Plant Dispersal, Introduced Species, and Vegetation Change in the South Pacific Kingdom of Tonga

Dispersal guilds hold key ecological implications for the vegetation history of islands. This study considers dispersal vectors in conjunction with species origin and growth form to characterize vegetation dynamics on the islands of Tonga in the South Pacific. Data for over 700 species compiled from published literature on the plants of Tonga support a comparative study of dispersal mechanisms and growth forms for native flora, species brought by Polynesian settlers, and taxa introduced since European contact. The indigenous flora, predominantly trees, is characterized primarily by endozoochorous (internal) dispersal through birds and bats. European introductions, primarily herbs, disperse commonly through epizoochorous (external) animal dispersal. Bat dispersal is most important for overstory indigenous and Polynesian trees and vines. In addition, rodents commonly eat seeds of native rain forest trees. The understory, which is overwhelmingly introduced, consists of wind-dispersed and externally animal-dispersed species, which are often early successional. Rain forest thinning encourages establishment of wind-dispersed species and nonnatives. Thus, the prospect of sustained native flora in Tonga would be enhanced by the preservation of bats, a particularly important dispersal vector for indigenous and endemic species, and by the eradication of introduced rats.