Ecological pyramids

The essentials

The relationship between producers and consumers in terms of number, biomass or energy can be shown as an ecological pyramid. The base is formed by producers (first trophic level) and the apex by the top-level consumers. The three types are:

• Pyramid of number: the number of individuals at each trophic level.
• Pyramid of biomass: the dry weight of organisms at each trophic level.
• Pyramid of energy: the energy content of each trophic level.

For calculations, all organisms at each trophic level must be included; considering only a few individuals would not be a true picture. Each pyramid deals with a trophic level, not with a species; a species may occupy more than one trophic level in different food chains.

Shapes: In most ecosystems, all three pyramids are upright: producers are more in number and biomass than herbivores, and herbivores more than carnivores. Exceptions:

• The pyramid of number can be inverted, for example when insects feed on a single large tree.
• The pyramid of biomass in the sea is generally inverted, because the biomass of fishes far exceeds that of phytoplankton at any moment (phytoplankton have a small standing crop but reproduce very fast).
• The pyramid of energy is always upright and can never be inverted, because some energy is always lost as heat at each step when it flows from one trophic level to the next.

Limitations of ecological pyramids:

• They do not take into account the same species belonging to two or more trophic levels.
• They assume a simple food chain, which almost never exists in nature; they do not accommodate a food web.
• Saprophytes are not given any place in ecological pyramids, even though they play a vital role in the ecosystem.

Common mistakes

• An inverted pyramid of biomass does not mean that energy increases upwards.
• Pyramids refer to trophic levels, not to individual species.