Unravelling β‐Diversity and the Role of Small Streams in Shaping Fish Communities Using a National‐Scale eDNA Database

The role of river size on fish communities remains underexplored in freshwater ecology. Small streams, despite accounting for 75% of stream length, are more vulnerable to human impacts and often lack legislative protection. Nevertheless, they can serve as refuges and rearing areas for fish, including rare or threatened species. To understand if small streams host distinct fish assemblages, we analysed fish β‐diversity across stream sizes using a publicly available eDNA database. At a national scale, we calculated ‘local contributions to β‐diversity’ (LCBD) to assess site distinctiveness and ‘species contributions to β‐diversity’ (SCBD) to evaluate species‐specific impacts on compositional variation. We further partitioned β‐diversity into turnover and nestedness components at the catchment scale to explore underlying mechanisms of community change. Our findings support the hypothesis that low‐order streams host more distinct fish assemblages than medium‐ or high‐order streams. At a catchment scale, compositional differences were more often driven by turnover and associated to stream order and distance to sea. We found a negative relationship between the proportion of diadromous species and community distinctiveness, suggesting these species may homogenize communities. Contrary to expectations, introduced species were associated with higher LCBD values, likely reflecting an invasion‐driven community reshuffling. Interpreting these patterns requires careful consideration of species’ traits, historical biogeography, and anthropogenic effects. Together, these results highlight the critical role of small streams in maintaining fish diversity and the importance of protecting them.