Browsing by Author "Richardson, David Mark"

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    Alien plant invasion on the Agulhas Plain : a detailed description of invasion patterns
    (2004) Blanchard, Ryan; Richardson, David Mark; Rouget, Mathieu
    Invasive alien species occur within several biodiverse regions of South Africa. It is not yet fully understood the extent to which invasive alien species affect natural vegetation. It is important to classify such species based on abundance and distribution in order to determine invasion patterns and set priorities for management within a region. Such a study was adapted for the Agulhas Plain of South Africa in the Cape Floristic Region. Eucalyptus lehmannii was recognised as the top invasive alien species, occupying both natural and transformed landscapes at high densities, >130% canopy cover. Acacia saligna and Acacia cyclops are the most widely distributed species, covering 41 % and 53% of the total area. Effectively determining invasive alien species richness required the use different variables to those used in this study. Invasive alien species invasion pattern was classified into two groups, namely coastal invasion and interior invasion. These characteristics differed with regards to vegetation types and species composition. Natural vegetation within the Mountain fynbos (100% invaded), Dune fynbos (91 % invaded) and Mountain top fynbos (99% invaded) vegetation types were largely threatened by invasive alien species. Vegetation types largely affected by transformation include Elim asteraceous fynbos (89% transformed) and Renoster fynbos. The invasion patterns described could be used in prioritisation of manageable land.
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    The ecology of invasions by Pinus (pinaceae) and Hakea (proteaceae) species, with special emphasis on patterns, processes and consequences of invasion in mountain fynbos of the southwestern Cape Province, South Africa
    (1989) Richardson, David Mark; Cowling, Richard M
    The fire-prone mountain fynbos of the southwestern Cape Province of South Africa has been severely invaded by introduced trees and shrubs. These invasions have transformed fynbos shrublands to dense thickets of trees and shrubs in many parts of the region, thus disrupting various natural ecosystem processes. The ecology of invasions by species of Pinus and Hakea (the most successful genera) was studied using a series of natural experiments in conjunction with autecological studies. The study was divided into main four parts: (i) case studies to elucidate major patterns and processes of invasion; (ii) studies of the life history and population ecology of selected invaders; (iii) studies of the determinants of invasibility; and (iv) assessments of the consequences of invasion and of control programmes. Bibliography: pages 235-272.
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    Ecosystem level impacts of annual and perennial N2-fixing invasive alien plants in the fynbos vegetation of South Africa
    (2000) Yelenik, Stephanie; Stock, WD; Richardson, David Mark
    In the nutrient-poor fynbos, changes in soil nutrients can affect species composition and community structure. The N2-fixing ability of invasive alien plants, therefore, may have impacts on ecosystem and community functioning within the fynbos vegetation type. This study investigated changes in fynbos N cycling regimes with the invasion of two different N2-fixing invasive alien plants, Acacia saligna and Lupinus luteus.
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    The effects of fragmentation of South Coast Renosterveld on vegetation patterns and processe
    (1997) Kemper, Jessica; Cowling, Richard M; Richardson, David Mark
    This thesis investigates the effects of fragmentation on vegetation patterns and processes of South Coast Renosterveld at three hierarchical levels. South Coast Renosterveld is a grassy shrubland derived from shales on the coastal forelands of the Western Cape Province, South Africa. The area forms part of the Cape Floristic Region. It occurs on fine-grained, moderately fertile soils. Thus, most South Coast Renosterveld has been replaced by agriculture. This vegetation type is particularly rich in geophytes, many of which have highly localised distributions. Almost no research has been carried out on the composition and structure of South Coast Renosterveld. However, fragmentation theory suggests that the fragmentation process iof South Coast Renosterveld would cause a loss of biodiversity. Although South Coast Renosterveld is one of the most threatened vegetation types in South Africa, only 0.8% of its area is formally conserved.
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    A model of a rangeland grazing system within a management procedure approach framework
    (2007) Soares, Muri; Plagányi, Éva; Hoffmann, Timm; Richardson, David Mark
    A model describing livestock grazing system dynamics was developed and fitted to available data. This study preliminarily explored the use of a formal management procedure (MP) approach to determine appropriate annual offtake in a terrestrial system. This approach has been applied in marine systems with great success, but has never been tried in terrestrial ecosystems. Rangelands and marine systems have in common the fact that there is often an offtake component to them and so stakeholders in those systems need knowledge of optimal harvesting strategies within defined management strategies. Three models were developed that described the growth of female goats in Paulshoek. The first (basic livestock model) depicted the growth of livestock as a logistic equation with an adult survival term and an annual growth term. The second model (rainfall-livestock model) added a rainfall component, with livestock productivity modelled as a function of rainfall. The final version of the model (vegetation-rainfall-livestock model) added a vegetation component that interacted with rainfall and livestock. The rainfall-livestock model provided a statistically significant better fit to the data, followed by the vegetation-rainfall model. The vegetation-rainfall-livestock model provided a reasonable representation of livestock population changes through time, with the largest deviations evident over the period 1975-1980. Results show that environmental factors alone are unable to fully explain observed system dynamics because anthropogenic factors for which no data are available may also play a role. Preliminary exploration with a simple MP suggested that a low offtake rate of 10% provided the highest average annual offtake. Our study has shown that a rangeland stocking system can be reasonably described by a simple model that uses only rainfall data and a rainfall-vegetation-livestock interaction. Rangeland livestock systems would benefit greatly from adopting an MP approach as it would allow stakeholders to make informed decisions on stocking rates and annual offtake.
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    Pattern and process of plant invasion in an African savanna ecosystem, with emphasis on multiple spatial and temporal scales
    (2007) Foxcroft, Llewellyn Courtney; Richardson, David Mark; Rouget, Mathieu
    Biological invasions are a significant ecological and economic global crisis. Protected areas also suffer from the increased burden that invasions place on their resources and the impacts placed on the ecosystem. However, management requires an ecological foundation which can inform best practice and optimize its responses. l explored the patterns and processes of invasion in Kruger National Park (KNP), South Africa; a large national park situated in the Lowveld savanna ecosystem. l used spatially-explicit alien plant data at various scales from a national database, the whole of KNP, a specific region, river system, and small scale plots in one invaded area of KNP. Using various statistical techniques, primarily classification and regression tree analysis, logistic regression, ANOVA, Nestedness and spatial pattern analysis, I assessed the relationship between the patterns observed at a specific spatial grain and extent, discussing the implications for invasion ecology and management. Using this knowledge and conceptual tools, I developed a new framework and model which contributes to invasion theory. Scale is a critical component in evaluating alien plant invasions. Without careful consideration of scale, studies from different scenarios cannot be compared and the science of invasion ecology will not advance. I provide a framework for assessing the risks of plant invasions in a watershed, using both an area- and species-approach, highlighting areas of current and future potential concern. I also explore the role of intentional introduction of ornamental plants and discuss management approaches for dealing with this. Evaluating a riparian system provides insights into how different patches in a landscape are differentially invaded, and how patch type characteristics need to be considered carefully for management and monitoring. I also describe how reconstructing the invasion history of a species, complemented by fine scale assessment, provides insights into species-specific spread models, and also how these types of studies can input into general theories, such as the role of propagule pressure. These components together provide insight into the dynamics of alien plant invasions in an African savanna and protected area system.
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    Patterns of parasitism and emergence in the gall midge Dasineura Dielsii (Diptera: Cecidomyiidae) : a biological control agent of Acacia cyclops in South Africa
    (2004) Wheat, Nicola M; Richardson, David Mark; Hoffmann, John H
    Acacia cyclops A. Cunn. ex G. Don is an invasive alien plant that invades fynbos and coastal dunes. Several acacia species are grown commercially in South Africa and this has limited biocontrol agents to those that reduce only reproductive capacity. Dasineura dielsii was released in 2002 as a biocontrol agent for A. cyclops. This gall-forming midge destroys inflorescences and prevents seedpods from forming, but allows continued harvesting. Insects overwinter as larvae within their galls. This study examined the levels of parasitism experienced by dormant D. dielsii larvae, as well as the trigger that causes them to break dormancy. Gall clusters were sampled over autumn and early winter, and were dissected to determine occupancy. The effects of temperature and light on dormant larvae were also examined. Observations showed that as the season progressed, more D. dielsii larvae entered dormancy. At the same time, a greater proportion of dormant larvae were parasitized. Parasitism was highest, at 18.5%, at the end of the study period, but this level of parasitism is not enough to reduce the biocontrol power of D. dielsii. A positive relationship between mass of gall clusters and the number of galls they contain was established. Larval dormancy could not be artificially broken by either light or temperature, and it appears that neither factor alone can trigger a break in dormancy.
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    The reconstruction of late quaternary vegetation and climatic patterns on the Cape West Coast, using a charcoal sequence from Elands Bay Cave
    (1998) Allsopp, Janet Christine Natalie; Cowling, Richard M; Parkington, John; Richardson, David Mark
    A taxonomically well resolved sequence of charcoal spanning the past 40 000 years from the Elands Bay Cave was used in this thesis to achieve two main objectives: firstly to determine the plant communities which occurred in the past to compare these to extant communities, and secondly to predict past climates by using extant species-climate relationships. This quantitative approach in predicting past climates is novel in the western Cape. Two scales of study were used, a local and a regional scale. The study area used in the local approach was a block of land in the surrounds of the cave where the present-day distributions of the species (that occurred in the charcoal) could be sampled. A larger study area including the whole of South Africa was also used as a study area so as to determine the distribution of species that do not occur on the Cape west coast today, and also to sample a wider suite of climatic conditions. A classification of the past, present and combined data was conducted in order to document the change in communities over time. Ordination techniques were used to asses the environmental variables controlling the distribution of species in the landscape.
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    Studies on aspects of the integrated control of Hakea sericea in the South-western Cape Province, South Africa
    (1985) Richardson, David Mark
    Invasion by alien woody plants is a major conservation threat in the sclerophyllous shrubland vegetation of the southwestern Cape Province which is known as fynbos. The Australian shrub, Hakea sericea (Proteaceae)is the most serious threat in mountain fynbos catchments. This study comprises four papers and aims to identify and quantify factors controlling the distribution and population dynamics of H. sericea. The results of a cartographic analysis of the distribution of Hakea spp. in the South-western Cape Province show that the most important physiographic factor characterising infested areas is the presence of quartzite and sandstone substrates of the Table Mountain Series. Other factors examined in this study are annual rainfall, altitude and aspect. The importance of two natural barriers is discussed and predictions are made on the range of potential spread of the species in the Fynbos Biome.