This post is old but I just stumbled on it.
Australia has no predatory animals that actively hunt. The Tasmanian Devil eats carrion and to get eaten by a crocodile an animal would have to enter its domain - primarily a northern salt water river. An occasional live catch isn't going to wipe out the kangaroo population. Or the other endemic animals most of which are also too big for the venomous snakes to eat. Also, marsupials carry their young in a pouch and that helps protect their joeys from snakes and raptors. A normal tree climbing carpet python won't go after an adult Koala again because it can't swallow an animal that size. And the endemic creatures don't live, certainly not in numbers, in the northern rainforests which is the habitat of the big scrub pythons.
Hence, the Australian continent has a unique population of endemic animals. The why is another matter. Of course, the reason has nothing to do with the fraud called, evolution. The continent does sit in the middle of the great southern water mass a long way from anywhere else except New Zealand that doesn't even have snakes much less predatory creatures, and New Guinea which is also free of predator carnivores.
Maybe the continent once had predators but they all died of starvation. The endemic animals, though cute and furry, aren't defenceless. The smaller ones live in burrows or trees, and if a lion could catch a kangaroo it wouldn't chase many after picking on an alpha male Grey but especially, Red.
Sure a lion, or lions, might win a hunt and the kangaroo made dinner but the consequences for the lion will be their guts on the ground. Kangaroos have a lethal claw on each foot and the big males can deliver a deadly kick to the stomach.
For people who have never seen one, an alpha male Grey kangaroo is big, and cut, but the alpha male Red is huge. And the fighting between the males is vicious. The Red kangaroo is too dangerous for Australian Zoos to allow people into their enclosures.
As for the fences - well, just as now, politicians and bureaucrats back then had families and friends that needed jobs.
There must have been a lot of those families and friends beecause there is an inordinate number of unused train tracks and tunnels in Australia. Some in places that defy belief. And the magnificence of Church buildings, often red brick, in small country towns is beyond belief.
