ALIEN PLANT INVASIONS ON THE TIWI ISLANDS - EXTENT, IMPLICATIONS AND PRIORITIES FOR CONTROL

The data from five vegetation surveys on the Tiwi Islands in north Aus tralia were compiled to examine the habitat preference of naturalised alien plant species. Eucalyptus forest, Callitris forest, Melaleuca wo odland, Acacia scrubland and monsoon rainforest had very few alien pla nt species. Of the ten alien plant species that have become naturalise d in native habitat, most were restricted to semi-deciduous Eucalyptus woodland and especially grassland and open scrubland on clay-rich sub strate. 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 s pecies. Despite few exotics in native vegetation, 95 alien species hav e colonised disturbed habitat around settlements, with species richnes s 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, som e alien species such as Pennisetum polystachion with small populations on the islands have elsewhere demonstrated ability to degrade large a reas of their habitat and should be urgently eradicated. (C) 1997 Else vier Science Ltd.