Pyramids of Number & Biomass (Cambridge (CIE) IGCSE Biology)

Revision Note

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Lucy Kirkham

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Pyramids of Number

  • 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:

 

Food chain, IGCSE & GCSE Biology revision notes

A food chain shows the transfer of energy

  • 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?

  • So, a pyramid of numbers for this food chain would look like this:

Pyramid of numbers 1, IGCSE & GCSE Biology revision notes

A pyramid of numbers

  • Despite the name (and the example above), a pyramid of numbers doesn’t always have to be pyramid-shaped, for example:

Pyramids of numbers 2, IGCSE & GCSE Biology revision notes

Pyramids of numbers are not always pyramid-shaped

  • This is because the size of the organism is also important - one large organism, like the oak tree in the pyramid above, contains enough energy to support many smaller organisms (the insects)

Rules to remember when drawing a pyramid of numbers:

  • You cannot change the trophic level of the organisms - they must stay in the same order as in the food chain with producers on the bottom, followed by primary consumers, then secondary consumers, then tertiary consumers

  • Generally, the larger an individual organism is, the less of them there are

Pyramids of Biomass

  • A pyramid of biomass shows how much mass the creatures at each level would have without including all the water that is in the organisms (their ‘dry mass’)

  • 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 has to decrease as you go up a food chain – if we take our first food chain as an example, it’s impossible to have 10kg of grass feeding 50kg of voles feeding 100kg of barn owls

A pyramid of biomass, IGCSE & GCSE Biology revision notes

A pyramid of biomass

  • Pyramids of biomass provide a much better idea of the quantity of the plant or animal material at each level of a food chain and therefore are a better way of representing interdependence within the food chain

Examiner Tips and Tricks

Pyramids of biomass are generally pyramid-shaped, so they are simple to draw. 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.

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. 

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Lucy Kirkham

Author: Lucy Kirkham

Expertise: Head of STEM

Lucy has been a passionate Maths teacher for over 12 years, teaching maths across the UK and abroad helping to engage, interest and develop confidence in the subject at all levels.Working as a Head of Department and then Director of Maths, Lucy has advised schools and academy trusts in both Scotland and the East Midlands, where her role was to support and coach teachers to improve Maths teaching for all.