Explain why energy is lost at each trophic level in a food chain, and describe how this affects the number of organisms that can be supported at higher trophic levels.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a grassland ecosystem, grass is eaten by rabbits, which are eaten by foxes. Only a small percentage of the energy stored in the grass is eventually available to the foxes.
Model answer (5 marks)
Energy is lost through respiration at each trophic level.
Energy is lost in faeces and undigested material that is not absorbed.
Only a small percentage of the energy stored in the grass is transferred to the next trophic level.
Because less energy is available at each successive trophic level, fewer organisms can be supported.
This results in a pyramid of biomass or numbers, with fewer organisms (or less biomass) at higher trophic levels.
Energy is lost in faeces and undigested material that is not absorbed.
Only a small percentage of the energy stored in the grass is transferred to the next trophic level.
Because less energy is available at each successive trophic level, fewer organisms can be supported.
This results in a pyramid of biomass or numbers, with fewer organisms (or less biomass) at higher trophic levels.
Examiner tips
- Use the exact phrases from the mark scheme (e.g., "respiration", "faeces", "small percentage"),
- Show the causal link: loss of energy → fewer organisms at higher levels.
- Keep each point concise and directly answer the question.
Mark scheme (5 marks)
- Energy is lost through respiration at each trophic level
- Energy is lost in faeces / undigested material that is not absorbed
- Only a small percentage / fraction of energy is transferred to the next trophic level
- Because less energy is available at each successive trophic level, fewer organisms can be supported
- This results in a pyramid of biomass / numbers, with fewer organisms (or less biomass) at higher trophic levels
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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