How does the beetle cross the road?

Not very successfully at all, according to a recent study which suggests that the high density of roads in many areas may be creating isolated populations of these key invertebrates, reducing genetic heterogeneity and increasing the possibility of local extinctions. Irene Keller and Carlo Largiader at the University of Berne, Switzerland, have carried out a study on the large, flightless ground beetle, Carabus violaceus, in a Swiss forest crossed by both longestablished roads and a relatively new highway (Proc. R. Soc. B, published online). These beetle are quite mobile and able to cover distances of a few hundred metres over days. However previous work suggests that ground beetles find paved roads a major barrier. One study in which individual beetles of the species Abax parallelepipedus were caught, marked, released and then recaptured, found only one of more than 700 marked individuals had crossed a sixmetre road. In another study, pitfall traps placed in the grass strip separating the lanes of a road failed to catch a single ground beetle over a four-week period. Against the background of such concerns, Keller and Largiader sampled eight sites differently separated from each other by the roads although none were more than 1.5 km apart. They analysed six microsatellite regions for polymorphisms from individuals collected at each site. Their results showed that genetic differences between the samples did not correlate with distances between the sites but were strongly linked to the number of roads between different sites. The differences also did not correlate with the age of the road, which suggested that some individuals had crossed the the older roads, which were recorded to be at least 90 years old. But the effect of the highway, which was built only 30 years ago appeared more dramatic, with as strong a genetic difference between populations divided by this road than those divided by the much older roads. The researchers estimate that it would take as few as 139 generations for a completely isolated population to lose 50 per cent of genetic heterozygosity and that a habitat of just a few hectares could be small enough for genetic factors to become important for long-term survival. Indeed, the researchers believe their results may already have revealed local extinctions as no beetles were found in two sites they sampled. They believe their results argue for the consideration of invertebrates in studies on the effects of habitat fragmentation and that future conservation efforts should improve links for these animals too. How does the beetle cross the road?