Island floras are diverse with exceptionally high rates of endemicity, and they are also severely threatened. Invasive plants are widespread on islands, but whether islands are particularly susceptible to invasion or island species are more vulnerable to displacement, or both, remains unclear. As part of the “island plant syndrome,” it has been predicted that island plants have convergently evolved conservative resource use, slow growth rates, and weak competitive abilities in response to moderate climates and the presumed absence of competition in communities with relatively low species richness. Yet, functional trait approaches have provided mixed evidence to support this prediction, and direct tests of competition as neighbour effects on plant performance are lacking. Considering the extensive environmental heterogeneity that exists within islands and among islands, it seems more likely that diverse functional strategies, spanning conservative to acquisitive, have evolved in island plants. Furthermore, assessing island plant syndrome predictions through comparisons with invasive species, which are nonrandom subsets of continental plants, is a flawed approach. Future studies that compare functional strategies of native island versus native continental plants and direct tests for competition between native and invasive island plants within the local scale at which competition occurs, and that consider non-additivities with other simultaneous global threats, are urgently needed to conserve these biodiversity hotspots.