Explain why the efficiency of energy transfer between trophic levels in a food chain is typically only 10%, with reference to the forms in which energy is lost.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Energy transfer between trophic levels is only about 10% efficient because:
1. Most of the energy that organisms ingest is used in cellular respiration and is released as heat.
2. Not all of the biomass of a trophic level is eaten by the next level – some organisms die and are decomposed, so their energy is lost.
3. A portion of the food is indigestible and is egested as faeces, which is not assimilated into new biomass.
4. These large losses (heat, egestion, decomposition) mean that only roughly 10 % of the energy at one level becomes biomass at the next, limiting food‑chain length.
1. Most of the energy that organisms ingest is used in cellular respiration and is released as heat.
2. Not all of the biomass of a trophic level is eaten by the next level – some organisms die and are decomposed, so their energy is lost.
3. A portion of the food is indigestible and is egested as faeces, which is not assimilated into new biomass.
4. These large losses (heat, egestion, decomposition) mean that only roughly 10 % of the energy at one level becomes biomass at the next, limiting food‑chain length.
Examiner tips
- Use the exact terms: cellular respiration, heat, decomposers, egestion, biomass.
- Show the cause–effect chain: loss mechanisms → low efficiency.
- Keep each point short and directly linked to the 10 % figure.
Common mistakes
- Confusing energy loss with energy transfer; forgetting to mention heat.
- Saying “only 10%” without explaining why.
Mark scheme (4 marks)
- A large proportion of energy ingested by organisms at each trophic level is lost as heat through cellular respiration
- Not all biomass at one trophic level is consumed/ingested by the next — some organisms die and are broken down by decomposers, so that energy does not pass up the food chain
- Some ingested material cannot be digested and is egested as faeces, so this energy is not assimilated into consumer biomass
- Because energy losses (heat, egestion, decomposers) are large at each step, only approximately 10% of the energy available at one trophic level is incorporated into the biomass of the next, limiting the length of food chains
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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