A woodland ecosystem contains oak trees, caterpillars, blue tits, and sparrowhawks arranged in a food chain. Explain why the number of sparrowhawks that the woodland can support is much smaller than the number of caterpillars.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Oak trees → Caterpillars → Blue tits → Sparrowhawks
Model answer (5 marks)
Energy moves up the food chain but is lost at each trophic level. When a sparrowhawk eats a blue‑titi, only about 10 % of the energy stored in the blue‑titi is transferred to the sparrowhawk; the rest is lost as heat during respiration and as waste (faeces, urine, undigested food). Because each step loses most of the energy, the amount of energy that reaches the top predator is very small. Consequently only a few sparrowhawks can be sustained, whereas many more caterpillars can be supported at the lower level where more energy is available.
Examiner tips
- Mention the 10 % transfer rule and the loss as heat and waste; link this to the small top‑predator population.
- Use the word ‘energy’ and ‘trophic level’ to show understanding of the concept.
Common mistakes
- Saying ‘sparse sparrowhawks’ without explaining energy loss; forgetting the 10 % rule; not mentioning heat and waste as specific loss mechanisms.
Mark scheme (5 marks)
- Energy is lost at each trophic level / as you move along the food chain
- Energy is lost through heat released during respiration
- Energy is lost in waste materials such as faeces / urine / undigested food
- Only approximately 10% of energy is transferred from one trophic level to the next
- By the time energy reaches the sparrowhawks, so little energy is available that only a small population can be sustained / there is not enough energy to support a large number of sparrowhawks
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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