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Invasives…

The Gamba Grass Nightmare

By Denise Lawungkurr Goodfellow

A pestilence is quietly but definitively creeping across the Top End of Australia, destroying the tropical woodlands that have dominated this landscape for eons. That plague is a weed, Gamba Grass Andropogon gayana. It leaves in its wake, a panorama of tall exotic grass waving silently in the breeze, and little else. The trees and shrubs, the birds and other fauna, all have gone. Little else can exist in an infestation of gamba grass.

Property
Gamba grass on our property… our dog Sarah in front of gamba at Darwin River

A perennial species native to the African savannah, gamba grass grows in clumps 4 m. high and a metre in diameter. Its stems can be as thick as bamboo, and it forms an almost impenetrable wall (Anonymous, 2018; Kennedy, 2018). Gamba was introduced to Australia in the 1930s and embraced with gusto by the cattle industry. Graziers considered it a godsend for its ability to support many more cattle than native pasture (Csurhes 2005). By 1986, the Northern Territory Department of Primary Industries had produced an ‘easily established’, ‘highly productive’, and ‘drought resistant’ cultivar that was ‘adapted to seasonally wet tropics’ (Petty, 2013).

Wall
…a wall of gamba grass

The first widespread trials of gamba grass were held in the late 1980s and 1990s in the Top End and in northern Queensland. Despite assurances that the plant could be safely contained within paddocks heavily grazed by stock it quickly escaped into surrounding savannah woodland (Petty, 2013). It is now known that gamba spreads easily “…through pasture, bushland, riverine systems and transport corridors, and has diverse invasion pathways including via wind, water, animals and vehicles” (Beaumont, Keily & Kennedy, 2018).

Often gamba-fuelled conflagrations are big enough for thermal updrafts to form pyrocumulus (literally fireclouds), their tops reaching several kilometres into the sky. These thermal air movements can carry gamba grass seed a long way (Lamb, 2008). And each plant can produce up to 250, 000 seeds per plant annually (Dept. of Environment & Natural Resources, n.d.)

Smoke
Gamba grass fire on Camp Creek station in October ©Bushfires NT

Presently, gamba grass infests up to 1.5 million hectares of the Northern Territory, with smaller outbreaks scattered from Western Australia to Queensland. Each outbreak, even when consisting only of a few tussocks, can become a major infestation in less than five years. It is predicted that gamba grass could invade not only woodland and open forest, but right to the edges of wetland and monsoon forest, representing an area of approximately 38 million ha (Douglas and Setterfield: 2005; Petty, Setterfield, Ferdinands, and Barrow: 2012). Tragically, the ranges of some of our most iconic avifauna, for example, Gouldian Finch and Hooded Parrot, lie totally within the potential reach of this weed.

Map
Potential distribution of gamba grass ©Beaumont, Keily and Kennedy, 2018 - Source: adapted from Setterfield et al, 2013 based on data from the NT Government)

Gamba is a ‘transformer’ weed (Rossiter, Douglas, Setterfield, & Hutley, 2003), that is, it “transforms” the natural environment from forest to monotypic grassland. It does this by crowding out native flora; by changing nutrient and water cycles; and by burning many times hotter than a typical Top End grass fire. Gamba-fuelled fires are spectacular. They have the fast-moving front typical of a grassfire, but with flames reaching tens of metres into the canopy.

Our eucalypt-dominated woodlands cannot withstand blazes fuelled by gamba grass. While most are protected from the cool fires common to this habitat by thick or reflective bark, gamba-fuelled fires will kill them. Sixty percent of trees burnt in a gamba-fuelled fire near our property a year ago are dead. Furthermore, gamba can carry two fires a year (Low, 2011). And every year the weed rises again like Lazarus every Wet Season ready to inflict another conflagration on the bush.

As the trees disappear so will much of the native fauna. Tree hollows, for example, are essential for 18% for birds, 40% for mammals, 20% for reptiles and 13% for frogs (Taylor, Woinarski, & Chatto, 2003). In the Top End avifauna utilising nest hollows include several species of duck including Green Pygmy-goose and Radjah Shelduck, cockatoos and parrots, including Red-tailed Black Cockatoo and Northern Rosella, owls and allies, kingfishers, Dollarbird, Black-tailed Treecreeper, woodswallows, and Gouldian Finch. The latter species nest in the hollow branches of a limited number of eucalypts, including Salmon Gum, E. tintinnans. The two most dominant eucalypts in the Top End landscape, E. miniateand E. tetrodonta, covering over 445,000 squ. km. (calculated from Fox & Clarke 1972 by Woinarski & Westaway, 2008) may be 250 years of age before they form hollows greater than 10 cm (Woinarski & Westaway, 2008) to form, that is a hollow

Even if there is no fire much fauna will suffer, and granivores are at a particular disadvantage from gamba grass. For example, Gouldian Finch and Partridge Pigeon feed largely on the seeds of native grasses, and both breed in the early Dry Season when grass seed is plentiful. When seed has fallen to the ground cool fires cleared the curing grasses from the ground allowing the birds to continue feeding; when carrying out surveys of Gouldian Finch in the late 1980s I looked for the bird on recently burnt ground.

Gamba grass crowds out the species of grass on which the birds feed. Furthermore, it burns so hot that any seed that does fall to the ground is destroyed. And the next year there is another crop of gamba, ready to spring into action leading to an indefinite cycle of hot, late fires that kill trees and other plants leading to further gamba grass invasion - the grass-fire cycle, and thus creating what one writer has called the ‘field of nightmares’ (Petty, 2013).

This ‘triffid of the plant world’ as Tim Low of the Invasive Species Council has called it, has been declared a weed of national significance, and landholders are expected to control it on their properties. However, the importance of gamba to the cattle industry has meant that various stations have been allowed to keep grazing cattle on the weed (Lawler, 2018) to the dismay of indigenous rangers who find themselves fighting a losing battle trying to control the weed and fight fires (Fitzgerald & Burton, 2018), and those of us who care about our wildlife. Graziers rely on their stock to keep gamba at a level where it will not seed, but given the uncertainties of the live cattle trade there is no guarantee that they will be able to physically or financially control the weed (LandcareNT, 2012).

New infestations keep appearing and it is very doubtful that governments will expend the millions of dollars needed to both control the weed and fight fires. Such blazes are often uncontrollable anyway. One firefighter faced with a firestorm remarked that if they couldn’t hold it “…we’re screwed” (anon. pers. comment, Oct, 2018). That comment could apply to a large number of birds and other fauna, as well as flora, across a huge swathe of the Australian continent. The genie is out of the bottle.

Fire
Fireman fleeing Gamba Grass wildfire ©cSu Lamb, Bushfires Council, 2007

Visiting birdwatchers need to avoid gamba grass. Firstly, it is easily spread by seeds caught in mud from someone’s car (Fitzgerald & Burton, 2018) or seeds blown into a vehicle. Secondly, a fire even some distance away could be upon one in a matter of minutes. Aboriginal rangers have said that gamba-fuelled fires “…are killing the country”, “everything, animals and trees” (Mimal Land Management, 2017). It goes without saying that such fires will also kill people. And that includes unwary birdwatchers.

REFERENCES

Anonymous (n.d.). Gamba Grass Andropogon gayana. Dept. of Environment & Natural Resources, Northern Territory Government.

Anonymous (2018). Vernon Arafura Regional Bushfire Management Plan, Dept. Dept. of Environment & Natural Resources, Northern Territory Government.

Beaumont, T., Keily, T. & Kennedy, S. (2018). Counting the cost: Economic impacts of gamba grass in the Northern Territory. Report commissioned by The Pew Charitable Trusts and Environment Centre Northern Territory.

Csurhes, S. (2005). Weed Risk Assessment: Andropogon gayanus (Gamba grass). Queensland Government Department of Natural Resources and Mines, Brisbane.

Douglas, M.M. and Setterfield, S.A. (2005). Impacts of exotic tropical grasses: lessons from gamba grass in the Northern Territory. In: Proceedings of the Eighth Queensland Weed Symposium, ed. W. Vogler. The Weed Society of Queensland, Brisbane. 69-73.

Fitzgerald, D. & Burton, L. (2018). Indigenous rangers dismayed as NT Government allows cattle station to graze gamba grass weed. ABC News. Retrieved from Webpage

Fox, R.E., and Clark, N.B. (1972). The incidence of termites in eucalypts of the Darwin area. Australian Forest Research 5, 29-36.

Gibbons, P., and Lindenmayer, D.B. (1997). Conserving hollow-dependent fauna in timber- production forests New South Wales. National Parks and Wildlife Service Environmental Heritage Monograph Series 3: 1- 110.

Kennedy, S. (2018). How researchers are mapping an invasive species advancing across an entire region. Ecos, CSIRO. Webpage

Lamb, S. (2008).Accessed 16 July, 2013 from Webpage .

Landcare (2012). Live export uncertainty complicates gamba management, 4 Nov. Retrieved from Webpage).

Lawler, E., 2018, Protecting our natural environment: New gamba grass management plan released, media statement. Northern Territory Government, Sept. 2018. Retrieved from Webpage

Low, T. (n.d.) Gamba Grass, Invasive Species Council Webpage Accessed 5 Sep, 2017.

Low, T. (2011). Climate change and Queensland biodiversity. Department of Environment and Resource Management, Queensland Government, Brisbane. Retrieved from Webpage.

Mimal Land Management (2017). Gamba: A weed that’s killing country. Retrieved from Webpage.

Petty, A. (2013) Field of nightmares: gamba grass in the Top End. Accessed 30 June 2013 from Webpage.

Petty, A., Setterfield, S.A., Ferdinands, K., and Barrow, P. (2012). Inferring habitat suitability and spread patterns from large-scale distributions of an exotic invasive pasture grass in north Australia, Journal of Applied Ecology. 49:742–752

Rossiter, N.A., Douglas, M.M., Setterfield, S.A. and Hutley, L.B. (2003). Testing the grass-fire cycle: Alien grass invasion in the tropical savannas of northern Australia. Diversity and Distributions, 9:169-176.

Setterfield, S. A., Rossiter-Rachor, N. A., Hutley, L. B., Douglas, M. M. & Williams, R. J. (2010). Turning up the heat: the impacts of Andropogon gayanus (gamba grass) invasion on fire behaviour in northern Australian savannas. Diversity and Distributions, 16(5), pp. 854–861. Retrieved from Webpage

Taylor, R., Woinarski, J. & Chatto, R. (2003) Hollow use by vertebrates in the Top End of the Northern Territory. Australian Zoologist: Vol. 32, No. 3, pp. 462-476.

Woinarski, J. & Westaway, J. (2008). Hollow formation in the Eucalyptus miniata – E. tetrodonta open forests and savanna woodlands of tropical northern Australia. Final report to Land and Water Australia (Native Vegetation Program). Project TRC-14. NT Department of Natural Resources Environment and The Arts. Retrieved from Webpage

24th November 2018