A population is a subgroup of individuals within the same species that are living and breeding within a geographic area. The number of individuals living within that specific location determines the population density, or the number of individuals divided by the size of the area. In the case of humans, population density is often described as the number of people per square kilometer (or per square mile) and is discussed in relation to urbanization, migration and demographics.
Globally, statistics related to population density are tracked by the United Nations Statistics Division. Many countries also collect their own data on population density, often through the use of a census. In the United States, for example, the Constitution requires population data to be collected every 10 years, a task carried out by the U.S. Census Bureau. China also conducts a census once a decade and uses the information for economic and social planning. In recent years, some countries have employed new technologies to confirm population density and predict changes. China, for example, analyzes nighttime light data collected from sensors in space to confirm current population density calculations and to predict how density will change in coming years. Other countries use computer algorithms to interpret census data and forecast future growth.
There are also independent research groups that track population density across the world. Independent groups are especially helpful in countries that have few resources or are facing challenges (such as conflict) that make it difficult for the government to collect accurate data.
Data on population density at the national and regional levels is not always useful, however. One reason for this is because census data can be flawed; in particular, it sometimes fails to count entire groups of people, such as nomadic populations, migrants, refugees, people experiencing homelessness, residents of informal settlements (such as favelas) and others who do not have clear citizenship status. Collecting census data often relies on census takers visiting people at home, so residents who live in areas where there is conflict or who live in remote, hard-to-access places may also not be counted. In many places, forms have replaced census takers, but this can contribute to inaccuracies in areas with low literacy rates.
Population density provides the number of people living on a given area of land (such as people per kilometer or per square mile), but it does not describe how people relate to the land of that country. For example, society tends to form clusters that can be surrounded by sparsely inhabited areas, and life might be very different for those who live in the cluster versus those who live outside of it. Similarly, the population density of an area that includes a large city and desert may be the same as that for a population that is more evenly spread in an area, despite the fact that the two areas have little in common in terms of crowdedness. As a result, population density is more useful when applied to small areas, such as neighborhoods, than to entire regions or countries.
Some methods of calculating population density are more helpful for certain kinds of analysis than a strict “people per unit of land area” formula. Physiological density is a calculation of how many people in the country are supported by a unit of arable land. This can reveal how much pressure is put on the country to produce enough food for the population. Agricultural density measures how many farmers or people living in rural areas there are versus how much arable land. These kinds of metrics help illuminate issues of population density that would otherwise be missed.
Dense population clusters generally coincide with geographical locations often referred to as city, or as an urban or metropolitan area; sparsely populated areas are often referred to as rural. While these terms do not have globally agreed upon definitions, they are useful in general discussions about population density and geographic location.
Population density data can be important for many related studies, including studies of ecosystems and improvements to human health and infrastructure. For example, the World Health Organization, the U.S. Energy Information Administration, the U.S. Global Change Research Program and the U.S. Departments of Energy and Agriculture all use population data from the U.S. Census or United Nations statistics to understand and better predict resource use and health trends.
Key areas of study include the following:
- Ecology: how increasing population density in certain areas impacts biodiversity and use of natural resources.
- Epidemiology: how densely populated areas differ with respect to incidence, prevalence and transmission of infectious disease.
- Infrastructure: how population density drives specific requirements for energy use and the transport of goods.
- Urbanization: how population density relates to overcrowding, housing shortages and the provision of services.
This list is not inclusive—the way society structures its living spaces affects many other fields of study as well. Scientists have even studied how happiness correlates with population density. A substantial area of study, however, focuses on demographics of populations as they relate to density. Areas of demographic breakdown and study include, but are not limited to:
- age (including tracking of older populations);
- race, ethnicity and cultural characteristics (ethnic origin, religion and language)
- socioeconomic characteristics (including poverty rates)
