Demographics of alien willows in the Grassland Biome of South Africa
| dc.contributor.advisor | Steenhuisen, Sandy-Lynn | |
| dc.contributor.advisor | Martin, Grant | |
| dc.contributor.advisor | Payne, Stephanie | |
| dc.contributor.author | Mashamba, Tapiwanashe | |
| dc.date.accessioned | 2025-06-04T22:28:24Z | |
| dc.date.available | 2025-06-04T22:28:24Z | |
| dc.date.issued | 2024 | |
| dc.description.abstract | Invasive alien species present a critical threat to biodiversity, ecosystem services, and agricultural productivity worldwide. This study examines the distribution, demographic dynamics, and ecological impacts of ๐๐ข๐ญ๐ช๐น ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข L. (weeping willow) and ๐๐ข๐ญ๐ช๐น ๐ง๐ณ๐ข๐จ๐ช๐ญ๐ช๐ด L. (crack willow) within the Grassland biome of the Eastern Free State, South Africa. These invasive species, introduced for ornamental and functional purposes, have shown extensive colonization, particularly in riparian zones, where they outcompete native vegetation and alter hydrological processes. The research combines historical and contemporary data sources to evaluate changes in the population structure and spatial distribution of these species over time. Using aerial photographs from 1984 and 2023, supplemented with roadside surveys and Google Street View (GSV) analysis, the study provides an assessment of the speciesโ demographics. Additionally, field-based verification of GSV-derived data validates its utility as a cost-effective tool for mapping and monitoring invasive alien plants. The study confirms that ๐. ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข and ๐. ๐ง๐ณ๐ข๐จ๐ช๐ญ๐ช๐ด remain prevalent along watercourses, roadsides, and disturbed areas, with ๐. ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข exhibiting a broader distribution along roadsides with approximately 50% of the sites having a population size of 1-5 trees. Although ๐. ๐ง๐ณ๐ข๐จ๐ช๐ญ๐ช๐ด is not as widely distributed as ๐. ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข it dominates the areas in which it is found, forming pure stands; 56% of the sites surveyed consisted of 6 or more trees. Gender analyses of ๐. ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข populations reveal a predominance of female trees which suggests that its propagation is mainly asexual. The demographic study done through road surveys and GSV reveals limited recruitment of ๐. ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข in recent decades, suggesting a potential decline in its invasive potential with almost 80% of the population consisting of adult or dying trees. In contrast, ๐. ๐ง๐ณ๐ข๐จ๐ช๐ญ๐ช๐ด populations display a continuous expansion with healthy recruitment, with 44% of the trees recorded as seedlings, young trees, or mature trees. This expansion indicates that ๐. ๐ง๐ณ๐ข๐จ๐ช๐ญ๐ช๐ด is now more invasive than ๐. ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข, which increases the need for a tailored management strategy to address this invasive species. The research also investigates interactions between invasive willows and invertebrate communities. Surveys at selected sites document the presence of 348 individual insects and one arachnid, representing 21 families, with 14 species associated with both willow species. Another finding was the presence of crown galls on 27.5% on the ๐. ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข population surveyed. Invasive willows provide habitat and resources for certain insect populations. Some of these insects, specifically ๐๐ญ๐ข๐จ๐ช๐ฐ๐ด๐ต๐ฆ๐ณ๐ฏ๐ข ๐ฅ๐ช๐ด๐ค๐ฐ๐ญ๐ฐ๐ณ, ๐๐ถ๐ฃ๐ฆ๐ณ๐ฐ๐ญ๐ข๐ค๐ฉ๐ฏ๐ถ๐ด ๐ด๐ข๐ญ๐ช๐จ๐ฏ๐ถ๐ด ๐ข๐ฏ๐ฅ ๐๐ฆ๐ญ๐ช๐ฐ๐ฏ๐ช๐ฅ๐ช๐ข ๐ฒ๐ถ๐ข๐ฅ๐ณ๐ช๐ฎ๐ข๐ค๐ถ๐ญ๐ข๐ต๐ข, along with gall-inducing bacteria (potentially ๐๐จ๐ณ๐ฐ๐ฃ๐ข๐ค๐ต๐ฆ๐ณ๐ช๐ถ๐ฎ ๐ต๐ถ๐ฎ๐ฆ๐ง๐ข๐ค๐ช๐ฆ๐ฏ๐ด), recorded within the ๐. ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข populations could be resulting in the decline of this invasive species. This study explored the use of ArcGIS and GSV, to assess invasive tree distributions and potential of these tools to monitor the spread of invasive trees over time. The comparison of historical and current data reveals significant efficiency of GSV as a monitoring tool, offering a cheap and quick approach to invasive species management in resource-limited settings. Policy implications of the findings are discussed with reference to South African legislation, including the Conservation of Agricultural Resources Act (CARA) of 1983 and the National Environmental Management: Biodiversity Act (NEMBA). ๐๐ข๐ญ๐ช๐น ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข and ๐. ๐ง๐ณ๐ข๐จ๐ช๐ญ๐ช๐ด have been classified as Category 2 invaders under CARA but are not found under the NEMBA Act. This may result in challenges in effectively controlling ๐. ๐ฃ๐ข๐ฃ๐บ๐ญ๐ฐ๐ฏ๐ช๐ค๐ข and ๐. ๐ง๐ณ๐ข๐จ๐ช๐ญ๐ช๐ด. This dissertation contributes to the field of biological invasions by providing an updated assessment of invasive willows in the Grassland biome, thereby offering insights for land managers, conservationists, and policymakers. The study concludes by emphasizing the importance of integrating historical data and modern geospatial tools, as an informative approach to develop effective management solutions for invasive alien plants. The methodological framework and findings presented herein offer valuable lessons for addressing invasive species challenges in similar ecological contexts globally. | |
| dc.identifier.other | Dissertation (M.Sc.(Botany))--University of the Free State, 2024 | |
| dc.identifier.uri | http://hdl.handle.net/11660/13089 | |
| dc.language.iso | en | |
| dc.publisher | University of the Free State | |
| dc.rights.holder | University of the Free State | |
| dc.title | Demographics of alien willows in the Grassland Biome of South Africa | |
| dc.type | Dissertation |