In scientific literature, r-selected species are occasionally referred to as "opportunistic" whereas K-selected species are described as "equilibrium". K-strategists have longer life expectancies, produce relatively fewer offspring and tend to be altricial, requiring extensive care by parents when young.īy 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). As with K-selection, below, the r/ K paradigm ( Differential K theory) has controversially been associated with human behavior and separately evolved populations.Ī Bald eagle, an individual of a typical K-strategist species. Organisms that exhibit r-selected traits can range from bacteria and diatoms, to insects and grasses, to various semelparous cephalopods and small mammals, particularly rodents. Organisms whose life history is subject to r-selection are often referred to as r-strategists or r-selected. 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. There is little advantage in adaptations that permit successful competition with other organisms, because the environment is likely to change again. In unstable or unpredictable environments, r-selection predominates due to the ability to reproduce rapidly. A typical r species is the dandelion (genus Taraxacum). R-selected species are those that emphasize high growth rates, 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). (Note that decrease is negative growth.) The choice of the letter K came from the German Kapazitätsgrenze (capacity limit), while r came from rate. Thus, the equation relates the growth rate of the population N to the current population size, incorporating the effect of the two constant parameters r and K. Where N is the population, r is the maximum growth rate, K is the carrying capacity of the local environment, and dN/dt, the derivative of N with respect to time t, is the rate of change in population with time. A life-history paradigm has replaced the r/ K selection paradigm but continues to incorporate many of its important themes. 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. Wilson in 1967 based on their work on island biogeography although the concept of the evolution of life history strategies has a longer history (see e.g.
The terminology of r/ K-selection was coined by the ecologists Robert MacArthur and E. In contrast, more stable environments allow parents to confidently invest in one offspring because they are more likely to survive to adulthood. An unstable environment would encourage the parent to make many offspring, because the likelihood of all of the majority of them surviving to adulthood is slim. The stability of the environment can predict if many expendable offspring are made or if fewer offspring of higher quality would lead to higher reproductive success. The concepts of quantity or quality offspring are sometimes referred to as "cheap" or "expensive", a comment on the expendable nature of the offspring and parental commitment made. The focus on either an increased quantity of offspring at the expense of individual parental investment of r-strategists, or on a reduced quantity of offspring with a corresponding increased parental investment of K-strategists, varies widely, seemingly to promote success in particular environments. 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. Whale reproduction follows a K-selection strategy, with few offspring, long gestation, long parental care, and a long period until sexual maturity. A North Atlantic right whale with solitary calf.
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