Fragmentation by artificial barriers is an important threat to freshwater biodiversity. Mitigating the negative aftermaths of fragmentation is of crucial importance, and it is now essential for environmental managers to benefit from a precise estimate of the individual impact of weirs and dams on river connectivity. Although the indirect monitoring of fragmentation using molecular data constitutes a promising approach, it is plagued with several constraints preventing a standardized quantification of barrier effects. Indeed, observed levels of genetic differentiation GD depend on both the age of the obstacle and the effective size of the populations it separates, making comparisons of the actual barrier effect of different obstacles difficult. Here, we developed a standardized genetic index of fragmentation (F_INDEX), allowing an absolute and independent assessment of the individual effects of obstacles on connectivity. The F_INDEX is the standardized ratio between the observed GD between pairs of populations located on either side of an obstacle and the GD expected if this obstacle completely prevented gene flow. The expected GD is calculated from simulations taking into account two parameters: the number of generations since barrier creation and the expected heterozygosity of the populations, a proxy for effective population size. Using both simulated and empirical datasets, we explored the validity and the limits of the F_INDEX. We demonstrated that it allows quantifying effects of fragmentation only from a few generations after barrier creation and provides valid comparisons among obstacles of different ages and populations (or species) of different effective sizes. The F_INDEX requires a minimum amount of fieldwork and genotypic data and solves some of the difficulties inherent to the study of artificial fragmentation in rivers and potentially in other ecosystems. This makes the F_INDEX promising to support the management of freshwater species affected by barriers, notably for planning and evaluating restoration programs.

中文翻译：

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量化淡水中人工屏障的个体影响：碎片化的标准化绝对遗传指数

人为障碍造成的破碎是对淡水生物多样性的重要威胁。减轻碎片带来的负面后果至关重要，现在对于环境管理人员来说，准确评估堰坝和水坝对河流连通性的影响至关重要。尽管使用分子数据间接监测片段化是一种有前途的方法，但是它受到一些阻碍阻碍屏障效应标准化量化的约束的困扰。实际上，观察到的遗传分化水平GD取决于障碍物的年龄及其分离的种群的有效大小，因此很难比较不同障碍物的实际障碍作用。在这里，我们开发了标准化的片段化遗传指数（F _INDEX），可以绝对独立地评估障碍物对连通性的影响。所述˚F _INDEX是所观察到的之间的标准化比率GD位于障碍物的任一侧种群对和间GD预期如果这个障碍完全防止基因流动。预期的GD是通过模拟计算得出的，其中考虑了两个参数：自创建壁垒以来的世代数和预期的种群杂合性，这是有效种群规模的代名词。使用模拟和经验数据集，我们探索了F _INDEX的有效性和局限性。我们证明了它仅允许量化障碍创建后几代内的碎片效应，并提供不同年龄和不同有效大小的种群（或物种）的障碍之间的有效比较。该˚F _INDEX需要实地考察和基因型数据的最小量，并解决了一些固有的人造碎片的河流研究，并可能在其他生态系统的困难。这使得F _INDEX有望支持对受障碍影响的淡水物种的管理，特别是用于规划和评估恢复计划。