5 min readPathway sheet2 or 3-mark questions, assertion-reason, and 4-mark case-based questions

Biological magnification

Harmful chemicals that cannot be broken down build up at each trophic level. Their concentration is highest in the top consumers, often human beings.

Biological magnificationIncrease in the concentration of harmful non-biodegradable chemicals at each higher trophic level

ExamplePesticides such as DDT

Highest inTop consumers, such as human beings, eagles and big fish

Why it happensThe chemicals are not broken down or removed, and each eater eats many organisms from the level below

How a pesticide reaches our plate

1
Farmers spray pesticides on crops to kill pests, and factories release some chemicals too.
2
Rain washes them into the soil and into water bodies such as ponds and rivers.
3
Plants absorb them along with water and minerals. Tiny water plants and animals take them in too.
4
The chemicals are non-biodegradable. They are not broken down and not removed from the body, so they stay there.
5
Each animal eats many organisms from the level below, so it collects more and more of the chemical.
6
The concentration is highest at the top: in big fish, eagles and human beings.

Draw it: a pesticide building up in a pond food chain

Energy shrinks at each step, but the amount of the chemical in each kilogram of body grows.

Why it matters and what we can do

These chemicals can harm health. In some fish-eating birds, for example, they make eggshells thin and weak.
Humans, at the top of many food chains, can get the largest dose.
Use pesticides only as advised, and try safer methods such as neem-based sprays and natural predators of pests.
Wash fruits and vegetables well before eating them, and support organic farming where possible.

In real life

Pesticide sprayed on cotton or vegetable fields can wash into village ponds and then pass through fish to the people who eat them. Washing vegetables well and using neem-based sprays in kitchen gardens are small steps that help.

Exam angle

2 marks: “Define biological magnification. In which organisms is it the highest, and why?” The rise in concentration of harmful non-biodegradable chemicals at each higher trophic level. It is highest in top consumers such as humans, because the chemicals are not broken down and each eater eats many organisms below it.

3 marks: “Explain, with a food chain, how a pesticide used on crops can end up in human beings.” Pesticide washes into soil and water; plants absorb it; herbivores, then carnivores and humans eat these. It is not broken down, so it builds up and is highest at the top.

Remember it

Up the chain, up the poison. Energy goes down tenfold at each step, but the poison goes up.