Abstract Aim In general, the plant communities of oceanic islands suffer more from exotic plant invasions than their continental equivalents. At least part of this difference may be contributed by differences in non-biological factors, such as the antiquity and intensity of human impacts and the absence of internal barriers to dispersal, rather than differences in inherent invasibility. We tested the resistance of species-rich continental rain forests to plant invasion on a small, continental island that has been subject to prolonged and intensive human impact.
Location Singapore is a 683-km2 equatorial island <1 km from the Asian mainland and with a population of 4 million people. It has a continental biota but has been subject to human impacts as intense as on any oceanic island.
Methods We sampled twenty-nine sites in seven vegetation types, ranging from urban wasteland to fragments of primary lowland rain forest. In each sample plot, all plant species were identified, exotic cover was estimated, and a range of environmental variables measured. Additional qualitative surveys for exotic invasion were made in other forest areas in Singapore. The data were analysed by Spearman's rank correlation coefficient.
Results The number of exotic species recorded at a site was unrelated to the number of native species. Across all sites, percentage canopy opening had the highest correlation with the number of exotic species, while soil pH (which largely reflects the incorporation of calcareous construction wastes) had the highest correlation if the mangrove sites were excluded. There were no exotics in mangrove forest and only a tropical American, bird-dispersed shrub, Clidemia hirta (L.) D. Don (Melastomataceae: Koster's Curse), in primary and tall secondary forest patches. The species-poor early stages of woody plant succession on highly degraded soils were also very resistant to exotic plant invasion.
Main conclusions Long-isolated rain forest fragments in an exotic-dominated continental island landscape resist invasion by exotic plants, suggesting that the problems on oceanic islands may reflect an inherently greater invasibility. This study also adds to the increasing evidence that the floras of tropical rain forest fragments in South-east Asia are remarkably resilient on a time-scale of decades to a century or more.