• Except for deep sea hydrothermal ecosystem, sun is the only source of energy for all ecosystems on earth.
  • Less than 50% of incident solar radiation is photosynthetically active radiations. (PAR).
  • Plants capture 2-10 % of PAR and used in photosynthesis.
  • All organisms depend on the producers, either directly or indirectly.
  • Energy flow in the ecosystem is unidirectional i.e. energy transferred from producer to consumers.
  • Energy transfer is not absolute, and spontaneous, unless energy is degraded it cannot be transfer. When energy transferred from one trophic level to another, lot of energy lost in the form of heat to the environment.
  • Only 10% of energy transferred from one trophic level to other.

Food chain:

  • Food chain is a succession of organism in an ecological community that are linked to each other through the transfer of energy and nutrients begginning with an autotrophic organism such as plant and continuing with each organism being consumed by one which is at higher trophic level in the chain.
  • Grazing food chain: it extends from producers through herbivore to carnivore.

  • Detritus food chain: Begins with dead organic matter (detritus) and pass through detritus feeding organism in soil to organisms feeding on detritus-feeders.

  • In aquatic ecosystem GFC is the major conduit for energy flow.
  • In terrestrial ecosystems a much larger fraction of energy flows through the detritus food chain than through GFC
  • Different food chains are naturally interconnected e.g. a specific herbivore of one food chain may serve as food of carnivores of other food chains. Such interconnected matrix of food chains is called food web.

  • Trophic level: A group of organism irrespective of their size having same source of energy or similar food habit constitute a trophic level.
  • Standing crop: each trophic level has a certain mass of living material at a particular time called as the standing crop.
  • The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.
  • The number of trophic levels in a food chain is restricted by 10 % flow of energy, less amount of energy available to the last trophic level.


  • The base of the pyramid is broad and it narrows down at the apex. The similar shape is obtained when food or energy relationship between organisms at different trophic level.
  • The relationship can be expressed in terms of number, energy or biomass.
  • The base of the pyramid represented by producer and apex is the top consumer; other trophic levels are in between.
  • In most ecosystems, all the pyramids, of number, of energy and biomass are upright.
  • The pyramid of number in a tree ecosystem is inverted.


  • The pyramid of biomass in sea also inverted because the biomass of fishes is far exceeds that of phytoplankton.

  • Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next, some energy is always lost as heat at each step.

Limitations of ecological pyramids:

  • It does not take into account the same species belonging to two or more trophic levels.
  • It assumes a simple food chain, it never exits in nature.
  • It does not accommodate food web.
  • Saprophytes are not given place in ecological pyramids.

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