Pyramids of Number & Biomass (Cambridge International IGCSE Biology)

Pyramids of numbers

Spec point — 19.2.11 · Draw, describe and interpret 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 food chain shows the transfer of energy

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

A pyramid of numbers

• 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

Pyramids of numbers are not always pyramid-shaped

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

Spec point — 19.2.11 · Draw, describe and interpret pyramids of biomass Discuss the advantages of using a pyramid of biomass rather than a pyramid of numbers to represent a food chain Describe a trophic level as the position of an organism in a food chain, food web or ecological pyramid Identify the following as the trophic levels in food webs, food chains and ecological pyramids: producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers

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

A pyramid of biomass

• 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

Exam Hint Pyramids of number can be any shape, so make sure you learn the rules for drawing a pyramid of numbers as it is more common to see unusual shapes in the exam.

Pyramids of biomass are generally pyramid-shaped, so they are simpler to sketch. Some ecosystems, such as an aquatic ecosystem, may show a different shape or inverted pyramid but it is very unlikely you will see this in an exam.