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Carrying capacity

The supportable population of an organism, given the food, habitat, water and other necessities available within an environment is known as the environment's carrying capacity for that organism. For the human population, more complex variables such as sanitation and medical care are sometimes considered as part of the necessary infrastructure.

As population density increases, birth rate often increases and death rates typically decrease. The difference between the birth rate and the death rate is the "natural increase." The carrying capacity could support a positive natural increase, or could require a negative natural increase. Carrying capacity is thus the number of individuals an environment can support without significant negative impacts to the given organism and its environment. A factor that keeps population size at equilibrium is known as a regulating factor. The origins of the term lie in its use in the shipping industry to describe freight capacity, and a recent review finds the first use of the term in an 1845 report by the US Secretary of State to the Senate (Sayre, 2007).

Below carrying capacity, populations typically increase, while above, they typically decrease. Population size decreases above carrying capacity due to a range of factors depending on the species concerned, but can include insufficient space, food supply, or sunlight. The carrying capacity of an environment may vary for different species and may change over time due to a variety of factors, including: food availability, water supply, environmental conditions and living space.

Temporary exceptions

It is possible for a species to exceed its carrying capacity temporarily. Population variance occurs as part of the natural selection process but may occur more dramatically in some instances. Due to a variety of factors, one determinant of carrying capacity may lag behind another. A waste product of a species, for example, may build up to toxic levels more slowly than the food supply is exhausted. The result is a fluctuation in the population around the equilibrium point which is statistically significant. These fluctuations are increases or decreases in the population until either the population returns to the original equilibrium point, or a new equilibrium is established. These fluctuations may be more devastating for an ecosystem compared with gradual population corrections, since if it produces drastic decreases or increases, the overall effect on the ecosystem may be such that other species within the ecosystem are in turn affected, and begin to move with statistical significance around their own equilibrium points. The fear is of a domino-like effect, where the final consequences are unknown and may lead to collapses of species or even whole ecosystems.

Examples

The moose and wolf population of Isle Royale National Park

in Lake Superior is one of the world's best-studied predator-prey relationships. Without the wolves, the moose would overgraze the island's vegetation. Without the moose, the wolves would die. The first scientists who studied the issue thought that the wolves would eventually overpopulate and kill all the moose calves, then die from famine. This has not occurred, however, and in fact the wolves appear to be "limiting their own population size".

Easter Island has been cited as an example of a human population crash. When fewer than 100 humans first arrived, the island was covered with trees with a large variety of food types. In 1722, the island was visited by Jacob Roggeveen, who estimated a population of two to three thousand inhabitants with very few trees, "a rich soil, good climate" and "all the county was under cultivation". Half a century later, it was described as "a poor land" and "largely uncultivated". The ecological collapse which followed has been variously attributed to overpopulation, slave traders, European diseases (including a smallpox epidemic which killed so many so quickly, the dead were left unburied and a tuberculosis epidemic which wiped out a quarter of the population), civil war, cannibalism and invasive species (such as the Polynesian rats which may have wiped out the ground nesting birds and eaten the palm tree seeds). Whatever the combination of factors, only 111 inhabitants were left on the island in 1877. For whatever reasons (whether Moai worship, survival, status or sheer ignorance), the question of how many humans the island could realistically support never seems to have been answered. This example, and others, are discussed at length in Jared Diamond's Guns, Germs, and Steel.

The Chincoteague Pony Swim is a human-assisted example.''

Both herds are managed differently. The National Park Service owns and manages the Maryland herd while the Chincoteague Volunteer Fire Company owns and manages the Virginia herd. The Virginia herd, referred to as the "Chincoteague" ponies, is allowed to graze on Chincoteague National Wildlife Refuge, through a special use permit issued by the U.S. Fish and Wildlife Service. The size of both herds is restricted to approximately 150 adult animals each in order to protect the other natural resources of the wildlife refuge.

A further example is the Island of Tarawa, where the finite amount of space is evident, especially since landfills cannot be dug to dispose of solid waste, due to constraints in the subsurface rock and lack of topographic elevations. With colonial influence and an abundance of food (relative to life before the year 1850), the population has expanded to the extent that overpopulation is transparently present.

Fertility and carrying capacity interaction

If environmental food supply is abundant (for humans, for example), twinning may result . As a consequence, parents then typically also devote less care to each offspring in other ways, as the young may manage on their own with abundant food supply. Such parents have as many offspring as possible, by starting early and repeatedly breeding. When environmental conditions deteriorate as a result of expanding population, they may K-shift (i.e. revert to smaller numbers of offspring) via the more conservative strategy of betting on a few well-placed shots. When a species is already exploiting the environment near the limits of carrying capacity (which includes food availability but also nesting sites, etc.), a wise strategy is to produce a limited number of offspring, devoting considerable care to each.

Since this also applies to humans, two questions immediately arise: How is the "boom time" R-shift (reversion to larger numbers of offspring) implemented? Is sexual maturity sped up, or is it juvenile growth rate, or perhaps both? And what is the trigger, which aspects of the environment are "read" for the forecast? If one is ever to replace this corner-cutting "quantity is better than quality" philosophy, and effectively combat its fatalistic "life is cheap" corollary, we need to understand what drives it (the "hangover" following a reproductive "binge" is better known as a population crash).

Mathematics

For a specific case example in the wild, see the Lotka-Volterra equation, which shows how limited resources will cause the predator population to decline, due to famine. Note that depending on the situation, the impact of famine could be moderate (where the prey is not the main source of food for the predator), or extreme (where the prey becomes extinct due to over-predation, such as when humans hunted mammoth populations to extinction; if the prey is the only source of food, the predator will also become extinct unless it can find another food source).

Humans

In the words of one researcher: "Over the past three decades, many scholars have offered detailed critiques of carrying capacity--particularly its formal application--by pointing out that the term does not successfully capture the multi-layered processes of the human-environment link, and that it often has a blame-the-victim framework. These scholars most often cite the fluidity and non-equilibrium nature of this relationship, and the role of external forces in influencing environmental change, as key problems with the term.

In other words, the relationship of humans to their environment may be more complex than is the relationship of other species to theirs. Humans can alter the type and degree of their impact on their environment by, for instance, increasing the productivity of land through more intensive farming techniques, leaving a defined local area, or scaling back their consumption; of course, humans may also irreversibly decrease the productivity of the environment or increase consumption (see overconsumption).

Supporters of the concept argue that humans, like every species, have a finite carrying capacity. Animal population size, living standards, and resource depletion vary, but the concept of carrying capacity still applies. The World3 model of Donella Meadows deals with carrying capacity at its core.

Carrying capacity, at its most basic level, is about organisms and food supply, where "X" amount of humans need "Y" amount of food to survive. If the humans neither gain or lose weight in the long run, the calculation is fairly accurate. If the quantity of food is invariably equal to the "Y" amount, carrying capacity has been reached.

Humans, with the need to enhance their reproductive success (see Richard Dawkins' "The Selfish Gene"), understand that food supply can vary and also that other factors in the environment can alter humans' need for food. A house, for example, might mean that one does not need to eat as much to stay warm as one otherwise would.

Over time, monetary transactions have replaced barter and local production, and consequently modified local human carrying capacity. However, purchases also impact regions thousands of miles away. Carbon dioxide from an automobile, for example, travels to the upper atmosphere. This led Paul R. Ehrlich to develop the IPAT equation:

I = P * A * T

where:

I is the impact on the environment resulting from consumption

P is the population number

A is the consumption per capita (affluence)

T is the technology factor

(Ehrlich and Holdren 1971)

This is another way of stating the carrying capacity equation for humans which substitutes impact for resource depletion, adding the technology term to cover different living standards. As can be seen from the equation, money affects carrying capacity - but it is too general a term for accurate carrying capacity calculation.

The concept of the "ecological footprint" was developed to examine differential consumption by humans. By calculating the average consumption of humans over a small area, projections can be made for that type of population's impact on the environment.

Carrying capacity 'averages' the blame for these impacts by blaming the rich for using too many resources, as well as blaming the poor for being too numerous. Carrying capacity calculates the 'average' use of food and resources, which of course is closer to the billions of poor in the world than to the hundreds of billionaires.

This type of discussion raises the question of whether or not it is possible to define a measure of sustainability which does not already contain implicit assumptions about solutions to the problems of resource over-exploitation and environmental degradation.

Reduction of Earth's carrying capacity in the 21st century

After an expansion of agricultural capability on the Earth in the last quarter of the 20th century, there are many projections of a continuation of the decline in world agricultural capability (and hence carrying capacity) which began in the 1990s. Most conspicuously, China's food production is forecast to decline by 37 percent by the last half of the 21st century, placing a strain on the entire carrying capacity of the world, as China's population could expand to about 1.5 billion people by the year 2050. This reduction in China's agricultural capability (as in other world regions) is largely due to the world water crisis and especially due to mining groundwater beyond sustainable yield, which has been happening in China since the mid-20th century.

It should also be noted that because of modern agriculture's reliance on hydrocarbon fuel, any decline (whether artificial or depletion-based) in those supplies could potentially impact the world's human carrying capacity. Industrial agriculture relies on petroleum-driven equipment, natural gas-based fertilizer, petrochemical pesticide, and diesel-fueled trucking. Organic farming can reduce this factor, but the tractors and trucks currently run on these fuels.

Possible expansion of carrying capacity

Not all social scientists and demographers are convinced of an imminent carrying capacity crisis for humans. The Danish economist Ester Boserup has shown how technological developments in agriculture can increase carrying capacity, although not without limitations. Her work is summarized in the AAAS Population & Environment Atlas as follows:

A more sophisticated adaptation approach was put forward by Ester Boserup in her classic book The Conditions of Agricultural Growth. Boserup suggested that population growth was the principal force driving societies to find new agricultural technologies (Boserup, The Conditions of Agricultural Growth, Allen and Unwin, 1965, expanded and updated in Population and Technology, Blackwell, 1980.).

Unlike Julian Simon, Boserup did not claim that the process ran smoothly. She acknowledged that population pressure could cause serious resource shortages and environmental problems, and it was these problems that drove people to find solutions. Nor did she claim that things were always better after the adaptation.

They could often be worse. For example, when hunter-gatherers with growing populations depleted the stocks of game and wild foods across the Near East, they were forced to introduce agriculture. But agriculture brought much longer hours of work and a less rich diet than hunter-gatherers enjoyed. Further population growth among shifting slash-and-burn farmers led to shorter fallow periods, falling yields and soil erosion. Plowing and fertilizers were introduced to deal with these problems - but once again involved longer hours of work and degradation of soil resources(Boserup, The Conditions of Agricultural Growth, Allen and Unwin, 1965, expanded and updated in Population and Technology, Blackwell, 1980.).

If agricultural innovation could increase with population density, carrying capacity might also increase in some areas, averting a crisis there. This hypothesis might find support in the work of Mike Mortimore and Mary Tiffen (1994) ) in high-density East Africa, and in several other studies which they and others have conducted across the continent. However, Africa is still subject to desertification and other such effects which suggest that population may be outpacing agricultural development.

Nonetheless, there have been concerns that carrying capacity has been exceeded in 2008, due to soaring prices for commodities and food.

Carrying capacity in tourism

The process of defining Tourism Carrying Capacity (TCC) is composed of two parts. It follows (in principle) the conceptual framework for TCC as described by Shelby and Heberlein (1986), and these parts are described as follows:

Descriptive part (A): Describes how the system (tourist destination) under study works, including physical, ecological, social, political and economic aspects of tourist development. Within this context of particular importance is the identification of:

Constraints: limiting factors that cannot be easily managed. They are inflexible, in the sense that the application of organisational, planning, and management approaches, or the development of appropriate infrastructure does not alter the thresholds associated with such constraints.
Bottlenecks: limiting factors of the system which managers can manipulate (number of visitors at a particular place).
Impacts: elements of the system affected by the intensity and type of use. The type of impact determines the type of capacity (ecological-physical, social, etc). Emphasis should be placed on significant impacts.

Evaluative part (B): Describes how an area should be managed and the level of acceptable environmental impacts. This part of the process starts with the identification (if it does not already exist) of the desirable condition or preferable type of development. Within this context, goals and management objectives need to be defined, alternative fields of actions evaluated and a strategy for tourist development formulated. On the basis of this, Tourism Carrying Capacity can be defined. Within this context, of particular importance is the identification of:

Goals and/or objectives: (i.e. to define the type of experience or other outcomes which a recreational setting should provide).

Differing definitions

First of all, the carrying capacity can be the motivation to attract tourists visit the destination. The tourism industry, especially in national parks and protected areas, is subject to the concept of carrying capacity so as to determine the scale of tourist activities which can be sustained at specific times in different places. Over the years, several arguments have been developed about the definition of carrying capacity by various scholars as follows: Middleton and Hawkins defined carrying capacity as a measure of the tolerance of a site or building which is open to tourist activities, and the limit beyond which an area may suffer from the adverse impacts of tourism (Middleton & Hawkins, 1998). Chamberlain, on other hand, defined it as the level of human activity which an area can accommodate without either it deteriorating, the resident community being adversely affected or the quality of visitors' experience declining (Chamberlain, 1997), whereas Clark defined carrying capacity as a certain threshold (level) of tourism activity, beyond which there will be damage to the environment and its natural inhabitants (Clark, 1997).

The World Tourism Organisation argues that carrying capacity is the maximum number of people who may visit a tourist destination at the same time, without causing destruction of the physical, economic and socio-cultural environment and/or an unacceptable decrease in the quality of visitors' satisfaction (http://ec.europa.eu/environment/iczm/pdf/tcca_material.pdf. Date assessed 08/03/07). In the publication, ‘Agenda 21 for the Travel and Tourism Venture: towards environmentally sustainable development’, the Secretary-General of the World Tourism Organization.

Carrying capacity as part of a planning system

The definitions of carrying capacity need to be considered as processes within a planning process for tourism development which involves:

Setting capacity limits for sustaining tourism activities in an area. This involves a vision about local development & decisions about managing tourism.
Overall measuring of tourism carrying capacity does not have to lead to a single number, like the number of visitors (http://ec.europa.eu/environment/iczm/pdf/tcca_material.pdf. Date assessed 08/03/07).
In addition, carrying capacity may contain various limits in respect to the three components (physical-ecological, socio-demographic and political–economic).

“Carrying capacity is not just a scientific concept or formula of obtaining a number beyond which development should cease, but a process where the eventual limits must be considered as guidance. They should be carefully assessed and monitored, complemented with other standards, etc. Carrying capacity is not fixed. It develops with time and the growth of tourism and can be affected by management techniques and controls” (Saveriades, 2000).

The reason for considering carrying capacity as a process, rather than a means of protection of various areas is in spite of the fact that carrying capacity was once a guiding concept in recreation and tourism management literature. Because of its conceptual elusiveness, lack of management utility and inconsistent effectiveness in minimising visitors' impacts, carrying capacity has been largely re-conceptualized into management by objectives approaches, namely: the limits of acceptable change (LAC), and the visitor experience and resource protection (VERP) as the two planning and management decision-making processes based on the new understanding of carrying capacity (Lindberg and McCool, 1998). These two have been deemed more appropriate in the tourism planning processes of protected areas, especially in the United States, and have over the years been adapted and modified for use in sustainable tourism and ecotourism contexts (Wallace, 1993; McCool, 1994; Harroun and Boo, 1995).

Notes

References

Gausset Q., M. Whyte and T. Birch-Thomsen (eds.) 2005. Beyond territory and scarcity: Exploring conflicts over natural resource management. Uppsala: Nordic Africa Institute
Tiffen, M, Mortimore, M, Gichuki, F. 1994. More People, Less Erosion. Environmental Recovery in Kenya. London: Longman.
Sayre, N.F. 2008. The Genesis, History, and Limits of Carrying Capacity. Annals of the Association of American Geographers 98(1), pp. 120-134.
Karl S. Zimmerer, 1994. Human geography and the “new ecology”: the prospect and promise of integration. Annals of the Association of American Geographers 84, p. XXX.