DOI: 10.5281/zenodo.4740310
title: Riparian plant community change and alien plant invasions following geomorphological change in the Sabie River, Kruger National Park, South Africa

description: Invasive species are among the biggest drivers of environmental change globally. Because of their ability to alter ecological and evolutionary processes they rank high among the most pressing environmental pressures facing protected areas. However, a major challenge in invasion ecology is identifying priority areas and species, and thus managers are unsure of which to prioritise. Two broad aims were examined in the thesis: 1) to assess if the distribution of alien plants is associated with the habitat template of the Sabie River and 2) to investigate the response of the native plant community to changes in alien plant density and diversity. Vegetation data were recorded in 2004 at 12 sites along the riparian zone of the Sabie River in Kruger National Park. Of these, 11 sites were resampled in February 2015, collated with the 2004 data and assigned trait information. Twenty-one modified Whitaker plots, placed parallel to the river channel and stratified by channel type, were sampled. Native and alien (tree, shrub and herbaceous) species abundance and species richness were recorded in each plot. Plant density, species richness, evenness and composition were examined to assess plant community changes. Alien plant density and species composition differed significantly between channel types, indicating that alien plants are not independent of the channel geomorphology. The dynamic braided channel type had the highest density of alien plants compared to the other channel types. This was not surprising given that most of the alien species in this study were herbaceous and annual species that tend to be ruderal in nature. Furthermore, there were two distinct types of alien plant communities. The first, a disturbance driven community that thrives in alluvial influenced channel types is dominated by ruderal species. The second is a combination of woody and short-lived species that is rooted in the water table within the bedrock influenced channel types. While the herbaceous species thrive in the disturbance driven community, they were not confined to alluvial influenced channel types. As expected, alien plants density was significantly higher than native species, while native species were more species rich. The high density of alien plants correlated with a decrease in native species evenness and species composition. Native and alien species were significantly dissimilar in plant strategies, suggesting that most alien species access or utilise differently to native species. I conclude that the decline in native species with the increase in alien species was an indirect consequence of the competitive dominance of alien species. The decline in native plant density when native and alien species shared a closer combination of plant trait strategies indicates that species with the highest potential to drive change are those that are similar in trait combinations to natives. This result supports the notion that alien species with the greatest potential to compete with natives are those that share a similar combination of traits to natives. By examining the patterns and effects of alien plants along the Sabie River, the study revealed some important considerations for land managers. Land manager could prioritise the bedrock-influenced channel types for alien plant, as they are most likely to be altered by the effects of alien plants. Managers should priorities those alien species that have a similar combination of plant trait strategies to native species, as they have the highest potential to drive competitive exclusion of the less common species.
description: Eleven sampling locations from a study conducted in 2004 and 2005 by Parsons et al. (2006) were re-sampled during the months of January and February of 2015.