Fair game with fairy circles

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#02 2023

Fair game with fairy circles

By Joh Henschel*, Research Associate, SAEON Arid Lands Node

Round bare patches that pockmark the eastern sand plains of the Namib Desert have been targets of more theories than one can shake a stick at, considered fair game for fairy tales. However, these fairy circles, as the bare patches are known, are actual abodes of dainty little fair-coloured creatures called Psammotermes allocerus, which translates to ‘sand termites with disparate teeth’.

Although these ingenious insects are not fairies, they exert powers far beyond them. Somehow, they convert scant temporary pastures into lusher long-lasting pastures that become fair game for game and other organisms, large and small.

This was recently revealed in fine detail in the book Fairy Circles of the Namib Desert Ecosystem – engineering by subterranean social insects (Jürgens, 2022) with fifteen contributing authors. Genetic analyses of 65 populations from Angola, Namibia and South Africa revealed that Psammotermes allocerus is actually a species complex. There are at least seven distinct genetic groups with distinctive morphological and phenotypical characteristics. Different clades construct distinctive tunnel systems, nests, royal chambers and fairy circles in different areas.

Fairy circles are regularly spaced bare patches bordered by rings of taller grasses on sandy plains of the eastern Namib Desert (Photo: Joh Henschel)

How do these industrious ecosystem engineers do it?

They kill grass seedlings by attacking the roots. This leaves bare sand patches where rainwater is not taken up and transpired by grass, so the water can percolate unhindered into the sand to deeper than 50 cm, where evaporation is minimal. There, the termites can live comfortably in moist surroundings even during prolonged droughts.

However, the fruits of their labour to secure water for themselves at these depths have not gone unnoticed by many other thirsty organisms. Most prevalent are the lush grasses growing at the edge of the bare circles, called perennial belts, that tap into the deep water to form conspicuous rings around the circles.

The book unfolds full evidence of termites being the founders of fairy circles, weighing in heavily against other assertions from opposing viewpoints by those who fail to observe these secretive termites at work or who do not like fairies being demystified.

The full details of the debates are complex, enhanced by sand termites not being the only organisms that form bare patches in the desert, though not all fit the definition of Namib Fairy Circles as regularly spaced circular bare patches on the eastern desert grassy plains. Read the book.

Sand termite nests (left), here shown with worker (centre) and soldier (right), are located at the edges of fairy circles next to the perennial belt of lush grass that they do not clear, unlike the grasses they clear from the circle’s bare patch (Photos: Joh Henschel)

What this engineering means

Fairy circles are like mini oases on desert plains where ephemeral grasses quickly take up most of the available rainwater to complete their life cycle by the time they run out of water. The pocket of moisture under the denuded sand patches of fairy circles and the associated perennial belt attracts many other organisms that invade the termites’ domain. Populations of herbivores – such as springbok, oryx, mountain zebra, ostrich and gerbils – as well as various insects, benefit from the grass of the perennial belt.

There is more, as some plants other than grass, such as the shrubby spider-plant (Cleome suffruticosa), somehow manage to grow in the centre of fairy circles, and the termites compensate by enlarging the circumferences of the circles. Sometimes harvester ants, Carebara kunensis, capitalise on the sand moisture and establish their nests in the middle of fairy circles, preventing the termites from evicting them. Some ants prey on termites or their eggs, while other predators such as the golden mole, thread snake and ammoxenid spiders go a step further by specialising in hunting these termites.

In turn, other predators recognise fairy circles as convenient targets to hunt the termite hunters. For some, fairy circles are convenient for other uses, such as nesting ostriches or sand-bathing zebra. Many more organisms – bacteria, fungi, plants and animals – relate to fairy circles. In a nutshell, fairy circles are important localised biodiversity hotspots.

Without sand termites, the Namib Desert, which extends into South Africa across the Namaqualand, would be less lively than it is. There would also be less lively discussion about it, as the conspicuous fairy circles not only attract attention but also provoke the imagination.

However, the termites in question extend across a much wider area of the arid western half of South Africa, across the Northern Cape and Karoo regions of the Western Cape, including Tierberg-LTER, where they are either ignored or considered a pest. There is no reason to believe that, even where they are less visible, they play lesser roles as ecosystem engineers affecting ecosystem processes.

SAEON should continue improving knowledge of dualities of this kind, especially because long-term ecological research facilitates the recognition of the underlying drivers and ultimate effects even of tiny agents such as termites.

* My contributions concerned the above-mentioned ‘game and other organisms, large and small’ (Henschel and Jürgens, 2022), and I also participated in a study of the circle-forming termites with PhD candidate Felicitas Gunter (Gunter et al., 2022a; Gunter et al., 2022c, b; Gunter et al., 2023).

Sand-swimming golden moles track the boundaries of fairy circles by listening to the activities of termites in their nests, shown here from the arrival and departure points, with traces all around the circle (Photo: Joh Henschel)

The book that puts fairy tales to rest (Photo: Klaus Hess Publishers)

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