The main reason for slow and fluctuating population growth prior to early 1800's was the prevalence of diseases such as small pox, diphtheria, measles and scarlet fever. In addition, epidemics of diseases such as typhoid fever, cholera and plague eliminated large number of adults. Famines also were not unusual. Biologically speaking, prior to 180s the population was essentially in a dynamic balance with natural enemies and other aspects of environmental resistance. High reproductive rates were largely balanced by high mortality.
Since the 19th centuary
� discoveries of vaccination provides protections to many of the infectious agents
� discovery of antibiotics is a major breakthrough in the medical history,
� improvements in agricultural techniques,
� improvements in the nutrition and
� better sanitation and personal hygiene
brought about spectacular reductions in mortality, especially among infants and children. The birth rate has remained high. So the human population entered into exponential growth, as they were freed from natural enemies and other environmental restraints.
Increasing numbers of people put increasing demands on the environment, both through demands for resources and through production of wastes. Most of the human population survived through subsistence agriculture to meet their needs. After the modern medicines and industrial revolution, the death rate plummeted and population growth increased. What are the impacts of rapid growth on a population that is largely engaged in subsistence agriculture? Five basic alternatives are being played out to various degrees,
� people can subdivide farms among the children or intensify cultivation of existing land to increase production per unit area.
� open up new land to farm.
� move to cities and seek employment.
� engage in illicit activities for income.
� emigrate to other countries legally.
The rapid population growth especially affects women and children. Increasing the average wealth of a population affects the environment both positively and negatively.