Spatial dynamics of beetles living on exposed riverine sediments in the upper River Severn: method development and preliminary results

1. Exposed riverine sediments (ERS) are habitats for a large number of rare and specialized invertebrates and, as such, are of considerable conservation importance. Actions that threaten ERS specialists operate on a variety of scales and include river engineering, flow regulation and livestock damage. Populations of specific species of ERS specialist beetles are likely to exhibit individual responses to these threats, depending on the spatial structure and dynamics of the population. An understanding of the spatial dynamics of ERS specialist beetles is, therefore, essential if conservation initiatives are to be successful. 2. A mark-recapture experiment was used to investigate the spatial dynamics of beetles on a section of the upper River Severn, mid-Wales, during June and August 2002. Two species of carabid, Bembidion atrocaeruleum Stephens and Bembidion decorum (Zenker), and one elaterid, Fleutiauxellus maritimus (Curtis), were individually marked and released on one of nine discrete patches of ERS demonstrating a range of patch size, sediment size, habitat heterogeneity, degree of shading, vegetation cover and grazing intensity. The methods used to trap, handle and individually mark beetles were all found to be suitable. 3. Population size of B. atrocaeruleum was mainly determined by ERS patch size, suggesting that, in contrast to F. maritimus, this species used all ERS microhabitats. 4. During the June study, 5.7% of marked and recaptured B. atrocaeruleum and 10% of marked and recaptured B. decorum actively moved between ERS patches over water or through thick vegetation in both upstream and downstream directions. Inter-patch movements of >65 m for B. atrocaeruleum and >135 m for B. decorum were detected. Movement rates in August were lower. F. maritimus showed no evidence of inter-patch movement. 5. The lower rate of dispersal and more specific habitat requirements are postulated as reasons for the greater rarity of F. maritimus.