# 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

> 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.

## 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

- [trophic levels](https://www.gradenine.co.uk/glossary/trophic-levels)
- [ecosystem](https://www.gradenine.co.uk/glossary/ecosystem)
- [energy transfer](https://www.gradenine.co.uk/glossary/energy-transfer)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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