On Christmas Eve, 1859, 24 rabbits arrived at Thomas Austins hunting estate near Sydney in eastern Australia. Just two months earlier, his brother William Austin had sent him 13 rabbits from southern England. Three years later, the local press reported that there were already thousands of European leporids in the country. Thomas himself acknowledged that he had slaughtered 20,000 of the animals on his property. By 1906, the rabbits had reached Australias west coast, 4,000 kilometers (about 2,485 miles) away from Thomas Austins property. Now, a genetic study confirms that the Austin brothers rabbits started the largest biological invasion ever recorded.

The contemporary press and history books note the Austins culpability in the disaster. Even Williams granddaughter, Joan Palmer, tells her version of the story in her memoirs. But many scientists and historians acknowledge that its more complicated than that: the English settled in Australia in 1788, when the ships of the First Fleet arrived on a mission for the British crown to turn the huge island into a penal colony. Five rabbits were aboard one of the ships that made the voyage. Records show that there were another 90 shipments of rabbits over the next 70 years. Some of the animals escaped or were released deliberately, but none of them caused a biological invasion. The proliferation of wild rabbits has damaged Australian ecosystems and cornered marsupials; rabbits are the continents main agricultural pests. For over a century and a half, Australians have tried everything to stop the invasion, from rifles and fences, to ferrets and phosphorus poisons, to viruses and bacteria. So, what happened on Christmas Eve in 1859?

A group of British, Portuguese and Australian researchers have used genetics to confirm the Austin brothers responsibility for the disaster. The scientists analyzed the genes of almost 200 rabbits from Spain (the common rabbits place of origin), France (where the animals were domesticated during the Middle Ages), England, Australia and two other countries that also suffered the scourge: neighboring New Zealand and Tasmania. Several specimens come from a few years after the invasion began. This data allowed the scientists to create a genetic tree to study how the rabbits proliferation occurred. The researchers published their findings in the scientific journal PNAS (Proceedings of the National Academy of Sciences of the United States of America).

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

Joel Alves, a researcher at the University of Oxford and the Research Center for Biodiversity and Genetic Resources in Portugal, and the studys lead author, explains what they hoped to find by recreating the rabbit gene tree: We looked for a combination of different genetic markers that we expect when populations expand. Importantly, they saw that almost all Australian rabbits are closely related, despite being separated by thousands of kilometers. Something like that would not have been possible if there had been other successful introductions, notes Alves. He adds that the further away the rabbit populations are from Victoria [the state where the invasion originated], the less genetic diversity they have. This is to be expected from a large significant expansion from a single place, because genetic diversity erodes as individuals expand rapidly. Biologists call this the founder effect.

Analysis of mitochondrial DNA, which is inherited exclusively through the maternal line, has allowed the researchers to estimate the number of females behind the proliferation of hundreds of millions of Australian rabbits: William sent only five females to Thomas. Genetics has also helped the scientists confirm the rabbits geographical origin. On the European branches of the genetic tree, the Australian rabbits are closest to the ones connected to the United Kingdoms southwest, where William Austins land was located. But what was so special about the Austin rabbits as compared to the ones that arrived in Australia earlier?

We have historical and genetic evidence that most of the previous introductions were domestic rabbits. Austins are the only ones explicitly described as wild and captured in a natural environment, which we have confirmed genetically, says Alves. One piece of historical proof is the memoir of Joan Palmer, William Austins granddaughter. She recalls that Thomas asked William to send him a dozen wild rabbits to release on his hunting grounds. The emigrant belonged to one of the acclimatization societies that sprang up in the 19th century. These associations imported species from their homelands to introduce them into the colonies, out of both longing and economic interests. But that had dire consequences in the many ecosystems exposed simultaneously to the actions of white settlers and invasive species. In the Austins case, William captured only six wild rabbits and bought another seven from neighbors who had trapped the animals as young and then domesticated them. The rabbits had to interbreed during the journey to match the 24 animals that appear in the historical records.

Alves explains their wild ancestry probably gave these rabbits an advantage, because they were better adapted to the harsh Australian landscape. In addition to their gray to brownish coloration, which is ideal for camouflage in the dry, semi-arid terrain, the wild rabbits retained their flight response to danger. Domestic rabbits are an easier target for predators because they have lost that reaction and are more docile and more strikingly colored.

In Australia rabbits have evolved with changes in body shape to help regulate their temperature

Francis Jiggins, a professor of genetics at the University of Cambridge and a senior author of the study, notes that there are numerous traits that could make domestic rabbits of wild origin poorly adapted for survival in the wild, but it is possible that they lacked the genetic variation needed to adapt to Australias arid and semi-arid climate. He concludes that to cope with this [environment], Thomas Austins wild rabbits and their offspring had a genetic advantage in adapting to these conditions.

Martn Nuez, a researcher at the University of Houston and an expert in invasion ecology, says that the investigations results are compelling, although they are at odds with previous research that finds rabbits spread throughout Australia from different places at different times. [Determining] why some species invade and others dont is invasion biologys holy grail. Weve been trying to find that out for decades. This study didnt find the holy grail, but it provides information about the processes through which it can happen, he explains. Nuez notes that the research shows that its not so much about the species, but about a population of potentially pre-adapted individuals. This work also demonstrates that the view of rabbits as a highly invasive species, at least outside of Europe, is wrong, since here it is very clear that a given population at a specific time was responsible [for the biological invasion], while other populations had much more limited success.

Alves acknowledges that genetics are not the only explanation for the invasion: It is likely that changes in the environment also played a role when the Austin rabbits arrived. Australia in 1788 was very different from Australia in 1859; by the mid-19th century, the country had extensive grazing land that could feed rabbits, and herders killed fewer predators. So, it was the perfect storm; the right rabbits were in the right place at the right time.

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It was a perfect storm: How 13 rabbits caused the largest biological invasion in history - EL PAS USA