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.