A level Biology exam revision resources written by A level Examiners

  • Populations are changed by - Births, Deaths, Immigrations or Emigrations.
    • If B + I = D + E then the population remains stable.
    • If B + I < D + E then the population decreases.
    • If B + I > D + E then the population shrinks.
  • The rate of natural increase (RNI) is the change in the size of a population as a percentage of the total population per year.
  • Doubling time is the time it would take a population to double assuming the RNI remains constant.
  • The demographic transition model is a model of how human populations change.
  • Population pyramids are a visual representation of the age structure of a population

This is the accepted model of how human populations change. The change in population is brought about by changes in the birth and death rates. For Countries that are in the advanced stages of demographic transition the earlier stages would have occurred in the past. Other countries are still in the earlier stages, and it is anticipated that these countries will progress to the later stages of demographic transition, in the future.

The figure 1 below shows the demographic transition model and the four main stages that occur as the population undergoes transition. We will examine these stages in the level of detail required for A level biology.

Figure 1 : The Demographic Transition Model
Cumulative (Total Growth)

Stage 1

Birth and death rates are high and fluctuating and the rate of population growth is low.

The high birth rates are often linked with parents having large families to offset high infant mortality rates, lack of available birth control methods, children required for labour, and to look after parents in old age. The death rates are high factors such as lack of medical care, famine caused by unstable food supplies and water-borne diseases.

Stage 2

Birth rates are high and death rates fall dramatically in this stage. This leads to rapid population growth.

The birth rates remain high for the same reasons as in stage one. However the death rates fall because of reasons such as:

  • clean water supplies established so fewer water-borne diseases
  • food supplies stabilised so less death due to famine
  • better knowledge of disease spread
  • introduction of early vaccines such as smallpox

Stage 3

Here the death rate remains low and the birth rate falls so the rate of population increase falls to a low level.

The falling birth rates are due to factors such as increased availability of contraception (in the UK stage three coincided with the advent of the pill). The need to have children to work the land is reduced so families have less children. Materialism and equal job opportunities for women, lead to women having less children, and families placing a greater emphasis on both parents working and increasing family wealth.

Stage 4

This is the final stage where the birth and death rates remain low and stable.

Case study: The UK

The timeline for the demographic transition in the UK can be seen in figure 2, below.

Figure 2 : The timeline for the demographic transition in the UK
The timeline for the demographic transition in the UK

This is a visual way of showing the age structure of a population. The different stages of the demographic transition model tend to have populations, that show structures similar to those shown in figure 3. One additional feature of demographic transition becomes apparent from these pyramids, that is that life expectancy increases as populations go through demographic transition.

Figure 3 : Population pyramids
Population pyramids

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