K Selected Species in Ecology and Life History Strategy

In order to define K selected species, one has to know that K selected species are the ones that are identified based on the r and K selection theory. The r/K selection theory states that the selection of the different combinations of traits in the organism is a trade-off between the quantity and quality of the offspring. 

The focus to choose whether an increased number of offspring at the expense of individual parental investment as in the case of r-strategists or on a reduced number of offspring but with significant increased parental investment as in the case of K-strategists, varies widely. It is based on the conditions required for success in particular environments. These concepts of quantity or quality of the offspring are sometimes said to be either cheap or expensive which is a comment on the expendable nature of the offspring and the parental commitment made to them.

Theory of K Selected Species

The naming of r and K selection was done by ecologists Robert MacArthur and E. O. Wilson in 1967 which was based on their work committed to understanding island biogeography. Even though introduced in 1967, another theory, the theory of evolution of life-history strategies has a longer history than the r and K selection theory. The r/K selection theory became popular during the 1970s and the 1980s when it was considered to be a heuristic device but gradually lost its importance in the early 1990s as it was criticised based on the results of several empirical studies. A life-history theory then replaced the r/K selection theory but it still continues to incorporate many of the important concepts and themes that are a part of the r/K selection theory. 

The difference between r and K selected species is that the K-selected species unlike the r-selected species, display the characteristics or traits associated with the living at densities that are close to the carrying capacity of the species and are usually strong competitors in crowded niches that are ready to invest heavily on fewer offsprings as each of the offsprings will have a relatively high probability of surviving to adulthood a condition described as low r and high K. Thus, in scientific literature, the r-selected species are sometimes referred to as an opportunistic species whereas the K-selected species are described as in equilibrium. 

The K-selected species predominates particularly in stable or predictable environments as they have the ability to compete with a high success rate for the limited resources which is crucial in such environments and the population of the K-selected species’ organisms typically are very constant in number and are close to the maximum possible number that their environment can bear unlike the conditions of r-selected species where the population sizes are liable to change much more rapidly. 

The traits or characteristics that are thought to be specific to K selected organisms, include large body size, longer life expectancy, and the reproduction of fewer offsprings (in number), which often requires a deep involvement of parental care and it continues until the offspring matures. Organisms whose life history shares traits or characteristics of K-selection theory are many times referred to as either K-strategists or K selected organisms. The examples of K strategists or organisms that exhibit the K-selected traits include large organisms such as elephants, humans and whales and smaller but long-living organisms such as the Arctic terns, parrots and eagles.

Although some of the organisms are identified and can be classified primarily as r or K strategist species, the majority of the organisms of different species do no follow this pattern of classification. One of the examples of organisms that do not follow this pattern is the example of trees having traits or characteristics such as longevity and strong competitiveness that are the factors used to characterize them as K strategist species. However, in reproduction trees usually produce many thousands of offspring and disperse them widely across a wide-spread ecosystem which can be used as a factor to characterize them as r strategists. 

This is also observed in the case of reptiles such as the sea turtles that display traits of both the r and K strategy species. As per the difference between r strategist and k strategist, the sea turtles are large organisms having longer lifespans, in the case that they reach adulthood (K strategist species), they are known to produce large numbers of offspring that are not properly nurtured (r strategist species). The r and K classification can be expressed as a continuous spectrum based on the economic concept of the discounted future returns such as in the case of r selected species it corresponds to the large discount rates and in the case of the characteristics of K selected species it corresponds to small discount rates. 

Ecological Successions and Application of K Selected Species Theory

In the areas that are known to have major ecological disruption or sterilization such as areas involving events such as the aftermath of a major volcanic eruption like the area at Krakatoa or Mount St. Helens, both the K selected and r selected species have a different role to play in the ecological succession that regenerates the ecosystem. Due to their higher reproductive rates favouring ecological opportunism, the primary colonisers in any area are typically the r strategists and they are then followed by a succession of increasingly competitive flora and fauna. The ability of an ecosystem to increase its energetic content via the photosynthetic capture of the solar energy usually increases with the increase in the complex biodiversity like the r species that proliferate or reproduce to reach a maximal level or a peak that is possible with the K strategists. 

Over time a new equilibrium is approached which is sometimes called a climax community in which the r selected organisms are slowly being replaced by the K selected organisms which are more competitive in nature and are better adapted to the emerging and developing micro-environmental conditions and characteristics of the landscape. Traditionally, biodiversity was considered to be maximized at this stage with introductions of newer and newer species that result in the replacement and the local extinction of the endemic species. However, the Intermediate Disturbance Hypothesis states that the intermediate levels of disturbance in a given landscape create different patches at different levels of succession, thus promoting the coexistence of different species that can be characterized into both the colonizers and competitors at the regional scale.

While the theory is usually applied at the level of the species, the K selection theory or the r selection theory is also useful in studying the evolutionary characteristics of the ecological and life-history differences that occur in between subspecies. For example, the African honey bee, A. m. scutellata, and the Italian honey bee, A. m. lingustica. Moving towards the other end of the spectrum, the theory has also been used to study the evolutionary ecology of whole groups of organisms like the bacteriophages. 

Some of the researchers such as Lee Ellis, J. Philippe Rushton, and Aurelio Jose Figueredo, have applied the r and K selection theory to different human behaviours that include crime, sexual crime, sexual promiscuity, fertility, IQ, and other traits that are related to life-history theory Rushton’s work that resulted in him developing the differential K theory in an attempt to explain the many different variations observed in human behaviour across geographic areas, a theory that has been criticized by many other researchers. Some other researchers have proposed that the evolution of the human inflammatory responses are the result of the responses that are related to r and K selected species adaptation.

In the year 2002, Reznick and colleagues reviewed the controversy regarding the r and K selection theory and said in their conclusive statement 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 a new paradigm that focuses 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. “