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Birth Rate
Human birth rates are declining, particularly in developed countries. Educated women tend to begin child bearing at a later age and have fewer children. However, although today's women have smaller families, there are so many young women that the population still expands; this phenomenon is called "population momentum." In poorer countries, births remain high: Children contribute labor and economic support, and families are less likely to have access to contraception.
Infant Mortality
In the animal kingdom, birth rates are high of necessity, because many die before reaching maturity. In human populations, however, improvements in sanitation and healthcare have reduced infant mortality. In Asia, Africa and South America, falling infant mortality combined with high birthrates is producing population booms. Because western population growth is slowing, the United Nations Population Fund (UNPF) predicts that while in 1850, 9 percent of humanity lived in Africa and 21 percent in Europe, by 2050 this pattern will have reversed: Africa will be host to 20 percent and Europe to 7 percent.
Life Expectancy and Disease
In human populations the main "predators" are disease, famine, war and natural disaster. According to UNPF, since 1950, the human death rate has halved; global life expectancy increased by 20 years and infant mortality dropped by two thirds. With these trends continuing, the human population -- below 1 billion in 1800 -- will reach almost 9 billion by 2050. The availability of clean drinking water has eradicated much epidemic disease. Vaccination is widespread; hygiene, surgery and medicine all have improved. Epidemics can still devastate local populations, particularly in poor, overcrowded conditions, or when infrastructure is disrupted after a natural disaster. Haiti experienced its first cholera outbreak in a century after the 2010 earthquake.
Food and Shelter
Populations are sustained only where there is sufficient food. With insufficient plant life, animal populations are limited; in lush regions, populations are large and varied. Improvements in food production and transportation have boosted human population growth. Unfortunately, food and the wealth to buy food are not equitably distributed. Third world population growth patterns are still limited by malnutrition. Life requires shelter, warmth and hospitable terrain. Physical geography both determines population locations and limits their growth. Expanding communities may run out of suitable land. Overcrowding puts pressure on food and water supplies and encourages disease. Geography may also be a source of natural disasters such as volcanoes, earthquakes, floods or tsunamis. These features both discourage settlement and cause sudden depopulation.
Migration and Immigration
Populations have always migrated in search of food. In humanity's search for opportunity, people migrate from countryside to city and from impoverished countries to wealthier ones. Migration has little effect on total population, but alters patterns of density and distribution. Humans increasingly gather in urban centers; rural areas are depopulated. Mass migration is also triggered by war, persecution and famine.