Measuring biomass
Biomass shows how much energy is stored at a trophic level.
- Measured as dry mass to remove water
- Water does not contain usable energy
- Units are usually g m⁻²
Always mention dry mass in exams when describing biomass.
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Ecological pyramids
Ecological pyramids always place producers at the base.
They help visualise how energy and matter decrease as you move up a food chain.
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Pyramid of numbers
A pyramid of numbers counts individuals, not energy.
- Often pyramid-shaped
- Can be inverted (e.g. insects feeding on one tree)
- Does not show energy transfer
Pyramid of biomass
A pyramid of biomass represents stored energy more accurately than numbers.
- Measured in g m⁻²
- Usually pyramid-shaped on land
- Can be inverted in aquatic ecosystems
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Pyramid of energy
A pyramid of energy is the most accurate pyramid.
- Always pyramid-shaped
- Measured in kJ m⁻² yr⁻¹
- Shows real energy losses
Pyramids of energy can never be inverted.
IB-style question — Why a pyramid of numbers can be inverted
Explain why a pyramid of numbers for a forest food chain may be inverted, while a pyramid of biomass for the same chain is usually upright. [3]
How to answer it, step by step
- Numbers can invert
• One large tree (producer) supports thousands of caterpillars → consumer bar wider than producer bar.
• Numbers count individuals, not mass, so organism size drives the shape. - Biomass stays upright
• The tree's dry mass far exceeds the caterpillars it feeds → producer bar remains widest.
• Energy (and biomass) is lost as heat at every trophic transfer — less biomass can be supported higher up.
Final answer
Full marks need both the reason for inversion (large individual producer) AND why biomass resists inversion (producer dry mass exceeds consumer dry mass). Classic error: saying 'more energy in producers' — correct idea, wrong pyramid type.
IB-style question — Measuring biomass: the dry-mass method
Outline the dry-mass method used to measure the biomass of a plant sample, and explain why organisms must be killed. [4]
How to answer it, step by step
- Collect → oven-dry → weigh to constant mass
• Harvest the sample; record fresh mass (includes water).
• Oven-dry at ~80 °C; re-weigh repeatedly until mass stops falling → that is the dry mass = biomass. - Why organisms must be killed + scale up
• Heating to remove water destroys the organism — unavoidable; water content varies and obscures true organic mass.
• Express result per unit area (e.g. g m⁻²) to compare trophic levels or sites.
Final answer
Examiners need: oven-drying, repeated weighing to constant mass, and an explicit reason organisms are killed (water removal destroys them). Just 'weigh them' with no mention of drying scores 0.