Construct a pyramid of biomass
A biomass pyramid represents the mass of living material at each trophic level using the same area and measurement basis.
Section B18.2 · Food chains and food webs
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Read energy-transfer arrows and distinguish energy from matter in the 19.1 Energy flow pathway.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a biomass-pyramid bar represent?
What you will learn
- Construct biomass pyramids using comparable measurements
- Interpret biomass as mass rather than organism count
Represent material, not individuals
Biomass is the mass of living material at each trophic level. A biomass pyramid uses bar widths proportional to that mass, with producers at the base. Comparisons should use the same area and a stated measurement basis so the levels can be compared fairly.
A biomass pyramid can show more about how much living material is present than a count alone. It is a snapshot of biomass at a stated time and place, not a direct measure of energy transferred over time. Depending on the ecosystem and sampling basis, it need not always be upright.
Build a fair comparison
Read values in the same units and check that every trophic level was sampled in a comparable area. In a sampled water ecosystem, producers may have a low standing biomass at one instant while reproducing rapidly; this can make a biomass pyramid appear inverted. Explain the measurement rather than assuming all diagrams must be triangular.
Do not compare a per-area value with an unscaled total. Biomass records how much living material exists at a time; energy pyramids represent energy transfer through levels and require a time basis.
Sources for this lesson
Pause and explain
Two levels were sampled using different areas. What must be checked before comparing their biomass?
Worked example
Measured biomass in a plot is 900 g m⁻² for producers, 180 g m⁻² for primary consumers and 35 g m⁻² for secondary consumers. State the order of bar widths.
Show the worked solution
- Use the common area unit.
- Compare the values at each trophic level.
Answer The producer bar is widest, the primary consumer bar is narrower, and the secondary consumer bar is narrowest. The widths should be in the ratio 900:180:35.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Biomass values are 500 kg ha⁻¹ at producers, 80 at primary consumers and 12 at secondary consumers. Describe how to draw the pyramid.
Give me a hint
Use the same scale and keep producers at the base.
Compare my reasoning
- Label each trophic level.
- Make widths proportional to 500, 80 and 12 using one scale.
Draw the widest base for producers (500 kg ha⁻¹), a narrower primary-consumer bar (80) and the narrowest secondary-consumer bar (12).
Look for these in your work
- Levels ordered
- Widths proportional to values
A water sample has a smaller producer standing biomass than primary-consumer biomass at the sampling time. Is an inverted biomass pyramid automatically invalid? Explain.
Give me a hint
Biomass is a measurement at a place and time; follow the recorded values.
Compare my reasoning
- Use the data as measured if sampling is comparable.
- An unusual shape can occur in some ecosystems.
No. If the measurements and sampling basis are comparable, the pyramid should show the observed values even if the producer bar is narrower.
Look for these in your work
- Data respected
- Shape not assumed
A graph reports total dry mass in one pond but biomass per square metre in another. Why can the values not be directly compared?
Give me a hint
Check whether the same area basis is used.
Compare my reasoning
- One is a total for the whole pond.
- The other is standardized by area.
They use different area bases, so they are not directly comparable until expressed on a common basis.
Look for these in your work
- Area mismatch noticed
- Fair comparison specified
Common mistakes
- Using organism counts instead of mass
- Treating biomass as energy transferred per year
- Forcing an inverted data set into an upright pyramid
What can you explain now?
What does a biomass-pyramid bar represent?
Compare with the explanation
Mass of living material at a trophic level
Biomass is mass, compared on a consistent basis.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How does a biomass pyramid differ from a number pyramid, and what must be comparable between levels?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.