Pyramid of Biomass (DP IB Environmental Systems & Societies (ESS))

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