Alien plant species invasive in Kakadu National Park, tropical Northern Australia

A survey of the distribution and abundance of invasive alien plant species in Kakadu National Park in Australia's tropical Northern Territory was undertaken to provide a basis for management. Some 5·8% (89 species) of the vascular flora of Kakadu were considered invasive aliens. The majority of these species were either rare in distribution or widespread but with low mean cover values. Most were associated with human activities, road-ways and other disturbed ground—habitats comprising a small proportion of the Park. In natural habitats, the most severe infestations occurred in riparian communities, especially those frequented by the feral Asian water buffalo, Bubalus bubalis. The most commonly found species were the annuals Hyptis suaveolens, Sida acuta, Sida cordifolia, Alysicarpus vaginalis and Euphorbia hirta and the perennial vine Passiflora foetida. However, the species considered most damaging to the integrity of the Park's biota were those capable of dominating relatively undisturbed native plant communities, especially the vulnerable wetlands. The most important of these perennial weeds remains Mimosa pigra in wetlands, under control in the Park but occuring in abundance elsewhere in the region. Also important are Brachiaria mutica and Salvinia molesta in wetlands and Pennisetum polystachion in the uplands. If fundamental changes to the nature and conservation status of this World Heritage Park are to be avoided, an ongoing commitment to controlling invasive alien species (especially Mimosa) both inside and outside the Park is required.