Geographic scaling and genetic differentiation in two highly mobile European saltmarsh beetles

Genetic structure and diversity are studied in two European saltmarsh beetles, Bembidion minimum and B. normannum, on a regional as well as a Western European scale. Results are based on allozymes, studied at four polymorphic loci for more than 1600 individuals from all remaining saltmarshes in Belgium and from a selection of European reference sites. Average gene diversity is not related to habitat or population size, but is larger in the more common B. minimum, in comparison to Atlantic samples of B. normannum. One Mediterranean sample of the latter species reveals a much higher diversity and suggests this region as the evolutionary centre of origin and/or as a possible glacial refugium of the species. Significant overall genetic structure is observed in the complete data of both species, with 2 to 6 \% of the total genetic variation explained by differentiation between populations. Genetic differentiation in both species is significant at different geographic scales, with higher values at a larger scale. A Mantel-test (isolation by distance) between geographic and genetic distance is significant in B. normannum. Our results indicate that habitat fragmentation has not yet resulted in genetic erosion, probably because of the large population sizes of both species, even in very small saltmarshes. The observed genetic differentiation suggests that metapopulations at a relatively large geographic scale are still functional in these highly mobile species. Re-establishment of even small saltmarshes is suggested as a positive conservation measure for long term survival of these specialised ground beetles.