Bare areas caused by Hodotermes mossambicus in the northern Namib Desert

Vegetation patterns such as rings, bands, and spots are recurrent characteristics of resource‐limited arid and semiarid ecosystems. One of the most recognizable vegetation patterns is the millions of circular patches, often referred to as “fairy circles,” within the arid grassland matrix extending over hundreds of kilometers in the Namib Desert. Several modeling studies have highlighted the role of plant‐soil interactions in the formation of these fairy circles. However, little is known about the spatial and temporal variabilities of hydrological processes inside a fairy circle. In particular, a detailed field assessment of hydrological and soil properties inside and outside the fairy circles is limited. We conducted extensive measurements of infiltration rate, soil moisture, grass biometric, and sediment grain‐size distribution from multiple circles and interspaces in the Namib Desert. Our results indicate that considerable heterogeneity in hydrological processes exists within the fairy circles, resulting from the presence of coarser particles in the inner bare soil areas, whereas concentration of fine soil occurs on the vegetated edges. The trapping of aeolian and water‐borne sediments by plants may result in the observed soil textural changes beneath the vegetation, which in turn, explains the heterogeneity in hydrological processes such as infiltration and runoff. Our investigation provides new insights and experimental data on the ecohydrological processes associated with fairy circles, from a less studied location devoid of sand termite activity within the circles. The results seem to provide support to the “self‐organization hypothesis” of fairy circle formation attributed to the antiphase spatial biomass‐water distributions.

Fairy circles (FCs) are barren circular patches of soil surrounded by grass species. Their origin is poorly understood. FCs feature in both the gravel plains and the dune fields of the Namib Desert. While a substantial number of hypotheses to explain the origin and/or maintenance of fairy circles have been presented, none are completely consistent with either their properties or their distribution. In this study, we investigated the hypothesis that FC formation in dunes and gravel plains is due to microbial phytopathogenesis. Surface soils from five gravel plain and five dune FCs, together with control soil samples, were analyzed using high-throughput sequencing of bacterial/archaeal (16S rRNA gene) and fungal (internal transcribed spacer [ITS] region) phylogenetic markers. Our analyses showed that gravel plain and dune FC microbial communities are phylogenetically distinct and that FC communities differ from those of adjacent vegetated soils. Furthermore, various soil physicochemical properties, particularly the pH, the Ca, P, Na, and SO4 contents, the soil particle size, and the percentage of carbon, significantly influenced the compositions of dune and gravel plain FC microbial communities, but none were found to segregate FC and vegetated soil communities. Nevertheless, 9 bacterial, 1 archaeal, and 57 fungal phylotypes were identified as FC specific, since they were present within the gravel plain and dune FC soils only, not in the vegetated soils. Some of these FC-specific phylotypes were assigned to taxa known to harbor phytopathogenic microorganisms. This suggests that these FC-specific microbial taxa may be involved in the formation and/or maintenance of Namib Desert FCs. Fairy circles (FCs) are mysterious barren circular patches of soil found within a grass matrix in the dune fields and gravel plains of the Namib Desert. Various hypotheses attempting to explain this phenomenon have been proposed. To date, however, none have been successful in fully explaining the etiology of FCs, particularly since gravel plain FCs have been largely ignored. In this study, we investigated the hypothesis that microorganisms could be involved in the FC phenomenon through phytopathogenesis. We show that the microbial communities in FC and control vegetated soil samples were significantly different. Furthermore, we detected 67 FC-specific microbial phylotypes, i.e., phylotypes present solely in both gravel plain and dune FC soils, some of which were closely related to known phytopathogens. Our results, therefore, demonstrate that microorganisms may play a role in the formation and/or maintenance of Namib Desert FCs, possibly via phytopathogenic activities.

Endemic to the Namib Desert, Fairy Circles (FCs) are vegetation-free circular patterns surrounded and delineated by grass species. Since first reported the 1970's, many theories have been proposed to explain their appearance, but none provide a fully satisfactory explanation of their origin(s) and/or causative agent(s). In this study, we have evaluated an early hypothesis stating that edaphic microorganisms could be involved in their formation and/or maintenance. Surface soils (0–5cm) from three different zones (FC center, FC margin and external, grass-covered soils) of five independent FCs were collected in April 2013 in the Namib Desert gravel plains. T-RFLP fingerprinting of the bacterial (16S rRNA gene) and fungal (ITS region) communities, in parallel with two-way crossed ANOSIM, showed that FC communities were significantly different to those of external control vegetated soil and that each FC was also characterized by significantly different communities. Intra-FC communities (margin and centre) presented higher variability than the controls. Together, these results provide clear evidence that edaphic microorganisms are involved in the Namib Desert FC phenomenon. However, we are, as yet, unable to confirm whether bacteria and/or fungi communities are responsible for the appearance and development of FCs or are a general consequence of the presence of the grass-free circles.

 Fairy circles are exceedingly regularly spaced barren circular patches in arid landscapes, typically encircled by a ring of taller grasses. These vegetation patterns occur in Southwestern Africa and Australia and have also been suggested to occur in North Africa, Middle East and Madagascar. The enigmatic origins of fairy circles in arid landscape shave intrigued ecologists and sparked heated debate about the two main competing hypotheses: the termite origin and vegetation self-organization hypotheses.In the southern part of the Giribes Plains, Kunene region, northwest Namibia, fairy circles form in a distinctive, chain-like arrangement along drainage lines that run from north to south, closely aligned along a slope. These fairy circles are unusual in their extreme elongation, with the most extreme case measuring 32.5 meters long and only 7.7 meters wide. In contrast, the fairy circles in the rest of the Giribes outside the drainage lines are typically circular and exhibit a highly ordered, hexagonal pattern. Based on field work, remote sensing and mathematical modeling we explain the formation of these unique fairy circles.The soil in the matrix between the circles is covered by physical crust, with some areas featuring a thin biocrust. This is the only place in Namibia where soil crust developed in the matrix. This crust causes the matrix soil to be nearly four times more compact than the soil within the fairy circles. The sand within the fairy circles is coarser (D50 ~600 µm) compared to the matrix soil (D50 ~300 µm), which supports the formation of the crust. Interestingly, sand in fairy circles not aligned with the drainage lines is also coarser (D50 ~450 µm). Hydraulic conductivity, measured using a mini-disk infiltrometer, is three to four times greater within the fairy circles than in the surrounded matrix.Building on these field observations, we hypothesize that the elongated shape of the fairy circles results from anisotropic soil water diffusion. Water diffuses more readily along the drainage lines than in the surrounding matrix, causing the fairy circles to expand more rapidly along the watercourses than laterally. To test this hypothesis, we used the mathematical model of Zelnik et al. (2015), which simulates biomass and soil water densities under varying water-soil diffusion coefficient ratios, r (r=1outside the drainage lines and r>1 inside the fairy circle) and precipitation rates .The simulations indicate that, for moderate diffusion ratios and varying precipitation rates, elongated fairy circles form along the drainage lines, while circular fairy circles emerge when the diffusion ratio is lower. The results agree with remote sensing analysis of images take from a drone.  The stability of the pattern to different precipitation rates and r values was also studied. These results support the hypothesis that anisotropic soil water diffusion contributes to the elongated shape of the fairy circles in the Girbies plain, although other factors may also play a role. Indirectly our work supports the self-organization hypothesis for the origin of fairy circles.  The formation of the crust in the matrix remains is still an open question for future research. 

ContextNamibia’s fairy circles (FCs) form an extraordinary vegetation pattern along the Namib Desert. Recent evidence from multiple fieldwork activities is increasingly supporting the view that FCs result from biomass-water feedbacks and plant self-organization.ObjectivesTo shed light on these biomass-water feedbacks, we focused here on a temporal analysis of the spatial FC patterns with regard to vegetation response after rainfall events.MethodsWe analyzed the distribution of FCs in 10 drone-mapped study plots of the Namib and related their spatial patterns to the soil infiltrability. Additionally, we mapped three plots repeatedly during the rainfall seasons 2020–2023 to assess how the emerging grasses within FCs changed the FC patterns after rainfall.ResultsWe found that the most regular, spatially periodic, FCs occurred in areas with deep aeolian sands where rain water infiltrates very quickly and homogenously across the study plot, which enables the most symmetric competitive interactions between the grasses. After ample rainfall following a drought period, between 58 and 34% of all mature FCs revegetated. These 1092 closing FCs were 73 times more than the 15 new FCs that formed during the same time. The closing FCs occurred in areas where there was locally a higher density of FCs, which act as underground water sources for the surrounding grasses.ConclusionsOur study shows that the dynamic vegetation response to rainfall and soil water is the key driver of the FC patterns. Overall, the research underlines that Namibia’s fairy circles are a self-organized emergent vegetation pattern that is driven by biomass-water feedbacks and the competition of grasses for limiting water resources.

Very high extinction risk for Welwitschia mirabilis in the northern Namib Desert

AimsThe fairy circles along the Namib Desert in southern Africa are round grassland gaps that have puzzled scientists for about 50 years. With the discovery of fairy circles in Australia in 2016, the debate on the origin of the circles has been extended to a new continent. Research interest on the topic has since then risen strongly but so has the use of the term “fairy circle”. This term has become more imprecise and, by analogy, has been applied to circular vegetation gaps or plant rings that are largely unrelated to fairy circles. For this reason, we define the concept of fairy circles by identifying their three main characteristics based on in situ field observations and soil excavations to larger‐scale spatial patterns, and regional‐scale distribution.ResultsFollowing this approach, fairy circles are defined by: (a) being “empty gaps” in grassland without a central insect‐nest structure; (b) their ability to form spatially periodic patterns, which are regular hexagonal patterns with an extraordinary degree of spatial ordering; and (c) their strongly regional distribution confined within a narrow arid climatic envelope. In these combined traits, fairy circles differ from other common vegetation gaps which, for example, always have a central insect‐nest structure and may occur across broad climatic gradients on continents. Also plant rings have their own specific characteristics that largely differ from the combined attributes of genuine fairy circles.ConclusionsThere are many other vegetation‐gap patterns in arid lands but if such gaps cannot jointly show the three characteristics defining the fairy circles, they should be carefully discussed on their own, rather than mixing them up with fairy circles. Our synthesis provides a new etymology for the different types of vegetation gaps and rings, aiming to guide the reader through various classes of circular plant patterns.

The generaEremohaplomydasBequaert, 1959,HaplomydasBezzi, 1924, andLachnocorynusHesse, 1969 (Diptera: Mydidae: Syllegomydinae) are revised. Currently, four species are known from southern Africa,i.e.,Eremohaplomydas desertorumBequaert, 1959 from north-western Namibia,Haplomydas crassipesBezzi, 1924 widespread in southern Africa,Lachnocorynus chobeensisHesse, 1969 from northern Botswana, andLachnocorynus kochiHesse, 1969 from northern Namibia. Four new species,Eremohaplomydas gobabebensissp. nov.andEremohaplomydas whartonisp. nov.from the central Namib desert of Namibia,Eremohaplomydas stomachorissp. nov.from the northern Namib desert in Namibia, andLachnocorynus stenocephalussp. nov.from north-eastern Zimbabwe are described.Lachnocorynus kochiis synonymized withLachnocorynus chobeensis. Distribution, biology, occurrence in biodiversity hotspotssensuConservation International and seasonal imago flight activity are discussed. Descriptions/redescriptions, photographs, specimen occurrence data, and identification keys (both dichotomous and matrix-based) to species are provided and made openly accessible in data repositories to support and accelerate future studies of the included taxa. An updated identification key to the Mydidae genera of the Afrotropical Region is provided. The placement of the three genera in the subfamily taxon Syllegomydinae is discussed and several morphological features, such as an extremely reduced proboscis in some species, a unique wing venation inEremohaplomydas gobabebensissp. nov., and the unique metathoracic coxa, are discussed.

Plant water stress, not termite herbivory, causes Namibia’s fairy circles

In the hyperarid Namib Desert, the sand termite Psammotermes allocerus Silvestri, 1908 (Isoptera: Rhinotermitidae) establishes colonies that create conspicuous, barren patches known as 'fairy circles' on permeable, sandy soils. The central bare areas of fairy circles serve the key function of storing moisture received from sparse rainfall. The sandy soil texture allows rapid infiltration and percolation of precipitation, while localized herbivory by the termites creates the bare patch, thereby reducing the rapid loss of soil moisture by the uptake and transpiration of water by plants. The resulting storage of rain water even during prolonged periods of drought enables perennial life in hyperarid desert environments and forms a globally unique example of ecosystem engineering by social insects. During the past decade, most publications primarily debated the origin of fairy circles. Here, we contribute to the special issue with a focus on the functional and evolutionary dimension of the structure of the Psammotermes colony with two differing nest types and two spatially separated key resources, as a successful adaptation to extreme desert environment. The paper is primarily a review and a synthesis of previous work, with the inclusion of new, relevant findings. This article is part of the theme issue 'The evolutionary ecology of nests: a cross-taxon approach'.

The Namib Desert, as represented in Iona National Park, provides a living museum of the adaptations of animals and plants to extreme environments. This chapter illustrates the morphological, behavioural and physiological adaptations resulting from natural selection in response to extreme conditions of aridity and temperature. The role of water is described as the ‘currency of life’ in the Namib. Surface area to volume ratios in animals and plants determine rates of water loss through evaporation, with many succulent plant species displaying the short, stout water conserving pachycaul life form. Water capture from fog is achieved by ‘fog basking’ and ‘sand trenching’ by beetle species, and by grazing at night (when water content of grasses is highest) by antelope. Competition for food in low productivity environments such as the Namib takes many forms, including habitat partitioning, cooperative behaviour and efficient food storage mechanisms. Unique Namib natural phenomena, such as the ‘fairy circles’ - bare patches in extensive desert grasslands - have received intensive study by multiple research groups. Arguments proposing divergent hypotheses demonstrate the scientific method in action.

Terricolous lichens are the dominant vegetation in expansive areas of the Namib Desert, where fog is the main source of moisture and other vegetation is scarce. They play several important roles in soil crust stabilization and in the primary production of the Namib Desert ecosystem, yet little is known about the diversity and distribution of lichens in the northern unexplored regions of the Namib. To our knowledge, this study is the first survey of terricolous lichens to be carried out in the northern Namib Desert. Seven soil crust habitat types were identified in the study area, and a total of twenty-eight soil crust lichen species was recorded. The survey uncovered lichen species that may be unique to the northern Namib, including vagrant species, and common species were frequently found in a vagrant form. The crustose group was the most widely represented. Overall community compositions differed from those found in other regions of the Namib, and distribution patterns suggest a link to gravel clast size and physical soil crust type.

The Namib Desert (Namibia) is home to fairy circles which are barren, circular to almost-circular patches of land surrounded by grasses. During a survey of the fungi associated with the most common grass species, Stipagrostis ciliata (Poaceae), and its rhizospheric soils associated with these fairy circles, Curvularia was commonly isolated (80 strains). Curvularia is a cosmopolitan fungal genus that occurs in diverse geographical locations and on a wide range of substrates, but particularly on foliar plants. Curvularia strains were identified based on multilocus sequence comparisons of their internal transcribed spacer rDNA region (ITS), and the partial gene regions of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and translation elongation factor 1-alpha (TEF1). The strains belonged to 13 species, including the discovery of five novel Curvularia species. The aim of this paper was to report on the identified species and to formally describe and name the new species as C. deserticola, C. gobabebensis, C. maraisii, C. namibensis, and C. stipagrostidicola.

Fairy circles are circular, barren structures in dry grasslands and the mechanisms generating and maintaining them are currently under intense discussion. Here, we analysed bacterial and fungal communities in Namib Desert fairy circle soils, in the tapetum lining of termite nests, the gut of the prevailing sand termite Psammotermes allocerus, and also in the neighbouring grass plants. The total and active microbial communities were analysed through amplicon sequencing of both DNA and RNA extracts, respectively, species-specific PCRs, and through cultivation of bacteria and fungi from the various samples. In the sandy soils, the active bacterial communities were substantially different from the total communities, which suggested the fairy circle soils to be highly selective environments in comparison to other drylands or even temperate soils. This appears to be even more true for fungi as amplification of fungal genes from RNA extracts was not possible. However, we observed a large diversity among cultured fungal species in comparison to cultured bacterial species, specifically in tapetum samples. Our results support the hypothesis that Psammotermes allocerus acts as a vector for plant pathogenic fungi and that their nests below the bare patches of fairy circles provide a refugium for the fungal plant pathogens.

Do bats seek clean water? A perspective on biodiversity from the Namib Desert