We discussed MacArthur and Wilson's theory of island biogeography extensively in the last chapter. Another application of their findings was the theory of r- and K-selection as applied to life histories. MacArthur and Wilson (1967) asserted that for "colonizing species" the ability to grow rapidly and to disperse was the major component of fitness. They theorized that new, unpopulated environments would be colonized first by these "r-selected" species. On the other hand, once an environment, either an island or the mainland, was completely colonized, it became densely populated (crowded or saturated), with all populations near their carrying capacities (K). They reasoned that such populations were exposed to "K-selection." The major component of fitness in a crowded environment was the ability, not to grow quickly, but to survive under these highly competitive conditions.
According to the theory, in an environment with no crowding (an r-selecting environment), those genotypes which harvested the most resources and produced the most offspring in the shortest period of time, even if they were not efficient at using their resources, would be favored. In the crowded, K-selecting environment, those genotypes
Table 6.3 Correlates of r- and K-selection. Adapted from Pianka (1970).
Climate Mortality Survivorship Population size Recolonization frequency Competitive ability Investment in defense
Parental care Length of life Stage of succession Rate of development r
Pre-reproductive period Body size
Number of offspring Size of offspring Dispersal ability r-selection
Variable or unpredictable Density-independent Type III
Variable, below K, Re-colonization common Usually poor Little energetic investment Minimal Short Early Rapid High Short Small Many Small Excellent
Predictable, less variable Density-dependent Type I or II
Fairly constant; at or near K, Re-colonization uncommon Usually keen
Great energetic investment
Fair to poor which replaced themselves at low resource levels would survive and reproduce. Evolution was said to favor efficiency in these ^-selecting environments.
Thus r-selected species would be favored in areas where there has been a disturbance of the established community. ^-selected species would be favored in areas, such as mature communities, where competition is high. It is natural to think of r- and ^-selection in terms of ecological succession, in which colonization and competition are assumed to be the primary forces determining the appearance of plant communities over time. r-selected species appear early in succession, to be eventually replaced by K-selected species in the later stages.
Biologists were led to consider what aspects of life histories and of the physical environment would be correlated with r- or ^-selection. In 1970 Pianka published a paper entitled "On r- and ^-selection," which laid out general expectations of r- and ^-selection for a number of life-history traits. Table 6.3 is adapted from that paper with modifications and embellishments.
In evaluating the theory of r- and ^-selection, we should recognize that it is based on the assumption that life histories have evolved as a response to the twin selective pressures of competition and colonizing or dispersal ability. As described in section 6.9, life histories that appear to be r-selected may have evolved in response to predation or to the uncertainties of the physical environment (Wilbur et al. 1974). Caution must therefore be used in making assumptions about the selective pressures that have shaped a particular life history. Nevertheless, the terms r- and ^-selection are ingrained in the ecological lexicon and have retained a certain utility as handy shorthand for a particular suite of life-history traits.
In a review of the r- and ^-selection theory, Reznick et al. (2002) gave it credit for stimulating a great variety of empirical and theoretical work on life-history evolution. An important aspect of the theory was its focus on density-dependent selection as an important force in the shaping of life histories. But r- and ^-selection was a starting point. Now it is clear that age-specific mortality, resource limitation, predation, environmental variation, metabolic rate, and density-independent factors all must be included in models of life-history evolution.
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