Carrying capacity began as a shipping term before ecologists made it their K
Every environment has a ceiling: the largest population of a species it can sustain with the food, water and space available. Ecologists write that ceiling as K, from the German for capacity limit, but the phrase itself seems to have started life in an 1845 government report.
Carrying capacity is the maximum population a particular environment can support over time, the point at which births and immigration balance deaths and emigration. Below it, a population can keep growing; above it, numbers generally fall as space, food or even sunlight run short. The ceiling differs for each species, and whatever specifically halts growth is called the limiting or regulating factor.
The word's history is murky. Some accounts link it to international shipping in the 1840s, others to early laboratory work with microbes; a 2008 review found the earliest English use in an 1845 report from the US Secretary of State to the Senate. Biologists adopted it in the 1870s. In 1923 Hadwen and Palmer defined it as how many stock could graze an area for a set time without damaging the range, and Aldo Leopold carried it into wildlife management in 1933, followed a year later by the wetlands specialist Paul Errington, who used it differently.
The mathematics is older than the phrase. In 1838 the Belgian mathematician Pierre François Verhulst published a population model in which growth slows as numbers approach a limit, though he did not use the term. In his equation the growth rate equals an intrinsic rate r times the population, multiplied by one minus the population divided by K. The solution is the familiar S-shaped logistic curve: near-exponential growth at first, levelling off as K is reached.
Applied to people, the idea grows contentious. Raymond Pearl and Lowell Reed used it for human populations in the 1920s, and neo-Malthusians and eugenicists popularised it in the 1950s. The 1972 book Limits to Growth examined global limits and drew heavy criticism, and in 2012, 22 researchers writing in Nature warned that the biosphere might be nearing a state shift that could lower the human carrying capacity.
Source: Carrying capacity
