Earlier this year, I discussed a paper about the collapse of stickleback species due to increasing hybridization. In a similar vein, I ran across this 2004 paper by Judith Mank reviewing the apparent breakdown of the distinction between American black ducks and mallards:
American black ducks (Anas rubripes) and mallards (A. platyrhynchos) are morphologically and behaviorally similar species that were primarily allopatric prior to European colonization of North America. Subsequent sympatry has resulted in hybridization, and recent molecular analyses of mallards and black ducks failed to identify two distinct taxa, either due to horizontal gene flow, homoplasy, or shared ancestry. We analyzed microsatellite markers in modern and museum specimens to determine if the inter-relatedness of mallards and black ducks was an ancestral or recent character. Gst, a measure of genetic differentiation, decreased from 0.146 for mallards and black ducks living before 1940, to 0.008 for birds taken in 1998. This is a significant reduction in genetic differentiation, and represents a breakdown in species integrity most likely due to hybridization. Using modern specimens, we observed that despite a lower incidence of sympatry, northern black ducks are now no more distinct from mallards than their southern conspecifics.
Turns out that this outcome was predicted a long time ago, as reflected in a 1967 paper by Paul Johnsgard:
Owing to its much smaller gene pool, the Black Duck is vulnerable to eventual swamping through hybridization and introgression, although the present hybridization rate is sufficiently low as to make this unlikely in the foreseeable future (Johnsgard 1967:51).
Hybridization of introduced mallards with endemic species is a problem all over the world. For instance, New Zealand grey ducks:
Small numbers of Mallard (Anas platyrhynchos) were introduced into New Zealand from Great Britain and North America over 100 years ago. Both sexes have undergone differentiation in size and plumage characters as a consequence of hybridization with the indigenous Grey Duck (A. superciliosa). Pure forms of both species, as documented by early descriptions, appear to be disappearing, particularly the Grey Duck. As a consequence of hybridization, two morphologically distinct hybrid populations have been produced: one resembles the Grey Duck and the other the Mallard. By 1981-1982 levels of hybridization, based on plumage analysis, had reached 51%, and the proportion of pure Grey Ducks had dropped to 4.5%, which is below the level suggested for the maintenance of a species. In the absence of reproductive isolation or antihybridization mechanisms between these two species, the Mallard and hybrid populations represent a potential threat to the conservation of the New Zealand Grey Duck (Gillespie 1985:459).
And a review entitled "Extinction by hybridization and introgression" by Rhymer and Simberloff, there is this passage:
Hybridization with introduced mallards has contributed to the decline of the endangered, endemic Hawaiian duck (A. wyvilliana) and has hampered attempts to reintroduce this species to Oahu and Hawaii. Domesticated nonmigratory mallards that escaped or were released for hunting breed with the endemic Florida mottled duck (A. fulvigula fulvigula), and the resultant introgression threatens the existence of the latter subspecies. Introgression also occurs between domesticated introduced mallards and the native Australian (Pasific) black duck, A. superciliosa rogersi (Rhymer and Simberloff 1996:86)
Mallards are not alone, apparently ruddy ducks of North American origin now threaten the endangered population of European white-headed ducks (Oxyura leucocephala).
Gillespie GD. 1985. Hybridization, introgression, and morphometric differentiation between mallard (Anas platyrhynchos ) and grey duck (Anas superciliosa ) in Otago, New Zealand. Auk 102:459-469.
Johnsgard PA. 1967. Sympatry changes and hybridization incidence in mallards and black ducks. American Midland Naturalist 77:1:51-65. DOI link
Mank JE, Carlson JE, Brittingham MC. 2004. A century of hybridization: decreasing genetic distance between American black ducks and mallards. Conservation Genetics 5:395-403. DOI link
Rhymer JM, Simberloff D. 1996. Extinction by hybridization and introgression. Ann Rev Ecol Syst 27:83-109.