Biotic differentiation in headwater creeks after the massive introduction of non-native freshwater aquarium fish in the Paraíba do Sul River basin, Brazil

Abstract This study evaluated fish beta diversity in six headwater creeks located in the area affected by the largest ornamental aquaculture center implemented in the Minas Gerais State, southeastern Brazil. We sampled fish assemblages in 2017 and 2018 to investigate changes in assemblage structure (species richness and beta diversity), comparing these data with the historic species pool. We recorded 60 fish species, of which 16 were native and 44 non-native with 19 translocated, and 25 exotic. The exotics Poecilia reticulata, Xiphophorus maculatus, X. variatus, Danio rerio, and Misgurnus anguillicaudatus were the most widely distributed in the headwater creeks. The Contamination Index showed that most creeks had high proportional contamination by exotic species (above 60%). Beta diversity increased from historical to contemporary periods in all creeks due to the introduction and differential colonization pressure of several non-native translocated and exotic species, indicating biotic differentiation. Temperature and number of ponds were the main preditors of change in beta diversity in the headwater creeks during the contemporary period. In summary, we observed that invaders have induced substantial changes to fish communities under influence of environmental filters. Our results support the hipothesis that aquaculture is a main driver of fish non-native fish introduction and native biodiversity loss in the Neotropics.

摘要
本研究针对巴西东南部米纳斯吉拉斯州境内规模最大的观赏水产养殖中心影响区域内的6条源头溪流，开展鱼类β多样性（beta diversity）评估。研究于2017年与2018年采集鱼类群落样本，以调查群落结构（包括物种丰富度与β多样性）的变化情况，并将所得数据与历史物种库（historic species pool）进行对比。
本次调查共记录鱼类60种，其中本土物种16种、非本土物种44种，包含19种移殖种（translocated species）与25种外来种（exotic species）。分布最为广泛的外来种为孔雀花鳉（Poecilia reticulata）、剑尾鱼（Xiphophorus maculatus）、杂色剑尾鱼（X. variatus）、斑马鱼（Danio rerio）及泥鳅（Misgurnus anguillicaudatus）。
污染指数（Contamination Index）结果显示，多数溪流的外来物种占比超过60%，污染程度较高。由于多种非本土移殖种与外来种的引入及差异化定殖压力，所有溪流的β多样性自历史时期至当代均有所提升，表明群落发生了生物分异（biotic differentiation）。在当代研究时段内，水温与池塘数量是调控源头溪流β多样性变化的主要驱动因子。
综上，本研究发现入侵物种在环境过滤（environmental filters）作用下，对鱼类群落造成了显著改变。本研究结果支持“水产养殖是新热带区（Neotropics）鱼类非本土物种引入与本土生物多样性丧失的核心驱动因素”这一假说。