During an era marked by species extinction, it’s easy to overlook how human expansion has hugely benefited certain species like the brown rat. Originating from the northern fields of China and Mongolia, the brown rat has practically taken over globally, facilitated by the growing trade routes. This includes remote islands where rats, along with other introduced mammalian predators like ferrets and mice, have wreaked sheer destruction.
Remote islands, typically void of mammalian predators, often host unique animal populations that evolved without the fear of terrestrial predatorial threats. However, the advent of rats on these islands posed a severe challenge for ground-nesting birds that either lost their ability to fly or simply did not recognise the rodents, thus lacking any defensive response. It effectively turned into a free dinner buffet for the invading rats.
According to some assessments, rats have now infiltrated nearly 90% of the world’s islands. A scientific report labelled them and other mammalian invaders as “the most severe menace to island biodiversity,” attributing the majority of extinctions and ecosystem changes to rodents. Rodents such as rats and mice pose a direct threat, feeding on bird eggs, hatchlings and even live adult birds, including the largest seabird, the albatross.
The ramifications aren’t just confined to the bird population; a ripple effect is seen throughout the ecosystem. As evidence from tropical regions suggested, fewer birds resulted in less faecal matter, leading to a decrease in nutrients flowing into the sea. This caused a decline in healthy fish populations, thereby impacting the health of coral reefs, as evidenced in the Chagos Islands in the Indian Ocean.
Further research from the secluded Hawadax Island in the North Pacific revealed that the removal of apex predators such as gulls and oystercatchers by rats drastically altered the coastal ecosystem. This lead to a surge in smaller grazing creatures like limpets and snails, which caused substantial reductions in seaweed cover.
In spite of the challenges, there’s a glimmer of hope. An impressive recovery was witnessed on Hawadax island where rats were removed successfully in 2008, leading to the restoration of the native bird population over the subsequent five years. An increase in signs of broader ecosystem revival was also observed after 11 years. Furthermore, this triumph on Hawadax is not an isolated incident. Data from the Nature Conservancy, a US-based environmental non-profit body, highlight 900 successful animal exterminations on approximately 800 islands since the 1950s. An exceptional conservation victory was achieved in 2018 when the largest rodent eradication project ever, located on South Georgia in the Atlantic Ocean, was successfully completed. These victories are crucial in the campaign to counteract the loss of biodiversity.
Moreover, the elimination of invasive species isn’t just happening on remote islands. David Tierney, an ornithologist with the National Parks & Wildlife Service’s scientific advice and research directorate, recently participated in a landmark publication, Seabirds Count: A Census of Breeding Seabirds in Britain and Ireland (2015-2021). There are over 4,000 smaller islands between the two nations, playing host to globally significant sea bird colonies. Non-indigenous mammals, such as rats, pose a problem, albeit according to Tierney, the magnitude of this issue is unclear.
Tierney indicates that offshore islands are often selected by seabirds as nesting grounds to escape mainland predators. Some species, like the Manx shearwaters, puffins and storm petrels, expose themselves to vulnerability since they nest on moderately sloped terrains. However, other species such as the gannets, by nesting on sheer cliff faces, remain less accessible to small rodents.
On Irish islands, invasive mink, in addition to rats, have raised concerns, says Tierney. While Ireland has 72 designated Special Protection Areas to safeguard a minimum of one seabird species, a comprehensive record of islands heavily affected by invasive species is yet to be assembled. Confirming the absence of such species is difficult, despite easily identifying when an island’s ecosystem has been disrupted by the presence of mink or rats.
Tierney highlights that the Blasket Islands and neighbouring Puffin Island have already eradicated mink populations, yet there’s no assurance of their non-return. “This is a perpetual effort,” he asserts. Skellig Micheal is suffering from an infestation of mice, a recognised nuisance in other areas, which may be adversely affecting burrow-nesting storm petrels. These small seabirds are highly mobile at sea, covering substantial distances, but are nearly powerless when they land. As of yet, Tierney affirms, “no obvious warning signs depicted”.
In 2018, an initiative was rolled out by NPWS, targeting the dispersion of rats occupying the Saltee Islands near County Wexford. Not only a renowned breeding site for diverse seabirds, the islands are particularly recognised for their puffin colonies. “We explored Great Saltee in 2017, only to find puffins completely missing from the burrows we investigated,” Tierney shares.
The resulting picture is potentially disastrous. In the year 2000, puffin estimates on Great Saltee suggested a population of 1,522, plummeting to a depressing 120 by 2017. The rat elimination process comprised of strategically baiting traps with a certified rodenticide. Extensive surveys post-eradication, supported by DNA testing of potential rat droppings, showed no rat presence. While it is premature to determine the effect of this on puffins and other bird species due to their significant lifespan, the future can be viewed with a glimmer of hope. The expectation is that similarly affected islands in the area could now benefit from comparable interventions.
Meanwhile, BirdWatch Ireland has been intricately involved in a project on the Dalkey Islands, a mere 300 meters off the Dublin coastline. Rats are not the only issue here, with feral goats and rabbits also causing problems. Tara Adcock, their urban birds expert, explains, “The devastation caused by rats on the Dalkey Islands’ seabird population is something we’ve directly witnessed. In the 2023 breeding cycle, we recorded 45 Arctic tern breed pairs, but not a single chick. With rats infiltrating the island again, this is the root of the problem.”
BirdWatch Ireland has been associated with an EU-financed project with the intention of eliminating rats from various islands, thereby providing a safe habitat for the threatened roseate tern. This species significantly breeds off Rockabill, an island situated to the county’s north and recognised as the primary breeding settlement for this bird within northwest Europe. The establishment of an additional breeding settlement would serve as a backup should any potential threats such as avian flu occur at Rockabill. The rat eradication project on the Dalkey Islands eventually led to the arrival of the Artic tern, a closely related species. A potential issue is that the islands are easily accessible to rats through swimming, and human visitors and boats unintentionally transport these rodents onshore. A single pregnant female rat has the capacity to obliterate an entire colony. It highlights the difficulty in maintaining these islands as rat-free zones.
In recent times, the responsibility for the operation has shifted to the Dún Laoghaire Rathdown County Council, the landowner of the area. A planned rodenticide treatment during the 2023-2024 winter season is expected to resolve the issue, however, it will be a continuous effort.
Funding for eradication projects, as well as ongoing financial support for long-term monitoring and problem-solving as issues emerge, are crucial, states Adcock. Volunteering at Dalkey Island in 2018, Adcock witnessed how effective these measures can be, as the island had 30 breeding pairs of terns but no offspring. After implementing rat eradication efforts that winter, they were greeted by an influx of chicks capable of survival and departure from the island, highlighting the rapid and significant results these interventions can yield.