Characteristics of successful cultivated alien plants in Africa

Anthropogenic activities have been changing biotic and abiotic ecosystem components throughout human history. The exchange of biota between different parts of the globe is one of the most distinct of these man-made changes. For example, over 13,000 species of vascular plant are known to have become well established outside their native geographical ranges. Some of these are considered invasive (i.e. become widespread and locally dominant). The unfavourable impacts of invasive species such as biological, economic, and social damages, have made it urgent to understand the mechanisms underlying observed ecological patterns and to determine the characteristics that promote invasion success of non-native taxa. While assessing characteristics that promote invasion success, many studies focus on only one stage along the invasion process —transport, introduction, establishment (i.e. naturalization) and spread (i.e. invasiveness)— mostly one of the two last stages. However, overlooking the earlier stages (e.g. introduction) by implicitly assuming that selection of plant species for introduction is random, may result in over- or under-emphasizing the characteristics of successful invasive species. Great efforts have been made to identify and document biological invasions data (e.g. checklists of cultivated flora), which provides a great opportunity to better understand drivers of biological invasions. However, accounting for all the stages along the invasion process is still hindered by the lack of such data (e.g. introduction history, traits, phylogenetic information). In chapter 1, I quantified and accounted for bias associated with the introduction of alien cultivated flora (i.e. introduction bias) when assessing the naturalization success of cultivated flora in 10 countries composing Southern Africa. To better understand patterns in the geographic origin, phylogenetic composition and functional traits of the naturalized flora. Overall, cultivated flora was not a random subset of the global flora, and this introduction bias largely contributed to patterns in geographic origin, phylogenetic composition and functional traits of the naturalized flora. The finding suggests that accounting for introduction biases is important to avoid over- or under-estimation of the characteristics of successfully naturalized alien plants. In chapter 2, I further accounted for introduction biases in regulating the phylogenetic pattern of naturalization and invasiveness of alien flora in Southern Africa. Specifically, I tested Darwin's Naturalization Conundrum (DNC; two alternative hypotheses on whether success of introduced species is positively or negatively associated to their relatedness to native floras) at the different stages of the invasion process (i.e. naturalization and invasiveness) in 18 sub-regions in Southern Africa. I showed that phylogenetically dissimilar species to the native flora of Southern Africa were more likely to be introduced from the global flora for cultivation. Once Introduced, phylogenetic relatedness to the native flora negatively influenced the subsequent naturalization stage and positively influenced the naturalized species transition into the invasiveness stage. This suggests that Darwin's alternative hypotheses are not mutually exclusive, but depend in part on the invasion stage. In chapter 3, I contributed to filling an important gap in alien plant data, as data missing hinder the consideration of the invasions stages and their associated biases. I compiled the first checklist of the alien flora in Sudan and South Sudan. This checklist should contribute to filling the data gap in these countries, and stimulate further research and recording of the alien flora to better understand the drivers of successful alien plants. Taken together, the results of my thesis show that patterns of naturalized flora largely reflect the biases in the selection of introduced species and that the mechanisms underlying invasion success could depend on the stage of the invasion. my thesis strongly suggests that accounting for all stages of biological invasions and associated biases is important to provide new insights into characterises underlying the observed invasion patterns. Lastly, by focusing on Africa, my thesis helps to fill important knowledge gaps that could biased our comprehensive understanding of the global invasion patterns.