Managing weeds for wildlife conservation - Gamba Grass Andropogon gayanus

Introduction | Impact | Classification | Control | References 

Introduction 

Gamba Grass is an African grass that was brought into Australia to replace native grasses as a pasture forage. It grows in the Top End, and is particularly well established in the Darwin Daly region, but there is a small outbreak in northeast Arnhem Land and isolated patches on the Tiwi Islands. While many pastoralists believe Gamba Grass is a useful pasture species, this support is not universal. Gamba Grass can be hard to control and, if not grazed continually in the growing season, can become rank and unpalatable. However, opposition to Gamba Grass is unanimous amongst environmental scientists.

Impact

Gamba Grass is so vigorous that little else grows near it. It is able to trap most of the available nitrogen to the exclusion of other plants. Small ground-layer plants are most at risk - the Endangered Helicteres being one of them. Two of the three known populations of this small shrub grow in areas that are heavily infested by Gamba Grass.

The voluminous bulk of Gamba Grass results in fires that are both intense and extensive. Even in the early dry season, they may cause complete ground cover and canopy loss. Several species may be both outcompeted by Gamba Grass and then incinerated in a Gamba Grass-fuelled fire. The Vulnerable Armstrong's Cycad is one threatened plant that could be lost in this process. Gamba Grass increases the fuel load to such an extent that even tree death from fire becomes a regular event.

While all tree death as a result of exotic grass fires is undesirable, the impact on rainforests is the most worrying. Rainforests in the Northern Territory largely persist in fire-protected pockets created by topography, rock formations and springs. Gamba Grass can encroach on these pockets, rendering them flammable instead of fire-protected.

The first rainforest plants to be displaced by Gamba Grass fires are likely to be species found at the rainforest edge, such as the epiphytic Luisia Orchid and the ground-dwelling orchid Malaxis marsupichila. If such fires cause further attrition of rainforest patches, even species presently found deep within them, such as the Darwin Palm, Pternandra and the ground orchid Zeuxine oblonga, could be affected. Large stands of Gamba Grass grow on the Mary River floodplain, where they abut hills containing the threatened rainforest species Yellow Star. If Gamba Grass is not controlled in this area, the habitat of this species is likely to get burnt, and Yellow Star along with it. On the Tiwi Islands, fires fuelled by Gamba Grass could also threaten rainforests containing Mitrella, Quandong, Xylopia, Native Walnut, Tarennoidea, Tiwi Islands Waxflower, Mapania and Dendromyza. Through its effect on fire regime Gamba Grass also has the potential to reduce the abundance of the two Typhonium species that are found only on the Tiwi Islands.

The impact on animals can be equally severe. Large Gamba Grass plants leave little or no spaces between grass clumps, taking away the feeding habitat of many species. Additionally, wildlife has nowhere to hide from the flames of a Gamba Grass fire; and, after the fire, cannot access the shelter that would have once been provided by unburnt patches of grass, leaf litter and logs. Hollows high up in the tree canopy may also be burnt. If these are occupied during a fire, the animals inside them will probably die. Once the fire has passed, any surviving animals could be deprived of shelter and nesting sites. Each of these processes can be equally devastating, as the following examples illustrate.

For much of the year, Gouldian Finches and Partridge Pigeons feed on seeds that fall on patches of bare ground between clumps of grasses and other herbaceous plants. Gamba Grass can completely destroy this habitat. First it replaces the seed-producing plants. Then, as the growing season progresses, it eliminates the bare ground, and, if burnt, later converts much of the habitat to little else besides bare ground. Were Gamba Grass to invade Gouldian Finch habitat, the fires it fuels may destroy nest trees. If lit early enough in the year, these fires may also kill birds in the nests. Similarly, the survival of any Northern Quoll, Brush-tailed Rabbit-rat, or Northern Brush-tailed Phascogale sheltering in a tree hollow burnt in a Gamba Grass fire would be at risk.

Yellow-snouted Gecko is a species of dappled environments, living amongst grass plants and fallen leaves, and feeding on insects taken from patches of bare ground. Its stronghold is in the catchment of the Wildman River, particularly in areas dominated by native perennial grasses. This habitat is threatened by Gamba Grass, which is well established in nearby pastures.

If the spread of Gamba Grass is not checked, then this weed poses one of the most significant threats to biodiversity across the Top End, even to species, such as Crested Shrike-tit, that are currently outside its range. Gamba Grass fires also pose significant risk to human life and property.

Classification

The Northern Territory Parks and Conservation Masterplan recommended listing Gamba Grass as a declared weed because its vigorous growth allows it to outcompete most other plants, and because of the fire hazard it creates. Gamba Grass is now a declared a weed in Western Australia, Queensland and Northern Territory. Attempts to have it listed as a Weed of National Significance in 2007 failed because at that time it was not listed as a weed by any State of Territory Government. Its sowing for pasture production is no longer recommended. It is listed as a high impact weed in the Field Guide to Assessing Australia’s Tropical Riparian Zones.

Control

Control of Gamba Grass can be achieved. While this requires considerable effort, the benefits in preventing loss of Australian species and landscapes in the long term make it worthwhile. Recommended practices include keeping machinery free of Gamba Grass seed to prevent spread beyond infested areas; containing existing large areas and eradicating small outlying infestations; and containing and actively managing Gamba Grass on pastoral lands where it has already been established. While Gamba Grass may be useful as hay, its should not be bailed once it has seeded.

Small outbreaks can be hand pulled, slashed or sprayed with Glyphosate. Fire can be used to reduce plant size before physical or chemical control. Early wet season burning in particular will reduce plant vigour, and encourage the production of new growth that is most receptive to pesticide. To be most effective all control should undertaken before seed production, when plants are actively growing. This will also reduce the likelihood of seeds being inadvertently spread either in the fire updraft or in machinery. Grazing helps to keep Gamba Grass in check, but high stocking rates and continual stocking may be necessary. Keeping native pastures in good condition and not overgrazing will help prevent the establishment of new Gamba Grass plants. If Gamba Grass becomes a declared weed in the Northern Territory, and subsequently listed as a Weed of National Significance, funding support may become available for these actions.