(a) Age-pyramid is a graphic representation representing geometrically the proportion of different age groups, i.e., pre-reproductive, reproductive and post-reproductive groups in the population of any organism. It is a vertical bar graph in which the number or proportion of individuals in various age ranges at any given time is shown from youngest at the bottom of the graph to oldest at the top.

 

(b) There are three basic types of age pyramids :

(i) Triangular Age Pyramid : Pyramid with broad base or triangular structure indicates a rapidly expanding population with a high percentage of pre- reproductive individuals. Number of reproductive individuals is moderate while post-reproductive individuals are fewer. It can be shown as:

(ii) Bell-Shaped Age Pyramid: The number of pre- reproductive and reproductive individuals is almost equal. Post-reproductive individuals are comparati- vely fewer. The population size remains stable, neither growing nor diminishing.

 

 

(iii)  Urn-shaped Age Pyramid :  Proportion  of reproductive age group is higher than the individuals in pre-reproductive age group. Number of post-reproductive individuals is also sizeable. It is declining or diminishing population with negative growth. It can be shown as: