R/K selection theory

In ecology, r/K selection theory relates to the selection of traits which promote success in particular environments. The theory originates from work on island biogeography by the ecologists Robert MacArthur and E. O. Wilson.

Overview

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 derived from standard ecological algebra, as illustrated in the simple Verhulst equation of population dynamics:

${\frac {dN}{dt}}=rN\left(1-{\frac {N}{K}}\right)$

where r is the growth rate of the population (N), and K is the carrying capacity of its local environmental setting. Typically, r-selected species exploit empty niches, and produce many offspring, each of which has a relatively low probability of surviving to adulthood. In contrast, K-selected species are strong competitors in crowded niches, and invest more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood. In the scientific literature, r-selected species are occasionally referred to as "opportunistic", while K-selected species are described as "equilibrium".

Unstable environments

In unstable or unpredictable environments r-selection predominates, as the ability to reproduce quickly is crucial, and there is little advantage in adaptations that permit successful competition with other organisms, because the environment is likely to change again. Traits that are thought to be characteristic of r-selection include: 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 with r-selected traits range from bacteria and diatoms, through insects and weeds, to various semelparous cephalopods and mammals, especially small rodents.

Stable environments

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 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 that 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, trees, humans and whales, but also smaller, long-lived organisms such as Arctic Terns.

As a continuous spectrum

Although some organisms are identified as primarily r- or K-strategists, the majority of organisms do not follow this pattern (some, such as sea turtles completely violate the theory and others fall between these two ecological extremes and may display traits considered characteristic of both ends of the r/K spectrum). 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 large organisms with long lifespans (should they reach adulthood), they produce large numbers of unnurtured offspring.

In ecological succession

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. Typically, biodiversity is maximised at this stage, with introductions of new species resulting in the replacement and local extinction of endemic species.

Status of the theory

Although r/K selection theory became widely used during the 1970s, it also began to attract more critical attention. In particular, an influential review by the ecologist Stephen Stearns drew attention to gaps in the theory, and to ambiguities in the interpretation of empirical data for testing it. Nonetheless, the concept of r/K selection is still used in biological and ecological research.

See also

• Adaptive capacity
• Evolutionary game theory