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Pyramids of numbers and biomass

Organisms and their environment · Trophic levels and ecological pyramids · note 2 of 4

Spec 19.2.11, 19.2.12Download all Trophic levels and ecological pyramids notes (PDF)Download all notes (PDF)6 pages

Pyramids of numbers and biomassSpec 19.2.11, 19.2.12

An ecological pyramid has one horizontal bar for each trophic level, with the producers at the bottom. The length of each bar is drawn to scale.

  • A pyramid of numbers shows the number of organisms at each trophic level.
  • A pyramid of biomass shows the mass of living material (biomass) at each trophic level, for example in g per m².

Drawing a pyramid

  1. Choose a scale so that the longest bar fits on the page.
  2. Draw a bar for each trophic level, with the producers at the bottom. Make the length of each bar proportional to its value.
  3. Centre the bars on a vertical line and label each bar with the organism.
Pyramid of biomass drawn to scale: a wide bar for grass (400 g per m²) at level 1, a narrower bar for grasshoppers (40 g per m²) at level 2 and a very thin bar for frogs (4 g per m²) at level 3, with a scale bar. (opens full size in a new tab)
A pyramid of biomass for grass, grasshoppers and frogs, drawn to scale.

Pyramids of numbers can be the wrong shape

A pyramid of numbers does not always get narrower. One large oak tree can support hundreds of caterpillars, so the bar for the producers is shorter than the bar above it. The pyramid is not pyramid-shaped and is often described as inverted. The size of the organisms is ignored.

For the food chain oak tree, caterpillars, blue tits: a pyramid of numbers drawn to scale with a very narrow bar for 1 oak tree, a wide bar for 500 caterpillars and a narrow bar for 10 blue tits; and a pyramid of biomass with the widest bar for the oak tree and narrower bars above. (opens full size in a new tab)
The same food chain: the pyramid of numbers is an odd shape because one large tree supports many caterpillars, but the pyramid of biomass is pyramid-shaped.

Advantage of a pyramid of biomass

A pyramid of biomass is usually a better way to represent a food chain than a pyramid of numbers, because:

  • it takes the size of the organisms into account, so one large tree is not shown as equal to one tiny insect;
  • it is usually pyramid-shaped, because there is less biomass at each higher trophic level.
Exam tip:

To explain an odd-shaped pyramid of numbers, say that the producer is one very large organism that supports many smaller consumers. Say that numbers ignore the size of organisms but biomass does not.