Taking second place to feral house cats, the red fox is one of Australia's most problematic invasive predator. They were originally brought over in 1855 from Europe for the purpose of recreational hunting. Since then, their numbers have exploded, estimated around 7 million animals. They are responsible for the decline of many small marsupials, including the numbat. The fox is a very adaptable, elusive and prolific animal, and as such, is nearly impossible to contain or control. It continues to decimate populations of native species and there is no foreseeable solution.
It's not because of the predators, it's because of the competition from the introduced herbivores like rabbits and mice. Small animals like Numbats and Bilbies have been hunted large predators for thousands of years no problem, but its the competition for their niches that's the issue....
Beautifully rendered, if really sad, picture, it gets people thinking. The fence and the background vegetation in particular is awesome.
While researching numbats and the conservation of them, I read that they have had success with baiting foxes, but now those feral cats are filling the hole left by the foxes. Feral cats are even more of a problem because they don't go for the bait as much. :\
Project Numbat: [link]