pyramid of numbers

Pyramids of numbers and pyramid of biomass. The pyramid of numbers is a useful way of representing a food chain as it shows how the number of consumers at each level decreases, with plants being the most numerous at the base of the pyramid and top carnivores the smallest group. Where the plant being eaten is a tree, however, the pyramid no longer works as a useful model. This is rectified by the use of the pyramid of biomass, where it is the mass of the levels rather than numbers that are represented.

In ecology, a diagram that shows quantities of plants and animals at different levels (steps) of a food chain. This may be measured in terms of numbers (how many animals) or biomass (total mass of living matter), though in terms of showing transfer of food, biomass is a more useful measure. Where biomass is measured, the diagram is often termed a pyramid of biomass. There is always far less biomass, or fewer organisms, at the top of the chain than at the bottom, because only about 10% of the food (energy) an animal eats is turned into flesh - the rest is lost through metabolism and excretion. The amount of food flowing through the chain therefore drops with each step up the chain, supporting fewer organisms, hence giving the characteristic ‘pyramid’ shape.

In a pyramid of biomass, the primary producers (usually plants) are represented at the bottom by a broad band, the plant-eaters are shown above by a narrower band, and the animals that prey on them by a narrower band still. At the top of the pyramid are the few ‘top carnivores’ such as lions and sharks.