Predicting Population Changes Using Age Structure Diagrams

The age structure (which is the proportion of population in each age class) of a population affects current and future birth rates, death rates, and growth rates; our impact on the environment; and current and future social and economic status. The overall shape of an age structure diagram indicates whether the population is increasing, stable, or shrinking. Demographers typically construct age structure diagram by plotting the percentages or numbers of males and females in the total population in each of three age categories: prereproductive (ages 0-14 yrs), reproductive (ages 15-45 yrs), and postreproductive (ages 45 yrs and up).

The age structure diagram of a country with a very high growth rate is shaped like a pyramid (Fig. 2a). In developing countries about 37% of the populations are under 15 years old (e.g. Kenya).

                      

                       Fig. 2: Age structure diagram.

Worldwide, 30% of the human population is under age 15. When these people enter their reproductive years, they have the potential to cause a large increase in the growth rate. Even if the birth rate does not increase, the growth rate will increase simply because there are more people reproducing.

The age structure diagram of stable population, one that is neither growing nor shrinking, demonstrates that the numbers of people at pre-reproductive and reproductive ages are approximately the same and a larger proportion belongs to post-reproductive ages. Many countries in Europe have stable populations.

In a population that is shrinking in size, the pre-reproductive group is smaller than either the reproductive or post-reproductive group. Russia, Bulgaria, Austria and Germany are examples of countries with slowly shrinking populations.      

Most of the world population increase since 1950 has taken place in developing countries as a result of the younger age structure and the higher-than-replacement-level fertility rates of their populations. Most of the population increase that will occur during the 21^st century will also take place in developing countries (because of high fertility rate), largely the result of their younger age structures. These countries are least able to support such growth.