Not to be confused with fairy rings, which are circles of mushrooms and other fungi. These fairy circles were photographed in Namibia, and they’re a feature of the semi-arid margin of the Namib Desert. They form on sandy soil in regions where the annual rainfall is between 50 and 150 mm. They have a bare centre, and a raised rim with a strong growth of grass. And they have a life cycle—appearing at about two metres diameter, growing to 10 or 12 metres over a period of decades, and then dissipating, to be replaced by new circles. There’s a region of (relatively) high soil water content immediately under the bare central area. Rainwater falling in the bare area quickly percolates down into the soil, and the soil here suffers less evaporative loss because the water isn’t being sucked up by plants and lost by transpiration from their leaves. A cross-section of a fairy circle looks like this, with the wetter region marked in blue:
When we saw them in 2009 they were a bit of a mystery, with multiple competing explanations for how they formed. The game guides in the NamibRand told us that an experiment was afoot in the reserve to try to narrow down the possible mechanisms, but they didn’t have any details at that time. It turned out to be a multifaceted, five-year experiment that was reported in PLoS One last year.
More of that in a minute. First, a summary of the competing explanations:
- Insect feeding (ants or termites)
- Residual plant toxins (from, for instance, Euphorbia)
- Poisonous gases
- Soil radioactivity
- Nutrient deficiency
- A “self organizing” emergent property of the vegetation
So Walter Tschinkel’s experiment in the NamibRand reserve was designed to test some of these possibilities. (Tschinkel WR. Experiment Testing the Causes of Namibian Fairy Circles. PLoS ONE (2015) 10(10): e0140099.) He buried an impervious membrane beneath some fairy circles, but this did not alter their density and growth compared to controls. He transferred soil from the circles to a cleared area to see if it would create new circles (it didn’t), and soil from areas outside the circles into the bare zones of existing circles, to see if that would “heal” them (it didn’t). And he tried adding fertilizer to the bare circles, with no effect.
So Tschinkel effectively eliminated hypotheses involving gases seeping from below, toxins in the circle soil, and nutrient deficiencies.
While Tschinkel’s study was on-going, Norbert Juergens published an investigation into the role of insects in the circles. (Juergens N. The Biological Underpinnings of Namib Desert Fairy Circles. Science 2013 339: 1618-21.) Juergens had sampled insect populations associated with fairy circles and had discovered that, although several species of ant and termite appear to be associated with fairy circles, only one species turned up everywhere these fairy circles form: the sand termite Psammotermes allocerus. And P. allocerus turned up frequently in the circles sampled (80-100%) and also early in their development (before the onset of water accumulation and the characteristic grassy rim). So Juergens proposed that the termites were effectively “farming” the circles—creating them by killing a patch of grass, exploiting the water that accumulated in the resulting bare patch, and then expanding the circle by eating the grass on its rim, which thrived because of the additional water in the soil.
But, as Tschinkel subsequently pointed out, association is not causation. Juergens had observed a correlation between fairy circles and P. allocerus nests, but had carried out no test interventions (removing the termites to see if a circle recovered, for instance). It’s possible that the termites were merely exploiting a patch of dying grass and an area of water accumulation caused by something else. It would also be interesting to know whether or not P. allocerus colonies typically develop with a scale and spacing that matches the behaviour of fairy circles.
Because the spacing of fairy circles is quite striking. Here’s a Google Earth view of the fairy circles near Wolwedans Dunes Lodge, where we stayed during our time in the NamibRand:
There’s a natural spacing to them, as if each circle somehow repels those around it. And this can be confirmed mathematically—on average, the circles are maximizing their own space in competition with their neighbours. And that’s what leads to the idea that the circles are somehow self-organizing—that simple local rules about how plant grow and utilize water leads to the emergence of a global pattern.
You can see a similar emergent pattern in the convection cells that form in a pan of boiling water:
There’s uniform heating at the bottom of the pan, which makes the water at the bottom less dense than the surface water. But hot water can’t rise to the surface everywhere—there’s got to be room for cold surface water to descend as well. So the water spontaneously sorts itself out into multiple cells with rising water in the middle and sinking water at the edges.
Likewise, we can imagine fairy circles arising because rainfall is insufficient to support a continuous carpet of grass. Instead, bare patches catch enough water to support a surrounding halo of grass, and by doing that inhibit the formation of nearby bare patches. As that situation plays out, a landscape dotted with fairy circles might be one stable solution that could emerge.
Hence the excitement, last month, at the report of fairy circles in Australia (Getzin et al. Discovery of Fairy Circles in Australia Supports Self-Organization Theory. PNAS 2016 113(13): 3551-6). We now have a second, strikingly similar pattern of bare circles with surrounding vegetation, occurring in a near-desert environment. Here’s an image from Google Earth:
But this pattern involves different vegetation (spinifex grass), no consistent association with termites, and a different mode of water collection—in the Australian case, the bare circular patch develops a hard crust and sheds water from its surface towards plants at its edges.
This lends credibility to the idea that this pattern is something that can emerge spontaneously when biomass is trying to make the best of scarce water resources. So those Namibian termites might well just be part of the pattern, rather than its cause.