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A Pacific island has recorded a significant recovery in several seabird populations following the removal of invasive rats in 2005. A study published in Conservation Biology examined how seven seabird species responded after rodents were eradicated from Surprise Island in New Caledonia.
According to the researchers, three species experienced substantial population increases, with their breeding populations eventually reaching between two and ten times their earlier levels.
The findings also revealed that recovery did not happen immediately for every species. While some seabirds increased within a few years, others showed improvements only after longer periods of monitoring, highlighting the importance of sustained conservation efforts.
Why invasive rats threatened seabirds
Surprise Island is a small, uninhabited island in the Pacific, covering approximately 24 hectares. Before the eradication programme, it was home to invasive black rats and house mice, which posed a threat to nesting seabirds. Rodents can prey on eggs and chicks, reducing the number of young birds that survive to adulthood. On islands where seabirds nest on the ground or in accessible locations, these introduced predators can have particularly damaging effects.
Native birds may have evolved without such predators and may lack effective defences against them.In 2005, conservationists eliminated the invasive rodents from Surprise Island. The eradication was confirmed as successful in 2006. Researchers subsequently examined whether removing the predators helped seabird populations recover and whether the benefits differed between species.
Scientists tracked seven seabird species
The research team monitored seven seabird species, including red-footed boobies, brown boobies, masked boobies, noddies and frigatebirds.
Their analysis combined surveys conducted before and after the eradication with observations collected over subsequent years. Annual surveys between 2002 and 2009 provided information about breeding populations before and shortly after the rodents were removed. Researchers also used earlier observations from 1995 and 1996 and later surveys conducted between 2017 and 2021.Rather than simply counting birds flying over the island, the team recorded active nests containing adults, eggs or chicks to estimate the number of breeding pairs.
This approach helped them assess whether the seabirds were establishing and maintaining breeding populations. The extended monitoring period was particularly important because population changes were not equally visible across all species. Some responded relatively quickly, while others took considerably longer to show clear signs of recovery.
Noddies and frigatebirds increased after eradication
Noddies and frigatebirds were among the species that showed relatively early improvements after the invasive rodents disappeared.
Their breeding populations increased within four years of eradication, suggesting that the island had become a more favourable place for nesting. The long-term results for noddies were especially striking. Researchers estimated that the island supported around 400 breeding pairs in 1995.
By 2021, the estimated number had reached 5,346 pairs, representing approximately a tenfold increase.Frigatebirds also experienced substantial growth. Researchers recorded 1,586 breeding pairs in October 2017, compared with 398 pairs estimated in December 1996.
The researchers suggested that birds arriving from nearby islands may have contributed to these increases. With invasive rodents no longer threatening their eggs and chicks, Surprise Island could provide safer nesting opportunities for seabirds looking to establish new breeding sites.
Red-footed boobies showed delayed recovery
Red-footed boobies responded differently. During the first four years after eradication, researchers did not observe a clear increase in their breeding population.
This showed why short-term surveys alone may not provide a complete picture of the effects of conservation work. Later observations revealed stronger growth. The estimated number of breeding pairs reached 4,698 in 2017 and 5,548 in 2019, before declining to 3,256 in 2021.
Before the rodents were removed, the estimated population had never exceeded 2,883 pairs.The delayed increase may be linked to the birds’ breeding biology.
Long-lived seabirds often take several years to mature, so improvements in breeding success may take time to translate into a larger adult population. However, not every species showed a clear long-term increase. Brown and masked boobies did not experience the same measurable recovery, indicating that removing invasive predators does not guarantee identical results for all seabirds.
Long-term monitoring reveals conservation benefits
The study provides evidence that eliminating invasive rodents can help support the recovery of island seabird populations, but the results may take years to become fully apparent.
Noddies and frigatebirds showed relatively early improvements, while red-footed boobies required longer monitoring before their population increases became clear.The researchers’ findings also highlight the importance of evaluating individual species rather than assuming that an entire seabird community will respond in the same way. Differences in breeding behaviour, survival and movement between islands can influence recovery.Surprise Island provides an example of how targeted conservation efforts can benefit wildlife after an invasive threat has been removed. The research also underlines why long-term surveys are essential for determining whether those gains continue and for identifying species that may need additional protection.