W. Richard J. Dean

The karoo: Ecological patterns and processes

Foreword P. W. Roux Preface Acknowledgements Part I. The Driving Variables W. Dean and S. Milton: 1. The climate of the Karoo - a functional approach P. Desmet and R. Cowling 2. Soils of the arid south-western zone of Africa M. Watkeys 3. Paleoenvironments M. Meadows and M. Watkeys 4. Plant biogeography, endemism and diversity R. Cowling and C. Hilton-Taylor 5. Biogeography, endemism and diversity of animals C. Vernon Part II. Form and Function B. Lovegrove: 6. Form and function in perennial plants G. Midgley and F. van der Heyden 7. Functional aspects of annual and biennial plants M. van Rooyen 8. Plant reproductive ecology K. Esler 9. Animal form and function B. Lovegrove 10. Animal foraging and food W. Dean and S. Milton Part III. Dynamics P. Novellie: 11. Population-level dynamics S. Milton, R. Davies and G. Kerley 12. Community patterns and dynamics A. Palmer, P. Novellie and J. Lloyd 13. Modelling populations and community dynamics in Karoo ecosystems K. Kellner and J. Booysen 14. Spatially explicit computer simulation models - tools for understanding vegetation dynamics and supporting rangeland management F. Jeltsch, T. Wiegand and C. Wissel Part IV. Human Impacts W. Siegfried: 15. Hunters and herders in the Karoo landscape A. Smith 16. Historical and contemporary land use and the desertification of the Karoo M. Hoffman, B. Cousins, T. Meyer, A. Petersen and H. Hendricks 17. Alien plant invaders of the Karoo: attributes, impacts and control S. Milton, H. Zimmerman and J. Hoffman 18. Comparison of ecosystem processes in the Nama Karoo and other deserts W. Whitford 18. The Succulent Karoo in a global context: plant structural and functional concepts comparison with North American winter rainfall deserts K. Esler, P. Rundel and R. Cowling 20. The Karoo: past and present S. Milton and W. Dean References Index.

Simulated pattern formation around artificial waterholes in the semi‐arid Kalahari

. Sinking boreholes to tap groundwater supplies facilitated expansion of all-year round livestock production into the semi-arid Kalahari. Increased grazing and trampling pressure around the boreholes often caused vegetation changes and range degradation. The long-term influences of cattle grazing on vegetation pattern around watering points in the southern part of the semi-arid Kalahari are investigated using a grid-based simulation model. Shrub-grass dynamics are modelled for two regimes with high and low rainfall and under various stocking rates. Results indicate the formation of distinct vegetation zones (‘piosphere’ zones) at the high rainfall site. Under all tested stocking rates distinct zones of bare soil, woody shrubs and a mixed grass-shrub savanna develop. The piosphere zones expand outwards at a rate correlated with the grazing pressure. At the lower-rainfall site zone development is limited and influenced by rainfall. Under abnormally high stocking rates an increase in shrub cover occurs within 50 yr under the low rainfall regime, leading to less distinct zones than under the high rainfall scenario. Modelling results suggest that the recovery potential of shrub-encroached piosphere zones after withdrawal of cattle is negligible in a time span of 100 yr.

South Africa's arid and semiarid rangelands : why are they changing and can they be restored ?

Since the mid-19th century settled livestock ranching has been the major form of land use in South Africa, occupying 68% of the land surface. Decreases in livestock densities and ranch numbers during the past century imply that carrying capacities for domestic herbivores are falling. Differences in carbon isotope signals with soil depth and abrupt shifts in dominant plant species across ranch boundaries reveal that southern African rangelands are changing. Case studies suggest ways to control altered grassland composition, bush encroachment in arid savanna, and dominance by toxic and halophytic shrubs in arid shrublands. But climatic and biological factors constrain rates of passive recovery, and guidelines for active restoration are poor and techniques costly. Moreover, conservation of remaining good rangeland is seldom enforced, and economic considerations usually outweight the land users desire to sustain diversity and productivity.

Economic incentives for restoring natural capital in southern African rangelands

Technical and economic factors hinder effective ecological restoration, especially in developing countries. Three examples show how social policy, economic threats and opportunities, and national and international development policy are driving the restoration of degraded landscapes in southern Africa. First, new opportunities in nature tourism, together with the declining profitability of traditional ranching, have led to diversification into game farming, tourism, and hunting, all initiatives that rely on properly functioning ecosystems. Second, new environmental legislation is forcing industries, particularly mining, to restore land upon termination of their activities. Third, through South Africa’s “Working for Water” program, an elegant solution to problems of excessive water use, local residents are developing skills in clearing alien plants and restoring rangelands.

Food selection by ostrich in Southern Africa

We observed ostrich (Struthio camelus) in southern African savanna, desert grassland, arid shrubland, and fynbos (Mediterranean shrubland) to determine physical and chemical factors that influenced food selection. Natural diet information was needed to provide guidelines on carrying capacity of natural vegetation for ostrich and for improving rations for captive ostrich. Ostrich fed on green annual grasses and forbs when available. When these were not available they consumed leaves, flowers, and fruits from succulents and woody plants. Dead or woody material and animal matter (other than bone) were absent from the diet. Ostrich selected (P < 0.05) for fiber and against (P < 0.05) phenolics in forbs

Simulated plant population responses to small‐scale disturbances in semi‐arid shrublands

We used a spatially explicit simulation model to examine the impact of small-scale disturbance (created by the digging of aardvarks, Orycteropus afer, and bat-eared foxes, Otocyon megalotis or as a management action) on the tempo- ral and spatial dynamics of a typical Karoo shrub plant com- munity, and to gather insight into the interplay between distur- bance structure and population dynamics. Establishment, growth, mortality, seed dispersal and competitive interactions were modelled over long time-scales in annual time-steps under the influence of stochastic and unpredictable rainfall. Three disturbance regimes were included, varying the type, rate and size of the small-scale disturbances. The impact of a disturbance regime on long-term community dynamics depends on complex interactions between disturbance charac- teristics and life-history attributes of component species. Plant density decreased with overall disturbance rates; this effect was independent of the type of disturbance. A given type and rate of disturbance did not influence all species within a guild (e.g. colonizer species) in the same way. The reason for these differences was that species responded not only to the distur- bance but to changes in competition intensity from other species and changes in their reproductive potential relative to other species as well. Such interactions resulted in a sequential change in dominant species within guilds as disturbance rates increased. An increase in the overall disturbance rate did not always produce the trend in evenness expected from the intermediate disturbance hypothesis, but was influenced by the relative abun- dance of different types of disturbance.

Plant invasions in arid areas: special problems and solutions: a South African perspective

Management of invasive alien plants in arid areas is complicated by the cryptic and stochastic nature of the invasion process, the low density of researchers, extension officers and farmers in these areas, the complex, delayed and sometimes, indirect, effects of alien invasive plants on these ecosystems, and by high and shifting values placed on goods and services derived from invasive alien plant species. Fluctuating vegetation cover together with convergent adaptations for dispersal and facilitation enables some desert aliens to invade intact vegetation. Invasive plants in arid areas are not all arid-adapted: the most problematic species globally are phreatic, wetland or oasis specialists that can colonise remote wetlands and springs through a combination of wind-dispersed seeds and vegetative reproduction. Their success is often linked to disturbance and facilitated by agricultural activities including water extraction, cropping and livestock management. Invasive alien plants in arid region wetlands have an impact on forage, water resources and biodiversity in these key resource areas, that is disproportionately great relative to the area they occupy. Management of arid region aliens could include pre-introduction biocontrol planning that makes it possible to use aliens while reducing invasion risks. An alternative is to replace the aliens with extralimital indigenous plants that can supply the rangeland services perceived to be absent from arid environments—but such interventions may carry even greater risks.

Introduced and Invasive Species in Novel Rangeland Ecosystems: Friends or Foes?

Abstract Globally, new combinations of introduced and native plant and animal species have changed rangelands into novel ecosystems. Whereas many rangeland stakeholders (people who use or have an interest in rangelands) view intentional species introductions to improve forage and control erosion as beneficial, others focus on unintended costs, such as increased fire risk, loss of rangeland biodiversity, and threats to conservation efforts, specifically in nature reserves and parks. These conflicting views challenge all rangeland stakeholders, especially those making decisions on how best to manage novel ecosystems. To formulate a conceptual framework for decision making, we examined a wide range of novel ecosystems, created by intentional and unintentional introductions of nonnative species and land-use–facilitated spread of native ones. This framework simply divides decision making into two types: 1) straightforward–certain, and 2) complex–uncertain. We argue that management decisions to retain novel ecosystems are certain when goods and services provided by the system far outweigh the costs of restoration, for example in the case of intensively managed Cenchrus pastures. Decisions to return novel ecosystems to natural systems are also certain when the value of the system is low and restoration is easy and inexpensive as in the case of biocontrol of Opuntia infestations. In contrast, decisions whether to retain or restore novel ecosystems become complex and uncertain in cases where benefits are low and costs of control are high as, for example, in the case of stopping the expansion of Prosopis and Juniperus into semiarid rangelands. Decisions to retain or restore novel ecosystems are also complex and uncertain when, for example, nonnative Eucalyptus trees expand along natural streams, negatively affecting biodiversity, but also providing timber and honey. When decision making is complex and uncertain, we suggest that rangeland managers utilize cost–benefit analyses and hold stakeholder workshops to resolve conflicts. Resumen Mundialmente, nuevas combinaciones de plantas introducidas e inducidas y especies de animales han cambiado los pastizales a nuevos ecosistemas. Mientras que muchos de los interesados en los pastizales (personas que usan o tienen interés en los pastizales) ven un beneficio en la introducción de especies para el mejoramiento de la producción de forraje y control de la erosión, otros se interesan en los costos no planeados tales como el aumento en el riesgo de fuego, pérdida de biodiversidad en los pastizales y amenazas en los esfuerzos de conservación especialmente en reservas naturales y parques. Estos puntos de vista conflictivos son retos para todos los interesados en los pastizales, especialmente para la toma de decisiones en cómo manejar mejor los ecosistemas nuevos. Para formular un modelo conceptual para toma de decisiones, examinamos un amplio rango de ecosistemas nuevos, creados de manera intencional y no intencional de especies no nativas y el uso de tierras que facilitan la expansión de especies nativas. Este modelo simplemente divide la toma de decisiones en dos tipos: 1) francamente–seguro y 2) complejo–no seguro. Discutimos que las decisiones de manejo para mantener ecosistemas nuevos son seguras cuando los bienes y servicios proporcionados por el sistema sobrepasan por mucho el costo de restauración, por ejemplo en el caso de las praderas intensivas de Cenchrus. Las decisiones para devolver ecosistemas nuevos a sistemas naturales son también seguras cuando el valor del sistema es bajo y la restauración es fácil y barata como en el caso del control biológico de las infestaciones de Opuntia. En contraste, las decisiones ya sea de mantener o recuperar ecosistemas nuevos se complican y son inciertas en casos donde los beneficios son bajos y los costos altos, por ejemplo en el caso de detener la expansión del Prosopis y Juniperus en los pastizales semiáridos. También las decisiones para mantener o renovar un ecosistema nuevo son difíciles e inciertas cuando por ejemplo, especies no nativas como el Eucalipto se extienden sobre arroyos naturales afectando negativamente la biodiversidad pero también proveyendo madera y miel. Cuando el proceso de toma de decisiones es complejo e incierto sugerimos que los manejadores de pastizales usen el análisis de costo beneficio y talleres entre los interesados para resolver conflictos.

The Ecological and Geographic Context of Morphological and Genetic Divergence in an Understorey-Dwelling Bird

Advances in understanding the process of species formation require an integrated perspective that includes the evaluation of spatial, ecological and genetic components. One approach is to focus on multiple stages of divergence within the same species. Species that comprise phenotypically different populations segregated in apparently distinct habitats, in which range is presently continuous but was putatively geographically isolated provide an interesting system to study the mechanisms of population divergence. Here, we attempt to elucidate the role of ecology and geography in explaining observed morphological and genetic variation in an understorey-dwelling bird endemic to southeastern Africa, where two subspecies are recognized according to phenotype and habitat affinity. We carried out a range-wide analysis of climatic requirements, morphological and genetic variation across southeast Africa to test the hypothesis that the extent of gene flow among populations of the brown scrub-robin are influenced by their distinct climatic niches. We recovered two distinct trends depending on whether our analyses were hierarchically structured at the subspecies or at the within subspecies level. Between subspecies we found pronounced morphological differentiation associated with strong reproductive isolation (no gene flow) between populations occupying divergent climatic niches characterized by changes in the temperature of the warmest and wettest month. In contrast, within subspecies, we recovered continuous morphological variation with extensive gene flow among populations inhabiting the temperate and sub-tropical forests of southern Africa, despite divergence along the climate axis that is mainly determined by minimum temperature and precipitation of the coldest months. Our results highlight the role of niche divergence as a diversifying force that can promote reproductive isolation in vertebrates.

Opportunistic and multiple breeding attempts in plants and vertebrates of semi-deserts with unpredictable rainfall events through the year

Quantity and timing of rainfall vary among years in semi-deserts on the inland edges of Mediterranean-climate regions on all continents. One way that plants and animals might cope with the lack of predictability in rainfall is to have many fixed-season breeding attempts over a long lifespan, some of which are likely to succeed. Another approach is to breed sporadically, whenever conditions favour reproduction. In this review we compare breeding phenology of semi-desert organisms with that in other more predictable ecosystems. We test the hypotheses that breeding is unsynchronised within (a) communities and (b) species. We also examine evidence that opportunistic and multiple within-year breeding attempts in plants and animals is characteristic of semi-deserts with episodic rainfall that may occur at any time of the year. We conclude that opportunistic and iteroparous breeding occurs in many unrelated taxa, but that there are life-history, metabolic or dietary constraints to opportunism.