Wind as a climatic driver of biotic communities in the sub-Antarctic

Thesis (PhD (Plant Science))--University of Pretoria, 2021.The effects of temperature and precipitation, and the impacts of changes in these climatic conditions, on biological communities have been investigated extensively. The roles of other climatic factors are, however, comparatively poorly understood, despite potentially also strongly structuring community patterns. Wind, for example, is seldom considered when forecasting species responses to climate change, despite having direct physiological and mechanical impacts on plants, soil, and animals. It is, therefore, important to understand the magnitude of the potential impacts of changing wind conditions on biological communities. This has become increasingly relevant given that wind speeds have accelerated globally over the past decade, with the largest changes taking place in the Southern Ocean. Therefore, the aim of this thesis was to examine the role of wind in shaping biological systems in the sub-Antarctic, testing the influence of wind, across multiple spatial and organizational levels, on: 1) island-scale vegetation distribution, and the occurrence of vegetation types; 2) plant species richness, vegetation cover and composition at a community scale; 3) the fine-scale distribution and cover of individual vascular plant species; and 4) nest site selection by a surface-nesting seabird across an entire island. At the broadest scale, across the whole of Marion Island, wind velocity was the second most important predictor (after elevation) driving the occurrence of vegetation types on the island, and the fourth most important predictor of total vegetation cover. Wind also affected a highly mobile species, the Wandering Albatross, at the island-scale. The nest-site selection of the world’s largest pelagic bird was most strongly influenced by elevation, distance from the coast, terrain ruggedness and wind velocity. Nests had the highest probability of occurring in areas with intermediate wind velocities, which present favourable conditions for take-off and landing. Wind turbulence was, however, not important for either Wandering Albatross nest-site selection or vegetation patterns, emphasising the importance of considering wind velocity and wind exposure into future models. At a finer spatial scale, using data from 1440 x 1 m2 quadrats, wind stress significantly affected plant species richness, vegetation cover, and community composition, even after accounting for other ecophysiologically-important predictors. Species richness was highest under intermediate wind stress conditions, while the highest vegetation cover occurred in plots that experienced the highest wind stress. The differences in community composition were driven by turnover due to species-specific responses to wind conditions. Wind stress had a significant effect on the occurrence of twelve out of sixteen species, and was a more important predictor than any temperature- or moisture-related variables for six of these species. Wind conditions are, therefore, strongly related to multiple aspects of biological communities in this ecosystem that experiences chronic winds. Based on these findings, it is clear that wind has been overlooked as a climatic driver of ecological patterns, and that wind characteristics need to be incorporated into studies investigating the links between climate and biological communities, as well as explicitly included when forecasting the ecological impacts of climate change.National Research FoundationPlant SciencePhD (Plant Science)Unrestricte