Abstract

Islands have always attracted considerable research effort due to their unique geography and biota. However, the biogeography of islands still presents substantial challenges. For instance, islands often sustain high rates of plant invasions. The distributional patterning of exotic species on islands though is still poorly understood. Additionally, while species vary strongly in their life histories, plant traits have been rarely integrated with the investigation of the island biogeography of plant species. Islands are also commonly struck by storms and strong winds of oceanic origin, yet how ocean-borne disturbances affect island plant communities is unclear. Finally, in the last 50 years, researchers investigated the insular distribution of virtually every known taxon, but very little is known about variation in human population sizes on islands. The goal of this thesis is to investigate these understudied aspects within the theory of island biogeography framework in vascular plant species and humans. To better understand plant invasions on islands, I compared the relationships between native and exotic species richness and island characteristics on 264 islands offshore Northern New Zealand. Native and exotic species displayed broadly similar biogeographical patterns; however, exotic species exhibited subtle, yet distinctive, invasion patterns. Trends in species richness were also scale-dependent, and increasingly for exotic species. Second, I integrated plant life-history traits within the theory of island biogeography framework to investigate how exotic species with different traits relate to island characteristics on 264 islands offshore Northern New Zealand. Exotic species with traits associated with high invasion rates (i.e. high island occupancy rates) were more similar to native species both in occupancy and in relationships with island characteristics. Moreover, they were less commonly associated with human-related variables. 6 Third, I assessed how distributional patterns of native and exotic plant species varied depending on different levels of ocean-borne disturbances on 97 small New Zealand islands. Overall results show that both native richness and composition varied with different levels of disturbance. In striking contrast, distributional patterns of exotic species remained unchanged. Differences between natives and exotics might reflect a lack of coastal specialists in the exotic species pool. Lastly, I explored relationships between human population sizes and island characteristics on 10 archipelagos worldwide, for a total of 486 islands. Overall results showed that, just like other animals, human distributions are associated to island characteristics. However, relationships between human population sizes and island characteristics varied markedly among archipelagos, often reflecting specific social, political and historical circumstances. This thesis combines with a growing body of research on plant invasions on islands. It provides a fresh perspective on the subject by assessing previously overlooked aspects of the invasion process, such as the scale-dependency of the relationships between exotic species richness and island characteristics. Additionally, it integrated a trait-based approach within the theory of island biogeography framework. It also provided a test of how species of different biogeographic origins respond to varying levels of ocean-borne disturbances. Lastly, this work includes what is, to my knowledge, the first global test of the island biogeography of humans.