Kangaroo Island (4350km2) lies 15km off the South Australian coast and is the third-largest offshore island in Australia (after Tasmania and Melville Island). With nearly half of its native vegetation still remaining, Kangaroo Island is nationally important for biodiversity conservation, primary production and tourism. The island is free of foxes and rabbits but has other feral pests including pigs, deer, cats and goats.
Feral goats (Capra hircus) were an early introduction to Kangaroo Island arriving with the sealers and later with the first settlers nearly 200 years ago. The coastal environment on the western end and north coast of the island became a population stronghold. Feral goats are hardy and overgraze ground plants, and browse shrubs and trees. Of particular concern was their destruction of the drooping she-oak or casuarina (Allocasuarina verticillata), the food source of the endangered glossy black cockatoo (Calyptorhynchus lathami). Feral goats also compete with grazing livestock, such as sheep, for pasture.
In 2006, a trial program to eradicate feral goats from Kangaroo Island began as part of a broader project — known as ‘Repel the Invaders’ — which aimed at effectively managing key feral animals on the island. The goat eradication program was implemented by the Kangaroo Island Natural Resources Management Board (KINRMB) with a small team of local staff and volunteers using a long-term strategic approach and an enhanced hunting method known as the ‘Judas goat’ technique. This case study describes the seven-year control program, which is now approaching eradication of feral goats from Kangaroo Island.
The program aimed to:
- systematically eradicate feral goats from Kangaroo Island
- establish measures to reduce the chance of reinfestation
- monitor changes in the vegetation after goat removal.
Partners and management
The eradication program of Kangaroo Island’s feral goats was a collaborative effort involving various partners. The KINRMB led the program in association with the Invasive Animals Cooperative Research Centre (IA CRC), the then Department of Environment and Natural Resources (DENR), Biosecurity SA (then DLWBC), Forestry SA, landholders, volunteers including Friends of Parks KI Western Districts and the local community.
To achieve eradication, population reduction rates must be faster than the reproduction rate, meaning over half of the population would need to be removed every year. It was not possible to do so across the whole area where goats were found on Kangaroo Island, but feasible if the area was first divided into Management Units (MUs) so that control could be focused on one unit at a time.
Seven MUs containing feral goat populations were defined over a total area of 1452 km2 based on natural barriers for containment and road lines for access. The creation of MUs helped limit reinvasion.
Control was done by coordinated ground and aerial shoots, and a small amount of dam and paddock trapping. Mobs of feral goats were located using Judas goats (see below and Judas goat factsheet), by monitoring with remote motion sensing cameras, and with information from landholders and the community. Professional ground shooting teams of two or three people were used to destroy mobs of up to 16 goats with no escapees. Typically, weapons with .308 calibre were selected, ensuring quick humane killing of feral goats at both close and long range. Aerial control with accredited shooters was carried out using a helicopter as an opportunistic event following a large fire in early 2008 across MU2, 3, 4 and 5, and at the end of the program when only a few individuals were left in difficult terrain.
The program used both locally captured and imported goats (from Salt Creek or Iron Knob, SA) for the Judas method. Judas goats were preferably 6—36 months of age, in good condition, not pregnant, and white in colour. All-white Judas goats were easier to spot at a distance or in thick vegetation, which helped identification and prevented accidental shooting. Pre-release procedure included acclimatisation and quarantine in a pen for at least one week. Judas goats were photographed, measured, weighed, ear-tagged, fitted with a Sirtrack VHF or Sirtrack Kiwi 101 satellite collar, and sterilised at a local veterinary clinic.
Judas goats were then released individually at a minimum of 2km apart (if they were from different groups), and 5km apart (if they originated from the same group). This reduced the chance of the goats re-associating with each other. Judas goats were located using a Titley 26k receiver, three-element Yagi antenna and an omnidirectional vehicle-mounted whip antenna. MUs were monitored with Judas goats for up to 18 months after the last feral goat was removed.
The program also carried out focus group meetings and distributed a survey to the island community seeking to understand local opinions relating to feral and domestic goats.
Throughout the program, information was collected on feral and Judas goats, including their use of the island landscape (distribution, watering points and shelter locations), the food they ate, mortality, mob formation and behaviour.
Features of the study
This program made use of the Judas goat technique, which is an approach that exploits the sociability of goats to enhance hunting and shooting efforts. Judas goats are captured, sterilised, radio-collared and then re-released to associate with and reveal the location of other feral goats in the area. Judas goats have been used successfully in Australia and around the world, particularly at low feral goat densities and when the aim is eradication3 (see also Judas goat factsheet).
Kangaroo Island was a well suited location for a large-scale goat eradication program. Border protection, biodiversity monitoring and management of feral animals were already underway, providing a good framework for control. Importantly, natural re-invasion by feral goats from the mainland was not possible.
Feral goats were found mostly along the coastal margins and creek systems of the north and western end of the island, from Snellings Beach around to Hanson Bay. Mob sizes generally varied between 1–16 goats, and population densities in MUs were up to 3.4 goats/km2.
Analyses of goat stomach contents identified 75 plant species eaten by feral goats. Most were native species but weeds were also present, including cape weed, phalaris and barley grass. She-oak or casuarina was the most commonly eaten plant.
Native vegetation in sensitive areas has already shown strong signs of recovery since the removal of feral goats (see cave photos above). Judas goats assisted in the location and destruction of over 1150 feral goats. Feral goats have been successfully eradicated from MU1, 2, 3, 4, 6 and 7. It is estimated that less than five feral goats remain on the island.
Cost of control varied as the program progressed, influenced by factors such as increasing proficiency of hunting teams and changing habitat conditions (bushfire clearing and regrowth). For MU2 (Flinders Chase National Park) the first 164 goats removed by ground shooting operations cost about $70 each, but this figure then rose to about $235 per animal for the last 18 feral goats removed by this method. Aerial control destroyed a total of 111 goats in MU2, 3, 4, and 5, at a cost of $363 per animal.
Survey of local public opinion on feral and domestic goats indicated that support for eradication of feral goats from Kangaroo Island was very high at 94%. Most respondents (>65%) believed that domestic goats should be identified and registered and/or be kept within more stringent fencing.
- The program was successful largely due to the partnerships maintained between project officers, landholders, parks staff and the island community. Use of local staff and hunters kept landholders and community onside, which meant continued access to diverse areas of the island where feral goats roamed. Sightings of feral and Judas goats were also very well reported throughout the program.
- Significant bushfires in December 2007 resulted in widespread reduction in vegetation cover and food availability, which accelerated control operations. Open terrain meant onground shooting was more efficient and the use of aerial control was possible.
What didn’t work
- During the final stages of eradication, Judas goats tended to join together and associate with few feral goats as a direct result of the very low population densities. Project officers managed this by carefully balancing the number and placement of Judas goats.
- Coordinating a helicopter with aerial shooters from the mainland, combined with typical strong winds and poor weather conditions on the island, meant aerial shooting was difficult, costly and less viable than ground shooting with Judas goats.
- The 2007 bushfires resulted in dense regrowth in later years, which hampered onground operations by blocking access and providing protective cover for remnant feral goats.
The goat eradication program on Kangaroo Island has successfully removed nearly all of the feral goat population, with less than five individuals expected to remain by late 2013. This program demonstrates that the use of Judas animals can be highly effective against feral goats, and when combined with a long-term strategic approach, the goal of eradication is achievable on an island as large as Kangaroo Island.
Continued monitoring confirms that goats no longer impact many of the sensitive areas of the island that were once goat strongholds. The casuarinas and other native plants are recolonising at a much greater density now the goats are gone.
Engagement between resource managers, landholders, domestic goat owners and the community is ongoing5, helping to reduce the risk of reinfestation in the future. Policies are being considered to require domestic goats to be maintained under a permit system, under Section 188 of the Natural Resources Management Act 2004 (South Australia) to prevent goats escaping and reinfestation.
|Author||Centre for Invasive Species Solutions|
|Publisher||Centre for Invasive Species Solutions|
|ISBN/ISSN||PestSmart code: GTCS1|
|Control method||Judas Technique|
|Links||PestSmart Toolkit for feral goats: www.pestsmart.org.au/pest-animal-species/feral-goat/|
|Documents||PestSmart Case Study: Feral goat eradication on Kangaroo Island [540 Kb PDF]|