Carrying capacity

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Carrying capacity usually refers to the biological carrying capacity of a population level that can be supported for an organism, given the quantity of food, habitat, water and other life infrastructure present. For the human population other variables such as sanitation and medical care are sometimes considered as infrastructure. As population density increases, birth rates often decrease and death rates typically increase. Carrying capacity is the point at which these two rates are equal. 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). It was never used, as is widely assumed, by Thomas Malthus.

What is best to remember about carrying capacity is that ultimately it is not a number but a relationship best described as a differential equation. With food availability, population size, and environmental factors, all varying inconsistently over time. While by this definition it is easy to see the need for computer modeling, early attempts at wildlife management often neglected this fact.

Below carrying capacity, populations often increase, while above, they can 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 can vary for different species, and can change over time due to a variety of factors including: food availability; water supply; environmental conditions; and space.

It is possible for a species to exceed its carrying capacity temporarily, until mass fatalities occur as shortages in food and water take effect. This outcome is more devastating for a population compared to gradual population corrections within the carrying capacity, since it produces mass killings as well as stress for the entire species; moreover, the population can then fall far below the carrying capacity in overcorrection.

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[edit] Examples

The moose and wolf population of Isle Royale National Park [1] in Lake Superior is one of the world's best studied predator-prey relationships. Without the wolves, the moose would overgraze the island's plants. Without the moose, the wolves would die. It seemed to the first scientists that studied the problem that the wolves would eventually overpopulate, kill all the moose calves and then die from famine. However, this has not occurred, and, in fact, the wolves appear to be "limiting their own population size".

Easter Island seems to be a very good example of humans exceeding their carrying capacity. When fewer than 100 humans first arrived, the island was covered with trees with a large variety of food types. Contrast this paradise with the first sighting of Jacob Roggeveen, who reported two to three thousand inhabitants with very few trees. The ecological collapse that followed has be attributed to overpopulation, introduction of european disease, cannibalism, and invasive species (such as the rats that ate the palm tree seeds). Whatever the reason, or combination of reasons, only 110 inhabitants were left on the island in 1877. For whatever reasons, Moai worship, survival, status, or pure ignorance, the question of how many humans the island could comfortably support never seems to have come up. Their known history includes a population crash that might have been avoided had they asked that simple question.

The Chincoteague Pony Swim [2] 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, [1] where the finite amount of space is evident, especially since landfills cannot be dug to dispose of solid waste. 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[2].

[edit] Fertility and carrying capacity interaction

If food supply of the environment is abundant, in humans for example, twinning may result [3]. As a result, parents then typically devote less care to each offspring in other ways as well, as the young may manage on their own with abundant food supply. Such parents have as many offspring as possible by starting early and quickly repeating breeding. When environmental conditions deteriorate with an expanding population, they may K-shift (resort to small numbers of offspring) toward 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 play it safe by raising a limited number of offspring, devoting considerable care to each.

Since this also applies to humans, then two questions immediately arise: How is the "boom time" r-shift (resort to large numbers of offspring) implemented? (Is sexual maturity sped up, or is juvenile growth rate, or perhaps both?) And is the trigger, what 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" that follows a reproductive "binge" is better known as a population crash).

[edit] Humans

In the words of one researcher attempting to interpret the carrying capacity concept: "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 multilayered processes of the human-environment link, and that it often has a blame-the-victim framework. These scholars most often cite the fluidity and nonequilibrium nature of this relationship, and the role of external forces in influencing environmental change, as key problems with the term." (Cliggett 2001)

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 consciously change the type and degree of their impact on their environment by, for example, 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 (e.g. 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 on its most basic level is about organisms and food supply: 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 constant at 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 one does not need to eat as much to stay warm.

Over time monetary transactions have replaced barter and local production. However, purchases impact regions thousands of miles away. Carbon dioxide from an automobile, for example, travels to the upper atomsphere. This lead Paul Ehrlich to develop the IPAT Equation where:

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 that substitutes impact for resource depletion and adds 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 calcuation.

The concept of 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. It blames the rich for using too many resources, as well as the poor for being too numerous. Carrying capacity calculates the 'average' use of food and resources, which is closer to the billions of poor in the world, than the hundreds of billionaires.

This type of discussion raises the question of whether it is possible to define a measure of sustainability that does not already contain implicit assumptions about the solution to the problem of resource over-use and environmental degradation. Only by showing these implicit assumptions can progress be made.

[edit] Mathematics of Carrying Capacity

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 (e.g. the prey is not the main source of food for the predator), or extreme (e.g. the prey becomes extinct due to over-predation, such as when humans stressed mammoth populations over the brink of extinction; if the prey is the only source of food, the predator will also suffer severe famine or become extinct).

[edit] United States Congress estimate of

In 1862 The United States passed the Homestead Act. It gave 160 acres to a family (a family of four to be sustainable) or 40 acres per person. With the total amount of arable land on Earth equal to about 12 million square miles (7,680 million acres), The carrying capacity of the Earth would have been 192 million people. In 1850, world population was estimated at between 1.1 and 1.4 billion people [3] obviously most people were living on considerably less than 40 acres and didn't have that high of a living standard.

If the average American today owned 40 acres of arable land, there would be about 11.3 million Americans, instead of over 300 million, conversely our living standard would be higher. Most Americans would probably be farmers, and there would be 100% employment if they were. One can also argue that we would have lost World War II had our population been that small compared to the rest of the world, so living sustainably when the rest of the world does not would have its drawbacks.

However, this act was not passed to estimate carrying capacity in the United States, it was passed for a decent standard of living in the 1860s, and to settle the frontier. However, whatever its orginial intent, the yardstick of land and standard of living seem to be remarkably pertinent.

This section of the article was paraphased from Wendell Barry's The Unsettling of America ISBN 0-87156-877-2.

[edit] Points of Contention

The concept has been comprehensively critiqued by social scientists, demographers, and scientists who are concerned that it is simplistic and Malthusian in its assumptions. Some important criticisms appear in Sayre (2007). Ester Boserup, the Danish economist, famously argues that higher population densities increased agricultural productivity rather than leading to the reverse, because "necessity is the mother of invention".

A more complete synopsis of Ms Boserup's work is given by the AAAS Population & Environment Atlas:[4]

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 (Boserup, The Conditions of Agricultural Growth, Allen and Unwin, 1965, expanded and updated in Population and Technology, Blackwell, 1980.).


2. If agricultural innovation increases with population density, then carrying capacity becomes something of a non-issue. Empirical justification for Boserups's view is found in the work of Mike Mortimore and Mary Tiffen (1994, [5]), working in high-density East Africa, and in several other studies they and others have conducted across the continent.

If this contention were true Africa would be self sustaining and Desertification would not be taking place at a rate higher than when the population was lower.

3.Centuries of evidence from Africa and Asia shows when local livelihood conditions deteriorate, migration and a multiplicity of livelihood strategies are developed as common human responses, well before any point of 'capacity' is reached.

This statement shows that the inhabitants are aware of carrying capacity, and their movements are in response to it. Also while these adaptations work with low density primitive populations, when densities increase as a result of abundant food supply over a long period and national boundaries stop natural migration, problems like Ruwanda occur. [6]

4.Furthermore, the concept is simplistic. It requires belief in bounded spaces in which population-environment relationships may be measured and monitored.

As Occam's razor clearly states simplicity is an advantage, not a hindrance. While it is easier to quickly see the results of population biology on an island, with human populations worldwide subject to national censuses, information on imports and exports of goods as well as people, much of our economic system depends on accurate information of food supply, what the average human eats, and how many humans there are. Humans even have 'clocks' of population and productive land. [7]

5.The faith that these spatial units can be determined and then measured is unwarranted given recent knowledge of non-linear rangeland ecologies, and of population-environment relationships in general.

This is an article based on biological fact not faith. Nonlinear rangeland ecological thresholds refer to overshoot of carrying capacity, it doesn't negate carrying capacity it verfies it.

6.The world is not a petri dish, and people's use of it, is simply too chaotic and messy (Gausset et al, 2005).

A statement that in itself cannot be proved or negated, cannot logically be used to prove or disprove another statement. Carrying capacity has been proven to exist in the animal world beyond all doubt, it is the basis of evolution. To say carrying capacity doesn't apply to humans is tantamount to saying humans didn't evolve.

[edit] See also

[edit] References

  1. ^ http://www.pacificislands.cc/pm22001/pmdefault.php?urlarticleid=0009
  2. ^ Troost, The Sex Lives of Cannibals, (non-fiction) (2006)
  3. ^ http://www.stuff.co.nz/stuff/0,2106,3678934a7144,00.html

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. 2007. Carrying Capacity: Genesis, History and Conceptual Flaws. Environmental Politics Seminar, U Cal Berkeley. http://globetrotter.berkeley.edu/EnvirPol/ColloqPapers/Sayre2007.pdf

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

[edit] External links

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