Managing the invasive aquatic plant Sagittaria platyphylla (Engelm.) J.G. Sm(Alismataceae): problems and prospects
- Ndlovu, Mpilonhle Sinothando
- Authors: Ndlovu, Mpilonhle Sinothando
- Date: 2020
- Subjects: Aquatic weeds -- Biological control -- South Africa , Sagittaria latifolia -- Biological control -- South Africa , Noxious weeds -- Biological control -- South Africa , Invasive plants -- Biological control -- South Africa , Listronotus , Insects as biological pest control agents
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/167121 , vital:41439
- Description: Sagittaria platyphylla (Engelm.) J.G.Sm. (Alismataceae), commonly known as Delta arrowhead, is an invasive aquatic macrophyte native to southern United States of America (USA) that has become a serious weed in freshwater systems in South Africa, New Zealand, Australia, and recently China. In South Africa, the plant was first detected in Krantzkloof Nature Reserve, KwaZulu-Natal Province in 2008, and due to its known impact in other countries, it was listed as a Category 1a invader species under the National Environmental Management: Biodiversity Act 2004 (NEM: BA). This listing required mechanical and chemical control methods to be implemented by the South African National Biodiversity Institute’s (SANBI), Invasive Species Programme (ISP), with the aim of eradicating the weed. Despite the eradication efforts, by 2016, the weed was recognized as one of the country’s top 10 worst and fastest spreading invasive alien plants. Since its introduction in 2008, the plant has spread both within and between sites in South Africa, increasing from one site in 2008 to 72 sites by 2019. Once introduced into lotic systems, the plant spread rapidly downstream, in some cases up to 120km within six years, with an average of 10 km per year. Extirpation over the last ten years was only possible at a limited number of sites. Under the current management approach, the invasion is foreseen to spread to new sites within a 5 km radius of the current populations. Due to the failure of conventional control mechanisms, biological control is currently being considered as a potential control option. Four potential biological control agents are under investigation, but none have been released. Amongst them is the fruit and flower feeding weevil Listronotus appendiculatus Bohm. (Coleoptera: Curculionidae) which showed most potential as a suitable biological control agent. This study demonstrated that L. appendiculatus herbivory negatively influenced the overall fitness of S. platyphylla by reducing the plant’s growth rate and above ground biomass. Listronotus appendiculatus herbivory also reduced the plant’s size and the potential to kill adult plants. Most importantly, L. appendiculatus larval feeding damage significantly reduce viable-germinating seeds, the weed’s primary dispersal mechanism. Therefore, a biological control programme is advised to be integrated within the current management plan.
- Full Text:
- Date Issued: 2020
- Authors: Ndlovu, Mpilonhle Sinothando
- Date: 2020
- Subjects: Aquatic weeds -- Biological control -- South Africa , Sagittaria latifolia -- Biological control -- South Africa , Noxious weeds -- Biological control -- South Africa , Invasive plants -- Biological control -- South Africa , Listronotus , Insects as biological pest control agents
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/167121 , vital:41439
- Description: Sagittaria platyphylla (Engelm.) J.G.Sm. (Alismataceae), commonly known as Delta arrowhead, is an invasive aquatic macrophyte native to southern United States of America (USA) that has become a serious weed in freshwater systems in South Africa, New Zealand, Australia, and recently China. In South Africa, the plant was first detected in Krantzkloof Nature Reserve, KwaZulu-Natal Province in 2008, and due to its known impact in other countries, it was listed as a Category 1a invader species under the National Environmental Management: Biodiversity Act 2004 (NEM: BA). This listing required mechanical and chemical control methods to be implemented by the South African National Biodiversity Institute’s (SANBI), Invasive Species Programme (ISP), with the aim of eradicating the weed. Despite the eradication efforts, by 2016, the weed was recognized as one of the country’s top 10 worst and fastest spreading invasive alien plants. Since its introduction in 2008, the plant has spread both within and between sites in South Africa, increasing from one site in 2008 to 72 sites by 2019. Once introduced into lotic systems, the plant spread rapidly downstream, in some cases up to 120km within six years, with an average of 10 km per year. Extirpation over the last ten years was only possible at a limited number of sites. Under the current management approach, the invasion is foreseen to spread to new sites within a 5 km radius of the current populations. Due to the failure of conventional control mechanisms, biological control is currently being considered as a potential control option. Four potential biological control agents are under investigation, but none have been released. Amongst them is the fruit and flower feeding weevil Listronotus appendiculatus Bohm. (Coleoptera: Curculionidae) which showed most potential as a suitable biological control agent. This study demonstrated that L. appendiculatus herbivory negatively influenced the overall fitness of S. platyphylla by reducing the plant’s growth rate and above ground biomass. Listronotus appendiculatus herbivory also reduced the plant’s size and the potential to kill adult plants. Most importantly, L. appendiculatus larval feeding damage significantly reduce viable-germinating seeds, the weed’s primary dispersal mechanism. Therefore, a biological control programme is advised to be integrated within the current management plan.
- Full Text:
- Date Issued: 2020
Quantifying ecosystem restoration recovery and restoration practice following the biological control of invasive alien macrophytes in Southern Africa
- Authors: Motitsoe, Samuel Nkopane
- Date: 2020
- Subjects: Salvinia molesta , Ceratophyllum demersum , Nymphaea mexicana , Invasive plants -- Biological control -- South Africa , Aquatic weeds -- Biological control -- South Africa , Restoration monitoring (Ecology) -- South Africa , Biolotical invasions -- Environmental aspects
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/167519 , vital:41488
- Description: Invasive alien aquatic plants (IAAP) species are known to have deleterious effects on the freshwater ecosystems they invade. This includes both socio-economic and ecologically important ecosystem goods and services. Thus, IAAP species are declared a serious threat, second only to habitat modification for causing a loss of aquatic biodiversity. Three control methods have been widely applied to control IAAP species invasion globally; mechanical, chemical and biological control. Both mechanical and chemical control methods are considered short-term and expensive, whereas biological control methods are regarded an effective and long-term solution for IAAP species control at the landscape level. But, little is known of the ecological recovery following the biological control of IAAP species, with mechanical control known to have had mixed success and chemical control to have non-targeted effects on aquatic ecosystems, causing harm to wildlife and human well-being. Biological control practitioners measure the success of biological control based on: (1) the biological control agents’ establishment and the negative impacts they impose on the targeted weed; and (2) the weeds biomass reduction and an increase in native macrophytes species. Arguably, measures of biological control success have been subjective and variable across the globe. Although some field studies have demonstrated biological control success to have positive socio-economic returns, there is little literature on ecological benefits. Furthermore, there is limited understanding on ecosystem recovery and possible restoration efforts following the biological control IAAP species, as compared to alien weeds in terrestrial and riparian ecosystems. Thus, this thesis aimed to quantify the ecological recovery i.e. aquatic biodiversity, ecosystem processes and trophic interactions following the management of Salvinia molesta in freshwater ecosystems. The research employed a suite of Before-After Control-Impact mesocosm and field studies to investigate the response of aquatic microalgae, macroinvertebrates and their interactions (food web structure and function) during S. molesta infestation and after mechanical and biological control. The mesocosm experiment (Before invasion, During invasion & After control) showed that both aquatic microalgae and macroinvertebrate diversity indices were reliable biological indicators of S. molesta ecological impacts and recovery following control. The restored treatment (100% S. molesta cover + biological control agents), demonstrated complete aquatic microalgae and macroinvertebrate recovery following biological control, similar to the control treatment (open water), where the degraded/impacted treatment (100% S. molesta cover with no biological control agents) showed a drastic decline in aquatic biodiversity and a complete shift in aquatic biota assemblage structure. Thus, the biological control effort by Cyrtobagous salviniae, the biological control agent for S. molesta, assisted in the recovery of aquatic biota following successful biological control. The field study (four field sites, two sites controlled mechanically and two biologically) investigated water quality, aquatic biodiversity and community trophic interactions (aquatic food web) “before and after” S. molesta control. The study showed a drastic decline in aquatic biodiversity (with three sites showing no record of aquatic macroinvertebrates, thus no biotic interactions during infestation) and poor water quality due to the shade-effect (light barrier due to floating S. molesta mats on the water surface) during the “before” S. molesta control phase. However, following both mechanical and biological control (“after” S. molesta control phase), there was a significant shift in abiotic and biotic ecosystem characteristics as compared to the “before” S. molesta control phase. Thus, rapid ecosystem recovery was apparent as a result of aquatic microalgae and macroinvertebrates recolonisation. Sites showed a normal functioning ecosystem where improved water quality, increased biodiversity, productivity and trophic interactions, was indicative of the return of biologically and functionally important species which were lost during the “before” S. molesta phase. Although the clear water state showed positive outcomes at Westlake River, these were short lived when the system was dominated by a cosmopolitan submerged Ceratophyllum demersum, and later replaced by a floating-leaved emergent IAAP Nymphaea mexicana. Each state was responsible for a significant shift in both biotic and abiotic characteristics, affirming macrophyte abilities to influence aquatic environments structure and functions. Furthermore, this event showed a clear example of a secondary invasion. Thus, a holistic IAAP species management strategy is necessary to restore previously invaded ecosystems and prevent subsequent secondary invasion and ecosystem degradation. In conclusion, the S. molesta shade-effect like any other free-floating IAAP species, was identified as the main degrading factor and responsible for water quality reduction, loss of aquatic diversity and shift in aquatic biota assemblage structure. Following S. molesta removal (or shade-effect elimination), there was a positive response to aquatic ecosystem species abundance, richness, diversity and community structure. Therefore, in combination, aquatic biota recolonisation rate and increases in biological and functional diversity were instrumental in the recovery of ecosystem structure and functions, following the control of S. molesta. Echoing existing literature, this thesis recommends: (1) IAAP species management programmes (mechanical and/or biological control) should not only aim to control the weed but also focus on ecosystems recovery and possible restoration goals; (2) biological control should be used where appropriate to combat free-floating IAAP species in freshwater ecosystems, followed by active introduction of native macrophyte propagules since they are limited by anthropogenic activities; and (3) more freshwater case studies are needed to add to our understanding of IAAP species management and restoration effort incorporating long-term monitoring.
- Full Text:
- Date Issued: 2020
- Authors: Motitsoe, Samuel Nkopane
- Date: 2020
- Subjects: Salvinia molesta , Ceratophyllum demersum , Nymphaea mexicana , Invasive plants -- Biological control -- South Africa , Aquatic weeds -- Biological control -- South Africa , Restoration monitoring (Ecology) -- South Africa , Biolotical invasions -- Environmental aspects
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/167519 , vital:41488
- Description: Invasive alien aquatic plants (IAAP) species are known to have deleterious effects on the freshwater ecosystems they invade. This includes both socio-economic and ecologically important ecosystem goods and services. Thus, IAAP species are declared a serious threat, second only to habitat modification for causing a loss of aquatic biodiversity. Three control methods have been widely applied to control IAAP species invasion globally; mechanical, chemical and biological control. Both mechanical and chemical control methods are considered short-term and expensive, whereas biological control methods are regarded an effective and long-term solution for IAAP species control at the landscape level. But, little is known of the ecological recovery following the biological control of IAAP species, with mechanical control known to have had mixed success and chemical control to have non-targeted effects on aquatic ecosystems, causing harm to wildlife and human well-being. Biological control practitioners measure the success of biological control based on: (1) the biological control agents’ establishment and the negative impacts they impose on the targeted weed; and (2) the weeds biomass reduction and an increase in native macrophytes species. Arguably, measures of biological control success have been subjective and variable across the globe. Although some field studies have demonstrated biological control success to have positive socio-economic returns, there is little literature on ecological benefits. Furthermore, there is limited understanding on ecosystem recovery and possible restoration efforts following the biological control IAAP species, as compared to alien weeds in terrestrial and riparian ecosystems. Thus, this thesis aimed to quantify the ecological recovery i.e. aquatic biodiversity, ecosystem processes and trophic interactions following the management of Salvinia molesta in freshwater ecosystems. The research employed a suite of Before-After Control-Impact mesocosm and field studies to investigate the response of aquatic microalgae, macroinvertebrates and their interactions (food web structure and function) during S. molesta infestation and after mechanical and biological control. The mesocosm experiment (Before invasion, During invasion & After control) showed that both aquatic microalgae and macroinvertebrate diversity indices were reliable biological indicators of S. molesta ecological impacts and recovery following control. The restored treatment (100% S. molesta cover + biological control agents), demonstrated complete aquatic microalgae and macroinvertebrate recovery following biological control, similar to the control treatment (open water), where the degraded/impacted treatment (100% S. molesta cover with no biological control agents) showed a drastic decline in aquatic biodiversity and a complete shift in aquatic biota assemblage structure. Thus, the biological control effort by Cyrtobagous salviniae, the biological control agent for S. molesta, assisted in the recovery of aquatic biota following successful biological control. The field study (four field sites, two sites controlled mechanically and two biologically) investigated water quality, aquatic biodiversity and community trophic interactions (aquatic food web) “before and after” S. molesta control. The study showed a drastic decline in aquatic biodiversity (with three sites showing no record of aquatic macroinvertebrates, thus no biotic interactions during infestation) and poor water quality due to the shade-effect (light barrier due to floating S. molesta mats on the water surface) during the “before” S. molesta control phase. However, following both mechanical and biological control (“after” S. molesta control phase), there was a significant shift in abiotic and biotic ecosystem characteristics as compared to the “before” S. molesta control phase. Thus, rapid ecosystem recovery was apparent as a result of aquatic microalgae and macroinvertebrates recolonisation. Sites showed a normal functioning ecosystem where improved water quality, increased biodiversity, productivity and trophic interactions, was indicative of the return of biologically and functionally important species which were lost during the “before” S. molesta phase. Although the clear water state showed positive outcomes at Westlake River, these were short lived when the system was dominated by a cosmopolitan submerged Ceratophyllum demersum, and later replaced by a floating-leaved emergent IAAP Nymphaea mexicana. Each state was responsible for a significant shift in both biotic and abiotic characteristics, affirming macrophyte abilities to influence aquatic environments structure and functions. Furthermore, this event showed a clear example of a secondary invasion. Thus, a holistic IAAP species management strategy is necessary to restore previously invaded ecosystems and prevent subsequent secondary invasion and ecosystem degradation. In conclusion, the S. molesta shade-effect like any other free-floating IAAP species, was identified as the main degrading factor and responsible for water quality reduction, loss of aquatic diversity and shift in aquatic biota assemblage structure. Following S. molesta removal (or shade-effect elimination), there was a positive response to aquatic ecosystem species abundance, richness, diversity and community structure. Therefore, in combination, aquatic biota recolonisation rate and increases in biological and functional diversity were instrumental in the recovery of ecosystem structure and functions, following the control of S. molesta. Echoing existing literature, this thesis recommends: (1) IAAP species management programmes (mechanical and/or biological control) should not only aim to control the weed but also focus on ecosystems recovery and possible restoration goals; (2) biological control should be used where appropriate to combat free-floating IAAP species in freshwater ecosystems, followed by active introduction of native macrophyte propagules since they are limited by anthropogenic activities; and (3) more freshwater case studies are needed to add to our understanding of IAAP species management and restoration effort incorporating long-term monitoring.
- 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
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