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Explanation:
The interactions between human population dynamics and the environment have often been viewed mechanistically. This review elucidates the complexities and contextual specificities of population-environment relationships in a number of domains. It explores the ways in which demographers and other social scientists have sought to understand the relationships among a full range of population dynamics (e.g., population size, growth, density, age and sex composition, migration, urbanization, vital rates) and environmental changes. The chapter briefly reviews a number of the theories for understanding population and the environment and then proceeds to provide a state-of-the-art review of studies that have examined population dynamics and their relationship to five environmental issue areas. The review concludes by relating population-environment research to emerging work on human-environment systems.
Humans have sought to understand the relationship between population dynamics and the environment since the earliest times (1, 2), but it was Thomas Malthus’ Essay on the Principle of Population (3) in 1798 that is credited with launching the study of population and resources as a scientific topic of inquiry. Malthus’ famous hypothesis was that population numbers tend to grow exponentially while food production grows linearly, never quite keeping pace with population and thus resulting in natural “checks” (such as famine) to further growth. Although the subject was periodically taken up again in the ensuring decades, with for example George Perkins Marsh’s classic Man and Nature (1864) (4) and concern over human-induced soil depletion in colonial Africa (5, 6), it was not until the 1960s that significant research interest was rekindled. In 1963, the U.S. National Academy of Sciences published The Growth of World Population (7), a report that reflected scientific concern about the consequences of global population growth, which was then reaching its peak annual rate of two percent. In 1968, Paul Ehrlich published The Population Bomb (8), which focused public attention on the issue of population growth, food production, and the environment. By 1972, the Club of Rome had released its World Model (9), which represented the first computer-based population-environment modeling effort, predicting an “overshoot” of global carrying capacity within 100 years.
The future size of world population is projected on the basis of assumed trends in fertility and mortality. Current world population stands at 6.7 billion people (12). The 2006 revision of the United Nations World Population Prospects presents a medium variant projection by 2050 of 9.2 billion people and still growing, although at a significantly reduced rate. All of the projected growth is expected to occur in the developing world (increasing from 5.4 to 7.9 billion), whereas the developed world is expected to remain unchanged at 1.2 billion. Africa, which has the fastest growing population of the continents, is projected to more than double the number of its inhabitants in the next 43 years—from 965 million to approximately 2 billion. Globally, fertility is assumed to decline to 2.02 births per woman (below replacement) by 2050; it is population momentum arising from a young age structure that will cause global population to continue to grow beyond 2050 (the 2006 revision does not make prognoses about ultimate stabilization). The medium variant is bracketed by a low-variant projection of 7.8 billion (and declining) and a high variant of 10.8 billion (and growing rapidly) by 2050. Fertility in the former is assumed to be half a child lower than the medium variant, and in the latter, it is assumed to be half a child higher.1 As Cohen (2) points out, minor variations in above- or below-replacement fertility can have dramatic long-term consequences for the ultimate global population size; hence, projections are highly conditional, and their sensitivity to the underlying assumptions needs to be properly understood. Finally, the impact of the HIV/AIDS epidemic on future mortality is assumed to attenuate somewhat on the basis of recent declines in prevalence in some countries, increasing antiretroviral drug therapy, and government commitments made under the Millennium Declaration (13).
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POPULATION-ENVIRONMENT THEORIES
