Australia is very stable geologically, apart from a bit on the S.E. corner, which is why it is flat. It is very old and hasn't had the evolutionary disruptions caused by mountain raising and flattening. The animals have evolved quite distinctly in their own niche. Foreign imports could seriously disturb the balance. See, for example, rabbits, foxes, cane toads.
Makes perfect sense. I guess I'm left wondering why smaller, remote islands don't have to do this. Places like Hawai'i, New Guinea, the Philippines, Iceland, or Japan. I'm no biologist, but my guess is that those places aren't known for large-scale agriculture.
The basic answer is: too little, too late. By the time they realised what was going on, the problems had already been caused. Hawaii has been devastated by imported plants and animals. Thousands of bird species have been lost, and huge parts of the islands are overrun with noxious weeds. They're fighting it as best they can, but it's a case of preserving what is left.
As for the phillipines - they've been trading with other nations for a long time, as have Japan. I'm guessing that the transfer of pests and diseases happened a few centuries back.
Australia is unique in that basically no trade of any kind happened until 200 years ago. There's been a lot of mistakes, but that's no reason to give up.
