Species Origins, Dispersal, and Island Vegetation Dynamics in the South Pacific

We characterize the vegetation of the Kingdom of Tonga, South Pacific, by exploring the effects of species-area relationships, species isolation, and long-distance dispersal. Published literature provides data for eleven main islands of Tonga on species richness, vegetation types, origins, and dispersal mechanisms. Island area, maximum elevation, distance from neighboring archipelagos, and island geology provide key variables with which to consider the configuration of Tonga's floral biodiversity. Vegetation communities are evaluated by dispersal mechanisms, species origins, and island age as reflected by geology. Species-area relationships are significant for all plant species and for ancient and modern introductions, and species richness shows significant relationships to maximum elevation for indigenous plants and modern introductions. In contrast, species richness on the islands of Tonga does not correspond to patterns expected for species-isolation relationships with the nearest archipelagos of Samoa and Fiji. Plant dispersal spectra vary significantly according to island topography, geology, vegetation types, and plant species origins. The youngest volcanic islands have the most wind-dispersed species, whereas the older limestone islands have more bird, water, and human-dispersed plants. Tongatapu, the largest and longest inhabited island, reflects the legacy of a deeply humanized landscape where more than half the plants are introduced. In contrast, on the sparsely populated, more remote islands, and in many vegetation types, 70 to 90 percent of the plants are native. Evidence for species introductions, varying impacts on different vegetation types, and associated changes in dispersal spectra reveal key aspects of biogeographic dynamics in Tonga and potentially for island biogeography elsewhere.