Because of the layering in the ecosystem the trophic structure and function at successive trophic levels, i.e. producers -> herbivores -> carnivores may be shown graphically by means of ecological pyramids, where base of the pyramid represents the producer level and the successive levels making the apex.
There are three types of the ecological pyramids: (i) pyramid of numbers, showing the number of individual organisms at each level or step, (ii) pyramid of biomass, showing the total dry weight and other suitable measure of the total amount of living mater; and (iii) pyramid of energy, showing the rate of energy flow at successive trophic levels.
It is to be noted the first two pyramids may be upright or inverted, whereas the pyramid of energy is always upright.
(i) The pyramid of numbers:
In this pyramid the relationship between producers, herbivores, and carnivores are shown in respect of their numbers. The pyramids of numbers do not give a true picture of the food chain. They are differently shaped in different communities having different food chain.
(ii) Pyramids of biomass:
This type of pyramid shows the biomass instead of the number of species at each trophic level.
This is usually measured as weight of living material (e.g. dry gm-2) and represents the standing crop. Sometimes biomass pyramids can also be reversed as in some aquatic ecosystem, where primary production by phytoplankton is low and these are heavily grazed by zooplankton.
The result is that the biomass represented by the base of the pyramid is smaller than the biomass of the consumers in the ecosystem.
(iii) Pyramid of energy:
The pyramid of energy shows the best picture of the overall nature of the ecosystem. Actually this pyramid gives an idea of ecological productivity. The pyramid of energy gives a picture of the rates of passage of food mass through the food chain.
This pyramid is upright in shape because there is a gradual decrease in the energy level at successive trophic levels from the producers to various types of consumers.
In the pyramids of energy a and b, pyramids are upright because the cumulative energy is always maximum at the base, and the energy is the least at the top.