RATES OF SPREAD OF AN INVADING SPECIES - MIMOSA-PIGRA IN NORTHERN AUSTRALIA

1 The invasion of the wetlands of tropical Australia by the woody weed Mimosa pigra L. was examined at two scales: within a single wetland system on the Adelaide River flood plains, 70 km east of Darwin, and across the entire western coastal region of the Northern Territory. 2 Modelling indicated that, if wind dispersal alone were involved, the fastest rate of linear increase of a stand would be 18.3 m year-1. Actual rates of spread in the wetland system were found from aerial photographs to average 76 m year-1 or greater in five out of six years. This suggests that dispersal of seeds by flotation is central to the observed rapid expansion of the weed in the region's wetlands. 3 Skellam's ( 1 95 1 ) model for areal spread was an inadequate descriptor of the spread of this invading species within the wetland system, probably because it assumes normally distributed dispersal distances, whereas actual distances were log-normally distributed, and because of the irregular, elongate shape of the infestation, where the model assumes roughly circular infestations. 4 There was a close correlation (r = 0.94, P = 0.005) between the increase in the area colonized by the plant and the rainfall in the previous wet season, largely perhaps because of greater dispersal distances. The doubling time over the period of study averaged out at 1.2 years. 5 Across the region as a whole, the doubling time for numbers of infestations was much slower, being 6.7 years, probably because of the separation of the plant's major wetland habitats by eucalypt savannas that it can colonize less readily. There is no evidence to support the widespread popular perception that feral buffalo grazing suppressed the weed, nor that the removal of grazing pressure has resulted in an upsurge of the weed. 6 A graphical model is used to show that seed predators have little effect on the rate of spread of invasive plants until very high levels of predation are achieved, but that seed predators combined with folivores can slow the rate of spread considerably.