Despite Australia's relatively small population, the human impact on wildlife has been dramatic. In this episode we meet Australia's natural history success stories - and those being left behind as the continent undergoes rapid change.Starting in Australia's second largest city Melbourne, we witness how possums - tree-dwelling marsupials - have taken to sharing trees in busy city parks. They are breeding successfully, but when pressure from the neighbours becomes too great they risk encounters with heavy traffic and domestic cats in order to find shelter in rooves and garden sheds.