Recovery or change? Differences between in seabird fauna in island ecosystems before alien mammal disturbance and after alien mammal eradication

Abstract

Seabirds are important to island ecosystems because of their ecological functions; however, they are threatened by alien mammal invasions. Where alien mammals have been eradicated, seabird numbers, and ranges have often increased. Whether or not this constitutes recovery, however, is debatable because the post‐eradication seabird fauna might be different from the pre‐disturbance fauna that in most cases has not been recorded. Thus, identifying the pre‐disturbance seabird fauna is essential to determine a conservation goal. With this as our aim, we examined and radiocarbon‐dated avian bone remains found in a limestone cave on Minamijima, an islet on the Ogasawara Islands. The islet was invaded by goats Capra hircus and black rats Rattus rattus after 1830, and seabirds had declined. Since the 1970s, the alien mammals have been eradicated, and numbers of Wedge‐tailed Shearwaters Puffinus pacificus, Bulwer's Petrels Bulweria bulwerii, and Brown Boobies Sula leucogaster have increased. However, the dominant species in the avian remains were Bonin Petrels Pterodroma hypoleuca, Bryan's Shearwaters Pu. bryani, and Bannerman's Shearwaters Pu. bannermani. The radiocarbon ages of the bones were approximately 6,000–600 years BP. Therefore, seabird fauna was largely changed by the invasion and eradication of alien mammals. Our results indicate that the post‐eradication increase in seabirds is not a recovery, although the current inhabitants are native seabirds. The recent dominant species are likely to have thrived because they are cosmopolitan and competitive, and they prefer open habitats produced by goat grazing. We suggest that avian remains surveys should be exploited for ecosystem restoration.