Is it worth eradicating the invasive pest Rattus norvegicus from Molene archipelago? Genetic structure as a decision-making tool

Genetic variability and structure were estimated by microsatellite analysis at 7 loci in brown rat populations (Rattus norvegicus) from the Iroise ins ular complex and neighbouring mainland (Brittany, France). Island genetic d iversity is lower than on the mainland and a highly significant positive co rrelation was found between mean heterozygosity and the logarithm of island area, which is consistent with theoretical expectations. Rattus norvegicus populations are substructured at a kilometric scale, both on the islands a nd on the mainland. Intra-island structuration is extremely high, suggestin g that no effective migration occurs between islands or with the mainland. Historical and genetical evidence suggest that R. norvegicus was introduced independently on Ouessant and Molene archipelago, with a low and a high fo under effect respectively. These results are discussed in terms of recoloni zation probability of islands that have been cleared of R. norvegicus, whic h illustrates the usefulness of genetic markers in determining parameters o f interest to the conservation biologist.