Browsing by Author "Walker, Gareth Alexander"
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- ItemAilanthus altissima : an assessment of its distribution at different spatial scales and options for management in South Africa(Stellenbosch : Stellenbosch University, 2017-03) Walker, Gareth Alexander; Richardson, D. M.; Robertson, M. P.; Gaertner, Mirijam; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.ENGLISH ABSTRACT: The constant flow of goods into and out of cities has resulted in the proliferation of invasive species in urban areas and, because of this, cities are viewed as ‘hotspots’ for invasive species. The rise of global trade and long distance transportation will ensure that further introductions will take place in urban ecosystems. Conditions that have been altered due to human activities enable some alien species to proliferate rapidly throughout urban centres, often also spreading into natural and semi-natural areas at the urban/wildland interface. Evaluating the current distribution of an invader by analysing potential patterns in its distribution, as well as determining the potential distribution of a species over multiple landscapes and identifying factors that promote its spread, is essential for determining the full extent of an invader. Understanding 1) where a species is; 2) where a species may spread to; and 3) the factors that promote the spread of a species are all needed to gain an understanding of the biological and biogeographical preferences that facilitate the spread of the species. This is best achieved by using a multi-scale analysis that studies all aspects of an invasive species over a variety of spatial scales. This study focusses on a notorious northern hemisphere invader - Ailanthus altissima – that, although having been introduced into South Africa in 1834, is yet to replicate similar levels of invasion success. I adopted for a multi-scale analysis of the species evaluating the current and potential distribution of the species at a global scale (Chapter 1), national scale (Chapter 2) and local scale (Chapter 3). Each chapter addresses a central aim in which a number of research questions are evaluated. In Chapter 1 I assess the biogeography, distribution, and habitat suitability of A. altissima by evaluating the current and potential distribution of the species (based on climatic suitability) at both a global scale and a national scale (in South Africa). The main conclusions of this chapter are that 1) online databases (although outdated at times) provide an accurate representation of the distribution of invasive species and should therefore be accessed. This was ascertained by cross referencing a number of online databases to evaluate whether they were largely in agreement with one another in terms of the distribution of an invader; and 2) modelling the potential distribution of an invasive species based on climatic suitability serves as a good first approximation of potential species spread. Although this may be true, it was also suggested in this chapter that to accurately quantify the potential distribution of an invasive species, other distribution drivers (such as land use and human mediated disturbances) need to be incorporated into the design of a species distribution model. Chapter 2 evaluates the degree of range filling exhibited by A. altissima in South Africa and identifies areas at risk of future invasion. This chapter also identifies specific factors that promote the spread of the species at a global and national scale. I determined that at the global scale, climate had the highest influence on the distribution of A. altissima whereas at the national scale, human mediated disturbances exerted a higher influence on the distribution of the species. This chapter builds on the concepts proposed in Chapter 1 by highlighting the importance of incorporating different environmental variables at various spatial scales to identify potential invasion ‘hotspots’. I conclude that the multi-scale approach presented in this chapter enables the early detection of invasive species, preventing damage associate with their potential spread. Also, the novelty of this approach is particularly effective when it comes to analysing urban invaders. The third chapter evaluates the distribution pattern of A. altissima within the City of Cape Town by using the suburb of Newlands as a case study. In addition, it determines potential options for management at a city scale. It can be concluded that fine-scale analysis of invasive species distributions is best achieved using Geographic Information System (GIS) techniques and that such these ultimately help to identify potential areas susceptible to future invasion while delimiting unsuitable areas. I determined that A. altissima has a preference for affluent urban areas with podzolic soils that are exposed to high levels of rainfall within the City of Cape Town. Using this information, I determined that there is a large capacity for further spread by the species throughout the city. The systematic approach that we used in Newlands is an effective way of gauging the full extent of an invader and is especially effective for evaluating the population structure of a particular invasive species. Future studies should adopt this approach in conjunction with remote sensing techniques to achieve the best possible results. The final chapter of this thesis provides the overall conclusions in which all the main findings are highlighted. Altogether, the approach presented in this thesis is an effective method that could be used on other emerging, urban invaders globally. The systematic, multi-scale analysis proposed in this project showed high levels of success and it is my belief that this project achieved its desired aims.