Masters Degrees (Botany and Zoology)

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    A comparison of mangrove and estuarine fish diversity using eDNA metabarcoding and baited remote underwater visual surveys in South Africa and Mozambique
    (Stellenbosch : Stellenbosch University, 2023-12) Janna, Jamila; Peer, Nasreen; Von der Heyden, Sophie; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Within estuaries, mangroves and seagrass beds are critical habitats that act as nurseries and refuge areas for fishes and provide food for coastal communities. However, in southern Africa, there are knowledge gaps regarding fish diversity in 1) mangrove ecosystems and 2) under-studied estuarine bays such as Inhambane Bay, Mozambique. Environmental DNA (eDNA) metabarcoding and baited remote underwater video systems (BRUVs) are used as complementary tools for biomonitoring as they provide a better insight into the presence of species and their interactions with the environment. This project investigates OTU (operational taxonomic unit) and genera richness in South Africa and Inhambane Bay using eDNA metabarcoding and BRUVs. The aims were to 1) identify whether mangrove fish diversity varies over a latitudinal gradient, 2) compare genera richness inside and outside the locally managed marine areas (LMMAs) in Inhambane Bay, and 3) compare the use of complementary tools BRUVs and eDNA in both sites. At all study sites, estuarine water was sampled and BRUVs were deployed for an hour. In South Africa, eDNA recovered 110 OTUs with 58 genera and BRUVs recovered 28 genera with only 12 common taxa detected using both methods. Fish diversity did not follow a latitudinal gradient in the mangroves as hypothesized. In Inhambane Bay, eDNA recovered 176 OTUs between the three locations. For the comparison between methods, 76 genera were recovered by metabarcoding and BRUVs only recovered 10 genera. There was no significant difference in fish diversity inside and outside the LMMAs. In both studies, the dual use of BRUVs and eDNA was valuable by detecting species unique to those methods. The results indicated a need to improve the study design, conduct more research on estuarine and mangrove fish diversity, and to improve reference databases and fish guides for southern Africa.
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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.
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    The role of the biological soil-root interface in seasonal variation of N and P recycling for Aspalathus linearis
    (Stellenbosch : Stellenbosch University, 2023-03) Du Toit, Elbe; Valentine, A. J. ; Kleinert, Aleysia ; Lotter, Daleen; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Aspalathus linearis is found in the diverse Cape Floristic Region, known for its nutrient poor soil with a low pH and weather conditions that consists of dry, hot summers and cold, wet winters. Aspalathus linearis is well adapted to the soil in this region by producing cluster roots, forming symbiotic relationships with nitrogen fixing bacteria and by increasing the soil pH through OH- secretion in the roots. Few studies have investigated the seasonal variation in A. linearis physiology with regards to N and P acquisition. This study aims to determine the seasonal variation in N and P acquisition for A. linearis grown in two different soils and determine the biological activities in the soil associated with A. linearis. It is expected that an increase in N and P enzyme activity will be seen for soil with higher nutrient levels. Two soils were identified, one with higher P and C concentrations and a higher clay percentage. Nitrogen and P acquisition was determining using enzyme assays, including GS, NR, GDH-D, GDH- A, Phytase, APase, RNAse and Pi. Soil chemical and physical properties were investigated to determine the differences in the soil and microbial community diversity were investigated in the study. The differences in soil showed a great impact on soil enzyme activity on enzymes such as NR and APase, while having little effect on other soil enzymes. Seasonal growth caused a significant increase in bacterial activity and diversity for both soils identified and an increase in wetter season was found in microbial diversity. Soil conditions such as higher C and P did not show a consistent impact on plant enzyme activity. Enzymes such as GS, GDH-A, GDH-D and Pi were affected by soil conditions, however, seasons affected activity more than soil. Amino acid concentrations increased in winter, with soil in higher C and P having significantly lower concentrations than amino acid concentrations from all winter material measured. The study found that an increase in nutrients including C and P does not lead to an overall increase in N and P acquisition and that enzyme activity varied in dependency on soil conditions. Further studies should investigate the effect of different soil conditions on drought for A. linearis.
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    Seasonal patterns of evapotranspiration, soil moisture, rainfall, and normalized difference vegetation index in a Nama-Karoo ecosystem
    (Stellenbosch : Stellenbosch University, 2023-03) Gqaleni, Nosipho Onwabile; Midgley, Guy; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH SUMMARY: This study mainly focused on the South African semi-arid region, the Eastern Karoo, which is located on an ecotone between the Nama Karoo and the Grassland Biomes. The Eastern Karoo is classified as a summer rainfall area, and is known for its highly variable rainfall patterns, which vary in time, space, amount and duration. Seasonal changes in rainfall patterns can affect the hydrological cycle, environmental processes, vegetation, and other biological processes of this region. Dryland ecosystems such as in the Eastern Karoo seem to be one of the most responsive ecosystems to climate change and rising atmospheric CO2, for example, leading to globally widespread desert “greening”. Despite these important changes underway globally and locally, and the value of semi-arid systems in southern Africa, much remains to be learned about their functioning and resilience in response to biophysical drivers. The main aim of this study was to determine if the soil moisture regime, and the related vegetation evapotranspiration and production response to rainfall and other climatic drivers in the Eastern Karoo are predictably linked, and if the ecosystem shows predominantly “pulsed” (eventdriven) or seasonal functioning. The inter-biome ecotonal location was specifically selected to enhance the likelihood of gathering data representing a range of climatic conditions over a succession of years. Results suggest that while soil moisture in this system is somewhat more strongly event-driven rather than seasonally driven, particularly for shallow soil depths, system evapotranspiration appears to be under strong plant physiological control, especially via a reliance of evapotranspiration on deeper soil moisture. These findings do not support the notion that the Nama-Karoo is strongly event-driven with respect to vegetation physiological activity. They also do not support the inference that ongoing climate change will necessarily increase evapotranspiration and cause system aridification. Results for vegetation productivity showed that average normalized difference vegetation index (NDVI) is driven by evapotranspiration and air temperature, not soil moisture, while short term shifts in NDVI (DNDVI) are driven by rainfall and soil moisture conditions. Finally, in-field daily average NDVI measurements on a dominant grass species strongly indicated that shallow soil moisture is a significant driver of the C4 grass productivity response. Taken together, these results show the value of using a range of approaches as lenses in answering questions of how climate conditions affect vegetation water use and production as a whole, and that in-field sampling adds value to the mechanistic understanding gained by studying the individual components of biodiversity. Future research could usefully build on this platform to develop a fuller picture of the interplay between these components of plant biodiversity, and how they contribute to the resilience of overall productivity under climatic variability and ongoing change.
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    Leaf-level physiological response and carbon allocation in two eucalypt hybrids with contrasting water use strategies under drought
    (Stellenbosch : Stellenbosch University, 2023-03) Tonkin, Christopher; Midgley, Guy F.; Drew, David; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Eucalypts are among the fastest growing trees on earth, with high production rates. However, the high species diversity of eucalypts has allowed this taxon to be successful across a broad climatic range, and allowed the forestry industry to match species with diverse sites around the world that may demand resilience to adverse conditions in addition to productivity. The creation of hybrids has further facilitated the selection of traits which can optimize productivity and resilience under adverse conditions. Stomatal regulation is thought to underpin this optimization, but a coherent predictive theory remains elusive, with a gain:risk framing now shaping new advances. Solutions to this challenge could help position eucalypts as a vital element of nature-based solutions to anthropogenic climatic change, addressing mitigation needs while considering optimal resource use, especially of water. This study explored drought responses of two eucalypt hybrids (Eucalyptus grandis x longirostrata (E.gl) and Corymbia henryi x torelliana (C.ht)) with contrasting water use strategies, to identify how key leaf-level functional traits (stomatal and light harvesting behaviour) influence the trade-off between growth rate and drought tolerance traits. Experimental plants were grown from rooted cuttings of each clone, and subjected to a 12-week drought period. Repeated harvesting quantified growth effect using biomass, stem height and stem diameter measures. Photosynthetic response analysis determined the differential effect of drought on the photosynthetic apparatus of the two varietals. Stomatal conductance and soil water potential were measured repeatedly while varietals were subjected to soil dry-down, and used to parameterize an optimization model to estimate hydraulic parameters for both varietals. The data showed distinct water-use strategies in the two varietals, resulting in a differential fit between responses predicted by a gain:risk approach in the two varietals. Model predictions more closely matched measurements in E.gl, which showed a more isohydric response to drought (i.e. more rapid stomatal closure) and less closely in C.ht which showed a more anisohydric response (i.e. lower but less drought- responsive stomatal conductance). Both varietals showed no damage to photosynthetic apparatus under drought conditions, revealing the resilience of light harvesting in both water use strategies. This suggests that stomatal optimization modelling approaches need to better incorporate longer term adaptive responses that affect the gain:risk trade-off. Varietal differences in growth measures reflected these leaf level functional traits, with C.ht showing a significantly higher relative increase in biomass than E.gl by the end of the experiment, regardless of water treatment. Prioritisation of stem diameter growth in C.ht may support xylem development that is more tolerant of cavitation, while the E.gl varietal prioritised apical stem growth which would likely support greater productivity under sustained ideal water supply conditions, but this accumulative effect would require more time to emerge than permitted by this experiment.