Human Population

A population of organisms changes in response to the balance of the rates at which new individuals are added by births and immigration and the rate at which they are lost by deaths and emigration. Zero population growth occurs when the growth and loss parameters are balanced. These demographic relationships hold for all species, including ours.

The history of Homo sapiens extends to somewhat more than one million years. For almost all of that time relatively small populations of human beings were engaged in subsistence lifestyles that involved hunting wild animals and gathering wild edible plants. The global population of human beings during those times may have been as large as a million or so individuals. However, colonization of new parts of the world (e.g., Asia, Europe, Australia, Polynesia, the Americas) and occasional discoveries of new tools and weapons allowed prehistoric human beings to grow in numbers and become more effective at gathering food. This in turn allowed the population to increase.

About 10,000 years ago, the first significant developments of primitive agriculture began to occur. These included the domestication of a few plant and animal species to achieve greater yields of food for human beings. The development of these early agricultural technologies and their associated sociocultural systems allowed enormous increases in environmental carrying capacity for human beings and their domesticated species, so that steady population growth could occur. Even primitive agricultural systems could support many more people than could a subsistence lifestyle based on the hunting and gathering of wild animals and plants.

Further enhancements of Earth's carrying capacity for the human enterprise were achieved through other technological discoveries. For example, the discovery of metals and their alloy—first copper and bronze, later iron and steel—allowed the development of superior tools and weapons. Similarly, the invention of the wheel and of ships made possible the easy transportation of large quantities of valuable commodities from regions of surplus to those of deficit. At the same time, increased yields in agriculture were achieved through a series of advances in breeding of domesticated plants and animals and in farming techniques. The evolution of human sociocultural systems has thus involved a long series of discoveries and innovations that increased the effective carrying capacity of the environment, permitting growth of the population. As a result of this process, there were about 300 million people alive in A.D. 0, and about 500 million in 1650, at which time the rate of population growth increased significantly. This trend has been maintained to the present. The recent explosive growth of the human population has several causes. Especially important has been the discovery of more effective medical and sanitary technologies, which have greatly decreased death rates (especially infant and child death rates) in most human populations. There have also been enormous advances in the technologies that allow effective extraction of resources, manufacturing, agriculture, transportation, and communications, all of which have allowed further increases in the carrying capacity of the environment.

As a result of these relatively recent developments, the global population of human beings increased from about 500 million in 1650 to over one billion in 1850, two billion in 1930, four billion in 1975, and five billion in 1987. In 2002, the human population was approximately 6.215 billion individuals.

More locally, there have been even greater increases in the rate of growth of some human populations. In recent decades some countries have achieved population growth rates of 4% per year, which if maintained would double the population in only 18 years. One third of all the world's births occur in India and China, the two most populous countries in the world (about 1.049 billion and 1.28 billion persons, respectively).

These sorts of population growth rates place enormous pressure on ecosystems. For example, the human population of central Sudan was 2.9 million in 1917, but it was 18.4 million in 1977, an increase of 6.4 times. During that same period the population of domestic cattle increased by a factor of 20 (to 16 million), camels by 16 times (to 3.7 million), sheep by 12.5 times (to 16 million), and goats by 8.5 times (to 10.4 million). Substantial degradation of the carrying capacity of dry lands in this region of Africa has been caused by these increases in the populations of human beings and their large-mammal symbionts, and there have been other ecological damages as well (e.g., destruction of trees and shrubs for cooking fuel).

Another example of the phenomenon of rapid population growth is the number of people in the province of Rondonia in Amazonian Brazil. This population increased twelvefold between 1970 and 1988, mostly through immigration, while the population of cattle increased by 30 times. These population increases were accompanied by intense ecological damage, as the natural rainforests were "developed" to sustain human beings and their activities. (The areas in question are not, for the most part, "developed" in the sense of being transferred from their wild state to a sustainable agricultural state, but in the sense of being stripped and degraded for short-term profit.)