LAUSR

Abstract Intermittent streams are subject to high levels of environmental variation. However, little is known about how biota responds to river drying across the channel-to-upland habitat gradient. This is an important shortcoming because assumes that intermittent river habitats and metacommunities are static. Here we studied how river drying affects the spatial and temporal variation in ground-dwelling arthropod communities (spiders, beetles, and ants) across the lateral habitat gradient. We asked whether particular habitats, moments, or species contribute disproportionally to beta diversity. To this end, we monitored two perennial-intermittent reach pairs during an entire drying cycle, and applied beta-diversity partitioning methods to highly-resolved taxonomy data. We predicted that: (i) intermittent reaches would accumulate higher levels of diversity (alpha and beta) than perennial reaches; (ii) dry riverbeds would harbor more species and more unique species; (iii) alpha and beta diversity would be temporally more variable in intermittent than in permanent reaches; and (iv) species dispersal limitation would explain variation in contributions to beta diversity. Intermittent reaches presented higher alpha and beta diversity, with dry channels harboring more unique species and more species than any other habitat. Arthropod metacommunities were more variable across space than over time, and temporal turnover was significant—with species dispersal limitation being positively associated with contributions to beta diversity. Our results elevate the role of dry channels in intermittent river ecology, and show that previous research has likely underestimated their contributions to river-wide biodiversity. Our findings call for the need to integrate the dry phase of intermittent streams into monitoring and conservation programs.