Physical ecology of hypolithic communities in the central Namib desert : the role of fog, rain, rock habitat, and light

[1] Hypolithic microbial communities are productive niches in deserts worldwide, but many facets of their basic ecology remain unknown. The Namib Desert is an important site for hypolith study because it has abundant quartz rocks suitable for colonization and extends west to east across a transition from fog- to rain-dominated moisture sources. We show that fog sustains and impacts hypolithic ecology in several ways, as follows: (1) fog effectively replaces rainfall in the western zone of the central Namib to enable high (≥95%) hypolithic abundance at landscape (1–10 km) and larger scales; and (2) high water availability, through fog (western zone) and/or rainfall (eastern zone), results in smaller size-class rocks being colonized (mean 6.3 ± 1.2 cm) at higher proportions (e.g., 98% versus approximately 3%) than in previously studied hyperarid deserts. We measured 0.1% of incident sunlight as the lower limit for hypolithic growth on quartz rocks in the Namib and found that uncolonized ventral rock surfaces were limited by light rather than moisture. In situ monitoring showed that although rainfall supplied more liquid water (36 h) per event than fog (mean 4 h), on an equivalent annual basis, fog provided nearly twice as much liquid water as rainfall to the hypolithic zone. Hypolithic abundance reaches 100% at a mean annual precipitation (MAP) of approximately 40–60 mm, but at a much lower MAP (approximately 25 mm) when moisture from fog is available.This work was partially supported through NASA’s ASTEP Programhttp://agupubs.onlinelibrary.wiley.com/agu/jgr/journal/10.1002/(ISSN)2169-8961hb201

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

Reflections on the Karoo Special Issue: towards an interdisciplinary research agenda for South Africa’s drylands

We begin this essay with reflections on major research themes highlighted by the Karoo Special Issue (KSI). These include concerns over land-use change, long-term monitoring, climate change, governance and the need for more interdisciplinary research. We also identify some of the novel contributions of the KSI around these themes and highlight research issues that require further attention. These include greater focus on the social-ecological impact of large-scale infrastructural developments and wildlife ranching, as well as the need for more longitudinal studies related to contemporary social issues. We conclude by outlining a simple framework for thinking about interdisciplinarity in future research efforts in the Karoo. The first axis relates to the natural science/social science continuum, while the second is concerned with the continuum between basic and applied science. Greater awareness of these two dimensions could not only encourage researchers to reflect more carefully on where their research fits in relation to the broader research needs of this region but could also promote interdisciplinarity within the research community. It could also bring researchers closer to the realities and needs of the people who live in and derive their livelihoods from Karoo environments.Keywords: applied research, interdisciplinary research, land-use change, social-ecological systems, sustainabilit

Introduction to the Karoo Special Issue: Trajectories of Change in the Anthropocene

The Karoo is an arid to semi-arid area across the western third of South Africa, comprising the Succulent Karoo and Nama-Karoo biomes. Its environment and people have experienced considerable changes, and now face new challenges as the Anthropocene unfolds. This Karoo Special Issue (KSI) brings together new information in 20 papers, a mixture of reviews, research articles and commentaries, significantly adding to previous syntheses of Karoo knowledge. The KSI comprises several sections focusing on different aspects of change, namely a lead article that provides an overview of social and environmental changes, followed by papers concerning changes over time from deep history to contemporary conditions (Xhaeruh to Karoo), insights from long-term studies at several sites across the area, different perspectives of ecosystem processes, and ending with a set of reflections and proposals for research priorities. We end this introduction by dedicating the KSI to two outstanding scholars of the Karoo: Dr Suzanne J Milton and Dr W Richard J Dean. These KSI papers, many of which were written by their colleagues, friends and former students, represents a Festschrift that celebrates and honours their research as well as the inspiration and leadership they gave to a generation of scientists.Keywords: Festschrift, Nama-Karoo, Richard Dean, social-ecological systems, Succulent Karoo, Suzanne Milto

Widespread Dieback of Riparian Trees on a Dammed Ephemeral River and Evidence of Local Mitigation by Tributary Flows

Ephemeral rivers act as linear oases in drylands providing key resources to people and wildlife. However, not much is known about these rivers’ sensitivities to human activities. We investigated the landscape-level determinants of riparian tree dieback along the Swakop River, a dammed ephemeral river in Namibia, focusing on the native ana tree (Faidherbia albida) and the invasive mesquite (Prosopis spp.). We surveyed over 1,900 individual trees distributed across 24 sites along a 250 km stretch of the river. General linear mixed models were used to test five hypotheses relating to three anthropogenic threats: river flow disruption from damming, human settlement and invasive species. We found widespread dieback in both tree populations: 51% mortality in ana tree, with surviving trees exhibiting 18% canopy death (median); and 26% mortality in mesquite, with surviving trees exhibiting 10% canopy death. Dieback in the ana tree was most severe where trees grew on drier stretches of the river, where tributary flow was absent and where mesquite grew more abundantly. Dieback in the mesquite, a more drought-tolerant taxon, did not show any such patterns. Our findings suggest that dieback in the ana tree is primarily driven by changes in river flow resulting from upstream dam creation and that tributary flows provide a local buffer against this loss of main channel flow. The hypothesis that the invasive mesquite may contribute to ana tree dieback was also supported. Our findings suggest that large dams along the main channels of ephemeral rivers have the ability to cause widespread mortality in downstream riparian trees. To mitigate such impacts, management might focus on the maintenance of natural tributary flows to buffer local tree populations from the disruption to main channel flow