Alien species eradication and the general topic of island biodiversity restoration. The special case of Brittany island mammals

In taking the decision whether or not to eradicate the Norway rat (Rattus norvegicus) from islands off the coast of Brittany in France, some aspects of the general subject of alien species eradication was examined, in an attempt to answer the following question: What is the significance of islands for conservation and restoration of the biodiversity? Can we accept the concept of the "naturalisation" of an alien species, and if so, how do we recognize it? What are the criteria on which we base a decision to eradicate a species in place for a "long time"? It appears in conclusion that, as far as we know, the house mouse (Mus musculus), the ship rat (Rattus rattus) and the Norway rat did not reach the coast of Brittany before 3 000, 2 000 and 500 years BP respectively, that is to say, after the last Quaternary transgression which induced the insularity of the Brittany islands. Consequently, their is a great probability that the spread of these species onto Brittany islands was caused by human activity. Moreover, the literature give no evidence that any of these species fills any of the criteria quoted by Usher (1989) and Chapuis et al. (1996) preventing its eradication.