However, human beings are reduced in numbers by following agencies:
1. Food shortage or famines due to crop failure as from unfavourable weather, especially in densely populated lands such as China and India where food reserves are scant or unequally distributed. Malnutrition may lead to physical impairment with greater susceptibility to disease and death.
2. Inadequate shelter, especially during cold of winter outside the tropics is a important mortality factor.
3. Calamities in nature, like floods, droughts, hurricanes, volcanic eruptions, earthquakes, etc., claim great toll of human population (Table 24.2).
4. Enemies: Large predatory animals like lion, tiger and poisonous snakes, cause many deaths in less civilised regions.
5. Wars: Warfare is a form of competition between human races or nations for land, natural resources, trade routes, or other desired features of human environment. Wars have remained a major mortality factor throughout human history, not only by causing direct battle casualities, but also by stimulating disease and famine.
6. Disease is the largest single factor in reducing populations. Diseases like bubonic plague, cholera, typhus, small pox, malaria, yellow fever, sleeping sickness, etc., have been among the great killers of man throughout history.
7. Explosions, fire, pollution, shipwrecks and transportation accidents cause much death in modern civilizations.
Table 24.2. Human deaths due to noteworthy tropical cyclones, (from Dube and Singh, 1981):
By performing experiments on the populations of Drosophila, deer, house mice (musculus), etc., biologists have predicted that increased size of human population will cause a lowered birth rate. This lowered birth rate might result from a lowered fertility occurring as a part of the stress syndrome, or it may result from social and economic changes.
One major social factor still to be evaluated is the shift in thought concerning birth control that is now occurring in many religious groups. Others predict the human population size will be limited by intra specific strife such as wars, revolutions and murders since the stress syndrome (adrenal gland deterioration, high blood pressure, hardening of the arteries, liver disease, and other breakdowns) is known in man stress will cause increase in the mortality rate that will control population size.
As biologists, we can only hope that modern man will solve this new problem by again applying his unique power of thought and reasoning and develop techniques of birth control instead of increasing death rates?