The big idea: Some pollutants — such as the pesticide DDT or methyl mercury — are not broken down in the environment or inside living things.
Because they are not broken down and not excreted, they build up inside an organism over time, and they get more and more concentrated as they pass up the food chain.
The result: the top predator ends up with the highest concentration of the pollutant — often a harmful dose, even though the level in the water or soil was tiny.
A persistent pollutant becomes more concentrated at each higher trophic level: each predator eats many contaminated prey and keeps the pollutant, so it multiplies up the chain and is highest in the top predator.
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- Persistent (non-biodegradable) pollutant
- A pollutant that is not broken down by enzymes or decomposers, so it stays in the environment and in organisms for a long time (e.g. DDT, mercury).
- Bioaccumulation
- The build-up of a pollutant inside a single organism over time, because it is taken in faster than it can be broken down or excreted.
- Biomagnification
- The increase in the concentration of a pollutant from one trophic level to the next, so that it is highest in the top predator.
- Trophic level
- A feeding position in a food chain (producer, primary consumer, secondary consumer, and so on).
- Toxin
- A substance that is harmful to living organisms, even in small amounts.
Two words, two scales: Bioaccumulation = building up inside one organism (the level rises as the organism ages).
Biomagnification = the level multiplying as you move up the food chain (highest in the top predator).
They work together: each organism accumulates the pollutant, then passes its whole load on when it is eaten.
To biomagnify, a pollutant has to have two features: it must be persistent (not broken down) and it must be stored, not excreted (often because it is fat-soluble).
If it could be broken down or removed in waste, it would never build up. Because it cannot, every mouthful an organism eats adds to the load it is already carrying.
| Property of the pollutant | What it means | Why it leads to biomagnification |
|---|---|---|
| Non-biodegradable (persistent) | It is not broken down by enzymes or decomposers | It stays in the body instead of being destroyed, so it keeps building up |
| Not excreted | It cannot be removed in urine or other waste | It is stored rather than lost, so the amount only rises |
| Fat-soluble | It dissolves in fats and is stored in fatty tissue | It is locked away in the body and passed on when that body is eaten |
| Toxic in small amounts | It harms organisms even at low concentrations | By the time it has multiplied up the chain, the dose at the top is harmful |
Step by step — how it multiplies: Follow the pollutant up a chain:
1. Producers (e.g. algae) absorb a tiny amount of the pollutant from the water or soil.
2. A primary consumer eats many producers and keeps all their pollutant — so its level is higher than any single producer.
3. A predator eats many of those contaminated consumers and again keeps all of it — so its level is higher still.
4. At each step the pollutant is stored, not lost, so the concentration multiplies and is highest in the top predator.
A persistent pollutant becomes more concentrated at each higher trophic level: each predator eats many contaminated prey and keeps the pollutant, so it multiplies up the chain and is highest in the top predator.
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Bioaccumulation (within one organism)
- Pollutant taken in faster than it is removed
- It is stored (often in fat) instead of excreted
- The amount rises as the organism ages
- Happens in every organism in the chain
Biomagnification (up the chain)
- Each consumer eats many contaminated prey
- It keeps all of their pollutant
- Concentration multiplies at each trophic level
- Highest in the top predator
Why energy falls but pollutant rises: Energy decreases up a food chain because most is lost as heat at each level.
A persistent pollutant does the opposite — it increases up the chain.
The reason is that the pollutant is not used up or lost the way energy is; it is stored and passed on, so it gathers in fewer and fewer top organisms.
A memory hook: Energy goes down the pyramid; a persistent toxin goes up it.
Think 'big fish eats many small fish' — the big fish inherits every small fish's pollutant.
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How this is tested: A favourite Paper 2 / Paper 3 task is to explain why a pollutant's concentration is highest in the predator at the top of a food chain — they want the cause-and-effect chain, not just the word 'biomagnification'.
You should be able to explain how a named persistent pollutant (such as methyl mercury or a crude-oil chemical) reaches harmful levels in fish or birds, and to outline the wider environmental effects of a pesticide like DDT.
Top-scoring answers always link persistent / not excreted to stored and passed on, and many prey eaten to concentration multiplies up the chain.
IB-style question — why is the top predator worst affected?
A persistent insecticide is sprayed on a lake. Its concentration is far higher in the fish-eating birds at the top of the food chain than in the algae at the bottom. Explain why. [3]
How to score all three marks
- Say why it is not removed. The insecticide is non-biodegradable and is not excreted, so once an organism takes it in, it is stored rather than broken down or lost.
- Explain the build-up between levels. Each consumer eats many contaminated organisms from the level below and keeps all of their insecticide, so its concentration is higher than in any single prey item — this is biomagnification.
- Reach the top. This is repeated at every trophic level, so the concentration multiplies up the chain and is highest in the top predator (the fish-eating birds). (Mark 1: persistent / not excreted, so stored. Mark 2: each consumer eats many prey and keeps the pollutant. Mark 3: concentration multiplies up the levels → highest at the top.)
Final answer
The insecticide is non-biodegradable and not excreted, so it is stored in each organism. Every consumer eats many contaminated prey and keeps all their insecticide, so the concentration multiplies at each trophic level (biomagnification) and is highest in the top predator.
✓ Why this scores full marks: It gives three separate, linked reasons: the pollutant is stored (persistent / not excreted), each consumer keeps many prey's worth, and so it multiplies up the chain.
An answer that just says 'because of biomagnification' names the process but does not explain it — and loses marks.
| Feature | Bioaccumulation | Biomagnification |
|---|---|---|
| Where it happens | Inside ONE organism, over its lifetime | BETWEEN organisms, up the food chain |
| What builds up | Pollutant collects in one body faster than it is removed | Pollutant concentration rises at each higher trophic level |
| Cause | The pollutant is not broken down or excreted, so it is stored (often in fat) | Each consumer eats MANY contaminated prey and keeps their pollutant |
| Result | An organism's pollutant level rises with age | The top predator carries the highest concentration of all |