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Danielle teaches high school science and has an master's degree in science education.
Demography is the statistical study of human populations. Demographers look at information in order to determine specific characteristics of a population.
In this lesson, we will use the fictitious population of Dharma Island that has recently formed in the ocean to look at the theory of demographic transition, which is basically a population change model that demographers can use to review how a population has changed in the past as well as how to make predictions about the future of the population.
Before we can look at the theory of demographic transition using our population on Dharma, we first need to go over a few terms that will be used. Fertility rate refers to the number of births in a population. This is normally given as a number out of another number. For example, the fertility rate on Dharma was around 31 births per 1,000 women age 15 - 44. You may also see it as the number of children per woman such as 1.6 children per woman. This is an average of children who were born to women generally of reproductive age (around 15 - 44 years old).
Opposite of fertility rate is mortality rate, which is the number of deaths in a population. Again, this is normally given as a number out of another number. On Dharma, the mortality rate was 8 out of 1,000 people.
Other terms that are used when talking about demography include age composition and life expectancy. Life expectancy refers to the age at which someone is predicted to die. Again, let's look at information about Dharma. The life expectancy of women was 81 years, and the life expectancy of men was 76 years.
The age composition of a population can best be described as the age structure of the population. This refers to the number of people in certain age groups. A population may be described as having a large number of young people or maybe a large number of older individuals. Demographers often use population pyramids in order to illustrate information about age composition and life expectancy. Let's take a look at a few pyramids now to understand the basics.
In this diagram, we have the number of males on the left in blue and the number of females on the right in pink. The overall shape of the population looks like an upside down ice cream cone. We can see from this that the age composition of the population is much larger at younger ages than older ages. The number of females and males is about the same. The life expectancy is not very high as there are few older people in the population.
This next diagram again has the males on the left in blue and the females on the right in pink. This looks more like a triangle, but again, we can see that the age composition of the population is heavily based in the younger population. The main difference between this pyramid and the first is that the life expectancy is longer. We can see this by viewing the top of the pyramid. In this case, there are more people alive in the older ages than on the first diagram.
This next population pyramid has a more even shape, indicating that the age composition is pretty well balanced between the younger and older groups within the population. The life expectancy is greater for this population than the previous population, which we can see by the large number of individuals in the older age groups. We can also tell that the fertility rate isn't as high because the number of young people is more even with the rest of the population.
In this last population pyramid, we can identify one main difference from the last graph. In this population, the birth rate is declining. It may be difficult to see, but if we look closely at the bottom of the diagram, we can see that there is a small dip indicating that the number of young people is lower than the rest of the population.
Now that we have the basic terms down, let's look at the theory of demographic transition using our population on Dharma. Demographers have identified four main stages of transition, but some have also identified a fifth stage. Let's start with stage 1.
The island of Dharma has recently become inhabited. The population is just starting out and does not yet have the best sanitation and water quality. Some of the water is contaminated, allowing diseases to spread. There isn't much health care available on this remote island, which does not help the stop of diseases that are being caused by the lack of sanitation. There aren't yet supermarkets from which the people can buy food and supplies, so they need to work the land. Larger families are better for this because the children can go to work and help support the family. Dharma is currently in stage 1, or the high stationary stage of demographic transition. Stage 1 has high fertility and high mortality rates.
Because both rates are high, the overall size of the population doesn't change much. As with the population on Dharma, mortality rates are high due to poor sanitation, poor water quality, high rates of disease and high rates of famine. The fertility rates are high due to the lack of family planning and the cultural need for children. Populations in stage 1 tend to be rural and agricultural, so families need kids in order to work the land and to survive. Relating this information back to the pyramids, in the first pyramid the number of births was very high and based on the small number of older individuals, we can assume that the death rate was also very high. This pyramid is an example of a population in stage 1 of demographic transition.
Because of the high death rates on Dharma, the population decides that it is necessary to improve the water quality, sanitation and health care system. While it is more than a small feat to do this, the people of Dharma know that it is needed. As these improvements are made, the population moves into stage 2, or the early expanding stage of demographic transition. Populations in this stage have a rapid drop in mortality rates and high fertility rates.
During this stage, the overall population starts to increase. As we see with the people of Dharma, the decrease in deaths in stage 2 is due to improved sanitation, water quality, health care and food production. The number of births is still high because populations at this stage still require large families for survival and there is still limited family planning. This stage was seen in the second population pyramid, which had a high fertility rate but a lower death rate than the first pyramid.
Now that the people of Dharma have access to health care and clean water, they begin to move into larger towns. There is no longer the need to live in a rural area as there is now the infrastructure for urban areas, which also means industrialization. While previously the people of Dharma wanted to have large families so that the children could help provide for the family, it is now becoming expensive to have so many kids in the city. Rather than have a large family and struggle financially, more family planning will take place so that it is more affordable to have a family. The population is now in stage 3, or the late expanding stage of demographic transition. The population has a decrease in fertility rates and steady mortality rates.
Populations in this stage, as with our population on Dharma, continue to grow as they did in stage 2. The decrease in births is due to changes in society, such as industrialization and urbanization. Remember that with stages 1 and 2, large families were required in order to work the rural land; however, with the increases in industrialization and urbanization, large families are no longer needed and can actually be a hassle because of the cost of raising children in cities. There is also an increase in family planning of populations in stage 3, which decreases the number of births. Death rates remain the same as there are not great advances seen in areas such as medicine and water quality. We didn't look at a pyramid in this stage; however, this diagram illustrates that the population is expanding, but slowly.
The population on Dharma continues to change. There are advances in medicine and the life expectancy gradually increases, meaning that people are living longer. Additionally, couples are waiting until they are a little bit older to have children. This causes a slight decrease in the fertility rate of the population. At this point, the size of the population is generally stable and is considered to be in stage 4, or the low stationary stage of demographic transition. The population has fertility and mortality rates that are equal and steady.
Remember that because of this balance, a population in stage 4 will remain constant. This type of population is seen in most post-modern or industrial societies, such as the United States. The number of births continues to drop from stage 3 due to an increase in family planning and to people waiting longer to have children. Deaths continue to decrease because of advances in technology and medicine. In this population pyramid that we looked at earlier, there were about the same number of younger and older individuals. This illustrates a population in stage 4 because of the equilibrium seen in the pyramid.
According to some demographers, the population on Dharma will continue to change even after stage 4. Stage 5 in demographic transition occurs when mortality rates are higher than fertility rates. This causes a decrease in the overall population. The drop in birth rates continues due to family planning, the high cost of children or in some cases, government regulations. Countries that have been identified as in stage 5 include Italy, Germany and Japan. The last pyramid that we looked at showed low birth rates and can be classified as stage 5 of demographic transition.
In order to review all stages, let's look at a graph of all stages at the same time. Remember that in stage 1, populations have high fertility and mortality rates, causing the population to remain about the same. As populations improve medicine and water quality, the number of deaths begins to decrease and the population moves into stage 2. During this stage, the birth rates remain high but the death rates drop, causing the overall population to increase. In stage 3, the birth rates begin to decrease due to industrialization and the cost of children, but death rates remain about the same. The population continues to increase through stage 3. In stage 4, the population will stay about the same because the number of births and the number of deaths are in balance. Remember that the United States is considered to be in stage 4. Some demographers have identified stage 5, during which the number of births decreases, causing the population to decline.
The theory of demographic transition predicts how a population will change over time in regards to the mortality and fertility rates as well as age composition and life expectancy. All factors must be taken into consideration when studying a specific human population.
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Back To CourseCLEP Biology: Study Guide & Test Prep
23 chapters | 211 lessons