Terrestrial alien ferns (Polypodiophyta): a global assessment of traits associated with invasiveness and their distribution and status in South Africa
- Authors: Jones, Emily Joy
- Date: 2019
- Subjects: Biodiversity -- South Africa , Alien plants -- South Africa , Botany -- South Africa , Plant ecology -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/39995 , vital:35643
- Description: Globally, invasive alien plants (IAPs) are one of the greatest threats to biodiversity, ecosystems and ecosystem services. The threats posed by IAPs have prompted inventory compilations and screening exercises which aim to understand why some taxa become invasive and others do not. Ferns are a diverse taxon that appear to have a high invasive potential, however ferns have been vastly understudied in the field of invasion biology. This study aimed to establish a basic understanding of terrestrial alien true ferns (Polypodiophyta, hereafter ‘alien ferns’) at both global and national (South African) scales. We developed a global inventory of alien ferns comprising 157 species using published literature and online inventories. Our global inventory indicated that there are significantly more alien fern species than previously estimated (60 species). We used generalised linear models with binomial response variables to determine which traits influenced the probability that an alien fern would become naturalised or invasive. Our models explained 30-40 % of the variance associated with invasiveness and showed that ground-dwelling life forms, reproductive plasticity, tolerance to disturbances and varied light conditions, and a broad introduced range (interpreted as high environmental tolerance and popularity in horticulture) were important determinants of invasiveness in alien ferns. Ultimately, we found that the probability of alien ferns becoming naturalised or invasive is more than 50 %. We further established which geographic regions and fern families had the highest incidences of alien ferns and identified species that were invasive over extensive introduced ranges. For the national scale study, we aimed to assess the distribution, abundance, invasion status, and habitat associations of terrestrial alien fern species present outside of cultivation in South Africa. Field surveys were conducted across the country in habitats suitable for ferns, guided by pre-existing records of alien fern occurrences. Thirteen species occurred outside of cultivation, all of which were classified as invasive, and among these, approximately 5000 plants were recorded across 300 localities. Species richness and abundance peaked along the Indian Ocean Coastal Belt and the Afro-montane phytogeographical regions. A multiple correspondence analysis revealed that alien ferns in South Africa were most strongly associated with indigenous forest habitats and the majority of species selected for shaded conditions, close to water. Only two species were closely associated with open habitats under high light conditions, with plantations also infrequently selected by alien ferns. Habitat generalists were closely associated with disturbance, but avoided open habitats, far from water. Compared to the most prominent angiosperm invaders in South Africa, alien ferns have a lower potential for widespread ii invasion and range expansion, probably due to their habitat specificity. We identified four species as potential targets for eradication and which should be listed as Category 1a under the Alien and Invasive Species regulations of the National Environmental Management: Biodiversity Act of South Africa. The remaining species should be listed as Category 1b since they occupied an extensive geographic range. The most prominent ferns included Adiantum raddianum, Sphaeropteris cooperi, Deparia japonica and Cyrtomium falcatum. Our study informed appropriate regulation for several species of alien ferns that were deemed to be data deficient and in need of assessment by a recent report on the status of biological invasions in South Africa. A risk analysis completed for S. cooperi confirmed that the species has a high invasion risk in South Africa and should be listed as Category 1b. Given that the outcomes of the risk analysis completed for S. cooperi matched the findings from our field surveys, this study supported risk analysis as a reliable (literature-based) method to evaluate IAPs. The findings of our assessments at the global and national scale corresponded in that various traits that promoted invasiveness in alien ferns globally were also facilitating invasion in South Africa. Additionally, of the eleven species found to be invasive over extensive introduced ranges at global scale, only two (Lygodium microphyllum and Angiopteris evecta) were not yet recorded in South Africa. Since horticulture was established as the primary pathway for the introduction of alien ferns, the management of alien ferns (globally and in South Africa) should focus on regulating trade. The sources of cultivation and the pathways of dissemination across the globe also need consideration. Basic environmental and biological associations of alien ferns were evident in this study, but to fully understand their invasive potential further research should focus on niche-based modelling to predict potential invasion ranges, the effects of biological traits on invasiveness, and the potential ecological impacts of these species.
- Full Text:
- Date Issued: 2019
- Authors: Jones, Emily Joy
- Date: 2019
- Subjects: Biodiversity -- South Africa , Alien plants -- South Africa , Botany -- South Africa , Plant ecology -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/39995 , vital:35643
- Description: Globally, invasive alien plants (IAPs) are one of the greatest threats to biodiversity, ecosystems and ecosystem services. The threats posed by IAPs have prompted inventory compilations and screening exercises which aim to understand why some taxa become invasive and others do not. Ferns are a diverse taxon that appear to have a high invasive potential, however ferns have been vastly understudied in the field of invasion biology. This study aimed to establish a basic understanding of terrestrial alien true ferns (Polypodiophyta, hereafter ‘alien ferns’) at both global and national (South African) scales. We developed a global inventory of alien ferns comprising 157 species using published literature and online inventories. Our global inventory indicated that there are significantly more alien fern species than previously estimated (60 species). We used generalised linear models with binomial response variables to determine which traits influenced the probability that an alien fern would become naturalised or invasive. Our models explained 30-40 % of the variance associated with invasiveness and showed that ground-dwelling life forms, reproductive plasticity, tolerance to disturbances and varied light conditions, and a broad introduced range (interpreted as high environmental tolerance and popularity in horticulture) were important determinants of invasiveness in alien ferns. Ultimately, we found that the probability of alien ferns becoming naturalised or invasive is more than 50 %. We further established which geographic regions and fern families had the highest incidences of alien ferns and identified species that were invasive over extensive introduced ranges. For the national scale study, we aimed to assess the distribution, abundance, invasion status, and habitat associations of terrestrial alien fern species present outside of cultivation in South Africa. Field surveys were conducted across the country in habitats suitable for ferns, guided by pre-existing records of alien fern occurrences. Thirteen species occurred outside of cultivation, all of which were classified as invasive, and among these, approximately 5000 plants were recorded across 300 localities. Species richness and abundance peaked along the Indian Ocean Coastal Belt and the Afro-montane phytogeographical regions. A multiple correspondence analysis revealed that alien ferns in South Africa were most strongly associated with indigenous forest habitats and the majority of species selected for shaded conditions, close to water. Only two species were closely associated with open habitats under high light conditions, with plantations also infrequently selected by alien ferns. Habitat generalists were closely associated with disturbance, but avoided open habitats, far from water. Compared to the most prominent angiosperm invaders in South Africa, alien ferns have a lower potential for widespread ii invasion and range expansion, probably due to their habitat specificity. We identified four species as potential targets for eradication and which should be listed as Category 1a under the Alien and Invasive Species regulations of the National Environmental Management: Biodiversity Act of South Africa. The remaining species should be listed as Category 1b since they occupied an extensive geographic range. The most prominent ferns included Adiantum raddianum, Sphaeropteris cooperi, Deparia japonica and Cyrtomium falcatum. Our study informed appropriate regulation for several species of alien ferns that were deemed to be data deficient and in need of assessment by a recent report on the status of biological invasions in South Africa. A risk analysis completed for S. cooperi confirmed that the species has a high invasion risk in South Africa and should be listed as Category 1b. Given that the outcomes of the risk analysis completed for S. cooperi matched the findings from our field surveys, this study supported risk analysis as a reliable (literature-based) method to evaluate IAPs. The findings of our assessments at the global and national scale corresponded in that various traits that promoted invasiveness in alien ferns globally were also facilitating invasion in South Africa. Additionally, of the eleven species found to be invasive over extensive introduced ranges at global scale, only two (Lygodium microphyllum and Angiopteris evecta) were not yet recorded in South Africa. Since horticulture was established as the primary pathway for the introduction of alien ferns, the management of alien ferns (globally and in South Africa) should focus on regulating trade. The sources of cultivation and the pathways of dissemination across the globe also need consideration. Basic environmental and biological associations of alien ferns were evident in this study, but to fully understand their invasive potential further research should focus on niche-based modelling to predict potential invasion ranges, the effects of biological traits on invasiveness, and the potential ecological impacts of these species.
- Full Text:
- Date Issued: 2019
Strategic adaptive management and the efficiency of invasive alien plant management in South African national parks
- Authors: Loftus, Wynand Johan
- Date: 2013
- Subjects: Biodiversity -- South Africa -- Management , Alien plants -- South Africa , Invasive plants -- South Africa , Ecosystem management
- Language: English
- Type: Thesis , Masters , MTech
- Identifier: vital:10755 , http://hdl.handle.net/10948/d1020089
- Description: It is well known that invasive alien plants (IAPs) pose a significant threat to natural biodiversity and human well-being. Through various pressures exerted on natural ecosystems, IAPs decrease and alter natural processes that provide important ecosystems services and livelihoods to human communities. These plants also displace and out-compete natural plant communities in the areas in which they invade. Management of IAPs is a complex issue with social, environmental and financial challenges. The long-term sustainable management of IAPs requires a management approach that monitors and measures the outcomes of current management practices, and considers the lessons learnt in future decision making. Adaptive management is such an approach and is characterised by monitoring, assessment, reflection and adaptation. In this dissertation I explore IAP management within the South African National Parks (SANParks) organisation. I focus on the quality of the available IAP clearing data for monitoring and environmental decision-making and explore how strategic adaptive management (SAM) is being applied to IAP management. Strategic adaptive management is a form of adaptive management that SANParks adopted to deal with the inherent unpredictability of the social ecological systems they manage, and is primarily a learning-by-doing approach. In chapter one I describe the various impacts that IAPs have on natural environments. I also discuss management of IAPs, the national Working for Water (WfW) programme and how SANParks control IAPs in partnership with WfW. Clearing of IAPs within the borders of SANParks is funded by the Department of Environmental Affairs‘ (DEA) Natural Resource Management programmes and carried out through the Working for Water programme, under SANParks‘ Biodiversity Social Projects programme and is done on a contract basis. Contract data are stored in the Working for Water Information Management System (WIMS) database. Although IAP clearing is well established, the efficiency of overall clearing operations within the parks, and quality and reliability of the data available in the WIMS, needs to be examined. In Chapter two, I (i) assess the status of IAP clearing and the direct clearing costs in each of the five national parks (costs are inflated to 2012 equivalents using the consumer price index), (ii) assess the accuracy of workload estimations for WfW IAP clearing, and (iii) discuss the utility of WIMS as a system to collate and store accurate data for monitoring purposes, focusing on the integrity and reliability of the data, the accuracy of density estimations and whether it can inform sound decision making. The planning of future clearing contracts lacks efficiency due to inaccuracies and unreliability of input data, specifically estimates of invasive alien plant density. The accuracy of density estimation and cost of clearing varies greatly between parks and does not appear to reflect the reality on the ground. In Chapter 3, through an interview process with IAP managers, I 1) unpack the steps in the SAM ‗cycle‘ and investigate the effectiveness of implementation of each step according to respondents; 2) assess the barriers that are currently keeping managers from implementing effective IAP management; 3) discuss whether learning and adaptation is taking place, and 4) discuss how the SAM process can be modified to promote its effectiveness.
- Full Text:
- Date Issued: 2013
- Authors: Loftus, Wynand Johan
- Date: 2013
- Subjects: Biodiversity -- South Africa -- Management , Alien plants -- South Africa , Invasive plants -- South Africa , Ecosystem management
- Language: English
- Type: Thesis , Masters , MTech
- Identifier: vital:10755 , http://hdl.handle.net/10948/d1020089
- Description: It is well known that invasive alien plants (IAPs) pose a significant threat to natural biodiversity and human well-being. Through various pressures exerted on natural ecosystems, IAPs decrease and alter natural processes that provide important ecosystems services and livelihoods to human communities. These plants also displace and out-compete natural plant communities in the areas in which they invade. Management of IAPs is a complex issue with social, environmental and financial challenges. The long-term sustainable management of IAPs requires a management approach that monitors and measures the outcomes of current management practices, and considers the lessons learnt in future decision making. Adaptive management is such an approach and is characterised by monitoring, assessment, reflection and adaptation. In this dissertation I explore IAP management within the South African National Parks (SANParks) organisation. I focus on the quality of the available IAP clearing data for monitoring and environmental decision-making and explore how strategic adaptive management (SAM) is being applied to IAP management. Strategic adaptive management is a form of adaptive management that SANParks adopted to deal with the inherent unpredictability of the social ecological systems they manage, and is primarily a learning-by-doing approach. In chapter one I describe the various impacts that IAPs have on natural environments. I also discuss management of IAPs, the national Working for Water (WfW) programme and how SANParks control IAPs in partnership with WfW. Clearing of IAPs within the borders of SANParks is funded by the Department of Environmental Affairs‘ (DEA) Natural Resource Management programmes and carried out through the Working for Water programme, under SANParks‘ Biodiversity Social Projects programme and is done on a contract basis. Contract data are stored in the Working for Water Information Management System (WIMS) database. Although IAP clearing is well established, the efficiency of overall clearing operations within the parks, and quality and reliability of the data available in the WIMS, needs to be examined. In Chapter two, I (i) assess the status of IAP clearing and the direct clearing costs in each of the five national parks (costs are inflated to 2012 equivalents using the consumer price index), (ii) assess the accuracy of workload estimations for WfW IAP clearing, and (iii) discuss the utility of WIMS as a system to collate and store accurate data for monitoring purposes, focusing on the integrity and reliability of the data, the accuracy of density estimations and whether it can inform sound decision making. The planning of future clearing contracts lacks efficiency due to inaccuracies and unreliability of input data, specifically estimates of invasive alien plant density. The accuracy of density estimation and cost of clearing varies greatly between parks and does not appear to reflect the reality on the ground. In Chapter 3, through an interview process with IAP managers, I 1) unpack the steps in the SAM ‗cycle‘ and investigate the effectiveness of implementation of each step according to respondents; 2) assess the barriers that are currently keeping managers from implementing effective IAP management; 3) discuss whether learning and adaptation is taking place, and 4) discuss how the SAM process can be modified to promote its effectiveness.
- Full Text:
- Date Issued: 2013
Estimating the willingness-to-pay for restoring indigenous vegetation at selected sites in South Africa
- Authors: Tessendorf, Sharon Erica
- Date: 2007
- Subjects: Water resources development -- South Africa , Water-supply -- South Africa , Alien plants -- South Africa , Restoration ecology -- South Africa
- Language: English
- Type: Thesis , Masters , MCom
- Identifier: vital:8999 , http://hdl.handle.net/10948/617 , Water resources development -- South Africa , Water-supply -- South Africa , Alien plants -- South Africa , Restoration ecology -- South Africa
- Description: The Working for Water (WfW) Programme is a public works programme designed to clear South Africa of invasive alien vegetation and to restore lowwater consuming indigenous vegetation in the areas that have been cleared. Funds to clear alien invasives were initially secured on the basis that such a programme would increase water runoff, facilitate biodiversity and ecosystem functioning, and provide social benefits through job creation. The economic merits of the Programme, in terms of increased water yields, has been established in the Western Cape and KwaZulu-Natal, but questioned in the Eastern and Southern Cape. However, there are economic aspects of the studies carried out in the Eastern and Southern Cape that merit more attention than was given them; one of these being the issue of non-water benefits. Preliminary figures emanating from contingent valuation pilot studies conducted at six WfW projects sites indicated that one of these non-water benefits, namely the biodiversity and ecosystem resilience benefit, could be substantial. As such, the primary objective of the present study was to apply the contingent valuation method (CVM) to value people’s preference for indigenous vegetation. This value was intended to serve as a proxy for increased biodiversity and ecosystem resilience at three WfW sites. Despite the controversy surrounding the CVM, it has been found that it is a credible valuation tool. The CVM’s merits lie in its versatility and in the fact that it is the only method available which is capable of obtaining estimates of both nonuse and use values, thus making it applicable for valuing biodiversity. The primary aim of a CVM study is to determine an estimate of the total willingness-to-pay (WTP). In this study, the total WTP figure was calculated by multiplying the median WTP for the local WfW Programme by the total number of user households. The respective total WTP amounts are shown in Table 1. It was anticipated that respondents would be willing to pay more for the national WfW Programme, than for the less inclusive good (i.e. the local WfW Programme). The results correspond with this expectation at the Port Elizabeth and Underberg sites. However, due to strategic factors Worcester respondents were willing to pay more for the local WfW Programme than for the national Programme.
- Full Text:
- Date Issued: 2007
- Authors: Tessendorf, Sharon Erica
- Date: 2007
- Subjects: Water resources development -- South Africa , Water-supply -- South Africa , Alien plants -- South Africa , Restoration ecology -- South Africa
- Language: English
- Type: Thesis , Masters , MCom
- Identifier: vital:8999 , http://hdl.handle.net/10948/617 , Water resources development -- South Africa , Water-supply -- South Africa , Alien plants -- South Africa , Restoration ecology -- South Africa
- Description: The Working for Water (WfW) Programme is a public works programme designed to clear South Africa of invasive alien vegetation and to restore lowwater consuming indigenous vegetation in the areas that have been cleared. Funds to clear alien invasives were initially secured on the basis that such a programme would increase water runoff, facilitate biodiversity and ecosystem functioning, and provide social benefits through job creation. The economic merits of the Programme, in terms of increased water yields, has been established in the Western Cape and KwaZulu-Natal, but questioned in the Eastern and Southern Cape. However, there are economic aspects of the studies carried out in the Eastern and Southern Cape that merit more attention than was given them; one of these being the issue of non-water benefits. Preliminary figures emanating from contingent valuation pilot studies conducted at six WfW projects sites indicated that one of these non-water benefits, namely the biodiversity and ecosystem resilience benefit, could be substantial. As such, the primary objective of the present study was to apply the contingent valuation method (CVM) to value people’s preference for indigenous vegetation. This value was intended to serve as a proxy for increased biodiversity and ecosystem resilience at three WfW sites. Despite the controversy surrounding the CVM, it has been found that it is a credible valuation tool. The CVM’s merits lie in its versatility and in the fact that it is the only method available which is capable of obtaining estimates of both nonuse and use values, thus making it applicable for valuing biodiversity. The primary aim of a CVM study is to determine an estimate of the total willingness-to-pay (WTP). In this study, the total WTP figure was calculated by multiplying the median WTP for the local WfW Programme by the total number of user households. The respective total WTP amounts are shown in Table 1. It was anticipated that respondents would be willing to pay more for the national WfW Programme, than for the less inclusive good (i.e. the local WfW Programme). The results correspond with this expectation at the Port Elizabeth and Underberg sites. However, due to strategic factors Worcester respondents were willing to pay more for the local WfW Programme than for the national Programme.
- Full Text:
- Date Issued: 2007
An economic analysis of eradicating alien vegetation as an alternative to conventional water supply schemes: a case study of the Krom and Kouga
- Authors: Carpenter, Robert Charles
- Date: 1999
- Subjects: Alien plants -- South Africa , Water-supply -- South Africa , Kouga River (South Africa)
- Language: English
- Type: Thesis , Masters , MCom
- Identifier: vital:1000 , http://hdl.handle.net/10962/d1002735 , Alien plants -- South Africa , Water-supply -- South Africa , Kouga River (South Africa)
- Description: South Africa is classified as an arid to semi-arid region and water scarcity in South Africa has been identified as a key factor limiting socioeconomic development in the next century. In the Algoa region, the total urban plus agricultural water demand is expected to exceed the supply by the year 2005. The Kouga Working for Water Project aims to increase the base flow to the existing dams which supply the Port Elizabeth metropolitan area through the eradication of invasive alien trees in the riparian areas of the Krom and Kouga catchments. This thesis analyses the economic efficiency of optimal catchment management as a water supply scheme. A cost-benefit analysis is conducted for the Kouga eradication programme, and its desirability is evaluated in terms of the net present value (NPV) and the internal rate of return (IRR) criteria. In order to compare the cost-e~i.ciency of the eradication programme to alternative water supply augmentation schemes the Unit Reference Value (URV) is calculated for the project. The NPV for the project is calculated using a discount rate of 7% and amounts to nearly R24 million. This positive NPV indicates that the project is economically desirable in that it results in the improvement of human welfare. The IRR decision rule supports this finding. The URV of the eradication scheme is found to be competitive to that generated by more conventional schemes. Considered in the evaluation of the project are a host of environmental benefits that accompany the eradication of alien vegetation. This is in contrast to alternative schemes which result in several detrimental impacts to the environment. The economic analysis concludes that the eradication of alien vegetation is an efficient and desirable alternative water supply augmentation scheme. The conclusions drawn from the analysis of the eradication programme in this catchment area can be extended to other catchment areas, with the aim of promoting the most efficient supply of water.
- Full Text:
- Date Issued: 1999
- Authors: Carpenter, Robert Charles
- Date: 1999
- Subjects: Alien plants -- South Africa , Water-supply -- South Africa , Kouga River (South Africa)
- Language: English
- Type: Thesis , Masters , MCom
- Identifier: vital:1000 , http://hdl.handle.net/10962/d1002735 , Alien plants -- South Africa , Water-supply -- South Africa , Kouga River (South Africa)
- Description: South Africa is classified as an arid to semi-arid region and water scarcity in South Africa has been identified as a key factor limiting socioeconomic development in the next century. In the Algoa region, the total urban plus agricultural water demand is expected to exceed the supply by the year 2005. The Kouga Working for Water Project aims to increase the base flow to the existing dams which supply the Port Elizabeth metropolitan area through the eradication of invasive alien trees in the riparian areas of the Krom and Kouga catchments. This thesis analyses the economic efficiency of optimal catchment management as a water supply scheme. A cost-benefit analysis is conducted for the Kouga eradication programme, and its desirability is evaluated in terms of the net present value (NPV) and the internal rate of return (IRR) criteria. In order to compare the cost-e~i.ciency of the eradication programme to alternative water supply augmentation schemes the Unit Reference Value (URV) is calculated for the project. The NPV for the project is calculated using a discount rate of 7% and amounts to nearly R24 million. This positive NPV indicates that the project is economically desirable in that it results in the improvement of human welfare. The IRR decision rule supports this finding. The URV of the eradication scheme is found to be competitive to that generated by more conventional schemes. Considered in the evaluation of the project are a host of environmental benefits that accompany the eradication of alien vegetation. This is in contrast to alternative schemes which result in several detrimental impacts to the environment. The economic analysis concludes that the eradication of alien vegetation is an efficient and desirable alternative water supply augmentation scheme. The conclusions drawn from the analysis of the eradication programme in this catchment area can be extended to other catchment areas, with the aim of promoting the most efficient supply of water.
- Full Text:
- Date Issued: 1999
A predictive biogeography of selected alien plant invaders in South Africa
- Authors: Youthed, Jennifer Gay
- Date: 1997
- Subjects: Alien plants -- South Africa , Biogeography -- South Africa , Acacia -- South Africa , Acacia mearnsii -- South Africa , Opuntia ficus-indica -- South Africa , Solanum -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4846 , http://hdl.handle.net/10962/d1005522 , Alien plants -- South Africa , Biogeography -- South Africa , Acacia -- South Africa , Acacia mearnsii -- South Africa , Opuntia ficus-indica -- South Africa , Solanum -- South Africa
- Description: Five techniques were used to predict the potential biogeography of the four alien plant species, Acacia longifolia, Acacia mearnsii, Opuntia ficus-indica and Solanum sisymbrifolium. Prediction was based on five environmental factors, median annual rainfall, co-efficient of variation for rainfall, mean monthly maximum temperature for January, mean monthly minimum temperature for July and elevation. A geographical information system was used to manage the data and produce the predictive maps. The models were constructed with presence and absence data and then validated by means of an independent data set and chisquared tests. Of the five models used, three (the range, principal components analysis and discriminant function analysis) were linear while the other two (artificial neural networks and fuzzy logic) were non-linear. The two non-linear techniques were chosen as a plant's response to its environment is commonly assumed to be non-linear. However, these two techniques did not offer significant advantages over the linear methods. The principal components analysis was particularly useful in ascertaining the variables that were important in determining the distribution of each species. Artifacts on the predictive maps were also proved useful for this purpose. The techniques that produced the most statistically accurate validation results were the artificial neural networks (77% correct median prediction rate) and the discriminant function analysis (71% correct median prediction rate) while the techniques that performed the worst were the range and the fuzzy classification. The artificial neural network, discriminant function analysis and principal component analysis techniques all show great potential as predictive distribution models.
- Full Text:
- Date Issued: 1997
- Authors: Youthed, Jennifer Gay
- Date: 1997
- Subjects: Alien plants -- South Africa , Biogeography -- South Africa , Acacia -- South Africa , Acacia mearnsii -- South Africa , Opuntia ficus-indica -- South Africa , Solanum -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4846 , http://hdl.handle.net/10962/d1005522 , Alien plants -- South Africa , Biogeography -- South Africa , Acacia -- South Africa , Acacia mearnsii -- South Africa , Opuntia ficus-indica -- South Africa , Solanum -- South Africa
- Description: Five techniques were used to predict the potential biogeography of the four alien plant species, Acacia longifolia, Acacia mearnsii, Opuntia ficus-indica and Solanum sisymbrifolium. Prediction was based on five environmental factors, median annual rainfall, co-efficient of variation for rainfall, mean monthly maximum temperature for January, mean monthly minimum temperature for July and elevation. A geographical information system was used to manage the data and produce the predictive maps. The models were constructed with presence and absence data and then validated by means of an independent data set and chisquared tests. Of the five models used, three (the range, principal components analysis and discriminant function analysis) were linear while the other two (artificial neural networks and fuzzy logic) were non-linear. The two non-linear techniques were chosen as a plant's response to its environment is commonly assumed to be non-linear. However, these two techniques did not offer significant advantages over the linear methods. The principal components analysis was particularly useful in ascertaining the variables that were important in determining the distribution of each species. Artifacts on the predictive maps were also proved useful for this purpose. The techniques that produced the most statistically accurate validation results were the artificial neural networks (77% correct median prediction rate) and the discriminant function analysis (71% correct median prediction rate) while the techniques that performed the worst were the range and the fuzzy classification. The artificial neural network, discriminant function analysis and principal component analysis techniques all show great potential as predictive distribution models.
- Full Text:
- Date Issued: 1997
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