In Hunt to Solve ‘Fairy Circle’ Mystery, One Suspect Is Dismissed

https://www.nytimes.com/2022/11/01/science/fairy-circles-cause-water-termites.html

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The strange, barren spots pepper the vast Namib Desert, which stretches from southern Angola to northern South Africa. They are known as “fairy circles,” and for a natural phenomenon with such a whimsical name, scientific debates over their origins have been heated.

“The to and fro between opposing camps has often been nothing less than vitriolic,” said Michael Cramer, an ecophysiologist at the University of Cape Town who has studied fairy circles.

Despite decades of research, no consensus exists about the origin of the mysterious formations. Theories have included poisonous gases, noxious bushes and plant-killing microbes or fungi. Two of the explanations — the circles are made by termites, or they result from plants competing over limited water — have dominated the scientific debate.

“Each publication has been hailed as having finally ‘solved’ the fairy circle ‘mystery’ in the popular media,” Dr. Cramer said in an email, calling such reporting an approach that “is not the norm for science.”

A rigorous study published in October will not end this fight, but it does seem to give the water-related hypothesis a clear lead over the termite theory.

“Plants are forced to create these circles to redistribute water to maximize their chances of survival,” said Stephan Getzin, an ecologist at the University of Göttingen in Germany and an author of the study. “We call it ecosystem engineering.”

The Namib Desert is one of the driest places in the world, usually receiving only a few inches of rain each year. Researchers first proposed in 2004 that plants, in competition for water in this harsh ecosystem, may self-organize into fairy circles — an idea originally adapted from pattern-formation theory developed by the mathematician Alan Turing.

Over the past decade, Dr. Getzin and others have published more than a dozen papers in support of the hypothesis, known as plant water stress.

For their latest study, Dr. Getzin and his colleagues spent three years examining fairy circles at 10 study sites across 620 miles of desert. One of those years, 2020, was a drought, while 2021 and 2022 were exceptionally rainy — a lucky break that permitted the researchers to compare different conditions, Dr. Getzin said.

They used soil moisture sensors to collect continuous readings every 30 minutes of water content in the sand in and around fairy circles. They also examined hundreds of individual grass shoots and roots excavated at various intervals from within the circles and the surrounding areas.

After rain, the researchers found that grasses germinated both inside and outside fairy circles, but that within about 20 days virtually all of the young shoots inside a circle had died. They also found that the top eight inches of soil within fairy circles quickly dried out, something they hypothesize is caused when established plants surrounding fairy circles actively draw water toward them.

Plants are constantly transpiring — or losing water — through their leaves. Their roots, meanwhile, take water in. In Namibia’s sandy soil, this creates a vacuum effect that moves water from the interior of fairy circles toward the plants’ roots at the circle’s fringe and beyond. “It’s similar to you opening your window in winter and the warm air instantly moving out,” Dr. Getzin said.

The new paper also speaks to the termite hypothesis, which has been championed by Norbert Jürgens, an ecologist at the University of Hamburg in Germany. He reported in 2013 that fairy circles were in fact generated by sand termites that damage grass roots.

In the new paper, Dr. Getzin and his colleagues noted that termites were conspicuously missing from their study sites, and that they found no signs of root damage in grass that died after rainfall.

“We can say the reason is not termites, because there were no termites present at all,” Dr. Getzin said. “The reason is desiccation.”

Dr. Jürgens declined a request to comment.

Walter Tschinkel, an entomologist at Florida State University who was not involved in the research but who has published papers in support of the water-stress hypothesis, said the new findings provided “more nails in the termite coffin.”

“The support for the hydrodynamic explanation is now very strong, and the support for the termite cause is very weak,” Dr. Tschinkel said.

Yvette Naudé, an analytical chemist at the University of Pretoria in South Africa who was not involved in the research, agreed that the new study seemed to confirm that, “contrary to popular belief, termite activity does not cause the fairy circles.”

But she does not consider the mystery to be solved. “Many questions are still left unanswered,” she said.

Advocates of the water-stress hypothesis still need to contend with other explanations, Dr. Naudé said. She continues to suspect, based on earlier studies, that something about the composition of fairy circle soil is inhibiting plant growth.

Marion Meyer, a plant scientist at the University of Pretoria, said that while the new study showed “conclusively” that termites were not a factor, fairy circles might result from another plant, Euphorbia. According to research Dr. Meyer has published, it produces a milky, toxic sap that might be killing the grass in spots where it once grew, causing fairy circles to form.

Dr. Getzin disagreed with these interpretations, pointing to research that he said undercut both the soil content explanation and the Euphorbia hypothesis.

One of the reasons so many different fairy circle theories persist, Dr. Cramer said, is that it is exceedingly difficult to prove causation for “a long-lived ecological pattern that cannot be replicated in the lab.” To finally put the debate to rest, he called for “some manipulative experiments to test the ideas in the field.”

But someone other than Dr. Getzin will have to conduct such challenging experiments, because he has decided to retire from fairy circle research.

“After more than 20 years, I consider this chapter closed,” he said.

He plans to switch his focus to investigating “plant rings,” another odd natural occurrence in the Namib Desert. Plant rings look similar to fairy circles but are a distinct phenomenon, Dr. Getzin said — and so far, they have escaped the notice of other scientists.