Examining the spatiotemporal dynamics of exotic plants in a montane grassland : mechanisms and management

Turner, Sophia Carmel (2019-04)

Thesis (MScConEcol)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Despite a surge of research on exotic species in alpine habitats, a lack of reliable baseline data has inhibited long-term understanding about exotic species dynamics in mountain ecosystems across the world. A long-term study of species invasion in the Drakensberg region of South Africa provides an important exception. Making use of historical data collected in this system, vegetation surveys and online sources, this thesis investigates the underlying mechanisms resulting in exotic species establishment in mountain ecosystems, and whether prioritizing invasion introduction pathways is an optimal management strategy for the area. To investigate the change in exotic plant species richness and composition in and adjacent to a mountain pass road verge; and explore the role of the road verge in exotic species establishment, I complemented data collected ten years prior with a re-survey of road verge and adjacent transects in semi-natural habitat (N = 80; 25x2m) across an elevational gradient of 1500-2874 m a.s.l.. Along all transects, exotic species richness, exotic species cover, indigenous species cover and bare soil cover was estimated. Generalized Linear Models were fitted to test whether exotic richness, vegetation cover and bare soil had changed over time, and a Canonical Correspondence Analysis was used to estimate changes in exotic species composition. Since the initial survey, exotic species richness increased significantly across the entire elevational gradient, particularly in the mid-elevational zones. This distribution pattern indicates small-scale jump dispersal, which is likely driven by human-mediated activities, rather than gradual range expansion. Exotic species composition became more homogenous between road verge and semi-natural transects, showing that exotic species are spreading into natural habitat. It is likely that propagule pressure is key for colonization success, while disturbance in the road verges fosters spread both in elevation and expansion into the natural area. Further expansion of exotic species into the natural area can be expected. Invasive species management can be executed either through prioritizing species, sites or introduction pathways. Pathway management is particularly useful when propagule pressure is the dominant driver of invasion success. Knowing the important role of propagule pressure in this study system, I then investigated whether prioritizing introduction pathways would be an efficient approach to reduce exotic species richness and expansion. I did this by identifying the likely vectors and introduction pathways of exotic species along the Sani Pass, to see if successful exotic species made use of a specific vector or pathway. The likely introduction pathways of all the exotic species was categorized using information from online sources. Generalized linear models (GLMs) were used to test whether successful exotic species were associated with specific introduction pathways. Extent of exotic species’ presence in the natural area was used as a proxy for success. I also tested whether the number of pathways used by exotic species was related to their success, using GLMs. Successful species in the area do not utilise multiple introduction pathways, and only unintentional transport stowaways are significantly associated with presence in the natural area. These results show that successful species enter through vectors such as vehicles, people and livestock. Adopting stringent control of these vectors at the border posts will likely reduce the introduction of new exotic species in the area.

AFRIKAANSE OPSOMMING:Ten spyte van die groeiende navorsing in uitheemse spesies in hoë berg-alpiene gebiede, beperk die gebrek aan betroubare basislyndata die langtermyn begrip van uitheemse spesiesdinamika in berg-ekosisteme. 'N uitsondering is egter die langtermyn studie van indringerspesies in die Drakensberg streek van Suid-Afrika. Deur gebruik te maak van historiese data wat in hierdie stelsel ingesamel is, plantegroei opnames en aanlynbronne, ondersoek hierdie proefskrif die onderliggende meganismes wat tot uitheemse indringer spesies in bergekosisteme lei, en of die prioritering van die oorspronklikke verspreidings meganismes 'n optimale bestuurstrategie vir die gebied is. Deur die verandering te ondersoek in die uitheemse spesierykheid en samestelling van die plantegroei in en aangrensend tot die bergpas; en die ondersoek van die rol van die padrand in uitheemse spesie vestiging, het ek die volgende gebruik. Data wat tien jaar gelede versamel is, is gekomplimenteer deur ʼn heropname van die padrand en aangrensende transekte in die semi- natuurlikke habitat (N = 80; 25x2m) oor 'n hoogtegradient van 1500- 2874 meter bo seespieël. Langs elke transekt is alle uitheemse spesies en die algehele inheemse spesie-dekking aangeteken. Algemene Lineêre Modelle is toegepas om te toets of uitheemse spesie rykheid, plantbedekking en kaal grond oor tyd verander het, en 'n Canonical Correspondence Analysis is gebruik om veranderinge in uitheemse spesiesamestelling te bepaal. Sedert die aanvanklike opname het uitheemse spesiesrykheid aansienlik toegeneem oor die hele hoogtegradiënt, veral in die sones van middel ligging. Hierdie verspreidingspatroon dui op sprong-verspreiding, wat waarskynlik deur mensgemedieerde aktiwiteite gedryf word, eerder as geleidelike verspreiding. Uitheemse spesiesamestelling het meer homogeen tussen padrand en semi- natuurlike transekte geword, wat bewys het dat uitheemse spesies in die natuurlike habitat indring. Dit is waarskynlik dat propagule druk (saad beskikbaarheid) die sleutel tot indringing is, terwyl versteuring in die padrandte beide verspreiding in die natuurlike gebied en verspreiding in hoogte be seespiël bevorder. Verdere verspreiding van uitheemse spesies in die natuurlike gebied kan dus verwag word. Indringerbestuur kan uitgevoer word deur die priotiseering van spesies, òf gebiede òf verspreidings meganismes. Laasgenoemde is veral handig wanneer propagule druk die dominante aanvoerder van indringing sukses is. Weens die belangrikke rol van propagule druk in hierdie studiestelsel, het ek dan ondersoek ingestel of die bestuur van plaaslike skaal inbringingsweë 'n effektiewe benadering sou wees om uitheemse spesiesrykheid te verminder. Die waarskynlike inbringingsweë van elke uitheemse spesie is gekategoriseer met behulp van inligting uit aanlynbronne. Algemene lineêre modelle is gebruik om te toets of suksesvolle uitheemse spesies geassosieer word met spesifieke inbringingsweë. Die omvang van uitheemse spesieteenwoordigheid in die natuurlike gebied is as 'n proxy vir sukses gebruik. Ek het ook getoets of die aantal indringingsweë wat deur uitheemse spesies gebruik word, met hul sukses verband hou deur middel van algemene lineêre modelle. Suksesvolle indringerspesies in die omgewing gebruik nie veelvuldige inbringingsweë nie, en slegs toevallige vervoersaamryers word aanmerklik geassosieer met die teenwoordigheid in die natuurlike omgewing. Hierdie resultate toon dat suksesvolle indringerspesies deur vektore soos voertuie, mense en vee die gebied binne kom. Met die toepassing van streng beheer van hierdie vektore by die grensposte, sal die waarskynlikheid van die inbring van nuwe uitheemse spesies in die gebied verminder.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106203
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