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Carrying Capacity

CARRYING CAPACITY

In ecological theory, the carrying capacity (K) of a geographical region, with respect to a particular species, is the maximum population size that the region can support. It is assumed that the birth and death rates are density-dependent, the former declining and the latter increasing as the population size (N) increases and food per individual decreases. Then the population will reach a stable maximum, the two rates intersect, and N = K. When a species is introduced into a region, it will experience a high, unconstrained growth rate. As N approaches K, the growth rate will fall. Thus N follows an S-shaped curve that rises steeply at first and then reaches a plateau with N = K. If the trends in the birth and death rates are linear, then this curve is the logistic function, first described by Verhulst in 1845. The model assumes a closed population (no immigration or emigration), no importation of food, and no improvement in the efficiency of food production. These assumptions are restrictive in the context of nonmigratory human populations, but anthropologists have estimated the carrying capacity for isolated hunter-gatherer tribes. Even where the assumptions hold, it has been observed that many animal species, including homosapiens, restrict their fertility to maintain the population density below the level at which mortality rises.

For the world population as a whole, migration is not an issue. Over the years, several authors have estimated the global carrying capacity based on maximum food production. In 1983, Bernard Gilland calculated a global carrying capacity of 7.5 billion. However, improvements in food production have permitted the world population to reach 6 billion in the year 2000 without any evidence of increase in mortality.

Gerry B. Hill

(see also: Population Density; Population Forecasts; Population Growth; Sustainable Health )

Bibliography

Gilland, B. (1983). "Considerations on World Population and Food Supply." Population and Development Review 9:203211.

Gotelli, N. J. (1995). A Primer of Ecology. Sunderland, MA: Sinaner Associates, Inc.

Weiner, J. S. (1975). "Tropical Ecology and Population Structure." In The Structure of Human Populations, eds. G. A. Harrison and A. J. Boyce. Oxford: Clarendon Press.

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carrying capacity

carrying capacity The maximum population of a given organism that a particular environment can sustain; the K (saturation) value for species populations showing S-shaped population growth curves. It implies a continuing yield without environmental damage. It may be modified by human intervention to improve environmental potential (e.g. by applying fertilizers to range-land and reseeding it with nutritious grasses).

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carrying capacity

carrying capacity The maximum population of a given organism that a particular environment can sustain; the K (saturation) value for species populations showing S-shaped population growth curves. It implies a continuing yield without environmental damage. It may be modified by human intervention to improve environmental potential, e.g. by applying fertilizers to range-land and reseeding it with nutritious grasses.

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carrying capacity

carrying capacity Symbol K. The maximum population of a particular species that can be supported indefinitely by a given habitat or area without damage to the environment. It can be manipulated by human intervention. For example, the carrying capacity for grazing mammals could be increased by boosting the yield of their grassland habitat by the application of fertilizer. See also K selection.

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carrying capacity

carrying capacity The maximum population of a given organism that a particular environment can sustain; the K (saturation) value for species populations showing S-shaped population-growth curves. It implies a continuing yield without environmental damage.

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Carrying Capacity

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 resulta 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."

[Gerald L. Young and Douglas Smith ]


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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Carrying capacity

Carrying capacity

Carrying capacity for humans

Resources

Carrying capacity refers to the maximum abundance of a species that can be sustained within a given habitat. When an ideal population is in equilibrium with the carrying capacity of its environment, the birth and death rates are equal, and size of the population does not change. Populations larger than the carrying capacity are not sustainable, and will degrade their habitat. In nature, however, neither carrying capacity or populations are ideal; both vary over time for reasons that may be complex and in ways that may be difficult to predict. Nevertheless, the notion of carrying capacity is very useful because it highlights the ecological fact that, for all species, there are environmental limitations to the sizes of populations that can be sustained.

Carrying capacity is never static. It varies over time in response to gradual environmental changes, perhaps associated with climatic change or the successional development of ecosystems. More rapid changes in carrying capacity may be caused by disturbances of the habitat occurring because of a fire or windstorm, or because of a human influence such as timber harvesting, pollution, or the introduction of a non-native competitor, predator, or disease. Carrying capacity can also be damaged by overpopulation, which leads to excessive exploitation of resources and a degradation of the habitats ability to support the species. Of course, birth and death rates of a species must respond to changes in carrying capacity along with changes in other factors, such as the intensities of disease or predation.

Carrying capacity for humans

Humans, like all organisms, can only sustain themselves and their populations by having access to the products and services of their environment, including those of other species and ecosystems. However, humans are clever at developing and using technologies; as a result they have an unparalleled ability to manipulate the carrying capacity of the environment in support of their own activities. When prehistoric humans first discovered that crude tools and weapons allowed greater effectiveness in gathering wild foods and hunting animals, they effectively increased the carrying capacity of the environment for their species. The subsequent development and improvement of agricultural systems has had a similar effect, as have discoveries in medicine and industrial technology.

Clearly, the cultural evolution of human socio-technological systems has allowed enormous increases to be achieved in carrying capacity for our species. This increased effectiveness of environmental exploitation has allowed a tremendous multiplying of the human population to occur. In prehistoric times (that is, more than 10,000 years ago) all humans were engaged in a primitive hunting and gathering lifestyle, and their global population probably amounted to several million individuals. In the year 2000, because humans have been so adept at increasing the carrying capacity of their environment, more than six billion individuals were sustained, and the global population is still increasing.

Humans have also increased the carrying capacity of the environment for a few other species, including those with which we live in a mutually beneficial symbiosis. Those companion species include more than about 20 billion domestic animals such as cows, horses, pigs, sheep, goats, dogs, cats, and chickens, as well as certain plants such as wheat, rice, barley, maize, tomato, and cabbage. Clearly, humans and their selected companions have benefited greatly through active management of Earths carrying capacity.

However, an enormously greater number of Earths species have not fared as well, having been displaced or made extinct as a consequence of ecological changes associated with the use and management of the environment by humans, especially through loss of their habitat and over harvesting. In general, any increase in the carrying capacity of the environment for one species will negatively affect other species.

In addition, there are increasingly powerful indications that the intensity of environmental exploitation required to sustain the large populations of humans and our symbionts is causing important degradations of carrying capacitythat the apparent technology-based increases in carrying capacity that humans have achieved may be temporary. Symptoms of this environmental deterioration include the extinction crisis, loss of soil and decreased soil fertility, desertification, deforestation, fishery declines, pollution, and increased competition among nations for scarce resources. Many reputable scientists believe that the sustainable limits of Earths carrying capacity for the human enterprise may already have been exceeded. This is a worrisome circumstance, especially because it is predicted that there will be additional large increases in the global population of humans. The degradation of Earths carrying capacity for humans is associated with two integrated factors: (1) overpopulation and (2) the intensity of resource use and pollution. In recent decades human populations have been growing most quickly in poorer countries, but the most intense lifestyles occur in the richest countries.

If it is true that the human enterprise has exceeded Earths sustainable carrying capacity for our species, then compensatory adjustments will either have to be made by the human economy or they will occur naturally, via mass famine and die-off. Those managed or catastrophic changes will involve a combination of decreased per-capita use of environmental resources, decreased birth rates, and possibly, increased death rates. However, technological optimists argue that the Earths human carrying capacity is unlimited, based on the notion that technological advances always increase effectively available resources faster than industrial activity depletes them.

See also Sustainable development.

Resources

BOOKS

Begon, Michael, et al. Ecology: From Individuals to Ecosystems. Malden, MA: Blackwell Publishing, 2005.

Smith, Robert Leo and Thomas M. Smith. Ecology (6th ed.). San Francisco: Benjamin Cummings, 2005.

Bill Freedman

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Carrying Capacity

CARRYING CAPACITY


Carrying capacity is the maximum population size that a species can maintain indefinitely in a given area–that is, without diminishing the capacity of the area to sustain the same population size in the future. Carrying capacity is thus a function of both the resource requirements of the organism and the size and resource richness of the area. The carrying capacity of an area with constant size and richness would be expected to change only as fast as organisms evolve different resource requirements.

Measuring Carrying Capacity

The concept is simple, but it is notoriously difficult to measure. The identity and dynamics of resources critical to a species, and the complex of other factors that regulate its population size, are typically poorly known. Moreover, as usage of the term has spread beyond its original context–to do with sustainable stocking levels of domestic livestock on rangeland–into disciplines such as ecology, carrying capacity has taken on subtle but substantive differences in meaning. For instance, in theoretical population ecology, carrying capacity is defined by the parameter K (the equilibrium density of a species) within the logistic equation of population growth. These variations in meaning make it difficult to apply the term consistently across disciplines. Despite these limitations, most authors consider at least the broad concept (as defined above) to be an important heuristic tool.

Carrying Capacity and Human Beings

Applied to human beings, the carrying capacity concept is further complicated by the unique role that culture, broadly interpreted, plays in our species. Three culturally linked factors stand out as critical: individual differences in types and quantities of resources consumed; rapid evolution in patterns of resource consumption; and technological and other cultural change. To take the case of energy consumption, which can be considered a proxy for over-all resource use, in 1990 an average person in a developed nation used about 7.1 kilowatts of energy per year, while the average person in the developing world used just 0.9 kilowatts per year. (These averages of course mask large variation at the individual level.) Moreover, economic, social, and technological development bring vast changes in patterns of energy consumption: Global energy consumption has increased more than 20 fold since 1850, and there have been dramatic changes in the composition of energy sources and technologies.

Carrying capacity for human beings is thus highly variable across space and time, depending on levels and styles of living and their supporting technologies and social systems. Ten thousand years ago, at the dawn of agriculture, the world's human population was somewhere between 2 and 20 million, perhaps an indication of global carrying capacity under those conditions. The cultural (including technological) advances associated with the development of agriculture allowed human populations to expand far beyond the levels possible under the resource demands of a hunter-gatherer lifestyle. A few populations have retained that preagrarian pattern of resource use.

Biophysical and Social Carrying Capacity

Biologists distinguish between biophysical carrying capacity–the maximum population size that could be sustained biophysically under given technological capabilities–and social carrying capacity–the maximum that could be sustained under a specified social system and its associated pattern of resource consumption.

At any level of technological development, social carrying capacities are necessarily lower than biophysical carrying capacity, because the latter implies a factory-farm lifestyle that would be both universally undesirable and also unobtainable because of inefficiencies inherent in social systems. For example, what is considered to be food by a society is largely culturally determined. A population that eats large quantities of grain-fed meat requires four to five times more grain than a population sustained by a solely vegetarian diet. A vegetarian diet is more efficient from a caloric point of view than a meat-oriented one, but is not widely acceptable in many societies. Further inefficiencies, from a biophysical standpoint, result from unequal resource distribution–at local, national, and international scales. The higher the level of inequality, the smaller the population that can be sustained. Many other aspects of culture play significant roles in determining carrying capacity, ranging from patterns of investment in education and social development to frequency and severity of warfare.

Sustainability

A sustainable condition, process, or activity is one that can be maintained without interruption, weakening, or loss of valued qualities. Sustainability is thus a necessary and sufficient condition for a population to be at or below carrying capacity. The wide appeal of sustainability as a societal condition or goal reflects the moral conviction that the current generation should pass on its inheritance of natural wealth–not unchanged but undiminished in potential–to support future generations.

Are the collective activities of today's human population sustainable? The answer is clearly no. Many essential activities, notably food and energy production, are maintained only through the exhaustion and dispersion of a one-time inheritance of natural capital. Maintenance of the world's present human population, and accommodation of its anticipated growth, requires safeguarding the critical resources and services that are provided by this natural capital. A partial list of these includes: generation and renewal of fertile agricultural soil; provision of fresh water, energy, construction materials, and minerals; purification of air and water; mitigation of flood and drought; waste treatment and nutrient cycling; seed dispersal; generation and maintenance of biodiversity; protection from ultraviolet radiation; stabilization and moderation of climate; and crop pollination. In many parts of the world, the natural capital stocks providing this stream of goods and services are being severely degraded and depleted.

The Ecological Footprint

Ecological footprint analysis is a heuristic tool that turns the carrying capacity issue around, asking what productive land area would be required to sustain a given population's activities. It is calculated that the productive land of about two and a half more planet Earths would be required to support a global population of 6 billion people at a consumption level comparable to that of the present-day inhabitants of Vancouver, Canada, the home base of the originator of the concept.

See also: Ecological Perspectives on Population; Land Use; Limits to Growth; Sustainable Development; Water and Population.

bibliography

Daily, Gretchen C., and Paul R. Ehrlich. 1992. "Population, Sustainability, and Carrying Capacity." Bio Science 42: 761–771.

Daily, Gretchen C., Tore Söderqvist, Sara Aniyar, Kenneth Arrow, Partha Dasgupta, Paul R. Ehrlich, Carl Folke, Ann Marie Jansson, Bengt-Owe Jansson, Nils Kautsky, Simon Levin, Jane Lubchenco, Karl-Göran Mäler, David Simpson, David Starrett, David Tilman, and Brian Walker. 2000. "The Value of Nature and the Nature of Value." Science 289: 395–396.

Dhondt, André A. 1988. "Carrying Capacity: A Confusing Concept." Acta ·cologica 9: 337–346.

Hardin, Garrett. 1986. "Cultural Carrying Capacity: A Biological Approach to Human Problems." Bio Science 36: 599–606.

Holdren, John P. 1991. "Population and the Energy Problem." Population and Environment 12: 231–255.

Rees, William, and Mathis Wackernagel. 1994. "Ecological Footprints and Appropriated Carrying Capacity: Measuring the Natural Capital Requirements of the Human Economy." In Investing in Natural Capital, eds. Ann Marie Jansson, Monica Hammer, Carl Folke, and Robert Co-stanza. Washington, D.C.: Island Press.

Jai Ranganathan

Gretchen C. Daily

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Carrying Capacity

Carrying capacity

Carrying capacity refers to the maximum abundance of a species that can be sustained within a given area of habitat . When an ideal population is at equilibrium with the carrying capacity of its environment, the birth and death rates are equal, and size of the population does not change. Populations larger than the carrying capacity are not sustainable, and will degrade their habitat. In nature, however, neither carrying capacity or populations are ideal—both vary over time for reasons that may be complex, and in ways that may be difficult to predict. Nevertheless, the notion of carrying capacity is very useful because it highlights the ecological fact that, for all species, there are environmental limitations to the sizes of populations that can be sustained.

Carrying capacity is never static. It varies over time in response to gradual environmental changes, perhaps associated with climatic change or the successional development of ecosystems. More rapid changes in carrying capacity may be caused by disturbances of the habitat occurring because of a fire or windstorm, or because of a human influence such as timber harvesting, pollution , or the introduction of a non-native competitor, predator , or disease . Carrying capacity can also be damaged by overpopulation, which leads to excessive exploitation of resources and a degradation of the habitat's ability to support the species. Of course, birth and death rates of a species must respond to changes in carrying capacity along with changes in other factors, such as the intensities of disease or predation.


Carrying capacity for humans

Humans, like all organisms, can only sustain themselves and their populations by having access to the products and services of their environment, including those of other species and ecosystems. However, humans are clever at developing and using technologies; as a result they have an unparalleled ability to manipulate the carrying capacity of the environment in support of their own activities. When prehistoric humans first discovered that crude tools and weapons allowed greater effectiveness in gathering wild foods and hunting animals, they effectively increased the carrying capacity of the environment for their species. The subsequent development and improvement of agricultural systems has had a similar effect, as have discoveries in medicine and industrial technology.

Clearly, the cultural evolution of human socio-technological systems has allowed enormous increases to be achieved in carrying capacity for our species. This increased effectiveness of environmental exploitation has allowed a tremendous multiplying of the human population to occur. In prehistoric times (that is, more than 10,000 years ago) all humans were engaged in a primitive hunting and gathering lifestyle, and their global population probably amounted to several million individuals. In the year 2000, because humans have been so adept at increasing the carrying capacity of their environment, more than six billion individuals were sustained, and the global population is still increasing.

Humans have also increased the carrying capacity of the environment for a few other species, including those with which we live in a mutually beneficial symbiosis . Those companion species include more than about 20 billion domestic animals such as cows, horses , pigs , sheep , goats , dogs, cats , and chickens, as well as certain plants such as wheat , rice , barley , maize, tomato, and cabbage. Clearly, humans and their selected companions have benefited greatly through active management of Earth's carrying capacity.

However, an enormously greater number of Earth's species have not fared as well, having been displaced or made extinct as a consequence of ecological changes associated with the use and management of the environment by humans, especially through loss of their habitat and over harvesting. In general, any increase in the carrying capacity of the environment for one species will negatively affect other species.

In addition, there are increasingly powerful indications that the intensity of environmental exploitation required to sustain the large populations of humans and our symbionts is causing important degradations of carrying capacity. Symptoms of this environmental deterioration include the extinction crisis, decreased soil fertility, desertification , deforestation , fishery declines, pollution, and increased competition among nations for scarce resources. Many reputable scientists believe that the sustainable limits of Earth's carrying capacity for the human enterprise may already have been exceeded. This is a worrisome circumstance, especially because it is predicted that there will be additional large increases in the global population of humans. The degradation of Earth's carrying capacity for humans is associated with two integrated factors: (1) overpopulation and (2) the intensity of resource use and pollution. In recent decades human populations have been growing most quickly in poorer countries, but the most intense lifestyles occur in the richest countries.

If it is true that the human enterprise has exceeded Earth's carrying capacity for our species, then compensatory adjustments will either have to be made by the human economy, or they will occur naturally. Those managed or catastrophic changes will involve a combination of decreased per-capita use of environmental resources, decreased birth rates, and possibly, increased death rates.

See also Sustainable development.


Resources

books

Begon, M., J.L. Harper, and C.R. Townsend. Ecology: Individuals, Populations and Communities. 2nd ed. London: Blackwell Sci. Pub., 1990.

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.

Ricklefs, R. E. Ecology. New York: W.H. Freeman and Co., 1990.


Bill Freedman

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