A woodland ecosystem contains oak trees, caterpillars, blue tits, and sparrowhawks. When humans began over-harvesting blue tits from this woodland, describe the effect this would have on the populations of caterpillars and sparrowhawks. Give reasons for your answers.

Cambridge International IGCSE Biology (0610) — 19.2 Food chains and food webs · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Caterpillar population increases because fewer blue tits are available to eat them, so predation on caterpillars decreases.
Sparrowhawk population decreases because they rely on blue tits as a food source; with fewer blue tits, sparrowhawks receive less energy, reducing their survival and reproduction.

Examiner tips

  • Use the word ‘because’ to link effect and reason clearly.
  • Show the food‑chain link: blue tits → caterpillars and blue tits → sparrowhawks.
  • Keep each point concise – one sentence per effect and reason.

Common mistakes

  • Writing the effect without a clear reason. Assuming blue tits are the only food for sparrowhawks. Using vague terms like ‘more food’ or ‘less food’ without specifying the link to the specific species.

Mark scheme (4 marks)

  1. Caterpillar population increases
  2. Reason: fewer blue tits means fewer caterpillars are eaten / predation of caterpillars decreases
  3. Sparrowhawk population decreases
  4. Reason: fewer blue tits means sparrowhawks have less food / sparrowhawks cannot get enough energy to survive and reproduce

Key terms in this question

over-harvesting

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