Tourist vehicles as vectors of weed seeds in Kakadu National Park, Northern Australia

A total of 1960 seeds was collected from 304 tourist vehicles entering Kakadu National Park at monthly intervals between May 1989 and May 1990. Individual cars carried up to 789 seeds and 15 species, but most cars carried one or no seeds. The proportion of cars carrying seeds, and the total number of seeds entering per month, did not vary strongly with seasons, despite the fall in numbers of cars entering during the wet season. The numbers of seeds, and number of occurrences, of different weed species on the cars were unrelated to the abundance of the weeds in the Park found previously. However, those weed species that were found on tourist cars occurred at three times as many sites in the Park as those that were not, suggesting that movement of seeds by tourist cars may be partly responsible for weed infestations. Ten species of known tropical weeds were found amongst the samples, including Pennisetum polystachion, Sida acuta and Hyptis suaveolens, as well as 14 species not known in the Park. Propagules of the major tropical weeds, Mimosa pigra and Salvinia molesta, were not found amongst the samples. In view of the low density of weed seeds on tourist cars, it is recommended that resources are best spent on detecting and eradicating existing weed infestations, rather than on attempting to prevent this form of seed movement.