Browsing by Author "Ndilenga, Natanael Shuudifonya"

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    Pollination biology of Welwitschia mirabilis in the central Namib Desert, Namibia
    (Stellenbosch : Stellenbosch University, 2023-12) Ndilenga, Natanael Shuudifonya; Ellis, Allan George; Marais, Eugene; Maggs-Kolling, Gillian; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Pollination is a vital ecological process that facilitates the reproduction of flowering plants. However, the characterization of pollination systems has often been limited to the observation of visiting insects and sporadic assessments of pollen loads. To have a more comprehensive understanding of pollination systems, it is important to consider multiple components of pollination effectiveness. Generalist plants are visited by many pollinators, while specialized pollination systems involve only a few species, and this is mostly because rewards are exclusively accessible to specific pollinators. Contrary to the common belief that animal pollination is predominantly associated with angiosperms, substantial evidence suggests that animal pollination is a characteristic of seed plants. This study focuses on Welwitschia mirabilis Hook. f., (Welwitschiaceae) an unusual dioecious lineage within seed plants found only in the Namib Desert of Namibia and Angola. The pollination system of Welwitschia has been the subject of significant controversy, but a single study determined flying insects as primary pollinators. However, that study was limited in terms of sampling and thus inconclusive as to whether these visitors are effective pollinators or not. My study built on these preliminary results and aimed to accurately characterize the pollination biology of Welwitschia, at three populations in the central Namib Desert. In Chapter 2, I conducted extensive pollinator observations to assess two things. First, I assessed the community of animal visitors associated with Welwitschia cones. I expected Welwitschia to have many visitors because it has a generalist phenotype where rewards (pollen and pollination droplets) are accessible to any visitor. I found that Welwitschia is visited by 24 insect morphospecies representing four orders, suggesting that Welwitschia is generalized in its pollination requirements. However, 86% of recorded visits were from only six insect species (four species of fly and two wasp species) that visited male and female cones at all three sites, and Mythicomyiidae flies accounted for 34% of all observed visits. This implies that most pollination in Welwitschia is likely attributed to a small group of fly and wasp insect species. Second, I investigated variation of visitors across male and female cones. Since male cones produce pollen in addition to sweet pollination droplets, providing two sources of potential food, while female plants only produce sweet pollination droplets that are consumed by insects, I expected that visitor species richness, composition and visitation rates should be higher on male cones, and that visitors to female cones should represent a subset of those visiting male cones. I found notable male-biased pollinator visitation where male cones received three times as many visits as female cones, with a significantly higher visitation rate. The visitor communities are also more species-rich on male cones, and the composition of visitors differs significantly between male and female cones. Lastly, I assessed visitor communities across different populations. As Welwitschia is generalist and populations occupy abiotically and biotically variable sites, I expected the community of available visitors to differ between sites. I found differences in pollinator communities and visitation rates across the three different study sites, suggesting the potential influence of both the abiotic environment and the local variation of co-flowering plant species at each site. In Chapter 3, I assessed the effectiveness of Welwitschia pollinators using two approaches. First, I quantified components of pollination efficiency for different visitor species, including visitation rates, visitation patterns to male and female cones, contact with female cone droplets, and pollen loads. Based on the differences in pollinator foraging behaviours and potential to carry pollen, I hypothesized that there would be differences in the components of pollination effectiveness between visitor species and that visitor species could be ranked along a continuum from inefficient to efficient visitors. I also expected that for a visitor species to efficiently transfer pollen between male and female cones there should be significant temporal overlap in visitation to male and female cones. Second, I assessed the contribution of different pollinator groups to seed production using selective exclusion experiments. I expected Welwitschia to require pollinators for seed set, and that large-bodied insects would contribute most to seed set due to their ability to fly longer distances and carry larger pollen loads. I found substantial variation amongst pollinators in all components of pollinator effectiveness measured. I found that insect pollinators contribute significantly to seed set of Welwitschia, and that a core group of primary pollinators, comprising of both small and large-bodied flies and wasps, are important. However, surprisingly, the exclusion experiments show that seed set occurs in the absence of pollinators, suggesting facultative apomixis or alternatively a contribution of wind pollination. While the previous and only Welwitschia pollination study focused on observing visiting insects and assessing pollen loads, my study went a step further by quantifying pollinator effectiveness, thus providing a more accurate characterization of the pollination of this plant. Above all, this thesis represents a significant contribution to the field, as it is among the few studies that classify pollinators based on their quantified effectiveness in gymnosperms.