Insecticides used in farming can enter nearby rivers and accumulate in the bodies of organisms within a food chain. Explain how the use of insecticides on farmland can lead to the death of top predators living in a river ecosystem.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Insecticides applied to farmland are washed off by rain or run‑off and enter the river.
The chemicals are taken up by small organisms such as invertebrates, insects or small fish and are not readily broken down or excreted, so they build up in their tissues.
Each higher‑trophic‑level organism eats many of these contaminated lower organisms, so the concentration of insecticide increases up the food chain.
Top predators therefore accumulate a lethal or toxic dose of insecticide, which ultimately causes their death.
The chemicals are taken up by small organisms such as invertebrates, insects or small fish and are not readily broken down or excreted, so they build up in their tissues.
Each higher‑trophic‑level organism eats many of these contaminated lower organisms, so the concentration of insecticide increases up the food chain.
Top predators therefore accumulate a lethal or toxic dose of insecticide, which ultimately causes their death.
Examiner tips
- Use the word ‘accumulate’ and ‘bio‑accumulation’ to show understanding of the process.
- Show the stepwise increase in concentration from lower to higher trophic levels.
- Mention that the final lethal dose is reached in top predators.
- Keep the answer concise – 4 points, one sentence per point.
Common mistakes
- Confusing ‘bio‑accumulation’ with ‘biomagnification’ – both are relevant but the scheme expects accumulation. Omitting the step that insecticides are not broken down or excreted. Using vague terms like ‘pollution’ instead of ‘insecticides’ or ‘toxic concentration’.
Mark scheme (4 marks)
- Insecticides are washed from fields into the river by rain / enter the waterway via run-off
- Insecticides are taken in / absorbed by small organisms (e.g. invertebrates / insects / small fish) and are not broken down / excreted, so they accumulate in body tissues
- Each organism at a higher trophic level consumes many organisms from the level below, so the concentration of insecticide increases at each stage of the food chain / with each trophic level
- Top predators accumulate a lethal / toxic concentration of insecticide, causing their death
Key terms in this question
insecticides · food chain · top predator
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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