A useful way to investigate the structure and energy flow of an ecosystem is to create a trophic pyramid, a diagram that stacks the trophic categories on top of one another.

The base of the pyramid, or first trophic level, is made up of all the producers. The second trophic level consists of herbivores, also called the primary consumers because they eat the producers. The third trophic level is made up of secondary consumers, or carnivores, organisms that eat the primary consumers. The fourth trophic level consists of tertiary consumers, carnivores that consume other carnivores.

The size of each level in a trophic pyramid can be derived in several different ways. Trophic pyramids usually display one of three measures – numbers, biomass, or energy. Trophic pyramids allow scientists to study and compare the structure of communities, and assess their stability and vulnerabilities.

Energy Pyramid
An energy pyramid compares the amount of energy present in each trophic level. In all viable ecosystems, the base of the energy pyramid is by far the biggest level, and each step upward gets progressively narrower, just like an archaeological pyramid. The upward-narrowing shape of an energy pyramid reflects the inescapable Laws of Thermodynamics.

The First Law says that energy is not created or destroyed, though it can change form.

The Second Law says that when energy is converted from one form to another form, some energy is lost as heat.

These two laws are operating in every ecological community, and because of them energy is lost from the living part of the ecosystem with transfer from one trophic level to the next. The base of the energy pyramid, the producer level, receives a constant and apparently unlimited input of solar radiation, so this first trophic level holds a large amount of energy. But the second trophic level, the level of primary consumers, holds much less energy because so much of it is lost as heat when plant tissue is converted to animal tissue.

With every step up the pyramid, energy is dissipated as heat as it is reworked during its passage from organisms to organism. More energy is used up in respiration, movement, excretion, and the other processes of life. On average, each trophic level retains only about 10% of the energy contained in the previous level. Because of the large energy drops, trophic pyramids and food chains can only have 4 or 5 steps at most.

Numbers Pyramid
A numbers pyramid represents the number of individuals in each trophic level. This is a little easier to measure or to estimate than energy. Like an energy pyramid, a numbers pyramid is usually broadest at the base – it takes many blades of grass to nourish a single cow. But the pyramid will be inverted or diamond-shaped if the organisms on higher levels are quite small – a single large tree can support many caterpillars chewing on its leaves.

Biomass Pyramid
A biomass pyramid compensates for size differences between organisms by measuring the dry weight, or biomass, of living matter on each trophic level. In most ecosystems, most of the biomass is on the bottom of the pyramid. However this type of pyramid exaggerates the presence of long-lived animals that slowly accumulate mass over many years.