SHRUB INVASION OF A TROPICAL WETLAND - IMPLICATIONS FOR WEED MANAGEMENT

Records of the distribution and control of the shrubby weed Mimosa pig ra in wetlands within, and adjacent to, Kakadu National Park (KNP), no rthern Australia, were examined to determine the effects of habitat va riation and distance from large stands on the density and persistence of the satellite outbreaks. The cumulative density of outbreaks decrea sed with distance from the large stands to the east and west of KNP Th ere was, however, no trend in the number of new outbreaks discovered e ach year from 1984 until 1993, despite the doubling in area of the lar ge stand of M. pigra to the east of KNP every 1.4 yr from the early 19 80s until 1991. The highest densities of outbreaks of M. pigra in KNP were found in the wetland margins that had been disturbed by feral ani mals, and in wetland habitats with a relatively long period of inundat ion, but where trees were absent. The dispersal gradient provided a us eful model for predicting the occurrence of outbreaks on wetlands to t he east of KNP that had not previously been surveyed systematically. M ore than 20% of outbreaks required sustained control for 7 yr or more because of the establishment of plants from the persistent seedbank. T he control of satellite outbreaks of M. pigra has successfully prevent ed large stands developing in KNP. Detailed record-keeping by the cont rol program has facilitated essential follow-up control and enabled th e analysis of the spread of M. pigra, which should contribute to its e fficient control elsewhere.