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In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity and quality of offspring. The focus upon either increased quantity of offspring at the expense of individual parental investment, or reduced quantity of offspring with a corresponding increased parental investment, varies widely, seemingly to promote success in particular environments.
The etymology is from an equation where r comes from rate and K comes from carrying capacity; in German, the word for capacity is Kapazität but in this case K may come from Konstante, the German word for a constant. r-selection makes a species prone to numerous reproduction at low cost per individual offspring, while K-selected species expend high cost in reproduction for a low number of more difficult-to-produce offspring. Neither mode of propagation is intrinsically superior, and in fact they can coexist in the same habitat, as in rodents and elephants. r/K selection theory is also useful in studying the evolution of ecological and life history differences between subspecies such as African honey bee, A. m. scutellata, and the Italian bee, A. m. ligustica.
The theory was popular in the 1970s and 1980s, when it was used as a heuristic device, but lost importance in the early 1990s, when it was criticized by several empirical studies. A life-history paradigm has replaced the r/K selection paradigm and continues to incorporate many of its important themes.
The terminology of r/K-selection was coined by the ecologists Robert MacArthur and E. O. Wilson based on their work on island biogeography, although the concept of the evolution of life history strategies has a longer history.
In r/K selection theory, selective pressures are hypothesised to drive evolution in one of two generalized directions: r- or K-selection. These terms, r and K, are drawn from standard ecological algebra as illustrated in the simplified Verhulst model of population dynamics:
where r is the maximum growth rate of the population (N), K is the carrying capacity of its local environmental setting, and the notation dN/dt stands for the derivative of N with respect to t (time). Thus, the equation relates the rate of change of the population N to the current population size and expresses the effect of the two parameters.
As the name implies, r-selected species are those that place an emphasis on a high growth rate, and, typically exploit less-crowded ecological niches, and produce many offspring, each of which has a relatively low probability of surviving to adulthood (i.e., high r, low K).
In unstable or unpredictable environments, r-selection predominates as the ability to reproduce quickly is crucial. There is little advantage in adaptations that permit successful competition with other organisms, because the environment is likely to change again. Among the traits that are thought to characterize r-selection are high fecundity, small body size, early maturity onset, short generation time, and the ability to disperse offspring widely.
Organisms whose life history is subject to r-selection are often referred to as r-strategists or r-selected. Organisms that exhibit r-selected traits can range from bacteria and diatoms, to insects and grasses, to various semelparous cephalopods and mammals, particularly small rodents.
By contrast, K-selected species display traits associated with living at densities close to carrying capacity, and typically are strong competitors in such crowded niches that invest more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood (i.e., low r, high K). In scientific literature, r-selected species are occasionally referred to as "opportunistic" whereas K-selected species are described as "equilibrium".
In stable or predictable environments, K-selection predominates as the ability to compete successfully for limited resources is crucial and populations of K-selected organisms typically are very constant in number and close to the maximum that the environment can bear (unlike r-selected populations, where population sizes can change much more rapidly).
Traits that are thought to be characteristic of K-selection include large body size, long life expectancy, and the production of fewer offspring, which often require extensive parental care until they mature. Organisms whose life history is subject to K-selection are often referred to as K-strategists or K-selected. Organisms with K-selected traits include large organisms such as elephants, primates and whales, but also smaller, long-lived organisms such as Arctic terns.
Although some organisms are identified as primarily r- or K-strategists, the majority of organisms do not follow this pattern. For instance, trees have traits such as longevity and strong competitiveness that characterise them as K-strategists. In reproduction, however, trees typically produce thousands of offspring and disperse them widely, traits characteristic of r-strategists.
Similarly, reptiles such as sea turtles display both r- and K-traits: although sea turtles are large organisms with long lifespans (provided they reach adulthood), they produce large numbers of unnurtured offspring. Mammalian males tend to be r-type reproducers, whereas females tend to have K characteristics.
The r/K dichotomy can be re-expressed as a continuous spectrum using the economic concept of discounted future returns, with r-selection corresponding to large discount rates and K-selection corresponding to small discount rates.
In certain species, the r/K dichotomy likewise manifests itself upon population-specific examination relative to the range of expressions of a given trait observed across that species. For example, among humans, populations having low socioeconomic status disproportionately exhibit r-type traits such as early pregnancies, large numbers of children, male dispersal of offspring across multiple households, and low parental investment (in part because of a lack of resources to invest), in keeping with their disproportionate occupation of niches featuring high levels of instability and existential risk to individual members, whereas populations having high socioeconomic status disproportionately exhibit K-type traits such as delayed pregnancies, small numbers of children, concentration of offspring within a single household, and high parental investment (in part because of the presence of more resources to invest), in keeping with their disproportionate occupation of niches featuring relative stability and lower levels of existential risk to individual members.
In areas of major ecological disruption or sterilisation (such as after a major volcanic eruption, as at Krakatoa or Mount Saint Helens), r- and K-strategists play distinct roles in the ecological succession that regenerates the ecosystem. Because of their higher reproductive rates and ecological opportunism, primary colonisers typically are r-strategists and they are followed by a succession of increasingly competitive flora and fauna. The ability of an environment to increase energetic content, through photosynthetic capture of solar energy, increases with the increase in complex biodiversity as r species proliferate to reach a peak possible with K strategies.
Eventually a new equilibrium is approached (sometimes referred to as a climax community), with r-strategists gradually being replaced by K-strategists which are more competitive and better adapted to the emerging micro-environmental characteristics of the landscape. Traditionally, biodiversity was considered maximized at this stage, with introductions of new species resulting in the replacement and local extinction of endemic species. However, the Intermediate Disturbance Hypothesis posits that intermediate levels of disturbance in a landscape create patches at different levels of succession, promoting coexistence of colonizers and competitors at the regional scale.
Although r/K selection theory became widely used during the 1970s, it also began to attract more critical attention. In particular, a review by the ecologist Stephen C. Stearns drew attention to gaps in the theory, and to ambiguities in the interpretation of empirical data for testing it.
In 1981, a review of the r/K selection literature by Parry demonstrated that there was no agreement among researchers using the theory about the definition of r and K selection, which led him to question whether the assumption of a relation between reproductive expenditure and packaging of offspring was justified. A 1982 study by Templeton and Johnson, showed that in a population of Drosophila mercatorum under K selection the population actually produced a higher frequency of traits typically associated with r selection. Several other studies contradicting the predictions of r/K selection theory were also published between 1977 and 1994.
When Stearns reviewed the status of the theory in 1992 he noted that from 1977 to 1982 there was an average of 42 references to the theory per year in the BIOSIS literature search service, but from 1984 to 1989 the average dropped to 16 per year and continued to decline. He concluded that r/K theory was a once useful heuristic that no longer serves a purpose in life history theory.
More recently, the panarchy theories of adaptive capacity and resilience promoted by C. S. Holling and Lance Gunderson have revived interest in the theory, and use it as a way of integrating social systems, economics and ecology.
In 2002, Reznick and colleagues reviewed the controversy regarding r/K selection theory and wrote that: "The distinguishing feature of the r- and K-selection paradigm was the focus on density-dependent selection as the important agent of selection on organisms’ life histories. This paradigm was challenged as it became clear that other factors, such as age-specific mortality, could provide a more mechanistic causative link between an environment and an optimal life history (Wilbur et al. 1974; Stearns 1976, 1977). The r- and K-selection paradigm was replaced by new paradigm that focused on age-specific mortality (Stearns, 1976; Charlesworth, 1980). This new life-history paradigm has matured into one that uses age-structured models as a framework to incorporate many of the themes important to the r–K paradigm."