Explain how the use of pesticides on farmland can reduce the population of organisms at higher levels in a food chain.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Farmers often spray crops with pesticides to kill insects that damage their plants. Some of these pesticides do not break down easily in the environment.
Model answer (4 marks)
Pesticides sprayed on crops are taken up by the small organisms that live on or in the plants, such as insects and other invertebrates. Because many pesticides are persistent, they are not broken down quickly and therefore accumulate in the tissues of these organisms. When a predator eats several of these contaminated prey, it also takes in the pesticide. The amount of pesticide in the predator is therefore higher than in the prey, and this process continues up the food chain. At the top predators the concentration of pesticide becomes very high, which can lead to death, reduced fertility or other health problems, ultimately causing a decline in the population of those higher‑level organisms.
Examiner tips
- Use the word ‘accumulate’ and ‘bioaccumulation’/‘biological magnification’ to show understanding of the process.
- Show the stepwise increase in concentration from prey to predator.
- Mention the effects on top predators (death, reproductive failure).
Common mistakes
- Confusing ‘accumulation’ with ‘degradation’; forgetting that pesticides are persistent.
- Failing to link the increase in concentration to the decline in predator populations.
- Using vague terms like ‘damage’ instead of specific effects such as ‘reproductive failure’.
Mark scheme (4 marks)
- Pesticides are absorbed by / accumulate in small organisms (e.g. insects/invertebrates) at the base of the food chain
- Pesticides are not broken down / are persistent, so they build up (accumulate) in body tissues
- The concentration of pesticide increases at each level of the food chain (bioaccumulation / biological magnification)
- High concentrations of pesticide in top predators cause death / reproductive failure / population decline
Key terms in this question
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Human impact on the environment questions
- Explain how deforestation can affect the amount of carbon dioxide in the atmosph…
- Explain how burning fossil fuels can lead to damage to trees and other plants.
- Explain how the use of fertilisers on farmland can lead to the death of fish in …
- Explain how the release of raw sewage into a river can lead to a decrease in the…