Pyramids of Number & Biomass | Biology for GCSE/IGCSE - Year 11 PDF Download

Pyramids of Number

• A pyramid of numbers illustrates the quantity of organisms present at each trophic level within a food chain. 
• The width of each box corresponds to the number of organisms at that specific trophic level. 
• For instance, let's examine the following food chain:

A food chain shows the transfer of energy

• Consider the following inquiries:
  • Is it probable that the number of voles in a given area exceeds the number of grass plants?
  • How many voles might be required for a single barn owl's daily sustenance? If the number exceeds one, is it plausible that there are more barn owls than voles in the area?
• Thus, a pyramid of numbers representing this food chain would appear as follows:

• Despite the name and common examples, pyramids of numbers do not always conform to a pyramid shape.

Pyramids of numbers are not always pyramid-shaped

• The size of an organism is crucial in a pyramid of numbers. A larger organism, such as an oak tree, holds enough energy to sustain numerous smaller organisms like insects.

Rules for Drawing a Pyramid of Numbers:

• You must maintain the trophic levels of the organisms in the same sequence as in the food chain, starting with producers at the base, followed by primary consumers, secondary consumers, and tertiary consumers.
• Typically, the population of larger individual organisms is smaller in comparison.

Pyramids of Biomass

• A pyramid of biomass illustrates the mass of organisms at each trophic level, excluding water content, known as their 'dry mass'.
• Pyramids of biomass are consistently pyramid-shaped, irrespective of the pyramid of numbers in a food chain, due to the necessity for decreasing organism mass up the chain.
• This shape is a consequence of the principle that as we move up a food chain, the mass of organisms must diminish. For instance, it's impractical to have 10kg of grass supporting 50kg of voles, which in turn feed 100kg of barn owls.

A pyramid of biomass

Pyramids of biomass offer a more accurate representation of the amount of plant or animal material at each trophic level of a food chain, making them a superior method for illustrating interdependence within the chain.