Carnivory submerged: aspects of the ecology and ecophysiology of the aquatic Utricularia stellaris L. fil. (Lentibulariaceae) in South Africa
- Authors: Marais, Alice-Jane
- Date: 2024-10-11
- Subjects: Submerged aquatic vegetation , Bladderworts South Africa , Carnivorous plants , Bladderworts Ecology , Bladderworts Ecophysiology , Plant-microbe relationships
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/464473 , vital:76514
- Description: The trapping structures produced by aquatic species of Utricularia have traditionally been interpreted to function as adaptations to capture and break down zooplankton prey, as in other carnivorous plants, to overcome nutrient limitations. However, an increasing number of studies have found that these plants may also rely on benefits derived from living mutualistic microbial communities contained within traps. This study documents aspects of the environmental, growth and physiological characteristics of U. stellaris to inform and to form a basis for future investigation into the plant-microbe interaction. The environmental conditions in which U. stellaris grows were documented to identify potential adverse conditions plants are subject to in situ, from which nutrient limitation was identified as a primary limitation. Plant growth and trapping structures were then assessed to identify possible adaptations of plants to overcome these limitations. The production of trapping structures likely constitutes an adaptive trait, with 30% of total biomass per node allocated to the production of these structures. Based on their capture function, traps may aid plants based on their contents, possibly supplementing plants with nutrients. Although assessments of the habitats of U. stellaris indicate that dissolved CO₂ concentrations in the ambient water are high, CO₂ may still be limiting to the photosynthetic rates of these plants due to viscous water resisting the diffusion of CO₂. The primary site of photosynthesis in U. stellaris is leaves and trap tissue’s contribution to photosynthetic output is negligible. U. stellaris plants are subject to CO₂ limitations in natural pond conditions, making the substantial allocation of resources to non-photosynthetic trapping tissue even more costly. Therefore, benefits gained from trapping structures are likely to be derived from trap contents; having ruled out the possibility that the trap tissue itself is photosynthetic. Trap contents of U. stellaris were assessed. The proportion of traps containing living microbial communities greatly exceeded the proportion containing zooplankton prey. In addition, these communities were found to be diverse, stable, and self-sustaining. These results suggest that trapping structures may be beneficial for both the carnivorous capture of prey and the housing of living microbial communities. These results indicate the plantmicrobe interaction in U. stellaris warrants further study. , Thesis (MSc) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-10-11
- Authors: Marais, Alice-Jane
- Date: 2024-10-11
- Subjects: Submerged aquatic vegetation , Bladderworts South Africa , Carnivorous plants , Bladderworts Ecology , Bladderworts Ecophysiology , Plant-microbe relationships
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/464473 , vital:76514
- Description: The trapping structures produced by aquatic species of Utricularia have traditionally been interpreted to function as adaptations to capture and break down zooplankton prey, as in other carnivorous plants, to overcome nutrient limitations. However, an increasing number of studies have found that these plants may also rely on benefits derived from living mutualistic microbial communities contained within traps. This study documents aspects of the environmental, growth and physiological characteristics of U. stellaris to inform and to form a basis for future investigation into the plant-microbe interaction. The environmental conditions in which U. stellaris grows were documented to identify potential adverse conditions plants are subject to in situ, from which nutrient limitation was identified as a primary limitation. Plant growth and trapping structures were then assessed to identify possible adaptations of plants to overcome these limitations. The production of trapping structures likely constitutes an adaptive trait, with 30% of total biomass per node allocated to the production of these structures. Based on their capture function, traps may aid plants based on their contents, possibly supplementing plants with nutrients. Although assessments of the habitats of U. stellaris indicate that dissolved CO₂ concentrations in the ambient water are high, CO₂ may still be limiting to the photosynthetic rates of these plants due to viscous water resisting the diffusion of CO₂. The primary site of photosynthesis in U. stellaris is leaves and trap tissue’s contribution to photosynthetic output is negligible. U. stellaris plants are subject to CO₂ limitations in natural pond conditions, making the substantial allocation of resources to non-photosynthetic trapping tissue even more costly. Therefore, benefits gained from trapping structures are likely to be derived from trap contents; having ruled out the possibility that the trap tissue itself is photosynthetic. Trap contents of U. stellaris were assessed. The proportion of traps containing living microbial communities greatly exceeded the proportion containing zooplankton prey. In addition, these communities were found to be diverse, stable, and self-sustaining. These results suggest that trapping structures may be beneficial for both the carnivorous capture of prey and the housing of living microbial communities. These results indicate the plantmicrobe interaction in U. stellaris warrants further study. , Thesis (MSc) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-10-11
Encroaching species are stronger anisohydric “water spenders” under elevated CO2 conditions: implications for savanna seedling establishment rates
- Authors: Reynolds, Liam Macleod
- Date: 2024-10-11
- Subjects: Xylem , Scrub encroachment , Isotopes , Photosynthesis , Plants Effect of atmospheric carbon dioxide on
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/465091 , vital:76572
- Description: Plant water transport systems play a fundamental role in the productivity and survival of terrestrial plants due to the vascular architecture placing a physical limit on metabolic function. Savannas have high variability in rainfall, leading multiple studies to suggest that plant water-use strategies are key mechanisms affecting seedling establishment rates. Many savannas are seeing a directional shift towards an increase in the abundance of certain woody species through a process known as bush encroachment, which has been largely attributed to the fertilising effect of rising atmospheric [CO2] on C3 trees. These species are classified as encroachers. While there have been multiple studies investigating changes in the physiology of savanna species under elevated CO2 (eCO2), few have examined how climate and eCO2 affects the fundamental water-use strategies in the seedling stage, a crucial demographic bottleneck. Here, this research provides valuable insights into the mechanisms behind bush encroachment in the context of eCO2 using results from a pot experiment at the Rhodes University Elevated CO2 facility and a field experiment. All species showed water use strategies characteristic of anisohydric “water-spenders”, however, the vulnerability to embolism and rates of water-use were different between encroachers and non-encroachers. Encroachers are better at taking advantage of water pulses, particularly under eCO2 and grass competition. This comes at the cost of higher xylem vulnerability during drought, leading to reductions in conductance when exposed to heavy water stress. The response of the photosynthetic parameters mirrored this, with encroaching species had higher rates of photosynthesis and photosystem II quantum yield than non-encroachers under the well-watered treatments. Field experiments revealed that small trees are particularly vulnerable to drought stress, when compared to medium and large trees. The outcomes of this complex response will largely depend on the extent of changes to biotic and abiotic factors across spatial and temporal zones caused by climate change. This research highlights potential hydraulic mechanisms contributing to the increase in bush encroachment, as well as providing important insights into the determinant factors that make a savanna species capable of encroachment. , Thesis (MSc) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-10-11
- Authors: Reynolds, Liam Macleod
- Date: 2024-10-11
- Subjects: Xylem , Scrub encroachment , Isotopes , Photosynthesis , Plants Effect of atmospheric carbon dioxide on
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/465091 , vital:76572
- Description: Plant water transport systems play a fundamental role in the productivity and survival of terrestrial plants due to the vascular architecture placing a physical limit on metabolic function. Savannas have high variability in rainfall, leading multiple studies to suggest that plant water-use strategies are key mechanisms affecting seedling establishment rates. Many savannas are seeing a directional shift towards an increase in the abundance of certain woody species through a process known as bush encroachment, which has been largely attributed to the fertilising effect of rising atmospheric [CO2] on C3 trees. These species are classified as encroachers. While there have been multiple studies investigating changes in the physiology of savanna species under elevated CO2 (eCO2), few have examined how climate and eCO2 affects the fundamental water-use strategies in the seedling stage, a crucial demographic bottleneck. Here, this research provides valuable insights into the mechanisms behind bush encroachment in the context of eCO2 using results from a pot experiment at the Rhodes University Elevated CO2 facility and a field experiment. All species showed water use strategies characteristic of anisohydric “water-spenders”, however, the vulnerability to embolism and rates of water-use were different between encroachers and non-encroachers. Encroachers are better at taking advantage of water pulses, particularly under eCO2 and grass competition. This comes at the cost of higher xylem vulnerability during drought, leading to reductions in conductance when exposed to heavy water stress. The response of the photosynthetic parameters mirrored this, with encroaching species had higher rates of photosynthesis and photosystem II quantum yield than non-encroachers under the well-watered treatments. Field experiments revealed that small trees are particularly vulnerable to drought stress, when compared to medium and large trees. The outcomes of this complex response will largely depend on the extent of changes to biotic and abiotic factors across spatial and temporal zones caused by climate change. This research highlights potential hydraulic mechanisms contributing to the increase in bush encroachment, as well as providing important insights into the determinant factors that make a savanna species capable of encroachment. , Thesis (MSc) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-10-11
Spatial pattern analysis of thicket expansion in a semi-arid savanna
- Authors: Putzier, Rachel Rayne
- Date: 2024-10-11
- Subjects: Arid regions South Africa Eastern Cape , Spatial analysis (Statistics) , Thicket , Lidar , Cluster analysis , Trees Mortality , Scrub encroachment
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/464484 , vital:76515
- Description: Woody thickening has negative economic and ecological impacts in savannas globally. While the increase of savanna trees as a form of bush encroachment has been well studied, less is known about the increase of thicket species in savannas, which is an important phenomenon resulting in the formation of closed-canopy clumps which may exclude the grass layer over time. The early stage of thicket expansion is often initiated by a nucleating savanna species which facilitates the establishment of woody thicket pioneer species, and as the thicket clump formation expands, bush clumps increase in dominance, thereby increasing the frequency of competitive interactions and leading to a possible switch from facilitative to competitive interactions. Spatial point pattern analysis provides a useful tool to elucidate these underlying patterns and ecological processes. I used high resolution LiDAR data combined with spatial point pattern analysis to understand tree-tree interactions in a semi-arid savanna in the Eastern Cape Province of South Africa. I conducted a cluster analysis based on vegetation structural variables to distinguish different stages of woody plant encroachment from open savanna to closed canopy thicket. Using the canopy height model, I quantified the change in the grass height from open savanna to closed canopy thicket clumps as an indicator of a possible biome shift. Additionally, I used spatial point pattern analyses to investigate the effect of thicket clump formation on the composition of savanna and thicket species, the overall patterns of trees, and the associations of small thicket species with large Vachellia karroo trees, which serve as clump initiators. Finally, I examined the mortality of savanna trees across increasing stages of thicket expansion using second order spatial statistics, namely the Mark- and Pair-Correlation Functions. Results confirmed that three vegetation states, influenced by elevation, are present at the study site, representing open canopy savanna (early-stage thicket encroachment), encroached savanna with low thicket dominance (intermediate-stage thicket encroachment), and highly encroached with dominant thicket clumps (late-stage thicket encroachment). These stages showed increasing tree height, canopy cover and canopy height density, as well as decreased (but not completely absent grass layer) as thicket encroachment progresses. Spatial point pattern analysis showed, as predicted, that there was an overall aggregation of trees at small-scales within early thicket clump formation, from which I inferred that facilitative relationships may exist between trees. Contrary to my predictions, at later stages of thicket clump formation I found dominant independent patterns between savanna adults and juvenile thicket species, which may result from a combination of facilitative and competitive effects. Lastly, as expected, I found that the density of V. karroo mortality increased as thicket encroachment increased, with an overall random spatial pattern of dead V. karroo across encroachment stages. As predicted, tree mortality was randomly distributed in space in the open savanna state, and as thicket clump formation increases, tree competitive mortality became more evident, as well as decreased tree performance. Overall, the study highlights the interplay between facilitation and competition in semiarid savanna where thicket clumps are expanding. Intervention strategies are suggested to target areas of intermediate thicket clump formation, as these areas provide an opportunity to remove V. karroo before the nucleation process has enabled the establishment and increase of thicket species and to ensure the grass layer is kept productive. I conclude that the use of remote sensing and LiDAR technology holds a wide range of possibilities for monitoring and managing woody encroachment in savanna systems, however these methods need to be further refined for effective use within African savanna and thicket context, which displays high spatial aggregation making typical segmentation methods difficult. , Thesis (MSc) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-10-11
- Authors: Putzier, Rachel Rayne
- Date: 2024-10-11
- Subjects: Arid regions South Africa Eastern Cape , Spatial analysis (Statistics) , Thicket , Lidar , Cluster analysis , Trees Mortality , Scrub encroachment
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/464484 , vital:76515
- Description: Woody thickening has negative economic and ecological impacts in savannas globally. While the increase of savanna trees as a form of bush encroachment has been well studied, less is known about the increase of thicket species in savannas, which is an important phenomenon resulting in the formation of closed-canopy clumps which may exclude the grass layer over time. The early stage of thicket expansion is often initiated by a nucleating savanna species which facilitates the establishment of woody thicket pioneer species, and as the thicket clump formation expands, bush clumps increase in dominance, thereby increasing the frequency of competitive interactions and leading to a possible switch from facilitative to competitive interactions. Spatial point pattern analysis provides a useful tool to elucidate these underlying patterns and ecological processes. I used high resolution LiDAR data combined with spatial point pattern analysis to understand tree-tree interactions in a semi-arid savanna in the Eastern Cape Province of South Africa. I conducted a cluster analysis based on vegetation structural variables to distinguish different stages of woody plant encroachment from open savanna to closed canopy thicket. Using the canopy height model, I quantified the change in the grass height from open savanna to closed canopy thicket clumps as an indicator of a possible biome shift. Additionally, I used spatial point pattern analyses to investigate the effect of thicket clump formation on the composition of savanna and thicket species, the overall patterns of trees, and the associations of small thicket species with large Vachellia karroo trees, which serve as clump initiators. Finally, I examined the mortality of savanna trees across increasing stages of thicket expansion using second order spatial statistics, namely the Mark- and Pair-Correlation Functions. Results confirmed that three vegetation states, influenced by elevation, are present at the study site, representing open canopy savanna (early-stage thicket encroachment), encroached savanna with low thicket dominance (intermediate-stage thicket encroachment), and highly encroached with dominant thicket clumps (late-stage thicket encroachment). These stages showed increasing tree height, canopy cover and canopy height density, as well as decreased (but not completely absent grass layer) as thicket encroachment progresses. Spatial point pattern analysis showed, as predicted, that there was an overall aggregation of trees at small-scales within early thicket clump formation, from which I inferred that facilitative relationships may exist between trees. Contrary to my predictions, at later stages of thicket clump formation I found dominant independent patterns between savanna adults and juvenile thicket species, which may result from a combination of facilitative and competitive effects. Lastly, as expected, I found that the density of V. karroo mortality increased as thicket encroachment increased, with an overall random spatial pattern of dead V. karroo across encroachment stages. As predicted, tree mortality was randomly distributed in space in the open savanna state, and as thicket clump formation increases, tree competitive mortality became more evident, as well as decreased tree performance. Overall, the study highlights the interplay between facilitation and competition in semiarid savanna where thicket clumps are expanding. Intervention strategies are suggested to target areas of intermediate thicket clump formation, as these areas provide an opportunity to remove V. karroo before the nucleation process has enabled the establishment and increase of thicket species and to ensure the grass layer is kept productive. I conclude that the use of remote sensing and LiDAR technology holds a wide range of possibilities for monitoring and managing woody encroachment in savanna systems, however these methods need to be further refined for effective use within African savanna and thicket context, which displays high spatial aggregation making typical segmentation methods difficult. , Thesis (MSc) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-10-11
The effect of biological control on the population dynamics of Pontederia crassipes Mart. (Pontederiaceae) and Salvinia minima Baker (Salviniales: Salviniaceae)
- Authors: Chikodza, Tressia
- Date: 2024-10-11
- Subjects: Pontederia crassipes , Salvinia minima , Pontederiaceae Biological control South Africa , Salviniaceae Biological control South Africa , Population dynamics , Secondary invasion
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/464462 , vital:76513
- Description: Pontederia crassipes is widely regarded as the most damaging floating aquatic weed in terms of its invasive traits, and its impact on aquatic ecosystems. Biological control using host-specific natural enemies is widely used for its control, with the most recent agent released being a planthopper, Megamelus scutellaris, in South Africa and the USA. In South Africa, inundative releases of M. scutellaris have been shown to control the weed even at eutrophic, and high elevation cold sites, such as Hartbeespoort Dam, arguably Africa’s most hypertrophic impoundment. However, subsequent to the control of P. crassipes on Hartbeespoort Dam, in 2021, a secondary invader, Salvinia minima, dominated the water system. Salvinia minima is only known from a handful sites near Hartbeespoort Dam and there is currently no approved biocontrol agent for it. The population dynamics of P. crassipes and S. minima at the Dam were overall interchanging due to insect feeding on M. scutellaris reducing P. crassipes populations thus allowing S. minima to become dominant. Intense feeding from M. scutellaris resulted in the reduction of P. crassipes populations. During late autumn and winter, remaining P. crassipes plants experienced frost damage from cold temperatures. The decrease in P. crassipes allowed S. minima to successfully invade, potentially as the result of less competition from P. crassipes, and therefore available resources, such as space, nutrients, and light. However, as spring approached, P. crassipes regenerated from its seed bank, and S. minima populations diminished. The changes in dominance were observed in 2021 and 2022 but in 2023, this trend was not evident as S. minima did not dominate as in previous years. This thesis investigated the combination of competition and herbivory by M. scutellaris on the vigour of P. crassipes and S. minima to understand the dynamics of these two highly invasive species. Competitive abilities of P. crassipes and S. minima were determined using an inverse linear model with plant weight as the yield variable. In the absence of herbivory, P. crassipes, was 4 times more competitive than S. minima, but as competitive when exposed to M. scutellaris feeding. Salvinia minima was 1.2 times as aggressive as P. crassipes in the absence of herbivory, but 2.6 times as competitive when M. scutellaris was established on P. crassipes. In the presence of herbivory on P. crassipes, interspecific competition coefficients from P. crassipes on S. minima were no longer statistically significant. These results indicate that the competitive ability of P. crassipes was reduced through herbivory when grown with S. minima, explaining the temporal dominance between the two species at Hartbeespoort Dam. Some studies have reported that P. crassipes releases allelochemicals as a competitive strategy to algae and phytoplankton. Previously identified allelochemicals include N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine and linoleic acid. This thesis investigated the presence or absence of key chemical compounds released by P. crassipes during allelopathy with microbes and phytoplankton to determine its potential to inhibit S. minima growth. Methanol extracts from P. crassipes roots, leaves, and field samples underwent LC-ESI-MS/MS analysis, creating a molecular network to match chemical profiles. Of the investigated compounds, N-phenyl-1-naphthylamine and N-phenyl-2-naphthylamine were absent in P. crassipes and field samples, while linoleic acid was consistently found. Its presence suggests its potential defensive role against S. minima. Future research should explore allelochemical production in the presence of S. minima and algae to confirm if S. minima elicits an allelochemical response by P. crassipes, or whether the methods used here were insufficient to detect allelochemical production. Understanding whether P. crassipes employs allelochemicals, especially in the presence of S. minima, could shed light on its competitive advantage beyond its invasive nature at the Dam. , Thesis (MSc) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-10-11
- Authors: Chikodza, Tressia
- Date: 2024-10-11
- Subjects: Pontederia crassipes , Salvinia minima , Pontederiaceae Biological control South Africa , Salviniaceae Biological control South Africa , Population dynamics , Secondary invasion
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/464462 , vital:76513
- Description: Pontederia crassipes is widely regarded as the most damaging floating aquatic weed in terms of its invasive traits, and its impact on aquatic ecosystems. Biological control using host-specific natural enemies is widely used for its control, with the most recent agent released being a planthopper, Megamelus scutellaris, in South Africa and the USA. In South Africa, inundative releases of M. scutellaris have been shown to control the weed even at eutrophic, and high elevation cold sites, such as Hartbeespoort Dam, arguably Africa’s most hypertrophic impoundment. However, subsequent to the control of P. crassipes on Hartbeespoort Dam, in 2021, a secondary invader, Salvinia minima, dominated the water system. Salvinia minima is only known from a handful sites near Hartbeespoort Dam and there is currently no approved biocontrol agent for it. The population dynamics of P. crassipes and S. minima at the Dam were overall interchanging due to insect feeding on M. scutellaris reducing P. crassipes populations thus allowing S. minima to become dominant. Intense feeding from M. scutellaris resulted in the reduction of P. crassipes populations. During late autumn and winter, remaining P. crassipes plants experienced frost damage from cold temperatures. The decrease in P. crassipes allowed S. minima to successfully invade, potentially as the result of less competition from P. crassipes, and therefore available resources, such as space, nutrients, and light. However, as spring approached, P. crassipes regenerated from its seed bank, and S. minima populations diminished. The changes in dominance were observed in 2021 and 2022 but in 2023, this trend was not evident as S. minima did not dominate as in previous years. This thesis investigated the combination of competition and herbivory by M. scutellaris on the vigour of P. crassipes and S. minima to understand the dynamics of these two highly invasive species. Competitive abilities of P. crassipes and S. minima were determined using an inverse linear model with plant weight as the yield variable. In the absence of herbivory, P. crassipes, was 4 times more competitive than S. minima, but as competitive when exposed to M. scutellaris feeding. Salvinia minima was 1.2 times as aggressive as P. crassipes in the absence of herbivory, but 2.6 times as competitive when M. scutellaris was established on P. crassipes. In the presence of herbivory on P. crassipes, interspecific competition coefficients from P. crassipes on S. minima were no longer statistically significant. These results indicate that the competitive ability of P. crassipes was reduced through herbivory when grown with S. minima, explaining the temporal dominance between the two species at Hartbeespoort Dam. Some studies have reported that P. crassipes releases allelochemicals as a competitive strategy to algae and phytoplankton. Previously identified allelochemicals include N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine and linoleic acid. This thesis investigated the presence or absence of key chemical compounds released by P. crassipes during allelopathy with microbes and phytoplankton to determine its potential to inhibit S. minima growth. Methanol extracts from P. crassipes roots, leaves, and field samples underwent LC-ESI-MS/MS analysis, creating a molecular network to match chemical profiles. Of the investigated compounds, N-phenyl-1-naphthylamine and N-phenyl-2-naphthylamine were absent in P. crassipes and field samples, while linoleic acid was consistently found. Its presence suggests its potential defensive role against S. minima. Future research should explore allelochemical production in the presence of S. minima and algae to confirm if S. minima elicits an allelochemical response by P. crassipes, or whether the methods used here were insufficient to detect allelochemical production. Understanding whether P. crassipes employs allelochemicals, especially in the presence of S. minima, could shed light on its competitive advantage beyond its invasive nature at the Dam. , Thesis (MSc) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-10-11
The enemy release hypothesis and beyond: Lagarosiphon major invasion dynamics and management options for New Zealand using native natural enemies from South Africa
- Authors: Baso, Nompumelelo Catherine
- Date: 2024-04-05
- Subjects: Enemy release hypothesis , Lagarosiphon major Biological control New Zealand , Hydrellia , Submerged aquatic vegetation , Invasion ecology
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/435627 , vital:73174 , DOI 10.21504/10962/435627
- Description: Numerous scientific investigations have demonstrated the destructive impact that exotic species can have on ecosystem services beyond a specific threshold. There are many explanations for why introduced plants are likely to be more successful outside their native range. One such explanation is offered by the Enemy Release Hypothesis (ERH), which states that plants automatically become superior competitors outside of their natural range due to release from top-down stressors (herbivory, parasites, and diseases) that is evident in the absence of their natural enemies. The underlying assumption of the ERH is that natural enemies are important regulators of plant species populations, and that the pressures from these natural enemies are felt more readily by native species compared to alien plants. Consequently, in the absence of such pressures, the ERH assumes that exotic plants can allocate more resources towards growth and reproduction, while effectively maintaining accumulated biomass. Classical biological control has previously been cited as evidence for the enemy release hypothesis. Therefore, the overarching aim and theme of this thesis was to investigate the role of ERH on the invasiveness of Lagarosiphon major (Ridl.) Moss ex Wager (Hydrocharitaceae) in New Zealand. Firstly, a literature search and a meta-analysis was used to synthesize existing studies in order to test for general applicability of this hypothesis to aquatic plant invasions. Furthermore, an empirical investigation was conducted in order to directly quantify enemy release in L. major populations invaded areas of New Zealand. To achieve this, various plant parameters of this plant, overall macrophyte and invertebrate diversity were measured and compared between sites in the native range in South Africa and the invaded areas in New Zealand. Although the meta-analysis showed variable evidence for this hypothesis depending on various modulating factors such as study type, plant growth form and measured parameters, for L. major, there was strong evidence of enemy release. The biogeographical comparisons showed that L. major exhibited increased fitness in most of the invaded sites, marked by elevated biomass accumulation, significantly higher shoot production, and the displacement of native plant species. The observed fitness advantages were directly correlated to a decrease in herbivory diversity and pressure upon the plant's introduction to New Zealand. Unlike the native populations, which contend with the presence of at least four co-occurring herbivores, including specialist herbivores, the invaded range had a substantially lower herbivore diversity, with only Hygraula nitens Butler (Lepidoptera: Crambidae) syn. Nymphula nitens, significantly damaging L. major. These findings emphasize the importance of understanding invasion ecology and theories such as ERH in order to advance aquatic plant management and also present valuable insights for developing effective strategies to mitigate the impact of invasive alien species on aquatic ecosystems. Specifically, results from the empirical investigation provide evidence in support of the ERH and highlight the suitability of implementing biological control strategies to manage the L. major invasion in New Zealand. Previous studies have shown the suitability of two specialist herbivores, Hydrellia lagarosiphon Deeming (Diptera: Ephydridae), and Polypedilum tuburcinatum Andersen (Diptera: Chironomidae), as potential biological control agents. This control strategy presents a sustainable and ecologically responsible approach, promoting coexistence between exotic plants and native species rather than displacement through competitive exclusion. With the apparent dominance of L. major at various New Zealand localities, the subsequent objective of this thesis was to investigate the competitive interactions between L. major and another invasive Hydrocharitaceae, Egeria densa Planchon, as driven by herbivory. Combinations of two host specific Ephydrid flies, H. lagarosiphon and H. egeriae, were used at eight different factorial combination of planting densities. The analysis of plant parameters and the application of inverse linear models revealed that L. major often exhibits relatively higher fitness, especially in low monoculture treatments when the two insects were isolated. However, multiple inverse linear models revealed that actual competitive outcomes are dependent on factors such as initial plant density and herbivory regime, with competitive interactions generally being mild. Nevertheless, the presence of H. lagarosiphon resulted in facilitation of E. densa growth. Thus, even at lower densities, these insects still had an impact on the observed interactions, further emphasizing suitability as damaging biological control agents. Lastly, focusing on the abiotic component of L. major invasion, Species Distribution Models (SDMs) were employed to map potential suitable habitat for this species, as well as predict the consequences of climate change on this. Correlative and mechanistic modelling was also used to simulate suitable habitat for potential biological control agents, thus addressing the potential for mismatches between host plant distribution and insect suitable range. The Maximum Entropy Species Distribution Modelling (MaxEnt) algorithm revealed that more than 90% of all freshwater ecosystems in New Zealand are susceptible to L. major invasion, with suitability projected to expand further under future climate scenarios. Moreover, correlative modelling using this method suggests limited suitable habitat for both herbivores. However, degree-day modelling, which also takes into account the physiological requirements, showed that H. lagarosiphon has the potential to produce viable populations in several parts of New Zealand. Overall, this thesis explored the intricate web of biotic and abiotic factors influencing the success of L. major outside its native range. The results emphasize the potential impacts of climate change on the invasion potential and management strategies for L. major. The findings also advocate for the implementation of sustainable and ecologically sound management solutions, such as biological control, to manage this species. , Thesis (PhD) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-04-05
- Authors: Baso, Nompumelelo Catherine
- Date: 2024-04-05
- Subjects: Enemy release hypothesis , Lagarosiphon major Biological control New Zealand , Hydrellia , Submerged aquatic vegetation , Invasion ecology
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/435627 , vital:73174 , DOI 10.21504/10962/435627
- Description: Numerous scientific investigations have demonstrated the destructive impact that exotic species can have on ecosystem services beyond a specific threshold. There are many explanations for why introduced plants are likely to be more successful outside their native range. One such explanation is offered by the Enemy Release Hypothesis (ERH), which states that plants automatically become superior competitors outside of their natural range due to release from top-down stressors (herbivory, parasites, and diseases) that is evident in the absence of their natural enemies. The underlying assumption of the ERH is that natural enemies are important regulators of plant species populations, and that the pressures from these natural enemies are felt more readily by native species compared to alien plants. Consequently, in the absence of such pressures, the ERH assumes that exotic plants can allocate more resources towards growth and reproduction, while effectively maintaining accumulated biomass. Classical biological control has previously been cited as evidence for the enemy release hypothesis. Therefore, the overarching aim and theme of this thesis was to investigate the role of ERH on the invasiveness of Lagarosiphon major (Ridl.) Moss ex Wager (Hydrocharitaceae) in New Zealand. Firstly, a literature search and a meta-analysis was used to synthesize existing studies in order to test for general applicability of this hypothesis to aquatic plant invasions. Furthermore, an empirical investigation was conducted in order to directly quantify enemy release in L. major populations invaded areas of New Zealand. To achieve this, various plant parameters of this plant, overall macrophyte and invertebrate diversity were measured and compared between sites in the native range in South Africa and the invaded areas in New Zealand. Although the meta-analysis showed variable evidence for this hypothesis depending on various modulating factors such as study type, plant growth form and measured parameters, for L. major, there was strong evidence of enemy release. The biogeographical comparisons showed that L. major exhibited increased fitness in most of the invaded sites, marked by elevated biomass accumulation, significantly higher shoot production, and the displacement of native plant species. The observed fitness advantages were directly correlated to a decrease in herbivory diversity and pressure upon the plant's introduction to New Zealand. Unlike the native populations, which contend with the presence of at least four co-occurring herbivores, including specialist herbivores, the invaded range had a substantially lower herbivore diversity, with only Hygraula nitens Butler (Lepidoptera: Crambidae) syn. Nymphula nitens, significantly damaging L. major. These findings emphasize the importance of understanding invasion ecology and theories such as ERH in order to advance aquatic plant management and also present valuable insights for developing effective strategies to mitigate the impact of invasive alien species on aquatic ecosystems. Specifically, results from the empirical investigation provide evidence in support of the ERH and highlight the suitability of implementing biological control strategies to manage the L. major invasion in New Zealand. Previous studies have shown the suitability of two specialist herbivores, Hydrellia lagarosiphon Deeming (Diptera: Ephydridae), and Polypedilum tuburcinatum Andersen (Diptera: Chironomidae), as potential biological control agents. This control strategy presents a sustainable and ecologically responsible approach, promoting coexistence between exotic plants and native species rather than displacement through competitive exclusion. With the apparent dominance of L. major at various New Zealand localities, the subsequent objective of this thesis was to investigate the competitive interactions between L. major and another invasive Hydrocharitaceae, Egeria densa Planchon, as driven by herbivory. Combinations of two host specific Ephydrid flies, H. lagarosiphon and H. egeriae, were used at eight different factorial combination of planting densities. The analysis of plant parameters and the application of inverse linear models revealed that L. major often exhibits relatively higher fitness, especially in low monoculture treatments when the two insects were isolated. However, multiple inverse linear models revealed that actual competitive outcomes are dependent on factors such as initial plant density and herbivory regime, with competitive interactions generally being mild. Nevertheless, the presence of H. lagarosiphon resulted in facilitation of E. densa growth. Thus, even at lower densities, these insects still had an impact on the observed interactions, further emphasizing suitability as damaging biological control agents. Lastly, focusing on the abiotic component of L. major invasion, Species Distribution Models (SDMs) were employed to map potential suitable habitat for this species, as well as predict the consequences of climate change on this. Correlative and mechanistic modelling was also used to simulate suitable habitat for potential biological control agents, thus addressing the potential for mismatches between host plant distribution and insect suitable range. The Maximum Entropy Species Distribution Modelling (MaxEnt) algorithm revealed that more than 90% of all freshwater ecosystems in New Zealand are susceptible to L. major invasion, with suitability projected to expand further under future climate scenarios. Moreover, correlative modelling using this method suggests limited suitable habitat for both herbivores. However, degree-day modelling, which also takes into account the physiological requirements, showed that H. lagarosiphon has the potential to produce viable populations in several parts of New Zealand. Overall, this thesis explored the intricate web of biotic and abiotic factors influencing the success of L. major outside its native range. The results emphasize the potential impacts of climate change on the invasion potential and management strategies for L. major. The findings also advocate for the implementation of sustainable and ecologically sound management solutions, such as biological control, to manage this species. , Thesis (PhD) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-04-05
The effects of short-duration overnight kraaling on herbaceous vegetation and soils in mesic grassland
- Authors: Mgwali, Nompendulo
- Date: 2023-10-13
- Subjects: Grazing South Africa , Communal rangelands South Africa , Corrals South Africa , Soil nutrient , Ground cover plants
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424721 , vital:72178
- Description: Land degradation is widespread in communal rangelands in the grassland biome of South Africa, and often attributed to overstocking and lack of coordinated management. Excessive pressure on the herbaceous component has contributed to the uncontrolled spread of opportunistic invasive alien woody species e.g. Acacia mearnsii in many degraded areas, resulting in significant loss of ecosystem service capacity, along with soil and land productivity. Short-duration overnight kraaling has been suggested as a tool for restoring degraded rangelands. Recent studies in semi-arid savannas and shrublands have reported increased grass cover, soil nutrients and palatability and concluded that short-duration kraaling is a low-cost and effective way of restoring degraded rangelands using livestock. However, the response of different plant functional types and communities to such intense livestock impact may vary depending on local context. This study used twelve paired kraal and control sites to investigate the effects of short-duration (7-24 days) overnight kraaling of livestock on herbaceous vegetation and soils in a mesic grassland. The study area is generally considered to be overgrazed but has considerable variation in grass composition and basal cover. Sites included relatively intact natural grassland and sites where wattle infestations had been cleared and where mostly bare ground remained. I tested the hypotheses that overnight kraaling would result in (1) increased basal cover due to introduction of grass seed and stimulation of germination through hoof action, (2) increased infiltration due to hoof action, and (3) increased soil nutrients and organic matter due to dung and urine deposition. I also hypothesized that factors such as a site’s initial grass cover, its slope, the occurrence and amount of rainfall before and during kraaling, and the kraaling intensity (number of livestock and duration of the kraaling event) would influence the magnitude and direction of the kraaling effect. The effect of kraaling on vegetation was strongly dependent on initial condition. Kraaling increased basal cover of grasses when sites had low initial basal cover, but decreased basal cover if initial values were over 50%. Infiltration increased if kraaling took place during or after rain but decreased if kraaling took place when soils were dry. Kraaling increased soil P and K. In mesic grasslands, short-duration overnight kraaling is promising as a tool for rehabilitating degraded sites but should be avoided where the grass sward is relatively intact. I recommend that the suitability of kraaling be further evaluated per vegetation type and local context. , Thesis (MSc) -- Faculty of Science, Botany, 2023
- Full Text:
- Date Issued: 2023-10-13
- Authors: Mgwali, Nompendulo
- Date: 2023-10-13
- Subjects: Grazing South Africa , Communal rangelands South Africa , Corrals South Africa , Soil nutrient , Ground cover plants
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424721 , vital:72178
- Description: Land degradation is widespread in communal rangelands in the grassland biome of South Africa, and often attributed to overstocking and lack of coordinated management. Excessive pressure on the herbaceous component has contributed to the uncontrolled spread of opportunistic invasive alien woody species e.g. Acacia mearnsii in many degraded areas, resulting in significant loss of ecosystem service capacity, along with soil and land productivity. Short-duration overnight kraaling has been suggested as a tool for restoring degraded rangelands. Recent studies in semi-arid savannas and shrublands have reported increased grass cover, soil nutrients and palatability and concluded that short-duration kraaling is a low-cost and effective way of restoring degraded rangelands using livestock. However, the response of different plant functional types and communities to such intense livestock impact may vary depending on local context. This study used twelve paired kraal and control sites to investigate the effects of short-duration (7-24 days) overnight kraaling of livestock on herbaceous vegetation and soils in a mesic grassland. The study area is generally considered to be overgrazed but has considerable variation in grass composition and basal cover. Sites included relatively intact natural grassland and sites where wattle infestations had been cleared and where mostly bare ground remained. I tested the hypotheses that overnight kraaling would result in (1) increased basal cover due to introduction of grass seed and stimulation of germination through hoof action, (2) increased infiltration due to hoof action, and (3) increased soil nutrients and organic matter due to dung and urine deposition. I also hypothesized that factors such as a site’s initial grass cover, its slope, the occurrence and amount of rainfall before and during kraaling, and the kraaling intensity (number of livestock and duration of the kraaling event) would influence the magnitude and direction of the kraaling effect. The effect of kraaling on vegetation was strongly dependent on initial condition. Kraaling increased basal cover of grasses when sites had low initial basal cover, but decreased basal cover if initial values were over 50%. Infiltration increased if kraaling took place during or after rain but decreased if kraaling took place when soils were dry. Kraaling increased soil P and K. In mesic grasslands, short-duration overnight kraaling is promising as a tool for rehabilitating degraded sites but should be avoided where the grass sward is relatively intact. I recommend that the suitability of kraaling be further evaluated per vegetation type and local context. , Thesis (MSc) -- Faculty of Science, Botany, 2023
- Full Text:
- Date Issued: 2023-10-13
Population dynamics, fire, and reproductive ecology of Oldenburgia grandis (Asteraceae), an unusual fynbos tree endemic to the Eastern Cape
- Authors: Mostert, Emma Charlotte
- Date: 2022-10-14
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/365268 , vital:65722
- Description: Thesis embargoed. Possible release date set for early 2024. , Thesis (MSc) -- Faculty of Science, Botany, 2022
- Full Text:
- Date Issued: 2022-10-14
- Authors: Mostert, Emma Charlotte
- Date: 2022-10-14
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/365268 , vital:65722
- Description: Thesis embargoed. Possible release date set for early 2024. , Thesis (MSc) -- Faculty of Science, Botany, 2022
- Full Text:
- Date Issued: 2022-10-14
The process of thicket encroachment in semi-arid savanna: community patterns and biotic interactions
- Authors: Nell, Rhys
- Date: 2022-10-14
- Subjects: Scrub encroachment , Savanna ecology , Biotic interaction , Plant nutrients , Plant-water relationships
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/364966 , vital:65666
- Description: Bush encroachment in savannas is widespread in South Africa and is concerning, from both socio-economic and conservation viewpoints, as it affects ecosystem services, functioning and productivity. This phenomenon depends on multiple factors such as history, vegetation, management and environmental conditions, and their interplay. Encroachment into savannas has been relatively well-documented, however understanding of the different roles of tree-tree interactions between species that occur during this process is still limited. This includes the interactions causing spatial patterning, or how interactions and outcomes change over time in terms of encroachment succession from open savanna to closed-canopy thicket. The main objectives of this research are to document thicket establishment in a savanna ecosystem and consider the ecological roles of the key woody species and the abiotic properties of their micro-sites. Determining interactive effects of species co-occurrence is critical to understanding or predicting patterns and changes in biodiversity, nutrient distribution and available water resources. It is also imperative in determining correct and effective land management practices, particularly for reducing bush encroachment and its negative effect on rangelands. All data were collected on Endwell farm, located in the Smaldeel region of the Eastern Cape, South Africa. Endwell farm is a semi-arid savanna with a mean annual rainfall of 730 mm. First, I examine and describe the thicket encroachment process by exploring the associations between species and their size classes in the field. This was done by using plot-based belt transects and looking at changes in species size-class compositions from early to late successional stages. Association rules (market basket) analysis was used to identify the most common species size-class association patterns. The association between the savanna tree Vachellia karroo and the thicket pioneer Scutia myrtina was the most prevalent at all stages, with V. karroo being central to all associations in the first stage of encroachment; during later stages of encroachment, associations shift to incorporating other thicket pioneer species. The demography and clump formation of S. myrtina was strongly linked to associations with V. karroo to initiate bush clump formation. Results suggest that mature V. karroo facilitate the establishment and growth of S. myrtina. These two species were the focus of more detailed investigations to explore the nature and magnitude of their interspecific interactions. I then examined the effects of pairwise tree interactions between V. karroo and S. myrtina on soil and leaf nutrient content. I measureddifferences betweeninter-canopy and sub-canopy soil nutrient content, and the effect of associations on plant leaf nutrients, between pair-size combinations and individual controls. Results confirmed that pair-size tree interactions affected both soil nutrient and leaf nutrient content. All individuals increased soil K, N and organic C in the sub-canopy, while association with V. karrooincreased S. myrtinafoliar N, Pand K. In contrast, association with S. myrtinaloweredV. karroofoliar N, P and K. Small S. myrtina individuals werefound to benefit most from establishing and growing next to a large V. karroo individual, through mechanisms affecting soil and foliar nutrients. Scutia myrtina individuals establishing in association with smaller size classes of V. karroo showed no significant effects. I tested for positive and negative effects of pairwise tree interactions between Vachellia karroo and Scutia myrtina on available soil water and plant water potential (Ψ). This was done by looking at differences betweeninter-canopy and sub-canopy soil moisture and bulk density and associations on plant water stress (pre-dawn and mid-day leaf Ψ), between pair-size combinations and individual controls. I also selectively removed large V. karroo individuals from pairs to confirm the effects of competition andfacilitation. Similar to other studies, results confirmed positive and negative effects of pairwise tree interactions. Small S. myrtina individuals weremost facilitated by establishing and growing up next to a large V. karroo individual, through mechanisms affecting soil water content, bulk density and leaf Ψ. Scutia myrtina establishing in association with other size classes of V. karroo were much less facilitated, showing no significant effects. In contrast, large S. myrtina showed competitive interactions with V. karroo. , Thesis (MSc) -- Faculty of Science, Botany, 2022
- Full Text:
- Date Issued: 2022-10-14
The process of thicket encroachment in semi-arid savanna: community patterns and biotic interactions
- Authors: Nell, Rhys
- Date: 2022-10-14
- Subjects: Scrub encroachment , Savanna ecology , Biotic interaction , Plant nutrients , Plant-water relationships
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/364966 , vital:65666
- Description: Bush encroachment in savannas is widespread in South Africa and is concerning, from both socio-economic and conservation viewpoints, as it affects ecosystem services, functioning and productivity. This phenomenon depends on multiple factors such as history, vegetation, management and environmental conditions, and their interplay. Encroachment into savannas has been relatively well-documented, however understanding of the different roles of tree-tree interactions between species that occur during this process is still limited. This includes the interactions causing spatial patterning, or how interactions and outcomes change over time in terms of encroachment succession from open savanna to closed-canopy thicket. The main objectives of this research are to document thicket establishment in a savanna ecosystem and consider the ecological roles of the key woody species and the abiotic properties of their micro-sites. Determining interactive effects of species co-occurrence is critical to understanding or predicting patterns and changes in biodiversity, nutrient distribution and available water resources. It is also imperative in determining correct and effective land management practices, particularly for reducing bush encroachment and its negative effect on rangelands. All data were collected on Endwell farm, located in the Smaldeel region of the Eastern Cape, South Africa. Endwell farm is a semi-arid savanna with a mean annual rainfall of 730 mm. First, I examine and describe the thicket encroachment process by exploring the associations between species and their size classes in the field. This was done by using plot-based belt transects and looking at changes in species size-class compositions from early to late successional stages. Association rules (market basket) analysis was used to identify the most common species size-class association patterns. The association between the savanna tree Vachellia karroo and the thicket pioneer Scutia myrtina was the most prevalent at all stages, with V. karroo being central to all associations in the first stage of encroachment; during later stages of encroachment, associations shift to incorporating other thicket pioneer species. The demography and clump formation of S. myrtina was strongly linked to associations with V. karroo to initiate bush clump formation. Results suggest that mature V. karroo facilitate the establishment and growth of S. myrtina. These two species were the focus of more detailed investigations to explore the nature and magnitude of their interspecific interactions. I then examined the effects of pairwise tree interactions between V. karroo and S. myrtina on soil and leaf nutrient content. I measureddifferences betweeninter-canopy and sub-canopy soil nutrient content, and the effect of associations on plant leaf nutrients, between pair-size combinations and individual controls. Results confirmed that pair-size tree interactions affected both soil nutrient and leaf nutrient content. All individuals increased soil K, N and organic C in the sub-canopy, while association with V. karrooincreased S. myrtinafoliar N, Pand K. In contrast, association with S. myrtinaloweredV. karroofoliar N, P and K. Small S. myrtina individuals werefound to benefit most from establishing and growing next to a large V. karroo individual, through mechanisms affecting soil and foliar nutrients. Scutia myrtina individuals establishing in association with smaller size classes of V. karroo showed no significant effects. I tested for positive and negative effects of pairwise tree interactions between Vachellia karroo and Scutia myrtina on available soil water and plant water potential (Ψ). This was done by looking at differences betweeninter-canopy and sub-canopy soil moisture and bulk density and associations on plant water stress (pre-dawn and mid-day leaf Ψ), between pair-size combinations and individual controls. I also selectively removed large V. karroo individuals from pairs to confirm the effects of competition andfacilitation. Similar to other studies, results confirmed positive and negative effects of pairwise tree interactions. Small S. myrtina individuals weremost facilitated by establishing and growing up next to a large V. karroo individual, through mechanisms affecting soil water content, bulk density and leaf Ψ. Scutia myrtina establishing in association with other size classes of V. karroo were much less facilitated, showing no significant effects. In contrast, large S. myrtina showed competitive interactions with V. karroo. , Thesis (MSc) -- Faculty of Science, Botany, 2022
- Full Text:
- Date Issued: 2022-10-14
The invasion autecology of Iris pseudacorus L. (Iridaceae) in South Africa
- Authors: Sandenbergh, Emma
- Date: 2022-04-06
- Subjects: Iris pseudacorus South Africa , Invasive plants South Africa , Aquatic weeds South Africa , Plant genetics South Africa , Freshwater ecology South Africa , Iris pseudacorus Geographical distribution , Phytogeography
- Language: English
- Type: Master's thesis , text
- Identifier: http://hdl.handle.net/10962/232272 , vital:49977
- Description: Iris pseudacorus L. (Iridaceae) is an emergent aquatic macrophyte originating from Europe, north Africa, and western Asia, and is becoming an increasingly problematic invader in South Africa. By forming dense rhizomatic mats in the absence of natural enemies, I. pseudacorus outcompetes co-occurring indigenous biota, causing serious environmental and socioeconomic challenges. Iris pseudacorus is a declared invader in South Africa, Argentina, New Zealand, the United States of America, and Canada, but little information is known regarding the species’ invasive potential, particularly in the southern hemisphere, hindering the effectiveness of control efforts. This study addresses this knowledge gap in a South African context, providing valuable insight into the invasion autecology of I. pseudacorus in South Africa. For effective management and control of I. pseudacorus in South Africa and the global south, its distribution and invasive potential must be determined, and its population genetics understood. Hence, this study aimed to map the current confirmed distribution of I. pseudacorus populations in South Africa, investigating the relative abundance of I. pseudacorus individuals in each population, and comparing their sexual reproductive outputs. Moreover, this study assessed the competitive interactions between I. pseudacorus and co-occurring native species T. capensis, and examined the genetic diversity present between and within South African I. pseudacorus populations. Through field surveys, I. pseudacorus infestations were confirmed in eight of the country’s nine provinces, with the highest number of infestations recorded in the urban hubs, and greatest population abundances reported in the warmer, wetter regions of South Africa. These surveys indicated that South African I. pseudacorus populations have enhanced their sexual reproductive output relative to native range populations, and a germination rate of ~ 83 % was determined in the laboratory. The results of a common garden competition experiment indicated that T. capensis may be a superior competitor over I. pseudacorus, but this was not supported by field observations, and may be a result of the short duration of the experiment. Using inter-simple sequence repeats (ISSRs), high genetic diversity was observed within and between populations of I. pseudacorus, indicating the employment of sexual reproductive strategies, and providing evidence for gene-flow between and within populations. Moreover, a weak negative correlation was observed between geographic distance and genetic similarity, ii indicating a largely anthropogenic spread of I. pseudacorus, and suggesting the occurrence of fewer founding events than reported in the United States. This study provides useful insight into the invasion autecology of I. pseudacorus in South Africa, contributing to the ongoing research surrounding I. pseudacorus invasions worldwide, particularly in the southern hemisphere. These results contribute to the development of appropriate adaptive and integrated management strategies to control I. pseudacorus invasions in South Africa, and should be implemented before South African I. pseudacorus infestations reach the severity observed elsewhere. , Thesis (MSc) -- Faculty of Science, Botany, 2022
- Full Text:
- Date Issued: 2022-04-06
- Authors: Sandenbergh, Emma
- Date: 2022-04-06
- Subjects: Iris pseudacorus South Africa , Invasive plants South Africa , Aquatic weeds South Africa , Plant genetics South Africa , Freshwater ecology South Africa , Iris pseudacorus Geographical distribution , Phytogeography
- Language: English
- Type: Master's thesis , text
- Identifier: http://hdl.handle.net/10962/232272 , vital:49977
- Description: Iris pseudacorus L. (Iridaceae) is an emergent aquatic macrophyte originating from Europe, north Africa, and western Asia, and is becoming an increasingly problematic invader in South Africa. By forming dense rhizomatic mats in the absence of natural enemies, I. pseudacorus outcompetes co-occurring indigenous biota, causing serious environmental and socioeconomic challenges. Iris pseudacorus is a declared invader in South Africa, Argentina, New Zealand, the United States of America, and Canada, but little information is known regarding the species’ invasive potential, particularly in the southern hemisphere, hindering the effectiveness of control efforts. This study addresses this knowledge gap in a South African context, providing valuable insight into the invasion autecology of I. pseudacorus in South Africa. For effective management and control of I. pseudacorus in South Africa and the global south, its distribution and invasive potential must be determined, and its population genetics understood. Hence, this study aimed to map the current confirmed distribution of I. pseudacorus populations in South Africa, investigating the relative abundance of I. pseudacorus individuals in each population, and comparing their sexual reproductive outputs. Moreover, this study assessed the competitive interactions between I. pseudacorus and co-occurring native species T. capensis, and examined the genetic diversity present between and within South African I. pseudacorus populations. Through field surveys, I. pseudacorus infestations were confirmed in eight of the country’s nine provinces, with the highest number of infestations recorded in the urban hubs, and greatest population abundances reported in the warmer, wetter regions of South Africa. These surveys indicated that South African I. pseudacorus populations have enhanced their sexual reproductive output relative to native range populations, and a germination rate of ~ 83 % was determined in the laboratory. The results of a common garden competition experiment indicated that T. capensis may be a superior competitor over I. pseudacorus, but this was not supported by field observations, and may be a result of the short duration of the experiment. Using inter-simple sequence repeats (ISSRs), high genetic diversity was observed within and between populations of I. pseudacorus, indicating the employment of sexual reproductive strategies, and providing evidence for gene-flow between and within populations. Moreover, a weak negative correlation was observed between geographic distance and genetic similarity, ii indicating a largely anthropogenic spread of I. pseudacorus, and suggesting the occurrence of fewer founding events than reported in the United States. This study provides useful insight into the invasion autecology of I. pseudacorus in South Africa, contributing to the ongoing research surrounding I. pseudacorus invasions worldwide, particularly in the southern hemisphere. These results contribute to the development of appropriate adaptive and integrated management strategies to control I. pseudacorus invasions in South Africa, and should be implemented before South African I. pseudacorus infestations reach the severity observed elsewhere. , Thesis (MSc) -- Faculty of Science, Botany, 2022
- Full Text:
- Date Issued: 2022-04-06
Thicket expansion in a vachellia karroo-dominated landscape and its effect on herbaceous communities
- Authors: Khoza, Marina Rindzani
- Date: 2022-04-06
- Subjects: Savanna ecology South Africa , Forbs South Africa , Grasslands South Africa , Herbaceous plants South Africa , Vegetation dynamics South Africa , Forest canopies South Africa
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/291015 , vital:56808
- Description: Grass and forb species found in savannas are highly diverse, contributing to the structure and function of the savanna system. Where mean annual rainfall is seasonal and high enough to support closed canopy vegetation such as forests or thickets, savannas can exist as an alternative stable state maintained by disturbances such as fire and browsing. Biotic and abiotic processes act on savanna and forest (or thicket) systems maintaining both their tree and herbaceous cover at levels that ensure their persistence in those states. Studies have shown that many semi-arid rangelands in South Africa have undergone a rapid increase in tree cover (of both native and non-native species) over the past several decades. This process of increasing tree cover in semi-arid savannas, termed bush encroachment, results in a biome shift, changing landscapes that were once grasslands with few trees to ones dominated by broad-leaved trees with fewer sun-adapted forbs and grasses. The aim of this study was to investigate the impact of changing woody cover and its associated changes in tree composition, tree canopy structure, light dynamics in the understory and herbaceous community composition on Endwell farm in the Eastern Cape. Canopy cover changes between the years 1949 and 2019 were analysed at 51 sites on the farm and related to historical rainfall patterns. There had been a general increase in tree cover over the past several decades on the farm, and many sites showed a change from open (0-15%) in 1949 to low (1635%), moderate (36-50%) and high (51-100%) canopy cover in 2019. In earlier years most sites had a canopy cover below 50%, and the higher canopy cover values (>65%) occurred in more recent decades. Canopy cover of ~ 50% was found to be rare in each decade. This suggests that ~50% canopy cover maybe a transient, unstable state. The period with the highest rate of canopy cover increase was 2002-2013, and this increase coincided with a high mean annual rainfall 10 years prior to 2002 and a high mean annual rainfall in most years between the 20022013 period. The period between 2002 and 2013 also had the highest number of sites transitioning from lower to higher tree canopy cover classes, indicating that rainfall may have been a factor driving bush encroachment during the past several decades. An increase in canopy cover (a decrease in light transmittance) was accompanied by changes in woody species composition during thicket formation. The low canopy cover (high light transmittance) sites were dominated by Vachellia karroo and Scutia myrtina trees, while high tree cover sites had fewer V. karroo and S. myrtina trees and were rather characterised by an abundance of thicket tree species. Species proportion, NMDS and dendrogram plots indicated that sites with a light transmittance range between 50-100% had similar tree species compositions, different from sites with light transmittances <50%. An increase in tree density was strongly correlated to an increase in canopy cover (from 2019 satellite imagery), density of trees > 3m, maximum height reached by trees, diversity of trees, total canopy volume, total canopy area and leaf area index (LAI), and a decrease in light transmittance. A structural equation model (SEM) was used to explore the relationships between canopy characteristics (maximum canopy area, canopy volume, tree diversity, density of trees, density of trees >3m, individual trees and maximum canopy height), aerial canopy cover in 2019, and light transmittance. The model explained 73% of the variation in light transmittance, mostly via the direct effect of canopy characteristics. Canopy characteristics had a strong influence on both aerial cover in 2019 and directly on light transmittance, but canopy cover in 2019 had a weak influence on light transmittance. The herbaceous layer was rich and dominated by C4 grasses such as Eragrostis plana, Sporobolus fimbriatus, Themeda triandra and Digitaria eriantha) and forbs including Hibiscus aethiopicus, Helichrysum dregeanum, Helichrysum nudifolium and Gerbera viridifolia at low canopy cover sites with high light transmittance. In contrast, high tree cover sites had fewer herbaceous species in general. Grass and forb species characteristic of these sites high canopy cover sites were Panicum maximum, Loudetia flavida, Pellaea viridis and Cyperus spp. Different sites with low light transmittance (<50%) had similar herbaceous species composition. Basal cover, richness, abundance and diversity of herbaceous plants decreased significantly with an increase in tree density, density of trees >3 m, canopy volume, canopy area, canopy cover, LAI, and increased significantly with increasing light transmittance. Most grasses had their highest densities at LAI <0.5, which was estimated to correspond to ~75% light transmittance and ~38% canopy cover and then started to decline thereafter. Herbaceous species basal cover was also highest at LAI <0.5. An SEM model indicated that herbaceous diversity, basal cover and richness responded both to light availability and to the structure of the woody vegetation directly (R2 = 0.53). While the effect of light transmittance on herbaceous communities was strong (0.41), there was little difference between the effect of light transmittance and canopy characteristics (-0.35) on herbaceous communities. Two possible threshold points, relating to two types of transitions in vegetation structure, could be deduced from this study. The first threshold occurred at canopy cover ~ 40% (LAI < ~ 0.5, light transmittance ~ 75%), at which point many of the common herbaceous species, including the dominant C4 grasses, began to decline in abundance while the composition remained characteristic of the savanna state. A canopy cover of less than ~ 40% at a site provides a suitable state for a high abundance of grass and forb species which help maintain an open system by facilitating fires. The second threshold marked a compositional shift between savanna and closed-canopy vegetation states. Savanna species (trees, grasses and forbs) dominated at high light transmittances (>50%) and were significantly reduced at low light transmittances (< 50%), indicating a possible species composition threshold at ~50% light transmittance at which a savanna state switches to a thicket (LAI ~ 1 and canopy cover ~70%). This point indicated the point where there was a significant difference in both tree and herbaceous plant compositions, with a marked reduction in the occurrence of C4 grasses at light transmittance <50%. Fire is supressed when the C4 grass layer is lost, and further thicket encroachment will take place causing complete canopy closure. Land managers in this system should start becoming concerned about a reduction in grass biomass when canopy cover reaches about 40% and would have to reduce tree cover before the threshold of 50% light transmittance (70% canopy cover from aerial photos) is reached to maintain a savanna system. , Thesis (MSc) -- Faculty of Science, Botany, 2022
- Full Text:
- Date Issued: 2022-04-06
Thicket expansion in a vachellia karroo-dominated landscape and its effect on herbaceous communities
- Authors: Khoza, Marina Rindzani
- Date: 2022-04-06
- Subjects: Savanna ecology South Africa , Forbs South Africa , Grasslands South Africa , Herbaceous plants South Africa , Vegetation dynamics South Africa , Forest canopies South Africa
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/291015 , vital:56808
- Description: Grass and forb species found in savannas are highly diverse, contributing to the structure and function of the savanna system. Where mean annual rainfall is seasonal and high enough to support closed canopy vegetation such as forests or thickets, savannas can exist as an alternative stable state maintained by disturbances such as fire and browsing. Biotic and abiotic processes act on savanna and forest (or thicket) systems maintaining both their tree and herbaceous cover at levels that ensure their persistence in those states. Studies have shown that many semi-arid rangelands in South Africa have undergone a rapid increase in tree cover (of both native and non-native species) over the past several decades. This process of increasing tree cover in semi-arid savannas, termed bush encroachment, results in a biome shift, changing landscapes that were once grasslands with few trees to ones dominated by broad-leaved trees with fewer sun-adapted forbs and grasses. The aim of this study was to investigate the impact of changing woody cover and its associated changes in tree composition, tree canopy structure, light dynamics in the understory and herbaceous community composition on Endwell farm in the Eastern Cape. Canopy cover changes between the years 1949 and 2019 were analysed at 51 sites on the farm and related to historical rainfall patterns. There had been a general increase in tree cover over the past several decades on the farm, and many sites showed a change from open (0-15%) in 1949 to low (1635%), moderate (36-50%) and high (51-100%) canopy cover in 2019. In earlier years most sites had a canopy cover below 50%, and the higher canopy cover values (>65%) occurred in more recent decades. Canopy cover of ~ 50% was found to be rare in each decade. This suggests that ~50% canopy cover maybe a transient, unstable state. The period with the highest rate of canopy cover increase was 2002-2013, and this increase coincided with a high mean annual rainfall 10 years prior to 2002 and a high mean annual rainfall in most years between the 20022013 period. The period between 2002 and 2013 also had the highest number of sites transitioning from lower to higher tree canopy cover classes, indicating that rainfall may have been a factor driving bush encroachment during the past several decades. An increase in canopy cover (a decrease in light transmittance) was accompanied by changes in woody species composition during thicket formation. The low canopy cover (high light transmittance) sites were dominated by Vachellia karroo and Scutia myrtina trees, while high tree cover sites had fewer V. karroo and S. myrtina trees and were rather characterised by an abundance of thicket tree species. Species proportion, NMDS and dendrogram plots indicated that sites with a light transmittance range between 50-100% had similar tree species compositions, different from sites with light transmittances <50%. An increase in tree density was strongly correlated to an increase in canopy cover (from 2019 satellite imagery), density of trees > 3m, maximum height reached by trees, diversity of trees, total canopy volume, total canopy area and leaf area index (LAI), and a decrease in light transmittance. A structural equation model (SEM) was used to explore the relationships between canopy characteristics (maximum canopy area, canopy volume, tree diversity, density of trees, density of trees >3m, individual trees and maximum canopy height), aerial canopy cover in 2019, and light transmittance. The model explained 73% of the variation in light transmittance, mostly via the direct effect of canopy characteristics. Canopy characteristics had a strong influence on both aerial cover in 2019 and directly on light transmittance, but canopy cover in 2019 had a weak influence on light transmittance. The herbaceous layer was rich and dominated by C4 grasses such as Eragrostis plana, Sporobolus fimbriatus, Themeda triandra and Digitaria eriantha) and forbs including Hibiscus aethiopicus, Helichrysum dregeanum, Helichrysum nudifolium and Gerbera viridifolia at low canopy cover sites with high light transmittance. In contrast, high tree cover sites had fewer herbaceous species in general. Grass and forb species characteristic of these sites high canopy cover sites were Panicum maximum, Loudetia flavida, Pellaea viridis and Cyperus spp. Different sites with low light transmittance (<50%) had similar herbaceous species composition. Basal cover, richness, abundance and diversity of herbaceous plants decreased significantly with an increase in tree density, density of trees >3 m, canopy volume, canopy area, canopy cover, LAI, and increased significantly with increasing light transmittance. Most grasses had their highest densities at LAI <0.5, which was estimated to correspond to ~75% light transmittance and ~38% canopy cover and then started to decline thereafter. Herbaceous species basal cover was also highest at LAI <0.5. An SEM model indicated that herbaceous diversity, basal cover and richness responded both to light availability and to the structure of the woody vegetation directly (R2 = 0.53). While the effect of light transmittance on herbaceous communities was strong (0.41), there was little difference between the effect of light transmittance and canopy characteristics (-0.35) on herbaceous communities. Two possible threshold points, relating to two types of transitions in vegetation structure, could be deduced from this study. The first threshold occurred at canopy cover ~ 40% (LAI < ~ 0.5, light transmittance ~ 75%), at which point many of the common herbaceous species, including the dominant C4 grasses, began to decline in abundance while the composition remained characteristic of the savanna state. A canopy cover of less than ~ 40% at a site provides a suitable state for a high abundance of grass and forb species which help maintain an open system by facilitating fires. The second threshold marked a compositional shift between savanna and closed-canopy vegetation states. Savanna species (trees, grasses and forbs) dominated at high light transmittances (>50%) and were significantly reduced at low light transmittances (< 50%), indicating a possible species composition threshold at ~50% light transmittance at which a savanna state switches to a thicket (LAI ~ 1 and canopy cover ~70%). This point indicated the point where there was a significant difference in both tree and herbaceous plant compositions, with a marked reduction in the occurrence of C4 grasses at light transmittance <50%. Fire is supressed when the C4 grass layer is lost, and further thicket encroachment will take place causing complete canopy closure. Land managers in this system should start becoming concerned about a reduction in grass biomass when canopy cover reaches about 40% and would have to reduce tree cover before the threshold of 50% light transmittance (70% canopy cover from aerial photos) is reached to maintain a savanna system. , Thesis (MSc) -- Faculty of Science, Botany, 2022
- Full Text:
- Date Issued: 2022-04-06
Evaluating the physiological, morphological and nutritional effects of elevated atmospheric CO2 and drought on select South African maize cultivars
- Authors: Bopape, Tebadi Mamadiga
- Date: 2021-10
- Subjects: Corn Varieties South Africa , Dry farming South Africa , Corn Effect of atmospheric carbon dioxide on South Africa , Corn Effect of drought on South Africa , Corn Morphology , Corn Nutrition , Corn Physiological effect
- Language: English
- Type: Master theses , text
- Identifier: http://hdl.handle.net/10962/189020 , vital:44807
- Description: Thesis embargoed. Expected Release date October 2022. , Thesis (MSc) -- Faculty of Science, Botany, 2021
- Full Text:
- Date Issued: 2021-10
- Authors: Bopape, Tebadi Mamadiga
- Date: 2021-10
- Subjects: Corn Varieties South Africa , Dry farming South Africa , Corn Effect of atmospheric carbon dioxide on South Africa , Corn Effect of drought on South Africa , Corn Morphology , Corn Nutrition , Corn Physiological effect
- Language: English
- Type: Master theses , text
- Identifier: http://hdl.handle.net/10962/189020 , vital:44807
- Description: Thesis embargoed. Expected Release date October 2022. , Thesis (MSc) -- Faculty of Science, Botany, 2021
- Full Text:
- Date Issued: 2021-10
The invasion ecology of Pontederia cordata L. (Pontederiaceae) in South Africa
- Authors: Wansell, Sage Nora-Lee
- Date: 2021
- Subjects: Plant invsions -- South AFrica , Invasive plants -- Ecology -- South Africa , Pontederiaceae -- South AFrica
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/172433 , vital:42200
- Description: Pontederia cordata L. (Pontederiaceae) is a tristylous invasive macrophyte – originating from North and South America – that has caused detrimental environmental, agricultural and socio-economic impacts in South Africa (SA). This novel study investigates the invasive ecology of P. cordata in SA by determining population genetics, pollination ecology and floral traits. Preliminary field surveys suggest that only one of three tristylous forms of P. cordata is invading SA and no seeds have been observed in any invasive populations. This study therefore determined the population genetics, mode of spread of P. cordata in SA and possible reasons for the lack of seed production, as well as providing suggestions for future control and management strategies. Inter Simple Sequence Repeats of leaf samples from invasive populations in SA and the native range of the United States of America (USA) were performed to determine the population genetics of P. cordata. The clarification of population structure of an alien invasive plant can provide insight into founder effects, introduction events and modes of spread and is important for the development of management plans such as biological control. Results from the genetic analyses indicated that P. cordata populations have low genetic diversity within and amongst invasive populations in comparison to native populations. This suggests that high gene flow and sexual reproduction is not present in invasive populations, and that only a single or very few introductory events have occurred in SA. Furthermore, invasive P. cordata populations shared the highest genetic similarity with native samples from Belle Haven, Virginia, USA, and thus further sampling and future genetic surveys should be conducted in this area to identify source populations to survey for potential biological control agents. Following these findings, I investigated whether sexual reproduction and seed production is absent from invasive P. cordata populations in SA as speculated. Floral traits from populations throughout all the invaded provinces were measured and, along with pollen grain measurements, it was determined that only short-morphed plants are present in SA. It was speculated that the absence of native pollinators in the invasive range may be responsible for the absence of sexual reproduction. However, a pollination study confirmed the presence of generalist insect pollinators. Thereafter, artificial pollination experiments on 8 865 flowers were conducted to determine whether an incompatibility system was present which prevented seed production. No seeds were produced and it was concluded that illegitimate pollination of the short-morphed plants prevented seed production and rhizomes are responsible for the invasion of P. cordata throughout SA. The implications of these findings and possible management strategies such as biological control is discussed in Chapter 4. These findings suggest that control programmes should target the plants rhizomes to prevent and reduce spread. Preventing the introduction of medium- and long-morphed plants into SA is crucial to prevent P. cordata from producing seeds and intensifying invasion further through both asexual and sexual spread.
- Full Text:
- Date Issued: 2021
- Authors: Wansell, Sage Nora-Lee
- Date: 2021
- Subjects: Plant invsions -- South AFrica , Invasive plants -- Ecology -- South Africa , Pontederiaceae -- South AFrica
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/172433 , vital:42200
- Description: Pontederia cordata L. (Pontederiaceae) is a tristylous invasive macrophyte – originating from North and South America – that has caused detrimental environmental, agricultural and socio-economic impacts in South Africa (SA). This novel study investigates the invasive ecology of P. cordata in SA by determining population genetics, pollination ecology and floral traits. Preliminary field surveys suggest that only one of three tristylous forms of P. cordata is invading SA and no seeds have been observed in any invasive populations. This study therefore determined the population genetics, mode of spread of P. cordata in SA and possible reasons for the lack of seed production, as well as providing suggestions for future control and management strategies. Inter Simple Sequence Repeats of leaf samples from invasive populations in SA and the native range of the United States of America (USA) were performed to determine the population genetics of P. cordata. The clarification of population structure of an alien invasive plant can provide insight into founder effects, introduction events and modes of spread and is important for the development of management plans such as biological control. Results from the genetic analyses indicated that P. cordata populations have low genetic diversity within and amongst invasive populations in comparison to native populations. This suggests that high gene flow and sexual reproduction is not present in invasive populations, and that only a single or very few introductory events have occurred in SA. Furthermore, invasive P. cordata populations shared the highest genetic similarity with native samples from Belle Haven, Virginia, USA, and thus further sampling and future genetic surveys should be conducted in this area to identify source populations to survey for potential biological control agents. Following these findings, I investigated whether sexual reproduction and seed production is absent from invasive P. cordata populations in SA as speculated. Floral traits from populations throughout all the invaded provinces were measured and, along with pollen grain measurements, it was determined that only short-morphed plants are present in SA. It was speculated that the absence of native pollinators in the invasive range may be responsible for the absence of sexual reproduction. However, a pollination study confirmed the presence of generalist insect pollinators. Thereafter, artificial pollination experiments on 8 865 flowers were conducted to determine whether an incompatibility system was present which prevented seed production. No seeds were produced and it was concluded that illegitimate pollination of the short-morphed plants prevented seed production and rhizomes are responsible for the invasion of P. cordata throughout SA. The implications of these findings and possible management strategies such as biological control is discussed in Chapter 4. These findings suggest that control programmes should target the plants rhizomes to prevent and reduce spread. Preventing the introduction of medium- and long-morphed plants into SA is crucial to prevent P. cordata from producing seeds and intensifying invasion further through both asexual and sexual spread.
- Full Text:
- Date Issued: 2021
CO2-induced woody thickening depends strongly on interacting abiotic and biotic factors
- Authors: Raubenheimer, Sarah Lynn
- Date: 2020
- Subjects: Uncatalogued
- Language: English
- Type: thesis , text , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/164646 , vital:41151 , doi:10.21504/10962/164646
- Description: Thesis (PhD)--Rhodes University, Faculty of Science, Botany, 2020
- Full Text:
- Date Issued: 2020
- Authors: Raubenheimer, Sarah Lynn
- Date: 2020
- Subjects: Uncatalogued
- Language: English
- Type: thesis , text , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/164646 , vital:41151 , doi:10.21504/10962/164646
- Description: Thesis (PhD)--Rhodes University, Faculty of Science, Botany, 2020
- Full Text:
- Date Issued: 2020
Growth and photosynthetic responses of Acacia (Vachellia) seedlings to atmospheric CO2 increased from glacial to current concentrations: underlying mechanisms and ecological implications
- Authors: Anderson, Bruce Maurice
- Date: 2020
- Subjects: Acacia vachellia , Acacia vachellia -- Growth , Acacia -- South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/138182 , vital:37604
- Description: The African Acacia species Vachellia karroo, V. robusta, V. nilotica, and V. tortilis are some of the most invasive species implicated in bush encroachment and woody thickening of historically open savannas in southern Africa. This is partially explained by historic increases in atmospheric CO2 concentrations, which are proposed to have promoted the growth and survivorship of C3 tree seedlings relative to C4 grasses. However, the uniformity of CO2 responsiveness and differences among Vachellia species remain largely undetermined. Here we investigate the growth and photosynthetic responses of four Vachellia species, all implicated in woody encroachment, but originating from distinct climatic niches. Exposing these species to a range of sub-ambient CO2 concentrations (12 – 40 Pa) showed that V. karroo, V. robusta, V. nilotica and V. tortilis all responded strongly and fairly consistently to increasing CO2 concentrations, acting as a ‘functional type’ despite being selected from different geographic regions and having different climatic niches. Combined average net CO2 assimilation rates increased by 130% despite significant, but low levels of down-regulation and decreased stomatal conductance. The increased photosynthetic rates stimulated growth and biomass production in all compartments, with no significant differences in interspecific above and below ground allocation. Growth rates and dry biomass increased by 50% and 186%, respectively, while leaf level water use efficiency (ratio of net CO2 assimilation rate to transpiration rate) increased by an average of 218%. When this was scaled to the whole plant level, this stimulation was decreased to 80%. The decrease was the result of the CO2 stimulated increase in canopy areas, which increased leaf area for water loss. The seedlings’ total number of spinescent physical defenses, as well as the average mass and spine mass fraction also increased with rising CO2. These thicker spines could act as better deterrents against vertebrate browsers. Spine density was unchanged, however, showing that the increased spine numbers were associated with larger seedlings at higher CO2 rather than an increase in the number of spines per stem length. The stimulatory effects of increasing CO2 concentrations since the last glacial maximum and resultant increases in seedling growth and biomass are likely to have had important consequences for the survival and establishment of Acacia seedlings. Tolerance of drought and disturbance has been related to seedling size, hence stimulating the growth rate could confer disturbance tolerance and this tolerance would develop more rapidly with increasing CO2 concentrations. Furthermore, increased nitrogen and water use efficiency have the potential to support seedling establishment in environments where these resources would otherwise be limited at lower atmospheric CO2 concentrations. Resulting in a larger proportion of CO2 fertilization responsive woody seedlings surviving the seedling size classes, and persisting within historically open savannas. Where interspecific differences occurred they are likely to have arisen from adaptation to specific climates where these species are native and selection would have been driven by factors such climate, resource availability, levels of disturbance and competitive interactions. V. karroo had the highest growth rates and strong CO2 driven increases in biomass accumulation, despite having the lowest inherent photosynthetic rates. V. karroo also had the lowest increase in water use efficiency and high transpiration rates could potentially increase access to soil nutrients through mass flow. This species had the highest mean spine mass and showed significant increases in spine mass fraction at elevated CO2 concentrations, which may be important for deterring herbivores. V. robusta’s distribution to the mesic east coast of Africa suggests that water is an important limitation to its distribution. Hence, the CO2 stimulated increase in water use efficiency at both leaf and whole canopy level allows speculation that this may be an important driver of this species’ range expansion, which might continue if increasing levels of CO2 continue to promote water use efficiency. V. nilotica occupies a broad range of habitats, inhabiting large areas of the subtropics both north and south of the equator, with the strongest climatic correlates being the precipitation of the wettest quarter followed by high temperature seasonality. In response to increasing CO2, V. nilotica showed overall strong increases in growth, water use efficiency, and physical defenses. These responses may explain why V. nilotica has been such a successful encroacher in a broad range of habitats where limitations are likely to include multiple climatic factors and disturbances. V. tortilis has the widest distribution of all the species studied, covering broad ranges of Africa and only being excluded from the wettest parts of the equator and driest parts of the deserts. In these experiments this species showed the lowest biomass responsiveness to CO2, but had especially large increases in water use efficiency at both the leaf and canopy level. This may have been an important driver for this species’ encroachment into the more arid parts of its distribution, however this link will need to be verified with further experimentation.
- Full Text:
- Date Issued: 2020
- Authors: Anderson, Bruce Maurice
- Date: 2020
- Subjects: Acacia vachellia , Acacia vachellia -- Growth , Acacia -- South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/138182 , vital:37604
- Description: The African Acacia species Vachellia karroo, V. robusta, V. nilotica, and V. tortilis are some of the most invasive species implicated in bush encroachment and woody thickening of historically open savannas in southern Africa. This is partially explained by historic increases in atmospheric CO2 concentrations, which are proposed to have promoted the growth and survivorship of C3 tree seedlings relative to C4 grasses. However, the uniformity of CO2 responsiveness and differences among Vachellia species remain largely undetermined. Here we investigate the growth and photosynthetic responses of four Vachellia species, all implicated in woody encroachment, but originating from distinct climatic niches. Exposing these species to a range of sub-ambient CO2 concentrations (12 – 40 Pa) showed that V. karroo, V. robusta, V. nilotica and V. tortilis all responded strongly and fairly consistently to increasing CO2 concentrations, acting as a ‘functional type’ despite being selected from different geographic regions and having different climatic niches. Combined average net CO2 assimilation rates increased by 130% despite significant, but low levels of down-regulation and decreased stomatal conductance. The increased photosynthetic rates stimulated growth and biomass production in all compartments, with no significant differences in interspecific above and below ground allocation. Growth rates and dry biomass increased by 50% and 186%, respectively, while leaf level water use efficiency (ratio of net CO2 assimilation rate to transpiration rate) increased by an average of 218%. When this was scaled to the whole plant level, this stimulation was decreased to 80%. The decrease was the result of the CO2 stimulated increase in canopy areas, which increased leaf area for water loss. The seedlings’ total number of spinescent physical defenses, as well as the average mass and spine mass fraction also increased with rising CO2. These thicker spines could act as better deterrents against vertebrate browsers. Spine density was unchanged, however, showing that the increased spine numbers were associated with larger seedlings at higher CO2 rather than an increase in the number of spines per stem length. The stimulatory effects of increasing CO2 concentrations since the last glacial maximum and resultant increases in seedling growth and biomass are likely to have had important consequences for the survival and establishment of Acacia seedlings. Tolerance of drought and disturbance has been related to seedling size, hence stimulating the growth rate could confer disturbance tolerance and this tolerance would develop more rapidly with increasing CO2 concentrations. Furthermore, increased nitrogen and water use efficiency have the potential to support seedling establishment in environments where these resources would otherwise be limited at lower atmospheric CO2 concentrations. Resulting in a larger proportion of CO2 fertilization responsive woody seedlings surviving the seedling size classes, and persisting within historically open savannas. Where interspecific differences occurred they are likely to have arisen from adaptation to specific climates where these species are native and selection would have been driven by factors such climate, resource availability, levels of disturbance and competitive interactions. V. karroo had the highest growth rates and strong CO2 driven increases in biomass accumulation, despite having the lowest inherent photosynthetic rates. V. karroo also had the lowest increase in water use efficiency and high transpiration rates could potentially increase access to soil nutrients through mass flow. This species had the highest mean spine mass and showed significant increases in spine mass fraction at elevated CO2 concentrations, which may be important for deterring herbivores. V. robusta’s distribution to the mesic east coast of Africa suggests that water is an important limitation to its distribution. Hence, the CO2 stimulated increase in water use efficiency at both leaf and whole canopy level allows speculation that this may be an important driver of this species’ range expansion, which might continue if increasing levels of CO2 continue to promote water use efficiency. V. nilotica occupies a broad range of habitats, inhabiting large areas of the subtropics both north and south of the equator, with the strongest climatic correlates being the precipitation of the wettest quarter followed by high temperature seasonality. In response to increasing CO2, V. nilotica showed overall strong increases in growth, water use efficiency, and physical defenses. These responses may explain why V. nilotica has been such a successful encroacher in a broad range of habitats where limitations are likely to include multiple climatic factors and disturbances. V. tortilis has the widest distribution of all the species studied, covering broad ranges of Africa and only being excluded from the wettest parts of the equator and driest parts of the deserts. In these experiments this species showed the lowest biomass responsiveness to CO2, but had especially large increases in water use efficiency at both the leaf and canopy level. This may have been an important driver for this species’ encroachment into the more arid parts of its distribution, however this link will need to be verified with further experimentation.
- Full Text:
- Date Issued: 2020
The effects of elevated atmospheric CO2 on the biological control of invasive aquatic weeds in South Africa
- Authors: Baso, Nompumelelo Catherine
- Date: 2020
- Subjects: Aquatic weeds -- Biological control -- South Africa , Plants -- Effect of atmospheric carbon dioxide on , Atmospheric carbon dioxide -- Environmental aspects
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/140772 , vital:37917
- Description: There has been a rapid increase in atmospheric CO2 concentration, from pre-industrial values of 280 ppm to more than 400 ppm currently, and this is expected to more than double by the end of the 21st century. Studies have shown that plants grown above 600 ppm tend to have an increased growth rate and invest more in carbon-based defences. This has important implications for the management of invasive alien plants, especially for the field of biological control which is mostly dependent on herbivorous insects. This is because insects reared on such plants have been shown to have reduced overall fitness. Nevertheless, most of the studies on potential changes in plant-insect interactions under elevated CO2 are based on agricultural systems, with only a limited number of these types of studies conducted on alien invasive weeds. However, climate change and invasive species are two of the most prevalent features of global environmental change. Therefore, this also warrants active research and experimental studies to better understand how these systems will be affected by future climates. Thus, the aim of this study was to investigate the effects of elevated atmospheric CO2 on the biological control of four invasive aquatic weeds (Azolla filiculoides, Salvinia molesta, Pistia stratiotes, and Myriophyllum aquaticum). These species are a threat to natural resources in South Africa but are currently under successful control by their biological control agents (Stenopelmus rufinasus, Cyrtobagous salviniae, Neohydronomus affinis, and Lysathia n. sp.). To achieve this, the selected plant species were grown in a three-factor experimental design in winter (CO2 X nutrients X herbivory), and another two-factorial design in summer (CO2 X herbivory). Atmospheric CO2 concentrations were set at ambient (400 ppm) or elevated (800 ppm), as per the predictions of the IPCC. As per my hypothesis, the results suggest that these species will become more challenging in future due to increased biomass production, asexual reproduction and a higher C: N ratio which is evident under high CO2 concentrations. Although the biological control agents were in some instances able to reduce this CO2 fertilisation effect, their efficacy was significantly reduced compared with the levels of control observed at ambient CO2. These results suggest that additional biological control agents and other management methods may be needed for continued control of these invasive macrophytes, both in South Africa and further afield where they are problematic.
- Full Text:
- Date Issued: 2020
- Authors: Baso, Nompumelelo Catherine
- Date: 2020
- Subjects: Aquatic weeds -- Biological control -- South Africa , Plants -- Effect of atmospheric carbon dioxide on , Atmospheric carbon dioxide -- Environmental aspects
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/140772 , vital:37917
- Description: There has been a rapid increase in atmospheric CO2 concentration, from pre-industrial values of 280 ppm to more than 400 ppm currently, and this is expected to more than double by the end of the 21st century. Studies have shown that plants grown above 600 ppm tend to have an increased growth rate and invest more in carbon-based defences. This has important implications for the management of invasive alien plants, especially for the field of biological control which is mostly dependent on herbivorous insects. This is because insects reared on such plants have been shown to have reduced overall fitness. Nevertheless, most of the studies on potential changes in plant-insect interactions under elevated CO2 are based on agricultural systems, with only a limited number of these types of studies conducted on alien invasive weeds. However, climate change and invasive species are two of the most prevalent features of global environmental change. Therefore, this also warrants active research and experimental studies to better understand how these systems will be affected by future climates. Thus, the aim of this study was to investigate the effects of elevated atmospheric CO2 on the biological control of four invasive aquatic weeds (Azolla filiculoides, Salvinia molesta, Pistia stratiotes, and Myriophyllum aquaticum). These species are a threat to natural resources in South Africa but are currently under successful control by their biological control agents (Stenopelmus rufinasus, Cyrtobagous salviniae, Neohydronomus affinis, and Lysathia n. sp.). To achieve this, the selected plant species were grown in a three-factor experimental design in winter (CO2 X nutrients X herbivory), and another two-factorial design in summer (CO2 X herbivory). Atmospheric CO2 concentrations were set at ambient (400 ppm) or elevated (800 ppm), as per the predictions of the IPCC. As per my hypothesis, the results suggest that these species will become more challenging in future due to increased biomass production, asexual reproduction and a higher C: N ratio which is evident under high CO2 concentrations. Although the biological control agents were in some instances able to reduce this CO2 fertilisation effect, their efficacy was significantly reduced compared with the levels of control observed at ambient CO2. These results suggest that additional biological control agents and other management methods may be needed for continued control of these invasive macrophytes, both in South Africa and further afield where they are problematic.
- Full Text:
- Date Issued: 2020
Plants, people, and place: complex, mutualistic, and co-evolving global patterns through time
- Authors: Van Wijk, Yvette Ethné
- Date: 2019
- Subjects: Ethnobotany -- South Africa -- Western Cape , Khoisan (African people) -- Ethnobotany , Human-plant relationships -- South Africa -- Western Cape , Plants -- Classification -- South Africa -- Western Cape , Plant remains (Archaeology) -- South Africa -- Western Cape , Ethnoscience -- South Africa -- Western Cape , Regression analysis
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/76575 , vital:30609
- Description: My thesis studies and analyses the suite of distinctive plant taxa which persist in small patches of vegetation growing in close association with archaeological habitation sites in the southern Cape, South Africa. The unexpected correlation and overlap between botanical taxa collected at 75 site complexes, and ethnobotanical data collected in collaboration with modern Khoi-San communities in the same area, is explored and interrogated. Although sparse, reports of the same suite of taxa recovered from archaeological excavations in the Cape provinces provides depth of time to the study, linking the past to the present. The three-way correlation of a suite of plants closely associated with humans and habitation sites through time, allows for triangulation of the data in order to validate and cross verify the results using more than one frame of reference. Both the plants and the knowledge about their uses have persisted in spite of historical attrition, and alienation of land and language, suffered by the Khoi-San over the past 300 years. Drawing on a large body of primary and secondary data, and using an interdisciplinary, abductive and pragmatic mixed methods approach, a pattern can be traced throughout Africa and globally. Regression analysis strongly indicates that the most ubiquitous taxa were selected for a purpose and are not randomly present in association with humans. Botanical, anthropological, and archaeological studies seldom focus on the inter-connectedness of people and plants at the sites they inhabited. Very little research into modern vegetation in close association with the sites has been undertaken, and vegetation mapping has not captured the occurrence of these site-specific small vegetation patches recorded during my surveys. The topographically, geologically, and vegetatively complex and varied southern Cape, and greater Cape area, is extremely rich in archaeological sites and history. This study suggests that the value of site-specific plant taxa to humans throughout the aeons of pre-agricultural history, persists into the present. Due to tolerance of a broad range of climatic and environmental variables, there is value in the study of these ancient and neglected useful plants in the face of climate change. That this vegetation is so closely associated with archaeological sites of cultural and historic importance confers an urgency to recognising the existence and significance of the distinctive and possibly anthropogenic vegetation surrounding the sites.
- Full Text:
- Date Issued: 2019
- Authors: Van Wijk, Yvette Ethné
- Date: 2019
- Subjects: Ethnobotany -- South Africa -- Western Cape , Khoisan (African people) -- Ethnobotany , Human-plant relationships -- South Africa -- Western Cape , Plants -- Classification -- South Africa -- Western Cape , Plant remains (Archaeology) -- South Africa -- Western Cape , Ethnoscience -- South Africa -- Western Cape , Regression analysis
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/76575 , vital:30609
- Description: My thesis studies and analyses the suite of distinctive plant taxa which persist in small patches of vegetation growing in close association with archaeological habitation sites in the southern Cape, South Africa. The unexpected correlation and overlap between botanical taxa collected at 75 site complexes, and ethnobotanical data collected in collaboration with modern Khoi-San communities in the same area, is explored and interrogated. Although sparse, reports of the same suite of taxa recovered from archaeological excavations in the Cape provinces provides depth of time to the study, linking the past to the present. The three-way correlation of a suite of plants closely associated with humans and habitation sites through time, allows for triangulation of the data in order to validate and cross verify the results using more than one frame of reference. Both the plants and the knowledge about their uses have persisted in spite of historical attrition, and alienation of land and language, suffered by the Khoi-San over the past 300 years. Drawing on a large body of primary and secondary data, and using an interdisciplinary, abductive and pragmatic mixed methods approach, a pattern can be traced throughout Africa and globally. Regression analysis strongly indicates that the most ubiquitous taxa were selected for a purpose and are not randomly present in association with humans. Botanical, anthropological, and archaeological studies seldom focus on the inter-connectedness of people and plants at the sites they inhabited. Very little research into modern vegetation in close association with the sites has been undertaken, and vegetation mapping has not captured the occurrence of these site-specific small vegetation patches recorded during my surveys. The topographically, geologically, and vegetatively complex and varied southern Cape, and greater Cape area, is extremely rich in archaeological sites and history. This study suggests that the value of site-specific plant taxa to humans throughout the aeons of pre-agricultural history, persists into the present. Due to tolerance of a broad range of climatic and environmental variables, there is value in the study of these ancient and neglected useful plants in the face of climate change. That this vegetation is so closely associated with archaeological sites of cultural and historic importance confers an urgency to recognising the existence and significance of the distinctive and possibly anthropogenic vegetation surrounding the sites.
- Full Text:
- Date Issued: 2019
The conservation, ecology, and distribution of the critically endangered Encephalartos latifrons Lehm
- Authors: Swart, Carin
- Date: 2019
- Subjects: Encephalartos , Cycadaceae , Cycads -- Conservation -- South Africa , Botany, Economic -- South Africa , Rare plants -- South Africa , Endangered plants -- South Africa , Wild plant trade -- Law and legislation -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/94483 , vital:31049
- Description: Cycads have attracted global attention both as horticulturally interesting and often valuable plants; but also as some of the most threatened organisms on the planet. In this thesis I investigate the conservation management, biology, reproductive ecology and distribution of Encephalartos latifrons populations in the wild and draw out conclusions on how best to conserve global cycad biodiversity. I also employ computer-modelling techniques in some of the chapters of this thesis to demonstrate how to improve conservation outcomes for E. latifrons and endangered species in general, where information on the distribution, biology and habitat requirements of such species are inherently limited, often precluding robust conservation decision-making. In Chapter 1 of this thesis I introduce the concept of extinction debt and elucidate the importance of in situ cycad conservation. I explain how the concept of extinction debt relates to single species, as well as give details on the mechanisms causing extinction debt in cycad populations. I introduce the six extinction trajectory threshold model and how this relates to extinction debt in cycads. I discuss the vulnerability of cycads to extinction and give an overview of biodiversity policy in South Africa. I expand on how national and global policies contribute to cycad conservation and present various global initiatives that support threatened species conservation. I conclude Chapter 1 by explaining how computer-based models can assist conservation decision-making for rare, threatened, and endangered species in the face of uncertainty. Chapter 2 of this thesis illustrates how a modelling approach, using limited available historical and present day locality information, is a feasible method to determine areas of suitable habitat for E. latifrons and other critically endangered cycad species where locality information is inherently uncommon. Results from this chapter show that conservation planning through structured decision-making may be improved by the use of computer models, even when locality data are limited. These results may be incorporated into biodiversity conservation plans or used to assist conservation-decision makers when undertaking recovery efforts for E. latifrons and may provide guidance to conservation planners and policy makers when undertaking conservation plans to improve cycad biodiversity both nationally and globally. There was limited information available in the biology and ecological requirements of E. latifrons. This information is important when making policy decisions such as the publication of non-detriment findings and compiling biodiversity management plans for this and other cycad species. Chapter 3 investigates the life-history, population structure, fire response and survival of an in situ E. latifrons population. A demographic census was undertaken between 2013 and 2017 on a previously undiscovered population. Population characteristics of the “new” population were compared to the demographics of a well-known and intensively managed population. Results of this chapter show that at least one in situ E. latifrons population is stable and increasing under current environmental conditions. Importantly, the population is naturally recruiting seedlings without the need for artificial pollination. Demographic information described in this chapter is a necessary precursor to undertaking a Population Viability Assessment for the species. This will assist conservation decision-makers when determining the best conservation management strategy for E. latifrons. It may also be useful to apply generalisatons to other cycad species (with similar life-histories and habitat requirements) where there is limited information available on the species biological and ecological requirements, restricting robust policy conservation decision-making. It was important for this study to determine the extent and variety of cone fauna within existing E. latifrons wild populations. Previous anecdotal evidence suggested that E. latifrons is functionally extinct as a species, but evidence to the contrary was found when a healthy, self-sustaining wild population was discovered to be naturally recruiting. It was important to establish the existence and diversity of male cone faunal species (an important breeding site for weevil pollinators) within wild populations. Chapter 4 set out to determine if potential pollinators exist in the wild and if so, how diverse are they and in what numbers. This is the first comprehensive analysis of cone fauna present in wild E. latifrons populations. Equally important was the need to determine if wild populations are capable of producing viable seeds under conditions conducive to natural pollination. Results of this chapter show that there is a relatively high diversity of insect fauna in the male cones of some wild E. latifrons populations. Furthermore, some wild populations are capable of producing viable seeds through natural pollination; even though they may not be naturally recruiting seedlings into the population. A staggered germination pattern displayed by one of the wild E. latifrons populations was studied, suggesting the evolution of an adaptive trait given the stochastic environment (climatically and disturbances such as fire) within which E. latifrons populations may be found. Species recovery (restoration and/or population augmentation) may be the only conservation solution remaining to save endangered species such as E. latifrons from extinction in the wild. Chapter 5 involves the return of 25 seedlings germinated as part of a seed viability experiment (see Chapter 4) back into a wild population from where they originated. The primary threat to seedling survival at the site was livestock activity (grazing/trampling). The population was subsequently fenced off to mitigate this threat and seedlings planted both inside and outside a fenced area to establish if there was a difference in seedling survival between the unprotected and protected sites. A high percentage (92%) of seedlings planted perished in total. None of the seedlings planted outside the fenced area survived over the monitoring period, while only two seedlings planted within the fenced area survived. Survival of the seedlings inside the fenced area was only after placing individual cages on the seedlings to prevent further losses. The primary causes of death for all seedlings included uprooting, and defoliation with some of the seedlings missing completely. This chapter found that the lack of natural seedling recruitment at the site was as a result of livestock activity. Grazing by livestock poses a significant threat to natural recruitment in some E. latifrons populations. Alternative restoration methods are suggested and protection of seedlings while undertaking a restoration/augmentation programme is emphasised. Developing conservation management plans for rare and/or endangered species is often met with high levels of uncertainty, particularly if there is limited information available on the biology and ecological requirements for the species concerned. Population viability analysis (PVA) is often suggested as a tool to determine conservation management scenarios that may enhance wild population persistence. The standard PVA approach is however problematic as it is a time-consuming process requiring the collection of demographic data over long time periods. In addition, the PVA approach does not take in to account non-biological factors which may impede the effective implementation of conservation plans. Chapter 6 of this thesis makes use of a Multi-Criteria Decision Making (MCDM) approach called the Analytical Hierarchy Process (AHP) to decide on the best conservation management strategy for an E. latifrons population. Sensitivity analysis was completed to test the robustness of the decision and to identify which criteria influenced the original results. In this study, the development of the decision tree and criteria judgements, were made solely by the researcher. It is emphasised that the decision outcome may be biased if not conducted as part of a multi-stakeholder workshop using the same approach. Nevertheless, it is recommended that a Population Viability Risk Management (PVRM) assessment be undertaken for E. latifrons using an MCDM approach such as AHP as a prestudy, before the revision of the Biodiversity Management Plan (BMP) for E. latifrons. This method is particularly useful when non-biological criteria are to be incorporated into the decision-making process. It is also a viable and holistic alternative to the standard PVA approach when developing conservation management plans for rare and endangered species. In Chapter 7 I review the concept of extinction debt in cycads using E. latifrons as an example. I assimilate historical information to understand mechanisms that may have impacted on E. latifrons populations in the past. This was done to understand the scale of extinction time lags on E. latifrons and to relate this to its present position on the exitinction trajectory. I recommend aligning South African policies and biodiversity assessments with international initiatives and draw out general conclusions for the conservation of global cycad biodiversity. I conclude by recommending further research for E. latifrons.
- Full Text:
- Date Issued: 2019
- Authors: Swart, Carin
- Date: 2019
- Subjects: Encephalartos , Cycadaceae , Cycads -- Conservation -- South Africa , Botany, Economic -- South Africa , Rare plants -- South Africa , Endangered plants -- South Africa , Wild plant trade -- Law and legislation -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/94483 , vital:31049
- Description: Cycads have attracted global attention both as horticulturally interesting and often valuable plants; but also as some of the most threatened organisms on the planet. In this thesis I investigate the conservation management, biology, reproductive ecology and distribution of Encephalartos latifrons populations in the wild and draw out conclusions on how best to conserve global cycad biodiversity. I also employ computer-modelling techniques in some of the chapters of this thesis to demonstrate how to improve conservation outcomes for E. latifrons and endangered species in general, where information on the distribution, biology and habitat requirements of such species are inherently limited, often precluding robust conservation decision-making. In Chapter 1 of this thesis I introduce the concept of extinction debt and elucidate the importance of in situ cycad conservation. I explain how the concept of extinction debt relates to single species, as well as give details on the mechanisms causing extinction debt in cycad populations. I introduce the six extinction trajectory threshold model and how this relates to extinction debt in cycads. I discuss the vulnerability of cycads to extinction and give an overview of biodiversity policy in South Africa. I expand on how national and global policies contribute to cycad conservation and present various global initiatives that support threatened species conservation. I conclude Chapter 1 by explaining how computer-based models can assist conservation decision-making for rare, threatened, and endangered species in the face of uncertainty. Chapter 2 of this thesis illustrates how a modelling approach, using limited available historical and present day locality information, is a feasible method to determine areas of suitable habitat for E. latifrons and other critically endangered cycad species where locality information is inherently uncommon. Results from this chapter show that conservation planning through structured decision-making may be improved by the use of computer models, even when locality data are limited. These results may be incorporated into biodiversity conservation plans or used to assist conservation-decision makers when undertaking recovery efforts for E. latifrons and may provide guidance to conservation planners and policy makers when undertaking conservation plans to improve cycad biodiversity both nationally and globally. There was limited information available in the biology and ecological requirements of E. latifrons. This information is important when making policy decisions such as the publication of non-detriment findings and compiling biodiversity management plans for this and other cycad species. Chapter 3 investigates the life-history, population structure, fire response and survival of an in situ E. latifrons population. A demographic census was undertaken between 2013 and 2017 on a previously undiscovered population. Population characteristics of the “new” population were compared to the demographics of a well-known and intensively managed population. Results of this chapter show that at least one in situ E. latifrons population is stable and increasing under current environmental conditions. Importantly, the population is naturally recruiting seedlings without the need for artificial pollination. Demographic information described in this chapter is a necessary precursor to undertaking a Population Viability Assessment for the species. This will assist conservation decision-makers when determining the best conservation management strategy for E. latifrons. It may also be useful to apply generalisatons to other cycad species (with similar life-histories and habitat requirements) where there is limited information available on the species biological and ecological requirements, restricting robust policy conservation decision-making. It was important for this study to determine the extent and variety of cone fauna within existing E. latifrons wild populations. Previous anecdotal evidence suggested that E. latifrons is functionally extinct as a species, but evidence to the contrary was found when a healthy, self-sustaining wild population was discovered to be naturally recruiting. It was important to establish the existence and diversity of male cone faunal species (an important breeding site for weevil pollinators) within wild populations. Chapter 4 set out to determine if potential pollinators exist in the wild and if so, how diverse are they and in what numbers. This is the first comprehensive analysis of cone fauna present in wild E. latifrons populations. Equally important was the need to determine if wild populations are capable of producing viable seeds under conditions conducive to natural pollination. Results of this chapter show that there is a relatively high diversity of insect fauna in the male cones of some wild E. latifrons populations. Furthermore, some wild populations are capable of producing viable seeds through natural pollination; even though they may not be naturally recruiting seedlings into the population. A staggered germination pattern displayed by one of the wild E. latifrons populations was studied, suggesting the evolution of an adaptive trait given the stochastic environment (climatically and disturbances such as fire) within which E. latifrons populations may be found. Species recovery (restoration and/or population augmentation) may be the only conservation solution remaining to save endangered species such as E. latifrons from extinction in the wild. Chapter 5 involves the return of 25 seedlings germinated as part of a seed viability experiment (see Chapter 4) back into a wild population from where they originated. The primary threat to seedling survival at the site was livestock activity (grazing/trampling). The population was subsequently fenced off to mitigate this threat and seedlings planted both inside and outside a fenced area to establish if there was a difference in seedling survival between the unprotected and protected sites. A high percentage (92%) of seedlings planted perished in total. None of the seedlings planted outside the fenced area survived over the monitoring period, while only two seedlings planted within the fenced area survived. Survival of the seedlings inside the fenced area was only after placing individual cages on the seedlings to prevent further losses. The primary causes of death for all seedlings included uprooting, and defoliation with some of the seedlings missing completely. This chapter found that the lack of natural seedling recruitment at the site was as a result of livestock activity. Grazing by livestock poses a significant threat to natural recruitment in some E. latifrons populations. Alternative restoration methods are suggested and protection of seedlings while undertaking a restoration/augmentation programme is emphasised. Developing conservation management plans for rare and/or endangered species is often met with high levels of uncertainty, particularly if there is limited information available on the biology and ecological requirements for the species concerned. Population viability analysis (PVA) is often suggested as a tool to determine conservation management scenarios that may enhance wild population persistence. The standard PVA approach is however problematic as it is a time-consuming process requiring the collection of demographic data over long time periods. In addition, the PVA approach does not take in to account non-biological factors which may impede the effective implementation of conservation plans. Chapter 6 of this thesis makes use of a Multi-Criteria Decision Making (MCDM) approach called the Analytical Hierarchy Process (AHP) to decide on the best conservation management strategy for an E. latifrons population. Sensitivity analysis was completed to test the robustness of the decision and to identify which criteria influenced the original results. In this study, the development of the decision tree and criteria judgements, were made solely by the researcher. It is emphasised that the decision outcome may be biased if not conducted as part of a multi-stakeholder workshop using the same approach. Nevertheless, it is recommended that a Population Viability Risk Management (PVRM) assessment be undertaken for E. latifrons using an MCDM approach such as AHP as a prestudy, before the revision of the Biodiversity Management Plan (BMP) for E. latifrons. This method is particularly useful when non-biological criteria are to be incorporated into the decision-making process. It is also a viable and holistic alternative to the standard PVA approach when developing conservation management plans for rare and endangered species. In Chapter 7 I review the concept of extinction debt in cycads using E. latifrons as an example. I assimilate historical information to understand mechanisms that may have impacted on E. latifrons populations in the past. This was done to understand the scale of extinction time lags on E. latifrons and to relate this to its present position on the exitinction trajectory. I recommend aligning South African policies and biodiversity assessments with international initiatives and draw out general conclusions for the conservation of global cycad biodiversity. I conclude by recommending further research for E. latifrons.
- Full Text:
- Date Issued: 2019
The invasion ecology of Nymphaea mexicana Zucc. (Mexican Water lily) in South Africa
- Authors: Naidu, Prinavin
- Date: 2019
- Subjects: Nymphaea Mexicana zuccarini , Nymphaea , Nymphaea -- Biological control -- South Africa , Water lilies , Nymphaea -- Ecology -- South Africa , Water lilies -- Biological control -- South Africa , Invasive plants -- Biological control -- South Africa , Aquatic weeds -- Biological control -- South Africa , Water lilies -- Ecology -- South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/92920 , vital:30763
- Description: The Mexican water lily, Nymphaea mexicana Zuccarini, is an aquatic perennial, native to southern USA and Mexico, and has been introduced to South Africa via the ornamental plant trade. This species has rapid growth rates and becomes weedy in dams, ponds and rivers. It is currently listed as a NEM:BA category 1b invasive plant in South Africa. One possible management measure for this weed is biological control, but it is a novel target because no biological control programme has been initiated against it anywhere in the world. This study is intended as a baseline for the biological control programme against this plant in South Africa. Assessing the population structure and mode of reproduction of invasive alien plants is an imperative aid to determining if biological control is a suitable management option. Using amplified fragment length polymorphism (AFLP) molecular markers, I compared the amount of genetic variability and differentiation of N. mexicana in its native range (USA), and invasive range (South Africa). Results indicated a large genetic distance between populations in the USA and South Africa, compared to populations within each country. The genetic variability of the invasive populations was higher than that found in the native distribution. This could be due to hybridization in the introduced range, and/or multiple introductions from different source populations. Differences in the morphology of N. mexicana plants in the invasive range and South Africa were also observed which confirm the results of the genetic analyses. I also assessed the reproductive mode of N. mexicana cultivars/hybrids by conducting breeding system experiments and field pollinator studies. Results indicated that the cultivars are sterile, suggesting that the primary mode of reproduction is asexual via fragmentation of tubers. The main pollinators that were found to be associated with the cultivars in South Africa were honeybees, sweat bees, flies and beetles. These insect groups were the same as those that were observed in another study which was conducted on the pollinators associated with the pure N. mexicana in the native range in southern USA. Mechanical and chemical control of N. mexicana and its multiple genotypes have been applied but have not been efficient due to the fast regeneration of shoots, especially in summer. Therefore, these two management options are not long–term solutions and will also be costly due to the widespread occurrence of the hybrids in South Africa. Thus the only cost–effective, environmentally friendly, self–sustainable and long–term management option is biological control. The significant divergence between native and invasive populations of N. mexicana, as well as the possibility of numerous invasive cultivars, may limit future prospects of biological control of this species. However the differences in the root structures between native South African waterlilies, such as N. lotus and N. nouchali, and the introduced waterlilies, such as N. mexicana and its associated hybrids, may play a pivotal role in the success of biological control of the N. mexicana hybrid complex in South Africa. Natural enemies which feed on the hard tuberous roots of N. mexicana and its hybrids, as opposed to the soft bulbs of the native N. nouchali and N. lotus, should be prioritised.
- Full Text:
- Date Issued: 2019
- Authors: Naidu, Prinavin
- Date: 2019
- Subjects: Nymphaea Mexicana zuccarini , Nymphaea , Nymphaea -- Biological control -- South Africa , Water lilies , Nymphaea -- Ecology -- South Africa , Water lilies -- Biological control -- South Africa , Invasive plants -- Biological control -- South Africa , Aquatic weeds -- Biological control -- South Africa , Water lilies -- Ecology -- South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/92920 , vital:30763
- Description: The Mexican water lily, Nymphaea mexicana Zuccarini, is an aquatic perennial, native to southern USA and Mexico, and has been introduced to South Africa via the ornamental plant trade. This species has rapid growth rates and becomes weedy in dams, ponds and rivers. It is currently listed as a NEM:BA category 1b invasive plant in South Africa. One possible management measure for this weed is biological control, but it is a novel target because no biological control programme has been initiated against it anywhere in the world. This study is intended as a baseline for the biological control programme against this plant in South Africa. Assessing the population structure and mode of reproduction of invasive alien plants is an imperative aid to determining if biological control is a suitable management option. Using amplified fragment length polymorphism (AFLP) molecular markers, I compared the amount of genetic variability and differentiation of N. mexicana in its native range (USA), and invasive range (South Africa). Results indicated a large genetic distance between populations in the USA and South Africa, compared to populations within each country. The genetic variability of the invasive populations was higher than that found in the native distribution. This could be due to hybridization in the introduced range, and/or multiple introductions from different source populations. Differences in the morphology of N. mexicana plants in the invasive range and South Africa were also observed which confirm the results of the genetic analyses. I also assessed the reproductive mode of N. mexicana cultivars/hybrids by conducting breeding system experiments and field pollinator studies. Results indicated that the cultivars are sterile, suggesting that the primary mode of reproduction is asexual via fragmentation of tubers. The main pollinators that were found to be associated with the cultivars in South Africa were honeybees, sweat bees, flies and beetles. These insect groups were the same as those that were observed in another study which was conducted on the pollinators associated with the pure N. mexicana in the native range in southern USA. Mechanical and chemical control of N. mexicana and its multiple genotypes have been applied but have not been efficient due to the fast regeneration of shoots, especially in summer. Therefore, these two management options are not long–term solutions and will also be costly due to the widespread occurrence of the hybrids in South Africa. Thus the only cost–effective, environmentally friendly, self–sustainable and long–term management option is biological control. The significant divergence between native and invasive populations of N. mexicana, as well as the possibility of numerous invasive cultivars, may limit future prospects of biological control of this species. However the differences in the root structures between native South African waterlilies, such as N. lotus and N. nouchali, and the introduced waterlilies, such as N. mexicana and its associated hybrids, may play a pivotal role in the success of biological control of the N. mexicana hybrid complex in South Africa. Natural enemies which feed on the hard tuberous roots of N. mexicana and its hybrids, as opposed to the soft bulbs of the native N. nouchali and N. lotus, should be prioritised.
- Full Text:
- Date Issued: 2019
Elevated CO2 determines cell damage and nitrogen allocation in barley subjected to aphid herbivory
- Authors: Gallagher, Sean
- Date: 2018
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/64337 , vital:28535
- Description: Expected release date-May 2019
- Full Text:
- Date Issued: 2018
- Authors: Gallagher, Sean
- Date: 2018
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/64337 , vital:28535
- Description: Expected release date-May 2019
- Full Text:
- Date Issued: 2018
Studies in leaf domatia-mite mutualism in South Africa
- Authors: Situngu, Sivuyisiwe
- Date: 2018
- Subjects: Insect-plant relationships , Mites , Mutualism (Biology) , Biological pest control agents
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/63334 , vital:28394
- Description: Plants have various traits which allow them to cope and resist their enemies including both insects and fungi . In some cases such traits allow plants to build mutualistic relationships with natural enemies of plant pests. This is the case in many dicotyledonous plants which produce leaf domatia. Leaf domatia are plant cavities usually found in the axils of major veins in the abaxial side of leaves. They are usually associated with mites and often mediate mutualistic relationships with predacious mites. Mites use leaf domatia primarily for shelter, to reproduce, and to develop. In turn, plants benefit from having predaceous mites on their leaves, because mites act as plant “bodyguards” and offer defence against pathogens and small arthropod herbivores. This phenomenon has been well documented all over the world, but Africa remains disproportionally understudied. The aim of this study was to fill the gap that exists in our knowledge of the extent of the distribution of leaf domatia-mite mutualisms and generate a better understanding of the diversity of mites found within leaf domatia from an African perspective. This was done by surveying plant species that bear leaf domatia from different vegetation types in South Africa. The plants with leaf domatia were examined for the presence of mites in order to determine patterns of mite abundance and diversity and, in so doing, address the following questions: • Does each tree species host have a specific mite or mite assemblage? • Do some mites prefer certain types of leaf domatia? • Do mites prefer a specific place in the tree canopy and does the microclimate in the tree canopy affect the distribution of mites? • Do different vegetation sites and types differ in their mite diversity and species composition? • Does mite abundance and diversity vary with seasons? Do coffee plantations have a different suite of mites than the adjacent forest? The anatomical structures of leaf domatia from six selected plant species(Coffea arabica, Gardenia thunbergia, Rothmannia capensis, Rothmannia globosa (Rubiaceae), Ocotea bullata (Lauraceae) and Tecoma capensis (Bignoniaceae) with different types of leaf domatia were also studied. The results from this study suggested that the key futures which distinguish domatia are the presence of an extra layer of tissue in the lower epidermis, a thick cuticle, cuticular folds, the presence of trichomes and an invagination. This study provides a better understating of the structure of leaf domatia. Leaf domatia bearing plants are widely distributed in South Africa, and species and vegetation-specific associations were assessed. Over 250 plant specimens with leaf domatia were collected and examined and more than 60 different mite species were found in association with the sampled plant species. The majority of mites found within the domatia of these tree species were predaceous and included mites from Stigmatidae, Tydeidae and Phytoseiidae. Furthermore, 15 new species were collected, suggesting that mites are understudied in South Africa. This study showed that the different vegetation types sampled did not differ markedly in terms of their mite biota and that similar mites were found across the region, and the association between leaf domatia and mites was found to be opportunistic and that mites had no preference for any particular domatia types. No host specificity relationship was observed between plants and mites. The assessment of mites associated with Coffea arabica showed that indigenous mites are able to colonise and establish a beneficial mutualism on exotic species. This is important as it ascertains that economically important plants that are cultivated outside their area of natural distribution can still benefit from this mutualism. This study also found that mite abundance and diversity in plants with leaf domatia were influenced by factors such as temperature, relative humidity and rainfall. Mite communities found in association with domatia changed as the year progressed and over the seasons. The seasonal fluctuations varied between the sampled plant species. In addition, this study found that mites were sensitive to extreme environmental conditions, and thus, mites preferred leaves found in the lower parts of the tree canopy and avoided exposed leaves. This study provides a better understanding of the distribution of domatia bearing plants in South Africa and their associated mites and contributes to our knowledge of the biodiversity of mites in the region. Furthermore, this study also adds to our understanding of the leaf domatia - mite mutualism in Africa. The applied example looking at the plant-mite mutualism in Coffea arabica highlights the importance of this mutualism in commercial plants.
- Full Text:
- Date Issued: 2018
- Authors: Situngu, Sivuyisiwe
- Date: 2018
- Subjects: Insect-plant relationships , Mites , Mutualism (Biology) , Biological pest control agents
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/63334 , vital:28394
- Description: Plants have various traits which allow them to cope and resist their enemies including both insects and fungi . In some cases such traits allow plants to build mutualistic relationships with natural enemies of plant pests. This is the case in many dicotyledonous plants which produce leaf domatia. Leaf domatia are plant cavities usually found in the axils of major veins in the abaxial side of leaves. They are usually associated with mites and often mediate mutualistic relationships with predacious mites. Mites use leaf domatia primarily for shelter, to reproduce, and to develop. In turn, plants benefit from having predaceous mites on their leaves, because mites act as plant “bodyguards” and offer defence against pathogens and small arthropod herbivores. This phenomenon has been well documented all over the world, but Africa remains disproportionally understudied. The aim of this study was to fill the gap that exists in our knowledge of the extent of the distribution of leaf domatia-mite mutualisms and generate a better understanding of the diversity of mites found within leaf domatia from an African perspective. This was done by surveying plant species that bear leaf domatia from different vegetation types in South Africa. The plants with leaf domatia were examined for the presence of mites in order to determine patterns of mite abundance and diversity and, in so doing, address the following questions: • Does each tree species host have a specific mite or mite assemblage? • Do some mites prefer certain types of leaf domatia? • Do mites prefer a specific place in the tree canopy and does the microclimate in the tree canopy affect the distribution of mites? • Do different vegetation sites and types differ in their mite diversity and species composition? • Does mite abundance and diversity vary with seasons? Do coffee plantations have a different suite of mites than the adjacent forest? The anatomical structures of leaf domatia from six selected plant species(Coffea arabica, Gardenia thunbergia, Rothmannia capensis, Rothmannia globosa (Rubiaceae), Ocotea bullata (Lauraceae) and Tecoma capensis (Bignoniaceae) with different types of leaf domatia were also studied. The results from this study suggested that the key futures which distinguish domatia are the presence of an extra layer of tissue in the lower epidermis, a thick cuticle, cuticular folds, the presence of trichomes and an invagination. This study provides a better understating of the structure of leaf domatia. Leaf domatia bearing plants are widely distributed in South Africa, and species and vegetation-specific associations were assessed. Over 250 plant specimens with leaf domatia were collected and examined and more than 60 different mite species were found in association with the sampled plant species. The majority of mites found within the domatia of these tree species were predaceous and included mites from Stigmatidae, Tydeidae and Phytoseiidae. Furthermore, 15 new species were collected, suggesting that mites are understudied in South Africa. This study showed that the different vegetation types sampled did not differ markedly in terms of their mite biota and that similar mites were found across the region, and the association between leaf domatia and mites was found to be opportunistic and that mites had no preference for any particular domatia types. No host specificity relationship was observed between plants and mites. The assessment of mites associated with Coffea arabica showed that indigenous mites are able to colonise and establish a beneficial mutualism on exotic species. This is important as it ascertains that economically important plants that are cultivated outside their area of natural distribution can still benefit from this mutualism. This study also found that mite abundance and diversity in plants with leaf domatia were influenced by factors such as temperature, relative humidity and rainfall. Mite communities found in association with domatia changed as the year progressed and over the seasons. The seasonal fluctuations varied between the sampled plant species. In addition, this study found that mites were sensitive to extreme environmental conditions, and thus, mites preferred leaves found in the lower parts of the tree canopy and avoided exposed leaves. This study provides a better understanding of the distribution of domatia bearing plants in South Africa and their associated mites and contributes to our knowledge of the biodiversity of mites in the region. Furthermore, this study also adds to our understanding of the leaf domatia - mite mutualism in Africa. The applied example looking at the plant-mite mutualism in Coffea arabica highlights the importance of this mutualism in commercial plants.
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- Date Issued: 2018