Introduction. The terms of r- and K-selection were introduced to ecology by MacArthur and Wilson (1967), although some aspects of these concepts were ...
Oecologia
Oecologia (Berl) (1981) 48:260-264
9 Springer-Verlag 1981
The Meanings of r- and K-Selection Gregory D. Parry Department of Zoology, Monash University, Clayton, Victoria, Australia, 3168
Summary. This paper catalogues several different dichotomies that have all been termed r- and K-selection. The status of the concept of r- and K-selection is discussed and a more restricted usage of the terms is recommended.
Introduction The terms of r- and K-selection were introduced to ecology by MacArthur and Wilson (1967), although some aspects of these concepts were considered earlier by Dobzansky (1950). Originally r-selection meant selection for high population growth in uncrowded populations and K-selection referred to selection for competitive ability in crowded populations (MacArthur and Wilson 1967; Hairston et al. 1970; Wilbur etal. 1974; Cody and Diamond 1975). However, during the last decade the meaning of these terms has been broadened, especially by Pianka (1970, 1972), but also by Gadgil and Solbrig (1972), Southward (1977) and others. The enlarged concept is well-summarized by Stearns (1976, 1977) who notes that r- and K-selection predicts an association of life-history traits into two groups : (1) r-selection: early age of maturity; large number of young; semelparity; no parental care; a large reproductive effort, (2) K-selection: delayed reproduction; small number of young; iteroparity; parental care; and a smaller reproductive effort. An impressive amount of empirical support for the enlarged concept has been accumulated. Aspects of the life histories of corals (Loya 1976), crustaceans (Clarke 1979; Hicks 1979), insects (Pianka 1970; Barbosa 1977; Stubbs 1977), mammals (Pianka 1970; Richardson 1975; Stubbs 1977; Tarnarin 1978; Leutonegger 1979), molluscs (Branch 1976 ; Grahame 1977, Calow 1978 ; Rex 1979), parasites (Jennings and Calow 1975 ; Esch et al. 1977), plants (Gadgil and Solbrig 1972; Abrahamson and Gadgil 1973; Gaines etal. 1974; McNaughton 1975; Law etal. 1977; Law 1979a) and starfish (Menge 1974; Moore 1978) have been interpreted in terms of r- and K-selection. Unfortunately, an evaluation of the empirical support for r- and K-selection is difficult as the above authors have used the terms r- and K-selection in several different senses. This paper considers the different meanings which have been termed "r and K-selection", or "r- and K-strategies" and notes some important differences between them. Whilst I recognise that too rigid a definition too soon may inhibit the development of concepts, I believe that a more consistent usage of the terms is required (c.f. Southwood 1977). The usage is now so broad that almost any life history dichotomy is likely to be termed r- and K-selection (Atkinson 1979).
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I have been able to recognise four different meanings for the terms r- and K-selection. These are briefly defined below.
Meaning I r-selection is selection for maximum population growth in uncrowded populations. K-selection is selection for competitive ability in crowded populations.
Meaning H r-selection is the density-independentcomponent of natural selection. K-selection is the density-dependent component of natural selection.
Meaning III r-species occur in habitats which are ephemeral. K-species occur in habitats with a long durational stability.
Meaning IV r-selection is the allocation of a large proportion of resources to reproduction. K-selection is the allocation of a small proportion of resources to reproduction. These four meanings are the most frequently used, but the terms r- and K-selection have been used occasionally in other senses. While it is clear that the four meanings above are not mutually exclusive, these meanings do differ in important respects, as the following discussion demonstrates. Considerable confusion has resulted from the terms r- and K-selection being used as a label or as an implied explanation, without the distinction between these usages being made clear (Stearns 1976). The first three meanings above suggest a causal relationship between associated life-history parameters and an environmental parameter (i.e. degree of crowding, amount of DD mortality, or habitat stability). Meaning IV defines r- and K-selection only in terms of a life-history parameter, and therefore this, and other similar definitions, imply no causal relationships; the terms r- and K-selection are used merely as labels. Table I provides a summary of the meanings used by various authors and illustrates that differences in usage are widespread. The compilation of this table proved difficult as it was not always possible to designate the usage of a particular author
261 to one of the above meanings with complete certainty. In addition, some authors defined the term in one sense but have used it in another. Where this has h a p p e n e d I have regarded the definition which they claimed to be using to be the one which they actually used.
Meaning I M e a n i n g I is that originally used by M a c A r t h u r and Wilson (1967) a n d thus has priority. It is clearly limited in applicability to comparisons of species or populations in which the only imp o r t a n t selective difference is population density with respect to resources (Wilbur et al. 1974; Livdahl 1979). There are three assumptions in the original M a c A r t h u r and Wilson (1967) model which restrict its generality. (1) Fitness in crowded populations in equated to K. This may be incorrect when the best competitor in crowded conditions is the largest genotype (Luckinbill 1979). (2) Crowding is the only environmentaI parameter to which an evolutionary response is allowed. This was a reasonable assumption in the original model which endeavoured to predict genotypic changes following the colonisation of a n island. But in other circumstances life-history parameters would be expected to respond not only to crowding but also to any factors which prevented crowding i.e. life-history parameters m a y be affected directly by different causes of mortality (e.g. predation and climatic extremes) in addition to any effects that these causes of mortality may have upon the degree of crowding. (3) In the island colonisation model the origin of the first propagule requires n o explanation. However, in most natural populations chance alone is not sufficient to explain the establishm e n t of a particular population, even one in a relatively patchy environment. Individuals would be expected to evolve a pattern of dispersal such that most suitable patches of h a b i t a t are located each generation. Thus life-history parameters may usually be influenced by the requirements for dispersal ( H o r n 1978). Whilst certain life-history parameters may result from differences in crowding, m a n y of these parameters are identical to those which might be expected as responses to different causes of mortality, or from the requirements for dispersal. Thus, as Wilbur et al. (1974) have convincingly argued, it m a y be very misleading to conclude, on the basis of life-history characteristics alone, that the selection pressure responsible was the degree of crowding (i.e. r- and K-selection).
Meaning H M e a n i n g II leads to predictions similar to those of Meaning I if crowding is prevented by density-independent (DI) mortality and if crowding results in density-dependent (DD) mortality t h r o u g h resource shortage. But, if the D D source of mortality is predation rather t h a n crowding, then the life-history consequences are difficult to predict. However it seems unlikely that the life-history consequences will be the same as those which result from crowding (Wilbur et al. 1974). Furthermore, the usefulness of M e a n i n g II is restricted by the difficulty of measuring the degree of DI versus D D mortality (Stearns 1977, but see Stubbs 1977).
Meaning III The permanence of a habitat will influence the time available for population growth a n d therefore may affect the degree of crowding and the a m o u n t of r- and K-selection ( M e a n i n g I). However, the permanence of a habitat will also directly affect
Table 1. The meanings of the terms r- and K-selection as used by various authors a Author
Abrahamson and Gadgil (1973) Allen (1976) Armstrong and Gilpin (1977) Atkinson (1979) Boyce (1979) Charlesworth (1971) Clarke (1972) Demetrius (1975) Derickson (1976) Esch et al. (1977) Fenchel (1974) Gadgil and Bossert (1970) Gadgil and Solbrig (1972) Gaines et al. (1974) Gould (1977) Grahame (1977) Hairston et al. (1970) Hickman (1975) Horn (1978) Jennings and Calow (1975) Jones (1975) King and Anderson (1971) Law et aI. (1977) Leon (1976) Livdahl (1979) Loya (1976) Luckinbill (1979) MacArthur (1972) MacArthur and Wilson (1967) McNaughton (1975) Maiorana (1976) Menge (1974) NeweI1 and Trainer (1978) Nichols et al. (1976) Pianka (1970) Pianka (1972) Richardson (1975) Ricklefs (1977) Roughgarden (1971) Solbrig and Simpson (1974) Southwood (1977) Southwood et al. (1974) Stearns (1976) Stearns (1977) Stubbs (1977) Wilbur et al. (1974) Wilson (1975) Wilson and Bossert (1971)
Meaning b I II
III
x
IV
x x
x x x x x x ?
x
? • ? x x x x x x x x
x x ?
? x
x
x x ? x
x x x x • x x x x x x x x
? x
x
x x x x x x x • x x x x • x
x x ?
~ x ? ?
x ?
x ?
x x
x
•
x x
Those papers in which the terms r- and K-selection were mentioned but were not defined have not been included b A question mark indicates that I am uncertain whether the author uses the terms in the sense designated
other life-history parameters such as the m a x i m u m longevity and the strategy of dispersal, as well as indirectly influencing such i m p o r t a n t factors as the distribution a n d a b u n d a n c e of predators. Therefore, whilst it may often be usefuI to differentiate between the life-histories of species from temporary a n d permanent habitats to call such species r-selected or K-selected is likely to obscure the selective factors which have caused the differences.
262 Meaning I V
Meaning I, and perhaps Meanings II and III, suggest that the high population growth rates characteristic of r-selection should occur through the production of many small young, and that the more competitive environment characteristic of K-selection should result in larger young. If Meanings I/II/III are to be compatible with Meaning IV then it must be predicted that high reproductive effort should be associated with small young (r-selection), and low reproductive effort with large young (K-selection). (See e.g. Pianka 1972; Stearns 1976). Within the assumptions of the MacArthur and Wilson model (Meaning I) such an association may occur, but the complicating effects of various causes of age-specific mortality would be expected to obscure any such correlation in nature. Theoretical considerations suggest that the total expenditure of energy on reproduction should not influence the way this energy is packaged into offspring (Williams 1966 a, b; Schaffer and Gadgil 1975; Pianka 1976), except where the number of young is very small (Smith and Fretwell 1974). Empirical data suggest that there is no correlation between reproductive effort and egg size (reviewed by Wilbur 1977, but see also Calow 1978; Calow and Woollhead 1977; Bostock and Benton 1979). Thus if reproductive effort is used as the criterion of r- and K-selection (Meaning IV) then the predicted life-history consequences resulting from this usage will differ from those based on Meanings I, II and III. (See also Forsyth and Robertson 1975; Hickman 1975). Measurements of reproductive effort in a variety of animals and plants (reviewed by Calow 1979; but see also Haukiouja and Hakala 1978; Bell et al. 1979; Bryant 1979; Primack 1979) and theoretical considerations (Cody 1966; Williams 1966a, b; Gadgil and Bossert 1970; Schaffer 1974; Hirshfield and Tinkle 1975; Pianka and Parker 1975; Calow 1979) have suggested that high reproductive effort is correlated with high adult mortality rates and low reproductive effort with low adult mortality rates. - Gadgil and Solbrig (1972) were the first to closely link the concept of reproductive effort with r- and K-selection. They argued that r- and K-selection can be measured by the degree of DI versus DD mortality (Meaning II) and that the degree of DI versus DD mortality correlates with the amount of reproductive effort, hence reproductive effort measures r- and Kselection (Meaning IV). Empirical support for this correlation has been provided by studies of plant populations. Populations of plants from more disturbed sites (Gadgil and Solbrig 1972; Law et al. 1977) and from early successional stages (Gadgil and Solbrig 1972; Abrahamson and Gadgil 1973; Gaines et al. 1974) show higher reproductive effort than do populations from less disturbed or later successional stages. However, in these studies the amounts of DI and DD mortality were inferred rather than measured (Solbrig and Simpson 1974; Hickman 1975; Stearns 1977) and other possible explanations for the differences in reproductive effort were not considered. In particular, those sites in which DI mortalities were the highest were also, almost certainly, those in which adult mortality rates were also highest (Law 1979b). Thus differences in reproductive effort may be explained by reference to differences in the demography of populations and without recourse to DI versus DD mortality. Consequently, in view of the rather poorly established relationship between reproductive effort and DI versus DD mortality, and the differences between the predictions of Meanings I/II/III and Meaning IV, I regard the use of reproductive effort to measure rand K-selection as a confusing expansion of the original concept.
The apparent correlation between reproductive effort and age-specific mortality rates also suggests that we should be wary of using differences in age-dependent mortality rates to infer r- and K-selection (c. f. Pianka 1970, 1972; Horn 1978). Similarly as increased environmental unpredictability affects life-history parameters in much the same way as an increase in the mortality rates of the age-classes affected (Schaffer 1974; Horn 1978) we should also avoid suggesting that 'environmental unpredictability' will necessarily lead to r-selection (Stearns 1976; c.f. Pianka 1970). Several recent authors (e.g. McNaughton 1975; Derickson 1976) have termed the reasonably well-established (c.f. Stearns 1977) correlation between reproductive effort and age-specific mortality, "r- and K-selection". I regard this usage of the terms as completely unacceptable because (1) the association of reproductive effort and age-specific mortality derives from 'stochastic' models, which give alternative explanations for life-history characteristics to r- and K-selection (Stearns 1976, 1977). (2) the trade-off between reproduction and mortality would be more accurately described by the concept of b- and d-selection (Hairston et al. 1970) than by the terms r- and K-selection. (3) MacArthur (1972) termed the dichotomy in question "youth versus old-age selection", which he regarded as distinct from r- and K-selection. The concept of r- and K-selection has been responsible for stimulating much of the recent research into life-history patterns. But I believe that this concept has now out-lived its usefulness. The, originally justified, broadening of the concept in search of a more general synthesis (Pianka 1970, 1972), has left a legacy of vagueness that has already led to considerable confusion. I would recommend that further confusion is best avoided by reverting to using the terms r- and K-selection only in their original sense (Meaning I). However, for understanding patterns of life-histories in nature the applicability of such a definition appears to be strictly limited. Despite the many field studies in which r- and K-selection have been implicated, there do not appear to have been any studies in which the selection pressure responsible for the observed life-history characteristics can be unequivocably attributed to the amount of crowding. While the amount of crowding clearly constitutes an important selection pressure within a population its importance relative to other pressures still needs to be established. It is now apparent that there are many dimensions to a life-history pattern in addition to r- and K-selection (MacArthur 1972; Wilbur et al. 1974). Life-histories are shaped by constraints imposed by seasonality (Boyce 1979), habitat stability (Southwood et al. 1974; Whittaker and Goodman 1979), the need for dispersal (Horn 1978) in addition to predation (Wilbur et al. 1974), severe climatic stresses (Schaffer and Gadgil 1975; Grime 1979) and probably other factors. Recognition of the complex tradeoffs which determine life-history patterns is more likely to provide insight than attempts to fit these patterns into a "simple" but ill-defined concept. Acknowledgements. This paper was written whilst I was on leave at the Zoology Department, University of Glasgow. I thank Professor K. Vickerman for providing space and library facilities and Drs. P. Calow and P.S. Lake for helpful comments on the manuscript. References
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