Ecological Pyramids

It is important to recognize that the second law of thermodynamics only applies to ecological productivity (and to the closely related variable of energy flow). Consequently, only the trophic structure of productivity is always pyramid shaped. In some ecosystems other variables may also have a trophic structure that is pyramid shaped, for example, the quantities of biomass (also known as standing crop) present at a particular time, or the sizes or densities of populations. However, these latter variables are not pyramid shaped for all ecosystems.

One example of plants having a similar, or even smaller total biomass as the herbivores that feed upon them occurs in the open ocean. In that planktonic ecosystem the phytoplankton (or single-celled algae) typically maintain a similar biomass as the small animals (called zooplankton) that feed upon these microscopic plants. However, the phytoplankton cells are relatively short-lived, and their biomass is regenerated quickly because of the high productivity of these microorganisms. In contrast, the herbivorous zooplankton are longer lived, and they are much less productive than the phytoplankton. Consequently, the productivity of the phytoplankton is much larger than that of the zooplankton, even though at any particular time their biomasses may be similar.

In some ecosystems, the pyramid of biomass may be inverted, that is, characterized by a larger biomass of herbivores than of plants. This can sometimes occur in grasslands, where the dominant plants are relatively small, herbaceous species that may be quite productive, but do not maintain much biomass at any time. In contrast, the herbivores that feed on the plants may be relatively large, long-lived animals, and they may maintain a larger total biomass than the vegetation. Inverted biomass pyramids of this sort occur in some temperate and tropical grasslands, especially during the dry seasons when there can be large populations, and biomasses, of long-lived herbivores such as deer, bison, antelopes, gazelles, hippopotamuses, rhinos, elephants, and other big animals. Still, the annual productivity of the plants in grasslands is much larger than that of the herbivores.

Similarly, the densities of animals are not necessarily less than those of the plants that they eat. For example, insects are the most important herbivores in most forests, where they can maintain very large population densities. In contrast, the densities of tree populations are much smaller, because each individual organism is large and occupies a great deal of space. In such a forest, there are many more small insects than large trees or other plants, so the pyramid of numbers is inverted in shape. However, the pyramid of productivity in the forest is still governed by the second law of thermodynamics, and it is much wider at the bottom than at the top.