Mimosa (Mimosa pigra) and Olive Hymenachne (Hymenachne amplexicaulis) are the most serious environmental weeds found in the Arafura Swamp region. Both are vigorous wetland invaders with the potential to irrevocably change the swamp’s ecosystem.
Mimosa is an Australian Government Weed of National Significance and one of the world’s worst 100 invasive species. It forms dense, thorny, impenetrable thickets, devastating wetlands and threatening livelihoods and culture. Gurruwiling (the Arafura Swamp) is a priority area for mimosa control because of its outstanding ecological and cultural values. In 2020, a three-year programme was developed with the NT Weed Management Branch to map and control the major mimosa infestation at Ballinga, along with three smaller infestations at Marrio, Maniwirrka and Gadawinga. Rangers will treat mimosa with a combination of aerial and on-ground spraying.
Olive Hymenachne poses a major management challenge by threatening not only wildlife, but traditional economies based on resources such as water chestnuts and Magpie Geese.
Wetland weeds currently found in western Arnhem Land that pose a threat to the swamp include the floating fern Salvinia (Salvinia molesta) and the highly invasive grass, Para grass (Urochloa mutica). Checking and washing down vehicles and boats arriving from infested areas is an important way to control the spread of wetland weeds.
In upland areas of the catchment Mission Grass (Cenchrus pedicellatus), Grader Grass (Themeda quadrivalvis) and Gamba Grass (Andropogon gayanus) are threats. These introduced grasses out-compete native grasses and increase fuel loads, resulting in more intense fires, the death of mature habitat trees and general loss of biodiversity. By altering fire regimes invasive grasses have the potential to disrupt lucrative and environmentally important savanna carbon projects. ASRAC has an integrated weed management plan and through education, vigilance, and priority eradication of outbreaks along roads rangers are controlling these grassy weeds.
“Gamba means terrible fires because it makes so much more fuel than our native grasses. It can burn twice in the same year. We are earning money for land management by our success in reducing greenhouse gas emissions. If Gamba spreads through our country, we will be producing more greenhouse gas emissions, not less. Gamba grass could cost us millions of dollars a year and take away money for employing Yolngu and Bi land managers.”