Dirk Hartog Island: Return to 1616
Return to 1616 aims to restore the vegetation and habitats of Dirk Hartog Island to how Dirk Hartog would have seen them in 1616.
The island has experienced significant changes since Dirk Hartog landed there on 25 October 1616. Sheep and goats changed the vegetation, their grazing habits and trampling reducing the food and shelter available for native species. Feral cat predation added to the pressures on native species and made it impossible for some to survive.
Ten species of small mammals and marsupials did not survive the changes to the island’s ecology – the western barred bandicoot, chuditch, mulgara, dibbler, greater stick-nest rat, desert mouse, Shark Bay mouse, heath mouse, woylie and boodie. A small bird, the western grasswren was also lost.
Return to 1616 brings hope for these species. With sheep declared eradicated in June 2016 and goats almost gone, habitats are recovering. Along with the goats, it is expected that feral cats will be declared eradicated by mid-2018. Then fauna reconstruction will begin – over twelve years these species and two additional marsupials will be translocated to Dirk Hartog Island.
The Dirk Hartog Island National Park Ecological Restoration Project is funded by the Gorgon Barrow Island Net Conservation Benefits Fund with additional funding from the Department of Parks and Wildlife.
Intensive and systematic eradication and monitoring programs began in 2012 and are in their final monitoring phases in 2016-2018.
Removing Goats
Major destocking efforts began on the island in 2007 when the pastoral leaseholders removed about 4000 sheep by barge. Along with destruction of about 2000 goats during the previous two years, this was in preparation for the change in tenure from pastoral lease to national park. Teams of Parks and Wildlife staff then culled the remaining sheep and most of the goats between 2008 and 2013.
Radio collars were fitted to fifteen female goats in order to track their whereabouts on the island. Female goats spend time with other females and attract the more solitary males. The radio collars were used to find the goats and their mobs during monitoring and culling operations.
More than 10,000 goats and 5,000 sheep were removed from Dirk Hartog Island between 2007 and 2016, with the last of the goats being the most challenging to remove. In November 2016 only seven goats are believed to remain on the island, all of them female radio collared goats.
When eradication is declared, Dirk Hartog Island will be the largest island in the world from which goats have been eradicated.
Removing cats
The cat eradication team started work by trapping cats, fitting them with satellite collars and tracking their movements. The information gathered helped determine that 1080 poison baiting would be effective on the island.
In 2014 the cat team constructed a fence to divide the island into two sections to make monitoring more efficient and effective. Located at the northern end of Herald Bay and extending nearly 13 km to the west coast, the 1.8 metre high fence is made of rabbit netting with an overhang at the top and three electric wires. A gate allows vehicles to continue travelling north-south along the main track.
After baiting, the southern area was intensively monitored from May 2014 until July 2015. Four types of traps were used: automated camera traps, sand pads, soft leghold traps and cage traps. All traps used scent lures to attract cats as food lures are more likely to attract other animal species.
Checks south of the fence were done using detector dogs during the winters of 2015 and 2016. While the dogs were checking south of the cat-proof fence, monitoring for cats north of the fence was in full swing and the cat team expect to have detector dogs checking the northern part of the island in winter 2017.
If there is no evidence of cats following these checks and two years of surveillance the island will be declared free of cats and safe for the return of native mammals.
Since 2007 the Department of Parks and Wildlife has monitored changes to vegetation on Dirk Hartog Island. During this time many sheep and goats have been removed, eliminating the only large herbivores on the island.
The vegetation monitoring program uses satellite images along with extensive ground validation to build a picture of how vegetation cover on the island is changing.
Changes were first detected and investigated using satellite imagery. Hundreds of images collected at near monthly intervals since 1988 were analysed to identify where vegetation cover had increased since destocking began in 2008. These areas were then validated on the ground to determine whether the increased ground cover was native vegetation or invasive weeds. Many of the areas shown by satellite imagery to have increased vegetation cover were found to be native species.
Annual field checking continues with a team of Parks and Wildlife staff, including experts in botany and satellite imagery analysis.
Download this poster to find out more about the vegetation monitoring program.
Parks and Wildlife staff have prepared a strategic framework for translocating animals to Dirk Hartog Island between 2018 and 2030. Animals for translocation were selected primarily for either species conservation or fauna reconstruction purposes, although most achieve both.
Species conservation is about increasing a species’ chances of survival. The more viable populations there are of a species in different locations, the greater that species’ chances of survival. The two hare-wallabies, western barred bandicoot, Shark Bay mouse, woylie, heath mouse, dibbler and chuditch are all being translocated to help their species survive because they each have only limited viable populations.
Fauna reconstruction as the primary purpose for species’ translocations acknowledges that animals also provide services to ecosystems. Pollinating plants, spreading seeds, or digging to improve the soil are ecosystem services. The primary purpose of translocating the greater stick-nest rat, western grasswren, desert mouse and mulgara to Dirk Hartog Island is fauna reconstruction.
Other factors considered when selecting the species for translocation included whether they formerly existed on Dirk Hartog Island or in the Yalgoo Bioregion; habitat suitability on the island; availability of source animals; and likelihood of successful translocation.
The many considerations for successful translocations range from sourcing sufficient numbers of the different species, to sequencing and timing their translocations, and ensuring animals are released at suitable sites.
Sequence and timing of translocations will take into account interactions between different species, including predator/prey relationships; habitat competition; conservation status; and availability of animals to translocate.
Interspecies relations
Some of the species proposed for Dirk Hartog Island have similar habitat needs and may compete with each other for food and/or shelter. Boodies are capable of outcompeting woylies in captive situations so these two species will be released at sites that are both far apart and initially separated by the cat fence.
Predator/prey relationships also need to be considered. The chuditch is a nocturnal predator that preys on small mammals as well as insects, reptiles and birds. This species therefore cannot be taken onto the island until potential prey like the Shark Bay mouse, western barred bandicoot and greater stick-nest rat are well established.
Conservation status
The banded hare-wallaby is one of two species with no evidence of having previously occurred on Dirk Hartog Island. However, it is considered a priority for introduction as it is vulnerable to extinction. Bernier and Dorre islands are the last refuges for this sub species of the banded hare-wallaby; the mainland sub species is extinct. Animals from Bernier Island supplied the Peron Captive Breeding Program and from there were translocated to Faure Island and to another mainland fenced site. Introducing the banded hare-wallaby to Dirk Hartog Island will further improve this species’ conservation status.
The animals for Dirk Hartog Island will come from various sources, including islands in Shark Bay. It is important to determine the health and genetic diversity of potential source populations before translocating any animals.
Health of source populations has several meanings. One is the size of the population – are there enough animals for the population to cope with animals being removed? Another is disease – are there any diseases within the source population that could pose problems? Genetic diversity is also important for the long term health of populations.
Source populations for translocation will vary with species. Nearby wild populations are ideal, but not available for all species. Potential sources include islands, various wildlife sanctuaries, mainland populations and the Perth Zoo. Some of these are wild populations, others are captive breeding programs. Potential nearby sources are Salutation, Bernier and Dorre islands.
Salutation Island
Salutation Island has a healthy population of greater stick-nest rats that has established since their introduction to the island in 1990. However, measuring only about one kilometre wide by two kilometres long, Salutation Island is quite small and genetic diversity on the island will need management to maintain the long term viability of the population.
A survey was done in September 2016 to assess the genetic diversity of greater stick-nest rats on Salutation Island. This will help management determine whether rats are needed from other populations on South Australian islands to supplement genetic diversity on Salutation Island while also informing planning for Dirk Hartog Island.
Bernier and Dorre islands
Bernier and Dorre islands are potential sources for five species – the boodie, western barred bandicoot, rufous hare-wallaby, banded hare-wallaby and Shark Bay mouse. The wild populations of these animals occur naturally on the islands – they were not introduced like the greater stick-nest rats were to Salutation Island.
Surveys were done on Bernier and Dorre islands in August 2016 as part of ongoing monitoring and assessing the islands as potential translocation sources for Return to 1616. Information collected over the years includes rainfall data and a range of details about species health and population sizes. The data is being analysed to answer questions like:
- Is the population of each species healthy enough to remove animals from?
- Is the genetic diversity of each species sufficient to repopulate Dirk Hartog Island from these islands alone?
- What is the best time to collect animals for translocation? How many years after good rainfall are population numbers peaking, and therefore best for removing animals?
Some species may need to be sourced from different places when these questions are answered. Generally the options are limited with Bernier and Dorre islands being the only source for banded hare-wallabies; and the main source for western barred bandicoots, Shark Bay mice and the island sub species of rufous hare-wallaby.
Invasive weeds threaten natural ecosystems by displacing native species. When weeds displace native plants, animals are left without their natural homes and food. Consequently managing weeds is an important part of land management and ecological restoration and all visitors to Dirk Hartog Island are encouraged to make sure their vehicles, trailers, boats and equipment are clean and free from soil, weeds and other potential pests.
The Dirk Hartog Island Weed Management Plan recommends the following management of weed species on Dirk Hartog Island:
Weeds to Eradicate
Couch, Cynodon dactylon
Castor oil plant, Ricinus communis
Japanese pepper, Schinus terebinthifolius
Lupin Lupinus cosentinii
Wild Radish, Raphanus raphanistrum
Weeds to Control
Ice plant, Mesembryanthemum crystallinum
Fourleaf allseed, Polycarpon tetraphyllum
False sowthistle, Reichardia tingitana
Weeds to Prevent
‘High risk alert species’ have been recorded in Shark Bay but are not currently known on Dirk Hartog Island. It is important that any sightings of these species on the island are reported so they can be checked and dealt with promptly.
Ruby dock, Acetosa vesicaria
Kapok bush, Aerva javanica
Mexican poppy, Argemone ochroleuca
African boxthorn, Lycium ferocissimum
Crownbeard, Verbesina encelioides
Return to 1616 has a community engagement program keeping the local Shark Bay community up-to-date with the project. Following are links to the most recent updates and activities.
Reports
Biosecurity Implementation Plan, June 2015
A pilot study for the proposed eradication of feral cats on Dirk Hartog Island, Western Australia. Algar et al.
Field efficacy of the Curiosity feral cat bait on three Australian islands. Johnston et al.
Population Structure and Management of Invasive Cats on an Australian Island. Koch et al.
Media
Program to restore historic WA Island (Northern Guardian)
Turning back the clock on feral island (Australian)