Overpopulation occurs when there is an excess of any species, overburdening its environment. For wild animals, overpopulation is frequently adjusted by an increase in the number of natural predators, or intervention by increased hunting by humans. Overpopulation can also occur with the introduction of a foreign species. A classic example of this was the introduction of rabbits to Australia, which then bred out of control and destroyed crops.
There are many who worry about overpopulation of human beings on Earth. Since the end of the last great plague, the Black Death of the 14th century, human population has grown steadily. The most significant advances in human population have occurred in the last half-century, due in part to advancements in medical science. Increase food production as a result of industrialized agriculture has also permitted population booms.
In 2013, the human population was 7 billion. For comparison, there were only one billion people in 1804. This number doubled by 1927, and doubled again by 1974. Some estimates are that population will reach 12 billion by the year 2100. Scientists believe that the Earth can realistically only support between 4 and 16 billion, indicating perhaps a strain on available resources. Certain environmental problems, such as carbon dioxide levels and other forms of pollution are exacerbated by overpopulation.