Extinction risk and invasion success are associated with different functional traits in a tropical island flora

Islands host a disproportionately high richness of threatened native and invasive alien species. Despite intense research efforts, mechanisms underpinning extinction risk or invasion success are still not fully understood, which hinders the development of effective conservation strategies. The two-sides-of-the-same-coin (2sotsc) hypothesis, which states that traits involved in extinction risk and invasion success are identical, but with opposite values, remains largely untested at local scales where management occurs. Using five functional traits (specific leaf area, leaf dry matter content, maximum height, seed mass and dispersal syndrome) for 187 plant species from Reunion Island (Indian Ocean), we compared (i) threatened and non-threatened native woody species and (ii) invasive and non-invasive alien species. Seed mass was greater in threatened than in non-threatened woody natives. We found no trait difference among woody aliens, while specific leaf area was lower in invasive than in non-invasive aliens (woody and non-woody). Since different traits capture species differences in terms of extinction risk and invasion success, the underlying mechanisms are likely to differ. The species-centred perspective of the 2sotsc hypothesis appears too simplistic, which calls for better integrating community- and ecosystem-level impacts to resolve the complexity of extinction and invasion dynamics on islands and support informed conservation decision-making.