Sourcing Eurasian beaver Castor fiber stock for reintroductions in Great Britain and Western Europe

Eurasian beavers Castor fiber, formerly threatened with extinction, have been widely reintroduced since the 1920s. Reintroductions and studies of possible reintroductions are continuing. The International Union for Conservation of Nature (IUCN) guidelines for reintroductions state that 'the source population should ideally be closely related genetically to the original native stock'. Palaeoecological studies suggest that the species survived the last Ice Age in two refugia: in the west in Iberia and Southern France and in the east in the Black Sea region. The post-Ice Age population of Western Europe, including Great Britain, recolonized from the western refugium. Recent mitochondrial deoxyribonucleic acid studies strongly support this view, and extant beaver populations are clearly divided into eastern and western evolutionarily significant units (ESUs). The western ESU is composed of three stocks which survived the 19th and early 20th century as very small, isolated populations. They are very closely related to each other. Each is genetically depauperate, apparently as a result of genetic drift at low population levels. There is evidence of inbreeding depression and of phenotypic abnormalities in beaver populations descended from unmixed stocks. The evidence suggests three coherent management options for sourcing reintroduction stock for Great Britain and for unoccupied areas of western continental Europe. These are (i) use animals from a single western ESU stock; (ii) intentionally mix animals from two or all three of the surviving western ESU stocks; (iii) make an informed exception to the IUCN guidelines and reintroduce animals of mixed eastern and western ESU provenance. These options are discussed with regard to IUCN guidelines, conservation biology and animal welfare considerations. It would be advantageous if a common policy on the origin of reintroduction stock were agreed by the national agencies responsible.