in sochi it was just a matter of passing convenience. as for oz, they're afflicted by invasives, such as rats, dingos and other non-marsupials. more a matter of survival for them.
one of the worst problems is the rats and cats taking a short-ish swim to nearby islands and decimating native fauna. example: there is one kind of large parrot that is flightless, called the kakapo, or owl parrot. it has no experience with placental predators, and is basically just a sitting meal for whoever comes by. they are from NZ and a couple small nearby islands, and there are only about 100+ left. probably doomed in the wild, not unlike orangutans and many other animals. :/