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Introduction: Many semiarid regions around the world are presently experiencing significant changes in both climatic conditions and vegetation. This includes a disturbed coexistence between grasses and bushes also known as bush encroachment, and altered precipitation patterns with larger rain events. Fewer, more intense precipitation events might promote groundwater recharge, but depending on the structure of the vegetation also encourage further woody encroachment. Materials and Methods: In this study, we investigated how patterns and sources of water uptake of Acacia mellifera (blackthorn), an important encroaching woody plant in southern African savannas, are associated with the intensity of rain events and the size of individual shrubs. The study was conducted at a commercial cattle farm in the semiarid Kalahari in Namibia (MAP 250 mm/a). We used soil moisture dynamics in different depths and natural stable isotopes as markers of water sources. Xylem water of fifteen differently sized individuals during eight rain events was extracted using a Scholander pressure bomb. Results and Discussion: Results suggest the main rooting activity zone of A. mellifera in 50 and 75 cm soil depth but a reasonable water uptake from 10 and 25 cm. Any apparent uptake pattern seems to be driven by water availability, not time in the season. Bushes prefer the deeper soil layers after heavier rain events, indicating some evidence for the classical Walter's two-layer hypothesis. However, rain events up to a threshold of 6 mm/day cause shallower depths of use and suggest several phases of intense competition with perennial grasses. The temporal uptake pattern does not depend on shrub size, suggesting a fast upwards water flow inside. δ2H and δ18O values in xylem water indicate that larger shrubs rely less on upper and very deep soil water than smaller shrubs. It supports the hypothesis that in environments where soil moisture is highly variable in the upper soil layers, the early investment in a deep tap-root to exploit deeper, more reliable water sources could reduce the probability of mortality during the establishment phase. Nevertheless, independent of size and time in the season, bushes do not compete with potential groundwater recharge. In a savanna encroached by A. mellifera , groundwater will most likely be affected indirectly. [ABSTRACT FROM AUTHOR]

Abstract: In this study we investigated the effect of different land use options (wildlife versus livestock) on species richness of plants and reptiles in the protected Kgalagadi Transfrontier Park (KTP) versus adjacent non-protected farmland within the same savanna habitat type (Aoub dune veld) in the southern Kalahari, South Africa. Our results show that both plant and reptile species richness as well as plant cover and reptile abundance was significantly higher in the protected KTP than in the non-protected farmland. The higher proportion of shrub but lower proportions of perennial grass cover, herb cover, and herb species richness in the farmland can be explained by higher stocking rates and the differences in feeding behaviour between native wild ungulates (e.g. Antidorcas marsupialis, Oryx gazella) and livestock (mainly sheep). The reptile’s prey availability and microhabitats (perennial grass tussocks and rodent burrows) for thermoregulation and protection against predators were significantly lower in the farmland. To conclude, our results clearly show that long term effects of different land use options (wildlife in protected KTP versus extensive livestock production in the non-protected farmland) even within the same habitat type have led to significant changes in vegetation composition, availability of microhabitat structures and in the reptile community. [Copyright &y& Elsevier]

1. Habitat fragmentation has major implications for demography and genetic structure of natural plant and animal populations as small and isolated populations are more prone to extinction. Therefore, many recent studies focus on spatial fragmentation. 2. However, the temporal configuration of suitable habitat may also influence dispersal and gene flow in fragmented landscapes. We hypothesize that short-term switching of inhospitable matrix areas into suitable habitat can mitigate effects of spatial fragmentation in natural and seminatural ecosystems. 3. To test our hypothesis, we investigated the hairy-footed gerbil ( Gerbillurus paeba, Smith 1836), a ground-dwelling rodent, in fragmented Kalahari savannah areas. Here, rare events of high above mean annual rainfall suggest short-term matrix suitability. 4. During the field survey in ‘matrix’ areas in the Kalahari (shrub encroachment by heavy grazing) we never observed the hairy-footed gerbil in years of average rainfall, but observed mass occurrences of this species during rare events of exceptionally high rainfall. 5. In a second step, we developed an agent-based model simulating subpopulations in two neighbouring habitats and the separating matrix. Our mechanistic model reproduces the mass occurrences as observed in the field and thus suggests the possibly underlying processes. In particular, the temporary improvement in matrix quality allows reproduction in the matrix, thereby causing a substantial increase in population size. 6. The model demonstrates further how the environmental trigger (rainfall) impacts genetic connectivity of two separated subpopulations. We identified seasonality as a driver of fragmentation but stochasticity leading to higher connectivity. 7. We found that our concept of temporal fragmentation can be applied to numerous other fragmented populations in various ecological systems and provide examples from recent literature. We conclude that temporal aspects of fragmentation must be considered in both ecological research and conservation management. [ABSTRACT FROM AUTHOR]

Shrub encroachment caused by overgrazing has led to dramatic changes of savannah landscapes and is considered one of the most threatening forms of rangeland degradation leading to habitat fragmentation. Although changes to plant assemblages are becoming better known, however, our understanding of how shrub encroachment affects rodent communities is low. In this study, we investigated relative abundance of five rodent species in sixteen southern Kalahari rangelands where shrub cover ranged from low (<5%) to high (>25%). Rodent abundance was determined on three trapping grids (40 × 100 m) for each site. Our results show that increasing shrub cover affected rodent species differently. The relative abundance of hairy-footed gerbil, short-tailed gerbil and bushveld gerbil declined with increasing shrub cover, whereas highveld gerbil and striped mouse exhibited hump-shaped relationships with shrub cover. Overall, species richness decreased with increasing shrub cover and a negative impact of high shrub cover above 15% on rodent abundance was congruent for all species. We conclude that our results support the hypothesis that long-term heavy grazing that results in area wide shrub encroachment, threatens the diversity of arid environments. [ABSTRACT FROM AUTHOR]

In arid and semi-arid savannas, species diversity is strongly influenced by structural diversity, which in these systems is characteristically formed by woody vegetation. Shrub encroachment, associated with overgrazing by domestic livestock, changes the structural diversity and hence affects biodiversity. Many investigations have been carried out with focus on the structure building trees, but little is known about the importance of shrubs. Here, we present a spatially explicit and individual-based population model for analysing the influence of shrub encroachment on the occurrence of yellow mongooses (Cynictis penicillata) in the Southern Kalahari, South Africa. This analysis is based on field investigations in Southern Africa. The results of the modelling study are in line with the results of a field study, in which the same unimodal relationship between shrub cover and yellow mongoose abundance was found. The model could explain this relationship by the need of shrubs as protective structures on the one hand, and the need of foraging habitat in the grass matrix on the other hand. To evaluate the factors influencing the population dynamics at different levels of shrub encroachment we carried out an extensive sensitivity analysis. Finally, we determined a range of existence of the yellow mongoose for various shrub covers. Model results indicate a lower and an upper limit of critical shrub densities at 1% and 38%, respectively. These limits indicate the range in shrub cover allowing the persistence of the yellow mongoose in the Southern Kalahari. [Copyright &y& Elsevier]