A saltmarsh ecosystem contains the following food chain: cordgrass → marsh grasshoppers → marsh wrens → marsh hawks. Explain why the marsh hawk population can only be supported by a large number of cordgrass plants.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 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).

In a saltmarsh ecosystem, energy is transferred through a food chain. At each stage, only approximately 10% of the energy from one trophic level is transferred to the next.

Model answer (5 marks)

Energy is lost at each trophic level, mainly as heat during respiration and as waste products such as urine and faeces. Only about 10 % of the energy from one level is transferred to the next. Thus, by the time the energy reaches the marsh hawk, very little remains. Consequently a large biomass of cordgrass is required at the base of the food chain to supply enough energy to support the hawk population.

Examiner tips

  • Use the 10 % rule explicitly; mention heat and waste loss; link loss to need for large base biomass; keep answer concise and structured

Common mistakes

  • Failing to mention the 10 % transfer rule; confusing energy loss with energy gain; not linking loss to need for many cordgrass plants

Mark scheme (5 marks)

  1. Energy is lost at each trophic level / between each stage of the food chain
  2. Energy is lost through heat released during respiration
  3. Energy is also lost in waste products such as urine or faeces / in materials not consumed or digested
  4. Only approximately 10% of energy is passed from one trophic level to the next, so by the time energy reaches the marsh hawk very little energy remains
  5. Therefore a large biomass / large number of cordgrass plants is needed at the start to provide enough energy to support the marsh hawk population

Key terms in this question

food chain

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