Climates a big one for sure. In WA there's a large offshore island (Dirk Hartog) which is about 80k's long and I suppose 5ish to about 15k's across which has had all of the wild sheep, goats and cats shot out and trapped - no small job in itself but as they've proven it's possible. The interesting bit was that the rat population was pretty much non existent before they even started because the place was just too dry to suite their needs or so it's believed.
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