Pervasive decline of subtropical aquatic insects over 20 years driven by water transparency, non-native fish and stoichiometric imbalance

Insect abundance and diversity are declining worldwide. Although recent research found freshwater insect populations to be increasing in some regions, there is a critical lack of data from tropical and subtropical regions. Here we examine a 20-year monitoring data set of freshwater insects from a subtropical floodplain comprising a diverse suite of rivers, shallow lakes, channels and backwaters. We found a pervasive decline in abundance of all major insect orders (Odonata, Ephemeroptera, Trichoptera, Megaloptera, Coleoptera, Hemiptera and Diptera) and families, regardless of their functional role or body size. Similarly, Chironomidae species richness decreased over the same time period. Increased invasions of non-native insectivorous fish, water transparency and changes to water stoichiometry (i.e., N:P ratios) over time were the main drivers of this pervasive insect decline. All these drivers represent human impacts caused by reservoir constructions. This work sheds light on the importance of long-term studies for deeper understanding of human-induced impacts on aquatic insects. We highlight that anthropogenic impact monitoring and mitigation actions are pivotal in maintaining freshwater ecosystem integrity.

全球范围内，昆虫的丰度与多样性正持续衰退。尽管近期研究发现部分区域的淡水昆虫种群数量有所上升，但热带与亚热带地区的相关数据仍严重匮乏。本研究针对一处亚热带泛滥平原的淡水昆虫展开了为期20年的监测数据集分析，该区域涵盖河流、浅湖、河道与回水湾等多样水生生境。研究结果显示，所有主要昆虫目（蜻蜓目（Odonata）、蜉蝣目（Ephemeroptera）、毛翅目（Trichoptera）、广翅目（Megaloptera）、鞘翅目（Coleoptera）、半翅目（Hemiptera）及双翅目（Diptera））及其下属科的丰度均出现普遍下降，且该趋势与其功能角色或体型大小无关。同样，摇蚊科（Chironomidae）的物种丰富度在同期也呈下降趋势。入侵性食虫鱼类种群扩张、水体透明度提升，以及水体化学计量学（water stoichiometry）随时间发生的改变（即氮磷比（N:P ratios）），是此次淡水昆虫普遍衰退的主要驱动因素。上述所有驱动因素均由水库建设引发的人类活动所导致。本研究凸显了长期监测研究对于深入理解人类活动对水生昆虫造成的影响的重要意义。我们强调，开展人类活动影响监测并采取相应减缓措施，对于维持淡水生态系统的完整性至关重要。