A food chain in a woodland ecosystem has three trophic levels: oak trees → caterpillars → blue tits. Explain why a pyramid of biomass is considered more useful than a pyramid of numbers for representing this food chain.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A pyramid of numbers counts individuals, not their mass, so it ignores the fact that one oak tree can support many caterpillars. Consequently the producer bar may be narrower than the primary consumer bar, giving a misleading shape.
A pyramid of biomass sums the total mass of all organisms at each trophic level. It therefore shows a true pyramid shape and reflects the decrease in available energy as it moves up the food chain, because biomass – and hence energy – is lost at each level.
A pyramid of biomass sums the total mass of all organisms at each trophic level. It therefore shows a true pyramid shape and reflects the decrease in available energy as it moves up the food chain, because biomass – and hence energy – is lost at each level.
Examiner tips
- Use the word ‘mass’ or ‘biomass’ when contrasting with ‘numbers’
- Explain the loss of energy at each trophic level to justify the pyramid shape
- Mention the example of one oak tree supporting many caterpillars
Common mistakes
- Confusing biomass with energy directly, or saying biomass is the same as energy
- Failing to note that a numbers pyramid can be non‑pyramidal
- Using vague terms like ‘size’ instead of ‘mass’
Mark scheme (4 marks)
- A pyramid of numbers does not account for the size or mass of organisms
- This means the pyramid of numbers may not have a true pyramid shape (e.g. one oak tree supports many caterpillars, so the producer bar would be narrower than the primary consumer bar)
- A pyramid of biomass measures the total mass of living material at each trophic level
- A pyramid of biomass gives a true pyramid shape and indicates the amount of energy being passed on at each trophic level, because biomass decreases at each level as energy is lost
Key terms in this question
pyramid of numbers · pyramid of biomass · trophic level · biomass
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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