Density and Distribution of Population
DENSITY AND DISTRIBUTION OF POPULATION
Population distribution refers to the way in which the members of a population or of a specified subgroup of a population (for example, defined by age, sex, or ethnic status) are dispersed physically in a specific area. Population density provides a comparative measure of distribution with respect to a geographic area that usually is expressed as persons per square kilometer (or per square mile) of land. More specialized density measures also may be defined, such as population per unit of cultivatable land.
The Distribution of the World's Population
Population distribution on a global scale is highly uneven, with the greater part of the world's population living in the northern hemisphere and in countries in the less developed world. Less than 10 percent of the world's population lives in the southern hemisphere, and 80 percent lives between 20 degrees and 60 degrees north latitude. Table 1 shows the growth of the world population since 1950 and its changing distribution projected to 2050. By the year 2000 approximately 74 percent of the world's population lived in Africa and Asia (excluding the Russian Federation) on only 40 percent of the world's land area. Europe accounted for 12 percent of global population, with a further 8.6 percent in Latin America and the Caribbean, 5.2 percent in North America
(the United States and Canada), and 0.5 percent in Oceania.
The increased population concentration in the less developed world reflects the exceptionally rapid growth of population in those areas since the middle of the twentieth century and lower growth and in some cases stability, and more recently even decline, in the more developed countries. Table 2 shows the ten most populous countries in the year 2000. The stylized maps presented in Figure 1 show how population is distributed by country and region and the broad changes in relative sizes over time.
The Environment, Society, and the Economy
Population density per square kilometer on a global scale is related to a number of factors both in the physical environment and in society and the economy. Although the physical environment does not play a straightforward deterministic role, extremes tend to discourage human settlement. Climate is a major factor. In very cold and very hot environments the range of crops that can be grown, if any, is limited, and this inhibits human survival. Accordingly, large areas of the globe are empty. Thus, in Lapland there is only 1 person per square kilometer, and in the Gobi Desert only 1.4.
Altitude is also significant. Mountain soils are usually thin, and at high altitudes temperatures and the oxygen content of the air decrease rapidly. This makes agriculture less productive, with additional problems created by difficulty of access and transport. Lowland areas tend to attract settlement more readily, with more intensive farming and industrial and commercial development. Coastal areas are often more attractive to settlement: Around two thirds of the world's population lives within 500 kilometers of the sea. Natural vegetation also may be a deterrent to human settlement, with, for example, the great rain forests such as the Amazon being poorly suited for high population densities. Negative factors in the environment do not always discourage settlement: For example Bangladesh, prone to major environmental hazards such as flooding, sustains a very high population density. A hot and humid environment near the equator permits cultivation to take place year-round.
Population distribution within continents and countries is also highly variable and is apt to change significantly over time. Within the countries of Western Europe, for example, population densities range from very high concentrations in the Netherlands to much lower densities in much of France and Spain. Within the United Kingdom, which is an area with overall high density, regional densities vary from over 600 persons per square kilometer in the urban counties of the southeast and northern England to well under 100 in large tracts of Wales and Scotland. Figure 2 illustrates the wide disparities in population density in the United States.
Population redistribution through migration, as well as population growth or decline, takes on increasing significance at smaller geographic scales. On a global scale migration has been of great importance historically in determining distributions of population, especially in relation to the great transatlantic migrations of the nineteenth and early-twentieth centuries. Redistribution of population also rewrote the world cultural map. Within countries industrialization and migration have gone hand in hand, entailing major redistribution from rural to
urban areas. In the countries of the more developed world, for example, in much of Western Europe, rural depopulation and urban growth have been a salient feature since 1850. In the less developed world rapid urbanization since 1945, compounded by high levels of overall population increase, has redrawn the map of population distribution in many countries. Distribution also can be affected directly by government policy, for example, by the encouragement or discouragement of international migration.
Mapping Population Density
Attempts to map population distribution and density date back to the early-nineteenth century. Graduated shading was used in a map of Prussian population densities in 1828, dots were used to represent population in France in 1830 and in New Zealand in 1863, and a variety of methods were employed to map population by the Irish railway commissioners in 1837. The later part of the nineteenth century saw the use of cartograms, in which regions are depicted as proportional to their population size rather than their geographic area. (See Figure 1.)
A simple and frequently used representation of population distribution that complements mapping is the Lorenz curve. A straight diagonal line represents an even distribution of population over the areas selected, and the larger the gap between the curve and the diagonal line, the greater the degree of concentration of population. Figure 3 illustrates both the method and the distribution of subgroups within a population compared to the population as a whole, in this case the distribution of two ethnic minority populations in Great Britain in 1991: persons of Irish and Bangladeshi origin. The horizontal axis indicates the cumulative percentage of these two groups, and the vertical axis indicates the cumulative percentage of the total population over the districts (in this case census wards) into which the country has been divided. Note the highly concentrated population of Bangladeshi origin compared to the more evenly spread Irish population.
Problems with Measures of Density
There are a number of general problems with measures of density. Population data are collected for highly variable geographic units that are rarely homogeneous in terms of economic and environmental characteristics. A density figure is simply an average with all the limitations that that implies, and care needs to be taken both in the definition of the population and in the areal or other units being used, particularly when comparisons are made at different geographic scales. Measures of population density extend beyond the crude density of population, the number of people per unit area. Useful national comparisons may be based on density defined in relation to cultivatable or cultivated land. For example, in Egypt overall population density is low in relation to the total national territory but high if population numbers are related to cultivated land, which is dependent on irrigation from the Nile.
Other calculations have been made to relate population numbers to levels of national income and standards of living. At the city level, measures such as the density of population per household or housing unit and the average number of persons per room provide a useful way of describing patterns of settlement. Thus, in the Paris agglomeration at the time of the 1999 census of population, for example, the number of persons per household varied from2.82 in the outer suburbs to 1.87 in the inner city. The mean number of persons per room in the central area declined from 1.02 in 1962 to 0.74 in 1999.
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Philip E. Ogden