Kakadu National Park Landscape Symposia Series 2007–2009

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3.4 Challenges and impediments to the strategic management of weeds in northern Australia

The development and implementation of the NT WRM system is a critical step in improving strategic weed management in the NT and across northern Australia in general. However, there are still a number of issues that pose challenges to the successful and strategic management of weeds in the region. Our current limited ability to predict where and when weeds may become established across the landscape is a major obstacle in the prioritisation and allocation of management resources and needs to be refined within the weed management process. Similarly, the explicit inclusion of cost benefit analysis in the WRM process is an important refinement to the national WRM approach and will allow a defensible appraisal of conflict species (those that have some economic value but pose a serious environmental/cultural/economic threat) such as exotic pasture grasses or biofuel species. Cost benefit analysis will also provide a useful method of deciding on the appropriate management response for a particular species (eg whether to aim for eradication or containment). Research projects aimed at addressing some of these fundamental issues, particularly in relation to grass weed species, are underway.

References

Northern Territory WRM Technical Working Group 2007. Developing a weed risk management system for the Northern Territory: Progress report. Department of Natural Resources, the Environment and the Arts, Darwin NT.

Setterfield S, Beilby A, Douglas M, Clarkson J, Barratt J, Ferdinands K, Grace B & Wirf L. 2006. A weed risk management system for the Northern Territory. In Proceedings of the 15^th Australian Weeds Conference: Managing weeds in a changing climate. eds C Preston, JH Watts & ND Crossman. Weed Management Society of SA Inc, Adelaide.

Standards Australia, Standards New Zealand & the CRC for Australian Weed Management 2006. National Post-Border Weed Risk Management Protocol. Standards Australia, NSW and Standards New Zealand, Wellington.

Virtue JG 2005. South Australia weed risk management guide. Department of Water, Land and Biodiversity Conservation, South Australia, Adelaide.

Acknowledgments

The NT WRM system was developed via a collaborative project between Charles Darwin University and the Department of Natural Resources, Environment, the Arts and Sport (NRETAS). The project was possible thanks to a NHT2 grant from the Natural Resource Management Board NT to Charles Darwin University. Our thanks to the project team: Natalie Rossiter, Laura Wirf, Jane Barratt, Blair Grace; the members of the NT WRA Technical Working and WRM Reference Groups who assisted in the development and implementation of the NT WRM system and assessment of species. Thanks to Alice Beilby for her support for both the project and the collaborative approach between CDU and NRETAS.

4 Weed management in Kakadu National Park

F Hunter^⁵, M Ibbett^⁶ & B Salau^⁷

4.1 Focus summary

Invasive weeds have the capacity to cause dramatic changes across the landscapes of Kakadu National Park
The Kakadu National Park Management Plan 2007–2014 identifies a number of priority actions in relation to weed management in the Park, including the development and implementation of management programs and strategies for high priority weeds, and the mapping of the distribution of weeds in the Park.
Two teams of rangers are employed to combat weeds in the Park: the mimosa team which primarily targets Mimosa pigra but also assists with other weeds in the wet season, and the grassy weeds team which targets para grass, mission grass, olive hymenachne and gamba grass.
Mimosa, salvinia and olive hymenachne are listed as Weeds of National Significance and are specifically targeted for action in the Park’s Plan of Management.
Grassy weeds, particularly mission grass, gamba grass and para grass have the capacity to fuel destructive fires which can cause significant impact to native vegetation and habitat for native wildlife. All weed species discussed in this paper have the capacity to restrict or prevent traditional hunting activities, limit or prevent access to traditional foods like yams, lilies and to make habitat unsuitable for traditionally significant species like magpie geese. These are important reasons for ongoing control of weeds in Kakadu National Park.

The Paper

4.2 Introduction

Invasive weeds have the capacity to cause dramatic changes across a variety of landscapes. In the Top End of the Northern Territory Mimosa pigra has converted thousands of hectares of floodplain habitat to sometimes impenetrable shrubland, simultaneously reducing biodiversity and rendering areas unsuitable for cultural, recreational and pastoral activities. Similarly, pasture grasses such as mission, gamba and para grass have the capacity to outcompete native species, in the process altering fire regimes and other essential ecosystem processes.

Kakadu National Park has a relatively impressive record of removing, controlling and preventing the invasion of weed species within the Park. Of the 700+ species of plants recorded in the Park, only 120 (approx 7.8%) are considered invasive in comparison with the average of 21% in other conservation reserves in Australia. Despite this, it is expected that the number of invasive species in the Park will continue to rise in the future, primarily as a consequence of increasing tourism and development. The impact of feral animals on the spread of weeds is also a matter of some concern. Weed control programs need to be undertaken with feral animal control programs for their effectiveness to be maximised.

The current Plan of Management for Kakadu (Director of National Parks, 2007) outlines a number of key actions in regards to the management of weed plants in the Park. Two of these, the development and implementation of management programs and strategies for high priority weeds, and the mapping of the distribution of weeds in the Park, are primarily the responsibility of two dedicated teams of rangers.

This paper describes the current progress against actions 5.11.11 and 5.11.13 of the Plan of Management, the development and implementation of management programs and strategies for priotiry weed species and the ongoing mapping of the distribution of weeds in KNP. The paper particularly focuses on the management of mimosa Mimosa pigra, salvinia Salvinia molesta, olive hymenachne Hymenachne amplexicaulis and para grass Brachiaria mutica but also briefly considers mission grass Pennisetum polystachion and gamba grass Andropogon gayanus which are also serios weed issues in Kakadu.

4.2.1 Mimosa pigra

Mimosa pigra (also known simply as Mimosa or the giant sensitive plant) is native to tropical America. It is thought to have entered Australia through Darwin, but the exact timing and mode of entry are not confirmed. Within its native distribution mimosa grows to a maximum of 1–2 m tall but in Australia, where there are no native predators of the plant, it can grow to up to 6 m. In its favoured floodplain habitat, mimosa forms dense stands that out-compete all other native vegetation. Mimosa is a major problem in the Northern Territory because it decreases the cultural and conservation value of wetlands and reduces pastoral productivity by replacing grass and sedges with an inedible, impenetrable wall of thorny vegetation. Mimosa is a prolific seeder and as such has the capacity to spread and become established rapidly. The seeds are readily dispersed by vehicles, livestock and other animals. However, the most important dispersal agent in floodplain habitats is water itself, as the seeds can be carried considerable distances downstream from the source plant.

Mimosa was first discovered in Kakadu in 1981, at the outflow end of Yellow Waters. Since then a dedicated team of four staff has worked to locate and record new outbreaks and then undertake control work at these sites. Hundreds of plots have been surveyed across Kakadu (Figure 1). The floodplains of Kakadu are largely free of mimosa.

The approach to controlling mimosa usually involves cutting tracks into the stand and the broad application of Velpar, a herbicide that sterilises the ground and reduces future germination from the seed bank. Some of the major outbreaks that have been targeted in Kakadu have been at Munmarlary, Bamboo Creek and Cattle Creek (Figure 2).

4.2.2 Salvinia molesta

Salvinia molesta, or salvinia as it is commonly known is a free-floating aquatic fern that can grow rapidly to form dense mats on the surface of still or slow-moving water bodies. It reproduces asexually, from pieces of leaf or stem material that are spread by floodwaters, animals, vehicles and boats. Salvinia was first recorded in Kakadu in 1983. It has been recorded in a number of waterways in Kakadu including Yellow Water.

Various methods have been used to control salvinia including herbicides, mechanical removal and biological control. Herbicides have limited success because the non-wettable upper parts of the plant prevent absorption of the chemicals. The ease with which plants break into fragments and spread also reduces the effectiveness of mechanical removal. In Kakadu the preferred method of control is the use of the biological control agent (the weevil Crytobagous salviniae). The weevils can be highly effective at removing salvinia (Figure 3) but because of the rapid nature of reproduction in this species, re-infestation and new infestations can develop quickly.

Figure 1 Location of Mimosa pigra plots in Kakadu National Park, November 2007

Figure 2 Mimosa outbreak at Cattle Creek before treatment (left) and after treatment (right)

4.2.3 Olive hymenachne

Olive hymenachne Hymenachne amplexicaulis is recognised as a weed of national significance that has the potential to smother native vegetation and form dense monospecific stands in riverbank and swampy, seasonally inundated areas. It is a native of tropical and central South America that was deliberately introduced and planted in Australia as a pasture grass. It produces an abundance of seed that can be spread by water flows and livestock, as well as some native wildlife like magpie geese. The root systems of the grass are also easily broken and can be spread to other areas by water and livestock. Olive hymenachne is very similar to the native hymenachne species Hymenachne acutigluma (Figure 4).

Olive hymenachne can cause major environmental impacts including reducing or preventing the flow of waterways, reducing plant diversity and habita availability for native wildlife and reducing the opportunity for traditional use of natural resources by indigenous people. Olive hymenachne has the potential to rapidly become a major weed issue in Kakadu as it has been recently found in a number of the remotest wetlands in the Park (Figure 5). Infestations of olive hymenachne are managed by physical removal and chemicals.

Figure 3 Salvinia infestation at Djabiluka Billabong (clockwise from top left): April 1992 (prior to release of weevils); September 1992 after weevil release; October 1992 other vegetation emerging through salvinia; and November 1992 billabong surface is clear of salvinia

Figure 4 Olive hymenachne (on the left of this photo) has much broader leaves than native hymenachne (right)

Figure 5 Olive hymenachne has been found in a number of remote wetlands in Kakadu

4.2.4 Para grass

Like many other grassy weeds para grass Urochloa mutica was introduced to Australia as an improved pasture grass. It has been present in Kakadu for many years, with infestations known to have become established in the Cannon Hill area as early as 1940. It grows in wet or seasonally flooded areas, drainage lines and creek banks where it can form dense floating mats 1–2 m thick. As a result it can choke out native species and prevent the establishment of more desirable native species. It can adversely affect wildlife and restrict traditional hunting. Dense infestations of para grass can provide a substantial fuel load for late season fires on the floodplains. These fires can be particularly intensive and destructive. Para grass has the capacity to re-establish after these fires but many other native species do not.

Infestations of para grass have the potential to rapidly spread by seed and through the dispersal of vegetative parts. Monitoring of an infestation on the Magela Floodplain has demonstrated the rapid spread and increase in density of the species. Predictive modelling suggests that the infestation will continue to develop if left unchecked (Figure 7). Para grass is a difficult weed to control. In Kakadu, control work is only undertaken in catchments where there is a reasonable likelihood of success.

4.2.5 Mission grass

Mission Grass Pennisetum polystachion (Figure 8) is a tall, tussocking perennial grass that was introduced to Australia from Africa as a pasture grass. It is listed as a noxious weed under Northern Territory legislation. It is common in disturbed areas such as roadsides, degraded pastures and waste sites. The seed is readily spread by water, wind, in the fur of animals and often in the radiators or other parts of vehicles. It is now common in many parts of Kakadu and in the town of Jabiru. Mission grass is the main species targeted by Kakadu’s grassy weed team. Grid surveys are undertaken to locate and spray mission grass, preferably before it sets seed in April. Mission grass stays green until late in the dry season and it can provide substantial fuel loads for late season fires. These fires can be particularly destructive, and may result in the loss of native vegetation and important habitat for native wildlife.

Figure 7 GIS-based modelling is being used to predict the likely development of para grass infestations in the Magela floodplain in Kakadu (Bayliss et al 2006)

4.2.6 Gamba grass

Like mission grass, gamba grass Andropogon gayanus (Figure 8) was introduced to Australia from Africa as a pasture grass. It is also a perennial tussock-forming grass that can grown up to 4 metres tall. It inhabits creek lines, flood plain fringes, Eucalyptus dominated savannas where rainfall is greater than 600 mm per year and degraded areas including roadsides. The seeds are easily spread short distances by wind as well as being carried by vehicles. It is a deep rooted grass that forms very dense stands. It cures later in the season than native grasses and like mission grass can fuel very hot, destructive fires that can often result in the death of native woody vegetation. Gamba grass is a priority species for the grassy weeds team and targeted spraying is regularly undertaken to control this species.

Figure 8 Mission grass (left) and gamba grass (right) form dense, highly combustible stands