Airlie Island is a 25 ha nature reserve and lies off the Pilbara Coast, 35 km northeast of Onslow. The island has been used in the last decade as a fuel storage depot by three successive companies. Imminent decommissioning of the petroleum activities on the island require that buffel grass control be undertaken to ensure that the weed is contained, controlled or eradicated. Though the lessee is only required under agreement to control buffel grass and revegetate in the leased area (approximately half of the island), they are undertaking a program to try to eradicate buffel grass from the entire island and revegetate areas, where necessary.
The aims of this project are to eradicate buffel grass (Cenchrus ciliaris) from Airlie Island, to develop and implement methods to restore indigenous vegetation, and to collect and store seed for future restoration works. The most effective herbicides under these conditions were determined, and extensive field trials indicated the main perennial shrubby species on the island are resistant to the two selected (Roundup and Verdict). The window of opportunity for spraying is only about two weeks. Results of the spraying on Airlie Island indicate that 98% of the original stands of buffel grass has been controlled. Replanting with greenstock is preferable after heavy rainfall, the main shrubby species planted after spraying with Roundup can then be oversprayed, when required, with Verdict. Two to four sprays a year, depending on rainfall events, are required for a period of at least three years (estimated age of soil seed bank) to control this weed with follow up monitoring and backpack spot spraying or hand removal.
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Australia & New Zealand
Country or Territory:
Coastal, Dune & Upland
Area being restored:
Primary Causes of DegradationInvasive Species (native or non-native pests, pathogens or plants)
Buffel grass seed was most likely brought to Airlie Island in soil used in the construction of the lighthouse in 1913. By 1987, buffel grass roughly occupied a 2.5 ha kidney-shaped area around the lighthouse. The initial spread of buffel was relatively slow; however, disturbance events appear to have enhanced its invasive capacity.
On Airlie Island the presence of buffel grass threatens the natural plant communities by replacing, almost entirely, the understory cover of indigenous grasses and herbs. Buffel grass has already become a dominant plant species on the island and other islands along the Pilbara coast. This environmental weed substantially increases the fire risk, which may impact the habitat of local fauna and may cause significant and permanent changes in vegetation structure and diversity. At the start of spraying in 1999, buffel grass formed a near monospecific stand over eight hectares (33%) of the island. This weed may release allelopathic chemicals into the soil that inhibit growth of other species, potentially acting as a key displacement agent for most of the native vegetation. Buffel grass may also be detrimental to the island’s fauna, especially the breeding cycle of shearwaters and the survival of herpetofauna.
Reference Ecosystem Description
The rainfall pattern, lack of permanent surface water and small size of the island means the vegetation must be drought tolerant and able to cope with salt-laden wind. Most of the island is dominated by two Acacia species (A. bivenosa and A. coriacea), with Rhagodia preissii, Eulalia aurea and now buffel grass (Cenchrus ciliaris) as the major perennials. Shorelines are dominated by Spinifex longifolius, Ipomoea pescaprae, Sporobolus virginicus and Eulalia aurea. During favourable seasons, a large number of annual species can be found in abundance, including Portulaca intraterranea, Euphorbia spp., Boerhavia repleta, Cleome viscosa, Cuscuta australis and Threlkeldia diffusa. The islands off of the Pilbara coast have a variety of seabird nesting records, including pelicans (Pelecanus conspicillatus), wedge-tailed shearwaters (Puffinus pacificus) and pied cormorants (Phalacrocorax varius). Mammal fauna includes the pale field-rat (Rattus tunneyi), little red kaluta (Dasykaluta rosamondae), western chestnut mouse (Pseudomys nanus), and short-tailed mouse (Leggadina lakedownensis). Sea turtle nesting is known to occur, but details are sketchy. Dugongs (Dugong dugon) are relatively common in inshore waters.
Phase one of this project was to investigate and research the biology of buffel grass and to develop a control program which will integrate eradication or sustainable control of buffel grass with the reinstatement of indigenous species.
Phase two of the project is the control of buffel grass over the whole island based on the results of Phase one, restoration of indigenous communities, and initiation of a seed collection and storage program for future revegetation works.
The project does not have a monitoring plan.
The island has been used in the last decade as a fuel storage depot for Western Mining Corporation Petroleum Division, Novus Petroleum’s offshore oilfield projects and by the present lease holder Apache Energy. Imminent decommissioning of the petroleum activities on the island require that buffel grass control be undertaken to ensure that the weed is contained, controlled or eradicated. Though the lessee is only required under agreement to control buffel grass and revegetate in the leased area (approximately half of the island), they are undertaking a program to try to eradicate buffel grass from the entire island and revegetate areas, where necessary.
Description of Project Activities:
Phase One Spray plot sizes were 1x1 m, each separated by a 0.5 m-wide corridor. Three replicas of each plot were made within each trial site. All plots were sprayed working systematically across the plot and back again in the opposite direction to ensure even coverage. Compressed sheeting was used as a barrier to ensure there was no drift onto other plots. Three multi-herbicide trials were conducted, with Roundup Biactive 6 l/ha and Verdict 8 l/ha achieving a high kill rate. Large-scale (100m2) trials were attempted with Roundup and Verdict, and were aimed at confirming the effectiveness of these herbicides sprayed on a larger scale prior to the implementation phase. Phase Two Roundup Biactive at 8 l/ha and Verdict at 6 l/ha were used. The strategy employed Roundup in the first spraying operation as there were few annual indigenous plants emerged that might be affected by the spray. Then Verdict used as a blanket spray as annuals and possibly some native perennial plants are at a susceptible stage. Each spraying unit requires two operators, one as a sprayer and the other to release and withdraw the hose. In dense stands we used a blanket spray technique, and in other areas spot spray. Three rounds of spraying were conducted from 1999-2000. The first round used Roundup for ease of operations and because it is more cost effective than using Verdict. Herbicide application was following six weeks of heavy soaking rain. Ideally the spraying program should have been initiated two weeks earlier, as most of the buffel grass had just reached first anthesis and early seeding. The buffel grass was in rapid growth with some plants beginning to dry out by the fourth day of spraying. Most of the application was blanket spraying of heavily-infested buffel areas, avoiding as many indigenous plants as possible with very few annual seedlings present. Previously-sprayed areas were spot sprayed, avoiding contact of Verdict on Eulalia to reduce damage to planted greenstock and annual indigenous plants. Though original trials indicated Eulalia aurea was resistant to the grass-selective herbicides, subsequent trials showed they were very sensitive and future spraying would need to avoid excessive contact with Eulalia. The second round of spraying was mainly blanket spraying resprouts and seedlings with Roundup, as there was such a large amount to spray and low levels of annual indigenous plants in the previously-sprayed area. Some spot spraying with Roundup in outlier areas and spot spraying Verdict over replanted areas was done. The third round of spraying followed six weeks of substantial rainfall on the island as a result of a cyclone. The buffel was in excellent condition for spraying. About half of the area (low impact area) was sprayed with Roundup, and the remainder (rich herb fields) with Verdict. Backpack sprayers were used on outlier populations. A thorough inspection after the spraying operation detected some other plants which were removed by hand. At this stage we estimate most of the seedlings and over 98% of mature buffel grass plants on the island were eradicated. All seed for the greenstock propagation portion of Phase two was collected from Airlie Island to ensure only local provenances were utilised. Plants of A. coriacea, A. bivenosa, Rhagodia preissii and Eulalia aurea were propagated from seed in glasshouses at Kings Park and Botanic Garden and transported to Airlie Island. Acacia seed were hot-water treated and left to soak overnight. Seed was sown into punnets and after about six weeks, seedlings were pricked out into tubes. Seed was sown in summer for planting in early winter of the following year. Benchmarking (using quadrats and transects) in pristine areas of the island gave a figure of natural plant density and the estimated number of plants required for revegetation of bare areas. This density was doubled to allow for assumed seedling death. The total area requiring revegetation is estimated at one hectare; this excludes the leased area which is to be revegetated after decommissioning the oil facilities. The first planting trial in April 1998 consisted of A. bivenosa, A. coriacea, Rhagodia preissii, and Eulalia aurea which were planted into moist soil. A further trial in June/July used only 200 A. bivenosa. A large trial in June 1999 used 1400 plants: A. bivenosa and E. aurea in high numbers; A. coriacea and R. preissii in low numbers. These were planted in very dry conditions. Planting was in a number of locations in areas previously heavily-infested with buffel grass. After cleaning with bleach and thoroughly rinsing, the spraying units and long hoses were utilised to water the plants in. The results from the first two planting trials were outstanding, with 89% to 97% of all species surviving and growing vigorously 10 weeks after planting. The large planting in June 1999 was a failure, with no rain for six months after planting. Almost all the plants of all species died. Though no exact count was done, a few plants of A. bivenosa are alive in two or three locations, with survival of <1%
Ecological Outcomes Achieved
Eliminate existing threats to the ecosystem:
We estimate most of the seedlings and over 98% of mature buffel grass plants on the island were eradicated. Seedlings may germinate from the soil seed bank. However, the viability of the soil seed bank is rapidly declining and sustainable control of buffel grass will depend upon careful monitoring and a judicious "˜mop-up' spray program.
Factors limiting recovery of the ecosystem:
Some of the key constraints that were addressed were: -Getting the equipment on the island. This was trucked up to the remote townsite of Onslow then placed on a barge which goes to the island once a week. No large, heavy or dangerous items can be carried on the helicopters which take personnel to the island from Barrow Island, the direct flight destination from Perth. -No internal combustion engines are allowed on the island (due to possible hydrocarbon contamination/spills), therefore we had to use battery-operated spraying units. Water supply for spraying is from a reverse osmosis supply generated on the island, rainwater also goes into the tanks. This supply is limited; on one occasion the pipe to the tanks burst and we only had just enough water for spraying operations. -Cyclones during spraying operations. When these develop the island is evacuated and all operations ceased. -The 250 litre spray tanks have to be carried to each station by hand, as no vehicles are allowed in the natural vegetation areas. It is therefore necessary to judge very carefully how much material you need in the tank to finish off the area. -Coping with the extreme weather conditions (e.g. 36Â°C and 80% humidity during summer spraying operations) and difficult working conditions (e.g. walking backwards when spraying) and abundant dead twigs which fouled boots and clothing.
Socio-Economic & Community Outcomes Achieved
Economic vitality and local livelihoods:
Outcomes of this study will be directly relevant to the decommissioning of the Airlie Island oil installation while providing benchmark data on the control of buffel grass. The study will therefore be of regional, national and international significance to land managers and conservation agencies where buffel grass is an environmental weed. Information on buffel grass control generated from this program is already being utilised by local land managers on adjacent islands and adjacent mainland as well as other areas in Australia such as Queensland and central Australia.
Key Lessons Learned
The research results from Phase one of the program underpinned the implementation phase. A good knowledge of the biology of the buffel plants opened up a narrow window of opportunity for spraying and optimizing kill rates. Knowledge of seed production, soil seed bank and longevity of seed enabled us to plan ahead with a work schedule.
The initial trials provided an appreciation of the problems and how adaptable you have to be with your implementation program, which is entirely dependent on the vagaries of the weather. The spraying results proved to be inconsistent as every trial was different. However, the results from the large-scale spraying program were used as the basis for the implementation phase. The authors are confident that the herbicides, and their concentrations, used in the implementation phase are ideal for the expected varying conditions of the plants. Future operations, due to the recruitment of herbs in the buffel areas, will be using Verdict to avoid damage to these plants. Planting Eulalia aurea should be delayed until the soil seed bank is eliminated, otherwise further spraying could kill or damage Eulalia.
The best time for spraying adult plants is within the range of three to five weeks after sufficient rain has fallen. If spray is applied too early the seedlings are too small to target and with extra rainfall there may be delayed germination. Also, it is not cost-effective to spray twice when one operation can achieve good results. Under quick drying conditions some plants are beginning to get stressed, and may occasionally drop some mature seed before they are sprayed.
Planting greenstock can give excellent results if the ground is moist and there is follow-up rain. We recommend the best time for planting is in late autumn or early winter, though it can also be dry at this time of the year as experienced in the 1999 trial.
The ongoing success of the control of buffel grass and revegetation of Airlie Island with indigenous species depends upon the good aegis of the funding sponsors who manage the island. The program does show that with careful, focussed research, it is possible to achieve effective and timely weed control in the arid zone.
Though the spraying program is in hand, the program has identified an urgent need to revegetate bare areas. The dead below-ground biomass of buffel does provide soil-binding to prevent wind erosion for at least three years, in which time revegetation needs to proceed.
High on the priority list is a comprehensive seed collecting and storage program of all species native to the island, with key areas being the rich herb fields between the Acacias and other perennial plants when decommissioning takes place. Some stabilization trials also need to be undertaken as cyclonic winds are a regular occurrence during the summer season. A recent cyclone altered the shape of the island and caused erosion near a flare installation which is being re-stabilized and revegetated by consultants based on the mainland.
Sources and Amounts of Funding
Funding was provided by Western Mining Corporation Petroleum Division, Novus Petroleum, and Apache Energy, all of whom were leaseholders on Airlie Island at some point.
Dixon, I.R., K.W. Dixon, M. Barrett (2002). Eradication of buffel grass (Cenchrus ciliaris) on Airlie Island, Pilbara Coast, Western Australia. Turning the Tide: The Eradication of Invasive Species. C.R. Veitch, M.N. Clout, Eds. Gland, Switzerland and Cambridge, U.K., IUCN. IUCN SSC Invasive Species Specialist Group: 92-101.
Not peer reviewed: