SYDNEY (Reuters Life!) - 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. Research suggests that one species of rat will not invade another's territory.

"We know from basic theory that if you have an intact native fauna then invaders can't get into those areas," said Peter Banks, Associate Professor in Conservation Biology at the University of Sydney.

"So we thought maybe we can use the same logic by taking black rats out and putting bush rats in."

Compared to black rats, bush rats don't climb trees as much and are thus less likely to disturb birds' nests or eat eggs, meaning bird numbers may increase. They also tend to stay outside and are less likely to enter houses.

In addition, should a disease-carrying black rat enter the harbor, the disease is unlikely to spread without a population of black rats in the area.

"That's how the plague got here in 1901, there was a population of black rats for it to take hold in," Banks said.

Using poison to control black rats isn't possible due to potential risks to the environment and living things in the surrounding urban area.

Of the bush rats released near the harbor, 20 are wearing electronic tags and evidence suggests they didn't stray far from the area where they were released.

"By using a native species they're going to do the pest control for us," Banks said.

"They're going to stop those black rats getting in there and hopefully we will get those bio-diversity benefits returning to the environment."

(Reporting by Pauline Askin)