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

研究了两种生境类型淡水蜗牛（Physa acuta）的生长和繁殖率。在Asabata栖息地，蜗牛生活在孤立的水池中，这些水池偶尔会结合成一个大水池；在Kakegawa栖息地，它们生活在缓慢流动的水中。利用三个微卫星位点进行的遗传结构评估支持稳定的泛疫种群占据Kakegawa栖息地的观点；而交替混合种群，个体在斑块内相互作用和繁殖，但偶尔在大型全球种群中混合，占据Asabata栖息地。从两个栖息地收集的蜗牛进行的实验室实验表明，浅坂栖息地的幼蜗牛比Kakegawa栖息地的蜗牛生长得更快，下蛋更多，下蛋时间也更早。高密度生境的生长速率低于低密度生境；浅坂栖息地的情况正好相反。钉螺个体繁殖的密度依赖性响应在两种生境中是相同的。研究结果支持了空间结构影响密度依赖性生长速率和繁殖时间进化的假设。

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.