Pyramids of Number & Biomass (Cambridge (CIE) IGCSE Biology): Revision Note

Pyramids of numbers

• A pyramid of numbers shows how many organisms are present at each level of a food chain.

• The width of the box indicates the number of organisms at that trophic level

• Consider the following food chain:

• A pyramid of numbers for the food chain above would look like this:

• A pyramid of numbers doesn’t always have to be pyramid-shaped; this is because the size of the organism is also important

• One large organism, like the oak tree in the pyramid below, contains enough energy to support many smaller organisms

Rules to remember when drawing a pyramid of numbers

• When drawing a pyramid of numbers, the trophic level of the organisms cannot be changed - they must stay in the same order as in the food chain:

  • producers on the bottom

  • followed by primary consumers

  • then secondary consumers

  • then tertiary consumers

• Each bar should be labelled with its trophic level

• If graph paper is provided then the pyramid should be drawn to scale

• Generally, the larger an individual organism is, the fewer individuals there will be

Pyramids of biomass

• A pyramid of biomass shows how much living matter is present at each trophic level of a food chain

• Biomass is often a measure of the dry mass of an organism, once the water has been removed from the tissues

• Pyramids of biomass are always pyramid-shaped, regardless of what the pyramid of numbers for that food chain looks like

• This is because the mass of organisms decreases as you go up a food chain

• It can be advantageous to use a pyramid of biomass rather than a pyramid of numbers because:

  • pyramids of biomass take into account the size of organisms at each trophic level, so provide a more accurate representation of the living matter within an ecosystem

  • energy is stored in biomass, so biomass can be used as an approximate representation of the energy available at each level of a food chain