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Birkhofer K.,Lund University | Henschel J.,Gobabeb Training and Research Center | Lubin Y.,Ben - Gurion University of the Negev
Oecologia | Year: 2012

Individuals of most animal species are non-randomly distributed in space. Extreme climatic events are often ignored as potential drivers of distribution patterns, and the role of such events is difficult to assess. Seothyra henscheli (Araneae, Eresidae) is a sedentary spider found in the Namib dunes in Namibia. The spider constructs a sticky-edged silk web on the sand surface, connected to a vertical, silk-lined burrow. Above-ground web structures can be damaged by strong winds or heavy rainfall, and during dispersal spiders are susceptible to environmental extremes. Locations of burrows were mapped in three field sites in 16 out of 20 years from 1987 to 2007, and these grid-based data were used to identify the relationship between spatial patterns, climatic extremes and sampling year. According to Morisita's index, individuals had an aggregated distribution in most years and field sites, and Geary's C suggests clustering up to scales of 2 m. Individuals were more aggregated in years with high maximum wind speed and low annual precipitation. Our results suggest that clustering is a temporally stable property of populations that holds even under fluctuating burrow densities. Climatic extremes, however, affect the intensity of clustering behaviour: individuals seem to be better protected in field sites with many conspecific neighbours. We suggest that burrow-site selection is driven at least partly by conspecific cuing, and this behaviour may protect populations from collapse during extreme climatic events. © 2012 Springer-Verlag.

Joubert E.C.,Stellenbosch University | Harms T.M.,Stellenbosch University | Muller A.,University of Namibia | Hipondoka M.,University of Namibia | Henschel J.R.,Gobabeb Training and Research Center
Environmental Fluid Mechanics | Year: 2012

In the Namib Desert seed distribution is greatly influenced by wind patterns. Existing literature regarding wind patterns over dunes focuses on two-dimensional simulations of flow over simplified dune structures. The three-dimensional geometries of the sand dunes suggests far more complex flow features exist, which are not captured by two-dimensional simulations. Computational fluid dynamics (CFD) was used to reproduce the three-dimensional near surface wind patterns around a dune with the aim to learn more about seed distribution. Field work included terrain mapping, wind speed, direction and temperature metering. The CFD results show the expected two-dimensional flow features of high pressure at the dune toe, low pressure at the crest and flow acceleration up windward slope. Also observed are some three-dimensional flow features such as a spiral vortex near the crest and transverse flow due to crest-line curvature of the dune. It was also observed how the wall shear stress differs due to the three-dimensional shape of the dune. The wall shear stress suggests that seed accumulation is more likely to occur behind trailing (down-wind) crest edges. Particle tracking showed how seeds tend to move over the dune crest and recirculate towards the crest on the lee-side. The study showed that adding the third dimension makes the simulations more complex, adds to computational requirements and increases simulation time but also provides vital flow information which is not possible with two-dimensional simulations. © 2011 Springer Science+Business Media B.V.

Wasiolka B.,University of Potsdam | Jeltsch F.,University of Potsdam | Henschel J.,Gobabeb Training and Research Center | Blaum N.,University of Potsdam
African Journal of Ecology | Year: 2010

Although the effects of grazing-induced savannah degradation on animal diversity are well documented, knowledge of how they affect space use or responding behaviour remains poor. In this study, we analysed space use of the spotted sand lizard (Pedioplanis l. lineoocellata) in degraded versus nondegraded habitats of southern Kalahari savannah habitats. Lizards were radio tracked, daily movement distances recorded and home range sizes calculated. In degraded Kalahari savannah habitats where plant diversity and perennial grass cover are low but shrub cover high, P. lineoocellata moves larger distances (40.88 ± 6.42 m versus 27.43 ± 5.08 m) and occupies larger home ranges (646.64 ± 244.84 m2 versus 209.15 ± 109.84 m2) than in nondegraded habitats (high plant diversity, high perennial grass cover and low shrub cover). We assume that this increase in daily movement distances and home range sizes is a behavioural plasticity to limited food resources in degraded savannah habitats. Although P. lineoocellata is able to adjust to resource-poor savannah habitats, the increase in the lizard's movement activities is likely to result in a higher predation risk. This is supported by the lower availability of protective vegetation i.e. perennial grass cover. Hence, we conclude that despite behavioural plasticity of P. lineoocellata, overgrazing has a severe negative impact on the space use of P. lineoocellata. © 2009 The Authors. Journal compilation © 2009 Blackwell Publishing Ltd.

Steckel J.,University of Marburg | Penrith M.L.,University of Pretoria | Henschel J.,Gobabeb Training and Research Center | Brandl R.,University of Marburg | Meyer J.,University of Marburg
African Zoology | Year: 2010

A systematic classification of Namib Desert darkling beetles (Coleoptera: Tenebrionidae) based on morphological characters is complicated as strong selection pressures exerted by desert conditions have led to a suite of convergent morphological characteristics. Here we present a first and preliminary insight into the relationships within the tribes Zophosini, Eurychorini and Adesmiini using molecular methods. We analysed partial sequences of the mitochondrial genes cytochrome oxidase II and cytochrome b of 16 individuals comprising 12 species. Minimum Evolution, Maximum Parsimony, Maximum Likelihood and Bayesian inference were applied for analysing sequence data. The genus Pimelia was used as outgroup and for calibrating divergence time estimates. Overall, results supported phylogenies constructed on morphological characters. The proposed monophyly of the artificially defined tribe Zophosini did receive sufficient support, Speciation events in Namibian darkling beetles likely occurred during periods of aridification about 35 Mya, 16 Mya and 510 Mya. Those periods could be related to geological events and climate change due to the glaciation of Antarctica and the development of the Benguela current.

Goldenberg M.,University of Vienna | Goldenberg M.,Gobabeb Training and Research Center | Goldenberg F.,University of Vienna | Goldenberg F.,Gobabeb Training and Research Center | And 3 more authors.
Folia Zoologica | Year: 2010

Black-backed jackals (Canis mesomelas, Schreber, 1775) survive in a wide range of environments. Their foraging strategies are highly variable in different habitats. Adaptations in foraging behaviour in relation to abundance and quality of food sources are expected to be highly pronounced in an extreme habitat like a desert. This study investigated the diet composition in black-backed jackals in the Namib Desert by analysing faecal samples collected between February 2004 andAugust 2005. Frequency of occurrence, relative dry mass and proportion of biomass consumed were calculated for different prey items. Insect parts, mainly of two species - The giant longhorn beetle (Acanthophorus capensis) and a locust (Anacridium moestum) - Were found in 72.2% of the samples and were estimated to have contributed 22% to the biomass consumed. Mammals, predominantly rodents and ungulates, represented the highest proportion of biomass consumed (42%), although theirremains were found in only one third of the samples. Based on biomass, mammals are assumed to be the jackal's preferred prey, but, probably due to lower abundance, more diffi cult to obtain than insects. More than 50% of the samples contained plant material, mainly seeds of !NARA plants (Acanthosycios horridus) and false ebony (Euclea pseudebenus), especially during their fruiting seasons. Although the abundance of A. capensis and of A. moestum varied annually, their remains were found in scats throughout the year, indicating a certain degree of specialization on these prey species.

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