One in six plant species grows on oceanic islands, yet one in every three endangered plants is an insular endemic (World Conservation Monitoring Centre, 1992). Habitat loss and fragmentation, together with the negative effect of exotic species, imperil island native plants (Davis et al., 1986; Frankel et al., 1995). As a preliminary step in the conservation of endemic island plants, an extinction risk assessment provides a framework for identifying and prioritising threatened species (Rodrigues et al., 2006). The World Conservation Union (IUCN) has recognised the rapid loss of the world’s biodiversity and has created a system to assess the threat of extinction faced by each species. This system, established in 1963, is known as the IUCN Red List (IUCN website, 2004). Under the IUCN Red List criteria, hundreds of thousands of species have been assessed and over 12 000 species are considered threatened with extinction (IUCN, 2003). Island plants account for a large number of IUCN threatened species and seven of the top 10 most threatened floras are thought to be on islands (Walter & Gillett, 1998). Red Lists using the system implemented by the IUCN in 2001 have been published for a small number of large islands or archipelagos such as the Canary Islands (Bañares et al., 2004), the Falkland Islands (Broughton & McAdam, 2002), the Galapagos Islands (Lawesson, 1990) and New Caledonia (Jaffre et al., 1998). Conservation assessments of plants from small- or single-island floras using the IUCN system are uncommon despite their importance to global biodiversity (but see Bacchetta & Pontecorvo, 2005; Gray et al., 2005; Kingston & Waldren, 2005).