Shrub invasion of a tropical wetland: implications for weed management

Records of the distribution and control of the shrubby weed Mimosa pigra in wetlands within, and adjacent to, Kakadu National Park (KNP), northern Australia, were examined to determine the effects of habitat variation and distance from large stands on the density and persistence of the satellite outbreaks. The cumulative density of out- breaks decreased with distance from the large stands to the east and west of KNP. There was, however, no trend in the number of new outbreaks discovered each year from 1984 until 1993, despite the doubling in area of the large stand of M. pigra to the east of KNP every 1.4 yr from the early 1980s until 1991. The highest densities of outbreaks of M. pigra in KNP were found in the wetland margins that had been disturbed by feral animals, and in wetland habitats with a relatively long period of inundation, but where trees were absent. The dispersal gradient provided a useful model for predicting the occurrence of outbreaks on wetlands to the east of KNP that had not previously been surveyed system- atically. More than 20% of outbreaks required sustained control for 7 yr or more because of the establishment of plants from the persistent seedbank. The control of satellite outbreaks of M. pigra has successfully prevented large stands developing in KNP. Detailed record-keeping by the control program has facilitated essential follow-up control and enabled the analysis of the spread of M. pigra, which should contribute to its efficient control elsewhere.