Vegetation change on subantarctic Macquarie Island

Macquarie Island has a high diversity of vascular plant species and vegetation communities compared to other subantarctic islands. The severe impact of feral rabbits on the tundra vegetation of this island over many decades has been well documented. The native vegetation is changing rapidly following the removal of rabbits, rats and mice in 2011, in one of the largest island eradication projects undertaken. In the absence of native vertebrate herbivores, plant species distribution and vegetation structure are largely determined by abiotic factors such as climate. Recent responses to climate change are evident in the changing abundance and dieback of vascular plant species in grassland and feldmark vegetation. This thesis aims to determine how topography, climate and release from grazing pressure influence the distribution of plants and vegetation communities on Macquarie Island by investigating the climatic, topographic and historical factors influencing the dominant vascular plant components of the vegetation. Climatic factors such as wind exposure, air temperature and cloud cover were investigated using field data, image interpretation and spatial modelling to better understand topographic variation in these factors, which may influence plant species distributions. Changes in vegetation preceding and following pest eradication were observed from a 34-year series of repeat photographs and a temporally equivalent vegetation monitoring sites dataset. Spatial analysis of these changes and of satellite imagery from before and after eradication revealed geographic variation in vegetation trends. The total range and core range of key plant species constituting different vegetation communities were predicted using species distribution models. South-westerly winds were more damaging to plants than the prevailing westerly and north-westerly winds on Macquarie Island. Air temperature lapse rates on Macquarie Island are steep and have limited diurnal and seasonal variation and are not related to the frequent presence of fog at higher elevations. Tussock grass and megaherbs present in 1980 were reduced or absent in 2009, but in most cases had subsequently increased by 2014, following three growing seasons without grazing pressure. Other floristic elements showed mixed trajectories, including high elevation feldmark vegetation which did not show directional trends consistent with rabbit impacts or climate change. Generalist plant species with a wide geographic range occur as subdominants over much of their range, with a smaller modelled 'core range' where they have the potential to be dominant species. The lowland vegetation of Macquarie Island has changed rapidly since the successful pest eradication project in 2011. This ecosystem recovery will continue as the shifting distributions and abundance of individual plant species adjusts to the absence of grazing and a changing climate.