Explain why energy transfer between trophic levels in an ecosystem is never 100% efficient, and describe TWO consequences of this inefficiency for the structure of food chains.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a deciduous woodland ecosystem, measurements of biomass at each trophic level consistently show a dramatic reduction at each successive level. Ecologists use this data to construct pyramids of biomass and to model energy flow through the system.
Model answer (4 marks)
Energy transfer is never 100 % efficient because:
1. Not all ingested material is assimilated – a large part is excreted as faeces or remains in the digestive tract.
2. The assimilated energy is largely lost as heat during cellular respiration, so only a small fraction is available to the next trophic level.
Consequences of this inefficiency:
1. Food chains are short – usually only four or five trophic levels can be sustained.
2. Biomass (and often the number of organisms) decreases at each successive trophic level, giving the ecosystem a pyramid‑shaped biomass distribution.
1. Not all ingested material is assimilated – a large part is excreted as faeces or remains in the digestive tract.
2. The assimilated energy is largely lost as heat during cellular respiration, so only a small fraction is available to the next trophic level.
Consequences of this inefficiency:
1. Food chains are short – usually only four or five trophic levels can be sustained.
2. Biomass (and often the number of organisms) decreases at each successive trophic level, giving the ecosystem a pyramid‑shaped biomass distribution.
Examiner tips
- Use the two main loss mechanisms (excretion and respiration) to justify the 100 % inefficiency. Show the two consequences explicitly: short food chains and decreasing biomass. Keep the answer concise and use the exact wording from the mark scheme.
Common mistakes
- Confusing energy loss with energy transfer; writing that energy is ‘used up’ instead of ‘lost as heat’. Failing to mention both excretion and respiration. Not linking the inefficiency to the two specific consequences (short chains and pyramid shape).
Mark scheme (4 marks)
- Energy is lost at each trophic level because not all ingested material is assimilated — some is lost in faeces/egestion
- Energy is lost through heat released during cellular respiration, so assimilated energy is not fully available to the next trophic level
- Consequence 1: food chains are limited in length / typically no more than four or five trophic levels
- Consequence 2: biomass / number of organisms decreases at each successive trophic level, producing a pyramid shape
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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