In this article we will discuss about ecological pyramids an its limitations.
These are the diagrammatic illustrations of connection between different trophic levels in terms of energy, biomass and number of an organism. The base of each pyramid represents the producers or the first trophic level. Apex represents tertiary or top level consumers. In general, all pyramids are upright, but there are few exceptions.
There are three ecological pyramids that are usually studies:
1. Pyramid of Number:
It represents the total number of organisms at each trophic level. It is always upright but in a tree ecosystem pyramid of number is inverted.
2. Pyramid of Biomass:
It represents total weight of the organisms in each trophic level.
It can be:
(i) Upright, e.g., in grasslands.
(ii) Inverted, e.g., in pond ecosystem.
3. Pyramid of Energy:
It represents total energy of the organisms in each trophic level. Pyramid of energy is always upright, i.e., it can never be inverted, because when energy is transferred from a particular trophic level to the next trophic level some energy is always lost as heat at each step.
Some important points about ecological pyramids are given under:
(i) A given organism may occupy more than one trophic level simultaneously.
(ii) Trophic level represents a functional level.
(iii) A given species may occupy more than one trophic level in the same ecosystem at the same time.
For example, a sparrow is primary consumer, when it eats seeds, fruit, peas, etc., and a secondary consumer when it eats insects and worms.
(iv) In most ecosystems, all the pyramids of number, biomass, energy are upright, i.e., producers are more in number and biomass than the herbivores and herbivores are more in number and biomass than carnivores.
(v) Also energy at lower trophic level is always more at higher trophic level. However, there are exceptions to this generalisation.
(vi) Pyramid of biomass in sea is inverted because the biomass of fishes far exceeds that of phytoplankton.
(vii) Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time or annually per unit area.
Limitations of Ecological Pyramids:
(i) It never takes into account the same species belonging to two or more trophic levels.
(ii) It assumes a simple food chain, something that almost never exists in nature.
(iii) It does not accommodate a food web.
(iv) Saprophytes are not given any place in ecological pyramids even though; they play an important role in ecosystem.