Pyramid of Numbers

The pyramid of numbers represents the number of individual organisms present at each trophic level of a food chain, usually counted per unit area or per unit volume of the ecosystem being studied, at a given point in time.

In many common ecosystems, the pyramid of numbers is upright -- broad at the base and narrowing toward the apex -- because the number of individual organisms genuinely decreases at each successive trophic level. A grassland ecosystem is the standard example: an enormous number of individual grass plants (producers) supports a smaller number of herbivorous insects and small mammals (primary consumers) such as grasshoppers and mice, which in turn support a still smaller number of small carnivores such as frogs or lizards (secondary consumers), which finally support only a very small number of top predators such as hawks (tertiary consumers).

However, the pyramid of numbers is not always upright, and two well-known situations produce a different shape. The first is a food chain built around a single large producer, most classically a single mature tree. One tree, though it is only a single individual at the base of the chain, is able to support a very large number of individual insects that feed on its leaves and bark as primary consumers; these insects, though fewer in number than the leaves and bark cells they consume, are still far more numerous than the smaller population of insect-eating birds that prey on them as secondary consumers; and those birds, in turn, support only a handful of parasites living on them as tertiary consumers. Counted strictly by number of individuals at each level, this chain therefore rises sharply from one producer to very many primary consumers before narrowing again through the higher levels, giving a pyramid of numbers that is INVERTED, or bottom-heavy, at its base -- the opposite shape to the grassland example. …