The night-time temporal window of locomotor activity in the Namib Desert long-distance wandering spider, Leucorchestris arenicola

Even though being active exclusively after sunset, the male Leucorchestris arenicola spiders are able to return to their point of departure by following bee-line routes of up to several hundreds of meters in length. While performing this kind of long-distance path integration they must rely on external cues to adjust for navigational errors. Many external cues which could be used by the spiders change dramatically or disappear altogether in the transition period from day to night. Hence, it is therefore imperative to know exactly when after sunset the spiders navigate in order to find out how they do it. To explore this question, we monitored their locomotor activity with data loggers equipped with infrared beam sensors. Our results show that the male spiders are most active in the period between the end and the beginning of the astronomical twilight period. Moreover, they prefer the moonless, i.e. darkest times at night. Hence, we conclude that the males are truly—and extremely—nocturnal. We further show that they are able to navigate under the very dim light conditions prevailing on moonless nights, and thus do not have to rely on the moon or on moon-related patterns of polarised light as potential compass cue

Use of local cues in the night-time navigation of the wandering desert spider Leucorchestris arenicola (Araneae, Sparassidae)

Adult male Leucorchestris arenicola can walk round-trips of several tens of meters in search of females. Most excursions end with the spiders returning to their burrow. For small animals homing over distances of several meters is theoretically impossible without the aid of external cues. It was investigated, whether the spiders use local cues or they rely solely on global cues. Individually marked male spiders were captured during their excursions and displaced several meters inside an opaque box. Ten out of twelve displaced spiders returned to their burrows. This shows that the male L. arenicola are using local cues during their homing, as the comparatively small displacement distances could not be detected by means of global, e.g. celestial cues. In order to test whether the spiders could be using olfactory guidance, the burrows were displaced by 2m while the spiders were out on their journeys. In 12 out of 15 experiments, the spiders did not find their burrows. These results show that the burrows do not function as olfactory beacons for the homing spider

Specific microbial communities associate with the rhizosphere of Welwitschia mirabilis, a living fossil

Welwitschia mirabilis is an ancient and rare plant distributed along the western coast of Namibia and Angola. Several aspects of Welwitschia biology and ecology have been investigated, but very little is known about the microbial communities associated with this plant. This study reports on the bacterial and fungal communities inhabiting the rhizosphere ofW. mirabilis and the surrounding bulk soil. Rhizosphere communities were dominated by sequences of Alphaproteobacteria and Euromycetes, while Actinobacteria, Alphaproteobacteria, and fungi of the class Dothideomycetes jointly dominated bulk soil communities. Although microbial communities within the rhizosphere and soil samples were highly variable, very few “species” (OTUs defined at a 97% identity cut-off) were shared between these two environments. There was a small ‘core’ rhizosphere bacterial community (formed by Nitratireductor, Steroidobacter, Pseudonocardia and three Phylobacteriaceae) that together with Rhizophagus, an arbuscular mycorrhizal fungus, and other putative plant growth-promoting microbes may interact synergistically to promote Welwitschia growth.S1 Fig. Welwitschia plants dotted across an arid landscape (left). The exposed radial root system of a Welwitschia plant (right).S2 Fig. Rarefaction curves. a) bacteria, b) fungi. Sample nomenclature is as in S1 Table.S3 Fig. Bar graph showing the phylum-level distribution of (a) bacterial and (b) fungal OTUs (97% cutoff). The taxonomic affiliation was performed using the Ribosomal Database Project Classifier (bacteria) and the UNITE database (fungi).S4 Fig. Venn diagram showing the number of shared phylotypes observed between the rhizosphere samples. a) bacteria, b) fungi. Sample nomenclature is as in S1 Table.S1 Table. Bacterial diversity. Sample nomenclature indicates the sample type (S = bulk soil; R = rhizosphere), replicate (S = 1 to 5, R = 1 to 3) and pseudoreplicate (a, b).S2 Table. Fungal diversity. Sample nomenclature is as in S1 Table.The National Research Foundation, South Africahttp://www.plosone.orgam2016Genetic

Long-Term Population Dynamics of Namib Desert Tenebrionid Beetles Reveal Complex Relationships to Pulse-Reserve Conditions

Noy-Meir’s paradigm concerning desert populations being predictably tied to unpredictable productivity pulses was tested by examining abundance trends of 26 species of flightless detritivorous tenebrionid beetles (Coleoptera, Tenebrionidae) in the hyper-arid Namib Desert (MAP = 25 mm). Over 45 years, tenebrionids were continuously pitfall trapped on a gravel plain. Species were categorised according to how their populations increased after 22 effective rainfall events (>11 mm in a week), and declined with decreasing detritus reserves (97.7–0.2 g m−2), while sustained by nonrainfall moisture. Six patterns of population variation were recognised: (a) increases triggered by effective summer rainfalls, tracking detritus over time (five species, 41% abundance); (b) irrupting upon summer rainfalls, crashing a year later (three, 18%); (c) increasing gradually after series of heavy (>40 mm) rainfall years, declining over the next decade (eight, 15%); (d) triggered by winter rainfall, population fluctuating moderately (two, 20%); (e) increasing during dry years, declining during wet (one, 0.4%); (f) erratic range expansions following heavy rain (seven, 5%). All species experienced population bottlenecks during a decade of scant reserves, followed by the community cycling back to its earlier composition after 30 years. By responding selectively to alternative configurations of resources, Namib tenebrionids showed temporal patterns and magnitudes of population fluctuation more diverse than predicted by Noy-Meir’s original model, underpinning high species diversity

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

Population changes of alien invasive plants in the Lower Kuiseb River

Abstract The status of six alien invasive plant species was investigated along the Lower Kuiseb River. The focus entailed a comparison of plants surveyed along transects located at five settlements of a rural community, with additional transects located between these settlements. While most living alien plants occurred in or adjacent to the main channel of the river, additional seed banks were noted in river areas further away from the channel. Nicotiana glauca was clearly on the increase along the length of the study area, while Argemone ochroleuca, Ricinus communis, Datura stramonium and D. inoxia appear to be declining, and Prosopis continues to be confined. While changing surface hydrology and near-surface geohydrology may underlie the increase in N. glauca, trampling action by increasing livestock numbers could perhaps explain the decreases by other species. More detailed studies and long-term monitoring are required to improve management of alien plants in the Kuiseb and other ephemeral rivers

Monitoring tenebrionid beetle biodiversity in Namibia

Different field methods of determining abundance and species diversity of darkling beetles (Coleoptera, Tenebrionidae) were tested. A combination of the use of pitfall traps and linear transect surveys served as the best rapid assessment of diversity, while pitfall traps alone are good for estimating abundance. Trap size (15cm diameter vs. 10cm diameter) and different degrees of exposure to sun did not significantly affect the capture rate of beetles, which was highly variable between traps at a site, but there were differences between sites and seasons. A minimum of a full year of trapping is required before the slope of the species-effort curve begins to flatten when the most abundant species have been recorded. The curve continues to increase over the course of the next 20 years, by which time all species at a location have been recorded. Furthermore, long trapping periods covers different climatic conditions, reflecting that in the Namib Desert, long-term records are required to study biodiversity