Course: BIS 105 Environmental Biology

Unit 5 Learning Module

Demographic Transition

Demographic Transition

Major changes in human population growth began during the 18^th century, but they affected different regions at different times. We will first consider Europe, and later compare Europe to other regions of the world. In 18^th century Europe, seed planters, improved ploughs, threshing machines, crop rotation, and selective breeding of animals led to major growth in food supplies, so death rates due to starvation declined. With increasing understanding of the causes of disease, people improved water supplies, sewers, and personal hygiene – and lowered death rates even more. The Industrial Revolution of the 19^th century developed new sources of energy, such as coal and electricity. These further increased the efficiency of new agricultural machines and promoted the development of new forms of transportation, mainly railroads, which improved distribution of food. Death rates fell – particularly for those 5 to 10 years of age, allowing many more children to survive to reproduce. The pattern of human survivorship shifted toward a Type III curve.

Although death rates fell, birth rates remained at earlier levels. The gap between birth and death rates increased, and population growth began to accelerate (remember that r = b – d). Although this change did not happen uniformly throughout the world, it was soon reflected in world population levels: it took 200,000 years for the human population to grow to 1 billion, 123 years to grow to 2 billion, and only 12 years to grow from 5 to 6 billion!

Birth Rates Fall

It wasn’t long before birth rates started to fall as well in Europe and North America. People started having fewer children because large families were no longer beneficial for several reasons.

- As child death rates fell and machines did more work, farming families no longer needed to have as many children to work in the fields.

- Laws were passed that required children to go to school. Therefore, they could no longer work and contribute to their own support. They became a drain on the family’s income.

Eventually, birth rates fell to match death rates. As a result, population growth slowed to nearly zero.

Demographic transition models hold that human populations pass through four stages of growth (and has since been modified to include a fifth stage). This pattern of observed changes was first described by the demographer Warren Thompson in 1919.

Stages of the Demographic Transition

These changes in population that occurred in Europe and North America have been called the demographic transition. The transition can be summarized in the following four stages:

- Stage 1—High birth and death rates lead to slow population growth.

- Stage 2—The death rate falls but the birth rate remains high, leading to faster population growth.

- Stage 3—The birth rate starts to fall, so population growth starts to slow.

- Stage 4—The birth rate reaches the same low level as the death rate, so population growth slows to zero.

- Stage 5—The original Demographic Transition model only had four stages, but recently there have been proposed additions. Some countries, primarily European and East Asian countries, have experienced sub-replacement fertility (below 2.1 children per female). However, many other advanced countries have experienced fertility increases in the last 10 years.

This somewhat idealistic theory suggests that societies pass through predictable changes which lead to population growth patterns resembling the logistic or S curve. As we have seen, world population growth has been in a period of rapid expansion that is just beginning to show signs of slowing growth. Most developed countries are in stage 4 of the model; the majority of developing countries have reached stage 3. Some exceptions are a few poor countries, mainly in sub-Saharan Africa and some Middle Eastern countries, which are poor or affected by government policy or civil strife (Pakistan, Palestinian Territories, Yemen and Afghanistan).

Recent Population Growth

Death rates have fallen throughout the world, so that no country today is considered to remain in Stage 1. Countries appear to vary with respect to the timing of Stages 2 and 3. Many less developed countries remain in Stage 2, including Yemen, Afghanistan, Bhutan, Laos, and part of Sub-Saharan Africa.

Angola’s age structure (shown below) reveals accelerating Stage 2 growth. Widest at its base, the structure indicates many youths who will survive to reproduce at their parents’ high fertility rates because death rates are declining. Some countries, particularly those in regions of Africa which have been devastated by AIDS, appear stalled in Stage 2 due to disease and stagnant development. The demographic transition model may not prove to fit population growth in developing countries. Poor, low-income people in undeveloped countries have the highest birth rates. If demographic transition requires wealth and education, the world’s unequal distribution of development and resources may mean that these high birth rates will merely maintain exponential growth, rather than precipitate the social change associated with industrialization.

Angola’s population pyramid reflects Stage 2 growth: The wide bars at its base show the many youths who will survive to reproduce at their parent’s high fertility rates because death rates (small steps moving up the pyramid) are declining.

However, many countries appear to have begun the shift to Stage 3. Fertility rates have dropped 40% throughout much of South America, the Middle East, and the Pacific Islands. Countries such as India, Bangladesh, and Zimbabwe have lowered birth rates between 25-40%, and others such as Pakistan, Saudi Arabia, and Haiti have reduced fertility to 10-25% of earlier rates. Populations in most of these countries are beginning to level off, although resistance to change in the social factors which reduce birthrate may delay or prevent this response. Ecologist Garrett Hardin has pointed out that voluntary birth control selects against people who use it; by itself, voluntary control is unlikely to limit population growth.

High levels of industrialization and development have led to replacement (or lower) fertility rates in most of Europe, the United States, Canada, Australia, Brazil, China, and Thailand. China, Brazil, and Thailand passed through demographic transition extremely rapidly due to rapid economic and social changes. Replacement fertility includes 2 children to replace parents and a fraction of a child to make up for early mortality and at-birth sex ratio differences. Because mortality rates vary, replacement fertility rate ranges from 2.5 to 3.3 in poor countries, but averages 2.1 in developed countries. Globally, replacement fertility is 2.33 children per woman. In Stage 3 countries, populations will eventually stabilize if replacement fertility continues. However, many - including the US – continue to grow rapidly due to the “youth bulges” of exponential Stage 2 growth. The age structures of China and the US show demographic transition, but also youth bulges which will mean continuing growth for some time.

Population pyramids for China (above) and the U.S. (below) show decreased birth rates which suggest they have reached Stage 3 of the demographic transition model. Both countries show a population bulge remaining from earlier growth, so populations will continue to grow for a number of years.

Some countries have lowered birthrates below death rates so that r is actually negative. Japan, Germany, Italy, Spain, Portugal, and Greece are not producing enough children to replace their parents; populations in some of the southern European countries have already begun to decline. Top-heavy age structures for Spain and Austria are shown below. In countries such as Russia, negative growth emerged suddenly from economic and political crises which caused emigration, declining fertility, and increased male mortality, rather than from development and wealth as the transition model predicts. Negative growth rates pose economic threats: growth-dependent industries decline, and the burden of a large aging, economically dependent population falls on a smaller group of young workers. These shrinking population conditions are sometimes referred to as Stage 5 of the demographic transition.

Examples of Countries Through Time

The following video discusses the pattern of change we see as countries go through the demographic transition, and what it means for the future.

What stops population growth? from Gapminder Foundation on Vimeo.