Population Density

Density in Ecology

The defining theme of the science of ecology is the distribution and abundance of organisms—that is, their population densities. Ecologists and biogeographers have developed concepts and methods for the analysis of density patterns applicable to individual animal and plant populations (e.g., species) and to interacting populations within a given area. They have developed and refined the inherently geographical notions of range, carrying capacity, competition for resources, ecological niche, habitat, invasion, succession, and dispersal. Ecologists examine both the effects of environment on animal/plant population densities and the impacts of population densities on the environment (particularly on food resources and competing populations).

Ecological perspectives on density have been applied in anthropology, sociology, and human geography; to examine human evolution and dispersal; to assess the density impacts of hunting, gathering, and agricultural systems; and to study spatial competition and conflict between human groups. The Chicago School of human ecologists (Park, Burgess, and others) applied ecological notions of competition, invasion, and succession to patterns of ethnic and racial groups in large cities.

Global and Rural Population Densities

The term ecumene is derived from the ancient Greek word oikoumene and refers to the inhabited (or inhabitable) portion of Earth's surface. This concept relates directly to population density, in that a cutoff density of 1 person/km2 (approximately 2.5 persons/mi.2 [square mile]) is frequently employed. Below this density level, human groups typically exist as widely scattered hunters, gatherers, or herders and lack permanent settlements. The major nonecumene areas clearly relate to climate and specifically to areas of desert climate (e.g., the Sahara, outback Australia) or high-latitude areas with insufficient growing seasons for agriculture (e.g., the Canadian north, Siberia). Exceptions occur in the lowland tropics (e.g., Amazonia, interior Africa), where tropical diseases and poor transport access have deterred agricultural settlement, at least until recently. Another low-latitude exception is the Tibetan plateau, which mostly exceeds 4,000 meters in elevation.

The world's major population concentrations were all initially based on subsistence sedentary agriculture, with rural population densities ranging from 25 to 500 persons/km2 or more. The three great islands of mankind (a term coined by the North American geographer William Bunge) are in East Asia, South Asia, and Europe. After several millennia of agricultural expansion, their limits are now clearly shaped by climate and terrain. Smaller concentrations of more than 25 persons/km2 occur in southeast Asia, West Africa, the Central African highlands, and the highlands of Mexico, Central America, and South America. Their limits are partly related to the physical environment, but they are also shaped by factors such as farming technology, warfare, transport access, and disease. Both Edward Ackerman and Ester Boserup have provided authoritative reviews of the environmental, economic, technological, and cultural factors shaping world and regional density patterns.

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