Pyramids of biomass — AQA GCSE Biology
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Pyramids of biomass explained
A pyramid of biomass is a diagram that shows the relative mass of living material at each trophic level of a food chain.
Read the full explanation
Each level is drawn as a horizontal bar whose width is proportional to the biomass present, and the bars are stacked with trophic level 1 (producers) at the bottom. Biomass is the mass of living material, often measured as dry mass per unit area, for example in grams per square metre (g/m²). Because energy is lost between trophic levels through respiration, movement, excretion and uneaten material, the biomass always decreases at each successive level. Consequently, at GCSE level, a pyramid of biomass is always a true pyramid shape and is never inverted, unlike pyramids of numbers which can take irregular shapes.
Trophic level 1 is at the bottom of the pyramid.
In a pyramid of biomass, trophic level 1 is always placed at the bottom. Trophic level 1 contains the producers, which are plants or algae that make their own food by photosynthesis. Producers capture light energy and convert it into chemical energy stored in biomass, forming the base of the food chain. Because they are the first level and have the greatest biomass, they are drawn as the lowest and widest bar. Higher trophic levels are stacked above in sequence: primary consumers at level 2, secondary consumers at level 3, and so on. Pyramids of biomass are always upright, showing the decrease in biomass at each successive stage and the direction of energy flow.
Students should be able to construct accurate pyramids of biomass from appropriate data.
Constructing a pyramid of biomass means drawing a stacked bar chart in which each bar represents one trophic level and the bar width is proportional to the biomass at that level. To do this accurately, first identify the trophic levels and their biomass values from the data, then choose a suitable scale so the largest value fits the space available. Draw horizontal bars with widths calculated from the scale, stack them with trophic level 1 at the bottom, and label each level with its organisms and biomass value, including units such as g/m². A ruler and sharp pencil help produce neat, proportional bars. Accuracy is assessed by whether the bar widths correctly represent the data and whether the pyramid is clearly and correctly labelled.
Your focus
- Describe what a pyramid of biomass represents and how it differs from a pyramid of numbers.
- Construct a pyramid of biomass using proportional bar widths and a consistent scale.
- Explain why biomass always decreases at higher trophic levels, resulting in an upright pyramid shape.
Show all 9 objectives
- Identify trophic level 1 as the producers at the bottom of a pyramid of biomass.
- Sequence trophic levels correctly from producers to consumers.
- Explain why pyramids of biomass are always upright.
- Select and use a suitable scale to represent biomass data.
- Draw proportional bars to construct an accurate pyramid of biomass.
- Label trophic levels and biomass values correctly, including units.
Pyramids of biomass exam tips
Marking Points
- A pyramid of biomass represents the relative mass of living material at each trophic level.
- The width of each horizontal bar is strictly proportional to the biomass at that level.
- Bars are stacked symmetrically with trophic level 1 (producers) at the bottom and higher levels above.
- Biomass is often measured as dry mass per unit area, for example g/m².
- Biomass always decreases at higher trophic levels because energy and mass are lost between levels (e.g., through faeces, urea, and respiration).
- Pyramids of biomass are always upright (pyramid-shaped) at GCSE level, reflecting this continuous loss of biomass.
- Trophic level 1 is the bottom bar of a pyramid of biomass.
- Trophic level 1 contains the producers, such as plants or algae.
- Producers make their own food by photosynthesis and form the base of the food chain.
- Higher trophic levels are stacked above level 1 in order.
- Pyramids of biomass are always upright, so the bottom bar is always the widest.
- Identify the biomass value for each trophic level from the data.
- Choose a suitable scale that allows all bars to fit and remain proportional.
- Draw each bar with a width proportional to its biomass value.
- Stack the bars with trophic level 1 at the bottom and higher levels above.
- Label each trophic level with the organism type and biomass value, including units.
- Use a ruler and pencil to produce neat, accurate, proportional bars.
Examiner Tips
- 💡Label each trophic level clearly, for example producer, primary consumer and secondary consumer, and ensure level 1 is at the base.
- 💡Use a ruler and a consistent scale when drawing bars so the widths are proportional to the data provided.
- 💡Include units such as g/m² or kg/m² when stating biomass values.
- 💡Always label the bottom bar as trophic level 1 and identify the organisms as producers.
- 💡Remember that pyramids of biomass must be drawn to scale and are always upright.
- 💡Show your scale clearly, for example 1 cm represents 10 g/m², so the examiner can check proportionality.
- 💡Use a ruler for every bar and keep the bars horizontal and evenly spaced.
- 💡Label each level with both the organism and the biomass value to make the pyramid fully accurate.
Common Mistakes
- Confusing a pyramid of biomass with a pyramid of numbers; biomass uses mass of living material, not a count of organisms, so it is always pyramid-shaped.
- Drawing bars with widths that are not proportional to the biomass values; each bar width must represent the actual biomass using a consistent scale.
- Believing a pyramid of biomass can be inverted; correct this by remembering that at GCSE, biomass pyramids are always upright due to biomass loss at each stage.
- Placing consumers at the bottom of the pyramid; trophic level 1 is always the producers.
- Numbering trophic levels from the top downwards; numbering starts at the bottom with level 1.
- Thinking pyramids of biomass can be inverted; unlike pyramids of number, pyramids of biomass are always upright.
- Drawing bars of equal width regardless of the biomass values; each width must be proportional to the data.
- Forgetting to include units such as g/m² when labelling biomass values.
- Placing trophic level 1 at the top; the producers must be at the bottom.