Explain why energy transfer between trophic levels in an ecosystem is never 100% efficient, and describe what this means for the number of trophic levels that can be supported.

OCR A-Level Biology A (H420) — 6.5 Ecosystems · Explain · 4 marks · View as Markdown

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

In a woodland ecosystem, ecologists measured the energy content at each trophic level. They found that the efficiency of energy transfer between consecutive trophic levels ranged from 5% to 20%. The top predator in this system was a sparrowhawk feeding on small birds.

Model answer (4 marks)

Energy transfer is never 100 % efficient because:
1. At each trophic level a large part of the ingested energy is used for respiration and maintenance, leaving less for growth and reproduction.
2. Only a fraction of the biomass is actually assimilated; the rest is lost as egestion, excretion or remains uneaten.
These losses mean that the total energy available to the next level is progressively reduced. Consequently, only a few trophic levels can be supported – beyond a certain level the energy is too low to sustain a viable population.

Examiner tips

  • Use the phrase ‘energy is lost through respiration’ and ‘not all biomass is assimilated’ – these are key terms in the mark scheme.
  • Explain the consequence (reduced energy for higher levels) before stating the limit on trophic levels.
  • Keep the answer concise – 4 marks, so one sentence per point is sufficient.

Common mistakes

  • Claiming 100 % transfer is possible; ignoring respiration losses.
  • Forgetting that egestion/excretion also removes energy.
  • Over‑expanding the answer with irrelevant details such as specific species names.

Mark scheme (4 marks)

  1. Energy is lost through respiration at each trophic level, so not all ingested energy is available to the next trophic level.
  2. Not all biomass at one trophic level is consumed or assimilated by the next — some is lost in egestion, excretion, or as material not eaten.
  3. Because energy is lost at each trophic level, the total available energy decreases progressively along the food chain.
  4. The progressive reduction in available energy limits the number of trophic levels that can be supported, because beyond a certain level there is insufficient energy to sustain a viable population.

Key terms in this question

trophic levels · ecosystem · energy transfer

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