Abstract The number of native grass species and exotic grass species present on 129 offshore islands of southwestern Australia is best predicted by island area and island disturbance, respectively. Isolation of islands and gull activity on islands only slightly improved these predictions. Species turnover on a subset of 30 islands indicated that exotic grass species were more prone to local extinction and more likely to immigrate than native grass species. The major conservation implication of this study is that habitat disturbance on these islands should be minimized to reduce establishment of exotic grass species.