Neglected dipterans in stream studies

Abstract

True flies comprise approximately one-tenth of all animal species on Earth, yet despite their prevalence and ecological significance in freshwater ecosystems, members of the insect order Diptera are frequently neglected in stream studies. This absence or inconsistency regarding Diptera in literature and taxonomic lists may leave readers with a sense of discrepancy. To illustrate this underrepresentation in quantitative ecological investigations, we conducted a targeted literature-based meta-analysis, assessing the average level of Diptera identification and the reported number of families. These findings were compared to data from 639 quantitative samples collected across six European ecoregions (Mediterranean, Alpine, Continental, Balkanic, Pannonian, Boreal) during six, bimonthly repeated sampling campaigns in 2021 and 2022. Our analysis revealed that, compared to other macroinvertebrate groups, Diptera were typically identified at a less detailed level, often only to the family level, thereby failing to fully represent Diptera diversity, especially regarding rare, less abundant families. In our review of literature studies, we identified references to a total of 40 families. Notably, Chironomidae, Ceratopogonidae, and Simuliidae were consistently represented across the majority of studies, whereas nearly half of the families were exclusively mentioned in one or two studies. No significant differences were found in the number of families across continents or various habitat types. In our case studies the number of families was significantly higher than in European stream studies, suggesting that several rare families occasionally completely neglected during sampling, sample sorting or identification. We explored potential connections among Diptera assemblages through correlation and coexistence analyses. Our results highlighted the significant influence of the more frequent Chironomidae, Ceratopogonidae, and Simuliidae on the presence or absence of other families. While correlations between Diptera families were identified, attempts to develop a predictive model for the diversity and occurrence of minor families based on the abundance of major ones proved inconclusive. For future quantitative studies on macroinvertebrate communities, it is essential to recognize, identify and incorporate less abundant Diptera families, even on family level, or in higher taxonomic resolution, if possible,  to enhance understanding and prevent the loss of information concerning this compositionally and functionally uniquely diverse insect group, which represent a significant part of the entire community, and gain a better understanding on their interactions with other aquatic groups.