Historical Development of the rK Concept

The r/K concept is based on the idea that environments differing in population abundance and fluctuation should select for different phenotypes. In a paper published in 1950, Theodosius Dobzhansky compared evolution in the tropics to evolution in temperate environments. The tropics are more stable and populated by different species than temperate environments, so ''interrelationships between competing and symbiotic species become the paramount adaptive problem'' (p. 220). On the other hand, ''Physically harsh environments, such as arctic tundras or high alpine zones of mountain ranges, are inhabited by few species of organisms. The success of these species in colonizing such environments is due simply to the ability to withstand low temperatures or to develop and reproduce during the short growing season'' (p. 220).

Box 1

The logistic equation with its parameters r and K

Time, t

Visualization of the logistic equation dN/dt = rN(1 - N/K), where N is population size, t is time, r is intrinsic growth rate, and K is carrying capacity. The time discrete analog is Nt+1 = e-rNt/(1 +aNt) with a = (e-r-1/K).

The parameter r, intrinsic growth rate, is the difference between per capita birth rate and death rate at very low population densities. It is part of the logistic equation and also of the Euler-Lotka equation 1 = xe-rxlxmx, where lx is the probability of surviving from birth to age x and mx is the number of daughters per female at age x. The Euler-Lotka equation links the parameter r with the life history of individuals. Hence, r is an individual trait that can be selected.

The parameter K, carrying capacity, can however hardly be considered an individual trait. It is really not more than a parameter representing density dependence in the logistic equation. It is phenomenological, thus not directly biologically interpretable.

The idea that environments differing in stability and population select for different phenotypes was formalized by MacArthur in a paper published in 1962 and by MacArthur and Wilson in their landmark book The Theory of Island Biogeography from 1967. In contrast to Dobzhansky, however, MacArthur and Wilson did not look at the population of environments by different species (i.e., biodiversity) but at the population density of species. Given the title of their book, it is no surprise that they looked at species colonizing islands. They formally found that successful colonizers should have a high intrinsic growth rate r, which is the difference between per capita birth rate and death rate at very low population densities. Looking at empirical evidence, they concluded: ''The evidence for birds and ants [...] points to a preference for unstable, scattered habitats as a preadaptation to successful colonization'' (p. 82). Regarding population persistence, they found that a high carrying capacity K equals a long expected persistence time, where K is the number of individuals the island maximally can hold in equilibrium. The two parameters r and Kform the basis of the logistic equation (Box 1). MacArthur and Wilson then extended their findings to populations beyond islands. Based on work by Fisher, Haldane, and Wright, it was already known that r generally is an appropriate measure of fitness at low and increasing population densities. MacArthur and Wilson added that Kis an appropriate measure of fitness at high densities and accordingly coined the terms r-selection and K-selection: in fluctuating environments, populations are repeatedly diminished, so r-selection will dominate. In stable environments, on the other hand, populations will grow to a rather constantly high size where K-selection will dominate. They argued that r-selection tends to lead to ''a shorter developmental time, a longer reproductive life, and greater fecundity, in that order of probability'' (p. 157). In other words, r-selection should lead to high and fast productivity, whereas K-selection should lead to efficiency, especially of resource utilization.

In a short note published in 1970, Pianka made the connection between r-selection, K-selection, and life history more explicit and thereby gave the r/K concept its final form (Table 1). He wrote

Certainly, no organism is completely 'r-selected' or completely 'K-selected,' but all must reach some compromise between the two extremes. [.. .] We can visualize an r—K continuum, and a particular organism's position along it. The r-endpoint represents the quantitative extreme -a perfect ecologic vacuum, with no density effects and no competition. Under this situation, the optimal strategy is to put all possible matter and energy into reproduction, with the smallest practicable amount into each individual offspring, and to produce as many total progeny as possible. Hence r-selection leads to high productivity. The K-endpoint represents the qualitative extreme - density effects are maximal and the environment is saturated with organisms. Competition is keen and the optimal strategy is to channel all available matter and energy into maintenance and the production of a few extremely fit offspring. Replacement is the keynote here. K-selection leads to increasing efficiency of utilization of environmental resources.

Table 1 Attributes of r/K-strategists r-Strategists

K-Strategists

Climate Mortality

Survivorship

Population size

Intra- and interspecific competition

Life history

Variable and/or unpredictable

Often catastrophic, nondirected, density independent

Variable in time, nonequilibrium:

Time

Time

Variable, often lax

Rapid development High /mx Early reproduction Small body size

Semelparity: single reproduction Short life span

Fairly constant and/or predictable More directed, density dependent

Fairly constant, equilibrium:

o CL

Time

Usually keen

Slow development, greater competitive ability Lower resource thresholds Delayed reproduction Large body size

Iteroparity: repeated reproduction Long life span

This is the reasoning behind Table 1, so Pianka gave no formal justification of the attributes of r-strategists and ^-strategists. Synonyms for r-strategists are r-selected species, fugitive species, and opportunistic species. For ^-strategists, the synonyms K-selected species and stable species have been used. As typical r-strategists, Pianka mentioned insects (with the exception of 17-year cicadas and similar species), whereas vertebrates were typical ^-strategists (except some amphibians). Within each taxon, of course, some species are more on the r-end of the continuum while others are more on the K-end.

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