Science

Satellites Show Mysterious Rings of Vegetation in More Parts of the World

Fairy circles inspire wonder in viewers and fuel contention among experts. For decades, scientists have hotly debated the origin of the strange, polka-dot-like patterns of barren earth, which have been found in the Namib Desert, stretching from Angola to northern South Africa. Some researchers also say they occur in the Australian outback.

Now, there’s something new to argue about: To what extent are fairy circles found around the world?

Findings based on satellite imagery published Monday in the Proceedings of the National Academy of Sciences raise the possibility that fairy circles are significantly more widespread, occurring in up to 263 sites in 15 countries across three continents.

“We discovered fairy circle locations in many other places that we didn’t know existed before, because most of the work on this topic has been carried out in just two countries, Namibia and Australia,” said Fernando Maestre, an ecologist at the University of Alicante in Spain and an author of the study.

Other researchers who have worked on fairy circles say that until field work is performed, it remains to be seen whether any of the newly identified circular, bare patches are true fairy circles.

“In all arid regions of the world various types of bare patches exist, which are caused by different processes,” said Norbert Jürgens, an emeritus ecologist at the University of Hamburg, who was not involved in the research.

Until this study, Dr. Maestre and his colleagues were not part of the sometimes fractious fraternity of fairy circle researchers. They got sucked into the mystery when Emilio Guirado, a data scientist also at the University of Alicante and one of the study’s authors, spotted something strange on Google Earth: patterns in Niger that appeared to be fairy circles. He wondered whether they might exist in other dryland habitats.

To find out, the researchers trained a pattern-recognition model with images of known fairy circles from Namibia and Australia. They applied the model across satellite imagery of 575,000 two-and-a-half-acre plots of dryland habitat around the world.

Fairy circles in the Namib Desert, Namibia, in 2015.Credit…Jen Guyton/Nature Picture Library, via Alamy

Although drylands cover 41 percent of Earth’s land surface, the researchers’ model pinpointed only a tiny fraction as potentially containing fairy circles: about 193 square miles. The researchers consulted satellite imagery to manually confirm that fairy-circle-like patterns occurred in nearly all the places the model identified, from Kazakhstan to Madagascar.

Based on their findings, they created a profile of the types of habitats where fairy-circle-like patterns are most likely to occur: hot, arid places with sandy soil that is low in nitrogen, and that receive four to 12 inches of annual rainfall.

Statistical tests confirmed that “the patterns we have found are exactly the same patterns as what people have found in Namibia and Australia,” Dr. Maestre said.

Dr. Maestre said that he and his colleagues went into their study well aware, however, that fairy circles were “a hotly debated topic.” Partly because of this, they chose to be conservative in describing their findings as “fairy-circle-like vegetation patterns.”

“We are not trying to fight with anyone,” Dr. Maestre said.

Nevertheless, the new findings have inspired strong reactions.

“Unfortunately, the study dilutes the term ‘fairy circle’ and it ignores the definition of fairy circles in the process,” said Stephan Getzin, an ecologist at the University of Göttingen in Germany.

In 2021, Dr. Getzin and colleagues argued that true fairy circles occur in a grid-like pattern with “extremely strong” ordering.

None of the newly identified fairy-circle-like gaps match this stringent pattern, Dr. Getzin said.

Walter Tschinkel, a biologist at Florida State University, who also was not involved in the research, agreed with Dr. Getzin. While the authors of the new paper “certainly found plenty of round or roundish gaps located in areas with arid climates and sandy soil,” he said, the patterning does not “truly meet the criterion for fairy circles.”

Dr. Maestre responded that Dr. Getzin’s definition was “not backed up by the whole of the scientific community working with fairy circles” and “does not undermine our findings in any way.”

Michael Cramer, an ecophysiologist at the University of Cape Town, who was not involved in the research, said the lack of a standard definition of a fairy circle was a problem for the whole field.

“Unfortunately, the only guardians of the term ‘fairy circle’ are self-appointed,” Dr. Cramer said. “Getting agreement on the naming of fairy circles would probably require the establishment of a Fairy Circle Convention on Nomenclature, which seems unlikely.”

Whatever the newly discovered gaps turn out to be, they give scientists plenty of work to do, said Hezi Yizhaq, an environmental physicist at Ben Gurion University in Israel, who was not involved in the research.

“Now we have 263 new sites to investigate,” he said. “This is what is so interesting and exciting in science: to solve natural puzzles.”

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