Alien plant invasions on the Tiwi Islands. Extent, implications and priorities for control

Abstract

The data from five vegetation surveys on the Tiwi Islands in north Australia were compiled to examine the habitat preference of naturalised alien plant species. Eucalyptus forest, Callitris forest, Melaleuca woodland, Acacia scrubland and monsoon rainforest had very few alien plant species. Of the ten alien plant species that have become naturalised in native habitat, most were restricted to semideciduous Eucalyptus woodland and especially grassland and open scrubland on clay-rich substrate. All of these habitats have relatively fertile soils and it is argued that this greatly facilitates the spread of exotics, which may also be exacerbated by the activities, of Asian water buffalo, Bubalus bubalis. The herb Hyptis suaveolens is the most abundant alien plant species. Despite few exotics in native vegetation, 95 alien species have colonised disturbed habitat around settlements, with species richness related to the age of the settlement. The relatively low numbers of naturalised exotics enhances the natural value of the islands and the sea barrier provides valuable opportunity for quarantine. However, some alien species such as Pennisetum polystachion with small populations on the islands have elsewhere demonstrated ability to degrade large areas of their habitat and should be urgently eradicated.