Determinants of epigaeic beetle assemblages along northern forest ditches and streams: Habitat matters more than waterway type

We examined how waterway type (ditch vs. stream) and associated environmental and habitat variables influence epigaeic beetle assemblages along small forest waterways across eight study areas in Sweden on moraine deposits. In total, 8,860 individuals representing 264 species from 34 families were collected from 320 pitfall traps. Half of the species were unique to a single area, while only six occurred across all areas, reflecting strong spatial turnover. Assemblage composition varied significantly among areas and habitats, but not between waterway types. After accounting for area-level effects, habitat remained the only significant predictor of beetle community structure, and further analyses confirmed that habitat explained a small yet consistent proportion of variation across areas. We identified 25 species with significant habitat associations, 11 linked to deciduous forests and 13 to recently disturbed open habitats, while Pella humeralis was characteristic of mixed coniferous forest. Furthermore, dissimilarity among traps was overwhelmingly driven by species turnover, with nestedness contributing negligibly. Turnover dominated both among and within areas and habitats and was unrelated to inter-trap distance or distance from the waterway, suggesting that assemblage differences primarily reflected species replacement rather than richness gradients. Beetle species richness and the abundance of water-associated species were influenced by habitat and ground-cover type. Overall, beetle diversity along waterways was structured mainly by terrestrial habitat and fine-scale vegetation features, underscoring the importance of maintaining diverse riparian habitats to support biodiversity in managed forests. Thus, riparian buffers in this production forestry-dominated landscape could be equally important to implement around ditches and streams.