Evidence for the rK concept

The r/K concept can certainly be useful in describing some of the general differences between taxa. For instance, amongst plants it is possible to draw up a number of very broad and general relationships (Figure 4.30). Trees, in relatively K-selecting woodland habitats (relatively constant and predictable), exhibit long life, delayed maturity, large seed size, low reproductive allocation, large individual size and a very high frequency of iteroparity. Whilst in more disturbed, open, r-selecting habitats, plants tend to conform to the general syndrome of r characteristics.

There are also many cases in which populations of a species, or of closely related species, have been compared, and the correspondence with the r/K scheme has been good. For instance, this is true of a study of Typha (cattail or reed mace) populations (Table 4.7). Individuals of a southerly species, T. domingensis, and a northerly species, T. angustifolia, were taken from sites in Texas and North Dakota, respectively, and were grown side by side under the same conditions. In addition, certain aspects of the habitats, with long and short growing seasons, in which these species are found were quantified. It is clear from Table 4.7 that the former were relatively K-selecting and the latter relatively r-selecting. It is equally clear that the species inhabitating these sites conform to the r/K scheme. T. angustifolia (which naturally has a short growing season) matures earlier (trait 1), is smaller (traits 2 and 3), makes a larger reproductive allocation (traits 3 and 6) and produces more and smaller offspring (traits 4 and 5) than does T. domingensis (long growing season).

There are, then, examples that fit the r/K scheme. Stearns (1977), however, in an extensive review of the data available at that time, found that of 35 thorough studies, 18 conformed to the scheme whilst 17 did not. We might regard this as a damning criticism of the r/K concept, since it undoubtedly shows that the explanatory powers of the scheme are limited. On the other hand, a 50% success rate is hardly surprising given the number of additional factors already described (or to be described) that further our understanding of life history patterns. It is therefore equally possible to regard it as very satisfactory that a relatively simple concept can help make sense of a large proportion of the multiplicity of life histories. Nobody, though, can regard the r/K scheme as the whole story.

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