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Australia's battle with the bunny

They may look cute and cuddly but rabbits have been a persistent pest in Australia for 150 years. So are we any closer to eradicating this ecological nightmare?

Gardeners and growers everywhere, watch your lettuce patches! Australia is being hit by a bunny invasion and these marauders aren't the chocolate kind.

After years of battling this pest, Australia is now facing a fresh increase in rabbit numbers. Rabbits have been spotted in rising numbers in the Atherton tablelands in far north Queensland, and the Northern Rivers region in New South Wales.

The latest battlefront is Macquarie Island, a subantarctic island halfway between Australia and Antarctica. Here, rabbit numbers have swelled from under 20,000 to 130,000 in only six years, and have eaten much of the native bushland.

"You could compare [the island] to a golf course," says Dr Arko Lucieer from the University of Tasmania, co-author of a recent paper published in the Journal of Applied Ecology analysing the effect of the growing rabbit population on the island.

Faced with a bunny explosion across the country, scientists are urgently looking for solutions to eradicate this ecological nightmare.

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Why are they here?

European rabbits first arrived in Australia with the First Fleet in 1788, but they only became a pest after 24 wild rabbits were released for hunting near Geelong in Victoria 150 years ago.

"Rabbits were introduced as part of a broad attempt by early colonists to make Australia as much like Europe as they possibly could," says Greg Mutze, research officer at the Department of Water, Land and Biodiversity Conservation in South Australia.

"It was hoped that they would flourish so that the owners could hunt them."

Flourish they did. Rabbits spread throughout Victoria and by 1880 had crossed into New South Wales. In 1886 rabbits were spotted in South Australia and Queensland, and by 1890 were hopping across eastern Western Australia.

To prevent the rabbits' westward spread, the WA government finished building three rabbit-proof fences across the state in 1907. Unfortunately the fences were a flop because rabbits had already moved into the areas being fenced off.

By the 1920s, Australia's rabbit population had swelled to 10 billion.

Currently, rabbits inhabit around 4 million square kilometres of Australia, stretching from southeast NSW to the WA wheatbelt.

They have adapted to Australia's diverse environments, establishing themselves in farmland, deserts, grasslands and wet coastal plains, and causing havoc to native flora and fauna.

"Rabbits are very good at finding the seedlings of shrubs when they are very small and grazing them out to the extent where the native shrubs are completely unable to regenerate," says Mutze.

Rabbits also threaten some of our native burrowing animals, such as the bilby and the burrowing bettong, by moving into their existing burrows and competing for food.

While increased rabbit sightings tell us that numbers are on the rise, accurate estimates of Australia's rabbit population today are difficult to make because there is no national reporting and mapping system.

The rate at which bunnies breed doesn't help either; a single female rabbit is able to produce between 18 and 30 young per year.

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Getting rid of the bunny

Conventional and biological controls have been used in Australia to eradicate rabbits.

Conventional controls include destroying rabbit burrows with poison and fire.

"Using poison, deep ploughing and then fuming burrows was highly cost effective [in] reducing rabbit numbers," says Mutze.

However, conventional controls are labour intensive and time consuming and, faced with the rate at which rabbits breed, cannot hold down numbers on their own.

So in 1950 the biological control agent, Myxoma virus, was introduced to Australia's mainland.

Myxomatosis, the disease caused by the Myxoma virus, occurs naturally in South American cottontail rabbits.

Once infected, the rabbits develop lesions filled with mucus. The mucus accumulates under the rabbit's skin, leading to internal swelling. Most rabbits die of haemorrhage and seizures within 10 days.

Initially, myxomatosis caused enormous reductions in rabbit numbers. In some areas 99 per cent of the rabbits were killed.

However, with the virus spread by mosquitoes, fatality rates varied across the country. In arid areas, where mosquitoes cannot survive, myxomatosis did not spread well.

The virus' toxicity has also reduced over time.

"Most of the strains circulating now kill about 40 per cent of rabbits that are infected," says Mutze.

This is partly because rabbits are developing genetic resistance to the virus, and the virus itself has changed and is not as virulent as the original strain.

To combat the reduced effectiveness of myxoma virus, calicivirus, or rabbit haemorrhagic disease (RHD), was released in Australia in 1995.

"RHD was first detected in China in 1984 and it spread through wild and domestic rabbits in Europe," says Mutze.

RHD causes blood clots to develop in the rabbit's lungs, heart and kidneys. The clots block blood vessels and death from heart and respiratory failure quickly ensues.

The virus reduced rabbit populations by 90 per cent in arid zones and held them down for around 10 years, says Mutze. However, like myxomatosis, its efficacy varied throughout Australia.

As it is spread by flies, the virus had very little impact in cooler, high rainfall regions in coastal south eastern Australia where flies are less abundant.

Also, two years ago Dr Tanja Strive from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) discovered that Australian rabbits carry a native calicivirus that may confer some immunity to the disease.

"[The native calcivirus is] non-pathological so it doesn't kill them, but it's very similar to calicivirus. We suspect it is acting as a natural vaccine," says Strive.

The rabbits are also developing resistance to the introduced calicivirus.

"There will always be a coevolution between the virus and the host, so biological controls will never be enough on their own," says Strive.

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Macquarie Island

This has certainly been the case on Macquarie Island. Rabbits were brought to the island in the late 1800s to provide food for shipwrecked sailors.

In the 1900s the rabbit population exploded, and in 1968 the myxomatosis virus was released. Initially, this project was successful and rabbit numbers reduced from 130,000 to 20,000 in the 1980s.

But after a cat eradication program began in 1985, rabbit numbers have risen to 130,000 again.

"The combination of the [reduced efficacy of the] myxomatosis virus and the absence of cats meant that the rabbit population started to expand after 2000," says University of Tasmania researcher Lucieer.

The increased rabbit population has also had a devastating impact on the island's native vegetation.

"In a lot of cases, the vegetation community had completely changed to what are essentially grazing lawns," says Lucieer.

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Recording rabbit numbers

While increased rabbit sighting tells us that rabbit numbers are on the rise, scientists don't know the precise numbers.

"We don't have a reasonable rabbit map in Australia," says Professor Tony Peacock, chief executive officer of the Invasive Animals Co-operative Research Centre (IACRC).

Currently recording systems rely solely on individual reporting.

"Local authorities will get together and report: yes we've got rabbits, no we don't," says Peacock.

But the reporting grid varies between states.

"In some states these reports are documented on a five kilometre grid, while in other states it is a 50 kilometre grid," says Peacock.

To tackle this, the IACRC is making this coming May RabbitScan month.

Volunteers are being asked to scan their landscape and mark areas where they have seen rabbits on an online map. Scientists from the IACRC will then use this data to assess rabbit activity across Australia.

Knowing where rabbits are across Australia will assist in better implementation of rabbit eradication strategies.

"In some cases it's possible that we will find that calicivirus is not circulating in the population, so we can reinfect rabbits," explains Peacock.

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The future of rabbit eradication

Any future rabbit removal strategy should take heed of the Macquarie Island experience and consider the wider implications on the environment, says Lucieer. Namely, whether another pest could take the place of the rabbits once they are gone.

"Rather than focusing on the one species we should research the whole ecosystem," says Lucieer.

In terms of specific approaches going forward, we need to practice 'integrated rabbit management' and use several different methods, says CSIRO researcher Strive.

Where they can, farmers should continue using conventional methods to remove rabbits from their land.

"This means ripping out the warrens and baiting if the land is accessible. But for the more remote areas we will always have to rely on biological control," says Strive.

Researchers are looking into new biological control agents, which could come from new viruses or more virulent strains of myxomatosis and calicivirus.

In 2000, researchers in Italy reported the discovery of a new and virulent strain of the calicivirus. It has since spread across Europe, the United States, Cuba and Uruguay.

The suitability of this strain as a new agent depends on whether it infects native fauna, and the ease with which it spreads across rabbit populations.

While identifying new suitable diseases is expensive (calcivirus research cost more than $20 million), the benefits are enormous: calicivirus has saved Australian agriculture over $1 billion in the past 13 years.

And the sooner we introduce a new rabbit-killing virus, the better.

"It is much easier to hold rabbit numbers down than to get them down again after rabbits have been re-established," says Mutze.

As we know, once bunnies start breeding, it's hard to make them stop.

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