The DTM has five stages that can be used to explain population increases or decreases. You need to be able to recognize these when looking at a population pyramid.
Stage one has a high birth rate and a high death rate. Because of this the natural increase rate is close to zero Zero population growth is when the crude birth rate and crude death rate are equal and the population remains the same.
No country in the world is currently in stage 1. There may be small tribes in the Amazon or Sub-Saharan Africa that are in stage one, but it is rare.
However, for most of human history, the entire world was in stage one. Remember that it took the world about 100,000 years to reach one billion people!
Stage two has a high birth rate, but the death rate drops. Because of this the natural increase rate goes way up!
A number of countries in Sub-Saharan Africa (Niger, Democratic Republic of the Congo, Somalia) are currently in stage two. The population of these countries is rising and their doubling time is short. However, the population will not continue to go up at the same rate.
Stage three the birth rate goes down, while the death rate remains low. The population continues to rise, but not nearly as much as stage two. The natural increase rate is still positive, but not as high as in stage two.
Stage three countries start to become more industrialized, which means they are more urbanized and the total fertility rate goes down. Countries that are currently in stage three are Mexico, India, Colombia, and South Africa.
Stage four countries the birth rates get lower, while death rates start to rise as people are getting older. The natural increase rates (NIR) in these countries is close to zero.
For example, in the United States the NIR would be zero if you took away the net-in migration that it has every year. Other countries currently in stage four are China, Brazil, and Argentina.
Stage five countries the birth rates remain low, and the death rates go up. Because of this, countries have a negative NIR, which leads to the population decreasing. These countries have graying populations, with less men and women in their childbearing years.
A number of these governments promote pro-natalist policies to try and stunt the population decrease. Countries currently in stage five are Japan and a number in Eastern Europe (Germany, Estonia, Ukraine).
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The ETM describes causes of death in each stage of the DTM.
Most people die because of pandemics, like infectious and parasitic diseases (the Black Plague and Malaria).
They will also die because of environmental factors like drought, earthquakes, floods, and also things like starvation and malnutrition.
People begin to live longer, because of improved medicine, nutrition, and sanitation. In this stage, not as many people die of infectious diseases because of epidemiology, which is the branch of science that studies disease, the causes and cures.
Because of better infrastructure (hospitals, sewers, better plumbing) less people die of parasitic diseases.
People are living much longer. However, chronic diseases associated with age become a challenge. Things like cancer and heart disease are the leading causes of death.
People are still living longer, but because of better healthcare, treatments, and medical technology people are able to survive cancer and heart disease.
Stage five has the highest death rates because the population is older. This leads to a negative NIR.
Experts cite three different reasons for this.
The first is disease evolution. Infectious diseases have evolved and established resistance to drugs and other treatments.
The second is poverty. Even the most modern societies have homelessness and poverty. Infectious diseases spread more easily in these pockets of society.
The last is increased connections. Through air travel, trains, highways the world is more connected than ever. Because of this, diseases like AIDS, which spread from person to person via blood or sexual fluids, can be unknowingly spread around the world.
🎥 Watch: AP HUG - Population Growth and Decline
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2.5The Demographic Transition Model