Mysterious fairy circles in the Namibian desert finally explained
The Namib Desert is covered in regular patterns of bare circles whose origin is fiercely debated by researchers – but now it seems that the two main explanations are correct.
One camp claims that the empty areas, known as fairy circles, are created by termites under the ground that clear the vegetation around their nests. By making the soil porous, the argument goes, they establish permanent reservoirs of rainwater 50 centimeters below the surface, which supports them and the surrounding ecosystem.
An alternative idea is that the circles are explained by plants competing for water. Plants help their closest neighbors by creating shade and keeping water on the surface of the soil, but annoy those farther away by growing long roots that pull water out of the soil.
The theory of competition in water may explain regular patterns perfectly, but has not been proven in any test, says Corina Tarnita from Princeton University. Meanwhile, the termite theory is supported by observations of termite nests in the circles, but could not explain why the patterns are so regular.
âEach brought what we thought were compelling arguments,â she says.
So Tarnita and her colleague Rob pringle turned to computer models. They first created one to determine if termites could end up with even spacing between their nests.
Termites feed in a circular area around their nest. When they encounter a smaller termite colony, they destroy it and take over the territory.
But when two colonies of similar size clash, neither can defeat the other and they establish a border.
The model showed that this competition between termite colonies can lead to a regular honeycomb pattern, with each colony surrounded by six neighboring colonies. In addition to Namibian fairy circles, Tarnita and Pringle say this motif can be seen in termite colonies in Arizona, Brazil, Kenya, Mozambique, and Australia.
The couple then wondered what would happen if the competition of plants for water was also at work. “Why does it have to be one or the other?” Tarnita asks.
Their model predicted that there would be two models: a large-scale model of bare circles, created by termites, and a smaller model in vegetation between circles, resulting from competition for water.
When they went to Namibia to see the fairy circles for themselves, they saw these smaller circles, about eight inches in diameter and eight inches apart. They had not been reported before.
âThe fairy circles got so much attention that people didn’t pay attention to the appearance of the vegetation between the circles,â says Tarnita. Their observations match the model’s prediction, suggesting that the two mechanisms act simultaneously.
Norbert JÃ¼rgens from the University of Hamburg, Germany, a longtime proponent of termite theory, is happy that the new study supports their involvement. âNow it’s a balanced discussion,â he says.
But not everyone is convinced. Last year, Stephane Getzin at the Helmholtz Center for Environmental Research in Leipzig, Germany, reported fairy-circle patterns in the Australian outback, much like those found in Namibia.
Getzin says there are no termites in parts of Namibia and Australia where circles are found, and in others there is no correlation between circles and termites. âLogically, if there are fairy circles without the presence of termites, the termite theory cannot be considered a strong explanation for the phenomenon,â he says.
JÃ¼rgens says in a decade of research he’s never seen fairy circles without at least some indicators of termites, and offers to help anyone who can’t find them. âI’m happy to help,â he says.
It seems that the turf war is not over yet.
Journal reference: Nature, DOI: 10.1038 / nature20801
Read more: What causes mysterious fairy circles to appear in the desert?
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