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

[摘要] 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.

[发布机构] Stellenbosch University