River floodplains and swamp forests in northern Australia are threatened by dense thickets of Mimosa pigra. The weed supports fewer numbers of birds and lizards, less herbaceous plants and fewer tree seedlings. It prevents traditional food gathering by Aborigines on otherwise resource rich wetlands.
M. pigra has the potential to harm a wide number and variety of different types of primary production. If large infestations occur over farmland, mimosa may threaten the health of pastoral industries by reducing the area of grazing land and the carrying capacity of the land. Furthermore, if livestock are reliant on natural water sources for drinking, their access to water may be blocked. As a result, meat production and income may be reduced (Praneetvatakul 2001).
M. pigra may reduce water flow and increase silt levels, as it commonly colonises water course edges. This may threaten the sustainability of reservoirs and canals and any livelihoods reliant on them. For example, the weed negatively impacts rice cultivation in Thailand by blocking irrigation inlets (as well as encouraging increases in the numbers of rats and crabs, which damage crops) (Praneetvatakul 2001).
M. pigra may interference with the cultivation of other economically-important plants. For example, M. pigra is able to compete with the young palm trees in immature oil palm plantations. This may cause a decrease in the production of palm oil (Praneetvatakul 2001).
Common along roadsides, mimosa may also increase the costs of maintaining power poles and cables used for electricity transmission. It may also decrease driver visibility, increasing the potential for traffic accidents (Praneetvatakul 2001).
Location Specific Impacts:
Habitat alteration: Mimosa has transformed grasslands and sedgelands to monospecific or near-monospecific tall shrublands and has invaded billabongs and swamplands of Melaleuca spp. (Lonsdale et al. 1995, in Frono Fichera and Prior 1999).
Reduction in native biodiversity: In general, mimosa thickets support fewer birds and lizards, less herbaceous vegetation, and fewer tree seedlings than native vegetation (Braithwaite et al. 1989, in Walden et al. 1999).
Threat to endangered species: The yellow chat (Alligator Rivers subspecies) (see Epthianura crocea tunneyi) listed as “endangered” is restricted to a small geographic area in northern Australia that encompasses the floodplains from the Mary River to the East Alligator River. It is probable that the yellow chat is being negatively affected by the expansion of exotic weeds in the floodplain habitats (notably by M. pigra, para grass (Brachiaria mutica) and gamba grass (Andropogon gayanus) in addition to habitat change. (Parks and Wildlife Commission UndatedA).
The bare-rumped sheathtail bat (see Saccoilamus saccoilamus) is listed as “critically endangered”. Little is known about the distribution of the species but vegetation change associated with invasion by exotic species (such as M. pigra) is though to be one of the factors reducing habitat suitability for this species (Parks and Wildlife Commission UndatedB).
The magpie goose is known to be threatened by the spread of M. pigra. It lives in shallow lowland grasslands and swamps and feeds on seeds, tubers and grass species. It is restricted to northern Australia and its breeding habitat is being invaded by para grass (Brachiaria mutica), M. pigra and introduced ponded pasture plants (such as Hymenachne amplexicaulis). These species are replacing its natural principal food sources (Garnett and Crowley 2000).
Also, the Arnhem Sheathtail Bat (Taphozous kapalgensis) (considered to be a “near threatened” species by the Northern Territory) is losing suitable habitat due to the vegetation change associated with the invasion of exotic species such as M. pigra (Parks and Wildlife Commission UndatedC).
Northern Territory (Australia)
Economic/Livelihoods: In northern Australia, as early as 1981, the economic effects of dense mimosa infestations started to affect the tourist industry (due to decreased access to wetlands) (Miller et al. 1981, in Walden et al. 1999).
Habitat alteration: The yellow-billed Egret (Mesophoyx intermedia) is listed as Least Concern (LC) in the IUCN Red List of Threatened Species. Its habitat in the Northern Territory of Australia is under threat due to burning, grazing and the spread of invasive alien plants Mimosa pigra and Salvinia molesta. The introduced feral buffalo (Bubalus bubalis) is also a threat causing breakes in the levees leading to salt intrusion and accumulation of salt settlement (BirdLife International 2009).
Papua New Guinea
Economic/Livelihoods: Mimosa pigra especially threatens the wetlands of the Sepik and Ramu river basins. The successful invasion of M. pigra into these basins would endanger thousands of livelihoods that depend on fishing.
Threat to endangered species: Mimosa pigra could out-compete virtually all other plant species in wetland areas and could make infested areas inaccessible to animals and people. In the Oro Province M. pigra could displace the world’s largest butterfly Queen Alexandra's Birdwing (see Ornithoptera alexandrae in IUCN Red List of Threatened species), already threatened by extinction due to habitat destruction. M. pigra could also threaten the biodiversity of the Sepik River, which is home to many endemic species including the Sepik blue orchid (Dendrobium lasianthera).
Economic/Livelihoods: Mimosa pigra stands obstructs irrigation canals, reducing water flow into rice fields. This negatively affects rice health and growth resulting in reduced crop yields and lower economic gains. As M. pigra is a suitable habitat for rats and crabs, it also encourages the presence of these animals, both of which also cause damage to rice plants.
Human nuisance: Mimosa pigra is expanding along the national highways obstructing the aesthetic value of the countryside and decreasing driver visibility (increasing the potential for traffic accidents).
Modification of hydrology: Mimosa pigra chokes waterways and irrigation ditches. This reduces water flow in canals and rivers and accelerates the build-up of silt in reservoirs. Economically important reservoirs may be threatened by the presence of the weed. A reservoir has the potential to last for about 100 years in the absence of M. pigra and for about 25 years in the presence of the weed (Robert 1982, in Praneetvatakul 2001).