Determining the trajectory of graminoid invasions in Southern Africa's mountains: The case of Nassella
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Date
2022
Authors
Mapaura, Anthony
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Nassella neesiana (Trin. & Rupr.) Barkworth, N. tenuissima (Trin.) Barkworth and N. trichotoma (Nees) Hack. & Arechav are three alien grass species that have become naturalised in South African montane grassland – mostly known historically from remote mountainous areas in the Eastern Cape. Currently, only N. trichotoma and N. tenuissima are recognised as serious invasive species in category 1b of the National Environmental Management: Biodiversity Act (NEM:BA, 10/2004): Alien and Invasive Species (AIS) Regulations of 1 October 2014. However, N. neesiana, which is currently not listed in NEM:BA, is a prolific invader in Australia and New Zealand, where it is causing serious damage to both natural and planted areas, and costing thousands of Australian dollars in control measures and lost land potential. In South Africa, some research and central government-facilitated management measures were undertaken from the late 1970s to the year 2000. Since then, very little data have been generated about the status of any of these species in South Africa. Literature pointed out, inter alia, the difficulty of identifying the species in the field, especially when plants were not fertile; this may be a reason for sparse distribution records including limited records from citizen science projects that record invasive species occurrences. Chapters 1 and 2 provide a synthesis of global knowledge on Nassella available at the start of this project.
The principal aims of the research were to (i) establish the current invasive status of the three Nassella species in South Africa, (ii) determine the impacts of N. trichotoma and N. neesiana on native flora, and (iii) to predict how their invasive status might change in the future under projected climate change scenarios. A combination of fieldwork, laboratory analysis, and modelling were used to answer these questions.
The impact of N. trichotoma and N. neesiana on co-occurring plant species and chemical properties of the soil was investigated using field surveys and modelling techniques (Chapter 3 and 4). The research focused on vascular plant biodiversity differences between dense patches of Nassella and matched nearby uninvaded areas – space-for-time substitution. Nassella trichotoma was found to reduce the diversity of native species in montane rangelands while increasing the concentrations of minerals in topsoil. Nassella neesiana, on the other hand, has little impact on native plants in a disturbed urban setting. However, the presence of both Nassella species is associated with increased concentrations of minerals in topsoils and an increase in ruderal vascular plant species. This study is the first known research on N. neesiana in South Africa. The possibility of using DNA markers to distinguish the three Nassella species and separate them from co-occurring morphologically similar species was investigated (Chapter 5). Two candidate gene loci, petL–psaJ (petL) and ETS, were evaluated to determine their ability to discriminate the three Nassella species from each other and from three co-occurring native grass species (Aristida diffusa Trin., Festuca caprina Nees and Koeleria capensis (Steud.) Nees), and the exotic species Jarava plumosa (Spreng.) S.W.L. Jacobs & J. Everett in South Africa. The three native grass species have been confused with Nassella species before. Jarava plumosa and the three Nassella species used to be in the same genus, Stipa, before the recent taxonomic revisions. The other minor objective was to find out if this method can be used as a viable option for field practitioners. The results indicate that the petL locus has no power to discriminate the three species, but can discriminate the different genera; in contrast, ETS has sufficient power to discriminate all the tested species, and can be outsourced at a reasonable cost, making it a viable identification method even for people without technical knowledge.
The current and future (2081 to 2100 period) potential distribution of the three Nassella species was investigated (Chapter 6). The bioclimatic variables were downloaded from the WorldClim database (http://www.worldclim.org), at a spatial resolution of 2.5 arc minutes. The distribution of all three species is mainly driven by temperature-based bioclimatic variables, especially Isothermality. The research found that – under current climatic conditions – all three species can potentially occupy more area than they currently do. Compared to currently suitable envelopes, future projected envelopes will be much smaller for the three species. The Maloti-Drakensberg mountains (including Lesotho) will remain at high risk of invasion across all future climate-change scenarios, and it should be a priority for keeping N. trichotoma out of the Maloti-Drakensberg, along with managing N. tenuissima and N. neesiana invasions in the Maloti-Drakensberg.
The overall finding of this research is that the three Nassella species remain very high-risk species for continued invasion of montane grasslands in South Africa, particularly in the most important regional water tower: the Maloti-Drakensberg. Nassella neesiana should be formally placed on an invasive “watch-list”, for potential addition to the AIS Regulations (Category 1b), as it appears to have a much wider distribution and be much more widespread in the landscape than records show. Considering their impact on vascular plant diversity, rangeland quality and difficulty of management, they should be cause for major concern by landowners and government. This research provides a platform for resurrecting a research and management focus on Nassella in South Africa (and Lesotho), with the aims of securing montane rangeland-based livelihoods, ensure conservation of endemic mountain biodiversity, and safeguard mountain catchment potential under climate change.
Description
Thesis (Ph.D.(Botany))--University of the Free State, 2022
Keywords
Invasion, cryptic invader, climate change, DNA markers, sdm R package