Doctoral Degrees (Botany and Zoology)

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Browsing Doctoral Degrees (Botany and Zoology) by Author "Castillo, Maria Loreto"

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    Processes and drivers of Prosopis invasions in Eastern Africa
    (Stellenbosch : Stellenbosch University, 2019-12) Castillo, Maria Loreto; Le Roux, Johannes J.; Van Wilgen, Brian W.; Schaffner, Urs; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Increased movement of humans and goods around the world has facilitated the transportation of many species into new geographic ranges. A significant number of these have become invasive, resulting in substantial ecological, social, and economic impacts. In order to develop effective management strategies, it is necessary to elucidate the drivers underlying invasion and to understand what determines species invasiveness. Progress in the understanding and management of biological invasions depends on proper taxonomic identification of invasive species. However, the taxonomy of many alien taxa remains problematic due to unresolved species relationships, geographic distributions and/or inter-specific hybridization, among others. Mequite trees from the genus Prosopis are problematic invasive species in many parts of the world. To resolve taxonomic uncertainty among Prosopis species globally, I used phylogenetic and population genetic approaches to examine evolutionary relationships and levels of genetic diversity and population genetic similarity among Prosopis species collected from four native regions (Argentina, Chile, Mexico and Peru) and six non-native regions (Australia, Hawaii, Kenya, Ethiopia, Tanzania and South Africa). The genetic analysis showed high phylogenetic similarity, low genetic differentiation between species from the native range and provided evidence for inter-specific hybridization between different Prosopis species in both native and non-native ranges. My findings suggest that hybridization between previoulsy allopatric species may occur frequently when they are co-introduced into new ranges. In addition, polyploid individuals were detected in both native and non-native areas, with tetraploid P. juliflora being highly differentiated from other diploid Prosopis species. Polyploidy is therefore proposed as an additional mechanism that facilitates reproductive isolation between some Prosopis species. Lastly, levels of genetic diversity suggest that invasive populations in Eastern Africa (Kenya and Ethiopia) probably resulted from multiple introductions of two species, P. juliflora and P. pallida. Prosopis invasion in Eastern Africa provided an excellent opportunity to examine how the ecological and genetic attributes of invasiveness, and drivers of invasion success, vary with context and taxon, because the founder trees of two species, tetraploid P. julifora and diploid P. pallida, are still present in the original plantations today. Here, I exploited these unique circumstances and examined the mechanisms – such as plasticity, rapid post-introduction evolution and hybridization – that contribute to the invasion success of these trees in Baringo County, Kenya and Afar Region, Ethiopia. I found that in Baringo County, despite the similar invasion history of P. pallida and P. juliflora and probable inter-species hybridization, only P. juliflora individuals became invasive in the region; indicating that the success of Prosopis invasion is not attributed to hybridization but potentially to the higher ploidy of P. juliflora. Similarly, in Ethiopia’s Afar Region, genotypes consisted exclusively of P. juliflora. In Kenya’s Baringo County, I performed common garden and reciprocal field transplant experiments that indicated that high levels of phenotypic plasticity and post-introduction evolution had contributed to the invasiveness of P. juliflora. Similar levels of plasticity were absent from introduced, but non-invasive, P. pallida. My results also showed that different demographic processes may be occurring in the Afar region (Ethiopia) and Baringo County (Kenya). In the latter, contemporary genetic change during the invasive spread, or founder effects during initial range expansion from plantations, may explain the genetic erosion I found along the range expansion of Prosopis. In Afar Region, successful spread may have been promoted by gene flow from “source” plantations to invasive genotypes, homogenizing standing genetic diversity across the invasion. Lastly, by using landscape resistance modelling in both areas, I showed that dispersal among Prosopis populations was not influenced by any of the attributes analysed: physical distance between populations, variables related to human and animal-mediated dispersal along roads and rivers, bioclimatic and altitudinal conditions. Therefore, the dispersal of Prosopis populations was not constrained by any landscape variable, and probably involved frequent human-assisted long-distance dispersal. Overal, this study showed that hybridization, polyploidy or both have contributed to the invasiveness of Prosopis. Finally, this study formed part of a larger international collaborative project entitled “Woody invasive alien species (IAS) in Eastern Africa: assessing and mitigating their negative impacts on ecosystem services and rural livelihoods” (hereafter referred to as Woody Weeds), with the overall objective of mitigating the impacts of woody IAS on biodiversity, ecosystem services and rural livelihoods in Eastern Africa. For this, basic knowledge about the invasion process and the impacts of woody IAS are being evaluated and diverse control and sustainable land management strategies are being proposed. In collaboration with PhD students involved in the Woody Weeds project, I propose key components of research projects addressing complex social-ecological topics that facilitate inter-disciplinary and, when interacting with stakeholders, trans-disciplinary research. Trans-disciplinary approaches should have a clear structure that transcends disciplines through amultidisciplinary team with common goals. To allow for integration and upscaling of findings, there should be a co-design of data collection using different methodologies in the same experimental units/scales. It is important to have the clear intention to identify management options with stakeholders, estimate their effects and test their implementation, as well as provide transdisciplinary training for all project participants. To facilitate the integration of the drivers of alien plant invasion into the development of effective management options, I concluded with a discussion of two principal questions: (i) How does an improved understanding of the eco-evolutionary drivers of invasiveness help us to better manage the problem? (ii) What implications does the better understanding of genetic and ecological drivers have for the use of particular control methods, especially biological control?