Fairy circle enigma expands with global map

Mysterious fairy circles – regular, circular patches of bare ground surrounded by grass – have fascinated and flummoxed scientists for decades.

They’re scattered across West Australian and Namibian deserts, and their origins are poorly understood – they may come from water distribution, or termites, or something else.

A team of Spanish scientists has thrown yet another mystery into the mix: it turns out Namibia and Australia aren’t the only places that house fairy circles.

The researchers have used satellite imagery and AI to find fairy circles in 263 sites from 15 countries around the world, mostly in dry sandy regions.

“Our study provides evidence that fairy circles are far more common than previously thought, which has allowed us, for the first time, to globally understand the factors affecting their distribution,” says Manuel Delgado Baquerizo, leader of the BioFunLab at the Spanish National Research Council (CSIC), and co-author on a paper describing the research, published in PNAS.

Sites housing fairy circles include West Asia, the Sahel, and Madagascar. The researchers also found them in central Australia, further east than they’d previously been described.

The researchers trained a convolutional neural network, an algorithm that’s particularly good for classifying images, on fairy circles. They then conducted a global survey of satellite images, using the neural network to find fairy circle-like patterns.

Combining these results with other geographic data, the researchers were able to establish a few common themes in places that had fairy circles: the soil is low in nutrients and high in sand, and the climate is arid, hot and has very seasonal rainfall.

They also found a few other features common among many but not all of the fairy circles, like termite nests.

“This study has taken into account multiple variables hitherto not considered, such as albedo or the state of the aquifers. This is a particularly relevant factor, since the massive use of groundwater in arid areas around the world, including deserts, could disturb these formations,” says co-author Jaime Martínez-Valderrama, also from the CSIC.

The researchers found that areas with fairy circles showed more stable productivity than surrounding, circle-less areas.

“These results provide the first empirical evidence of increased stability of fairy circle productivity, a key property of ecosystems that is related to the stable provision of ecosystem services such as the amount of forage,” says co-author Fernando Maestre, a researcher at the Universidad de Alicante, Spain.

“These results also open the door to research on whether these spatial patterns can be indicators of ecosystem degradation with climate change, as is the case for other spatial patterns of vegetation in arid zones,” adds co-author Miguel Berdugo, from the Universidad Complutense de Madrid.

The researchers have made a “fairy circle atlas” – a database of fairy circle locations that other scientists can use to analyse the phenomena further.

“We hope that these unpublished data will be useful for those interested in comparing the dynamic behaviour of these patterns with others present in arid areas around the world,” says first author Emilio Guirado, a researcher at the Universidad de Alicante, Spain.