Environmental Carrying Capacity

The environmental carrying capacity of Earth is a subject of global debate. This is the concept [Page 493]that there is a maximum or an optimal burden of organisms that the environment can support indefinitely, rather like a ship's payload or the capacity of transmission lines to carry electricity. Exceeding this limit leads to some form of collapse. The concept has been applied in environmental and resource management at various scales and contexts, for example, the capacity of rangelands to support grazing livestock or of marine ecosystems to support fisheries catches. It is perhaps most closely associated with human-environment interactions, where the key question is Earth's capacity to support a growing human population. It is particularly in this global context that it has generated a great deal of critical debate.

The idea that the population of a species will always press against the limits of available resources (food, water, habitat, etc.) is intuitively obvious, and indeed it is almost a defining characteristic of life. There is also plenty of evidence—from bacterial colonies right through to human societies—that, when resources are overconsumed, species populations can and do collapse. However, extending the concept from this very general description to a quantitative assessment that can inform policy and practice relating to human systems is one of the challenges that provokes debate.

The population behavior of any single species involves a complex web of interactions with its physical environment and all other organisms present. Information about the historic populations in an area of habitat can give an indication of the numbers it can support, but unlike the volume of a ship's hold or the conductivity of a power cable, this measure of carrying capacity is not fixed and static. When any aspect of the environment changes, the carrying capacity may well change too. Ecosystem modeling efforts can often explain observed changes in terms of the most significant interactions, but they have limited skill in predicting the changes in complex systems, and some system changes are not predictable. Experience in fisheries modeling is a stark example.

Many people associate ideas of carrying capacity with T. R. Malthus (1766–1834), a political economist, whose essay “Principles of Population” was first published in 1798. Malthus did not propose a number for an optimal or maximum population; he just pointed out that human population cannot grow indefinitely. When numbers of people grow too great, outstripping access to resources, they will exceed the “power in the Earth to provide subsistence for man” and famine, disease, or conflict (“sickly seasons, epidemics, pestilence and plague”) will ensue.

Human population has grown by about tenfold in the past 300 years; estimated world population was about 600 million in 1700 and in 2011 stands at over 6.5 billion. Although human resourcefulness and technological innovation have averted Malthusian disasters repeatedly in the past, this rapid expansion in human numbers is once again focusing many people's attention on the availability of fundamental life support for human society: food, water, material resources for shelter, and even usable land area.

Unfortunately, human resourcefulness and technological innovation are also part of the current problem. The impact of people on Earth is not a simple function of population numbers. As people have become more affluent they have tended to consume much more per person. Technological developments enable them to extract more resources, notably the fossil fuels that have not just enabled travel and transport of goods around the globe but also supported agricultural production that, in turn, has sustained a higher population. The supply of energy to fuel this consumption and transport is looming as a new constraint on human activity.

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