A Population Pyramid is basically a graphical representation of the distribution of various age groups in males and females in a region. It is also frequently called the “age-sex pyramid”. The male population is shown on the left of the pyramid and the female on the right. It can be represented with numerical values of the actual number of the population or in terms of percentage of the total population. This is an effective tool used in Ecology to determine the age distribution of a population which helps to indicate the reproductive capacity and the possibility of the continuation of the species.
Representation of the Population Pyramid:
The population pyramid is often represented as a horizontal continuous stacked histogram bar diagram. The population is represented on the horizontal x-axis while the age groups are represented on the vertical y-axis. The size of the population is often given in terms of percentage of the total population in the region or by the actual number of individuals in the population thereby making it extremely easier so as to determine in the population pyramid the ratio of the male to the female. These pyramids are a great way to ascertain the age distribution and a great deal of information about the population can be obtained from these graphical representations. The y-axis represents the age typically in 5 year age groups with the youngest age group represented by the bottom bar and the oldest by the topmost bar. The length of the horizontal bar represents the number or percentage of individuals in that age group. Often the years of birth for each age category are also represented on the graph. The age axis is usually truncated at age group 80-85years due to the lower number of individuals in that age group. Population pyramids that are used for the purpose of comparison should be drawn on the same scale with the same age categories.
When assessing a population pyramid, many a thing needs into consideration such as the central tendency, mean, median and mode as the data is not absolutely accurate. The average age of a population can be determined using these pyramids. A population with an average age in the 50s or 60s shows an aging population while an average age of 15 or in the early 20s shows a young population with greater potential for growth. A population pyramid can be used to represent additional information about the population in a region such as the race and ethnicity, marital status and Geographic location. In such cases, the bar for the age-sex is subdivided to include the additional categories. The same format is applied consistently throughout the bar diagram on both sides of the Y-axis.
How to interpret the Population Pyramid
The Population Pyramid is usually represented through the following procedure. The shape of the bar diagram gives enough information about the age group and the sex of a specific population. The broad base of the pyramid indicates a large number of younger populations in the region as compared to the tapering top which indicates the relatively lower proportion of the older age group.
It is not necessary that the two sides of the Y-axis will be equal. The number of males and females in a population vary considerably in a region. For example, the number of females in the older age group is frequently larger than the males in the same age group. This is reflected in the shape of the pyramid with the bars on the right side of the Y-axis being longer than those on the left for the specific age groups.
The median age group of the population is the bar on mid of the Y-axis that equally divides the bar graph into equal halves within the pyramid (above and below).
The Population pyramid is good to represent the fertility and mortality in a region. It is easily reflected in the shape of the pyramid. A broad base and sharply tapering sides indicate a high rate of mortality as well as fertility in the lower age groups. Any unusual rate like high fertility or high mortality in any of the age groups is indicated by a bulge or an indentation on the pyramid. These aberrations can be caused due to migration in the region.
Population Pyramids and Demographic Transition
According to demographers who study the historical changes in the age and gender variation along with the study of the fertility and mortality of the population, the transition of the demography can be represented by population pyramids. The stages of the transition can be indicated by population pyramids wherein the shape of which gives useful information on the demography.
A pyramid with a broad base and sharply tapering sides reflect high fertility and high mortality rates in the younger age groups which results in a population that increases very gradually and remains relatively low. This is followed by a pyramid that reflects a dip in the mortality rates in the younger age groups along with a high fertility rate. There is a rapid growth in the population which however remains relatively young. The third stage in the transition of the demography is represented by a pyramid that has almost vertical sides with a broad base and peak. This would indicate a low fertility and mortality rates. A considerable section of the population comprises of older people as compared to the previous two stages and the population reflects noticeable stability.