An Ancient Desert in Africa
Namib, the oldest desert in the world, is located in southern Africa. Daily mean temperature is about 40°C with an annual precipitation less than 50 millimetres. This arid and semi-arid conditions have been existed for at least 55 million years . The desert stretches for more than 2,000 kilometres from south to north and 160 kilometres from east to west across Angola, Namibia and South Africa. It is unusual to have a desert next to the ocean. The unique geographic feature of Namib Desert is ascribed to a cold current called the “Benguela Current“. Flowing along the west coast of South Africa from Antarctic, humid air from the Atlantic Ocean is cooled down by the current. As cool and humid air is denser than warmer and lighter air, no clouds are formed in Namib, making this place to be one of the driest areas in the world . Water is essential for every life on Earth, a place like this would be extremely hard for organisms to survive. Surprisingly, Namib Desert is the only arid biodiversity hotspot in the world where more than 9,000 animal and 4,000 plant species could be found here included some endemic species like golden mole (family Chrysochloridae), Peringuey’s adder (Bitis peringueyi) and welwitschia (Welwitschia mirabilis) (Fig. 1).
Figure 1: Namib Desert is a coastal desert along Atlantic ocean. As one of the biodiversity hotspots, some creatures can be only found here (from left to right) like golden mole, Peringuey’s adder and welwitschia (Photograph by Lernidee Erlebhisreisen GmbH).
Fairy Circles in endless desert
However, the most spectacular scenario in Namib Desert is neither the living organisms nor the endless desert, but thousands of circular patches over the land. These circles have been given with a beautiful name, the Fairy Circle. Approximately 5 metres in diameter, plant is absent at the centre with perennial grasses growing at the margin (Fig. 2). The cause of fairy circles was considered to be one of the nature’s greatest mysteries. Since the first observation in 1920s, different hypotheses had been proposed but they were tested and rejected one by one. Currently, only two hypotheses remain.
Figure 2: As one of the Nature’s greatest mysterious, the fairy circles have been mentioned since 1920s. Until 2015, plant competition hypothesis and sand termite hypothesis are the most convincing explanations (Photograph by George Steinmetz, National Geographic).
One is “plant competition hypothesis”. It suggested competition among plants for water forming a gap between them. Mathematical model seems to have good match with the spatial pattern of the circles. Nevertheless, the model was argued to be too-generalised as similar prediction can be reached by territorial interactions as well. Most importantly, if this hypothesis is correct, a lot of plants should be found within patch with wet soil. But no such observation was reported so far . It is therefore the alternative hypothesis was thought to be more convincing, which is called the “sand termite hypothesis”.
Sand termite, the Guardian of Fairy Circle
Sand termite hypothesis suggested the fairy circles are created by sand termites (Psammotermes allocerus). These tiny creatures are highly specified in building tunnels that penetrate soil and form a complex network around their nest . Science study revealed their foraging behaviour of eating plant’s root and leading the foraged plant to death. According to the hypothesis, a nest with intense network is built beneath short-lived vegetation. Termites use the tunnels to search and eat every plant root they found. Eventually, all short-lived plants around the colony will be killed as they cannot absorb water and nutrition without roots. A bare patch of sand is then formed (Fig. 3).
Figure 3: Sand termite hypothesis suggests sand termite, P. allocerus, is the cause of fairy circles in Namib Desert. Top: A complex network built beneath short-lived vegetation. Middle: Using the tunnels, sand termites eat every plant’s root they found. Bottom: Plant dies, leaving a bare sand land at the centre (Image from NHK, Life Force).
The hypothesis was supported by strong and direct evidences. First, while few species were observed to live within fairy circle, only sand termite is found in every circle. Second, cemented sand layers represent where the sand termites foraged. As a result, they were found with high frequencies (80-100%) in every circle examined . Third, tunnels were found beneath every bare patch from a few centimetres to decimetres in length. What is more, they are the only species found in a newly formed circle. Last but not least, adult termites were observed to widen the circle by slowly consuming the perennial grass at the margin .
Ecosystem Function of Bare Patch
Most groundwater in desert will be rapidly absorbed by neighbouring plants. Hence, it is unlikely to have a lot of water in soil. However, things are different in fairy circle where vegetation is absent. Lacking transpiration by plants, water can be retained in soil for a long time after a rainfall event. For instance, more than 5% volumetric water content (approximately 98% relative air humidity) was reported at 40-cm depths in fairy circles over four years . Additionally, this termite-made reservoir allows neighbouring perennial grasses to be thrived. These grasses were thought to be consumed by termites during dry reasons and drought years. In order words, they are the “backup” food supply .
The Oasis in Namib Desert
A significant increase of species richness has been observed in locations with fairy circle . Water stored by sand termites allows plants to grow strong and tall. The thriving vegetation becomes a new habitat for various insects and herbivores. Insects included sand termites are vital for namaqua chameleon (Chamaeleo namaquensis), bat-eared fox (Otocyon megalotis), aardwolf (Proteles cristata), aardvark (Orycteropus afer) and many other endemic species in Namib. As a key role in the hotspot, fairy circle in Namib Desert created a lush oasis to arid land and saved countless life.
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