Inter- and intraspecific niche differences of four co-occurring Baetidae (Insecta: Ephemeroptera) in a mountain stream

Abstract Quantifying niche occupation and breadth among co-occurring aquatic species is a key challenge in ecology, especially in alpine streams where environmental variability can mask or reduce the effects of competition. This study investigated the niche position, breadth and overlap of four co-occurring mayfly morphospecies, Baetis alpinus (F. J. Pictet, 1843), Baetis melanonyx (F. J. Pictet, 1843), Baetis rhodani (F. J. Pictet, 1843) and Alainites mutic us (Linnaeus, 1758), across size classes in a pristine Italian mountain stream. The Outlying Mean Index (OMI) analysis showed that photoperiod and water temperature drove temporal segregation among the species, consistent with different life cycles, while a gradient of current velocity, depth, and substrate granulometry structured spatial partitioning. Alainites muticus and B. melanonyx showed the greatest niche marginality, while B. alpinus and B. rhodani behaved as generalists with substantial overlap offset by complementary temporal dynamics. At the intraspecific level, significant differences in ecological niches between size classes of A. muticus and, to a lesser extent, between larger nymphs of B. alpinus and B. rhodani , were observed. These results provide new ecological insights into widespread mayfly species, supporting the important role of life-history differences and fine-scale microhabitat use in promoting species coexistence and structuring aquatic macroinvertebrate communities.