Paul Ehrlich & Anne H. Ehrlich: The Population Explosion. Touchstone Books, 1991.
A useful overview of the facts and issues and still relevant despite being written more than twenty years ago.
Overpopulation occurs when population density is more than the carrying capacity of an area's resouces. The consequences of human overpopulation are: traffic, refuse disposal, smog, epidemics, soil erosion, lower water table, overuse of water resources, crime rates, air pollution, loss of genetic diversity in lost species and the effect on renewable and non-renewable resources.
There is a deep feeling of need to out-reproduce other members of a species. Early human females probably bore several young in their lifetimes but at intervals of several years. Since available food was not easily digested by the very young, they were perhaps breastfed for around 3 years, thus delaying return of fertility of mother.
Demographic momentum is the tendency of a previously growing population to keep expanding after reproductive rates have been reduced. In 1989, around 40% of the population of less developed nations were under 15 years old and had yet to reproduce. The prime reproductive years are between 15 and 30 years old. When an 'average couple' has slightly more than two children, a population has reached replacement level.
Human migration features: rural to urban, developing to developed nations, refugees fleeing from wars and ecological disasters. About a third of the world's grain harvest is fed to animals to produce eggs, milk and meat for American-style diets. Artificial hybrids are often vulnerable to pest, need chemical controls and are often sterile, so farmers have to buy each year. Fertilisers only replace two or three basic minerals and do nothing for soil structure. Over fishing and ocean pollution change the marine environment.
Climatic changes, soil changes, sea water level changes. Developed countries rely on smooth functioning of transport to keep food available. Rainwater is distilled water; irrigation water contains salts which are left behind and may build up to high levels (the Aral Sea has shrunk in area by a third and the irrigated cropland around it has become a salt desert). It is difficult to use marginal lands. Tropical rainforest nutrients are largely stored in vegetation not soil. If vanilla is produced in a laboratory, what happens to the c.70,000 Madagscan vanilla farmers?
Epidemics are related to population size and density. Many bacterial and viral diseases depend on densely packed large communities, or run out of susceptible individuals to infect and so die out (e.g. measles cannot persist in populations of fewer than 300,000 people). Populations with no exposure to a specific infection are likely to have a huge proportion of people affected. Poor areas in large cities with bad sanitation and unclean water provide an ideal target. Faster transport can spread disease as people may be infectious before showing signs of infection.
The factors which help reduce fertility are: adequate nutrition, proper sanitation, basic health care, education of women and equal rights for women. The first four reduce infant mortality. Women apply schooling to improve family life, are more open to contraception and better able to use it; while men use schooling to earn a better income. When women have sources of status other than children, family sizes decline. The fear of being out-reproduced by other populations needs to be reduced. Outlawing abortion only increases illegal abortion figures. Islam has no moral objections to contraception, but there is a generally low status of women in traditional patriarchal societies.
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