Riparian plant community change and alien plant invasions following geomorphological change in the Sabie River, Kruger National Park, South Africa

Thesis (MScConsEcol)--Stellenbosch University, 2019.ENGLISH ABSTRACT: 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.AFRIKAANSE OPSOMMING: Indringerspesies is wêreldwyd een van die grootste oorsake van omgewingsverandering. Weens hulle vermoë om ekologiese en evolusionêre prosesse te beïnvloed, word indringerspesies gereken onder die dringendste omgewingsdrukfaktore wat beskermde gebiede bedreig. Een van die grootste uitdagings in indringerekologie is egter om gebiede en spesies te prioritiseer; grondbestuurders is onseker waaraan hulle moet voorkeur gee. Hierdie tesis het twee algemene oogmerke: 1) om te assesseer of die verspreiding van uitheemse plante met die habitattemplaat van die Sabierivier ooreenkom; en 2) om te ondersoek hoe die inheemseplantgemeenskap reageer wanneer die digtheid en diversiteit van uitheemse plante verander. Plantegroeidata is in 2004 by 12 persele in die oewersone van die Sabierivier in die Kruger Nasionale Park aangeteken. In Februarie 2015 is monsters weer by 11 van die 12 persele geneem, met die 2004-data saamgestel en eienskap-inligting toegeken. Monsters is ingesamel by 21 aangepaste Whittaker-persele wat parallel met die rivier lê en volgens kanaaltipe gestratifieer is. Die weligheid van elke inheemse en uitheemse spesie (bome, struike en kruidagtiges) sowel as die rykdom aan spesies in elke perseel is aangeteken. Plantdigtheid sowel as die verskeidenheid, eweredige verspreiding en samestelling van spesies is ondersoek om veranderings in die plantgemeenskap te assesseer. Die digtheid en spesiesamestelling van uitheemse plante het aansienlik gewissel volgens kanaaltipe, wat aantoon dat uitheemse plante nie onafhanklik van kanaalgeomorfologie is nie. Dinamies gevlegte kanale het die hoogste digtheid van uitheemse plante vergeleke met die ander kanaaltipes gehad. Dit was te verwagte, aangesien die meeste uitheemse spesies in hierdie ondersoek kruidagtige en eenjarige spesies was, wat van nature neig om ruderaal te wees. Verder was daar twee duidelik onderskeibare tipes uitheemse plantgemeenskappe: (1) ’n versteuringsgedrewe gemeenskap wat in alluviaal-beïnvloede kanaaltipes floreer en waar ruderale spesies domineer; en (2) ’n samestelling van houtagtige en verganklike spesies met wortels in die watertafel binne die bodemrots-beïnvloede kanaaltipes. Hoewel die kruidagtige spesies in die versteuringsgedrewe gemeenskap floreer, is hulle nie tot alluviaal-beïnvloede kanaaltipes beperk nie. Soos te verwagte, was uitheemse plantdigtheid aansienlik hoër as dié van inheemse spesies, terwyl die inheemse spesies groter verskeidenheid getoon het. Die hoë digtheid van uitheemse plante korreleer met verlaagde eweredigheid en samestelling van inheemse spesies. Inheemse en uitheemse spesies verskil aansienlik wat plantstrategieë betref, wat daarop mag dui dat die meeste uitheemse spesies ander toegangs- of benuttingstrategieë as inheemse spesies volg. Die tesis kom tot die gevolgtrekking dat inheemse plantdigtheid afneem wanneer uitheemse spesies toeneem, as ’n onregstreekse gevolg van uitheemse spesies se mededingende dominansie. Die afname in inheemse plantdigtheid wanneer inheemse en uitheemse spesies ’n meer soortgelyke stel planteienskapstrategieë vertoon, dui daarop dat spesies met die sterkste potensiaal om verandering te veroorsaak dié is wat oor soortgelyke eienskapstelle as inheemse spesies beskik. Hierdie bevinding ondersteun die gedagte dat die uitheemse spesies met die sterkste potensiaal om met inheemse plante mee te ding dié is wat ’n soortgelyke stel eienskappe met inheemse plante gemeen het. Deur die patrone en uitwerking van uitheemse plante langs die Sabierivier te ondersoek, het hierdie navorsing ’n paar belangrike oorwegings vir grondbestuurders aan die lig gebring: (1) Grondbestuurders kan aan die bodemrots-beïnvloede kanaaltipes voorkeur gee wat uitheemse plantegroei betref, aangesien sulke kanale die grootste gevaar loop om deur die uitwerking van uitheemse plante verandering te ondergaan. (2) Bestuurders moet die uitheemse spesies prioritiseer wat ’n soortgelyke stel planteienskapstrategieë met inheemse spesies gemeen het, aangesien daardie uitheemse spesies die sterkte potensiaal het om die minder algemene spesies deur mededinging te verdryf.Master