Mary Seely

Irregular Fog as a Water Source for Desert Dune Beetles

SummaryThree methods of fog-water uptake have been observed in three tribes of Namib desert dune tenebrionid beetles, Adesmiini, Eurychorini and Zophosini. The methods used correlate with distribution and gross morphology of each species but cut across phylogenetic affinities. Of the three methods described, none involve obvious fine anatomical or physical adaptations of the beetles for fog-water uptake. Rather, the beetles have evolved specific behavioural patterns for drinking water condensed on vegetation, their own dorsum or sand.Use of fog-water necessitates surface activity at times when surface temperatures and wind velocities are not optimal for these diurnal or crepuscular species. Behavioural adaptation has enabled these beetles to use irregular and unpredictable fogs as a moisture source.

PREDATION AS A MAJOR COST OF REPRODUCTION IN NAMIB DESERT TENEBRIONID BEETLES

Male tenebrionid beetles from the Namib Desert are captured by predators more frequently than females. Males exhibit several behaviors that predispose them to an increased risk of mortality from predation. In attempts to find and monopolize females, males are more active on the surface, move more frequently, and travel farther than females. One result of such activity is a bias in captures by predators and pitfall traps. These biased sex ratios differ significantly from those in censused natural populations and an assumed 1:1 sex ratio. We suggest that such differential predation is a major cost of reproduction and can constrain sexual selection.

Cyanobacteria drive community composition and functionality in rock-soil interface communities

Most ecological research on hypoliths, significant primary producers in hyperarid deserts, has focused on the diversity of individual groups of microbes (i.e. bacteria). However, microbial communities are inherently complex, and the interactions between cyanobacteria, heterotrophic bacteria, protista and metazoa are likely to be very important for ecosystem functioning. Cyanobacterial and heterotrophic bacterial communities were analysed by pyrosequencing, while metazoan and protistan communities were assessed by T‐RFLP analysis. Microbial functionality was estimated using carbon substrate utilization. Cyanobacterial community composition was significant in shaping community structure and function in hypoliths. Ecological network analysis showed that most significant co‐occurrences were positive, representing potential synergistic interactions. There were several highly interconnected associations (modules), and specific cyanobacteria were important in driving the modular structure of hypolithic networks. Together, our results suggest that hypolithic cyanobacteria have strong effects on higher trophic levels and ecosystem functioning.

Non-Rainfall Moisture Activates Fungal Decomposition of Surface Litter in the Namib Sand Sea

The hyper-arid western Namib Sand Sea (mean annual rainfall 0–17 mm) is a detritus-based ecosystem in which primary production is driven by large, but infrequent rainfall events. A diverse Namib detritivore community is sustained by minimal moisture inputs from rain and fog. The decomposition of plant material in the Namib Sand Sea (NSS) has long been assumed to be the province of these detritivores, with beetles and termites alone accounting for the majority of litter losses. We have found that a mesophilic Ascomycete community, which responds within minutes to moisture availability, is present on litter of the perennial Namib dune grass Stipagrostis sabulicola. Important fungal traits that allow survival and decomposition in this hyper-arid environment with intense desiccation, temperature and UV radiation stress are darkly-pigmented hyphae, a thermal range that includes the relatively low temperature experienced during fog and dew, and an ability to survive daily thermal and desiccation stress at temperatures as high as 50°C for five hours. While rainfall is very limited in this area, fog and high humidity provide regular periods (≥ 1 hour) of sufficient moisture that can wet substrates and hence allow fungal growth on average every 3 days. Furthermore, these fungi reduce the C/N ratio of the litter by a factor of two and thus detritivores, like the termite Psammotermes allocerus, favor fungal-infected litter parts. Our studies show that despite the hyper-aridity of the NSS, fungi are a key component of energy flow and biogeochemical cycling that should be accounted for in models addressing how the NSS ecosystem will respond to projected climate changes which may alter precipitation, dew and fog regimes.

Metagenomic analysis provides insights into functional capacity in a hyperarid desert soil niche community

In hyperarid ecosystems, macroscopic communities are often restricted to cryptic niches, such as hypoliths (microbial communities found beneath translucent rocks), which are widely distributed in hyperarid desert environments. While hypolithic communities are considered to play a major role in productivity, the functional guilds implicated in these processes remain unclear. Here, we describe the metagenomic sequencing, assembly and analysis of hypolithic microbial communities from the Namib Desert. Taxonomic analyses using Small Subunit phylogenetic markers showed that bacterial phylotypes (93%) dominated the communities, with relatively small proportions of archaea (0.43%) and fungi (5.6%). Refseq-viral database analysis showed the presence of double stranded DNA viruses (7.8% contigs), dominated by Caudovirales (59.2%). Analysis of functional genes and metabolic pathways revealed that cyanobacteria were primarily responsible for photosynthesis with the presence of multiple copies of genes for both photosystems I and II, with a smaller but significant fraction of proteobacterial anoxic photosystem II genes. Hypolithons demonstrated an extensive genetic capacity for the degradation of phosphonates and mineralization of organic sulphur. Surprisingly, we were unable to show the presence of genes representative of complete nitrogen cycles. Taken together, our analyses suggest an extensive capacity for carbon, phosphate and sulphate cycling but only limited nitrogen biogeochemistry.

TEMPORAL AND SPATIAL VARIABILITY OF GRASS PRODUCTIVITY IN THE CENTRAL NAMIB DESERT

The production of grass was investigated on the gravel plains of the Central Namib Desert, Namibia, during 10 rainfall seasons sampled between 1989-2003. The aim was to evaluate the rainfall-productivity relationship, to elucidate the relationship between temporal and spatial variability, and to examine the spatial scale of patchiness. We compared two different methods and found that a less accurate rapid assessment of grass cover correlat- ed well with measurements of biomass. Our data were in agreement with previous determina- tions of the desert end of the curve of grassland productivity, and productivity was closely related to the rainfall of the particular season. There was high variability between years at study sites, especially in the west (CV=279%), where it rained more seldom than in the east (CV=86%). During all years rainfall was very patchy at a spatial scale of 5 km, which appar- ently reflected the storm path of individual rain clouds. Long-term monitoring should be continued in order to detect changes of pattern in this rainfall-driven system.

Water regime history drives responses of soil Namib Desert microbial communities to wetting events

Despite the dominance of microorganisms in arid soils, the structures and functional dynamics of microbial communities in hot deserts remain largely unresolved. The effects of wetting event frequency and intensity on Namib Desert microbial communities from two soils with different water-regime histories were tested over 36 days. A total of 168 soil microcosms received wetting events mimicking fog, light rain and heavy rainfall, with a parallel “dry condition” control. T-RFLP data showed that the different wetting events affected desert microbial community structures, but these effects were attenuated by the effects related to the long-term adaptation of both fungal and bacterial communities to soil origins (i.e. soil water regime histories). The intensity of the water pulses (i.e. the amount of water added) rather than the frequency of wetting events had greatest effect in shaping bacterial and fungal community structures. In contrast to microbial diversity, microbial activities (enzyme activities) showed very little response to the wetting events and were mainly driven by soil origin. This experiment clearly demonstrates the complexity of microbial community responses to wetting events in hyperarid hot desert soil ecosystems and underlines the dynamism of their indigenous microbial communities.

The effects of desert pavements (gravel mulch) on soil micro-hydrology

The effect of desert pavements (gravel mulch) on near surface soil micro-hydrology has been inadequately studied. Micro-hydrology in arid ecosystems occurs due to a daily non rainfall atmospheric water cycle, consisting of an input phase (dew, fog, vapour adsorption) and an evaporation phase. A winter comparative study between a bare soil (control) and gravel mulch using the automated microlysimeter approach was conducted in Stellenbosch, South Africa in 2008. Results showed that dew deposition and direct water vapour adsorption were significantly higher into bare soil compared to gravel mulch. In contrast, however, soil moisture from rain persists for a longer time under gravel mulch compared to bare soil. This result suggests that the greatest impact of gravel mulch on soil micro-hydrology is towards conserving moisture and could explain why the treatment is used in dry-land agriculture in Mediterranean regions.

Niche-Partitioning of Edaphic Microbial Communities in the Namib Desert Gravel Plain Fairy Circles

Endemic to the Namib Desert, Fairy Circles (FCs) are vegetation-free circular patterns surrounded and delineated by grass species. Since first reported the 1970s, 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.

The Surface Geology and Geomorphology Around Gobabeb, Namib Desert, Namibia

Abstract This paper serves as a brief review and overview of the surface geology and geomorphology around obabeb in the entral amib esert. It introduces the major lithologies associated with the amara rogen of Precambrian and ambrian age, followed by ertiary sandstones and conglomerates and uaternary uiseb valley conglomerates, silts and gravels. To celebrate the 50th anniversary of the obabeb research station and the recent designation of the amib and ea as a world heritage site, we are presenting a map to inform and guide future research and educational activities around obabeb. The contrast between an ancient gravel plain to the north, a dynamic aeolian sand sea to the south and ephemeral river environments in between become apparent. These natural laboratories have attracted investigations of contemporary sand dune movement, hyper saline waters, evaporites and duricrusts and ephemeral flooding and recharge processes as well as environmental change and human impacts.