Forests as dispersal barriers for Erebia medusa (Nymphalidae, Lepidoptera)

The nymphalid butterfly Erebia medusa is a typical species of meadows that are poor in nitrogen and fallow land. In forests, it is only found on clearings with meadow-like structures. However, it is missing on wind exposed places, too. 239 imagoes of E. medusa were sampled in the Aggtelek Karst region (north-eastern Hungary) at six different localities at the end of May 1997. The sample sizes varied between 33 and 46. The studied karst area has a typical forest steppe vegetation and is characterised by a mosaic-like distribution of meadows and forests. The sampled butterflies were analysed by allozyme electrophoresis. The level of polymorphism was significantly higher in the samples than the mean of the populations on European scale. Significant genetic differentiation was found between populations but the level of differentiation was low. Genetic distances varied between 0.013 and 0.020 with a mean of 0.017 (+/- 0.002 S.D.). The estimated F-ST value was about 0.005 (+/- 0.005 S.D.). An isolation by distance analysis showed no significant correlation between geographical and genetic distances (p > 0.05; r = 0.48). Contrarily, a significant correlation between genetic distances and the minimum forest distances between sampling sites was found (p < 0.05; r = 0.70). Therefore we assume that the meadows which are more or less continuously populated by E. medusa seem to be of little influence on the formation of local genetic structuring, whereas the distribution of forests which might act as migration barriers for this species seems to be an important factor.