“It was a perfect storm”: thirteen rabbits caused the largest biological invasion in history

Of 13 rabbits shipped just two months earlier from the south of England by William Austin, 24 arrived on Christmas Eve 1859 at his brother Thomas's hunting estate near Sydney in eastern Australia.

Three years later, the local press reported that these European leporids already numbered in the thousands and Thomas himself acknowledged having slaughtered 20,000 specimens on his property.

By 1906, they had already reached the Australian west coast, 4,000 kilometers from Thomas Austin's estate.

Now a genetic study confirms that it was the Austin brothers' rabbits that initiated the greatest biological invasion on record.

The Austins' culpability in the disaster appears in the press of the time and in the history books.

Even a granddaughter of William, Joan Palmer, tells her version in her memoir.

However, for many scientists and historians, things could not be so simple: the English settled in Australia in 1788, when the ships of the First Fleet arrived, a mission from the British crown to turn the huge island into a prison.

Already on that trip five rabbits were on board one of the ships.

The records show another 90 shipments of leporids in the following 70 years.

But despite the fact that some escaped or were released on purpose, none of those introductions led to a biological invasion.

An invasion that has caused great damage to Australian ecosystems, cornering the marsupials,

and that it is the main agricultural pest of the continent.

An invasion against which everything has been tested for more than a century and a half, from rifles and fences, to ferrets and phosphorus poisons, through viruses and bacteria.

So what happened on Christmas Eve 1859?

A group of British, Portuguese and Australian researchers have relied on genetics to confirm the responsibility of the Austin brothers in the disaster.

They have analyzed the genes of almost 200 rabbits from Spain (origin of the common rabbit) France (land where they were domesticated during the Middle Ages), England, Australia and two other countries that also suffered from the scourge, neighboring New Zealand and Tasmania.

Several of the specimens are from a few years after the beginning of the invasion.

With these data they have been able to create a genetic tree that they have published in the scientific journal

and with which they have been able to study the expansion of the rabbits.

"The wild ancestry probably gave these rabbits an advantage, as they were better adapted to the harsh Australian landscape."

Joel Alves, researcher at the University of Oxford (United Kingdom) and the CIBIO Institute (Portugal)

The researcher from the University of Oxford (United Kingdom) and the CIBIO Institute (Portugal) Joel Alves, main author of the study, explains what they intended when recreating the genetic tree of rabbits: "We are looking for a combination of different genetic marks that are expected when populations expand.

One of the key features they saw in this tree is that almost all Australian rabbits are closely related, despite being thousands of miles apart.

"Something like this would not have been possible if other successful introductions had taken place," highlights Alves, who adds: "Not only that, but the further away the rabbit populations are from Victoria [state of origin of the invasion], the less diversity genetics they have.

This is what is expected in a significant large expansion from a single point,

since genetic diversity erodes as individuals expand rapidly.”

This is what in biology is called the founder effect.

Analysis of mitochondrial DNA, which is inherited exclusively through the maternal line, has allowed researchers to estimate the number of females behind the hundreds of millions of Australian rabbits: William sent only five females to Thomas.

Genetics has also helped them confirm their geographical origin.

Among the European branches of the genetic tree, those of the Australian rabbits are closest to those that connect with the south-west of the United Kingdom, the region where William Austin's lands were located.

But what was so special about the Austins' rabbits that the previous ones didn't?

The image, from 1938, was taken in South Australia during the release of the virus that causes myxomatosis to reduce rabbit numbers. MW Mules/CSIRO

“We have historical and genetic evidence that most previous introductions were from domestic rabbits.

Those from Austin are the only ones explicitly described as wild and captured in a natural environment, which we have confirmed genetically," says Alves.

One of the historical proofs is the memoirs of Joan Palmer, the granddaughter of William Austin.

In them, she recalls that Thomas asked her to send him a dozen wild rabbits to release on her hunting ground.

The emigrant belonged to one of the acclimatization societies that proliferated in the 19th century.

These associations imported species from the metropolis to introduce them in the colonies,

in a combination of longing and economic interests that would have dire consequences in many ecosystems exposed to the parallel action of white settlers and invasive species.

In the Austins' case, William only caught six wild rabbits and bought another seven from neighbors who had caught them as pups and then tamed them.

Both had to meet during the journey to complete the 24 that appear in the records.

For Alves, "this wild ancestry probably gave these rabbits an advantage, as they were better adapted to the harsh Australian landscape."

The wild ones, in addition to a color between gray and brown, ideal for mimicking dry and semi-arid terrain, would retain their flight reaction to danger that domestic rabbits would have lost, more docile and with more striking colors, which make them a target. easier for predators.

“Australian rabbits have evolved body shape changes to help control their temperature”

Francis Jiggins, professor of genetics at the University of Cambridge, UK

Professor of genetics at the University of Cambridge (UK) and senior author of the research, Francis Jiggins, comments that "there are numerous traits that could make wild domestic rabbits not well adapted to survive in the wild, but they may have lacked the genetic variation needed to adapt to Australia's arid and semi-arid climate."

He and he concludes in a note: “To cope with this, rabbits in Australia have evolved changes in body shape to help control their temperature.

So it is possible that Thomas Austin's wild rabbits and his offspring had a genetic advantage in adapting to these conditions."

For Martín Nuñez, a researcher at the University of Houston (USA) and an expert in ecology of invasions, the results of this work are convincing, although they conflict with previous research that defends that rabbits spread throughout Australia from different points and Different moments.

“Why some species invade and others don't is the holy grail of invasion biology.

Something we have been trying to find for decades.

This study didn't find the holy grail, but it does provide insight into the processes of how it can happen,” he says.

One of these aspects that he highlights is that this research "shows that it is not so much about the species, but about a population of individuals (preadapted?)".

Alves, the main author of this work, acknowledges that not only genetics would explain the invasion: “It is likely that changes in the environment also influenced when the Austin rabbits arrived.

Australia in 1788 was very different from Australia in 1859, with extensive grazing land that could feed rabbits and fewer predators killed by herders.

So it was a case of the perfect storm, with the right rabbits, in the right place, at the right time."

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