Rapid increase in seabird breeding population after feral goat eradication on Mukojima, Ogasawara Islands, Japan

Abstract

During inhabitation from 1881 to 1994 grazing and agricultural lands were widely established on the Mukojima Group of islands. Although Mukojima, Nakoudojima and Yomejima, the main islands of the group, were widely covered by forest before human settlement, damage caused by feral goats, such as grazing and trampling, caused deforestation and soil erosion. Feral goats were eradicated from the islands during the period from 1999 to 2003. Alien Black Rats were also eradicated from Mukojima in 2010, although they still survive on Nakoudojima and Yomejima. In order to evaluate changes in seabird breeding status after goat removal, the numbers and distribution of nests of each species were recorded at three to four year intervals. After goat eradication, large-size ground nesters such as Brown Booby Sula leucogaster and Black-footed Albatross Phoebastria nigripes, have increased remarkably on islands where moderate populations had survived, whereas only limited increase has occurred on islands where initial populations were small. The Wedge-tailed Shearwater Puffinus pacificus, a mid-sized burrow nester, has shown outstanding increases regardless of the initial population sizes. Bulwer s Petrel Bulweria bulwerii, a small-sized burrow nester, which did not initially occur on all islands, has colonized two islands. The populations were, however, relatively small, and the limited increase may have been limited by rat predation. Because these population increases and range expansions have occurred over a relatively short time span, direct disturbance by feral goats, such as trampling, seems to have affected seabird breeding, rather than their indirect effects through environmental alteration. Responses to goat eradication varied among species. The three species that showed greatest recoveries each preferred open habitats such as grass and rocky areas. Before human settlement, forest breeding seabirds would have been common in natural forest environments. Thus the current avifauna is biased towards those species that breed in open habitats. To recover the original avifauna, it is necessary to promote forest restoration and rat eradication.

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

“…Wedge‐tailed Shearwaters, Brown Boobies, and Bulwer's Petrels share a biogeographic feature that differentiates them from the other species: they are cosmopolitan, occurring widely in the Pacific, Indian, and/or Atlantic Oceans (Onley & Scofield 2007). They are also currently the most widespread seabirds in the Ogasawara Islands (Chiba et al 2007; Suzuki et al 2019). Such species should frequently migrate among colonies and easily be able to expand their breeding distribution.…”

“…While the cosmopolitan seabirds have expanded their ranges in the Ogasawara Islands since feral goat eradication (Chiba et al 2007; Suzuki et al 2019), some original seabird fauna, that is, the light‐weight species with high endemism, have yet to recolonize. Given the apparent habitat change on these islands (Suzuki et al 2019), which were altered from forests and shrubland to grassland and bare ground, we can understand the faunal change. However, while the habitat has recovered to a certain extent on Minamijima, the seabird fauna has not.…”

“…In order to restore the Ogasawara Islands' ecosystem, national and local governments have implemented programs to eradicate feral cats, goats, wild pigs Sus scrofa , and rodents, control and restore the vegetations, and conduct translocation of endangered species on several islands, and seabird numbers have subsequently increased. For instance, on Mukojima, Nakodojima, and Yomejima, breeding populations of seabirds such as the Brown Booby Sula leucogaster ), the Black‐footed Albatross Phoebastria nigripes and the Wedge‐tailed Shearwater Puffinus pacificus have increased following feral goat eradications (Chiba et al 2007; Suzuki et al 2019). Suzuki et al (2019), however, remarked that the “recovered” seabird fauna might not be the same as that which originally inhabited the islands because the habitats were altered from forests to grasslands.…”

“…For instance, on Mukojima, Nakodojima, and Yomejima, breeding populations of seabirds such as the Brown Booby Sula leucogaster ), the Black‐footed Albatross Phoebastria nigripes and the Wedge‐tailed Shearwater Puffinus pacificus have increased following feral goat eradications (Chiba et al 2007; Suzuki et al 2019). Suzuki et al (2019), however, remarked that the “recovered” seabird fauna might not be the same as that which originally inhabited the islands because the habitats were altered from forests to grasslands. On the Ogasawara Islands, some petrels such as the Bonin Petrel Pterodroma hypoleuca and Bannerman's Shearwater Puffinus bannermani are known to breed in the well‐preserved forest (Kawakami et al 2018; Kawakami 2020).…”

Predicting ecosystem changes by a new model of ecosystem evolution

“…Nakoudojima Island, the subject of this study, is uninhabited, has an area of 1.4 km 2 and is located approximately 50 km north of Chichijima Island. Prior to human settlement, all of the Ogasawara Islands were covered with forest 10 – 13 . Among the islands, Minamiioujima Island, which has never been settled by humans, is still almost entirely covered with forest 14 .…”