Guest blog: Invasive Non-Native Species Challenges in Tristan da Cunha

Stephanie Martin, Environment & Conservation Policy Officer, Government of Tristan da Cunha.

Tristan da Cunha, deep in the South Atlantic Ocean, is part of the UK’s Overseas Territories. Home to tens of millions of seabirds and several unique land birds, plants and invertebrates, the islands are the only places on Earth where you can find Britain’s rarest bird, Britain’s only two Critically Endangered birds and the world’s smallest flightless bird.

Tristan da Cunha. Image: Government of Tristan da Cunha.

Invasive non-native species are an ever-increasing threat and a challenge to biodiversity in many parts of the world. This includes the world's most remote inhabited island, Tristan da Cunha, and its neighbouring uninhabited islands, where even extreme remoteness has failed to offer adequate protection.

The historical legacy of (most probably) 19th-century sealers bringing invasive non-native house mice (Mus musculus) onto Gough Island has had a devastating impact on the ecosystem of this World Heritage Site. The mice attack the chicks (and more recently adults) of Gough’s many species of seabird, which offer a rich and abundant food resource. This includes the Critically Endangered Tristan albatross (Diomedea dabbenena), a bird 300 times their weight! Mice are thought to cause the loss of a staggering 2 million eggs and chicks each year on the island – a patch of land smaller than the London Borough of Enfield. Gough Island was the first place where the true potential impact of house mice as an invasive non-native species on large seabirds was revealed – often the impacts of mice are masked or suppressed by the presence of invasive rats, but rats never reached Gough.

Hear from project partners in this short video

After many years of planning with the RSPB, Tristan da Cunha, and several other partners, and the support of many generous funders including the UK Government, a 50-person strong team is about to embark on a truly ambitious project to eradicate the mice from Gough Island and reverse the fortunes of millions of birds. The final team members will, hopefully, set sail across the Atlantic Ocean in the next few days. This will be amongst the most challenging rodent eradication projects ever to be attempted. You can follow their progress via the website: goughisland.com (external link) or on social media @GoughIsland (external link) and @NatureTristan (external link).

The small community on Tristan is also working on other invasive species challenges with the support of three DawinPlus projects. One seeks to tackle an invasion of the invasive non-native brown soft scale insect (Coccus hesperidum). The insect and resulting mould (that grows as a result of the scale insects’ prolific honeydew production) have infested Tristan's only native tree species, the endemic Phylica arborea, smothering and killing many on Nightingale Island.
The endemic Wilkins' bunting (Nesospiza wilkinsi), evolved to only feed on Phylica fruit and is, therefore, threatened with possible imminent extinction.

Endemic Wilkins' Bunting. Image: Government of Tristan da Cunha.

Carefully identified biological control agents have been successfully used on scale insects in other regions. The project, led by Tristan’s Conservation Department, is working with experts from CABI and Fera to see if these methods can also be applied on Nightingale Island. Together, we have identified a promising agent in a small, host-specific parasitoid wasp which are being released in small numbers. This project is also removing the non-native New Zealand flax (Phormium tenax), which is spreading on Inaccessible Island and threatening native plant species.
“It has been very exciting to receive the wasps from CABI in the UK, I had no experience of rearing insects in captivity before so it has been a steep learning curve and a difficult but rewarding experience to finally have enough to release on Nightingale to help save and protect our endangered Wilkins bunting” – “It is something that I have really enjoyed doing, it has made it exciting to go into the office every morning to see how many new wasps there are each day” 

Kirsty Repetto, Tristan’s Conservation Department.

Image: Government of Tristan da Cunha.

Invasive species are also a challenge in Tristan’s marine environment. In 2006, a free-floating semi-submersible oil platform PXXI was stranded on the southeast coast of Tristan. It had started its journey off the coast of Brazil and had various non-native invertebrates and fish species associated with it by the time it stranded in Tristan's waters. One of these fish, the South American silver porgy (Diplodus argenteus argenteus), is now found off Tristan year-round. Tristan's Fisheries Department is hoping that a second DarwinPlus project currently underway will help to answer questions surrounding its impacts on native populations – particularly as the local economy depends heavily upon the MSC-certified Tristan lobster fishery.

Invasive species are a constant threat to all the UK’s Overseas Territories and their unique ecosystems. The Tristan community is working with various government agencies and partners to try to address these challenges and protect Tristan’s unique ecosystems – importantly this includes working on a DarwinPlus project to enhance biosecurity for the entire island group, in the hope that we can prevent any more invasive species from arriving on Tristan’s shores in future.
Find out more about Tristan da Cunha (external link) and the impacts of invasive non-native species on the UK Overseas Territories.

Thin strip of image show tree trunk and bark