Case study: Warren ripping on Thackaringa Station, western NSW

Thackaringa Station is a long-established wool-producing property of 66,000 ha near Broken Hill in western NSW, owned by David Lord and his family. Historically, rabbits have persisted here in large numbers, causing vegetation and biodiversity loss, and increased grazing pressure. These impacts are comparable across many parts of arid-zone Australia, which are suffering the same devastation.

In past decades, Thackaringa’s owners took the opportunity to capitalise on the initial impact of biological control on the rabbit population. Following the establishment of myxomatosis in the 1950s, sodium monofluoroacetate (1080) baiting and some warren ripping was used in an attempt to keep rabbit numbers down on the property. As the myxoma virus became less effective, the rabbit population steadily increased (although never to pre-1950 levels) and warren density was 0.47 warrens/ha over the majority of the property. In the late 1980s, a trial warren-ripping program was run in the region, destroying 72,000 warrens over 22 properties, including 6000 on Thackaringa.

Rabbit haemorrhagic disease (RHD) dramatically reduced rabbit numbers on Thackaringa in 1995, with an estimated 750,000 rabbits dead within six weeks. The owners observed an immediate positive response by native plant species and wildlife. However, rabbits survived at low densities (less than 10% of the pre-RHD population) and continued to restrict the regeneration of arid-zone plants.

By 1999, David could see that biological control alone was not enough to significantly reduce the effect of rabbits on plant growth and survival. His trials with rabbit-proof enclosures showed that where there was no rabbit grazing, the rare purple-wood wattle Acacia carneorum (previously A. carnei; listed as a nationally vulnerable plant species) regenerated 16 times more often than in areas that rabbits could access. Before the trials, only mature trees aged between 180-300 years old were present, suggesting that there had been no recruitment of this plant species since rabbits arrived in the area in the early 1880s. It was considered likely that other native plant species would have been similarly affected.

Thackaringa’s owners were able to secure funding from the Commonwealth Endangered Species Program in 2001–2002, a program of the former Natural Heritage Trust (NHT). With this funding, they were able to design and implement a comprehensive warren-ripping program to finish earlier works carried out on the property and to promote best practice rabbit control.

Aim

To maximise past investments to control rabbit populations and reduce rabbit impacts on Thackaringa using an intensive warren-ripping program, and enhance the regeneration of A. carneorum and native species diversity within the region.

Partners and management

A key component of this project was the large personal financial and physical contribution made by the Lord family, in addition to the day-to-day running of the station. Initial funding to conduct rabbit control works was sourced through the then Australian Government’s National Soil Conservation Program. The NHT supplied funds, which enabled ripping of the remaining 22,000 warrens.

Process

Before ripping, an observer on a motorbike identified and mapped all warrens on the property using a GPS. When ripping was due to start, the observer used the GPS to relocate the warrens and lead the bulldozer to each warren. While the bulldozer was ripping, the observer searched for any missed warrens nearby.

A local contractor was hired to conduct the ripping works. A large tractor with rubber tracks was initially used, but was found to be unsuitable on stony ground. A larger D8 bulldozer was then used to rip the remaining warrens. Extensions on either side of the ripper bar enabled the bulldozer to pull five tines instead of three, with 600 mm spacing between the tines, which increased ripping productivity. Winged boots (extensions on each of the five tines) eliminated the need to cross rip. Up to 32 large warrens each hour (26 warrens each hour on average) were able to be ripped using the larger bulldozer.

Features of the study

This program featured a large financial contribution from the landholder, and the use of a heavy bulldozer modified with winged boots to effectively rip warrens.

Results

Monitoring after ripping showed no re-opening of warrens. However, 500 missed warrens (ie not ripped) have since been mapped and will be ripped in the near future.

In recent years, there has been sustained growth of seedlings and perennial grasses on the property. Of particular importance is the regeneration of the rare A. carneorum. Plants that were once considered winter/spring flowering on Thackaringa, such as the Sturt’s desert pea (Swainsona formosa), have continued to flower throughout the summer months. More wildlife has been observed on the property since warren ripping, including a large number of little button-quail, echidnas and small native marsupials. The owners have also reported fewer foxes, and feral cat sightings are rare.

After intensive warren ripping, Thackaringa Station has been able to keep rabbit numbers low and maintain its core flock of breeding ewes without resorting to agistment during a 20-year period of drought. The property’s average lambing rate, excluding years when ewes were unable to be joined due to the drought, has been maintained at 75%. Control measures have reduced potential rabbit grazing pressure ten-fold: from the equivalent of 140,000 dry sheep before RHD to 14,000 dry sheep equivalents (DSE) post-RHD.

Following the ripping program, rabbits have had limited impact on Thackaringa’s production and biodiversity but David maintains “it’s important not to become complacent”.   

Costs

Ripping costs in 2002 were $145 (GST exclusive) for every hour, or about $5.58 per warren. There were also costs associated with mapping the warrens and having an observer escort the bulldozer during ripping. On a good 12-hour working day, the observer could map up to 200 warrens at a cost of the wage and motorbike plus incidentals (eg a few dollars for batteries for the GPS).

What worked

  • Warrens were located for ripping after they had been mapped using a GPS. Up to 200 warrens could be GPS-mapped by one person in a day, compared with 100 warrens recorded and marked manually by two people in one day. The observer who led the ripping machine from one warren to the next was also able to seek out any missed warrens before the machine moved on. This process saved time and improved efficiency.
  • A skilled bulldozer operator was paramount to the efficiency of the operation.
  • Efficiency was improved when a larger bulldozer was brought in. On average, a Cat D5 bulldozer could rip 12.5 warrens per hour and the rubber-tracked machine ripped 18 warrens per hour, compared with 26 warrens per hour using a larger 250 HP Cat D8 bulldozer.
  • Winged boots on the bulldozer helped to completely collapse warrens without needing to cross-rip.
  • Warren ripping was strictly done when conditions were hot and dry, during the summer months (December—March). This is when rabbit populations are already under significant stress due to the heat and reduced feed availability. The soil is also loose and dry at this time, which causes warrens to collapse more easily.
  • It was found to be more effective to rip those warrens that were out in the open during the cooler part of the day, before moving into the areas of heavy cover. The noise of the machine and the rising heat would drive rabbits underground before ripping, as per the standard operating procedure.
  • Using 1 m-long ripper tines enabled the bulldozer to rip to a sufficient depth to prevent foxes and dingoes digging out and exposing rabbit holes in their quest for an easy feed.
  • A thorough knowledge of the area ensured that the most efficient route was taken by the bulldozer when travelling between warrens.
    A strong commitment to attempt to destroy every warren was important to the success of the operation.

What didn’t work

  • Low angle sunlight and dense vegetation sometimes made it difficult to see warrens from a motorbike, despite the GPS showing a marked warren.
  • Rubber tracks on the smaller bulldozer were damaged while working in the stony country. A larger, heavy-duty bulldozer was better suited to the rocky terrain.

Conclusion

Integrated rabbit control using heavy machinery to destroy warrens can maximise the effect of RHD and reduce rabbit populations to very low densities. In this case study, reducing rabbit numbers helped to minimise the effects of drought on pastures and livestock, and provided benefits to both agricultural production and the natural environment. By investing in rabbit control and using a best practice approach, Thackaringa’s owners have been able to increase biodiversity without varying livestock carrying capacity.

Author Invasive Animals CRC
Year 2011
Publisher Invasive Animals CRC
Pages 4
ISBN/ISSN RABCS3
Region NSW
Documents

PestSmart Case Study: Warren ripping on Thackaringa Station, NSW [400 kb PDF]

Links

PestSmart: Rabbits