Draw and interpret a pyramid of numbers
A pyramid of numbers shows how many individual organisms occur at each trophic level in a stated area and sample.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
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 the width of a number-pyramid bar represent?
What you will learn
- Represent organism counts by trophic level
- Interpret upright or non-upright number pyramids using the data
Convert counts into levels
A pyramid of numbers represents the number of individual organisms at each trophic level in a food chain. Place producers at the bottom, then primary consumers and higher consumers above. Draw each bar so its width is proportional to the number in that level; state the scale and use the same width scale throughout.
Counts need a defined area, time and chain or sampling rule. Different organism sizes are not reflected: one tree and many insects feeding on it can produce a pyramid that is narrow at the producer level and wider above. Some number pyramids therefore are not the familiar upright triangle.
Interpret the shape from counts
Read each level’s width as a count, not biomass or energy. Check how many producers and consumers were counted and whether the sample is comparable. A number pyramid shows population counts in a pathway; it does not by itself explain why counts differ or prove how much energy is transferred.
An unusual shape is not automatically an error. The width must follow the data, and the diagram should identify trophic levels and units or sampling basis.
Sources for this lesson
Pause and explain
A chain has one tree, many insects and several birds. What shape may its number pyramid have?
Worked example
A chain has 1 oak tree, 120 caterpillars and 6 songbirds in the stated sample. Describe the pyramid of numbers shape.
Show the worked solution
- Put oak tree at the producer base, caterpillars above and songbirds at the top.
- Compare counts: 1, then 120, then 6.
- Make the caterpillar bar widest.
Answer The producer bar is narrow, the caterpillar bar is much wider, and the songbird bar is narrower than the caterpillar bar. This number pyramid is not upright at its base because one tree supports many caterpillars.
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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.
Counts are 40 grass plants, 20 grasshoppers and 4 frogs. Which trophic-level bar is widest, and what is the shape overall?
Give me a hint
Compare counts, not the organisms’ mass.
Compare my reasoning
- 40 producers is the largest count.
- 20 primary consumers and 4 secondary consumers are progressively fewer.
The producer bar is widest, followed by the grasshopper bar, then the frog bar; this sample produces an upright narrowing shape.
Look for these in your work
- Counts compared
- Level order used
A sample contains 2 trees, 300 caterpillars and 15 birds. Explain why the number pyramid may widen above the producers.
Give me a hint
A number pyramid counts individuals regardless of their size.
Compare my reasoning
- Each tree is one producer in the count.
- Many small consumers can feed on one large producer.
It may widen because only two trees are counted but hundreds of caterpillars can feed on them; organism size is not represented by number.
Look for these in your work
- Count basis explained
- Size limitation noted
A learner draws a pyramid of numbers but labels the axis “kg of living material”. Identify the mismatch and name the correct representation for that unit.
Give me a hint
Match what is counted with the unit.
Compare my reasoning
- Counts are organisms, not kilograms.
- Biomass is a mass of living material.
The label does not match the number pyramid. Kilograms of living material describe biomass, so the diagram would need biomass data and a pyramid-of-biomass label.
Look for these in your work
- Mismatch identified
- Correct measure named
Common mistakes
- Drawing every number pyramid as an upright triangle
- Making bar width unrelated to the count
- Calling number of organisms “biomass”
What can you explain now?
What does the width of a number-pyramid bar represent?
Compare with the explanation
The number of organisms at that trophic level
Widths are proportional to counts.
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Make it stick
What information sets the width of each number-pyramid level, and why may its shape be non-upright?
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.