Carrying Capacity
Carrying capacity is a general concept based on the idea that every ecosystem has a limit for use that cannot be exceeded without damaging the system. Whatever the specified use of an area might be, whether for grazing, wildlife habitat, recreation, or economic development, there is a threshold that cannot be breached, except temporarily, without degrading the ability of the environment to support that use. Examinations of carrying capacity attempt to determine, with varying degrees of accuracy, where this threshold lies and what the consequences of exceeding it might be.
The concept of carrying capacity was pioneered early this century in studies of range management and wildlife management. Range surveys of what was then called "grazing capacity" were carried out on the Kaibab Plateau in Arizona as early as 1911, and this term was used in most of the early bulletins issued by the U.S. Department of Agriculture on the subject. In his 1923 classic, Range and Pasture Management, Sampson defined grazing capacity as "the number of stock of one or more classes which the area will support in good condition during the time that the forage is palatable and accessible, without decreasing the forage production in subsequent seasons." Sampson was quick to point out that the "grazing capacity equation has not been worked out on any range unit with mathematical precision." In fact, because of the number of variables involved, especially variables stemming from human actions, he did not believe that the "grazing-capacity factor will ever be worked out to a high degree of scientific accuracy." Sampson also pointed out that "grazing the pasture to its very maximum year after year can produce only one result—a sharp decline in its carrying capacity," and he criticized the stocking of lands at their maximum instead of their optimum capacity. Similar discussions of carrying capacity can be found in books about wildlife management from the same period, particularly Game Management by Aldo Leopold, published in 1933.
Practitioners of applied ecology have calculated the number of animal-unit months that any given land area can carry over any given period of time. But there have been some controversies over the application of the concept of carrying capacity. The concept is commonly employed without considering the factor of time, neglecting the fact that carrying capacity refers to land use that is sustainable. Another common mistake is to confuse or ignore the implicit distinctions between maximum, minimum, and optimum capacity. In discussions of land use and environmental impact, some officials have drawn graphs with curves showing maximum use of an area and claimed that these figures represent carrying capacity. Such representations are misleading because they assume a perfectly controlled population, one without fluctuation, which is not likely. In addition, the maximum allowable population can almost never be the carrying capacity of an area, because such a number can almost never be sustained under all possible conditions. A population in balance with the environment will usually fluctuate around a mean, higher or lower, depending on seasonal habitat conditions, including factors critical to the support of that particular species or community.
The concept of carrying capacity has important ramifications for human ecology and population growth. Many of the essential systems on which humans depend for sustenance are showing signs of stress, yet demands on these systems are constantly increasing. William R. Catton has formulated an important axiom for carrying capacity: "For any use of any environment there is a use intensity that cannot be exceeded without reducing that environment's suitability for that use." He then defined carrying capacity for humans on the basis of this axiom: "The maximum human population equipped with a given assortment of technologies and a given pattern of organization that a particular environment can support indefinitely."
The concept of carrying capacity is the foundation for recent interest in sustainable development, an environmental approach which identifies thresholds for economic growth and increases in human population. Sustainable development calculates the carrying capacity of the environment based on the size of the population, the standard of living desired, the overall quality of life, the quantity and type of artifacts created, and the demand on energy and other resources. With his calculations on sustainable development in Paraguay, Herman Daly has illustrated that it is possible to work out rough estimates of carrying capacity for some human populations in certain areas. He based his figures on the ecological differences between the country's two major regions, as well as on differences among types of settlers, and differences between developed good land and undeveloped marginal lands.
If ecological as well as economic and social factors are taken into consideration, then any given environment has an identifiable tolerance for human use and development, even if that number is not now known. For this reason, many environmentalists argue that carrying capacity should always be the basis for what has been called "demographic accounting."
Resources
Books
Edwards, R. Y., and C. D. Fowle. "The Concept of Carrying Capacity." In Readings in Wildlife Management, edited by J. A. Bailey, W. Elder, and T. D. McKinney. Washington, DC: The Wildlife Society, 1974.
Periodicals
Budd, W. W. "What Capacity the Land?" Journal of Soil and Water Conservation 47 (January-February 1992): 28-31.
Catton, W. R., Jr. "The World's Most Polymorphic Species: Carrying Capacity Transgressed Two Ways." BioScience 37 (June 1987): 413-419.
Graefe, A. R., J. V. Vaske, and F. R. Kuss. "Social Carrying Capacity: An Integration & Synthesis of Twenty Years of Research." Leisure Sciences 6 (December 1984): 395-431.
Nilsson, S. "The Carrying Capacity Concept." Interdisciplinary Science Reviews 9 (June 1984): 137-148.
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