The population started to increase following the transition to farming (the agricultural revolution) 10,000 years ago, but really accelerated following the industrial and scientific revolutionsstarting in Europe in the 18th century.
The increase in population, or growth rate, depends on four factors:
growth rate = (birth rate – death rate) + (immigration rate – emigration rate)
The equation shows that growth rate can increase by increasing the birth rate or decreasing the death rate (ignoring migration).
The staggering human population growth over the last two centuries is entirely due to
a massively decreased death rate caused by the improvements in farming, and in medicine.
The increased growth rate has therefore happened atdifferent times for different countries.
Growth rate can also be expressed as a percentage change:
Annual percent change =
(population at end of year - population at start of year)/ population at start of year X 100
If the population decreases the population change will be negative.
The Demographic Transition
History has shown that the birth and death rates of a country change in particular ways as the country develops.
These changes are called the demographictransition, and there are four stages:
Stage 1 (pre-industrial)
- In pre-industrial societies, birth and death rates are both high.
- The lack of medicine and poor sanitation means that child mortality is high and life expectancy is short.
- Children work from a young age, so are useful and cheap to bring up, so parents choose to have many children.
- Countries in sub-Saharan Africa, such as Angola and Ethiopia are still stage 1 societies.
Stage 2 (Developing)
- Developing societies the death rate (especially child mortality) decreases due to
improved farming, nutrition, heath care, sanitation and education.
The Demographic Transition 2
- Life expectancy increases, but birth rate remains high, so the population increases.
- Most European countries went through stage 2 in the 18th century, but developing countries such as Afghanistan and Laos are in stage 2 today.
Stage 3 (Industrial)
- In industrial societies the death rate is low and the birth rate starts to fall as parents
choose to have fewer children.
- This family planning often results from urbanisation, so children are no longer needed to work the land, and the cost of their upbringing and education increases.
- The population still increases, but at a slower rate.
- Many countries in the world today are in stage 3 including Mexico,India, Pakistan, Saudi Arabia and South Africa.
Stage 4 (Post-industrial)
- In post-industrial societies the birth and death rate are both low, so the population is
stable, but high.
The Demographic Transition 3
- Countries like the United States,Canada, Australia and most of Europe
are in stage 4.
- In some countries (such as the UK, Germany and Japan) the birth rate falls below the death rate, so the population starts to decrease.
- This decrease is sometimes described as stage 5.
The demographic changes also lead to changes in life expectancy and the age structure of a population, i.e. the proportions of young and old people.
These changes can be shown as population pyramids and survivorship curves.
- (Or age pyramids) show the population of a country as a bar chart with a bar for
each age group (usually 5 or 10 years per bar).
- Males (y) and females (¥) are usually shown separately on the two sides of the chart.
- To interpret these charts, it helps to put the ages into three bands: pre-reproductive (0–14); reproductive (15–44) and post-reproductive ages (over 45).
The shape of an age pyramid tells us about the future growth of the population and about life expectancy.
- The wider the base of the pyramid, the faster the population growth. Imagine the wide bottom bars moving up the pyramid over time into the reproductive age band. A pyramid with a narrow base indicates a falling population.
Age Structure 2
- The steeper the pyramid, the longer the life expectancy. An age pyramid with a wide base that declines quickly and has a narrow tip indicates high infant mortality and a short life expectancy. A more rectangular shape with a broader tip indicates long life expectancy
- (Or survivor curves) are constructed by tracking a group of individuals born around
the same time (a cohort) from birth until the last one has died, and recording each individual’s age at death.
- The percentage of the cohort surviving is plotted at each age.
- So all survivorship curves start at 100% and finish at 0%, but have different shapes in between.
- Survivorship curves allow us to calculate life expectancy– the mean lifespan of a cohort – by simply readingoff the age at which 50% survive.
Three kinds of shape are found.
Age Structure 3
Type I Curves
- There is a long life expectancy, with low infant mortality and most of the cohort dying
in old age.
- Type I curves are shown by large mammals and Industrial human societies, where families are small and there is a high investment in parental care.
Type II curves
- There is an intermediate life expectancy and a roughly constant death rate regardless
- Type II curves are shown by animals that are equally susceptible to predation or disease at any age, such as small mammals and many birds.
- They are also shown by human societies facing a serious epidemic, such as the AIDS epidemic in Botswana.
Age Structure 3
Type III Curves
- There is a short life expectancy, with most of the cohort dying in infancy and few
surviving to old age.
- Type III curves are shown by animals that do little or no parenting and produce large numbers of offspring to compensate, such as insects and fish.
- They are also shown by pre-industrial human societies with poor health care and high infant mortality.