The distribution area of a species is determined by a combination of factors and since each species of aquatic insect has adapted to drastically changing environments along river channels, they are excellent targets for study. Considering these factors, we focused on three Ephemera mayflies inhabiting the Japanese Archipelago and investigate their niche differentiation patterns. A total of 29 study sites were established within the Asahi-gawa River System to study the relationship between various environmental factors and these mayflies' population structures. From the results of RDA and/or GLMMs, Ephemera japonica tended to distribute in locations with a small degree of canopy openness and a lot of cobbles and boulders. In contrast, Ephemera orientalis tended to distribute in locations with small riverbed slope degree, and small grain size. The distribution area of Ephemera strigata was intermediate between that of E. japonica and E. orientalis. At several study sites, although the distribution patterns appeared irregular due to surrounding environments affected by artificial structures, these patterns can be explained by examining the environmental factors around dams. These results on geographical fine-scaled niche differentiation within a closely related species provide significance knowledge to support understanding of the mechanisms of species coexistence and biodiversity creation.
