Describe how biomass changes at each trophic level in a food chain, and give two reasons why biomass is lost between trophic levels.

Cambridge International IGCSE Biology (0610) — 19.1 Energy flow · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Biomass is greatest at the producer level and decreases at each successive trophic level.
1. Producers have the largest biomass.
2. At the next level, primary consumers have less biomass.
3. Secondary consumers have even less, and so on.
4. Biomass is lost because:
a. Energy is used in respiration and released as heat.
b. Undigested material is excreted or remains as waste (e.g. bones, fur).

Examiner tips

  • Use the word ‘decreases’ to show the trend; include the two loss mechanisms explicitly.
  • Show the order of trophic levels (producers → primary consumers → secondary consumers).
  • Mention that biomass is greatest at the lowest level.
  • Keep the answer concise – 4 marks only.

Mark scheme (4 marks)

  1. Biomass decreases at each successive trophic level (going up the food chain).
  2. Producers have the greatest biomass.
  3. Energy / biomass is used up in respiration (and released as heat), so less is passed on to the next level.
  4. Not all material can be digested / some material is lost through excretion or in faeces / material such as bones or fur cannot be digested.

Key terms in this question

biomass · trophic level · food chain

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