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

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  • 小笠原諸島聟島列島におけるノヤギ排除後の海鳥営巣数の急激な増加
  • オガサワラ ショトウ セイトウ レットウ ニ オケル ノヤギ ハイジョ ゴ ノ カイチョウ エイソウスウ ノ キュウゲキ ナ ゾウカ

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Abstract

<p>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.</p>

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