POPULATION

in biology, the number of individuals of a species living in a restricted area, or the total number or combined weight of members of a plant species present in a given area. The study of biological populations involves analysis of the distribution and abundance of individuals of a species within a defined environmental region, as well as of changes in the genetic composition of the population and its age structure, sex ratio, density, and rates of birth, death, and migration over time. The population biologist's task is complicated by the ambiguity of natural boundaries, the frequent difficulty of direct observation, the sparse evidence generally available to support conclusions about population changes, and the number of factors whose combined impact may be responsible for those changes. A decrease in the population of a bird species, for instance, may be the result of a diminished food supply, an increase in the population of predators, climatic changes, or an interaction among these and other factors. The means available to biologists for monitoring population sizes include actual counting-most practical for plant species and large animals active during the day-and capturing and marking representative samples whose characteristics can be compared with those of samples recaptured later. Population density and breeding patterns are often assessed in fish and insect species by capturing or killing the entire population of a highly restricted, representative area, without affecting the population of the region as a whole. Genetic markers are used to examine patterns of breeding within populations and patterns of movement among populations. Most population studies take place under conditions that have been modified in some way by human presence. Local population sizes fluctuate little in some species and much more in others. Some population fluctuations are highly erratic, and others are cyclical, reaching peaks and crashing to low levels every few years, as occurs in some rodent populations. Many local populations that occur across a region are interconnected and are called metapopulations. Even when local population sizes fluctuate greatly, the overall metapopulation may be more stable. Elimination of the metapopulation structure of some species may render local populations more vulnerable to extinction. It is generally agreed that factors that maintain stable growth rates are closely linked to a species' population density in a given region relative to the numbers the region can support. Thus, periods of expansion or decline are usually followed by compensatory periods of change in the reverse direction. In most species, reproductive rates and the rate of recruitment (the onset of breeding in adults) are closely related to both current population density and environmental conditions. Birds may respond to a scarcity of food, for instance, by maturing later and producing fewer but larger (or smaller but more numerous) eggs than when sustenance is more plentiful; mammals in such a circumstance will reproduce later in life and have smaller or fewer litters. Since most fish mature according to size rather than age, lack of nutrients will naturally impede their recruitment rate and may induce limitations on the number of eggs they lay; similarly, insects lay fewer eggs when food supplies are limited, and the proportion of larvae that reach maturity and breed will also be reduced. For any population in a limited area, an increase in natality (birth) rates reduces the amount of food available to each individual, which results either in expansion outside the area or in an equivalent increase in mortality (death) rates. Seasonal cycles can also induce fluctuations in population density, especially in species that reproduce once per year during a particular season, or that only live for a single year. Under controlled conditions, populations of predators and their prey can exhibit parallel density cycles, though other naturally interacting forces produce less predictable variations. Population sizes are determined not only by birth and death rates but also by four types of movement: dispersal, which helps to expand the range of a species; dispersion, by which breeding adults redistribute themselves within a region; migration, a recurring cycle of movement from and back to a specific area in response to seasonal changes; and emigration, or relocation to more fertile, less heavily predated, or less densely populated areas, without a pattern of return. Population biology is also concerned with modes of interaction among members of a species, such as the cooperation displayed in flocking and herding behaviour and the competition sometimes involved in feeding and mating. Interactions between species can also be cooperative-such as in the symbiotic exchange between some microbes and their hosts-or negative, as in predatory and parasitic relationships. Competition between species that require the same types of food and shelter generally results in the exclusion of all but one from a habitable territory. The study of biological populations is crucial for an understanding of the natural balances in the environment of which humans are a part and for the prevention of unexpected and potentially disastrous upsets that can result from their modifications. Knowledge of the patterns of interaction among species and of the factors that influence population variables has enabled people to control and preserve numerous plants and animals and has facilitated agricultural and other advances that would otherwise be unattainable. in human biology, the whole number of inhabitants occupying an area (such as a country or the world) and continually being modified by increases (births and immigrations) and losses (deaths and emigrations). As with any biological population, the size of a human population is limited by the supply of food, the effect of diseases, and other environmental factors. Human populations are further affected by social customs governing reproduction and by the technological developments, especially in medicine and public health, that have reduced mortality and extended the life span. Few aspects of human societies are as fundamental as the size, composition, and rate of change of their populations. Such factors affect economic prosperity, health, education, family structure, crime patterns, language, culture-indeed, virtually every aspect of human society is touched upon by population trends. The study of human populations is called demography-a discipline with intellectual origins stretching back to the 18th century, when it was first recognized that human mortality could be examined as a phenomenon with statistical regularities. Demography casts a multidisciplinary net, drawing insights from economics, sociology, statistics, medicine, biology, anthropology, and history. Its chronological sweep is lengthy: limited demographic evidence for many centuries into the past, and reliable data for several hundred years are available for many regions. The present understanding of demography makes it possible to project (with caution) population changes several decades into the future. in human biology and physical anthropology, the whole number of people or inhabitants occupying an area (such as a country or the world) and continually being modified by increases (births and immigrations) and losses (deaths and emigrations). The size of any biological population is limited by the supply of food, the effect of diseases, and other environmental factors. Human populations are further affected by social customs governing reproduction and by the technological developments, especially in medicine and public health, that have reduced mortality and extended the life span. Changes in population are traced by keeping track of fertility rates, mortality rates, and migration. The fertility rate, or the actual birth rate, is lower than the biologically possible birth rate in all populations because of physical circumstances and social custom. Stillbirths and abortions reduce the number of fetuses that come to term. Late marriage, widowhood, and celibacy limit a woman's childbearing years, while the use of contraception can reduce the chance of pregnancy during sexually active periods. Human populations have widely varying fertility rates, ranging from an average of 10 children per woman in some Third World communities to 2 children per woman in the developed Western countries. As with fertility, mortality in different populations varies in degree and kind. The risk of death is generally high for infants, decreases to its lowest point in young children, and increases significantly after the age of 25. Developments in medical technology over the past two centuries have dramatically decreased the infant mortality rate and extended the average life span, especially in the developed countries. Whereas the average life span in ancient Greece was probably in the vicinity of 28 years, the average life span in modern industrialized nations has approached as high as 80 years for females and the low 70s for males. Migration, or permanent change of residence, also influences population size. Migration patterns differ in societies with different levels of technological development. For example, when human populations became predominantly agricultural, the pattern of continuous migration typical of hunter-gatherer and slash-and-burn societies dwindled. Mass migrations that have significantly altered large populations are often the result of threatening ecological or social circumstances. Modern migration patterns show that regular migration occurs from the less-developed nations to the more-developed and from rural areas to urban centres. The rate of natural increase, measured by comparing the rate of fertility and the rate of mortality, usually does not exceed 4 percent per year for any national population. Even a 4- or 3-percent figure, however, signifies rapid natural increase over a period of years. For example, a rate of 3 percent will cause a population to double in only 23 years. Population growth statistics take into account both the rate of natural increase and the effects of migration. In addition to being examined in terms of numerical change, populations can be studied for their composition by age, sex, ethnicity, or geographic distribution. Different kinds of populations have different age profiles. Aberrations in these profiles are most often due to dramatic excesses in the fertility rate or dramatic declines in the mortality rate, as in the late-20th-century population explosions in Kenya, Ethiopia, Mexico, and a number of other Third World countries. Changes in the age profile have social ramifications when it alters the proportion of the working population to the dependent population of children and older people. Dramatic differences in the sex ratio, caused by large-scale wars or the hardship of migration, can affect marriage patterns and therefore affect the growth rate of a population. More detailed analyses by race, ethnicity, and geographic distribution provide important basic information for sociological study of any population. Various theories about population have tried to explain its cultural influence and the reasons for dramatic population changes. Most ancient theories, conceived in societies where mortality rates were high, were pronatalist ("Be fruitful and multiply"). In the mercantilist theories of the 16th to the 18th century, large populations were also deemed beneficial, because they provided a large and therefore cheap labour force as well as a large market. The physiocrats of the 18th century viewed population growth as the result, not the cause, of economic wealth, because their economic theory related wealth to abundant land rather than population. Utopian thinkers of the same period assumed that humans would determine their own best population levels just as they would perfect other aspects of their society. It was not until the English economist Thomas Robert Malthus developed his theory of population growth in the late 18th century that the pessimistic implications of continued growth became commonplace. In his pamphlet An Essay on the Principle of Population, Malthus saw overpopulation as a primary cause of the poverty that accompanied urbanization and industrialization in Britain. He considered a large population a potential burden on the economy and prescribed a conscious limitation on reproduction. Malthus advocated such moral restraints on fertility as delayed marriage and warned that institutionalized charity could be counterproductive. Neo-Malthusians used his ideas to promote contraception as a means of limiting fertility, marking the beginning of the modern birth-control movement. Socialist thinkers led by Karl Marx rejected Malthus' claim that poverty was the result of overpopulation and contended that any population could be sustained as long as the society's material wealth was evenly distributed. One of the most significant trends in modern populations has been the decline in fertility that followed industrialization. The theory of demographic transition demonstrated how, in industrialized, urbanized societies, the functions of family and children were so altered that the average fertility rate for individual families was affected. The fertility decline was explained in socioeconomic terms. More recent theories have tried to incorporate cultural belief systems into the theory, showing how traditions have, in practice, modified the socioeconomic model. The growth of the human population and its ecological implications have become an increasing concern since medical technology has decreased the mortality rate. Population growth has accelerated since about 1750. The most rapid growth from the 18th to the early 20th century occurred in Europe, North America, Australia, and New Zealand. After World War II the highest population growth rates yet documented occurred in the already densely populated countries of Asia, Latin America, and Africa. Additional reading General works Roland Pressat, The Dictionary of Demography (1985; originally published in French, 1979); John A. Ross (ed.), International Encyclopedia of Population, 2 vol. (1982), a comprehensive reference work that contains articles on topics ranging from classic demography to current problems and that provides coverage of world regions and countries as well as organizations and agencies active in the field; Peter R. Cox, Demography, 5th ed. (1976), an examination of methods used in the study of population; Dennis H. Wrong, Population and Society, 4th ed. (1977), an introduction to the main aspects of the population dilemma; and Warren S. Thompson and David T. Lewis, Population Problems, 5th ed. (1965), a comprehensive sociological study. Population change Arthur A. Campbell, Manual of Fertility Analysis (1983), a methodological study: Gerry E. Hendershot and Paul J. Placek (eds.), Predicting Fertility: Demographic Studies of Birth Expectations (1981), a survey of the concepts, knowledge, and methods of fertility; Norman E. Himes, Medical History of Contraception (1936, reissued 1970), a classic historical treatise; John Bongaarts and Robert G. Potter, Fertility, Biology, and Behavior: An Analysis of the Proximate Determinants (1983), a scholarly examination of such aspects of the population problem as family planning, family size, birth intervals, and fertility; Ansley J. Coale and Susan Cotts Watkins (eds.), The Decline of Fertility in Europe (1985), a summary of studies of fertility transition in Europe during the 1970s; Michael S. Teitelbaum and Jay M. Winter, The Fear of Population Decline (1985), the history and bases of past and current concerns about low fertility, with special emphasis on major Western countries; Pranay Gupte, The Crowded Earth: People and the Politics of Population (1984), a study of the effects of Western development programs on overpopulation; Samuel H. Preston. Mortality Patterns in National Populations: With Special Reference to Recorded Causes of Death (1976), a discussion of the determinants and consequences of national mortality patterns, with attention to the role of standards of living, sex differences, and major causes of death; Alan A. Brown and Egon Neuberger (eds.), Internal Migration: A Comparative Perspective (1977), a collection of scholarly articles; International Migration Policies and Programmes: A World Survey (1982), a study of immigration and refugee programs, one of the series of population studies conducted by the United Nations; David Grigg, Population Growth and Agrarian Change: An Historical Perspective (1980), a study of the relationship between demography and economics; Eli S. Marks, William Seltzer, and Karol J. Krtki, Population Growth Estimation: A Handbook of Vital Statistics Measurement (1974), an examination of methods for demographic estimates; and Nathan Keyfitz, Population Change and Social Policy (1982), a compendium of insightful essays. The 1984 issue of the annual World Development Report of the World Bank focuses on population changes in underdeveloped countries. Population composition Colin McEvedy and Richard Jones, Atlas of World Population History (1978), comparisons of population statistics; Nathan Keyfitz and Wilhelm Flieger, World Population: An Analysis of Vital Data (1968), a statistical analysis of demographic data for different countries; Nathan Keyfitz (ed.), Population and Biology: Bridge Between Disciplines (1984), a collection of papers on biological aspects of demography; Dorothy L. Nortman, Population and Family Planning Programs: Compendium of Data Through 1983, 12th ed. (1985), including an analysis of demographic and social characteristics; David M. Heer, Society and Population, 2nd ed. (1975), a brief study of the population of nation-states; Philip M. Hauser et al., Population and the Urban Future (1982), which examines various demographic topics, including the quality of life of urbanized populations; Raymond F. Dasmann, Environmental Conservation, 5th ed. (1984), a study of the human influence on nature and its consequences for human population; and Colin Clark, Population Growth and Land Use, 2nd ed. (1977), an analysis of specific problems. Population theories Ronald Freedman (ed.), Population: The Vital Revolution (1964), is a collection of authoritative essays on major aspects of contemporary demographic analysis. The history of population theory is presented in Charles Emil Stangeland, Pre-Malthusian Doctrines of Population: A Study in the History of Economic Theory (1904, reprinted 1967); Joseph J. Spengler, French Predecessors of Malthus: A Study in Eighteenth-Century Wage and Population Theory (1942, reprinted 1980); William Petersen, Malthus (1979); Patricia James, Population Malthus, His Life and Times (1979); E.P. Hutchinson, The Population Debate: The Development of Conflicting Theories up to 1900 (1967); and J. Dupquier, A. Fauve-Chamoux, and E. Grebenik (eds.), Malthus Past and Present (1983). Other works include Alfred Sauvy, General Theory of Population (1969; originally published in French, 1952-54), a study of the relationship between the demographic-biological characteristics of societies and their economic and social circumstances; Philip M. Hauser (ed.), The Population Dilemma, 2nd rev. ed. (1969), a collection of papers by leading theoreticians; Garrett Hardin, Population, Evolution, and Birth Control: A Collage of Controversial Ideas, 2nd ed. (1969), a survey of the history of views and opinions; Jonas Salk and Jonathan Salk, World Population and Human Values: A New Reality (1981), which traces changes in attitudes to developments in population; and Julian L. Simon, The Ultimate Resource (1981), an optimistic perspective for the world's population. Development trends The spatial distribution of population characteristics is studied in Glenn T. Trewartha, The Less Developed Realm: A Geography of Its Population (1972), and Glenn T. Trewartha (ed.), The More Developed Realm: A Geography of Its Population (1978). Alfred Sauvy, Fertility and Survival: Population Problems from Malthus to Mao Tse-Tung (1961; originally published in French, 1959), presents an analysis of problems and proposes solutions. Other works include The Determinants and Consequences of Population Trends: New Summary of Findings of Interaction of Demographic, Economic, and Social Factors, 2 vol. (1973-78), a monumental compendium of theory and evidence on population trends compiled by the United Nations; the Demographic Yearbook, published by the Statistical Office of the United Nations, a basic source for more than 200 countries and territories; and Bernard Berelson (ed.), Family Planning Programs: An International Survey (1969), and Family Planning and Population Programs: A Review of World Developments (1966), comprehensive assessments of data. Later sources on trends in population development include Ester Boserup, Population and Technological Change (1981); Gavin W. Jones, Population Growth and Educational Planning in Developing Nations (1975); John Cleland and John Hobcraft (eds.), Reproductive Change in Developing Countries: Insights from the World Fertility Survey (1985); Gavin W. Jones (ed.), Demographic Transition in Asia (1984); C. Alison McIntosh, Population Policy in Western Europe: Responses to Low Fertility in France, Sweden, and West Germany (1983); Donald J. Bogue, The Population of the United States: Historical Trends and Future Projections (1985); John L. Andriot (ed.), Population Abstracts of the United States (1980); Richard L. Rubenstein, The Age of Triage: Fear and Hope in an Overcrowded World (1983); Jane Menken (ed.), World Population & U.S. Policy: The Choices Ahead (1986), papers by leading experts prepared for the second American Assembly on population issues; and National Research Council, Committee On Population, Population Growth and Economic Development: Policy Questions (1986). Population projections World Population Prospects: Estimates and Projections as Assessed in 1982 (1985), and Patterns of Urban and Rural Population Growth (1980), are publications of the United Nations Department of International Economic and Social Affairs. See also Just Faaland (ed.), Population and the World Economy in the 21st Century (1982), a collection of papers with economic and demographic forecasts; and Michael D. Bayles, Morality and Population Policy (1980), and Reproductive Ethics (1984), which discuss the moral aspects of long-term population policies.For current discussions of relevant topics and reports of recent research the following periodicals may be recommended: Contraception (monthly); Demography (quarterly); Economic Development and Cultural Change (quarterly); Economic History Review (quarterly); Family Planning Perspectives (bimonthly); Human Biology (quarterly); International Labour Review (bimonthly); International Migration Review (quarterly); Journal of Biosocial Science (quarterly); Journal of Economic History (quarterly); Journal of Interdisciplinary History (quarterly); Monthly Labor Review; Population and Development Review (quarterly); Population Bulletin (quarterly); Population Index (quarterly); Population Studies (three times a year); Science (weekly); and Social Biology (quarterly). Many non-English periodicals in the field appear with systematic summaries in English: Annales: conomies, Socits, Civilisations (France, bimonthly); Demografia (Hungary, quarterly); Demografa y economa (Mexico, quarterly); Demografie (Czechoslovakia, quarterly); Genus (Italy, weekly); Journal of Population Problems (Japan, quarterly); and Population (France, bimonthly). Michael S. Teitelbaum