Ecological Pyramids

An ecological pyramid is a graphic representation of the relationship between the organisms of successive trophic levels in an ecosystem, expressed in terms of number, biomass, or energy — a concept first developed by C. Elton in 1927. Every such pyramid has a broad base, formed by the producers (the first trophic level), narrowing upward toward an apex representing the top or tertiary consumers; any calculation at a given trophic level must include all the organisms present at that level. Three kinds of ecological pyramids are usually studied: the pyramid of numbers (relative number of individuals per unit area at each trophic level), the pyramid of biomass (biomass per unit area at each level), and the pyramid of energy (accumulated energy per unit area at each level).

In most well-balanced ecosystems, all three pyramids are upright — producers exceed herbivores in both number and biomass, and herbivores in turn exceed carnivores. There are, however, notable exceptions. The pyramid of numbers can be inverted, as when a single tree (producer) supports a very large number of insects (primary consumers), which support a smaller number of birds (secondary consumers), which in turn support only a handful of parasites (tertiary consumers). The pyramid of biomass can also be inverted, as in oceanic ecosystems, where the standing biomass of fast-reproducing, short-lived phytoplankton is smaller than that of the zooplankton and fish that consume them, even though the phytoplankton's rate of production is high.

The pyramid of energy, by contrast, is always upright and can never be inverted, because at every transfer from one trophic level to the next only about 10% of the energy passes on as net energy, with a large fraction always lost as heat — consequently, shorter food chains retain more available energy overall than longer ones. …