Plant invasions in Taiwan: Insights from the flora of casual and naturalized alien species

Data on floristic status, biological attributes, chronology and distribution of naturalized species have been shown to be a very powerful tool for discerning the patterns of plant invasions and species invasiveness. We analysed the newly compiled list of casual and naturalized plant species in Taiwan (probably the only complete data set of this kind in East Asia) and found that Taiwan is relatively lightly invaded with only 8% of the flora being casual or naturalized. Moreover, the index of casual and naturalized species per log area is also moderate, in striking contrast with many other island floras where contributions of naturalized species are much higher. Casual and naturalized species have accumulated steadily and almost linearly over the past decades. Fabaceae, Asteraceae, and Poaceae are the families with the most species. However, Amaranthaceae, Convolvulaceae, and Onagraceae have the largest ratios of casual and naturalized species to their global numbers. Ipomoea , Solanum and Crotalaria have the highest numbers of casual and naturalized species. About 60% of all genera with exotic species are new to Taiwan. Perennial herbs represent one third of the casual and naturalized flora, followed by annual herbs. About 60% of exotic species were probably introduced unintentionally onto the island; many species imported intentionally have ornamental, medicinal, or forage values. The field status of 50% of these species is unknown, but ornamentals represent noticeable proportions of naturalized species, while forage species represent a relatively larger proportion of casual species. Species introduced for medicinal purposes seem to be less invasive. Most of the casual and naturalized species of Taiwan originated from the Tropical Americas, followed by Asia and Europe.