Resource diversity promotes among-individual diet variation, but not genomic diversity, in lake stickleback

Many generalist species consist of specialized individuals that use different resources. This within-population niche variation can stabilize population and community dynamics. Consequently, ecologists wish to identify environmental settings that promote such variation. Theory predicts that environments with greater resource diversity favor ecological diversity (via disruptive selection or plasticity). Alternatively, niche variation might be a side-effect of neutral genomic diversity in larger populations. We tested these alternatives in a metapopulation of threespine stickleback. Stickleback consume benthic and limnetic invertebrates, focusing on the former in small lakes, the latter in large lakes. Intermediate-sized lakes support generalist stickleback populations using an even mixture of the two prey types, due to greater among-individual variation in diet and morphology. In contrast, genomic diversity increases with lake size. Thus, phenotypic diversity and neutral genetic polymorphism are decoupled: trophic diversity being greatest in intermediate-sized lakes with high resource diversity, whereas neutral genetic diversity is greatest in the largest lakes.许多广食性物种由利用不同资源的特化个体组成。此种种群内的生态位分化可稳定种群与群落动态。据此，生态学家致力于识别能够促进此类分化的环境条件。理论预测，资源多样性更高的环境可通过干扰选择（disruptive selection）或表型可塑性（plasticity）推动生态多样性的形成。另有假说指出，种群内的生态位分化或许是大种群中性基因组多样性的附带效应。我们在三棘刺鱼（threespine stickleback）的集合种群（metapopulation）中对这两种假说进行了验证。三棘刺鱼以底栖（benthic）与浮游（limnetic）无脊椎动物为食：在小型湖泊中它们主要捕食底栖无脊椎动物，在大型湖泊中则优先取食浮游无脊椎动物。中型湖泊栖息着广食性三棘刺鱼种群，该种群的个体均匀利用两类猎物，这源于个体间在食性与形态上存在更高的变异水平。与之相反，基因组多样性随湖泊面积增大而升高。由此可见，表型多样性与中性遗传多态性（neutral genetic polymorphism）呈现解耦关系：营养多样性在资源多样性较高的中型湖泊中达到峰值，而中性遗传多样性则在最大的湖泊中最高。