A number of countries and communities do not have large enough a population to survive. In some cases this is due to birthrates being below 2.1 children per woman of childbearing age, the acceptable figure for sustained reproduction of a population. In other cases, a net out migration has caused the underpopulation.
In Russia in the 1990's, the birth rate declined to 1.3 births per woman while the life expectancy of men fell to 57 years, which last has been blamed partly on rising alcoholism.
Deaths per year in the early 1990's exceeded births by 800,000, so the population has fallen in absence of war, famine or epidemic disease.
Despite rapid population growth, Africa remains under-populated: its population density of 249 per 1,000 hectares is low compared to the world average of 442 or the 1,130 found in Asia (WRI, UNEP, UNDP and WB 1998).
Fertility in modern Western democracies has fallen below the replacement level. Unless there is a sharp increase in the size of its families, the West will lose out to other regions of the world, notably to the Third World.