Pyramids of biomass

Biomass is the amount of organic matter contained in living things. A graph showing the amount of organic matter in the organisms at each trophic level is a pyramid of biomass. It is given as grams per square metre per year (g m⁻² yr⁻¹), using the dry mass of the organisms.

Usually the biomass of consumers is less than the biomass of producers, so pyramids of biomass are usually upright (figure 15.12). But now and then, in aquatic environments, the biomass of consumers is greater than that of producers, and the pyramid of biomass can then be inverted.

Figure 15.12 — a pyramid of biomass (g m⁻² yr⁻¹).

Why weigh them dry

Weighing puts the coconut palm back in proportion: one tree weighs far more than all its beetles and their parasites together, so the pyramid that was inverted by counting becomes upright by mass. Dry mass is used because the water in a body varies — a jellyfish is almost all water, a seed very little — and water is not food energy. The rare inverted pyramid of biomass comes from water: tiny algae are few at any moment, but they reproduce so fast that they feed a larger mass of small animals that live longer.