Carabid beetles as indicators of stream zonation

• Carabid community structure and trait composition changed along the stream course. • Communities in downstream zones were most rich in species and individuals. • Small and long-winged carabids dominated in downstream. • Upstream communities were dominated by large and short-winged species. • Carabid beetles are found to be excellent indicators for stream zonation. Rivers and streams are characterised by longitudinal changes in hydrology and aquatic communities. Due to the strong connectivity between watercourses and adjacent terrestrial ecosystems, riparian communities might also show characteristic changes along stream courses. Although carabid beetles are often used as ecological indicators in riparian habitats (e.g., to monitor restoration success), little is known about how they indicate stream zonation. We conducted a study of riparian habitats at five streams, each sampled in three near-natural sections upstream, midstream and downstream, to analyse changes in carabid beetle abundance, species richness, indicator species and functional trait composition along stream courses. Carabid beetles were sampled using pitfall traps and hand collections in May and September 2023. Carabid abundance, species richness and functional diversity increased significantly from upstream to downstream. Carabid communities shifted from a dominance of large-bodied and short-winged species in the upstream zone towards smaller and long-winged species downstream. In addition, 11 indicator species were identified in the downstream zone, compared to only two in the upstream and one in the midstream zone. In contrast, the percentage of riparian habitat specialists peaked midstream. Changes in carabid communities were likely driven by increasing size of the riparian area and soil pH along the course of the streams, likely reflecting the increasing hydrological connectivity and sediment deposition. Our findings clearly showed longitudinal changes in the community structure and trait composition of carabid beetles along streams. We conclude that carabids are therefore excellent ecological indicators of the zonation of the riparian zone of small streams.