NCERT · Class 12 · Biology · EcosystemDefine ecological pyramids. Describe the pyramid of number, pyramid of biomass, and pyramid of energy with suitable examples and mention the limitations of ecological pyramids.

Definition of Ecological Pyramids\nThe representation of a chain of organisms in terms of energy, biomass, or number, starting from producers at the base to top carnivores at the apex, is known as an ecological pyramid. The base of a pyramid represents producers or the first trophic level, while the apex represents tertiary or top-level consumers.

Types of Ecological Pyramids

1. Pyramid of Number:

   • Represents the total number of individuals at each trophic level.
   • It is generally upright in grassland and pond ecosystems, where producers are maximum in number.
   • It can be inverted in a tree ecosystem (parasitic food chain) where a single tree supports numerous birds and parasites.

2. Pyramid of Biomass:

   • Represents the total dry weight or biomass of organisms at each trophic level.
   • It is upright in terrestrial ecosystems because producer biomass exceeds consumer biomass.
   • It is inverted in an aquatic ecosystem (e.g., open sea), where the biomass of phytoplankton is much less than that of zooplankton at any given time.

3. Pyramid of Energy:

   • Represents the amount of energy present at each trophic level per unit area over a specific time.
   • It is always upright, because energy flows unidirectionally and follows the 10% law of Lindeman, where energy decreases significantly at each successive trophic level.

Limitations of Ecological Pyramids

• Ignores Food Webs: It assumes a simple food chain, whereas nature has complex food webs.
• Assumes Simple Trophic Levels: It does not accommodate organisms belonging to two or more trophic levels simultaneously.
• Saprophytes Excluded: Saprophytes or decomposers are not given any place in ecological pyramids, despite their vital role in ecosystems.