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Round discs of barren dirt known as "fairy circles" look like rows of polka dots that can spread for kilometres over the ground.
The phenomenon's mysterious origins have intrigued scientists for decades — and they may be far more widespread than once thought.
Fairy circles were previously spotted only in the arid lands of Southern Africa's Namib Desert and the outback of Western Australia.
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But a new study has used artificial intelligence to identify vegetation patterns resembling fairy circles in hundreds of new locations across 15 countries on three continents. This could help scientists understand fairy circles and their formation on a global scale.
For the new survey, published Monday in the journal Proceedings of the National Academy of Sciences, the researchers analysed datasets containing high-resolution satellite images of drylands, or arid ecosystems with scant rainfall, from around the world.
The search for patterns resembling fairy circles used a neural network — a type of AI that processes information in a manner similar to that of a brain.
"The use of artificial intelligence based models on satellite imagery is the first time it has been done on a large scale to detect fairy-circle like patterns," said lead study author Dr Emilio Guirado, a data scientist with the Multidisciplinary Institute for Environmental Studies at the University of Alicante in Spain, in an email.
Hundreds of potential fairy circle sites
First, the study authors trained the neural network to recognise fairy circles by inputting more than 15,000 satellite images taken over Namibia and Australia. Half of the images showed fairy circles, and half did not.
The scientists then fed their AI a dataset with satellite views of nearly 575,000 plots of land around the world, each measuring about one hectare.
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The neural network scanned vegetation in those images and identified repeating circular patterns that resembled patterns of known fairy circles, evaluating the circles' sizes and shapes as well as their locations, pattern densities and distribution.
Output of this analysis then required a human review, Guirado said.
"We had to manually discard some artificial and natural structures that were not fairy circles based on photo-interpretation and the context of the area," he explained.
The results showed 263 dryland locations where there were circular patterns similar to fairy circles in Namibia and Australia.
These arid spots were distributed across Africa (the Sahel, Western Sahara and the Horn of Africa) and were also clustered in Madagascar and Midwestern Asia, as well as central and south-west Australia.
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Circle pattern recognition
Fairy circles aren't the only natural phenomenon that can produce round, repeated bare spots in a landscape. One factor that sets fairy circles apart from other types of vegetation gaps is a strongly ordered pattern between the circles, said Dr Stephan Getzin, a researcher in the department of ecosystem modelling at the University of Göttingen in Germany.
Getzin and colleagues published a November 2021 paper defining fairy circles and what made them unique, emphasising details of the overall pattern structure, he told CNN in an email. And according to Getzin, who was not involved in the latest study, the newfound patterns fall short.
"Fairy circles are defined by the fact that they have, in principle, the ability to form a 'spatially periodic' pattern", which is "significantly more ordered" than other patterns — and none of the patterns in the survey clear that high bar, Getzin said.
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But in fact, there is no universally accepted definition of fairy circles, Guirado said. He and his coauthors identified potential fairy circles — gauging the size and shape of individual circles, as well as the patterns they formed collectively — by referencing guidelines established across multiple published studies. The metrics of those spatial patterns, in fairy circles old and new "are virtually the same", he said.
Of the new locations that were identified, some passed muster with Dr Fiona Walsh, who as part of an international team has investigated fairy circles in the Australian outback.
"Pattern distribution in Australia appears to be congruent with some of what we previously reported," said Walsh, an ethnoecologist at the University of Western Australia. Walsh was not involved in the new survey.
Fairy circles' mysterious origins
The study authors also compiled environmental data where circles were spotted, collecting evidence that might hint at what causes them to form.
The researchers determined that fairy circle-like patterns were most likely to occur in very dry, sandy soils that were high-alkaline and low in nitrogen.
The scientists also found the fairy circle-like patterns helped stabilise ecosystems, increasing an area's resistance to disturbances such as floods or extreme drought.
But the question "What shapes fairy circles?" is complex, and factors that create fairy circles may differ from site to site, the study authors reported.
Getzin previously wrote that certain climate conditions, along with self-organisation in plants, generated fairy circles in Namibia, and while insects such as termites take advantage of the dry patches, their activities don't directly produce the patterns, he said in the email.
Walsh, however, said that Australia's fairy circles are inextricably linked to termite activity. Their team's research, conducted in close collaboration with indigenous peoples, determined that in Western Australia and in the Northern Territory, termites are intrinsic to the functioning of fairy circles, called "linyji" in the Manyjilyjarra language, and "mingkirri" in the Warlpiri language, she told CNN in an email.
"Aboriginal people illustrated these patterns at least since the 1980s and said they knew of them for generations, probably millennia earlier," Walsh said.
"In Australia, termites do not simply 'play a role'," she added.
"They are the primary mechanism and interpretations need to be centred on termite-grass-soil-water dynamics."
Many questions about fairy circles have yet to be answered, and the authors of the new study are optimistic that their global atlas will open a new chapter in the study of these peculiar barren spots.
"We hope that the information we publish in the paper can provide scientists around the world with new areas of study that will solve new puzzles in the formation of fairy-circle patterns," Guirado said.
