is uneven. As we will see, human populations are clustered and thus impact some locations more than others. In addition, rates of population growth vary substantially from place to place, so that the spatial distribution of clusters can shift. Population centers move over time, as people are born in particular places and move to new locations.
Population patterns and growth rates have a profound impact on places, as seen in their spatial relationships with economic and social conditions. As you can imagine, some countries face fast-growing populations, where women have many children and there is a sensation of overpopulation. In parts of the world, governments struggle to grow their economies and provide enough jobs, housing, and food for rapidly expanding populations. Shortages in these areas can sometimes lead to political instability, as peoples’ anger over unmet needs erupts into street protests, coups and revolutions, and crime.
Less obviously, some places are seeing very slow population growth or even negative growth. This can create a completely different set of problems, as governments face conditions where there may not be enough people to work and grow (or even maintain) an economy. Low population growth can lead to a situation where places have too many elderly residents relative to young, active workers.
There is a strong spatial relationship between economic and urban development, gender roles, culture, and differing rates of population growth. As people move away from farms and into cities, economic and social change has a profound impact on population dynamics. As the reader of this book, you are most likely a resident of an affluent country and live in a city or town. Given your life experience, how many children do you plan to have? Why did you choose that number? If you lived in a poor rural setting, your life experience would be very different and would have a significant impact on the number of children you would want. Where you live has a strong relationship with how much you will contribute to population growth.
Population issues also lead to important discussions on spatial interaction. When population growth is high in some places and low in others, forces of migration can come into play. Migration is covered in more detail in chapter 3, but let it suffice that population pressures on economies and environments can push people to leave some places and move to new countries or cities.
Spatial distribution of population
World population numbers
For much of human history, populations grew slowly, but once humans moved from hunting and gathering to agriculture, improved food supplies allowed population growth to speed up. Later, as more consistent food supplies were complemented by improvements in human health through medical innovation and sanitary conditions, population growth rates increased even more as in Tokyo, Japan (figure 2.1). The global human population did not reach one billion until around 1800 (figure 2.2). After that, however, the population growth rate increased quickly. In just 123 years, by 1927, the population reached two billion. Then, in just thirty-three years, by 1960, world population reached three billion. Four billion was reached by 1974, and five billion by 1987. In 1999, there were six billion, and by 2011, there were seven billion people. The human population as of 2017 was over 7.3 billion, and growing at over eighty million people per year.
These growth patterns appear to be following the S-shaped curve (figure 2.2), whereby a population grows slowly during an initial lag period and then sees a rapid increase of exponential growth.
Figure 2.1.Tokyo, Japan. Understanding human population patterns is a first step in understanding human geography. Photo by aon168, Stock photo ID: 519265849. Shutterstock.
But before getting too worried about the fact that over eighty million people are added to our planet each year, it is important to see how the growth rate is changing over time (figure 2.3). Globally, peak growth was reached between 1965 and 1970. However, since that time, the growth trend has typically sloped downward. So, while the earth’s population is still growing, it is growing less quickly than in the past. Population geographers and others believe that this downward trend will continue, with population reaching 11.2 billion by the year 2100. After that time, projections become more uncertain. The global population could continue to gradually increase, or it could ultimately reach zero growth, where the human population returns to a relatively stable equilibrium. This pattern would represent the later stages of the S-shaped curve, where population growth slows and possibly reaches a more stable plateau.
While the world’s population is over 7.3 billion people, these people are not evenly distributed over the planet. As pointed out earlier, humans are clustered, resulting in very distinct levels of population density.
Figure 2.2.World population over time. Human population size remained relatively flat until the past one-hundred years or so. Data sources: United Nations, 1999; United Nations, 2015.
Figure 2.3.Population growth rate. The human population continues to grow but at a slower rate than in the past. Date source: United Nations, 2015.
Population density
Population density is used to describe the concentration of people in different parts of the world and can be calculated in several ways (figure 2.5). Arithmetic density, the most commonly used measure of population density, is the number of people per unit area, such as people per square mile. Arithmetic density (in countries of one million or more people) ranges from a high of over 7,500 people per square kilometer in Singapore to a low of less than two people per square kilometer in Mongolia (figure 2.4). As a comparison, the United States has about thirty-four people per square kilometer.
Another measure of population density is physiological density, which measures the number of people per unit of arable land and is intended to compare the number of people in an area with the amount of land available to feed them. Arable land is defined in a couple of different ways and thus can be confusing when mapping and analyzing physiological density. One definition holds that arable land is land that is suitable for cultivation. It includes land with the proper soils, elevations, slopes, and climates for growing crops. A narrower definition, which is used by the World Bank and the United Nations, holds that it is land used for annual crops, such as corn, wheat, rice, and vegetables, in contrast to permanent crops planted once, such as coffee, fruit, and nuts.
Figure 2.4.Singapore, which is essentially urban, has one of the highest population densities in the world. Mongolia, a sparsely populated country, has one of the lowest population densities in the world. Singapore photo by Martin Ho Smart. Stock photo ID: 559395556. Shutterstock. Mongolia photo by Jan Peeters. Stock photo ID: 225794263. Shutterstock.
Nevertheless, the idea behind physiological density is that a high value reflects a large population with a limited amount of agricultural land. Singapore, with virtually no agriculture, has an astonishing physiological density of over 945,000 people per square kilometer of arable land. At the other extreme lies Australia, with a physiological density of only about fifty people per square kilometer of arable land. Obviously, the strategies for feeding the people of Singapore are different than those for feeding the people of Australia: either they grow food domestically, they import it, or they do some combination of the two.
A third measurement is agricultural density, the number of rural residents per unit of arable land, which indicates how many people are involved with agricultural production. This measure helps illuminate which countries are efficient at growing food and which are not. For instance, Egypt’s agricultural
