Summary1. Shrub encroachment has been widely observed in savanna regions. This study analysed the causes of shrub encroachment in the lowveld savanna of north-eastern Swaziland, southern Africa, and highlighted management regimes that can be used to reduce or prevent it. 2. The rates and dynamics of shrub encroachment were quantified for the period 1947-90 using aerial photographs, and for 1997 using a ground survey. Five similar areas with different land-use histories were compared to investigate the relative importance of fire, herbivory, rainfall, soil type and shrub density in driving shrub dynamics. 3. In the study area as a whole, shrub cover increased from a mean of 2% in 1947 to 31% in 1990. Dichrostachys cinerea accounted for most of the increase in cover, contributing 81% to total shrub cover during 1997. Shrub cover was strongly correlated with shrub density and weakly negatively correlated with tree cover. 4. Shrub encroachment varied across land-use fence lines. The key determinants of shrub dynamics were grazing, through its negative effect on fire frequency and an interaction between drought frequency and high shrub cover. Browsing pressure had a significant but minor impact on dynamics, while soil type had no significant effect. High grazing pressures through their effect on fire frequency were critical throughout the study period in promoting shrub encroachment, while the interaction between drought and high shrub cover produced declines in the later stages. Browsing had an impact on encroachment only in the early stages. 5. Frequent fires, facilitated by low grazing pressures, were capable of preventing shrub encroachment. When coupled with drought, frequent fires could reduce high shrub densities. 6. As cover and density were strongly correlated, it can be inferred from the negative correlation between change in cover (density) and initial cover (density) that the rate of shrub encroachment was cover (density)-dependent and that there was a shrub equilibrium of 40% cover, approximating to 2400 plants ha -1 . Shrub population growth was driven by events (fire, drought) as well as continuous agents (density dependence, mean browsing and grazing pressure). 7. Bush encroachment can be reversed by a combination of management (frequent fires) and climatic events (drought). The implications for savanna management are discussed.

Extensively managed grasslands are recognized globally for their high biodiversity and their social and cultural values. However, their capacity to deliver multiple ecosystem services (ES) as parts of agricultural systems is surprisingly understudied compared to other production systems. We undertook a comprehensive overview of ES provided by natural and semi‐natural grasslands, using southern Africa (SA) and northwest Europe as case studies, respectively. We show that these grasslands can supply additional non‐agricultural services, such as water supply and flow regulation, carbon storage, erosion control, climate mitigation, pollination, and cultural ES. While demand for ecosystems services seems to balance supply in natural grasslands of SA, the smaller areas of semi‐natural grasslands in Europe appear to not meet the demand for many services. We identified three bundles of related ES from grasslands: water ES including fodder production, cultural ES connected to livestock production, and population‐based regulating services (e.g., pollination and biological control), which also linked to biodiversity. Greenhouse gas emission mitigation seemed unrelated to the three bundles. The similarities among the bundles in SA and northwestern Europe suggest that there are generalities in ES relations among natural and semi‐natural grassland areas. We assessed trade‐offs and synergies among services in relation to management practices and found that although some trade‐offs are inevitable, appropriate management may create synergies and avoid trade‐offs among many services. We argue that ecosystem service and food security research and policy should give higher priority to how grasslands can be managed for fodder and meat production alongside other ES. By integrating grasslands into agricultural production systems and land‐use decisions locally and regionally, their potential to contribute to functional landscapes and to food security and sustainable livelihoods can be greatly enhanced.

The dynamics of arid and semiarid grazing systems are prone to the effects of highly variable rainfall, with droughts causing frequent episodic mortality in herbivore populations. This has led to the suggestion that they are nonequilibrium systems, in which animal impacts on plants are strongly attenuated or absent. We examine the utility and appropriateness of nonequilibrium concepts for understanding ecosystem processes in African rangeland, attempt to distinguish disequilibrium from nonequilibrium, and argue that such concepts do not justify the view that herbivory has little impact in climatically variable systems. We present evidence for an alternative view of African rangeland function. We argue that, despite the apparent lack of equilibrium, animal numbers are regulated in a density‐dependent manner by the limited forage available in key resource areas, which are utilized in the dry season. This model asserts that strong equilibrial forces exist over a limited part of the system, with the animal population being virtually uncoupled from resources elsewhere in the system. Spatially and temporally, the whole system is heterogeneous in the strength of the forces tending to equilibrium, these diminishing with distance from watering and key resource areas and during the wet season. We argue that wet‐season range is more heavily utilized by animal populations sustained by key resource areas than would apply in the absence of key resources, and that uncoupling of the animal population from vegetation carries an increased risk of degradation. Droughts may impose intense and localized defoliation on vegetation, and this may result in altered species composition, reduced rain‐use efficiency, soil erosion, and loss of productive potential. Rather than ignoring degradation, policy‐makers and ecologists should seek to identify the characteristics of grazing systems that predispose some systems toward degradation, while others appear to be resistant. Development policies should focus on the spatial heterogeneity in susceptibility to grazing impacts and on preserving the productive capacity of key resource areas.

No abstract

Illius, A. W. and O'Connor, T. G. 2000. Resource heterogeneity and ungulate population dynamics. -Oikos 89: 283 -294.It has been suggested that climatic variation has the effect on the dynamics of arid and semi-arid grazing systems of reducing animal numbers below the level at which they have much impact on vegetation or soils, and that spatial heterogeneity in resource availability serves to buffer herbivores against climatic variation. Modelling was used to test these hypotheses and to examine the interacting effects of temporal and spatial variability in plant production on animal population dynamics and defoliation intensity. The model distinguishes areas of the range that are accessible during wet and dry seasons, and examines the effect of seasonal restrictions in foraging area. It was established that the animal population is in long-term equilibrium with dry-season resources, on which it depends for survival; that dry season resource areas and outlying areas thus operate in a source-sink manner; and that the ratio of these areas determines the strength of consumer-resource coupling outside the dry-season range. A high ratio of dry season to wet season resources may support a sufficiently large animal population to impose non-trivial defoliation impacts on the outlying range. Increasing degrees of variability in primary production on areas used by animals for surviving the dry season increased the annual variation in animal abundance and reduced the mean. By comparison with a stable environment, for which the model predicts virtually stable animal numbers and constant, low defoliation intensity, variation in annual rainfall causes wide fluctuations in animal numbers and defoliation intensity. Under climatic variation, animal numbers can build up enough to impose much higher defoliation intensities than under a constant regime. Periodic intense defoliation is a consequence of climatic variability which is likely to make these environments more, not less, prone to ecological change.A. W. Illius,