At a national level rates of population growth vary from country to country.
The growth rates of less economically developed countries are very rapid
compared to more economically developed countries. In fact some countries even
have negative population growth. Japan's population has experienced negative
rates of natural increase since 2007 and is expected to lose 21 percent of its
population by the year 2050. One way we measure population growth is by
calculating the doubling time. This is the length of time it takes for
population to double in size. It is calculated by dividing 70 by the growth
rate. There are two main factors that influence population change. They are
natural increase and net migration. Natural increase is the birth rate minus
the death rate. The birth rate is the number of live births per 1000 people per year.
The death rate is the number of deaths per 1000 people per year. The rate
of natural increase can be expressed as a percentage. For example in 2014
Thailand had an estimated birth rate of 11.26 births per thousand
population. The death rate was 7.72 deaths per thousand population.
Therefore the natural increase equals 11.26 minus 7.72
equals 3.54. That is, 0.354%.
On average a total fertility rate of 2.1 children per woman is considered to be
the replacement rate. Today in many European countries women are giving
birth to less than this figure. In many cases the only thing stopping the
population from shrinking has been high rates of migration. Migration is the
term we use for the movement of people in and out of an area. We use the term
emigration when people enter a country and immigration when people leave a
country. Net migration is immigration minus emigration. Changes in population are
best represented using a line graph. For example this graph shows population
change over time in Australia. On a global scale the world's population will
grow simply if the birth rate is greater than the death rate. Until the 19th
century the world's population has grown slowly. But since the Industrial
Revolution there has been an explosion in the
growth of the world's population as can be seen from this line graph. By 1804 the
world's population reached 1 billion. By 1959 the world's population was 3 billion
and by 1999 it had doubled to 6 billion. Today there over 7 billion people on the Earth.
The changes in birth and death rates and their effect on the total
population of a country can be shown on the demographic transition model.
The highlighted gap between the birthrate and death rate on the model is
the natural increase. In the first stage of the model both birth rates and death
rates are high and the population is fluctuating. The high birth rates are due
to factors such as a lack of contraceptives. Or in many poor
communities large families are seen as desirable because children can be put to
use working on the family farm. The high death rates are due to factors such as
the lack of health care and rampant disease or famine, the lack of clean
water and sanitation, wars, or the lack of an education. This stage in the model
is typical or pre-industrial Britain in the 18th century.
Today it is associated with the least economically developed countries in the
world today. In the second stage of the model birth rates remain high but the
death rate begins to fall. As a result the overpopulation begins to rise.
Factors influencing the falling death rate include improved health care,
improved hygiene,
the boiling of drinking water, improved sanitation through the construction of
essential water systems and collecting garbage. Improved food production and
storage. Key developments here include things like the Agricultural Revolution
of the 18th century, the invention of refrigeration, tractors replacing horse
and plow, genetically modified crops and modern irrigation methods. Also
improved transport for food. All these things also lead to a decrease in infant
mortality rates, that is, the number of deaths for children under one year old
per 1000 per year. This stage was typical of Britain in the 19th century and today
in countries such as Nigeria and Bangladesh which are experiencing
similarly rapid population growth. In the third stage of the model birth rates
start to fall while the death rates continue to fall. Population continues
to rise but the rate of that growth starts the slow down. Death rates have declined
due to increased overall living standards, better sanitation and healthcare
which in turn have also led to lower rates of infant mortality.
The declining birth rates can be attributed to the following sorts of
reasons. The availability of contraceptives. Since the 1960s when the
contraceptive pill became widely available. Further when people staying in
education longer and more woman entering the workforce people are delaying having
children which in turn shortens a woman's childbearing years.
Attitudes towards having large families change in societies when mechanization
of farms has reduced the need for so many workers in rural areas. Today the
third stage is typical of countries such as India and South Africa, In stage
four of the model both birth rates and death rates are low thus causing a
stabilization of the total population growth. Most more economically developed
countries are in stage four, For example New Zealand, Australia, the United States
and the United Kingdom. These countries have strong economies, a highly educated
population, strong healthcare systems and more woman in the workforce. These
countries are also highly urbanized due to the previous migration of people from
rural to urban areas. In recent years some demographers have
added a fifth stage to the model. This is because some more economically developed
countries such as Japan have total fertility rates below the 2.1
replacement level resulting in a declining population. Some European
countries such as Germany also have total fertility rates below the
replacement level but have managed to maintain their population through
immigration. The negative population growth rate does not have an immediate
effect though, due to demographic momentum. This phenomena occurs when a large
proportion of a country's population has of childbearing age. Even if the total
fertility rate of people in a country drops below replacement level it could
be a generation or two before the absolute numbers of people being born
still will exceed the numbers of people dying.
For example from 1979 to 2015 China had its one-child policy. Despite this policy
population growth continued anyway. This is because the number of people becoming
adults in 1979 was based on the number of births around the 1950s, not 1979. As a
result the Chinese population growth maintained the same momentum of increase
for 20 years before finally starting to stabilize. The different stages of the
model could also be compared to the three types of population pyramids.