How flower visitation of remnant grassland patches is affected by commercial timber plantations and an invasive alien species (Rubus cuneifolius)

Thesis (MSc)--Stellenbosch University, 2015.

ENGLISH ABSTRACT: Our planet is in the midst of a biodiversity crisis, with factors such as land transformation, climate change, anthropomorphic disturbance and invasive species acting together to threaten biodiversity. In South Africa, with minimal natural wood resources, commercial forestry is one of the most abundant forms of landscape transformation. However, a third of the land inside many plantations has been set aside for conservation as unplanted remnant grassland patches (RGPs). These areas are subjected to an additional negative impact by invasive alien species, namely Rubus cuneifolius (American bramble), a weed that is particularly problematic in and around forestry plantations in South Africa. The grassland biome of South Africa is extremely diverse and is of vital importance for the ecosystem services it supplies. Despite this, the grassland biome is under threat as this is where much of South Africa’s forestry plantations are located. Driven by anthropomorphic disturbance, pollinators are in decline. Landscape transformation of natural areas for forestry plantations is likely to affect plant-pollinator interactions which will affect ecosystems and biodiversity. However, it is not known to what extent these ecosystems are affected. It is thought that the impact depends on the complexity of the ecosystem in question, and analyses at the network-level provide insights into the robustness of ecosystems in the face of biodiversity loss. Thus, this study evaluates the effect of natural habitat fragmentation and invasion of the alien species, R. cuneifolius, on flower visitation networks of South African grasslands. The study was conducted in the KwaZulu-Natal Midlands within a commercial timber plantation and a neighbouring protected area (PA). Flower-visitor observations were carried out in uninvaded protected areas and RGPs and in protected areas and RGPs invaded by R. cuneifolius. I found that RGPs within commercial forestry plantations successfully decrease the negative effects of land transformation on the grasslands of the KwaZulu-Natal Midlands, and flower visitation network patterns are largely maintained in these habitat fragments. However, within RGPs, invasion by R. cuneifolius affected the composition and the interaction network structure of flower-visitor and plant communities. The fact that there are unplanted areas within commercial forestry plantations is positive for biodiversity conservation in South Africa. Research has indicated that these areas successfully aid in the conservation of biodiversity and ecosystem functioning. Due to the positive influence that RGPs have on conservation in fragmented and transformed landscapes, it is critical that these unplanted areas are retained. However, the effects of bramble invasion are more intense within RGPs than within protected areas, and therefore, it must be a priority to keep these areas undisturbed. R. cuneifolius has been found to have devastating effects on ecosystem function and network structure. It is also a category 1 invasive plant within South Africa, and its removal is required by law. Therefore, the removal of bramble must be a management priority.

AFRIKAANSE OPSOMMING: Ons planeet is in die middel van 'n biodiversiteit krisis, met faktore soos grond transformasie, klimaatsverandering, antropomorfiese versteuring en indringerspesies wat gesamentlik werk om biodiversiteit te bedreig. Suid-Afrika, besit minimale houtbronne. Daarom is kommersiële bosbou een van die mees algemene vorme van landskap transformasie. Tog is 'n derde van die land binne baie plantasies opsy gesit vir bewaring as oorblywende grasveld kolle (OGKs). Hierdie gebiede word ongelukkig blootgestel aan die bykomende negatiewe impak van die uitheemse spesies, Rubus cuneifolius (Amerikaanse steekdoring), wat veral problematies is in en rondom bosbouplantasies. Die grasveldbioom van Suid-Afrika is baie divers en is van kardinale belang vir die ekosisteem dienste wat dit lewer. Ten spyte hiervan, word die grasveldbioom bedreig waar dit op dieselfde areas as die meerderheid van Suid-Afrika se bosbouplantasies geleë is. Antropomorfiese versteuring lui daartoe dat bestuiwergetalle daal. Landskap transformasie vir bosbou plantasies raak dus plant-bestuiwer interaksies, wat ekosisteme en biodiversiteit beïnvloed. Dit is nie bekend tot watter mate hierdie ekosisteme geraak word nie. Daar word vermoed dat die impak af hang van die kompleksiteit van die ekosisteem. Ontledings van ekosisteme op netwerk vlak kan insigte bied oor die robuustheid van hierdie ekosisteme in die aangesig van biodiversiteitverlies. Dus, die studie evalueer die effek van fragmentasie van natuurlike habitatte en inval van die indringer spesie, R. cuneifolius, op blom-besoekings netwerke van Suid-Afrikaanse grasvelde. Hierdie studie is uitgevoer in die KwaZulu-Natal Midlands binne 'n kommersiële hout plantasie en 'n naburige beskermde gebied (BG). Blom-besoeker waarnemings was in BGs en OGKs sonder R. cuneifolius, en in BGs en OGKs met R. cuneifolius uitgevoer. Ek het gevind dat OGKs binne kommersiële bosbouplantasies suksesvol is om die negatiewe uitwerking van land transformasie te verminder, en blom-besoeking netwerk patrone grootliks gehandhaaf word in hierdie habitat fragmente. Egter, binne OGKs, het R. cuneifolius die samestelling en die interaksie netwerk struktuur van blom-besoekers en plant gemeenskappe negatief geraak. Die feit dat OGKs ongeplant gelaat word, is positief vir die bewaring van biodiversiteit in Suid- Afrika. Navorsing dui aan dat hierdie gebiede suksesvol is om te help met die bewaring van biodiversiteit en ekosisteemfunksionering. As gevolg van die positiewe invloed van OGKs op bewaring in gefragmenteerde en omskepte landskappe, is dit krities dat hierdie areas ongeplant bly. Egter, die gevolge van steekdoring inval is meer intens binne OGKs as binne beskermde gebiede, en daarom moet dit 'n prioriteit wees om hierdie gebiede ongestoord te hou. R. cuneifolius se verwoestende uitwerking op ekosisteem funksie en netwerk struktuur was baie duidelik. Dit is ook 'n kategorie 1 indringerplant in Suid-Afrika, en sy verwydering word deur is die wet vereis. Daarom moet die verwydering van steekdoring ‘n bestuursprioriteit wees.