Prickly acacia (Acacia nilotica subsp. indica) is a Weed of National Significance (WoNS). It is one of 20 plants regarded as the worst weeds in Australia because of their invasiveness, potential for spread, and economic and environmental impacts. This species has largely spread from deliberate plantings as a fodder and shade plant for livestock, but may also have spread from garden plantings in some areas. Many of these plantings were often located near water points (i.e. near waterways and other water sources), from which it has spread out into adjacent grasslands and pastures. This spread was accelerated by the introduction of cattle to these areas, and in some areas infestations now cover entire properties. As well as being one of the most serious weeds in this country, this species is also regarded as one of Australia's worst environmental weeds. It forms dense thorny thickets in grasslands, open woodlands and open forests. Little grows under the canopy in these thickets, as this larger shrub outcompetes pasture grasses for water. Such loss of groundcover then facilitates water and wind erosion and leads to soil degradation. Grassland and open savanna communities in northern Australia are under the most serious threat, and vast areas of the north-eastern sector of the mitchell grass plains in Queensland are suffering from a very serious invasion by this species. In fact, aproximately seven million hectares of the mitchell grass plains in northern Australia have been infested to varying degrees by prickly acacia (Acacia nilotica subsp. indica). It has also been predicted that this species has the capacity to invade a further 50 million hectares of native grassland communities in northern Australia. Failure to adequately manage this species in the future could lead to much of these grasslands being converted into a dense shrubland dominated by this exotic woody weed.The establishment of this woody species in grassland communities causes a significant change in vegetation structure, and is likely to result in substantial changes in the species composition of both the flora and fauna. The increased density of trees eliminates much of the normal ground vegetation from grassland communities and threatens their integrity and biodiversity. Prickly acacia (Acacia nilotica subsp. indica) has been observed to change grasslands from being dominated by a relatively dense cover of perennial grasses to having a relatively sparse groundcover of short-lived ephemeral forbs and annual grasses. It is very likely that this change will seriously disadvantage numerous perennial grass species endemic to the mitchell grass plains and ultimately threaten their survival.Such changes to the vegetation are also thought to pose a threat to the survival of about 25 rare and threatened animal species that are found in the area. The Julia Creek dunnart (Sminthopsis douglasi), a small carnivorous masrupial, is one of these threatened animal species. It is restricted to the mitchell grass plains of north-western Queensland and requires these grasslands as a habitat in which to live. Research has shown that several native grassland lizards, that rely on adequate groundcover for their survival, were absent from areas heavily infested by prickly acacia (Acacia nilotica subsp. indica). This included at least one species of skink (i.e. Ctenotus agrestis) that is only found on the mitchell grass plains. Similar patterns have also been observed in the bird popluation of these areas, with predominantly grassland species being replaced by woodland species in infested areas. The observed changes in animal populations are thought to be caused by the loss of food sources and shelter. Similarly, the spread of prickly acacia (Acacia nilotica subsp. indica) along waterways may also reduce the availability of nesting sites for waterbirds, which cannot nest in these prickly thickets. Dense infestations may also restrict the movement of wildlife and harbour feral animals.