Intriguing question. I think we would all agree that foxes, cats and dogs are feral, except for the dingo, of course. Even though it was brought to Australia by humans, for some reason the dingo is claimed as a native. Perhaps when the fox has been here for 3500 years it too will be considered native?
More contentious are our friends the grey headed flying fox and the rainbow lorikeet. While they are obviously Australian should they be darkening the skies around Melbourne? While the occasional grey headed flying fox has wandered south for a number of years now it wasn’t until 1986 that a colony took up permanent residence in the Botanic Gardens (http://www.dse.vic.gov.au/plants-and-animals/flying-foxes-home-page/flying-foxes-melbournes-flying-foxes). These animals are as feral to the Botanic Gardens as the fox is to Australia and are only there because of abundant food trees. While native trees were planted they were not native to the Melbourne area and so attracted the bats, in the same way that putting bird feeders in gardens has brought in the lorikeets, which now plague orchardists and grape growers. Unfortunately people have once again allowed emotions to rule over logic, refusing to have the bats removed or dispersed and even trying to have them declared as an endangered species.
But are all feral species inherently detrimental?
The Maremma is a large (30-45 kg) white Italian dog that has been used for centuries to protect sheep from predation by wolves. In an interesting variation on this theme two of these dogs now protect a colony of little penguins on Middle Island, near Warrnambool, Victoria, from predation by foxes (http://www.janedogs.com/big-dogs-save-little-penguins). Colony numbers had declined from 600 to less than 10 birds by the 2005/2006 breeding season. Since the introduction of the dogs fox predation has ceased and penguin numbers have rebounded to over 180.
The project has been so successful that another pair of Maremmas was deployed in 2007 to Point Danger, near Portland, Victoria, to guard Australia’s only mainland breeding colony of Australasian gannets.
Dr. F. Bunny