Revisiting a 50-year-old ecological study of Ephemeroptera, Plecoptera, and Trichoptera species in Estaragne River (French Pyrenees): why taxonomy matters?

Long-term ecological surveys (LTES) and historical species inventories are essential for assessing changes in biodiversity. However, comparisons over time may be heavily biased if they disregard the evolution of taxonomic knowledge. To investigate, at a later date, changes in the distribution of aquatic species over the last 50 years, we revisited the Ephemeroptera, Plecoptera, and Trichoptera (EPT) species inventory conducted by Pierre Lavandier in the 70’s in the Estaragne, a subalpine and alpine catchment area in the French Central Pyrenees. In 2022, we sampled the same twelve stream sites during three seasonal campaigns, collecting both aquatic (larvae, nymphs and pupae) and aerial (imago) stages. Taxa were identified morphologically by EPT experts and cross-validated by external specialists for challenging species. Molecular analyses were performed to barcode unreferenced species and examine uncertain species affiliations. Finally, we proposed an approach to assess the autochthony of EPT species at the twelve sites. We thus updated the EPT species inventory performed by Lavandier according to advances in taxonomic knowledge, providing a 2022 inventory with detailed justification of taxa assignment. Of the 55 EPT taxa (52 at the species level) validated in the 70’s inventory, only 64% retained an unchanged taxonomic status; 36% required modifications, including reassignment (70%), revision (15%), and reduction to a higher taxonomic level (15%). In 2022, we validated 80 EPT taxa (65 at the species level), although several affiliations may change as taxonomic knowledge advances. Here, we provide detailed comments on our taxonomic assignments and distribution observations for 44 challenging and/or poorly known taxa. This reveals historical taxonomic errors, uncertain affiliations, potential undescribed, complex, or cryptic species, and highlights significant gaps in taxonomic knowledge for these aquatic insect orders, which are among the most widely studied in Europe. Based on current taxonomic knowledge, 70 EPT taxa collected in 2022 can be compared with the 55 updated EPT taxa collected in the 70’s. Our study demonstrates that LTES critically depend on transparent and revisable taxonomy. We therefore advocate for aquatic inventories that document species autochthony, preserve reference specimens from all life stages, and provide FAIR taxonomic datasets, including complete sampling metadata and the taxonomic references used for morphological identification. Finally, this work underscores the pivotal role of taxonomy in biodiversity science and LTES.