Ecological pyramids

An ecological pyramid shows how much is present at each trophic level, drawn as bars stacked on each other: producers at the base, top carnivores at the top. Three kinds are drawn:

• Pyramid of numbers: how many organisms at each level.
• Pyramid of biomass: the weight of living matter at each level.
• Pyramid of energy: the energy at each level.

Pyramid of numbers. In a grassland, millions of grass plants support fewer grasshoppers, fewer birds, and only a handful of top carnivores. The pyramid is upright. (But consider one large tree with thousands of insects on it: the pyramid of numbers is then partly inverted.)

Pyramid of biomass. It is usually upright, but in the sea it is inverted: the total weight of fish is greater than that of phytoplankton at any moment. The phytoplankton have a small standing crop, but they reproduce so fast that they can still feed the larger mass of animals above them.

Pyramid of energy. This one is always upright, and can never be inverted. Energy passes from one level to the next, and at every step some is lost as heat, so each higher level must have less energy than the one below. Producers themselves fix only about 1% of the sunlight that falls on them as NPP.

Pyramids have limits:

• A species can feed at more than one level at once: a sparrow is a primary consumer when it eats seeds and fruits, and a secondary consumer when it eats insects and worms. Pyramids cannot show this.
• They assume simple food chains, which never exist in nature; they do not show food webs.
• They leave out saprophytes (the decomposers), even though decomposers play a vital role in the ecosystem.