shifted westward and slightly southward since 1790. This shift is in response to westward agricultural settlement, development of the industrial economy of the Great Lakes region, migration to the West Coast with the railways, and later growth of the Sun Belt states.
Go to ArcGIS Online to complete exercise 2.1: “Spatial distribution of population.”
Figure 2.10.Population centroid of the United States. The centroid has shifted over time in response to population growth west and south from the original colonies. Data source: US Census.
The components of population
As seen so far in this chapter, the spatial distribution of population varies substantially from place to place. Environmental and economic forces play an important role in these patterns, as people cluster in areas with land suitable for agriculture or along important trade routes. But these forces tell only part of the story. As you can imagine, birth and death rates greatly vary from place to place as well. In some countries, women have many children, while in others, women have very few. Likewise, death rates can be high in some places and low in others. Naturally, the relationship between birth and death rates plays a role in population distributions. When more people are born than die, populations increase, while populations decrease when death rates are greater than birth rates. The reasons for the variation in birth and death rates include additional economic and political forces as well as cultural attitudes and beliefs.
Population growth or decline in a specific place can be calculated with a simple demographic equation:
Population change = Births − Deaths + Immigration − Emigration
This intuitive equation simply states that the population of a place changes as babies are born, people die, immigrants move in, and emigrants move out.
This chapter deals with the birth and death portion of the equation, and chapter 3 covers immigration and emigration.
Births
The crude birth rate (CBR) is the number of births per 1,000 people in a given year. The lowest CBR in 2015 was in Monaco with 6.65 births per 1,000, and the highest was in Niger with 45.45 (table 2.1). The CBR for the United States falls within the lower third of national rankings, at 12.49 births per 1,000. This measure is useful for calculating how quickly countries’ populations are growing, but its weakness is that it can be affected by the proportion of women in the population (those who may give birth) and by the age structure of the population (the proportion of young people to old people). These factors are discussed in more detail later in the chapter.
Table 2.1.Measurements of fertility vary greatly from place to place. Some countries face rapidly growing populations while in others births are not sufficient to replace previous generations. Data source: World Bank.
A somewhat more intuitive way of understanding population change, and one that accounts for differences in age structures and the proportion of sexes, is the total fertility rate (TFR) (table 2.1, figure 2.11). The TFR represents the average total number of children a woman will have during her lifetime. TFRs vary significantly. In Singapore, women in 2015 were having an average of just 0.81 children—less than one child per woman! The United States’ 2015 TFR fell in a moderate range of 1.87 children per woman. At the high end, Niger’s TFR was 6.76. As seen in the map, clusters of low TFRs can be found in Europe, and clusters of high TFRs are seen in sub-Saharan Africa.
Replacement fertility is the TFR necessary for a population to replace itself from one generation to the next without growing or shrinking. Replacement fertility of 2.0 would be the theoretical value, since 2.0 children would replace their two parents, resulting in no net gain or loss in population. However, because some women will die prior to reaching their reproductive years, replacement fertility is slightly over 2.0. In general, a TFR of 2.1 is used for replacement fertility. However, in developing countries with high mortality rates for infants and young adults, the rate can be a few decimal points higher. If a country has a TFR above replacement fertility, its long-term trend is toward an expanding population. Conversely, if a country’s TFR is below replacement fertility, its long-term trend is toward a shrinking population.
Look at figures 2.11 and 2.12. The countries represented in white have TFRs close to 2.1 and are in the ballpark range of replacement fertility. Those in the highest category have very high TFRs and should have rapidly growing populations. Most interesting, though, is the lowest category, those with TFRs substantially below 2.1. All those countries are facing the prospect of shrinking populations, since women are having fewer children than required for replacement. It is very rare for a species to voluntarily reproduce below the replacement level, but humans appear to be doing exactly that in many parts of the world.
Figure 2.11.Total fertility rate, 2015. Explore this map at http://arcg.is/2lDAgd5. Data source: World Bank.
Figure 2.12.Total fertility rates in Europe. Europe has some of the lowest fertility rates in the world, with most falling well below replacement levels. This is especially acute in Southern and Eastern Europe. Explore this map at http://arcg.is/2lDHFJs. Data source: World Bank.
The rate of births in a country results from a complex mixture of variables. Obviously, access to contraception is an important variable, but other factors include the spatial relationship of where a woman lives, economic conditions, political and social stability, gender equality, education, and levels of urbanization. These are discussed in more detail later in the chapter.
Go to ArcGIS Online to complete exercise 2.2: “Fertility rates.”
Deaths
Just as populations grow when babies are born, they shrink when people die. For this reason, the second essential component of population to understand is death and the different measurements used for quantifying it.
The crude death rate (CDR) is the number of deaths per 1,000 people in a given year (figure 2.13). As with the CBR, there is great spatial variation in the CDR. At the low end, Qatar had a 2015 rate of 1.53 deaths per 1,000, while Lesotho had a rate of 14.89. The United States falls close to the top third, with 8.15 deaths per 1,000.
When looking at figure 2.13, you can see that the CDR can lead to some surprising results. As you may expect, sub-Saharan Africa includes a large cluster of countries with high CDRs. Poverty, hunger, disease, and lack of medical care are prevalent in many parts of this region, so most people will not find it surprising that there are many deaths. But from there, the map illustrates death rates that many would not expect. Mexico and other Latin American countries have lower CDRs than the United States and Canada, for example. It is also apparent that Germany and Italy have CDRs higher than Iraq’s (figure 2.14). In fact, nearly all developed countries, such as those of North America and Europe, have higher CDRs than less developed countries in Latin America, Asia, and the Middle East.
Figure 2.13.Crude death rate 2015. Explore this map at http://arcg.is/2lDNfM0.
