Although habitat fragmentation is suspected to pose a major threat to biodiversity, its impact on abundant invertebrate species has been little investigated. We assessed the genetic population structure of the flightless ground beetle Abax parallelepipedus in a forest fragmented by two main roads and a highway using five microsatellite loci. We detected low levels of genetic differentiation, which was concordant with the high population densities of 632–1707 individuals/ha estimated with a mark–recapture method. A Mantel test detected a highly significant increase of pairwise FST-values with the number of roads between sampling locations. As expected, the most pronounced effect of the isolation due to roads was observed in the sample from the smallest fragment (highway exit loop), which was differentiated significantly from most other locations. However, no signs of a recent bottleneck or a loss of genetic variability were detected in this population, indicating a still relatively large effective population size (Ne). Computer simulations confirmed that the observed FST-values were indeed compatible with a Ne of a few hundred individuals in this fragment, assuming strong or absolute isolation since the construction of the roads. We discuss the implications of our findings for the conservation of abundant but poorly dispersing species in fragmented habitats.
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