GCSEBiologyAQARevision NotesEcologyTrophic Levels in an EcosystemPyramids of Biomass

Pyramids of Biomass (AQA GCSE Biology): Revision Note

Exam code: 8461

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Dr Natalie Lawrence

Updated on

Pyramids of biomass

Biomass

  • Biomass is the mass of living tissues (or recently dead tissues that have not yet been decomposed) that a plant or animal is made up of

    • For example, the wood of a recently fallen tree is still biomass, even though the organism is no longer alive

  • It is the ‘dry mass’ of the organism (how much mass the plant or animal would have without including all the water that is in the organism)

  • Biomass decreases at each successive trophic level

  • For example, for the simple food chain below, adding up the total biomass of all the grass in this ecosystem would give a greater figure than the total biomass of all the grasshoppers, which would in turn be greater than the total biomass of all the frogs

Diagram of biomass over trophic levels

Diagram of a three-level food chain showing biomass at each trophic level. Grass is the producer at trophic level 1, with a biomass of 50 kg. An arrow leads from the grass to a grasshopper, the primary consumer at trophic level 2, with a biomass of 10 kg. An arrow leads from the grasshopper to a frog, the secondary consumer at trophic level 3, with a biomass of 2.5 kg. Biomass decreases at each successive trophic level.
Biomass decreases along the food chain from 50 kg in the producer to 10 kg in the primary consumer and 2.5 kg in the secondary consumer

Use this image

Pyramids of biomass

  • A food chain can be represented as a type of modified bar chart called a pyramid of biomass

  • A pyramid of biomass shows the relative biomass (mass of living or recently dead material) at each trophic level in a food chain

  • A pyramid of biomass is always a pyramid shape (as seen in the image below)

  • For example, the pyramid of biomass below shows that the total biomass of clover plants is greater than the total biomass of snails, which is greater than the total biomass of thrushes, which is greater than the total biomass of sparrowhawks

Diagram showing a four-level food chain alongside a pyramid of biomass. Clover is the producer and is eaten by a snail, the primary consumer. The snail is eaten by a thrush, the secondary consumer, which is eaten by a sparrowhawk, the tertiary consumer. Beside the food chain is a four-tier pyramid that becomes progressively narrower towards the top, representing decreasing biomass at successive trophic levels.
A food chain and corresponding pyramid of biomass, showing that biomass decreases at higher trophic levels

Use this image

  • Being able to construct accurate pyramids of biomass from appropriate data is an important skill for a biologist

Worked Example

Below is a food chain with four trophic levels:

Clover → Snail → Thrush → Sparrowhawk

  • The total biomass of the organisms at each trophic level (kg):

    • Clover: 80

    • Snail: 30

    • Thrush: 10

    • Sparrowhawk: 2

Question 1: Draw a pyramid of biomass for this food chain.

Answer:

Biomass pyramid for a food chain of clover, snail, thrush and sparrowhawk. Horizontal bars are centred on a midpoint and decrease in width at each trophic level, showing decreasing biomass from clover to sparrowhawk. Annotations indicate that bars should touch, be equally spaced around the midpoint, each trophic level should be labelled, and the x-axis should use a suitable regular scale and be labelled “Biomass (kg)

Question 2: Calculate the percentage of the biomass lost between the clover and the sparrowhawk. Give your answer to 2 significant figures.

Answer:

Biomass of clover: 80
Biomass of sparrowhawk: 2
Total original biomass (biomass of clover): 80

(80−280)× 100 = 97.5

= 98%

Examiner Tips and Tricks

Calculate the difference in biomass first, then work this out as a percentage of the total original biomass.

Examiner Tips and Tricks

As there is always less and less biomass as you move up through the trophic levels in a food chain, pyramids of biomass are always pyramid-shaped: the higher the trophic level the smaller the bar will be.

Remember: trophic level 1 organisms (producers) are at the bottom of the pyramid. The next bar up is primary consumers, the next is secondary consumers and so on up the food chain.

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Author
Reviewer
Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Content Creator

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.

Dr Natalie Lawrence

Reviewer: Dr Natalie Lawrence

Expertise: Content Writer

Natalie has a MCantab, Masters and PhD from the University of Cambridge and has tutored biosciences for 14 years. She has written two internationally-published nonfiction books, produced articles for academic journals and magazines, and spoken for TEDX and radio.