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Pyramid of Numbers

  • This deals with the relationship between the numbers of primary producers and consumers of different levels.
  • It is a graphic representation of the total number of individuals of different species, belonging to each trophic level in an ecosystem.
  • Depending upon the size and biomass, the pyramid of numbers may not always be upright, and may even be completely inverted.

a) Pyramid of Numbers – Upright

􏰀  In this pyramid, the number of individuals is decreased from lower level to higher trophic level.

􏰀  Example of this pyramid – Pond ecosystem.

􏰀  Lowest trophic level occupied by snails because of their abundance.

􏰀  Next higher trophic level – primary consumer – herbivore (example – Small fishes)

􏰀  Number of small fishes is less than that of snails.

􏰀  The next energy level is primary carnivore (example bigger fishes).

􏰀  The numbers of bigger fishes are less than small fishes because, they feed on smaller fishes.

􏰀  The next higher trophic level is secondary carnivore (example- Crane). They feed on bigger fishes.

􏰀  With each higher trophic level, the number of individual decreases.

b) Pyramid of Numbers – Inverted

􏰀  In this pyramid, the number of individuals is increased from lower level to higher trophic level.

􏰀  Example of this pyramid – A tree in forest ecosystem.

􏰀  In the above diagram you can see that a big tree (primary producer) supports many birds (herbivores).

􏰀  These birds eat lots of tapeworms, fleas, and barnacles (parasites) which live on this tree. Number of parasites is more than number of birds.

􏰀  These parasites eat hyperparasites. Number of hyperparasites is greater than number of parasites.

􏰀  With each higher trophic level, the number of individual increases and so the resulting pyramid is of inverted shape.

􏰀  A pyramid of numbers does not take into account the fact that the size of organisms being counted in each trophic level can vary.

􏰀  It is difficult to count number of trees, birds and grasses etc. so the pyramid of no. doesn’t completely define the trophic structure of ecosystem.

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