As a population grows, it's put under pressure. This pressure can come from a lack of resources to feed, house and provide services; disease; war; or lack of enough space. The pressure can be relieved by migration. Wars, diseases and famine also reduce the pressure by killing off a portion of the population. In fact, the basis for Thomas Malthus' famous population theories is that human population will inevitably grow beyond the ability of the Earth to sustain it, resulting in self-correcting (and unpleasant) pressures.
Malthus' idea is sometimes known as "the Population Bomb" (or Malthusian Population Theory), and it gained popularity with the growth of the environmental movement in the 1970s. Fears of worldwide overpopulation are based on several factors:
- We won't be able to produce enough food to feed everyone.
- There isn't enough space for everyone to live.
- Humans cause harm to the environment. Too many humans will virtually destroy the ecosystem, further reducing our ability to produce food.
- We can't provide the societal infrastructure to care for all the people.
Our vulnerability to these factors is based on population density, the number of humans per unit of area. Since the Industrial Revolution, urbanization has caused a tremendous increase in population density in cities. The highest population density ever probably occurred in the Kowloon Walled City area of Hong Kong. At one point, about 50,000 people lived in a megablock that was about 150 meters by 200 meters in area [source: Tofu Magazine]. The virtually lawless district was evacuated and torn down to make way for a park. Today, the areas of greatest population density are obviously in major urban areas. India and China have large areas of intensely high population density [source: NASA Visible Earth].
As population density in a given area increases, it approaches what's known as the carrying capacity. This is the maximum number of humans an area is capable of supporting in terms of available resources. For animals, this is easy to calculate. A goat, for example, might need 50 square yards of grass to survive. An area of 200 square yards therefore has a carrying capacity of four goats. Calculating carrying capacity for humans is much more complex. We can use technology to improve our resource production. We can ship in resources from other areas. We can create sanitation systems and other infrastructure to support higher density.