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Life History Theory and the Equilibrium Status of Populations

Hal Caswell
The American Naturalist
Vol. 120, No. 3 (Sep., 1982), pp. 317-339
Stable URL: http://www.jstor.org/stable/2461057
Page Count: 23
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Life History Theory and the Equilibrium Status of Populations
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Abstract

Life history traits are often used to distinguish equilibrium from nonequilibrium populations. This is invalid, both within the framework of r- and K-selection theory and within a demographic model which takes age structure into account. In both cases, the patterns favored in an equilibrium population are even more intensely favored in a nonequilibrium population which happens to spend most of its history in a state of population decline. The only characteristic which seems to distinguish equilibrium populations is a lower absolute magnitude of selective pressure on some traits. The traits favored in declining populations include long lifespan, slow development, delayed reproduction, iteroparity, low degree of senescence, and perhaps a relatively high degree of investment in whatever offspring are produced. The traits favored in increasing nonequilibrium populations are largely the opposite: short life, fast development, and early reproduction, semelparity, senescence, and perhaps less investment in individual offspring. There appears to be an evolutionary instability in the center of the gradient from predominantly increasing to predominantly decreasing populations. Over evolutionary time, nonequilibrium populations will diverge in one direction or the other. This model predicts the existence of suites of species with distinctly "declining" (i.e., K-selected) characteristics, and other groups with distinctly "increasing" (i.e., r-selected) characteristics. Because the same traits that increase the proportion of its history that a population spends in decline also increase the time required for extinction, the former group will occupy relatively stable habitats, the latter group more disturbed habitats. This requires no equilibrium assumptions; all of the species involved may be local losers. The predictions of the model are supported by an analysis of the relation between competitive tolerance, age at first reproduction, and life span in North American trees.

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