Good and bad news: eradication of pests on islands is working – New Zealand and Australia lead the way

A global study looking at more than 100 years of pest eradication attempts across 998 islands has seen New Zealand leading the world in invasive mammal eradication on islands with Australia second. The results of the research are published in the Scientific Reports journal.

Islands are considered important pockets and testing grounds for global biodiversity conservation efforts. In a rare success story for species’ preservation, the international team’s findings show a high rate of invasive species eradication from attempts around the world.

The researchers looked at 1550 invasive mammal eradication efforts using the publicly available Database of Island Invasive Species Eradications (DIISE). Across the globe, there was an 88% success rate from the attempts analysed, with significant improvements coming since the 1980s.

It must be said that the bulk of the success stories are coming out of countries with greater resources. The team found that eight countries were responsible for 80% of all documented eradications: New Zealand, Australia, France, UK, USA, Mexico, Seychelles, and Ecuador. New Zealand accounted for nearly a quarter of these pest eradications with Australia second at just over 12%.

“Our study found that success rates from invasive species eradications are high and have remained stable over time,” says lead author Dena Spatz, senior conservation scientist at Pacific Rim Conservation based in Hawaii. “This is a testament to the hard work of people and partnerships seeking to prevent species’ extinctions and restore island ecosystems.”

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Though covering just five percent of Earth’s land area, islands have experienced 61% of extinctions since the 1500s. They are home to 40% of today’s highly threatened vertebrate species.

Invasive species like rats, cats and goats introduced by humans are a prime cause of these extinctions by competing with and eating native animals. One of the most effective ways to halt and reverse this damage is the removal of invasives.

“There is an amazing global impact by summing these local conservation interventions,” says co-author Nick Holmes. “This synthesis shows the remarkable conservation gains that have been collectively made on islands and builds on past work showing the tangible benefit to biodiversity.”

In 2016 a global partnership formed to remove invasive mice from Antipodes Island (Maōri: Moutere Mahue; “Abandoned island”) which are inhospitable and uninhabited volcanic islands in subantarctic waters to the south of – and territorially part of – New Zealand. The island is home to the endangered Antipodes parakeet and Reischek’s parakeet.

“The success on Antipodes Island motivated New Zealand to pursue more ambitious eradication programmes currently under development, such as on the larger and inhabited Stewart Island, continuing to pave the way for key invasive mammals to be removed from the entirety of New Zealand by 2050,” says co-author James Russell, professor at New Zealand’s University of Auckland.

“Breakthroughs in rodent eradications on islands have occurred in three waves. In the 1980s New Zealanders fine-tuned ground-based rodent eradication methodology; in the 1990s we pioneered the use of helicopters to treat much larger islands; and in the 2000s we gained the confidence to detect and remove rats reinvading islands.

“The next breakthrough will be the ability to work on the largest islands which also have human inhabitants.”

Throughout this time New Zealanders have always been exporting globally this knowledge, capability and technology.

“New Zealand has reached the halfway mark for mammal eradications, over the past century having cleared invasive mammals from half the islands on which they exist. We have to continue, indeed even accelerate, this commitment to remove invasive mammals from the remaining islands and achieve our Predator Free goal by 2050.”

But the tide is far from turned. Even island nations like New Zealand struggle when it comes to invasive species particularly on the mainland where there are large human populations.

Bats, including last year’s New Zealand “Bird” of the Year winner the long-tailed bat (pekapeka-tou-roa), are in danger of becoming the next meal of feral and domestic cats according to new research published in the New Zealand Journal of Zoology.

The research, conducted by New Zealand’s Department of Conservation (DOC), confirmed the long-held suspicion that bats were prey for the cats.

Science Advisor Dr Kerry Borkin examined the gut contents of a feral cat trapped in Pureora Forest Park on New Zealand’s north island and found the remains of a lesser short-tailed bat.

Injured juvenile male long-tailed bat with its wing extended to show two wing punctures. Defects were ∼4–5 mm in diameter and ∼20 mm apart (with wing fully extended). This bat was euthanised due to poor prognosis. Credit: Borkin et. al.

Borkin also recorded the reoccurring hunting of long-tailed bats by a pet cat owned by an Ōtorohanga household living on a rural property.

“We’ve known for some time cats have been attacking pekapeka,” Borkin says. “The two cases we report on are just the tip of the iceberg. What we found is it’s not just feral cats killing and eating our precious pekapeka – pet cats are killing and injuring them too.

“We’ve now got conclusive evidence a pet cat will attack and kill native bats over several years, and that’s important to know when protecting New Zealand’s only native land mammals.”

The pet cat had been preying on native bats near their rural property for at least two years – an indication of the repeated predation cats will undertake if near bat habitat.

“The Ōtorohanga household’s pet cat was a serial pekapeka killer, with seven dead or injured bats discovered on their property over the course of two years,” Borkin adds.

Borkin says the two cases demonstrate how a single cat can impact on a local bat population. “Although not all cat attacks on pekapeka will result in its death, they will reduce the overall likelihood of survival for individual bats and populations. That’s a real concern when our pekapeka are under significant threat. Native bats can be found in towns, cities, farms, and forests – if cats are there too, then bats are at risk of being killed.”

The example of the bats preyed upon by feral and domestic cats show that even the best-performing countries on the biodiversity front have a long way to go.

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