Biotic and abiotic factors and invasive species success in California grassland communities

In the last century humans have introduced thousands of new species into North America and the rate of introduction is ever increasing. Exotic species invasions can alter critical ecosystem processes and are considered one of the primary threats to biodiversity. Early attempts to study the problem began by elucidating the attributes of exotic species that allow their success in novel environments. Ecologists are now turning these studies around to look at the attributes of ecological communities and ecosystems that determine their susceptibility to new species introductions. In this thesis, I investigate the influence of two environmental factors, plant community diversity and a soil anomaly, on exotic plant species colonization and fitness.; Concomitant with a global increase in exotic species introductions is the loss of native species diversity in virtually every habitat on Earth. In chapters 1 & 2, I test the hypothesis that species poor areas of a community are more susceptible to new species introductions. In the investigation in chapter 1, local species diversity was reduced by removing species from a natural community. The hypothesis was also tested under controlled conditions using an assembled community study (chapter 2). These experimental approaches allowed separation of biotic (diversity) and abiotic (e.g., nutrients and disturbance) affects on the invasion process. The results suggest that higher levels of diversity, even among uncommon species, can deter exotic species invasion.; Exotic species introductions have resulted in enormous alteration California's perennial dominated grasslands. In stark contrast to this trend, serpentine grasslands have remained relatively pristine. The final chapter of this thesis (chapter 3) describes a series of studies that address the adaptation of A. triuncialis, a noxious weed, to a novel edaphic environment. The investigation also addresses conservation concerns regarding the occurrence of A. triuncialis in Northern California serpentine communities.; Observational studies demonstrate that A. triuncialis grows successfully and perhaps preferentially on serpentine but spreads slowly on severe, undisturbed sites. Results of an experimental greenhouse study on ecotypic differentiation suggest that A. triuncialis may be subject to selection pressure by serpentine substrates. These findings, however, were not supported by common garden field studies.