Browsing by Author "Richardson, Dave"

Now showing 1 - 6 of 6
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    The impacts of different methods of alien plant control on the recovery of Fynbos ecosystems
    (2002) Parker, Fatima; Richardson, Dave; Van Wilgen, Brian; Robertson, Hamish
    This study investigated the effects of past management aimed at clearing invasive alien plants in South African fynbos ecosystems. The treatments included "bulldozing and aerial-spraying", "aerial-sprayiing", "boom-spraying" and "spot-spraying". Both an ant study and vegetation survey was conducted to test the hypothesis that fynbos is resilient to all kinds of pertubation (including the most severe alien plant management treatments employed) and that natural recovery over time is adequate to restore fynbos (without human intervention).
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    Modelling the current and potential distribution of woody plants, with special emphasis on the importance of spatial scale and environmental factors
    (2002) Rouget, Mathieu; Richardson, Dave; Cowling, Richard M; Lavorel, Sandra; Milton, Suzanne Jane
    The relationship between species distribution and environment has always been a central issue in ecology. The primary aim of this thesis is to explore the role of abiotic and biotic factors in mediating the distribution of woody plants, particularly invasive alien species. Predictions of the future distribution of invasive species are required for management, conservation actions, and legislation. Using predictive geographical modelling, this thesis assesses the role of spatial scale in understanding the determinants of species distribution, modelling invasion spread, and taking conservation decisions. The species distribution databases were collated in a Geographic Information System from various sources (from field mapping to remote-sensing data), and at various spatial scales (from local to sub-continental). Classification and regression trees, a flexible non-parametric statistical technique, were developed for each case study. I show that static modelling approaches, such as classification trees, are appropriate for modelling species distribution at regional to sub-continental scales. More mechanistic approaches, which include biological attributes, are required for accurate modelling of species abundance at local scale. A hierarchy of environmental factors was observed. Abiotic factors such as climate were more important for modelling species distribution at broad scales, whereas biotic factors were the fine-scale drivers of species distribution and abundance. This highlights the scale- dependence of prediction accuracy and of environmental drivers. Predictive habitat modelling was also applied to modelling future land use changes (including invasive alien species) in the context of conservation planning for the Cape Floristic Region. Effective conservation planning requires a detailed assessment of current land use patterns and future land use scenarios. Conservation planning is generally derived at regional scale but implementation takes place at finer scales. I investigated the transition from broad-scale planning to fine-scale implementation. This work emphasises the importance of diverse and fragmented habitats in measuring conservation value. This thesis concludes by linking predictive habitat modelling, species determinants, and application types to the spatial scale analysed.
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    Modelling the influence of rainfall variability and different grazing systems on the spatiotemporal dynamics and productivity of semiarid rangeland
    (2010) Kazembe, John Abdul-Aziz; Richardson, Dave
    This study pursued a two-fold objective: firstly, to broaden our knowledge about the rainfall-plant-animal system dynamics; and secondly, to re-examine the rotational grazing system with modelling as a substitute to field trials.
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    Predicting rates and patterns of alien plant spread
    (1998) Higgins, Steven Ian; Richardson, Dave
    The invasion of alien plants into natural ecosystems is a widespread phenomenon that impacts negatively on ecosystem structure and functioning. The invasion and subsequent spread of an alien plant population is equivalent to the processes of colonisation and migration. This implies that the existing toolbox of techniques developed for plant succession research should be useful for predicting plant invasions. Practitioners of invasion biology . have, however, found biological invasions frustratingly difficult to predict. The aim of this thesis was to use succession models to develop a modelling protocol for predicting rates and patterns of alien plant spread. The rationale was that such a model would both improve our understanding of the determinants of invasions and allow us to make predictions on the rates and patterns of alien plant spread. Such predictions are likely to be extremely valuable for the tactical and strategic management of plant invasions. Many modelling approaches could be .. adopted: the need to transcend the gap from general models of plant spread to management models led me to select a spatially explicit simulation modelling approach. The modelling approach is developed by comparing the behaviour of an individual based spatially explicit simulation (SEIBS) model of plant spread to the behaviour of the classic Skellam reaction diffusion model. This process also served to define the model's sensitivity and data requirements. The model's heuristic value is demonstrated by exploring why it is so difficult .to predict which plant will invade which environment. The model also ·provides a useful tool for exploring the role of long-distance dispersal in determining invasion rates. I show that long-distance dispersal is extremely difficult to define statistically, but is a key determinant of invasion rates. The model is validated using independent data on the spatial demography of two invasive species, Acacia cyclops and Pinus pinaster, and independent historical reconstructions of invasions. This validated model was then used to develop a dynamic landscape-extent model. This scaled-up model explores the optimal strategies for clearing alien plants and the ability of different clearing strategies and funding schedules to mitigate the threat that alien plants pose to native species. I conclude that models that are tightly linked to understanding of ecological processes and to field data can be used to rapidly develop predictive models. The development of these models challenges our fundamental ecological understanding and, therefore, emphasises the interplay between data, theory and prediction.
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    The restoration of riparian plant communities following alien plant clearing in the Western Cape
    (2002) Prins, Natalie; Van Wilgen, Brian; Richardson, Dave
    Few of the riparian zones of the Western Cape Province, South Africa, can still be described as 'natural', as many, if not all, have been impacted upon in one way or another. They are, therefore, frequently disturbed environments that are highly susceptible to invasion by alien plants. the vegetation of riparian zones controls the flow of water, nutrients and sediments into streams. Plan communities in these habitats are often rish in species, and serve as corridor facilitating the movement of organisms.
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    The vegetation and restoration potential of the arid coastal belt between Port Nolloth and Alexander Bay, Namaqualand, South Africa
    (1996) Desmet, Philip George; Cowling, Richard M; Richardson, Dave
    This thesis introduces the environment and the vegetation of the Namaqualand coastal belt between Port Nolloth and Alexander Bay. Aspects of the abiotic environment are discussed and related to patterns and processes observed in the vegetation of the study area. The restoration of the natural vegetation impacted by diamond mining activities is discussed. 1-4 The study area, located within the winter rainfall area of the Namib Desert, is one of four global fog deserts. The area is characterised by a near ubiquitous covering of Recent to Tertiary amorphous dunes of marine origin. The dunes can be divided into two broad categories: Recent, mobile white dunes, and Tertiary to Late Quaternary, semi-mobile red dunes. The red dune soils are considered arenosols, underlain by dorbank and calcrete hardpans, whereas the white dunes generally lack this structure, unless they are superimposed on an older dune series. The dune landscape is interrupted by outcrops of bedrock, such as river canyons (Holgat River); inselbergs (Buchu Twins); and koppies. Gravel plains and rocky outcrops cover much of the area on the south bank of the Orange River, as far south as Cape Voltas. The low rainfall ( <70 mm) is offset by frequent fog and dew. Summers are dominated by high energy, southerly winds and winters by gentle land-sea breezes interrupted by occasional, warm-easterly "berg" winds. A complete bio-inventory of higher plants of the study area was undertaken. Patterns of diversity and endemism were analysed in relation to plant growth form and habitat. 300 plant species were collected in the study area representing 40 families, with 28 being endemic to the coastal region. The flora is dominated by the Asteraceae (53 species), Mesembryanthemaceae (47), Crassulaceae (28), Poaceae (17) and Aizoaceae (15). Endemic species are over-represented in the Mesembryanthemaceae (60 endemic) and Crassulaceae (44), and under represented in the Asteraceae (8), Poaceae (0) and Aizoaceae (0). Rocky outcrops have the highest species:area ratio (3.77). They are characterised by their own distinct flora as well as representing a significant proportion ( 46) of the species from the surrounding dune landscape. Endemic species are concentrated on these, as well as on the gravel plain habitats. Dune habitats are the most widespread. However, they are characterised by a widespread, generalist flora with low species:area ratio (0.81), few endemics, and share an expected number of species with other habitat types (21 ). An endemic species in the southern Namib can be characterised as being a dwarf leaf succulent in the Mesembryanthemaceae which is most likely to be encountered on a rocky outcrop.