Long-term impacts of Chromolaena odorata (L.) invasion and ungulate grazing on ant body size and communities in Hluhluwe-iMfolozi Park, South Africa

Abstract

The present study investigated the impacts of Triffid weed Chromolaena odorata infestation and ungulate grazing on native ant communities in Hluhluwe-iMfolozi Park (HiP), in KwaZulu-Natal province, South Africa. HiP is a highly valued park because of its unique biodiversity. However, management of biodiversity in this park is intimidated by restructuring disturbances produced by invasions by alien plants and grazing. C. odorata is a highly invasive plant known to invade to an impenetrable extent, out shading and eliminating all competition of indigenous vegetation to many parts of the world. The rapid invasion and spread of C. odorata in natural environments, including large parts of HiP, is making ecosystems unstable as indigenous species decline, thereby resulting in hampered delivering of ecological services. The re-vegetation of C. odorata even after control measures has heightened, raises concerns regarding sustainability of indigenous species. The increase in populations of herbivores in HiP is of concern as vegetation is restructured resulting in interruptions in flora and fauna relations. To increase the understanding of changes associated with these habitat altering disturbances, the effects of invasions by C. odorata and herbivory on indigenous communities were investigated. Ants play diverse roles in terrestrial ecosystems and influence composition, distribution and abundance of other species. Ants are sensitive to changes that influence food abundance, shelter and microclimate. Ants are also strongly linked to a variety of vegetation. Therefore, disturbances that result in displacement of indigenous flora impede the diversity and abundance of indigenous ant species. Grazing had no effects on ant species diversity but combination of reserve and grass had a significant influence, while rainfall influenced ant abundance. The presence of high rainfall promoted high ant diversity and evenness indicating increased foraging and nesting resources for a variety of ant species. Ant diversity and evenness in areas of high precipitation could also have been caused by moist soils that encourage vegetation growth and cover. Ant abundance was significantly affected by the combination of reserve, grazing and grass type. High ant abundance was associated with high rainfall, light grazing, and greater vegetation biomass. High ant abundance in lightly grazed exclosures contributed to high vegetation diversity. The lack of ants specializing in lawn grass suggests that this habitat does not contain unique ant communities. However, greater ant diversity found in these habitats, compared to tall grass, confirms that lawn grass provides optional foraging areas and is important for management of ants. This study concludes that rainfall or altitude variations influence ant composition through changes in vegetation growth and diversity more than herbivory.