Carrying capacity
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As population density increases, birth rates decrease and death rates increase. The 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, 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 presentTroost, The Sex Lives of Cannibals, (non-fiction) (2006).
Fertility and Carrying Capacity interaction
If the food supply of the environment is of high quality, in humans for example, twinning resultshttp://www.stuff.co.nz/stuff/0,2106,3678934a7144,00.html.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 usehttp://williamcalvin.com/bk5/bk5ch6.htm.
See also
References
External links
- http://www.chincoteague.com/pony/ponies.html
- http://www.amazon.com/gp/product/1572230312/103-7334099-0640636?v=glance&n=283155
- http://www.hartford-hwp.com/archives/24/042.html
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