Dams and their associated impoundments may restrict dispersal and gene flow among populations of numerous freshwater species within stream networks, leading to genetic isolation. This can reduce effective population sizes and genetic diversity, increasing the risk of local extinction. We studied crayfishes from multiple up‐ and downstream sites in three impounded and two unimpounded streams in the Bear Creek and Cahaba River drainages, Alabama, U.S.A. Using mitochondrial DNA (cytochrome oxidase subunit I gene) sequence data generated from population‐level sampling of two abundant native crayfishes, Faxonius validus and Faxonius erichsonianus (Decapoda: Cambaridae), we assessed species’ spatial genetic structure and genetic diversity, estimated the magnitude and directionality of gene flow, and compared results between the species. For both species, levels of genetic diversity (number of haplotypes, and haplotypic and nucleotide diversity) were the same or higher in impounded compared to unimpounded streams. Conversely, crayfish populations in up‐ and downstream sections of unimpounded streams displayed high genetic similarity and bidirectional gene flow, whereas in impounded streams, crayfish populations typically had greater up‐ and downstream genetic differentiation and predominantly unidirectional, downstream gene flow. Although impoundments were associated with lower connectivity between up‐ and downstream sections for F. validus and F. erichsonianus, the magnitude of genetic effects was species‐specific, with greater differentiation between F. validus populations up‐ and downstream of impoundments. In an ecologically short timeframe, impoundments appear to have fragmented stream crayfish populations, and even species with relatively high abundances and large ranges had lower gene flow among populations in impounded streams compared to unimpounded streams. In addition, feedbacks between genetic and demographic effects on fragmented populations may decrease the probability of long‐term persistence.

中文翻译：

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小龙虾种群在被蓄水 36-104 年的溪流中基因破碎

大坝及其相关的蓄水池可能会限制河流网络内众多淡水物种种群之间的扩散和基因流动，从而导致基因隔离。这会减少有效种群规模和遗传多样性，增加局部灭绝的风险。我们研究了来自美国阿拉巴马州 Bear Creek 和 Cahaba 河流域的三个蓄水和两个未蓄水河流中多个上游和下游地点的小龙虾。 Faxonius validus 和 Faxonius erichsonianus (Decapoda: Cambaridae)，我们评估了物种的空间遗传结构和遗传多样性，估计了基因流的大小和方向，并比较了物种之间的结果。对于这两个物种，与未蓄水的流相比，蓄水的遗传多样性水平（单倍型的数量，以及单倍型和核苷酸的多样性）相同或更高。相反，未蓄水流上游和下游部分的小龙虾种群表现出高度的遗传相似性和双向基因流，而在蓄水流中，小龙虾种群通常具有更大的上下游遗传分化，并且主要是单向下游基因流。尽管蓄水与 F. validus 和 F. erichsonianus 上下游部分之间较低的连通性有关，但遗传效应的程度具有物种特异性，蓄水上游和下游 F. validus 种群之间的差异更大。在生态上很短的时间内，蓄水的溪流小龙虾种群似乎支离破碎，即使是丰度相对较高和分布范围较大的物种，与未蓄水的溪流相比，蓄水溪流中种群之间的基因流动也较低。此外，遗传和人口统计对分散种群的影响之间的反馈可能会降低长期持续存在的可能性。