Mallard x Hawaiian Duck hybrid, Ko'Olina, Oahu (Hawaii, USA), 6th August 2012 - copyright Steve Mlodinow
(photo ID: 1257)
(photo ID: 1257)
There are a number of examples from around the world where species are threatened by hybridisation. Typically this arises where a non-native species is introduced to an area where a closely-related species already exists. The two species interbreed and the hybrid young are fertile. If there are sufficient numbers of the introduced species, especially if these are more dominant, genetically pure examples of the native species can in due course be wiped out completely. Hybridisation may be one of a number of factors, with others like habitat loss playing a contribution.
Many examples involve the introduction of domesticated Mallards. An example is the Hawaiian Duck which is listed as Endangered because it is "inferred to have a very small and fragmented range on a few islands, where wetlands are being lost and degraded, and where hybridisation is slowly reducing the number of pure individuals" (source: BirdLife International).
Another example involving Mallards is the American Black Duck,
American Black Duck x Mallard hybrid, Everett (Washingtonhere). The same issue exists in New Zealand (where the name Grey Duck is often used for this species) - see the PhD thesis by Wiebke Muller: "Hybridisation, and the Conservation of the Grey Duck in New Zealand" (available here)."Feral Mallards: A Risk for Hybridisation with Wild Pacific Black Ducks in Australia?" by P-J Guay and J P Tracey (in The Victorian Naturalist 126. 3:87-91, 2009) (abstract
Hybridisation with domestic Mallards may also be one of a number of threats to the African Black Duck in southern Africa (mentioned in the BirdLife International factsheet).
Famously in the UK a cull of accidentally-introduced Ruddy Ducks has recently taken place. This cull proved highly controversial though supported by the RSPB (they explain their decision here). Birds originating in the UK had spread across western Europe reaching Spain where they began hybridising with the already threatened White-headed Duck. It was hoped that by eradicating them from the UK (and I think other countries have deployed similar programmes) the threat of hybridisation in Spain could be reduced.
Ruddy Duck, Whitlingham Broad (Norfolk, UK), 17th January 2010 - copyright Dave Appleton
(photo ID: 1770)
It is not necessarily the introduction of non-native species that cause a threat to endangered species through hybridisation. In some cases two species may co-exist with no difficulty when both species have strong populations but when one declines for other reasons it becomes suscpetible to hybridisation (with insufficient same-species mates related species are chosen instead), and that then compounds its problems. This is the situation with Black Stilts in New Zealand. Their decline was initially caused by predation from introduced mammals and habitat loss, but hybridisation with Pied Stilts is now a significant additional factor. It is known that hybridisation used to occur when population levels were higher but the incidence was low compared to the total population so it did not generate significant issues for the species. See the Black Stilt Recovery Plan (Reed & Murray, 1993) for more information.
Pied Stilt x Black Stilt hybrid, Ashley River Mouth (Canterbury, New Zealand), on or before 13th January 2008 - copyright Ian McHenry
(photo ID: 2086)
African Black Duck Anas sparsa
Mallard Anas platyrhynchos Hawaiian Duck Anas wyvilliana
American Black Duck Anas rubipres
Pacific Black Duck Anas superciliosa
Ruddy Duck Oxyura jamaicensis
White-headed Duck Oxyura leucocephala
Pied Stilt Himantopus leucocephalus
Black Stilt Himantopus novaezelandiae