Greve, Michelle2026-01-282026-01-282019-04-242018-08*A2019http://hdl.handle.net/2263/107667Dissertation (MSc)--University of Pretoria, 2018.Invasional meltdown occurs when one invasive species facilitates the establishment, spread and increase of other invasive species, and subsequently increases their impacts. Most studies on invasional meltdown have assessed facilitation of a single invasive species by another invader, but few have assessed meltdowns across different taxa. Therefore, the aim of this study was to evaluate whether an invasive plant, Sagina procumbens L. (procumbent pearlwort, Caryophyllaceae), causes an invasional meltdown in two different taxa (vascular plants and Collembola) on sub-Antarctic Marion Island. Additionally, some direct impacts of S. procumbens on native taxa were assessed. Sagina procumbens is an invasive cushion or mat-forming vascular plant that has spread extensively in the sub-Antarctic. While little is known about the species’ ecological impacts, it is thought to negatively impact biodiversity on Marion Island. Invasional meltdown was tested by comparing the richness of plants growing epiphytically on S. procumbens to those growing on two indigenous plant species (Azorella selago and Clasmatocolea humilis) that are being locally outcompeted by S. procumbens. Additionally, the richness, abundance, and biomass of native and of invasive Collembola extracted from these three focal plant species were compared between S. procumbens and the two indigenous plant species. The direct impacts of S. procumbens on native species were assessed by comparing the composition of plants growing epiphytically and of Collembola associated with S. procumbens to those of the two indigenous plant species. Additionally, native and invasive Collembola richness and abundance in S. procumbens were compared. Hardly any invasive plants grew on any of the three focal plant species. The richness of native plants did not differ between S. procumbens and the indigenous focal species. Invasive Collembola had a higher abundance, richness and biomass in S. procumbens than in both A. selago and C. humilis. However, native Collembola were also more abundant in S. procumbens than in the two native focal plant species. In contrast, the richness of native Collembola was not significantly different between the focal species, and the biomass of native Collembola did not differ significantly between S. procumbens and A. selago, although it was higher in S. procumbens than C. humilis. The composition of plants growing on S. procumbens did not differ significantly from those of plants growing on the two native focal species, though native Collembola assemblages did differ somewhat between the three focal species. Additionally, there was no significant difference between the richness of native and invasive Collembola assemblages in S. procumbens, although the abundance of native Collembola was significantly higher than that of invasive Collembola. Therefore, while I found no evidence for invasional meltdown in plants brought about by S. procumbens, some evidence for invasional meltdown in Collembola exists: S. procumbens leads to an increase in the species richness and biomass of invasive Collembola, but not native Collembola. Therefore, S. procumbens impacts Collembola community structure and composition, and may affect ecosystem processes, like decomposition, which are strongly linked to Collembola.en© 2024 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria.UCTDBiological invansionsCollembolaFacilitationPrince Edward IslandsSynergistic impactsDissertation13345908N/A