In Australia, it may take a rat to control a rat. Scientists have high hopes that introducing native Australian bush rats to the area around Sydney Harbour will help control black rats, a non-native species, that have long been a scourge to flora and fauna. Black rats, which carry diseases such as plague and lung worm, capable of spreading to humans and animals, were introduced to Australia more than 200 years ago, possibly on the first fleet of ships carrying white settlers. The bush rats once lived in Sydney but the last confirmed sighting was more than a century ago. They may have been eliminated due to a government bounty for each rat killed during an epidemic of bubonic plague. Roughly 40 of the bush rats were released on a hectare of land on Thursday night, after the same number of black rats were trapped and removed.