Genetic Threat to Pacific Black Ducks in Tasmania

The following was published in Birdlife Tasmania's Yellow-Throat Newsletter (Issue 115 Spring 2021, page 13):

Hybridisation is a relatively unknown impact on native duck species. But as proven in other parts of the world, it can be a threat to a species, especially when combined with other threats such as shooting and habitat loss, as it can result in the species’ loss of genetic identity.

In Tasmania, hybridisation (or cross-breeding) occurs between introduced Mallards and native Pacific Black Ducks (PBDU). Mallards and PBDUs are closely related and can interbreed easily, resulting in fertile hybrid offspring. There are commonly expressed traits (eg orange legs, blotchy bills, varying feather patterns), but the traits can be subtle and variable. Due to the success of these hybrids, it can be difficult to spot a genetically pure PBDU in the Derwent Estuary and many other Tasmanian waterways.

PBDUs can’t be shot in residential areas, but it seems that the vast majority of urban waterways in Tasmania are dominated, or at least populated by, numerous feral ducks and hybrids. This means that when PBDU populations are safe from shooters, they are still vul- nerable to competition and interbreeding with the more dominant and aggressive Mallards. Male PBDU courtship displays can’t compete against the more dominant male Mallards, who are much larger than PBDUs and will force themselves on native female PBDUs.

Mallard numbers around Tasmania are supplemented by dumped pets and supported by inappropriate feeding by members of the public. They tame easily and readily thrive in human landscapes. When there is a stable food source provided by humans, their numbers can increase rapidly and quickly exclude smaller native ducks.

In New Zealand, it is now very difficult to find a genetically pure PBDU due to the extent of Mallard hybridisation, and PBDUs on Lord Howe Island have been replaced by hybrids. Several other duck species around the world are endangered by Mallard hybridisation, and species on islands or smaller geographical areas are at greater risk of extinction.

Here in Tasmania, we are in a unique situation where we can create safe habitats for native ducks close to urban centres. Often, we need to create reserves and national parks to protect habitats, but in this case the impact is oc- curring in suburbs and town parks. Residential areas provide refugia for native ducks, but feral ducks (including feral geese and Muscovy ducks) need to be removed to minimise the threat from hybridisation.

Important urban wetlands such as Gould’s Lagoon and Lake Dulverton are already being impacted by feral waterfowl, and this impact is only going to increase if Mallard numbers are not controlled. All native ducks will benefit from the removal of the larger, aggressive Mallards, but PBDUs in particular will have the bonus of reducing the threat to their genetic identity.

To help our native ducks, it is critical that people stop feeding them! No human food can beat a native duck’s natural diet, and swapping bread for greens still supports feral ducks and geese. Placing containers of water out also only supports feral ducks. To best look after our native Tasmanian ducks, we need to enjoy them by identifying them, learning about them, and by simply watching them going about their business.

Posted by jason_graham jason_graham, 11 September, 2021 23:56


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