Syllabus Edition
First teaching 2015
Last exams 2025
Pyramid of Biomass
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Pyramids of Biomass
A pyramid of biomass measures the amount of biomass present at each trophic level at a specific point in time and represents the standing stock of each trophic level
Biomass can be measured in grams of biomass per square metre (g m–2) or units of energy, such as joules per square metre (J m–2)
As per the second law of thermodynamics, the quantities of biomass generally decrease along food chains, so the pyramids become narrower towards the top
This means that pyramids of biomass are usually pyramid-shaped but can occasionally be inverted and show higher quantities at higher trophic levels
Pyramids of biomass are snapshots of the ecosystem and may not indicate productivity over time
A fertile intensively grazed pasture may have a lower standing crop biomass of grass but higher productivity than a less fertile ungrazed pasture because biomass is constantly removed by herbivores, resulting in an inverted pyramid of biomass
Inverted pyramids sometimes occur due to marked seasonal variations
For example, in some marine ecosystems, the standing crop of phytoplankton, the major producers, is lower than the mass of the primary consumers, such as zooplankton
This is because the phytoplankton reproduce very quickly and are constantly being consumed by the primary consumers, which leads to a lower standing crop but higher productivity
This can occur because phytoplankton can vary greatly in productivity (and therefore biomass) depending on sunlight intensity
Constructing a Pyramid of Biomass
Being able to construct accurate pyramids of biomass from appropriate data is an important skill
Worked Example
Figure 1 shows:
A food chain with four trophic levels
The total mass of organisms at each trophic level
Figure 1
| Clover → | Snail → | Thrush → | Sparrowhawk |
Biomass in kg | 80 | 30 | 10 | 2 |
Draw a pyramid of biomass for the food chain in Figure 1.
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