the density of population in a given area. Population density is expressed in terms of the number of permanent inhabitants per unit area, usually per sq km. Uninhabited territory and large inland water areas are sometimes excluded when computing density. Separate density indexes are applied for urban and rural populations. Population density varies greatly by continent, country and region; it depends on the distribution, proximity, and size of settlements. Density is ordinarily much higher in large cities and urban areas than in the countryside. The population density of an area therefore represents the average level of population density in the individual parts of the area when the size of the parts has been taken into account.

As one of the conditions for population reproduction, population density has some effect on a population’s rate of growth. However, population density does not determine population growth, much less the development of society, Increase in density and uneven population growth in particular parts of a country result from the development of productive forces and the concentration of production. Marxism rejects contentions that population density defines absolute overpopulation.

In 1973 the population density of the inhabited continents was 28 persons per sq km. Figures broke down as follows: Australia and Oceania—2 persons per sq km; the Americas—13 (North America—14, Latin America—12); Africa—12; Asia—51; Europe—63; USSR—11 (34 in the European part, about 4 in the Asian part).

the number of individuals (animals, plants, microorganisms) per unit volume (water, air, or soil) or area (soil or bottom of a body of water).

Population density is an important ecological indicator of the spatial distribution of members of a population, the dynamics of animal numbers, the conditions of variation, and the manifestation of natural selection. It is determined predominantly by the degree of favorableness of living conditions for the species in a given biotope or by the most important ecological factors of the environment, especially those that are present at a minimum and are called limiting factors. For this reason, the favorableness of a habitat may be judged by the average population density. Places of temporary and permanent habitation (“survival stations”) where the remaining individuals of a population are preserved in particularly unfavorable years) may be identified from their permanency and the degree of population fluctuation in various seasons and years. The survival stations of rodent species having a high rate of fecundity usually constitute no more than 3 to 10 percent of the animals’ range. By identifying the survival stations of agricultural and forest pests and those of reservoirs and carriers of diseases of man and animals useful to man, including domestic animals, one may economically and effectively control harmful animals in their survival stations, thus avoiding contamination of extensive areas.

The population density and spatial distribution of animals vary regularly by corresponding population mechanisms. The growth of population density in most species is accompanied by their discharge into the environment of metabolic products, including special signaling substances that inhibit or accelerate growth and development, limit or even arrest reproduction, increase mobility, and change behavior. As a result, with high population density, resettlement is intensified and mass emigration may begin. When population density decreases, emigration is curtailed and mobility somewhat decreases, increasing anew when the population becomes very sparse and threatens intra-population groupings (families, flocks, herds, colonies) with destruction. Intensity of reproduction is simultaneously increased.

Every species, depending on its way of life and mobility (sessile, settled, nomadic, migratory), has an optimal population density and permissible limits for its fluctuations, which vary with each biotope (maximum and minimum population density). In nonmobile organisms (plants, microorganisms, sessile animals), which obtain food and oxygen by means of water and air currents and soil solutions, it is possible and, in many cases, even feasible for organisms to join together (seeCOLONIAL ORGANISM). Such is the significance of colonies or families of social insects, such as bees, ants, and termites. Colonial breeding sites of birds, especially bird rookeries, and colonies of mammals (susliks, marmots, pikas, bats) are also characterized by high population density.

The majority of species live singly or in small groups (families), occupying definite areas (individual or family plots) that, as a rule, are adjacent to one another or, sometimes, partially or completely overlap. Population density, corresponding to the way of life of a species and the conditions of its existence, is maintained and regulated by many mechanisms that have developed through evolution. Of major significance is territoriality, that is, the animal’s ability to demarcate its territory and protect it from intrusion actively and through the use of warning signals (chemical, visual, acoustic). There are signals of opposite purpose, for example, to attract individuals of the same family or herd, in order to maintain groups. (See also.)