Habitat Structure and Evolution of density-dependence ; with regard to evolutionary significance of Population fluctuation

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We examined the growth and reproductive rates of freshwater snails, Physa acuta, in two habitat types. In the Asabata habitat, snails lived in isolated water pools, which occasionally joined to form a single large pool ; in the Kakegawa habitat, they lived in a slow-running water way. Genetic structure assessments using three microsatellite loci supports the idea that a stable panmictic population occupies the Kakegawa habitat ; whereas alternate mixing populations, with individuals interacting and reproducing within patches, but occasionally mixing within a large global population, occupy the Asabata habitat. Laboratory experiments, using snails collected from the two habitats, showed that juvenile snails grew faster, laid more eggs, and laid them earlier in the Asabata habitat than in the Kakegawa habitat. Growth rates were lower at high density than at low density in the Kakegawa habitat ; the inverse was true in the Asabata habitat. Density-dependent response of individual snail reproduction was the same for the two habitats. The results support the hypothesis that spatial structure affects the evolution of density-dependent growth rates and of timing for reproduction.