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UBC Theses and Dissertations

Patterns and ecological consequences of exotic plant invasion in Canada's endangered antelope-brush ecosystem Symonds, Josephine Elizabeth


Exotic species invasion is a serious threat to ecosystem structure and function throughout the world. In an effort to understand and limit the future effects of invasion, recent research has focused on quantifying and predicting patterns of exotic plant invasion based on abiotic and biotic features, particularly native species diversity, over multiple spatial scales. I investigated native-exotic richness relationships (NERRs) and their scale-sensitivity and predictability within the antelope-brush (Purshia tridentata Pursh (DC)) shrub-steppe grasslands of the South Okanagan Valley, British Columbia, one of Canada’s four most endangered ecosystems and a national hot spot of biodiversity and endemism. I applied for the first time the concept of focus, which describes one aspect of observational scale, to the NERR and built upon previous studies to determine how different components of scale influence the NERR. I found that the NERR was affected by changing the grain but not the focus or extent of analysis in the antelope-brush grasslands. Contrary to the prevailing hypothesis, I found a highly significant positive NERR at the finest grain and focus (i.e., 1 m²) that appeared to be influenced at least in part by preferential facilitation of exotic plants by antelope-brush shrubs. Also contrary to expectations from the literature, I found no association between exotic and native species richness at the broadest grain and focus (i.e., 1,000 m²), and that mean environmental conditions and variation in these conditions failed to account for significant variation in broad focus exotic species richness in this system. These unexpected results challenged me to re-examine my data in light of other possible hypotheses and to develop a novel interpretive framework that provides a theoretical explanation for all possible NERR results given a particular focus of analysis and study system. Although correlative, the patterns observed in this study may simplify the scope of exotic plant management in the antelope-brush grasslands, as they suggest that broad focus environmental heterogeneity has limited influence on species richness. However, the indication that biotic factors, particularly facilitation by antelope-brush shrubs, influence exotic species richness and abundance at fine focuses poses unique challenges to conservation efforts in this endangered ecosystem.

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