Rats took over this Pacific island. Now drones are leading the fightback

When the people who would become the first Polynesian islanders ventured out into the remote Pacific some 3,000 years ago, they took three main animals with them; pigs, chickens, and dogs. Expanding their territory over the next few thousand years, from New Zealand north to Hawaii and east to Easter Islands, the Polynesians flourished. Surpassing even their ability to prosper in some of the world’s most remote locations was a fourth addition to their animal crew: rats. According to research by James Russell, a conservation biologist at the University of Auckland, Polynesians introduced R. exulans (the Polynesian rat) to Tetiaroa atoll – a six-kilometre-square island in French Polynesia – around 1,000 years ago, while European explorers brought the R. rattus (black rat) variety to the atoll in the 1970s. Which just happened to be around the same time Marlon Brando built a small village on the atoll after filming Mutiny on the Bounty near Tahiti.The rats are now everywhere on Tetiaroa atoll. Sally Esposito, speaking on behalf of the Tetiaroa Society, a non-profit designated by the Brando Family Trust as environmental stewards of the islands, explains that the nearby island of Motu Reiono experienced 65-153 rats/hectare. Extrapolating that data to the surface area of Tetiaroa suggests that there are currently anywhere between 28,000 and 65,000 rats on the island.Rats have a knack for taking over isolated parts of the world from the moment they arrive. Wherever they’re found, these crafty stowaways rapaciously feast on the eggs and hatchlings of the islands’ native bird and reptile species, knocking the pre-ordained flow of nature out of sync. Because most island species have evolved without the presence of mammal species (who, with the exception of bats have historically struggled to make it to islands from the nearest mainland under their own power), most native species have evolved without an evolutionary response to these scurrying interlopers. With a reduced bird population, fewer outside nutrients are brought onto the island creating a closed loop in which rats dominate all resources. This, in turn, reduces the amount of subsidy nutrients being washed out to support coral reefs around the islands.But now the rats’ reign over Tetiaroa may be about to come to an end. Beginning in August 2021, the conservation group Island Conservation will employ a novel approach in an attempt to clear the rat population from Tetiaroa Atoll (as well as two other islands in French Polynesia): drones. By using specially-engineered drones to blanket the islands with rat poison, the charity will implement the world’s first scalable, heavy-lift drone operation to remove invasive rats.The method was trialled on Seymour Norte in the Galapagos in 2019 where the swallow-tailed gull – the only nocturnal gull on Earth – was at risk of extinction due to the local rat population. Launched from boats and flying autonomously along predetermined routes, a drone was able to drop rodenticide with extreme precision while minimising the impact on non-rat species. Two years later, Seymour Norte was declared 100 per cent rat free – a resounding success for conservationists everywhere.Founded in 1994, Island Conservation and its partners have so far successfully restored 65 islands worldwide, benefiting 1,218 populations of 504 species and subspecies. Protecting our islands is important because they play an outsized role in the planet’s biodiversity. They make up only five per cent of our planet’s land area, but are home to an estimated 20 per cent of all plant, reptile and bird species. Unfortunately, 75 per cent of all amphibian, bird and mammal extinctions occur on islands, with invasive species such as rats the primary cause.Removing an island’s rat population can significantly help restore its biodiversity. A 2018 study of the Chagos Archipelago in the Indian Ocean found that islands without rats not only boasted much larger seabird communities, but had substantially higher nutrient levels in their coral reefs. Meanwhile, researchers at Lancaster University found evidence that a more robust seabird population can even help coral reefs recover even after coral bleaching has occurred.“More than 80 percent of the world’s islands have rodents,” explains David Will, innovation program manager at Island Conservation. “There’s emerging research on the benefits to marine ecosystems and coral reefs from removing invasive rodents,” Will says. “It’s a holistic ecosystem-wide approach.”Island Conservation landed on Tetiaroa as its next target by surveying where the most endangered species occur, and where most invasive species live. This venn diagram indicated that taking action in French Polynesia would have the biggest impact.Tetiaroa supports thousands of nesting seabirds, including four IUCN locally threatened species, making it one of the key seabird breeding sites within French Polynesia. The atoll is also a major nesting site for green sea turtles – all of which are threatened by rat predation.“Unfortunately we don’t have any exact numbers on sea turtle populations,” explains Esposito. “But we do have video evidence that the rats are eating the turtle eggs and hatchlings, so we know the turtle numbers would be much higher without rats.” What’s more, the atoll is the last stronghold for many seabirds in the Society Islands with at least nine species recorded – all of which are at risk from the island’s rats. “Every day invasive rats remain on the atoll means more seabirds and turtles will perish,” Esposito says.With each restoration project undertaken in consultation with local communities, land users and governments, Will hopes projects such as this will help his team understand the direct and indirect benefits to island conservation, including benefits to tourism, food security and the ecosystem as a whole.Previously, helicopters were used to drop the poison pellets, but advances in drone technology have opened up the playing field. For one thing, drones are cheaper, lighter and more portable, meaning they can be broken down and transported to the islands as hand luggage where they can then be reassembled and piloted from offshore boats. The cost of hiring international helicopter pilots and transporting them to remote islands versus training locals to fly drones has also been attractive.“We’ve been watching drone technology for a number of years with the idea that it can dramatically reduce cost and also democratise island restoration by allowing local experts to be able to fly them using precision automating processes,” Will explains.A previous effort to rid Tetiaroa of rats in 2012 failed. For an eradication to be successful, 100 per cent of the rats on an island have to be killed. According to Russell, a single pregnant rat can overrun a 1,000 hectare island within two years. In 2012, it took 35 volunteers four months to spread poison pellets by hand over 100 hectares.This year, by using drone technology, Will and his team will treat five times this area, dropping 30 tonnes of poison onto the islands over a total two week period using a human staff of just five or six.An initial application will be applied on Tetiaroa, then the drone will be broken down and transported to the Gambier Archipelago at the opposite end of French Polynesia. There it will give those islands a first application, be transported back to Tetiaroa and the process repeated. Usually, Will explains, these would be separate projects taking place in separate years, but using drones instead of helicopters has allowed them to do multiple pellet drops in a single year.The work will be carried out by a three metre by three metre hexacopter drone developed by New Zealand-based Envico Technologies. It’s an ever-evolving technology; the drones used on Seymour Norte carried a 12kg payload, this time around the drone will have an effective payload capacity of around 50kg and will utilise gas-powered engines for the first time, providing at least a full hour of flight time.And where Seymour Norte is a predominantly flat and low-lying island, on Tetiaroa the drones will be pre-programmed to avoid cliffs and coastlines, allowing human pilots to launch the drones on a predetermined pathway instead of maintaining manual control throughout each flight.By maximising flight efficiency, and using smaller drones over larger, nosier helicopters, the team will minimise the disruption to other native species. What’s more, the poison pellets are coloured blue to reduce their attractiveness to bird species. The project will also take place while migratory birds are away from the island with certain other animals taken into temporary captivity for its duration.Success in small scale projects such as Seymour Norte and Tetiaroa Atoll helps Will and his team build the case for attempting eradications on more complex islands, as well as driving funding to help them develop newer tools. “Generally, the larger the island the more biodiversity it has, so scaling it up to those larger islands will have the largest impact on ecosystems,” Will explains. “We have a pipeline of projects. Over the next five years we expect drones can reduce costs by $4 million and we can restore ten more islands with those funds and benefit twice as many species populations.”Recently, Leonardo DiCaprio and the charity Re:wild announced a £31m plan to rewild all of the islands in Latin America, beginning in 2023. Will hopes to take the learnings from the next two years worth of work – including using medium sized drones on Tetiaroa Atoll – and apply it to this project, hopefully employing a large helicopter drone with a 200kg capacity to help them scale up their operations.Alongside its drone projects, Island Conservation is looking at other ways to carry out species-specific removals, including developing species-specific toxicants and creating a genetically modified rodent that would only be able to produce a single offspring, eventually killing off the entire population.“We’re moving very cautiously to understand the social and ethical problems,” Will explains. “We’re outcome-driven and we know we need new tools to accomplish our goals, especially when we’re working on islands that are large and have human communities. From a timeline perspective, species are going extinct at a rapid pace. We know that if we don’t do anything, species will go extinct.”