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 dataset 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. The main drivers of this pervasive insect decline were increased concurrent invasions of non-native insectivorous fish, water transparency and changes to water stoichiometry (i.e. N : P ratios) over time. All these drivers represent human impacts caused by reservoir construction. This work sheds light on the importance of long-term studies for a deeper understanding of human-induced impacts on aquatic insects. We highlight that extended anthropogenic impact monitoring and mitigation actions are pivotal in maintaining freshwater ecosystem integrity.

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The role of multitrophic diversity in sustaining multiple functions simultaneously (multifunctionality) is still poorly understood in natural communities, especially in highly diverse aquatic ecosystems. Existing studies have focused on the effect of single trophic group on ecosystem function and on individual ecosystem functions, neglecting the fact that multiple trophic groups are needed to maintain ecosystem multifunctionality. Here, using a 16‐year database from tropical shallow lakes, we combined species richness of nine single trophic group into a unique measurement of multitrophic richness. We then investigated the influence of the richness within separate single trophic group and in a multitrophic context on ecosystem multifunctionality. We also investigated how removal of any single trophic group influence the effect of the multitrophic richness on multifunctionality; and how the interactions among multiple single trophic group affect multifunctionality. We showed that the multitrophic richness had a stronger positive effect on multifunctionality than the richness of single trophic group. The removal of each single trophic group decreased the effect of the multitrophic richness on multifunctionality. The larger predatory vertebrates and primary producers had stronger positive effects on multifunctionality, but the richness of basal trophic groups fuelled the large‐sized predators, thus indirectly contributing to increase multifunctionality. Our study highlights the need for preserving multiple trophic groups to sustain multifunctionality in highly diverse aquatic ecosystems; thus, trophic degradation of the ecosystems should strongly impair their functioning. A free Plain Language Summary can be found within the Supporting Information of this article.

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