Impacts of urbanization and landscape change on native plant diversity and the distribution of exotic forest invaders.

摘要

This dissertation addresses three questions focused on enhancing our understanding of the impacts of urbanization and land cover change on patterns of native biodiversity, the distribution of exotic plant species in forests, and the resistance of forest communities to invasion. First, I examine whether human-mediated invasion pressure, quantified using a novel variable, the residential force of invasion (rFOI) can be used to improve species distribution models (SDMs) of exotic forest understory invaders. My results show that human mediated invasion pressure influences the distribution of forest invaders and that including rFOI significantly improves model performance. This research also demonstrates that high invasion pressure substantially increases the risk of invasion in habitats that prior to considering invasion pressure, were identified as unsuitable. Second, using a multi-scalar approach, I investigate the influence of multi-temporal trajectories of forest cover change on patterns of native plant diversity in forests. Temporal trajectories of forest cover change are derived from a longitudinal assessment of forest cover across four time intervals (1938-1956, 1956-1975, 1975-1997, 1997-2009) and classified into distinct types of forest cover change reflecting the timing, extent and nature of the change (deforestation, reforestation or no change). My results show that while measures of forest cover change derived using a single time interval (1938-2009) did not influence native diversity, the typology of multi-temporal forest cover change is a significant explanatory factor of patterns of native diversity, after controlling for other environmental and landscape covariates. Furthermore, from these results, I have identified a specific type of trajectory, "rapid reforestation between 1957 and 1975", that is associated with the highest native diversity observed within in the study extent. Finally, I investigate the direct and indirect effects of urbanization, environmental factors, and landscape proximity on the ability of forest communities to resist invasions using a structural equation modeling framework. My results support the hypothesis that urbanization has negative direct and indirect effects on invasion resistance via its interaction with propagule pressure and habitat eutrophication, thereby providing a mechanism to explain the increased susceptibility of urban forests to invasions. Taken together, the results of this work provide much needed empirical evidence linking anthropogenic factors with patterns of native biodiversity, the distribution of exotic species and decreased resistance to invasions and show that careful consideration of how anthropogenic factors may influence specific ecological processes can both advance our knowledge of both invasion and urban ecology, as well as result in better predictive models.