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.