The theory of r-selection and K-selection [...] helped to galvanize the empirical field of comparative life-history and dominated thinking on the subject from the late 1960s through the 1970s. [. ..] By the early 1980s, sentiment about the theory had changed so completely that a proposal to test it or the use ofit to interpret empirical results would likely be viewed as archaic and nai've. (Reznick etal. 2002, p. 1509)
Similarly, Roff wrote: ''it may be preferable to avoid use of the terms [r- and K-selection] altogether'' (2002, p. 79). As these quotes show, the r/K concept was very important in the past but today, it is in its whole basically not used anymore. It has fallen into disfavor due to the recognition of several problems, especially the following three: (1) The concept's assumption of a tradeoff between r and K is often not valid. (2) The parameter K is not directly biologically interpretable. (3) The life-history traits attributed to K-strategists are not justifiable, neither theoretically nor empirically.
1. The assumption of a tradeoff between r and Kis often not valid. The r/K concept assumes that r selection and K selection are in opposition although there is no logical necessity to this assumption. A tradeoff between r and K has not often been found empirically, either. Although it has received some support in experiments with Drosophila melanogaster, this was not the case for experiments with
Escherichia coli, the rotifer species Asplanchna brightwelli, or the cladoceran Bosmina longirostris.
2. The parameter K is not directly biologically interpretable. While r is the difference between per capita birth rate and death rate at very low population densities and can be directly related to the life history of individuals, K is quite a complex parameter: it is meant to give the maximum number of individuals that a given environment can sustain under constant conditions. This phenomenological parameter cannot be determined in natural populations and is thus not directly biologically interpretable. In models, K is defined as the unstable or stable point of equilibrium where death rates equal birth rates (dN/dt = 0 in time-continuous models, Nt+1 = Nt in time-discrete models). In real populations, such points of equilibrium are rarely constant over time. How K relates to life-history traits is indefinable, too. Stearns in 1977 wrote: ''K is not a population parameter, but a composite of a population, its resources, and their interaction. Calling K a population trait is an artifact of logistic thinking, an example of Whitehead's Fallacy of Misplaced Concreteness. Thus r and K cannot be reduced to units of common currency.'' In other words, the r/K concept is comparing apples and oranges.
3. The life-history traits attributed to K-strategists are not justifiable. There is no reason why species living in constant environments should have the combination of traits proposed by Pianka (Table 1). The linkage between the environment and the life history made by Pianka is at the heart of the r/K concept but has never been theoretically justified, neither by Pianka nor anybody else. To achieve a high r, a species can either maximize its birth rate and/or minimize its death rate, and the corresponding strategies will result in different life-history traits. This ambiguity questions the life-history attributes of r-strategists. But while these attributes can still be logically defended, the main reason for the traits of K-strategists seems to have been the intuitive assumption that they should be the opposite of those of r-strategists. As mentioned above, however, there is no necessary tradeoff between r and K. When we take a closer look at the attributes, we may for example ask why, as claimed by Pianka, a population of large aggressive individuals should have a higher carrying capacity than a population of small peaceful individuals. Larger individuals need more resources than smaller ones, so a given amount of resources provided by the environment can be used either by a small number of large individuals or a large number of small individuals. Under many circumstances we can therefore expect a smaller carrying capacity for larger individuals, while the r/K concept claims the opposite. In defending the concept, we could reinterpret K and measure it in biomass rather than individuals. This trick does not help us with the problem of aggressiveness, however. Intraspecific aggression should often lead to a smaller population size and thus a smaller carrying capacity, again in contrast to the r/K concept. The linkage between the environment and life history made by Pianka does not only lack theoretical but also empirical support. For example, when fruit flies (D. melanogaster) were reared at low or high densities, the low-density lines evolved a higher capability to increase in population size at low densities but a lower capability to increase at high densities. In the high-density lines, the fly larvae were more competitive due to a higher feeding rate and pupation at a greater height above the medium compared to low-density lines. These experimental results are in accordance to the general predictions of the r/K concept about the differences between selection at low versus high densities. However, they are not in accordance to the explicit predictions about the linkage of these differences to specific life-history traits. The same is true for experiments with pitcher-plant mosquitoes (Wyeomyia smithii) where differences in population densities again led to differences in competitive ability but not to differences in life-history traits. Thus, a direct relationship between population density or fluctuation on the one hand and life-history characteristics on the other hand, as proposed by the concept, has not been established. Although it is true that life-history patterns do exist, the r/K concept cannot explain them.
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