Pyramids of biomass — AQA GCSE Biology
Test yourself on Pyramids of biomass with AQA GCSE practice questions.
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Pyramids of biomass explained
A pyramid of biomass is a diagram that shows the relative mass of living material at each trophic level of a food chain.
Read the full explanation
Each level is drawn as a horizontal bar whose width is proportional to the biomass present, and the bars are stacked with trophic level 1 (producers) at the bottom. Biomass is the mass of living material, often measured as dry mass per unit area, for example in grams per square metre (g/m²). Because energy is lost between trophic levels through respiration, movement, excretion and uneaten material, the biomass always decreases at each successive level. Consequently, at GCSE level, a pyramid of biomass is always a true pyramid shape and is never inverted, unlike pyramids of numbers which can take irregular shapes.
Your focus
- Describe what a pyramid of biomass represents and how it differs from a pyramid of numbers.
- Construct a pyramid of biomass using proportional bar widths and a consistent scale.
- Explain why biomass always decreases at higher trophic levels, resulting in an upright pyramid shape.
Pyramids of biomass exam tips
Marking Points
- A pyramid of biomass represents the relative mass of living material at each trophic level.
- The width of each horizontal bar is strictly proportional to the biomass at that level.
- Bars are stacked symmetrically with trophic level 1 (producers) at the bottom and higher levels above.
- Biomass is often measured as dry mass per unit area, for example g/m².
- Biomass always decreases at higher trophic levels because energy and mass are lost between levels (e.g., through faeces, urea, and respiration).
- Pyramids of biomass are always upright (pyramid-shaped) at GCSE level, reflecting this continuous loss of biomass.
Examiner Tips
- 💡Label each trophic level clearly, for example producer, primary consumer and secondary consumer, and ensure level 1 is at the base.
- 💡Use a ruler and a consistent scale when drawing bars so the widths are proportional to the data provided.
- 💡Include units such as g/m² or kg/m² when stating biomass values.
Common Mistakes
- Confusing a pyramid of biomass with a pyramid of numbers; biomass uses mass of living material, not a count of organisms, so it is always pyramid-shaped.
- Drawing bars with widths that are not proportional to the biomass values; each bar width must represent the actual biomass using a consistent scale.
- Believing a pyramid of biomass can be inverted; correct this by remembering that at GCSE, biomass pyramids are always upright due to biomass loss at each stage.