Ecological pyramids summarize how quantities are distributed across trophic levels, but different pyramid types measure different things.

An energy pyramid compares the rate of energy production available at successive trophic levels, often in units such as $\mathrm{kJ,m^{-2},yr^{-1}}$. Because some energy is dissipated as heat at every transfer, energy pyramids are always upright: production at a higher trophic level cannot sustainably exceed the production supplying it from below.

A biomass pyramid compares the amount of living material present at one moment. Biomass pyramids can be inverted. For example, rapidly reproducing phytoplankton may maintain a small standing biomass while supporting a larger standing biomass of zooplankton because producer biomass is replaced and consumed quickly.

A pyramid of numbers compares counts of organisms and can also be inverted. One large tree can support thousands of herbivorous insects.

The distinction is therefore:

• energy pyramid: rate of usable energy flow;
• biomass pyramid: standing mass;
• numbers pyramid: individual abundance.

Confusing these quantities leads to apparent paradoxes. A low standing producer biomass does not imply low productivity, and many consumers do not imply that more energy exists at the consumer level. The pyramid must be interpreted according to the quantity actually measured.