Dispersal, defined as the movement of individuals among local communities in a landscape, is a regional process determining metacommunity dynamics in ecosystems. Whereas both natural and anthropogenic ecosystem fragmentations can limit dispersal, previous attempts to measure such limitation have faced considerable context-dependency, due to a combination of spatial extent and associated environmental variability, the wide range of dispersal modes and abilities of organisms or variation in network topologies. Therefore, it is today unclear what role dispersal plays compared to local environmental filtering in explaining metacommunity dynamics in dendritic ecosystems. We quantified alpha and beta-diversity components of invertebrate metacommunities across ten fragmented headwater stream networks and tested the hypothesis that dispersal is the primary determinant of biodiversity organisation in these dynamic and spatially-constrained ecosystems. Alpha-diversity was much lower at intermittent reaches compared to perennial’s, including long upon rewetting, indicating an overwhelming effect of drying including a legacy on local communities. Beta-diversity was never correlated with environmental distances but predominantly explained by spatial distances accounting for river network fragmentation. The nestedness proportion of beta-diversity was considerable and echoed compositional differences that communities from intermittent reaches were subsets of perennial ones. Altogether, these results indicate dispersal is the primary process shaping metacommunity dynamics in these ten headwater stream networks, where local communities recurrently undergo extinction and recolonization events. This challenges previous conceptual views that local environment filtering is the main driver of headwater stream metacommunities. As the fragmentation of river networks is increasing due to global change, our results suggest that some freshwater ecosystems currently driven by local environment filtering could gradually become dispersal-limited. In this perspective, shifts from perennial to intermittent flow regimes represent tipping points that should not be crossed to not jeopardise river biodiversity, functional integrity and the ecosystem services they provide to society.

Environmental_data.xlsx: The file includes raw environmental data sampled for each of 10 intermittent headwaters studied for both sampling period (before and after the drying season). References_morphological_identification.docx This file includes all references used to identify each organism to the lowest practical taxonomic level (identification keys, articles, books, websites).