Explain why the biomass available at each trophic level decreases as you move up a food chain.

AQA GCSE Biology (8461) — 4.7.4 Trophic levels in an ecosystem (Bio only) · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Only a small percentage of the biomass that an organism eats is converted into new biomass.

When food is ingested, most of it is used for the organism’s own metabolism – energy is released by respiration and heat is produced.

The remainder is expelled as waste (faeces, urine, etc.).

Consequently, the amount of biomass that can be passed on to the next trophic level is much smaller, so biomass decreases up the food chain.

Examiner tips

  • Use the word ‘only a small percentage’ to show you understand the 10 % rule.
  • Explain both energy loss (respiration) and waste loss.
  • Show the sequence: ingestion → respiration → waste → less biomass for next level.
  • Mention that this is why each higher trophic level has less biomass.

Common mistakes

  • Saying ‘all biomass is lost’ – ignores the small amount that is transferred.
  • Using vague terms like ‘energy loss’ without linking to respiration and waste.
  • Not explaining that waste products are a major route of biomass loss.

Mark scheme (4 marks)

  1. Not all biomass/material eaten is absorbed/converted into new biomass by the organism
  2. Biomass is lost through respiration (releasing energy for movement/heat/life processes)
  3. Biomass is also lost in waste products such as faeces and urine
  4. Therefore less biomass is available to pass on to the next trophic level / only a small percentage of biomass is transferred between trophic levels

Key terms in this question

trophic level · biomass · food chain

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