r/K selection theory

An unstable environment would encourage the parent to make many offspring, because the likelihood of all (or the majority) of them surviving to adulthood is slim.

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 that exhibit r-selected traits can range from bacteria and diatoms, to insects and grasses, to various semelparous cephalopods, certain families of birds, such as dabbling ducks, and small mammals, particularly rodents.

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.

In reproduction, however, trees typically produce thousands of offspring and disperse them widely, traits characteristic of r-strategists.

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.

[15] 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.

[31][32][33][34] In particular, a review in 1977 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.

[35] 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.

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;[31] Stearns 1976,[45] 1977[35]).

Alternative approaches are now available both for studying life history evolution (e.g. Leslie matrix for an age-structured population) and for density-dependent selection (e.g. variable density lottery model[47]).

A North Atlantic right whale with solitary calf. Whale reproduction follows a K -selection strategy, with few offspring, long gestation, long parental care, and a long period until sexual maturity.
A litter of rats with their mother. The reproduction of rats follows an r -selection strategy, with many offspring, short gestation, less parental care, and a short time until sexual maturity. The same applies to mice.
A bald eagle , an individual of a typical K -strategist species. K -strategists have longer life expectancies, produce fewer offspring, and when young tend to be altricial , requiring extensive care by parents.