Islands can be powerful demonstrations of how destructive invasive species can be on
endemic faunas and insular ecologies. Oceanic islands in the eastern Indian Ocean have
suffered dramatically from the impact of one of the worldâ€™s most destructive invasive
species, the black rat, causing the loss of endemic terrestrial mammals and ongoing
threats to ground-nesting birds. We use molecular genetic methods on both ancient
and modern samples to establish the origins and minimum invasion frequencies of black
rats on Christmas Island and the Cocos-Keeling Islands. We find that each island group
had multiple incursions of black rats from diverse geographic and phylogenetic sources.
Furthermore, contemporary black rat populations on these islands are highly admixed to
the point of potentially obscuring their geographic sources. These hybridisation events
between black rat taxa also pose potential dangers to human populations on the
islands from novel disease risks. Threats of ongoing introductions from yet additional
geographic sources is highlighted by genetic identifications of black rats found on
ships, which provides insight into how recent ship-borne human smuggling activity to
Christmas Island can negatively impact its endemic species.