S.J. Milton

Analysing shrub encroachment in the southern Kalahari: a grid-based modelling approach

Shrub encroachment is reducing the carrying capacity of arid grasslands in southern Africa for cattle. Although shrub-encroachment is known to occur as a result of the selective overgrazing of grasses by cattle, the interactions between rainfall and grazing are not well understood. Both the quantity and sequence of rainfall events are likely to influence the growth rates and competitive abilities of shrubs and grasses. Shrub encroachment is a slow process and animals are stocked at low densities in arid regions. Thus, field experiments for determining stocking rates that avoid shrub encroachment under various rainfall scenarios are almost impossible to replicate. We used a grid-based simulation model to investigate the shrub-grass dynamics of the southern Kalahari under various realistic rainfall scenarios and stocking rates of domestic livestock. The simulation experiments addressed the following questions: Does simulated cattle grazing lead to shrub encroachment? Over what time scale does the process take place? Are the dynamics of vegetation-change continuous in relation to grazing pressure or do they show a threshold behaviour? Simulation results indicated that the answers to all three questions depended on the quantity and sequence of rainfall. Simulated cattle grazing led to shrub encroachment under all rainfall scenarios, once stocking rates exceeded a threshold determined by long-term mean annual rainfall. The stocking rate threshold for shrub encroachment was less distinct (i.e. shrub cover in different simulation experiments had a higher coefficient of variation) under xeric than mesic climatic scenarios. This is because either competition from the herbaceous layer or rain may limit shrub establishment. In relatively mesic scenarios, where shrub encroachment was limited mainly by grass competition, the grazing of grasses beyond a certain threshold led to an almost deterministic increase in shrub cover. However, under xeric climates, where rainfall was lower and more stochastic, the rate of shrub encroachment in response to a given intensity of grazing became less predictable. The most significant finding of the simulation experiments was that, although the stocking rates currently recommended by pasture scientists are unlikely to lead to shrub encroachment within 20 years, they have a high probability of bringing about shrub encroachment within a century. These findings applied to most of the rainfall scenarios found in the southern Kalahari and are therefore of particular interest to rangeland policy makers in this semi-arid region.

Predicting invasion dynamics of four alien Pinus species in a highly fragmented semi-arid shrubland in South Africa

This study explored the determinants of spread of four alien Pinus species and the ability of models to predict invasion dynamics in a complex fragmented landscape. The role of environmental factors, natural and anthropogenic disturbance in relation to invasion history was assessed for different stages in the invasion process using a Geographic Information System. Pines escaped from plantations over the past 30 years and spread into the natural semi-arid shrubland (renosterveld). The pattern of spread was compared with a simulated random distribution using two different techniques, a standard logistic regression, and a new recursive modelling approach (Formal Inference-based Recursive Modelling; FIRM). FIRM analysis improved the accuracy of predictions and revealed interactive effects of variables hidden by the logistic regression analysis. More than 80% of isolated pine individuals were found in 20% of the habitat classified as suitable by the models. Soil pH was the most important predictor for the distribution of isolated trees, whereas the establishment of dense pine stands was largely determined by fire history. Differences in invasive behaviour could be explained by species attributes such as limited dispersal for P. canariensis, and better drought-tolerance for P. halepensis. Sixty-five percent of the current pine distribution was accurately predicted by the spatial distribution of the first trees to have invaded. Such models could be used to predict potential spread of invasive plants and gain a better understanding of the main factors driving the invasion process. However, the spread of invasive species in fragmented landscapes, strongly modified by human activities, is very complicated, and the spread remains difficult to predict in the long term. The dynamics of invasion are discussed in relation to changes in land use and disturbance regime.

The application of state‐and‐transition models to rangeland research and management in arid succulent and semi‐arid grassy Karoo, South Africa

Abstract We evaluate the ability of existing models of vegetation dynamics to explain how drought and herbivory by domestic livestock can change Karoo vegetation, and why the vegetation does not always return to its original composition when livestock are withdrawn. Using published data for the arid southern and semi‐arid eastern Karoo, we provide new conceptual models of the dynamics of these vegetation types and discuss their relevance for research into the conservation and rehabilitation of Karoo rangelands.

Desertification in the semi-arid Karoo, South Africa: review and reassessment*

The concept of desertification has recently been re-evaluated with two major outcomes. Firstly, the term itself has been reviewed, with most authors concluding that the emotive implications of ‘desertification’ (i.e. advancing deserts per se) are inappropriate in many cases and that the general term ‘dryland degradation’ is a more accurate description of reality. Secondly, it has been argued that the extent, nature and apparent irreversibility of the process has been over-estimated and that degradation in some areas may only be temporary, consequent upon an unfavourable combination of climatic and anthropogenic factors. In southern Africa, a reassessment of the concept of desertification is long overdue; the traditional view has prevailed to the extent that grazing management policy in the semi-arid parts of the sub-continent is essentially founded on the premise that widespread, irreversible degradation has occurred in post-colonial times. In this paper, we examine this position in the light of the available published evidence. It is suggested that some changes in the vegetation of the semi-arid Karoo occurred prior to the onset of European colonization, a function of phases of climatic aridification and the impacts of both Kho San hunter-gatherers and Khoi Khoi herders. This argument contextualizes the alleged historical impacts, which are shown to be interpreted along the lines of three competing models accounting for the pattern of assumed vegetation change. The literature concerning Karoo degradation is reviewed and it is revealed that much of the research throws little light on which, if any, of the three competing models most closely approximates reality. Instead, it becomes clear that the recognition of dryland degradation in the Karoo requires further investigation, and several possible approaches are presented by which the hypothesis of irreversible change may be tested and the pattern of vegetation change identified. It is concluded that a more systematic monitoring programme, including the investigation of vegetation and other environmental variables, should be instigated in South Africa.

Growth, flowering and recruitment of shrubs in grazed and in protected rangeland in the arid Karoo, South Africa

A three-year study involving 1300 marked shrubs of three species differing in palatability to sheep provided evidence that changes in the demographic structure of shrub populations in arid rangelands are a function of seed availability. Sheep reduced canopy size and flowering of their preferred forage species but had no effect on survivorship of seedlings or established plants during the study. Although annual turnover in all populations was low (<6%), abundant recent recruitment of non-forage species was observed on degraded rangeland. Natality of all three species was positively correlated with flower abundance. Mortality was correlated with natality because seedlings had a low probability of surviving their first year. Recruitment of both forage and non-forage species, appeared to be inhibited by competition and seedlings seldom survived near adult plants of the same growth form.

Linking ecosystem services and water resources: landscape-scale hydrology of the Little Karoo

There is growing acknowledgement of the dependence of human society on ecosystem services and of the fact that service delivery is being compromised by human impacts on ecosystems. This paper describes the linkage between landscape-scale hydrology and ecosystem services, and how degradation of the landscape is believed to have altered the delivery of those services. The Little Karoo, an arid environment in South Africa that encompasses a remarkable diversity of plant species, has been degraded by inappropriate agricultural practices, mainly overgrazing, cultivation, and irrigation. Landscape linkages, such as hydrological flows and the recycling of organic matter and nutrients, have been disrupted, resulting in net losses at all scales, from the shrub patch to the river basin. Land rehabilitation, while in most cases too expensive at the farm scale, may be economically feasible at the river basin scale, provided that some of the economic benefits are used to rehabilitate and manage areas as socioecological systems.

The importance of roads and road verges for raptors and crows in the Succulent and Nama-Karoo, South Africa

The behaviour and activities of 6 458 raptors (Accipitridae and Falconidae) and 1 947 Corvidae in the Karoo, South Africa, were recorded during the period January 1988 to June 2000 (n = 208 surveys) over a total distance of 90 012km. A significantly larger proportion of raptors and corvids were recorded foraging in road verges, or flying along roads than foraging or flying over rangelands. About 21% of all observations of crows feeding or foraging were associated with road-kills, whereas less than 2% of observations of feeding and foraging Accipitridae and Falconidae were associated with road-kills. The most frequent Accipitridae feeding on road-kills were Pale Chanting Goshawk (Melierax canorus), Yellow-billed Kite (Milvus migrans parasitus) and Jackal Buzzard (Buteo rufofuscus). There was no seasonal pattern in the number of crows feeding on roadkills, although there was a trend for more mammals to be killed on the roads, and thus more available food, in winter (June). It is likely that Accipitridae and Falconidae are attracted to roads by the availability of perches and the relatively productive road verges rather than the availability of road-killed animals, whereas crows may be attracted by road-kills as well.

Effects of rainfall, competition and grazing on flowering of Osteospermum sinuatum (Asteraceae) in arid Karoo rangeland

The shrub Osteospermum sinuatum, an important fodder plant in Karoo rangelands, flowered after heavy rain in autumn, winter and spring. The number of flowers produced per bush was positively conelated with basal stem diameter and rainfall in the 12 weeks before anthesis. Flowering increased when neighbouring plants of all species were removed. Grazing during flowering and seed set, by sheep stocked at recommended densities, reduced potential seed set by as much as 90%. Birds, hares and small antelope consumed 10–50% of the flowers. Size‐class distributions indicated that little recent recruitment had taken place on a ranch where palatable plants were scarce and where O. sinuatum flower production was severely depressed by grazing sheep.

The role of disturbance in the establishment of indigenous and alien plants at Inaccessible and Nightingale Islands in the South Atlantic Ocean.

Disturbances play an important role in the establishment of vegetation at Inaccessible and Nightingale Islands in the South Atlantic Ocean. Type, altitude and age of disturbance all influenced the species richness, life-form and abundance of seedlings. A few indigenous species appear to be dependent on small disturbances, such as the clearings around albatross nests, for their perpetuation, e.g. Acaena sarmentosa and Gnaphalium thouarsii. Alien forbs and grasses, e.g. Conyza albida, Holcus lanatus, Pseudognaphalium luteo-album and Rumex obtusifolius, tended to dominate large recent disturbances such as soil-slips and depressions, and to colonize man-modified sites. Alien plants were less common on the plateau at Inaccessible Island than on the lowlands. Their success appears to be enhanced by large-scale disturbances. The highest density and diversity of seedlings occurred on large soil-slips where mineral soil was exposed and vegetation cover was minimal.

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.