R and K selection

Some of the predictions of the previous sections can be brought together in a scheme that has been particularly influential in the search for life history patterns. This is the concept of r and K selection, originally propounded by MacArthur and Wilson (1967; MacArthur, 1962) and elaborated by Pianka (1970) (but, see Boyce, 1984). The letter r refers to the intrinsic rate of natural increase (above) and indicates that r-selected individuals have been favored for their ability to reproduce rapidly (i.e. have a high r value). The letter K will not be introduced properly until intraspecific competition is discussed fully in the next chapter, but for now we need note only that it refers to the size ('carrying capacity') of a crowded population, limited by competition. Thus, K-selected individuals have been favored for their ability to make a large proportional contribution to a population that remains near that carrying capacity. The concept is therefore based on there being two contrasting types of habitat: r-selecting and K-selecting. It originally emerged (MacArthur & Wilson, 1967) from the contrast between species that were good at rapidly colonizing relatively 'empty' islands (r species), and species that were good at maintaining themselves on islands once many colonizers had reached there (K species). Subsequently, the concept was applied much more generally. Like all generalizations, this dichotomy is an oversimplification - but one that has been immensely productive.

A K-selected population lives in a K selection habitat that imposes few random environmental fluctuations on it. As a consequence, a crowded population of fairly constant size is established. There is intense competition amongst the adults, and the results of this competition largely determine the adults' rates of survival and fecundity. The young also have to compete for survival in this crowded environment, and there are few opportunities for the young to become established as breeding adults themselves. In short, the population lives in a habitat that, because of intense competition, is both high CR and offspring size-sensitive.

The predicted characteristics of these K-selected individuals are therefore larger size, deferred reproduction, iteroparity (i.e. more extended reproduction), a lower reproductive allocation and larger (and thus fewer) offspring. The individuals will generally invest in attributes that increase survival (as opposed to reproduction); but in practice (because of the intense competition) many of them will have very short lives.

By contrast, an r-selected popular selection tion lives in a habitat that is either unpredictable in time or short lived. Intermittently, the population experiences benign periods of rapid population growth, free from competition (either when the environment fluctuates into a favorable period, or when a site has been newly colonized). But these benign periods are interspersed with malevolent periods of unavoidable mortality (either in an unpredictable, unfavorable phase, or when an ephemeral site has been fully exploited or disappears). The mortality rates of both adults and juveniles are therefore highly variable and unpredictable, and they are frequently independent of population density and of the size and condition of the individuals concerned. In short, the habitat is both low CR and offspring size-insensitive.

The predicted characteristics of r-selected individuals are therefore smaller size, earlier maturity, possibly semelparity, a larger reproductive allocation and more (and thus smaller) offspring. The individuals will invest little in survivorship, but their actual survival will vary considerably depending on the (unpredictable) environment in which they find themselves.

The scheme is thus a special case of the general classification of habitats in Figure 4.25c. Note, therefore, first, that adult and offspring habitats need not be linked in the way the r/K scheme envisages, and second, that the life history characteristics associated with the r/K scheme can arise for all sorts of reasons beyond its scope (e.g. predation of diminutive adults as opposed to intense competition amongst adults).

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