A woodland ecosystem contains oak trees, caterpillars, blue tits and sparrowhawks arranged in a food chain. Explain why the number of sparrowhawks that can be supported in the woodland is much smaller than the number of caterpillars.

Pearson Edexcel A-Level Biology A (9BI0) — 3.2 Energy flow and ecosystems · Explain · 5 marks · View as Markdown

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

Oak trees are the producers in the woodland food chain. The food chain is: oak tree → caterpillar → blue tit → sparrowhawk.

Model answer (5 marks)

Energy is lost at each trophic level, mainly as heat during respiration and as waste (faeces, urine, undigested food). Consequently, only about 10 % of the energy from oak leaves is transferred to caterpillars, and only about 1 % reaches blue tits. By the time the energy reaches sparrowhawks it is far less, so fewer sparrowhawks can be supported than caterpillars.

Examiner tips

  • Use the 10 % rule to show energy loss between levels; mention heat and waste; explain why biomass decreases; keep answer concise and use correct terminology.

Common mistakes

  • Saying energy is conserved; ignoring heat and waste; not linking energy loss to fewer sparrowhawks.

Mark scheme (5 marks)

  1. Energy is lost at each trophic level / between each organism in the food chain
  2. Energy is lost through heat released during respiration
  3. Energy is lost in waste materials such as faeces / urine / undigested food
  4. Less energy is available to sparrowhawks than to caterpillars because energy has been lost at multiple trophic levels before reaching the sparrowhawk
  5. Therefore less biomass / fewer organisms can be supported at the level of the sparrowhawk compared to the caterpillar

Key terms in this question

food chain

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