Ecosystem - Notes | Class 12 | Part 4: Ecological Pyramids



The representation of a food chain in the form of a pyramid is called ecological pyramid.

The base of a pyramid represents producers (first trophic level). The apex represents tertiary or top-level consumer.

Ecological pyramids are 3 types: Pyramid of number, Pyramid of biomass and Pyramid of energy.

a) Pyramid of number:

E.g. grassland ecosystem. 

b) Pyramid of biomass:

It shows a sharp decrease in biomass at higher trophic levels.

c) Pyramid of energy: 

Primary producers convert only 1% of the energy in the sunlight available to them into NPP.
Any calculations of energy content, biomass, or numbers has to include all organisms at that trophic level.

A trophic level represents a functional level, not a species as such. A species may occupy more than one trophic level in the same ecosystem at the same time. E.g. A sparrow is a primary consumer when it eats seeds, fruits, peas. It is a secondary consumer when it eats insects & worms.

In most ecosystems, all the pyramids are upright, i.e., producers are higher in number, biomass and energy than the herbivores, and herbivores are higher in number, biomass and energy than the carnivores.

But in some cases, inverted pyramids for number and biomass are present.

Inverted pyramid of number: E.g. Insects feeding on a tree.

Inverted pyramid of biomass: E.g.
  • Small standing crop of phytoplankton supports large standing crop of zooplankton.
  • Pyramid of biomass in sea is inverted because the biomass of fishes far exceeds that of phytoplankton.

Pyramid of energy is always upright because some energy is always lost as heat at each trophic level. So energy at a lower trophic level is always more than at a higher level.

Limitations of ecological pyramids 
  • It does not consider the same species belonging to two or more trophic levels.
  • It assumes a simple food chain that never exists in nature. It does not accommodate a food web.
  • Saprophytes are not included.
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