CARRYING CAPACITY
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 typically decrease and death rates typically increase. carrying capacity is at the point when these two rates are equal. It is usually a range of values, rather than a single point in time. It is the number of individuals an environment can support without negative effects.
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 will tend to increase, while above, they will tend to 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 will vary for different species in different habitats, and can change over time due to a variety of factors including trends in food availability, environmental conditions and space.
It is possible for a species to exceed its carrying capacity, in which case mass fatalities will occur as shortages in food and water take effect. This is often considered more devastating for a population as it produces stress for the entire species, and populations can fall far below the carrying capacity.
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.
The carrying capacity assessment is essentially the same. They never reached it. Unlike most North American Indian tribes who did reach a more or less stable population size, until the white settlers arrived. 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].
Fertility and carrying capacity interaction
If the food supply of the environment is of high quality, in humans for example, twinning results[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 such improving environmental conditions. 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 your species is already exploiting the environment near the limits of its carrying capacity (which includes food availability but also nesting sites and such), play it safe by waiting until you are better prepared, then raise only a few offspring and devote a lot of care to them.
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).
Used in accordance with Fair use[4].
Humans
The concept of carrying capacity is most easily used on other life forms. Applying the concept to humans is more difficult but doable. The reasons for this are obvious. All other animals and plants have to live off the environment in their immediate vicinity. Many humans (much of the so called third world) also have this limitation, however the first world, and second world do not.
With transportation, refrigeration, and preservation technologies some humans can literally have food from half a world away before it spoils. How to easily calculate how humans in each country damage the nearby ecosystem as a result of this, proves a daunting task.
The method used most of the time for this calculation is to simply calulate food and other resources used by humans over the entire earth, 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 [5] does one no use 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 method of calculation proves that 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? we must also ask if we want an American standard of living or a Mexican standard.
Critiques of application to humans
Carrying capacity has come under heavy 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 is 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.
Ecological footprint is a more recent (albeit also criticized) attempt to address 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 rather than birth rate, which guides the discussion away from population-oriented solutions.
Avoiding population-oriented solutions is considered preferable by people in three categories:
- those who consider global economic systems as 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);
- those who oppose population-based policies in all cases, based on what they consider to be past abuses, such as forced sterilization;
- those who oppose the use of contraception (or the encouragement of its use) based on religious reasons.
In any case, 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. Ecological footprint defines the problem and solution in terms of consumption, but does not have any measure or variable for population; carrying capacity defines the problem and solution in terms of population, but does not have any parsimonious way to calculate in different resource consumption levels.
See also
References
External links
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