The pyramid structure which is obtained when we illustrate the number or biomass or energy of different organisms at different trophic levels in an ecosystem also known as the ecological pyramid. Trophic levels represent the position of organisms in the food chain. The base of an ecological pyramid represents the first trophic level or producers. The next trophic level comprises primary consumers, followed by secondary consumers and finally tertiary consumers at the top. Each of these trophic levels represents a functional level and not a single species as such. Also, all the organisms at a particular trophic level must be considered for any calculation on number, biomass or energy.
Based on the types of parameters used, ecological pyramids are of three types: pyramid of numbers, pyramid of biomass and pyramid of energy. A pyramid of numbers is a graphic representation of the number of individuals per unit area present at various trophic levels in an ecosystem and its shape varies from one ecosystem to another. In a parasitic food chain the pyramid of numbers is inverted.
In some cases the pyramid of numbers is spindle shaped.
Pyramid of biomass, which is a graphic representation of biomass present per unit area in each trophic level in an ecosystem. In a terrestrial ecosystem, producers have maximum biomass, which decreases progressively from lower to higher trophic levels. Therefore, the pyramid of biomass in a terrestrial ecosystem is always an upright one. The pyramid of biomass in an aquatic ecosystem is either inverted or spindle-shaped.
Pyramid of energy is a graphic representation of the amount of energy trapped per unit time and unit area at different trophic levels of an ecosystem. Some energy is used for the metabolic activities of organisms and some energy is dissipated as heat in the process. That is why a pyramid of energy is always upright and never inverted.
In general, an ecological pyramid shows the feeding relationship between groups of organisms and also depicts the amount of biomass or energy present at various trophic levels in an ecosystem.