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C Bell shaped age pyramid - Stable population
A population is composed of individuals belonging to different age groups at any given time. The individuals in a population can be divided into three age groups: pre-reproductive, reproductive and post-reproductive age groups.
The graphical representation of the age composition of a specific population is called the age pyramid.
Age pyramid is obtained when the per cent individuals of a given age or age group is plotted for the population.
The base of the pyramid consists of individuals of the pre-reproductive age group, the individuals in the reproductive age group are present in the middle and the post-reproductive individuals are placed at the top of the pyramid.
The shape of the age pyramid gives us an idea about the growth status of the population. It is used by demographers to study the human populations.
For the human population, the age pyramids generally also include the age distribution of males and females (sex) and thus is also referred to as age-sex pyramid. The shape of the pyramid reflects whether the population is growing, stable or declining.
Age pyramids can be of the following shapes:
- Bell-shaped age pyramids are formed when the pre-reproductive and reproductive individuals are almost in equal numbers or percentages and the post-reproductive individuals are relatively fewer. Such a population is predicted to remain stable.
- Triangular shaped age pyramids are formed when the individuals in the pre-reproductive and reproductive age group are high and very smaller number of individuals are present in the post-reproductive age group. Such a population is predicted to grow or expand.
- Urn shaped age pyramids display low numbers or percentages of pre-reproductive individuals, maximum number of individuals in the reproductive age group and a few numbers of individuals in the post-reproductive age group. Such a population is said to decline.
Figure: Representation of age pyramids for human population
- Triangular shaped age pyramid represents expanding population.
- Bell shaped age pyramid represents stable population.
- Urn shaped age pyramid represents declining population.