Ecological pyramids

Food or energy relationships between trophic levels, drawn as bars, give a pyramid shape: broad at the base, narrow at the apex.

The base stands for producers (first trophic level) and the apex for the top consumers.

Three kinds are usually studied: the pyramid of number, of biomass and of energy.

In a grassland, the pyramid of number shows only three top carnivores supported by the production of nearly 6 million plants.

The pyramid of biomass usually falls sharply at higher trophic levels.

In most ecosystems all three pyramids are upright: producers outnumber and outweigh herbivores, which outnumber and outweigh carnivores.

Exceptions: counting the insects on one big tree, and then the small and larger birds that depend on them, gives an inverted pyramid of number.

The pyramid of biomass in the sea is generally inverted: a small standing crop of phytoplankton supports a much larger biomass of zooplankton and fishes.

The pyramid of energy is always upright and can never be inverted, because some energy is lost as heat at every transfer. Producers convert only about 1 per cent of the sunlight available to them into NPP.