25 Oct 2017

New study reveals why islands are our biggest extinction battlegrounds

New research has found that nearly half of the earth’s highly threatened vertebrates live on islands – and two thirds of them overlap with invasive species. With this information, we’re better equipped than ever before to focus conservation where it’s most needed. Like Gough Island, where removing invasive mice could save six threatened bird species in one go.

A success story: invasive rats have already been successfully eradicated from Anacapa Island in the Channel Islands © Abe Borker
A success story: invasive rats have already been successfully eradicated from Anacapa Island in the Channel Islands © Abe Borker
By Jessica Law

Earlier this month, a paper was published in Scientific Advances that will benefit conservationists for years to come. Spearheaded by Island Conservation, working with BirdLife International and partners across the world*, the authors analysed data from the Threatened Island Biodiversity Database. The first of its kind, the database documents which of the IUCN Red List’s 1,189 Endangered and Critically Endangered vertebrate species occur on each of the world’s islands.

Dr Stuart Butchart, Chief Scientist at BirdLife International and co-author on the paper, calls it “The most comprehensive assessment yet of the distribution of threatened species on islands”. And the results were invaluable. We now know that 41% of highly threatened land vertebrates live on islands, and we know which islands hold the largest concentrations. And, as the saying goes, knowledge is power.


A screen grab from the Threatened Island Biodiversity Database website


“The opportunities to prevent extinctions are now laid out right in front of us".

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“The opportunities to prevent extinctions are now laid out right in front of us. This knowledge base of threatened island biodiversity can really empower more efficient and better-informed conservation efforts, which is exactly what our planet needs right now,” commented Dr Dena Spatz, Conservation Biologist at Island Consevration and lead author on the paper.

So how can we best conserve these species? The database offered a vital insight into the solution. As well as mapping highly threatened species, it also documented whether damaging invasive vertebrates, such as rats and cats, occurred on the same islands. And there was indeed a big overlap, with 60% of the islands suffering from the impacts of such invaders. In fact, the paper estimated that eradicating invasive aliens could benefit 95% of the threatened species.


Floreana Island in the Galapagos Archipelago is no longer home to its namesake, the Floreana Mockingbird, thanks to invasive species © Island Conservation


Islands have always been precarious places for a species to live. Although they make up only 5.3% of the world’s land area, they have hosted 61% of all recorded extinctions since the 16th century. We all know about the plight of the Dodo Raphus cucullatus on Mauritius – but what about the Mysterious Starling Aplonis mavornata on Mauke in the Cook Islands? Or the Pagan Reed-warbler Acrocephalus yamashinae on the Northern Mariana Islands? And we know that 44% of those extinctions were caused by invasive species. (The “mystery” of the Mysterious Starling’s disappearance was invasive rats).

Separated from the mainland, many native species evolved without natural predators, which makes them defenceless when an invader does arrive. The Floreana Mockingbird is a tragic example. In the 19th century, mere decades after human colonisation, it was wiped out on the island of Floreana in the Galapagos by invasive rodents and feral cats. A few hundred individuals remain on two tiny predator-free islets nearby, with none surviving on their namesake island.


The Floreana Mockingbird was wiped out from its namesake island by invasive rodents and feral cats. © Island Conservation / Paula Castano

But there is hope: invasive rats were successfully eradicated on Anacapa Island

But there is hope. On some invasive species-free islands, biosecurity measures can be put in place to prevent them from arriving in the first place. And on others, it is possible to completely remove the interlopers. When invasive rats were successfully eradicated on Anacapa Island in the Californian Channel Islands, the hatching success of the vulnerable Scripp’s Murrelet Synthliboramphus scrippsi shot up from 20% to 90%. Excitingly, the Endangered Ashy Storm-petrel Hydrobates homochroa was also recently discovered on the island for the first time.


Scripp's Murrelet have dramatically increased in number thanks to the removal of rats from Anacapa Island © Shaye Wolf


“By managing invasive species in priority islands, we can significantly reduce the current dramatic rate of biodiversity loss”, says Dr Piero Genovesi, co-author, Head of Wildlife Service – ISPRA and chair of the IUCN SCC Invasive Species Specialist Group.

Removing invasive mice from Gough island could benefit all six endangered bird species in one go

The paper also highlights islands where conservation action would make an especially big difference. Gough Island, a UK Overseas Territory in the Atlantic Ocean, is home to millions of breeding birds. These include six endangered species, the Tristan Albatross Diomedea dabbenena among their number. But the island is also home to a giant strain of invasive house mice, which threaten the survival of thousands of seabirds each year by devouring their chicks. Removing them could benefit all six species in one go.


Video credit: The Royal Society for the Protection of Birds (BirdLife in the UK)


The next step is clear. “[The database] provides national governments and NGOs with critical biodiversity information assembled on a global platform, which can inform and stimulate promotion, fundraising, and action to protect species from extinction,” concludes Dr Spatz.


The endangered Ashy Storm Petrel was discovered breeding on Anacapa Island for the first time ever following the removal of rats © Annie Schmidt


A global collaboration is now underway, combining the database’s findings with information on which invasive species would be the most feasible to eradicate. This will pinpoint which islands to start working on.

We hope that this will help to turn Gough Island into the new Anacapa Island, and lead to many more such successes in the future.

* The paper was led by Dena Spatz at Island Conservation in close collaboration with the Coastal Conservation lab at the University of California, Santa Cruz (UCSC). The TIB database was developed by Island Conservation, UCSC, BirdLife, and IUCN-ISSC.