Human Population Growth

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University of Massachusetts Amherst
Environmental Science

Human Populatio nhapters 8 and 9 I. The principles of population ecology apply to humans. A. The world population was 6.07 billion in 2000 and increased by 85 million from 1999 to 2000. 1. Although our numbers continue to increase, the growth rate (r) has declined slightly over the past several years, from a peak in the mid-1960s of 2.2% per year to 1.4% per year in 2000. 2. Demographers, scientists who study population statistics, project that the world population will become stationary (r = 0), that is, zero population growth, by the end of the 21st century. B. Highly developed countries have the lowest birth rates, lowest infant mortality rates, longest life expectancies, and highest per-capita GNPs. Developing countries have the highest birth rates, highest infant mortality rates, shortest life expectancies, and lowest per-capita GNPS. In recent decades, it has become useful to recognize a distinction within the developing countries between: 1. Moderately developed countries (e.g., Argentina, Brazil, Chile, Mexico, Thailand, Turkey, and Venezuela.) 2. Less developed countries or “LDCs” (e.g., Bangladesh, Cambodia, Laos, Ethiopia, and Niger.) C. The Demographic Transitionrefers to a commonly observed sequence of population- related events through which populations or nations almost invariably pass during the course of industrialization: 1. In the preindustrial stage, birth rates and death rates have been in equilibrium for many generations during which the level of the technology used by a society remained essentially unchanged. Infant mortality rates are characteristically high during this stage, and disease is often an important regulator of population as well. 2. In the transitional stage, technologies which can offer the largest reduction in human misery in return for the smallest investment of capital are introduced. These early transitional technologies include public health measures ensuring access to clean water and reduced human contact with sewage. The great European plagues were ended not by the developing of modern medicine but by improvements in public health measures. Vaccinations also offer a lot of “bang for the buck” and are among the earliest misery-reducing technologies to be introduced. Birth rates do not begin to decline until late in the transitional stage, so the imbalance between r and d generates a period of tremendous population growth. 3. In the industrial stage, the population finally begins to enjoy the benefits of affluence. For example, most costly measures to reduce human suffering become affordable, such as high-tech medicine. Certainly, most individuals in such societies can afford the modest costs of measures to reduce infant and maternal mortality. Another benefit of affluence is an almost guaranteed reduction in birth rates. a.“Affluence is the best contraceptive.” Moderately developed countries typically have lower fertility rates than LDCs (Table 8-3). Appreciable population growth may still occur during this stage, especially as a result of population growth momentum (see below) remaining from transitional times. b. Once a society reaches the industrial stage, restricting family size leads to an increase in standard of living and ability to educate the next generation. This situation is a reverse of the case in preindustrial societies where offspring go to work at an early age to contribute to the support of their families. c. Points 3a and 3b above form a kind of positive feedback loop that virtually ensures that industrialization leads to an eventual decrease in total fertility rate (number of offspring per woman in her lifetime). 4. Postindustrial societies have either equilibrated at their maximum population density or have entered a period of population decline following a peak at the end of the industrial period. Even in postindustrial societies, the inverse relationship between wealth and fertility can result in changes in the relative population sizes of different groups within a society if discrimination, recent immigration, or other factor produces a less wealthy subpopulation. 5. Most future growth of the world’s population is predicted to occur in countries that are in the earliest portions of the transition stage, i.e., in less-developed countries. (Fig. 8-16) 6. In modern times, the contraceptive effect of affluence is, of course, actually produced through the use of contraceptive technologies (Table 8-3 and Fig. 9- 9) D. The age structure of a population greatly influences population dynamics. Age structure diagrams illustrate the distribution of a population into different age and gender classes (Fig. 8-14), using a pyramid-like double histogram with number of males per age class on the left and females on the right. 1. It is possible for a country to have replacement-level fertility and still experience population growth if the largest percentage of the population is in the pre-reproductive years. a. Replacement-level fertility is the number of children a couple must have to contribute an average of two reproductive-age adults to the next generation. b. Replacement-level fertility is not a built-in property of the human species but, instead, varies markedly as a function of the exact shape of the survivorship curve applicable to a particular homogenous country or subpopulation in a heterogeneous country. c. Replacement-level fertility in highly equitable, postindustrial countries is about 2.1 children per couple, and in developing countries it is about 2.7 children. The last figure your instructor saw for the U.S. was 2.3 children per couple. 2. Such an age structure causes a positive population growth momentum. If you ignore the male side of a country’s age-structure diagram and reverse the X- and Y-axes, the resulting histogram should have exactly the same shape as the survivorship curve (on arithmetic axes!) for that country, IF the population is to have no population growth momentum. If the age-structure is too “rich” at the young end of the histogram (e.g., Fig. 8-13, left side and Fig 8-14a, Nigeria), then built in population growth is assured when the children of the present reach their reproductive ages, even if they all reproduce at exactly replacement levels. If the age structure is too “lean” at the young end (e.g., Fig. 8-13 right third, and Fig. 8-14c, Germany), then the population will shrink even if all the new couples reproduce at replacement levels. The percentage of a country’s population below reproductive age gives a good indicator of how much built in momentum its age structure is likely to have (cf. Fig. 8-15). E. The United States has one of the highest rates of population increase (0.6% in 2000) of all the highly developed countries. 1. The U.S. population has a positive population growth momentum because of the Baby Boom (the large wave of births that occurred after World War II) and immigration. 2. Compared to countries like Sweden where college professors earn the equivalent of $1000 to $1500 more per year than garbage collectors, income and wealth in the U.S. are distributed in a much less uniform fashion. Areas and populations can be found within the U.S. that resemble those in the developing world, so we should not be surprised that those areas and populations have population growth rates that are more like those of developing countries than more homogeneous highly developed countries. 3. The Immigration Reform and Control Act (IRCA), the basic immigration law in effect in the United States, gives three groups of people priority when migrating to the United States: those with family members living in the United States, those who can fill vacant jobs, and refugees seeking asylum. II. Many human problems, such as hunger, resource depletion, environmental problems, underdevelopment, poverty, and urban problems,
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