Secrets of Namibia’s fairy circles demystified: Plants organize themselves

Scientists have been puzzling over the origin of the fairy circles in Namibia for almost half a century. It boiled down to two main theories: either termites were responsible, or plants somehow self-organized. Now, researchers at the University of Göttingen, who benefited from two exceptionally good rainy seasons in the Namib Desert, show that the grasses in the fairy circles immediately after the rain died, but termite activity did not cause the bald patches.

Instead, continuous soil moisture measurements show that the grasses around the circles have severely drained the water within the circles, likely causing the grasses within the circles to die off. The results were published in Perspectives in plant ecology, evolution and systematics.

About 80 to 140 kilometers offshore in the Namib there are millions of fairy circles – circular gaps in the grassland, each a few meters wide, which together form a distinctive pattern across the landscape and are visible for miles. The researchers tracked the sporadic rain events in several regions of this desert and examined the grasses, their roots and shoots, and possible root damage by termites.

Termites, tiny insects that live in large colonies around the world, have often been blamed for the death of grasses. The researchers carefully studied the circumstances of the death of grasses within fairy circles just after the rains that triggered new growth of the grasses. Additionally, they installed soil moisture sensors in and around the fairy circles to record soil water content at 30-minute intervals from the 2020 dry season to the end of the 2022 wet season.

This allowed the researchers to capture exactly how the growth of the emerging grasses around the circles affected the soil water inside and around the circles. They examined the differences in water infiltration between the interior and exterior of districts in ten regions in the Namib.

The data shows that about ten days after the rain, the grasses within the circles had already begun to die back, while most of the interior of the circles showed no grass germination at all. Twenty days after the rain, the fighting grasses in the circles were completely dead and yellowish in color, while the surrounding grasses were vital and green.

When the researchers examined the roots of the grasses inside the circles and compared them to the green grasses outside, they found that the roots inside the circles were as long or even longer than those outside. This indicated that the grasses were striving for root growth in search of water. However, the researchers found no evidence that termites feed on roots. It was not until fifty to sixty days after the rains that root damage to the dead grasses became more clearly visible.

dr Stephan Getzin, Department of Ecosystem Modeling at the University of Göttingen, explains that “the sudden absence of grass in most areas within the circles cannot be explained by termite activity, as there was no biomass available for these insects to feed on could feed. But more importantly, we can show that the termites are not to blame because the grasses die off immediately after the rains, with no evidence of root-feeding creatures.

When the researchers analyzed the data on soil moisture fluctuations, they found that the decline in soil water inside and outside the circles was very slow after initial rains, when the grasses were not yet established. However, when the surrounding grasses were well established, the decline in soil water after rainfall was very rapid in all areas, although there were almost no grasses in the circles to absorb the water.

Getzin explains: “Under the intense heat in the Namib, the grasses constantly sweat and lose water. Therefore, they create a vacuum of soil moisture around their roots and water is drawn towards them. Our results are in strong agreement with those of the researchers, who showed that water in the soil diffuses rapidly and horizontally in these sands, even over distances of more than seven meters.

Adds Getzin, “By forming highly patterned landscapes of evenly spaced fairy circles, the grasses act as ecosystem engineers, directly benefiting from the water resource provided by the vegetation gaps. In fact, we know of related self-organizing vegetation structures from various other harsh arid regions in the world, and in all these cases the plants have no other chance of surviving than growing in precisely such geometric formations.”

This research has implications for understanding similar ecosystems, particularly with regard to climate change, as plant self-organization buffers negative effects of increasing desiccation.