Explain why the number of organisms at each trophic level decreases as you move along a food chain from producer to tertiary consumer.

AQA GCSE Biology (8461) — 4.7.2 Organisation of an ecosystem · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Biomass (or energy) is lost at each trophic level.
Biomass is used for respiration, movement and heat release by the organism.
Biomass is also lost in waste materials such as faeces, urine and uneaten parts.
Less biomass is available to support fewer or smaller organisms at higher trophic levels.

Examiner tips

  • Use the exact wording from the mark scheme – e.g. "biomass is lost". Show the chain of loss: respiration, waste, heat. Link loss to fewer organisms at the next level.
  • common_mistakes
  • :
  • Confusing energy with biomass. Omitting the waste component. Using vague terms like "energy loss" without specifying respiration or waste.

Mark scheme (4 marks)

  1. Biomass (or energy) is lost at each trophic level
  2. Biomass is used for respiration / movement / heat release by the organism
  3. Biomass is also lost in waste materials such as faeces / urine / uneaten parts
  4. Less biomass is available to support fewer / smaller organisms at higher trophic levels

Key terms in this question

trophic level · producer · tertiary consumer · food chain

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