2.4.1. 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.

♤ This type of pyramid can be seen in grassland ecosystem.

♤ The grasses occupy the lowest trophic level (base) because of their abundance.

♤ The next higher trophic level is primary consumer -herbivore (example - grasshopper).

♤ The individual number of grasshopper is less than that of grass. The next energy level is primary carnivore (example - rat).

♤ The number of rats are less than grasshopper, because, they feed on grasshopper. The next higher trophic level is secondary carnivore (example - snakes). They feed on rats.

♤ The next higher trophic level is the top carnivore. (Ex. Hawk).

♤ 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.

♤ A count in a forest would have a small number of large producers, for e.g. few number of big trees.

♤ This is because the tree (primary producer) being few in number and would represent the base of the pyramid and the dependent herbivores (Example - Birds) in the next higher trophic level and it is followed by parasites in the next trophic level. Hyper parasites being at higher trophic level represents higher in number.

♤ And the resulting pyramid is in 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 very difficult to count all the organisms, in a pyramid of numbers and so the pyramid of number does not completely define the trophic structure for an ecosystem.