Island eradication in New Zealand

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Aotearoa is an island nation with four large islands surrounded by hundreds of offshore islands. We have 890 islands bigger than one hectare and 334 of those are over five hectares. It is on these islands that the vision of a predator free New Zealand begins.


Small beginnings

Introduced predators spread across the North and South Islands in the late 19th and early 20th century devastating Aotearoa’s native species. Some islands were protected from invasion due to their distance from the larger islands, 123 islands have to the best of our knowledge never had rats, stoats or possums present. Sadly, often with human help, introduced predators still made it to many of our islands.  European settlers attempted to establish farms on islands nowadays considered inhospitable, the failed human settlement on the Auckland Islands in the 19th century introduced the cats and pigs that live there still. 

A handful of early conservationists, concerned about the dramatic loss of bird song on the main islands, began advocating for the establishment of island sanctuaries. One of these advocates was Richard Henry

Henry arrived in New Zealand around 1874 and settled near Lake Te Anau. There he became a bush guide and developed a fascination with the habits of kākāpō.  He also became aware that their numbers were declining due to mustelid predation. In 1894 Henry became the caretaker of our first island sanctuary, Resolution Island in Fiordland. Between 1894 and 1908 Henry translocated over 700 native birds (mostly kiwi and kākāpō) to Resolution Island. Tragically the arrival of weasels in 1900 led to the decimation of the bird population on the island. Devastated, Henry moved on to Kāpiti Island to become the caretaker of the new bird sanctuary there. 

An imagined vision of kākāpō and kiwi enjoying life in Aotearoa free of introduced predators. Kakapo & Kiwi, 1889, Dunedin, by Burton Brothers studio. Te Papa (C.018271)
An imagined vision of kākāpō and kiwi enjoying life in Aotearoa free of introduced predators. Image credit: Burton Brothers studio (Te Papa C.018271)

During the early 20th century, eradication programmes were undertaken haphazardly. Examples include the removal of stoats using shooting from Otata (Hauraki Gulf) and cats being removed from Kāpiti Island and Stephen’s Island in the 1920s. But it wasn’t until the 1960s that eradication became a serious conservation goal. 


Shifting perspectives on predation 

In the mid-20th century, predator eradication was not considered a priority. Many ecologists, as recollected by the pioneering conservationists Don Merton and Brian Bell, believed that:

  1. predator eradication could not be achieved and 
  2. predation did not lead to extinctions (based on the assumption that a predator species would not hunt a food source to extinction). 

As a result, minimal resources were committed to eradication efforts. However, two significant events in the late 1950s and early 1960s changed how scientists viewed the impact of predator numbers on native species.


Ruapeke/Maria Island (1964)

Ruapeke/Maria Island (2 ha), part of the Noise Islands in the Hauraki Gulf, experienced a rat invasion in 1959. The result was the death of nearly 1,000 white-faced storm petrels. A group of dedicated Forest & Bird volunteers responded by laying rat bait across the island. Their work paid off and in 1964 the island was officially recognised as rat-free – making it the first successful island predator eradication in New Zealand. (For more details, read NZ Geo’s 2015 article ‘Nineteen sixty-four’.)


Taukihepa/Big South Cape (1964)

At the same time the rat eradication was underway on Ruapeke/Maria Island, a tragedy was playing out off the coast of Stewart Island at Taukihepa/Big South Cape Island (939 ha). Free from introduced predators, Taukihepa had become a haven for wildlife driven from the mainland, including the South Island saddleback/tīeke, Stewart Island snipe, the greater short-tailed bat and Stead’s bush wren

In March 1964, rats where found on the island. Efforts to lay bait and translocation at risk species were attempted but only the tīeke survived the translocation, while extinction due to rat predation followed for the snipe, bat and bush wren.  This was not the first time that rats, and other introduced predators, had led to the extinction of a native species. But this time scientists were able to demonstrate clearly that predation was the primary factor leading to extinction. This provided justification for pushing island eradication techniques forward.

Specimen of Stead's Bush Wren, Xenicus longipes variabilis Stead
The bush wren became extinct after its remnant population was predated by rats on Taukihepa. Image credit: Field Collection 1948 – 1966 (Te Papa)

Pushing the boundaries 

From the mid-1970s to the early 1990s, key innovations enabled island eradications to become larger in scale while also tackling more complex terrain. For example, the development of second generation toxins (read our summary of baits) meant target species such as rats could consume a lethal dose in a single serve. Advances in the accuracy of GPS technology in the 1990s meant that aerial drops (the application of bait using helicopters or small planes) over larger islands became more feasible.

Breaksea Island (1985)

Rowley Taylor and Bruce Thomas, both from the DSIR Ecology Division, believed intensive use of brodifacoum (a 2G toxin) in bait stations could lead to eliminations on larger offshore islands. To test this theory they chose Breaksea Island/Te Au Moana (170h) in Fiordland, an island covered in temperate rainforest that had been infested with Norway rats for decades. 

Across the island, 743 bait stations were laid out on a grid and filled with brodifacoum tablets. The stations were checked daily, and re-baited if needed, for 22 days. After returning one month later, they found no evidence of rat activity (and plenty of evidence of dead rats!). Read the full story of how this island became predator free.

Field workers prepare a huge pile of yellow bait stations for Breaksea Island
Making rat tunnels for rat eradication, Breaksea Island, Breaksea Sound, Fiordland National Park, 1985. Image credit: Department of Conservation.

Kāpiti Island (1996)

In 1996 a groundbreaking rodent eradication project was undertaken on Kāpiti Island using aerial application. 

The project was more complex than previous island eradications. At 2,000 hectares Kāpiti Island is seven times the size of Ulva island, which at that time was the largest island where pests had been eradicated. It was also the first project that sought to eradicate two species at the same time (Norway rat and kiore) and it was the home of bird species that might consume the bait, for example weka and robin. Additionally, unlike other islands where eradication had taken place, Kāpiti Island has mixed land ownership (Crown, iwi and local whānau) and had a small settlement on the northern tip of the island.

tīeke on a branch
Only after rats were eradicated from Kāpiti Island did tīeke successfully establish themselves on the island. Image credit: Duncan-Wright (Wikimedia)

Bait (brodifacoum) was applied over the island using helicopter drops supplemented by ground operations. A population of weka, the species most at risk of poisoning, was removed from the island and then reintroduced after the operation was completed. By 1998 the island was officially declared rat free and the number of birds on the island was booming.

Campbell Island/Motu Ihupuku (2001)

Campbell Island/Motu Ihupuku (11,300 ha) is our southernmost island located 700km south of mainland New Zealand. The island became a nature reserve in 1954 and a population of feral sheep and cattle were removed by the Wildlife Service over the following decades. 

Campbell Island used to have the highest population density of Norway rats in the world – before 2001 about 200,000 rats lived on the island. In 2001 a DOC-led project began to eradicate the rats and a suspected population of feral cats. It was an ambitious goal for a large, isolated island in the ‘furious fifties’ latitude. 

To their surprise, no traces of cats were found on the island – something caused them to naturally die out. This meant the project only needed to focus on rats, but there were still significant challenges to overcome – including getting the bait and helicopters to the island!

The project was successful, and in 2006 Campbell Island was officially declared rat-free. This meant the tiny snipe and flightless teal could return from their exile on smaller islets to live safely on the main island again, and the southern royal albatross could nest in peace. Today Campbell Island remains the largest pest-free island in New Zealand, but when Predator Free Rakiura is successful Rakiura/Stewart Island (174,600 ha) will dwarf it in size!

The Campbell Island snipe amongst low foliage and ferns
The Campbell Island snipe prefers a predator free environment. Image credit: Alex Fergus (iNaturalistNZ)