Those circles in the sand

For many years, examples were only known to exist in the Namib Desert. How I’d have liked to see these beauties when I was in the country!

I refer to “fairy circles”: round formations in barren ground, devoid of vegetation inside the circle but with sparse shrubs outside. In the Namib Desert, they are scattered over an area measuring more than 2,500 sq. km, resembling a polka-dot pattern when seen from the air (take a look: https://t.ly/K_pgF). The stark contrast between inside and outside the circle strongly suggests that they are man-made. Maybe some prankster sweeps the inside clean? But they are too numerous, and the circles actually too irregular, for that to be true.

So scientists have wondered, for years, about the origin of fairy circles. One suggestion was that there once were certain poisonous plants in these spaces, and when they died they left behind chemicals that prevented other plants from growing there. A scientist found “evidence of recent occupancy by gerbils”, underlined by how the circles “correspond closely in size to active or recently active gerbil colonies”, There are more theories, too.

But none of them seemed satisfactory or conclusive. In 2021, a German researcher called Stephan Getzin published a paper about fairy circles in which, based on careful research in the Namib Desert, he spelled out three characteristics they all have:

* These are “empty gaps” in grassland, without a central insect nest structure.

* They form surprisingly ordered patterns

* They require arid climates.

(Definition of ‘fairy circles’ and how they differ from other common vegetation gaps and plant rings, Stephan Getzin et. al, Journal of Vegetation Science, 19 November 2021).

A year earlier, Getzin had published another paper that offered a fascinating explanation for these circles. He suggested that they are formed by a mechanism that the great mathematician and computer scientist Alan Turing described in 1952.

In his paper, The Chemical Basis of Morphology, Turing suggested how patterns in nature—such as a leopard’s spots or stripes in the sand on a seashore—might arise. This is the “reaction-diffusion” mechanism, also called the “activator-inhibitor” principle. It features two chemicals, one an “activator” and one an “inhibitor”. As they diffuse through a medium (foetus, desert, whatever), they also react to each other, and that produces patterns. (See my All Kinds of Patterns, https://t.ly/qhY66).

Getzin’s hypothesis was that the fairy circle patterns are created by “the reaction-diffusion, or Turing mechanism…rooted in physics and pattern-formation theory”.

The idea here is that there’s only so much water to go around in these desert conditions. Plants seek to maximize their access to water. So, these circles form a certain spatial order that maximizes each circle’s access to water. That is, plants—desert grasses, mostly—on the perimeter of the circles have evolved to have the best possible access to water.

Getzin and colleagues spent three years looking for evidence for this hypothesis. They set sensors into the soil about 20cm deep, to record how much water the plants absorbed from the soil.

Following a dry 2020 and 2021, 2022 saw plentiful rainfall. As Getzin remarked, this allowed the team to “really follow how the growth of the new grasses was redistributing the soil water”. Something intriguing was happening with the circles. Water within them was getting used up fast, even though there were no plants to actually use it. The grasses on the perimeter and outside, on the other hand, were healthy and certainly not lacking for water.

Apparently those plants had evolved to create a vacuum of sorts around their roots, which drew water from inside towards themselves. In contrast, any grasses within the circle could not find enough water to survive. And, we get a circle because of an old geometric truth: It’s the shape that encloses the greatest area for a given circumference. If each individual plant is seeking—like the others—to get the maximum water, a circle is the most logical shape for a collection of such plants.

Getzin called this process an example of “ecohydrological feedback”. The dry circles effectively become water tanks for plants on their circumference, though by inhibiting the growth of plants within. In a real sense, the plants on the circumference have “self-organized” to collectively battle dry conditions.

For me, all this is a lot to—well—absorb and comprehend. But never mind. As with pretty much every scientific theory ever proposed, this one has been challenged—in Australia, at any rate—and in a way that’s fascinating in its own right.

In 2016, fairy circles were also found in Western Australia. Are they also a result of Getzin’s “self-organization” process? Well, a team of Australian researchers have other ideas. They make the case that “these regularly spaced, bare and hard circles in grasslands are pavement nests occupied by Drepanotermes harvester termites. These circles [are] called ‘linyji’ [in an aboriginal language].” (First Peoples’ knowledge leads scientists to reveal ‘fairy circles’ and termite linyji are linked in Australia, Fiona Walsh et. al, Nature, 3 April 2023, https://t.ly/1HI2t).

Drawing on Aboriginal (First People) art and customs, the paper makes a compelling case for this pavement (meaning close to the surface) termite-nest theory.

For one, there is plenty of Aboriginal art that shows regularly spaced termite nests, and the paper actually puts some of it side-by-side with images of the circles, for comparison.

For another, termites were and remain crucial to Aboriginal lives. They are an important food source, they are revered, and Aborigines used pavement nests as “surfaces to thresh seed and process other materials”.

Aborigines are “clear that the bare circles are occupied by termites”. Besides, the soil under the circles is denser than the surrounds. This means that after any rain, water tends to stay on the surface longer in a fairy circle than outside—and so these circles become water sources.

There’s much more. I doubt we have heard the final word on fairy circles. For now, I’m just delighted that efforts to understand them have drawn on termites, Aboriginal art and Alan Turing.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

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Updated: 19 Oct 2023, 10:35 PM IST