Ecological pyramid is the graphic representation of the numbers, biomass and energy of the successive trophic levels of an ecosystem. Charles Elton (1927) developed the ecological pyramids and are therefore, also termed as Eltonian pyramids. In an ecological pyramid, producers form the lower most trophic level and the top most level is that of the carnivores.
An ecological pyramid can be upright (with larger base and gradually tapering towards the tip), inverted (narrow base, gradually becoming broader towards the tip) or spindle shaped (narrow both at the base and tip with a broader part in the middle). The most common ecological pyramids are the pyramid of number, pyramid of biomass, and pyramid of energy.
Pyramid of number:
It is a graphic representation of the number of individuals per unit area of various trophic levels stepwise with producers at the base and top carnivores kept at the tip.
This pyramid shows a relationship between the numbers of producers, herbivores and carnivores. The organism of an area is first counted and then grouped into their trophic levels. The diagram drawn shows largest number in trophic levels of producers. The number shows gradual decrease from the second trophic levels up to the highest trophic levels.
Pyramid of number in terrestrial and aquatic ecosystem
The three common ecosystems which we study are grass land, pond ecosystem and forest ecosystem. In grass land ecosystem, grasses are the producers. These outnumber the organisms occurring in any trophic level of consumers. The number of consumers decreases towards the top of the pyramid and that of primary consumers of herbivores like rabbits, rats etc., is lesser than the number of grasses. The number of secondary consumers like snakes, lizards etc., lesser than the number of primary consumers like rats and rabbits. The numbers of last or tertiary consumer are still less than those of secondary consumers. In this way, the number of organisms decreases progressively from the first trophic level to the last trophic level. The pyramid of numbers of grass land ecosystem is upright.
In pond ecosystem also, the number of organisms decreases gradually from the first trophic level to the last trophic level. Thus, the pyramid of number in pond ecosystem too is upright or straight. In forest ecosystem, the shape of pyramid is rhomboidal. A single large tree represents the producers. Many food-eating birds, herbivores animal depend on this tree. Thus, number of primary consumers is more than the number of producers. Thereafter, the number of secondary and tertiary consumers decrease progressively.
Pyramid of biomass:
The amount of living or organic matter present in an organism is termed as biomass. Biomass is the total mass of organisms. It can be estimated in terms of wet mass, dry mass, or ash-free dry weight. The biomass at the time of sampling is termed as standing biomass or standing crop biomass.
Pyramid of biomass is a graphic representation of biomass present sequence-wise per unit area of different trophic levels with producers at the base and top carnivores kept at the tip.
Pyramid of biomass is more real than the pyramid of numbers. It is because the pyramid of numbers does not take into consideration the size of the individual. For example, mouse, shrew, squirrel, rabbit, deer, antelope, bison and elephant are all herbivores. The same amount of vegetation will support a large number of mice, fewer rabbits, still smaller number of deer and very few elephants. However, their total biomass shall be equal and will relate to the biomass of the vegetation as well. Maximum biomass occurs in producers. There is a progressive reduction of biomass in herbivores, primary carnivores, secondary carnivores etc.
Pyramid of biomass in terrestrial and aquatic ecosystem
In forest ecosystem and grassland ecosystem, the pyramid of biomass is upright or straight. The amount of biomass continues to decrease progressively from the trophic level of producers to the last trophic level of carnivores.
In aquatic habitats the biomass of a trophic level depends upon reproductive potential and longetivity of its members. In pond, the number of producers (algae) is very large but their biomass is the least of all, being very small in size. The amount of biomass continues to increase with primary, secondary and tertiary trophic levels. Thus, the pyramid of biomass in pond ecosystem is inverted.
Pyramid of energy:
It is a graphic representation of amount of energy trapped per unit time and area in different trophic levels of a food chain with producers forming the base and top carnivores the tip. This is the most ideal and fundamental way of representing, energy relationship between organisms and different trophic level. The energy content is expressed as Kcal/m2/Yr or KJ/m2/Yr. Maximum energy content is present in producers.
In every ecosystem, only producers have the capacity to use the energy from the sun and convert it to food system. The energy converts in chemical form and gets stored inside the organic matter manufactured by the producers. The energy in the form of food transfers from one trophic level to another. Thus, the flow of energy is always unidirectional.
As the energy passes into higher trophic levels along with food, its amount decreases because of its dissipation as heat and use in overcoming entropy as well as for performing various body activities. The amount of energy that reaches the next trophic level is less than it was present in the earlier trophic level. Therefore, the amount of energy decreases with each higher trophic level. Such a pyramid would be straight or upright in all the ecosystem.
Pyramid of energy in terrestrial and aquatic ecosystem
The data given by Odum (1971) shows that in a fish pond, phytoplankton trap 31080 KJ/m2/Yr of energy. Zooplankton and other herbivores dependent upon phytoplankton have an energy content of 7980 KJ/m2/Yr. The primary carnivores (insect larvae and small fish) possess an energy content of 2100 KJ/m2/Yr. They support secondary carnivores with an energy content of 126 KJ/m2/Yr. Thus the pyramid of energy is always upright and is more accurate than pyramid of biomass or the pyramid of numbers.