How Do You Read a Pyramid of Biomass?
Learn to read a pyramid of biomass for AQA GCSE Biology: identify trophic levels, compare bar widths and calculate biomass transfer with a worked example.
Read a pyramid of biomass from the bottom up. Each bar represents the biomass at one trophic level in a food chain. The bottom bar is the producer, usually a plant or alga. Above it are the primary consumer, secondary consumer and, if there is one, tertiary consumer.
What does the width show?
Biomass is the mass of living material. A wider bar shows more biomass at that trophic level. Compare the widths to see how much biomass is available at each stage of the food chain. Trophic level means an organism’s position in that chain: producers are level 111, and the organisms that eat them are level 222.
This is biology-only content in AQA GCSE Biology, for both Foundation and Higher tier. On a diagram, check the biomass values and units before comparing bars. The widths should be proportional to the values, so you can compare levels fairly.
Usually, each bar is narrower than the one below because only some biomass is transferred when organisms eat other organisms. Not all the material eaten is absorbed; some leaves as faeces. Organisms also use glucose in respiration and lose materials as waste.
Worked example
Suppose a pyramid shows 1000 g1000\,\text{g}1000g of producer biomass, 100 g100\,\text{g}100g of primary consumer biomass and 10 g10\,\text{g}10g of secondary consumer biomass. To find the percentage transferred from producers to primary consumers, divide the biomass at the higher level by the biomass at the level below:
1001000×100=10%\frac{100}{1000}\times 100=10\%1000100×100=10%So 10%10\%10% of the producer biomass was transferred to the primary consumers in this example. The same calculation gives 10%10\%10% from primary to secondary consumers. These figures are example data: the percentage is not guaranteed to be exactly 10%10\%10% in every food chain.
Exam mistake to avoid: bar width shows biomass, not the number of organisms. A narrow bar does not, by itself, tell you how many organisms are at that level.
