The World’s Population. Population Density. Physiological and Agricultural Density

Where is the world's population concentrated? What processes are responsible for changes in population distribution over time? Why do different places have different rates of population growth? What factors influence the migration of people from one place to another? The determinants and consequences of population distribution and change are fundamental to the study of human geography.

Most anthropologists accept the theory that the human species, Homo sapiens, originated in East Africa two or three million years ago. Since that time, human beings have moved into every habitable place on the earth's surface (Figure 3-1).

Figure 3-1. The Diffusion of Homo-sapiens. Our species is believed to have originated in East Africa some three million years ago. Human beings soon moved across the Eurasian and African continents. In more recent times, boats and ships have enabled people to populate the Americas, Australia, the Pacific Islands, and the isolated islands of the world

Most of Africa and Eurasia had already been colonized by humans hundreds of thousands of years ago. More recently, our species has colonized most other areas of the world. Ancestors of Native Americans and the Inuit are believed to have migrated across a land bridge between Siberia and Alaska that formed during the most recent ice ages. Boats and ships expedited the settlement of Indonesia, Australia, and the Pacific Islands in more recent times. The isolated islands of Hawaii and New Zealand were settled by Polynesians only a few centuries before European settlers arrived during the Age of Exploration.

In the contemporary world, the human population is distributed in five major clusters (Figure 3-2). Three of these are located in Asia and the others are in Europe and North America. The world's largest population cluster is found in East Asia. It encompasses most of eastern China. Japan, Taiwan, and South Korea. The next largest includes countries in the Indian subcontinent—India, Bangladesh, Pakistan, and Sri Lanka. A third major cluster in Southeast Asia includes much of Indonesia, Vietnam, and the Philippines, along with the island of Singapore. Together, the Asiatic clusters contain more than half of the world's population.

A majority of the population in each of these Asiatic clusters lives in rural areas. Although places like Japan and Singapore are highly urbanized, the majority of Asians are farmers. The two remaining population clusters are found in highly urbanized and more uniformly developed countries. The European cluster encompasses much of Western and Central Europe, including England, Germany, France, the Netherlands, Belgium, and their neighbors. The North American cluster encompasses the large cities of the Atlantic seaboard and the Great Lakes, including New York, Boston, Washington, Detroit, Toronto, and Chicago.

Population Density. The five major population clusters are all characterized by high population density. Population density is a measure of the number of people per unit of land. For example, the population density of the United States is found by dividing its population of approximately 250 million people by its land area of approximately 3.6 million square miles, or 9.4 million square kilometers. Division yields a population density of approximately 70 people per square mile, or 27 per square kilometer.

Within the United States, population density varies considerably. Alaska's population density is less than one person per square mile, while those of Rhode Island and New Jersey are more than 800 per square mile. Naturally, population-density values are even higher in cities. In Washington, D.C., approximately 600,000 people live within its territory of 70 square miles, so its density is nearly 9,000 per square mile.

Physiological and Agricultural Density. The concept of population density provides a useful basis for comparing densely populated and lightly populated places, density measures tell us nothing about the ability of a population to support itself. A large population living on highly productive land will thrive, while a large population cannot support itself in unproductive territory.

In order to take the quality of land into account, geographers calculate physiological density. The physiological density of a place is its population divided by the amount of its arable land, or land suitable for cultivation. Countries in which arable land is scarce may have high physiological densities even if their actual population densities are low. Egypt is a prime example. Nearly all Egyptians are concentrated along the fertile strip adjacent to the Nile River. Farmers in the rainless Egyptian desert depend on the Nile for water. Hence, arable land in Egypt is limited to the irrigated Nile Valley and the outlying deserts are sparsely populated.

Physiological density values cannot always account for differences in standards of living. Japan, like Egypt, has a high physiological density. Arable land in Japan is limited by steep slopes associated with Japan's mountainous terrain. While Egypt has suffered in recent years from serious food shortages, Japan maintains an impressive per-capita level of food consumption. Japan feeds its large population on limited agricultural land because of the high productivity of Japanese farmers and because Japan's high level of economic development allows the importation of massive quantities of food. By contrast, most Egyptian farmers lack the technology needed to grow highly productive crops. At the same time, Egypt's weaker industrial base restricts its ability to import food.

A third density measure captures the distinction between the cases of Egypt and Japan. Agricultural density measures the number of farmers per unit of arable land. A low agricultural density relative to physiological density implies that a country's agricultural production is highly efficient. Countries like Japan have highly efficient agricultural production systems, with each farmer able to produce enough food to feed large numbers of people. By contrast, countries with high agricultural and physiological densities often run into difficulty feeding their populations (Figure 3-3).

Figure 3-3. The Measurement of Population Densities. Each of these measurements produces a different relationship between humans and the land.