Write a short note on pyramid of numbers | Class 12 Biology Chapter Ecosystem, Ecosystem NCERT Solutions

1. Pyramid of numbers: Graphical representation of the total number of the individuals of different species that belongs to each trophic level in the ecosystem is known as pyramid of number. It consists of horizontal bars that shows the specific trophic levels which are arranged in the sequential manner from producer to consumers level and the length of each bar represents the total number of individuals at each trophic level. In case of grassland ecosystem or pond ecosystem, it always shows upright pyramid because the numbering of the organisms decreases at successively higher trophic level. But in the tree ecosystem it always shows inverted pyramid, in this tree is a primary producer, so it would represent the base of the pyramid and the dependent insect population will represent the second trophic level.

Diagram of Upright pyramid of numbers in grassland ecosystem:

Diagram of Inverted pyramid of numbers in tree ecosystem:

2. Pyramid of biomass: This pyramid represents the total dry weight of living beings of different species at each trophic level at the particular time. In this all kinds of organisms at a trophic level are weighted. The biomass of the producer is much greater than the herbivore’s biomass and this is greater than the carnivore’s and so on. In forest ecosystem, it shows the shape of upright pyramid but in some aquatic ecosystem, like large lakes and oceans this pyramid usually shows in inverted form. The reason behind the inverted pyramid is, microscopic phytoplanktonic algae are the primary producers and being single- celled they do not get accumulate much biomass; and at a given time the total weight of the standing crop of phytoplankton is less when compared to herbivores or other consumers, thus the base of the pyramid being smaller than the upper structure in the aquatic ecosystem.

Diagram of Upright pyramid of biomass in forest ecosystem:

Diagram of Inverted pyramid of biomass in pond ecosystem: