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.
