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Carrying capacity is the 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.

Carrying capacity is therefore the largest size of a density-dependent population for which the population is at equilibrium (population size neither increases nor decreases). A factor that keeps population size at equilibrium is known as a regulating factor.

Below carrying capacity, populations often increase, while above, they will 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.

1 Examples
2 Fertility and carrying capacity interaction
3 Humans
4 See also
5 References
6 External links

[edit] Examples

The Chincoteague Pony Swim is an excellent human assisted example. The Pony swim is used to limit the population of ponies on the island to 150 to make sure they will not overgraze the island.

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 islands plants. Without the moose, the wolves would die. It seemed to the first scientists that studied the problem that the wolves would eventally 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".

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 overgrown 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.

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 the food supply of the environment is abundant, in humans for example, twinning may result^[3]. In addition to doubling up, parents may devote less care to each offspring in other ways as well, as the offspring may be able to manage on their own with abundant food supply. Instead such parents have as many offspring as they can by starting early and repeating just as quickly as possible. When prospects turn sour, they may K-shift [resort to small numbers of offspring] back toward the more conservative strategy of sinking one's bets on a few well-placed shots. When a species is already exploiting the environment near the limits of its 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 consierable care to each.

If 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 what triggers it, what aspects of the environment are "read" for the forecast? If we are ever to replace this corner-cutting "Quantity is Better than Quality" philosophy of nature 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

Carrying capacity has come under critique as a useful model for assessing the relationship between human populations and their environment. In the words of one scholar who is attempting to re-interpret the concept in new ways, "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, or scaling back their consumption.

If one accepts the premise that humans are animals, which evolution requires, then their carrying capacity can be calculated. Population size, living standards, and resource depletion vary with all animals, but the tool of carrying capacity is still useful, it may not be as accurate as for other animals, but for tracking the human/environment relationship no more accurate method has been formulated. Even 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 origin of the food can continue to produce Y amount of food indefinitely, carrying capacity has been reached. Provided the number of humans stay the same.

Humans with the need to enhance their reproductive success (see Richard Dawkins 'the Selfish Gene' ) understand that food amounts can vary and also that other factors in the environment can alter humans need for food. A house for example might mean you don't need to eat as much food to stay warm. This can also be calculated.

Over time, this system has evolved, the calculation has become commonplace, it is called money. Money can buy food, a house, land, pets, virtually everything that impacts the carrying capacity of an area. However, it can also impact regions thousands of miles away. Carbon dioxide from your car's tailpipe travels all the way 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.

Simple carrying capacity calculation calulates food and other resources used by humans over the entire earth, for an 'average carrying capacity' of the so called 'average human'. For individual humans, cities, provinces, and countries, this method is useless. The fact that food production has outpaced population growth worldwide ^[4] does not feed people in the mountains of Tibet.

In order to correct the 'average human' problem the concept of Ecological footprint was developed. 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.

This attempt (albeit also criticized) addresses the parameters of the sustainability of human life on earth. The key shift that is made when changing the focus from "carrying capacity" to "ecological footprint" is that the emphasis is not on the number of people in an area (or on the planet) but on their use of resources and the speed with which they use those resources. This frames the solution in terms of use and distribution of resources and proves that (for example) fewer Americans than Mexicans can live on the planet without degrading the environment ( see Sustainability). Simply because the 'average' Mexican use fewer resources per person than the 'average' American does. Thus if we want to know How Many People Can the Earth Support? ISBN 0-393-31495-2 we must also ask if we want an American standard of living or a Mexican standard.

There are some in the social services field that avoid this debate on the grounds that the global economic system is responsible for creating poverty by distributing wealth unequally, and therefore consider population-based discussion to be inaccurately "blaming the victim" (i.e., blaming poor people for their poverty) however, this is not what carrying capacity is about. Carrying capacity is about calculating a number, not about what to do after one calculates the number.

If carrying capacity 'blames' anyone, it 'averages' the blame. 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 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.

Ecological footprint defines the problem and solution in terms of consumption. For example, The Netherlands (area: 33,920 sq km) serves to illustrate: We estimate that the people of Holland require a land area more than 14 to 15 times larger than their country to support current domestic consumption of food, forest products, and energy (Rees & Wackernagel, 1994). The food footprint alone is more than 100,000 square kilometers, based on world average productivities. Indeed, Dutch government data suggest that the Netherlands appropriates 100,000 to 140,000 km2 of agricultural land, mostly from the third world, for food production (including value-added food products produced in the Netherlands for export) (RIVM, 1991, cited in Meadows et al. 1992). [Most of the imported "food" is fodder for domestic livestock. This is a sufficient "Second Law of Thermodynamics" explanation of the fact that animal manure represents one of the most pressing waste disposal problems confronting the Netherlands!] This "imported" land is five to seven times the area of Holland's domestic arable land. (Rees1996)

If we used ecological footprint as our guide we would say the Netherlands needs to be 15 times as big in area to sustainably support its present population. In carrying capacity terms we could say the population of the Netherlands should be one fifteenth its present size. Neither of these statements would enable policy makers to carry them out. Similarily it doesn't help Americans [population size almost 300 million] understand that willingly controlling the birthing ability of White-tailed Deer [population size about 30 million] isn't the only way to address an overpopulation problem.