Long‐term monitoring highlights the positive responses of the seabird community to rat eradication at Tromelin Island, Western Indian Ocean

Abstract

The eradication of rats (Rattus norvegicus, R. exulans and R. rattus) on islands is essential for the preservation of island ecosystems, including seabird populations, which are particularly vulnerable to rat predation. However, the long‐term positive effects of rat eradication on seabird colonies and populations are often understudied. Brown rats (R. norvegicus) were eradicated from Tromelin Island in 2005. No other significative active restoration actions (such as artificial social attraction, translocation or habitat manipulations) have been implemented, which provides a unique opportunity to investigate natural seabird recolonization and recovery processes after rat eradication. We used seabird annual nest counts conducted from 2005 to 2022 and georeferenced data of occupied nests, to precisely describe the rebuilding of the seabird community of Tromelin Island. Despite the lack of any active restoration actions after the eradication of rats, and the remoteness of the island, 17 years after rat eradication, the seabird community increased from two to seven breeding species, and from 353 to 4758 breeding pairs (total for all species). The recovery of masked and red‐footed booby populations was mostly due to the improved breeding success and in fine to auto‐recruitment. Inter‐ and intra‐specific social attraction accelerated the arrival of new species and boosted their population growth. On a finer spatial scale, recolonizing species showed different patterns of colony settlement linked to intra‐specific attraction. The dynamics of the community of Tromelin Island after rat eradication can be regarded as a natural experiment that informs the processes of colony settlement, population dynamics and connectivity in tropical seabirds. Our results demonstrate the huge benefits of eradicating rats from seabird islands, even when no additional active restoration actions are feasible.