 in five sentences without looking.
