Testing the performance of bats as indicators of riverine ecosystem quality

Rivers host a wealth of biodiversity and play critical ecological functions, so monitoring their environmental status and tracking its changes has paramount importance for appropriate management. Although some biological groups, and especially benthic macroinvertebrates, are employed routinely to achieve this goal, the quest for bioindicators of river quality is far from over, because finding further suitable organisms may improve indication performances and inform habitat management. Using organisms that are at risk for bioindication also fulfills the goal of providing important information for the conservation of the taxon (or taxa) used for bioindication. Bats are a diverse, threatened mammal group whose characteristics make them potentially suitable bioindicators in many ecosystem types, but research in this field is still limited. In this study, set in Central and Southern Italy, we hypothesize that assemblages of foraging bats will respond to environmental status and quality of riverine ecosystems and that therefore bats may serve as effective bioindicators. We established the environmental status of 50 sampling sites selected along 10 rivers using two indices officially adopted in the country, i.e. the STAR_ICMi (evaluating water quality from macrobenthic invertebrate assemblages) and the fluvial functionality index (Indice di Funzionalità Fluviale, IFF), which incorporates several biotic and abiotic components and represents a functional indicator of river ecosystem health. At the sampling sites, we also recorded bat activity with operator-independent real-time bat recorders and classified bat passes to species or phonic groups. We examined 167,371 macroinvertebrates and 55,157 bat passes, corresponding to 15 species or phonic groups. The activity of Miniopterus schreibersii/Pipistrellus pygmaeus and Myotis daubentonii/capaccinii declined with increasing values of STAR_ICMi and IFF, while the activity of Nyctalus/Eptesicus serotinus increased with both indices. The activity of P. kuhlii also declined as IFF values increased, while we observed the opposite for Pipistrellus pipistrellus, Myotis emarginatus, Myotis nattereri and Barbastella barbastellus. Pooling together species whose activity respectively increased or decreased as the values of quality indices increased improved indication performances by strengthening statistical significance. Our work constitutes a significant step towards the use of bats as bioindicators in river ecosystems as we show that differences in bat activity may reveal changes in environmental conditions and may thus demonstrate the effects of habitat alteration on the river biota. We highlight that locally adapted bat populations may show differences in foraging behaviour and food preferences; hence our findings warrant confirmation from other regions. Further constraints are given by the variable degree of taxonomic resolution achieved in bat sound analysis, which may represent an issue especially in species-rich bat assemblages such as those of southern Europe.