Explain why biomass tends to decrease at successive trophic levels in most food chains.

IB DP Biology Higher Level (2023 syllabus) — C4.2 Transfers of energy and matter · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Energy and biomass are lost as heat during cellular respiration at each trophic level.
Only about 10 % of the biomass consumed is assimilated and transferred to the next level.
The remainder is egested as faeces or used for growth, reproduction and other metabolic processes.
Thus, at each step only a small fraction of the previous level’s biomass is available, so biomass decreases up the food chain.

Examiner tips

Use the 10 % rule to justify the drop in biomass.
Mention both heat loss and egestion to cover all points.
Show the chain of loss from one level to the next.
Use clear, exam‑style language (e.g., ‘only a small proportion is transferred’).

Common mistakes

Confusing energy with biomass or vice versa.
Forgetting to mention egestion or metabolic use as additional losses.
Using vague terms like ‘much’ instead of the 10 % figure.

Mark scheme (4 marks)

Energy/biomass is lost as heat through cellular respiration at each trophic level
Not all biomass consumed is assimilated — some is egested as faeces
Only a small proportion (typically around 10%) of energy/biomass is transferred to the next trophic level
Biomass is also used for growth, reproduction, and other metabolic activities, meaning less is available for the next trophic level / biomass accumulates only in the living tissues that survive to be consumed

Key terms in this question

trophic level · biomass