Masters Degrees (Botany and Zoology)

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The applicability of radiocarbon dating to understanding lifespan and mortality patterns in the quiver tree, Aloidendron dichotomum (Masson) Klopper & Gideon F.Sm.
(Stellenbosch : Stellenbosch University, 2024-03) Murray, Kayleigh; Midgley, Guy F. ; Foden, Wendy Bernardina; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
ENGLISH ABSTRACT: This thesis investigates the use of accelerator mass spectrometry (AMS) radiocarbon dating as a means of estimating the maximum longevity and time-of-death across the range of the quiver tree (Aloidendron dichotomum (Masson) Klopper & Gideon F.Sm.). The quiver tree was identified as an indicator species for climate change impacts due to measured increases in population mortality and decreased juvenile recruitment in regions of its range that have experienced warming and drying over the past century. Dead quiver trees persist in situ due to slow decay rates meaning a metric of mortality rate can be estimated as a proportion of the number of live to dead individuals in the population. The use of this metric was subsequently challenged due to doubt over the true time-of-death of trees in situ. The attribution of mortality to climate change was disputed citing instead centennial and millennial-scale population extinction debt as the cause for increased mortality and adult-heavy population structure. Discerning historical and contemporary causes of population mortality is, thus, of great importance for understanding and mapping climate change impacts on the quiver tree and co-occurring species. Doing so requires population demographic modelling which depends on metrics for the age of maximum longevity of the tree and verified time-of-death of the mortality class. In this thesis I aimed to develop radiocarbon dating methods for accurate estimation of these two metrics. After a first chapter reviewing the literature relevant to the quiver tree as an indicator species, my second chapter estimated the maximum age for four trees from the arid north of the species’ range in Namibia by analysing the radiocarbon ages of samples collected across the radius of the tree base. The oldest sample originated between 1277 - 1452 (95% CI) placing the maximum age of this tree at more than double that of the previous maximum estimates (350 years old). I discovered that the core tissue of the trees was consistently contemporaneous with the date-of-death, and that other samples along the radius deviated from the expected linear model of growth. This suggests material turnover and potential mixing of old and new carbon - a process which needs further investigation to understand how this impacts interpretation of results. As the core material is not present, Bayesian age-depth modelling should be used to estimate the true maximum age of the trees, and this can be supported by increasing the number of samples analysed. The third chapter of my thesis aimed to test whether trees in the mortality class died before or after 1950 using the presence or absence of a specific radiocarbon signal known as “bomb carbon”. This would test a hypothesis that the mortality is a relict of an early 1900’s drought. The majority of samples from the core and all of the samples from the edge of the tree post-date 1950, but five trees analysed only at the core pre-date 1950. I have deemed these results inconclusive considering material turnover at the core, and thus the inference which can be drawn from this chapter is incomplete. Despite this, radiocarbon dating shows promise as a forensic method for estimating an accurate year of death of the quiver tree if sampling is expanded to two proximal samples from the edge of the tree.
A comparative study on the role of colour and contrast discrimination in the predatory behaviour of oceanic blacktip and bronze whaler sharks
(Stellenbosch : Stellenbosch University, 2024-03) Nagel, Cicely; Cherry, Michael; Andreotti, Sara; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
ENGLISH ABSTRACT: Despite the low number of yearly fatalities caused by sharks, negative shark-human interactions have been at the centre of national and international debates. Specifically, controversial policies such as shark culling have been adopted in South Africa and Australia to reduce shark populations in the proximity of recreational beaches. This project was designed to determine whether some colours might attract sharks more than others, to help minimize the chances of negative shark-human interactions. Sharks, together with rays, skates and chimaeras are part of the class Chondrichthyans, one of the oldest most prolific vertebrate lineages extant today. Despite their ecological importance and historical longevity, chondrichthyan vision remains poorly studied, especially in regard to their ability to perceive colours. Colour vision in sharks has been debated within the literature for decades, with behavioural experiments providing conflicting results. Recent microspectrophotometry analysis has provided evidence that sharks do not possess multiple cone pigments within their eyes. This suggests that they are colour-blind, and that colour preferences reflect differences in contrast with their background. Utilising a novel in-field behavioural approach, the present study aims to determine whether oceanic blacktip sharks (Carcharhinus limbatus) and bronze whaler sharks (Carcharhinus brachyurus) possess the ability to discriminate between five differently coloured bait tubes (white, yellow, silver, red, and black), in terms of how these colours contrast against a background of sea water. Results show that both C. limbatus and C. brachyurus were able to discriminate between the five coloured tubes, demonstrating a preference for the brighter colours (white, yellow, and silver) over the darker colours (red and black). Shark reactions can be explained in terms of luminance contrast between different coloured bait tubes and sea water. This study is the first behavioural study on C. limbatus and C. brachyurus simulating visual predation and provides empirical evidence on which colours are best avoided when engaging in activities in shark-inhabited areas. Both C. limbatus and C. brachyurus are classified as potentially dangerous to humans, having been involved in 35 and 16 unprovoked attacks, respectively, worldwide since 1540. These findings provide an insight into the visual ecology of sharks, showing that visual cues are important in shark predatory behaviour. Based on the results of this study, to reduce the chances of negative interactions between these two species and humans, dive gear, wetsuits, and bathing suits should avoid high-contrast colours such as white, yellow, and silver.
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.
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.
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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