Population Characteristics - IV

4.     Age distribution

Age distribution of a given population can be defined as the total number of organisms distributed in different age groups. It influences other population characteristics such as natality, mortality and reproductive rate. Age can be expressed in terms of pre- and post-reproductive or else in terms of years, months or even days. By understanding the age structure or distribution we can predict about the future population and categories into expanding population, stable and declining population. If individual are more distributed in reproductive phase than post-reproductive phase, the population is termed as expanding population. If the individuals are distributed almost equally in both reproductive and post reproductive phase, the given population is stable while if the distribution is more in post-reproductive phase, the population is considered as declining population.

Age pyramids

Age polygons or pyramids are representing model to depict the age distribution in population where the relative width of successive bar represent the number of individuals distributed in different age groups at a particular time. The age pyramids are hypothetically categorized into three types:

i.       Bell shaped Age Pyramids

It’s a representative form of expanding population. The broad horizontal base of polygon represent the higher distribution of young organisms and thus, expanding population. It is characterized by exponential growth and high birth rate in a population at a particular time.

ii.                 Dome shaped Age Pyramid

It represent a stable population where the distribution of young to old is in equal proportion depicted by the equal width of horizontal zones of pre-Reproductive and reproductive phase. The post-reproductive phase is smallest in this form of representation.

iii.              Urn shaped Age Pyramid

It represents a declining population where the smallest distribution of organism is in pre-reproductive phase. 

Figure 7: Age distribution in Age pyramids, (a) Bell shaped, (b) Dome shaped and (c) Urn shaped age pyramids.