A hedgerow ecosystem contains the following organisms: bramble plants, aphids, ladybirds, blue tits, and sparrowhawks. Describe how a significant reduction in the number of aphids would affect the populations of blue tits and sparrowhawks. Give a reason for each effect.

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)

Blue tit population decreases because they have less food available – aphids are a main food source for them.
Sparrowhawk population decreases because they have less food available as blue tit numbers have fallen – sparrowhawks depend on blue tits as a food source.

Examiner tips

  • Use the word ‘decreases’ or ‘falls’ to match the mark scheme. Explain the food‑chain link: aphids → blue tits → sparrowhawks. Keep the answer concise – one sentence per effect.
  • Use the exact terminology: ‘food source’ and ‘depend on’ to gain full marks.

Common mistakes

  • Writing ‘increase’ instead of ‘decrease’ for the populations. Omitting the reason that blue tits rely on aphids. Using vague terms like ‘less food’ without linking to the specific predator–prey relationship.

Mark scheme (4 marks)

  1. Blue tit population decreases / falls
  2. Because blue tits have less food available (as aphids are their food source / prey)
  3. Sparrowhawk population decreases / falls
  4. Because sparrowhawks have less food available as blue tit numbers have fallen / sparrowhawks depend on blue tits as a food source

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