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 density is over 1,800, whereas it is 39 for the United States. This means that there are forty-six times more people in rural areas of Egypt relative to arable land compared to the United States. Presumably, these people are involved directly or indirectly in the agricultural economy. A lower proportion of Americans in rural areas relative to arable land implies that they are much more efficient than the Egyptians in agricultural production, possibly because of the availability of agricultural technology such as farm machinery, agricultural chemicals, and precision agricultural mapping and monitoring with geographic information systems.
Figure 2.5.Measures of population density. Common measures of population density include agricultural density, physiological density, and arithmetic density. Explore population densities in ArcGIS Online at https://arcg.is/1LaKue. Data source: World Bank.
When physiological and agricultural densities are high, pressure to expand arable land into new areas may increase. This expansion can cause negative environmental impacts as natural landscapes are converted to agricultural uses, displacing wild plant and animal life. One way to reduce this pressure is to import food. For example, Egypt was the world’s largest importer of wheat in 2016. If a country such as Egypt, with high physiological and agricultural densities, has a weak economy and is unable to earn foreign currency through exports, then it will be difficult to import food, and hunger can ensue. With Egypt’s economy weakened from political instability and violence in recent years, foreign currency shortages and a declining exchange rate have strained the country’s ability to import food (figure 2.6). Prices have increased for consumers, forcing the state to spend more on subsidizing bread. In contrast, countries with high physiological and agricultural densities and relatively strong economies, such as South Korea and Japan, can import food more easily and face virtually no risk of hunger.
Population clusters
As can be seen, the spatial distribution of people around the world varies greatly. Some places have high densities, known as population clusters, whereas others have very low densities. Population clusters are found in several key locations (figure 2.7). East Asia, South Asia, and Europe form the largest population clusters, with other localized clusters found in parts of the Americas, Africa, and Southeast Asia. In East Asia, China alone has over 1.3 billion people, making it the largest country in the world in terms of population size. The region also includes large populations in South and North Korea as well as Japan. South Asia is dominated by India, with a population of just under 1.3 billion, the second largest in the world. Bangladesh, Pakistan, and Sri Lanka also have substantial populations in South Asia. Dense populations can be found in Europe as well, stretching from the Iberian Peninsula (Portugal and Spain) into the western portions of Russia and northwest Turkey. In the United States, dense populations can be found along the northeast seaboard, stretching from Boston to Washington, DC. Smaller population clusters can be found throughout the world in the form of large urban agglomerations, the result of ongoing urbanization of human society.
Figure 2.6.Waiting to buy bread in Aswan, Egypt. Egyptians rely heavily on imported wheat for their bread. With a weakened economy, prices rise, and many people struggle to purchase this staple food. Photo by Olga Vasilyeva. Stock photo ID: 418291645. Shutterstock.
Figure 2.7.Major world population clusters in Europe, South Asia, and East Asia. Sign in to your ArcGIS Online account and explore this map at http://arcg.is/2lDz8WW. Data sources: World Population Estimated Density 2015, Esri.
The distribution of population can be partially explained by the natural environment. As humans migrated out of Africa millions of years ago and spread to the far reaches of the earth’s surface, some environments proved more suitable than others for supporting large populations. Temperate climates (those that are not too hot or too cold, too wet or too dry) tend to form soils well suited for agriculture, a prerequisite for large populations. Environments with more extreme climates, such as deserts and subarctic regions, are suited only for small populations, which tend to be nomadic. Larger human populations also tend to be located at lower elevations. Exceptions to this rule are the mountain valleys of tropical regions, such as Central America, which have milder climates and better soils than the lowland tropics. Other environmental characteristics, such as natural resources or waterways for trade, can influence population distributions. For instance, arid locations with large mineral deposits or tropical rainforests with timber resources can attract people to small local clusters. Likewise, populations can cluster along coastal and river locations that facilitate trade with other areas.
While the natural environment has been an important force shaping where human populations clustered for much of human history, it is no longer the most important determinant. The differing rates of fertility, mortality, and migration that determine world population distributions now depend on a wider range of cultural, environmental, political, technological, and economic forces. For instance, since food can be easily imported from far away, the importance of agricultural potential in the growth of population clusters is diminished. Also, population growth in trade centers can be just as fast along human-built routes such as railroads and highways as along natural rivers and harbors. Airports can now replace natural seaports. With fewer restrictions tied to the natural environment in terms of food production and trade, population clusters can form in many more locations than in the past.
There are several good examples of places where populations are growing despite the natural environment, not because of it. Phoenix, Arizona, with its arid landscape and over 100 days per year when temperatures reach 100 degrees or higher, is now a thriving metropolis. The same goes for Las Vegas, Nevada (figure 2.8). In both cases, economic development and technology have allowed for populations to grow in places with difficult natural environments. Air conditioning and complex systems to deliver food and