Pyramid of Numbers (DP IB Environmental Systems & Societies (ESS))

Pyramids of Numbers

• Ecological pyramids include:

  • Pyramids of numbers

  • Pyramids of biomass

  • Pyramids of productivity

• They are quantitative models usually measured for a given time and area

• A pyramid of numbers shows how many organisms we are talking about at each level of a food chain

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

• For example, consider the following food chain:

Grass → Vole → Owl

• Ask yourself the following questions:

  • Is it likely that there would be more voles in an area than grass plants?

  • How many voles might one barn owl need to eat per day? If it’s more than one, is it likely that there are more barn owls in an area than voles?

• A pyramid of numbers for this food chain would look like the one shown below

  • In accordance with the second law of thermodynamics, there is a tendency for number of organisms to decrease along food chains, as there is a decrease in available (chemical potential) energy, since some energy is lost to the surroundings at each trophic level

  • Therefore the pyramids become narrower towards the apex

A pyramid of numbers

• Despite the name, a pyramid of numbers doesn’t always have to be pyramid-shaped

• For example, consider the following food chain:

Oak tree → Insects → Woodpecker

• The pyramids of numbers for this food chain will display a different pattern to the first food chain

• When individuals at lower trophic levels are relatively large, like the oak tree, the pyramid becomes inverted

Pyramids of numbers are not always pyramid-shaped (they can be inverted, like the one shown above)