The size of human populations, like those of other organisms, is controlled by the environment. Throughout history, however, humans have expanded the carrying -pacity of the habitats in which they lived because of their ability to develop technic innovations. Earlier in our history, our populations were regulated by both densitydependent and density-independent effects, including food supply, disease, and predators there was also ample room on earth for migration to new areas to relieve overcrowding in specific regions. To a lesser extent, migration still plays a role in the adjustment of human populations to particular areas. Unusual disturbances, including floods, extreme temperatures, and droughts have also affected the pattern of human population growth.
Gradually, changes in technology have given humans more control over their food supply and enabled them to develop superior weapons to ward off predators, as we as the means to cure diseases. Improvements in transportation and housing have increased the efficiency of migration. At the same time, improvements in shelter an storage capabilities have made humans less vulnerable to climatic uncertainties.
As a result of our ability to manipulate these factors, the human population has grown explosively to its present level of more than 5.4 billion people. It is continuing to grow at the rate of approximately 1.8% per year, so that nearly 95 million people are added to the world population annually. At this rate, the human population will double in 40 years.