By most standards, Homo sapiens is a successful species. There are currently 7 billion humans living on this planet. Even so, we and all other multicellular organisms contribute only a small fraction of all the cells on earth—most of which are bacteria. So if we see life ultimately as a competition among reproducing organisms, bacteria are the winners, hands down.
Nevertheless, no matter what criterion for success is used, there is no question that humans have had an inordinate impact on the earth and all other forms of life. In the past, our ancestors had to respond primarily to challenges posed by nature. Today the greatest challenges for our species (and all others) are the vastly altered environments of our own making. Through our actions, which have caused widespread devastation of ecosystems all over the world, from every continent, from the deep seas to the upper atmosphere, we have disconnected our species from its long evolutionary legacy. But at the same time we are still dependent on the ecosystems of which we remain a part. Can we survive as a species if we continue to challenge Mother Nature? Perhaps, but things will certainly be different, and undoubtedly the planet will be able to support far fewer humans ,Increasing population size is perhaps the single most important reason that our impact has been so great. As human population pressure increases, more and more land is converted to crops, pasture, construction, and human habitation, providing more opportunities for still more humans and fewer (or no) habitats for most other species.
Scientists estimate that around 10,000 years ago, only about 5 million people inhabited the earth (almost half as many live in Los Angeles County or New York City today). By the year 1650, there were perhaps 500 million, and by 1800, around 1 billion (Fig. 17-1). Today we add 1 billion people to the world’s population approximately every 13 years (Barnosky et al., 2012). That comes out to about 77 million every year and roughly 200,000 every day— or just about 9,000 an hour.
The rate of population growth is not equally distributed among all nations. The most recent United Nations report on world population notes that 95 percent of this growth is occurring in the developing world. Likewise, resources are not distributed equally among all nations. Only a small percentage of the world’s population, located in a few industrialized nations, controls and consumes most of the world’s resources. A 2009 study estimated that 48 percent of the world’s population survives on less than $2 per day (Population Reference Bureau, 2009).