Riparian beetles, biodiversity, and stream flow regulation - the examples of Svartaen and Mjaellaen streams, Central Sweden

The beetle (Coleoptera) species composition along two sandy, meandering streams in central Sweden is presented. In addition, the impact of the regulation of water flow on the long term survival of riparian beetle species along streams is discussed. The Svartaen stream in Vaermland province is 17 km long and has a mean width of 8 m. In the past it had been subject to moderate flow regulation to facilitate timber transportation. Today the water level fluctuates seasonally in a natural way, with sediment deposition on the point bars during high spring floods. Using pitfall traps and hand collection, beetles were sampled on open or sparsely vegetated sand within ca 7 m of the water's edge at 10 point bars along a 10 km section of the stream, in June 1990 and July 1997. A total of 133 species were found, of which 50 belonged to Carabidae and 50 to Staphylinidae. About 63% of the collected species are characteristic of riparian habitats. In the light of previous studies of riparian beetles along a broader riparian zone, the total number of beetle species regularly occurring along the Svartaen can be estimated to be at least 385. Two species, Ischnopoda scitula and Anthicus bimaculatus, are included in the Swedish red list. Another ten species are local and rare in Sweden, including Bembidion litorale, Philonthus subvirescens, Bledius tibialis, Ischnopoda constricta, I. coarctata, Aloconota insecta and Psammodius sulcicollis. Also of interest is the isolated population of Cicindela hybrida, which is the northernmost in Scandinavia. Here, C. hybrida occurs on small and open point bars with dry sand and sparse grass vegetation. In contrast, the more continuous populations in south Sweden inhabit sparsely vegetated dune areas or some times, sand pits.