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University of Stellenbosch (2009)

Invasive potential of the Peruvian pepper tree (Schinus molle) in South Africa

Midoko Iponga, Donald

Titre : Invasive potential of the Peruvian pepper tree (Schinus molle) in South Africa

Auteur : Midoko Iponga, Donald

Université de soutenance : University of Stellenbosch

Grade : PhD (Conservation Ecology and Entomology) 2009.

Résumé
Natural and semi-natural ecosystems and human communities worldwide are under siege from a growing number of destructive invasive alien species. Alien species are those whose presence in an area is due to intentional or accidental introduction as a result of human activities. Some alien species become invasive, and some cause tremendous destruction to the ecosystem and their stability, but we do not yet understand fully the many factors that determine the levels of invasiveness in alien species. However, management of alien plants requires a detailed understanding of the factors that make them invasive in their new habitat. The aim of this study was to explore in detail the processes and potential for invasion of Schinus molle (Peruvian pepper tree) into semiarid savanna in South Africa and to examine the potential for this species to invade further in these ecosystems, and in other South African biomes. In this thesis I explored the patterns and processes of invasion of S. molle in semiarid savanna using small-scale experiments to investigate physical and ecological barriers to invasion that prevent or accelerate the invasion of this species. I examined factors such as pollination ; seed production ; seed dispersal ; seed predation and viability, all known to contribute to invasiveness. I highlighted the critical role of microsite conditions (temperature, humidity, water availability) in facilitating S. molle seedling establishment in semi-arid savanna and demonstrated that microsite type characteristics need to be considered for management and monitoring of the species in South Africa. I demonstrated the ability of S. molle to out-compete indigenous woody plants for light and other resources and also showed that disturbance of natural ecosystems was not a prerequisite for invasion, although human activities such as tree planting have played a major role in disseminating this species in South Africa. Predicting the future distribution of invasive species is very important for the management and conservation of natural ecosystems, and for the development of policy. For this reason, I also assessed the present and potential future spatial distribution of S. molle in South Africa by using bioclimatic models and a simulation-based spread model. I produced accurate profiles of environmental conditions (both biophysical and those related to human activities) that characterize the planted and naturalized ranges of this species in South Africa, by linking species determinants, potential habitat suitability and likely spread dynamics under different scenarios of management and climate change. All those components provided insights on the dynamics of invasions by fleshy-fruited woody alien plants in general, and on S. molle invasions in South Africa in particular. I developed a conceptual model that described S. molle population dynamics leading to an understanding of the processes leading to the invasive spread of this species in South Africa. This work also emphasized the need for policy review concerning the invasive status of S. molle in South Africa, and recommendations are made for future research.

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