Damming of large rivers has promoted well-documented ecosystem changes that constitute critical threats to freshwater fish biodiversity. However, it is unclear how trait–environment relationships drive community assembly in older (>50 years) tropical river-reservoir systems. We assessed physical features filtering functional traits (RLQ and fourth-corner approaches) in seven tropical reservoirs along a major river, in southeastern Brazil, and correlated taxonomic and functional counterparts of local and beta diversity to assess niche-based assembly processes under prevailing environmental conditions (null model approach). Lower littoral habitat availability (smaller area, volume, and riverine influence) favored species inhabiting open areas that feed on the bottom (mostly migratory species). Downstream position (lower elevation and inflow of water from the mainstem) allowed migrators to enter the reservoirs and promoted environmental heterogeneity, favoring detritivores, invertivores, and carnivorous generalists. Total blocking of the main channel excluded most migratory species from reservoirs at upstream reaches, whereas higher riverine influence and hydrological stability (larger area, volume, and water residence time) favored small-sized sedentary species (omnivores and piscivores with parental care, mostly non-native species) in the littoral zone. Correlations differing from null expectations for alpha diversity and nestedness revealed that niche-based processes support species richness and loss under higher riverine influence and hydrological stability. Our study supports the importance of physical features to set functional spaces available for fish species, but random processes that boost taxonomic differences between reservoirs, largely related to non-native species introductions, are the primary drivers of fish assemblage structure in tropical reservoirs long-term disconnected from riverine dynamics.

中文翻译：

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揭示热带水库中基于性状的鱼类群落组合

大河流的筑坝促进了有据可查的生态系统变化，对淡水鱼类生物多样性构成了严重威胁。然而，目前尚不清楚性状与环境的关系如何驱动较古老（> 50 年）的热带河流-水库系统中的群落组装。我们评估了巴西东南部一条主要河流沿岸七个热带水库的物理特征过滤功能特征（RLQ 和第四角方法），并关联了本地和 beta 多样性的分类学和功能对应物，以评估当前环境条件下基于生态位的组装过程（空模型方法）。较低的沿海栖息地可用性（较小的面积、体积和河流影响）有利于栖息在开放区域、以海底为食的物种（主要是迁徙物种）。下游位置（海拔较低，水从干流流入）允许洄游者进入水库并促进环境异质性，有利于食碎屑动物、倒食动物和肉食性通才。主河道的完全堵塞将大多数迁徙物种排除在上游水库之外，而较高的河流影响和水文稳定性（较大的面积、体积和水停留时间）有利于小型定居物种（杂食动物和食鱼动物，有父母照顾，大多是非- 沿海地区的本地物种。与阿尔法多样性和嵌套性零预期不同的相关性表明，基于生态位的过程在较高的河流影响和水文稳定性下支持物种丰富度和损失。我们的研究支持物理特征对于设置鱼类可用功能空间的重要性，但增加水库之间分类差异的随机过程（很大程度上与非本地物种的引入有关）是热带水库鱼类组合结构长期的主要驱动因素与河流动态脱节。