Departmental Bulletin Paper 外来種による小笠原在来植物の送粉系撹乱
Disruption of local plant-pollinator ecosystems in the Ogasawara Islands by alien species

辻村, 美鶴  ,  清水, 晃  ,  苅部, 治紀  ,  大林, 隆司  ,  村上, 勇樹  ,  村上, 哲明  ,  加藤, 英寿

(42)  , pp.23 - 64 , 2016-05 , 首都大学東京小笠原研究委員会
小笠原諸島の送粉生態系は北米原産のグリーンアノールによる在来昆虫の捕食と、養蜂用に導入されたセイヨウミツバチの高頻度の訪花により、大きな撹乱を受けていると考えられている。本研究は、小笠原に広く分布するヒメフトモモSyaygium cleyerifolium (小笠原固有種)とアカテツ Planchonella obovata(広域分布種)を対象として、アノールとミツバチが侵入した島(父島・兄島)と未侵入の島(聟島)において、デジタルカメラのインターバル撮影により訪花者の定量的な調査を行い、その種構成や頻度を島間で比較することを目的とした。その結果、2種の植物にはそれぞれ50種を超える多様な訪花者が確認されたが、島によって訪花者の種構成と頻度は大きく異なっていた。ヒメフトモモは早朝に開花し、聟島では在来の小型ハナバチ類とハエ類が多数訪花していたのに対して、父島では在来小型ハナバチ類の訪花は全く見られず、代わりにセイヨウミツバチが頻繁に訪花していた。また、在来の昆虫相が残存する兄島においても、在来ハナバチ類よりも高い頻度でセイヨウミツバチが訪花していた。一方、アカテツは夕方に開花し、いずれの島でもハエやカミキリモドキ・ガなどの夜行性昆虫が頻繁に訪花した。さらに聟島では昼行性のハエ・ハナアブ類なども高頻度で訪花した。両植物種とも夜間よりも昼間の訪花者相が島間で大きく異なり、これは夜行性昆虫が昼行性のグリーンアノールの捕食圧を受けにくいことによると考えられる。そのため、昼行性昆虫に送受粉を強く依存する植物種は、グリーンアノールやセイヨウミツバチによる送粉系撹乱の影響をより強く受けやすいと推察される。なお本研究により、目視では観察しづらい小型の訪花昆虫や、夜行性動物の訪花行動を数多く撮影することができたことから、インターバル撮影が無人島や夜間などの調査が困難な条件下でも、モニタリングなどにおいて有効な手法であると期待される。
Native plant-pollinator systems have been disrupted especially in inhabited Chichi-jima and Haha-jima Islands of the Ogasawara Islands due to predation pressure by the alien green anole, Anolis carolinensis since 1980s. Moreover, the honeybee, Apis mellifera was introduced to Chichi-jima Island for bee-keeping in 1880s, and is the main flower visitor in the island now. In this study, we quantitatively investigated the flower visitors of the Ogasawara native plants using interval photography function of a waterproof digital camera. We observed flowers of Syzygium cleyerifolium (endemic to the Islands) and Planchonella obovata (cosmopolitan), which abundantly grow together in the similar habitats. Study sites in this study were set in inhabited Chichi-jima Island as well as in uninhabited Ani-jima and Muko-jima Islands. The results of our observations showed that more than 50 species of animals visited the flowers of the two plant species. Species composition of flower visitors and frequencies of each visitor greatly differed among the three islands. The flowers of S. cleyerifolium, which open early in the morning, were frequently visited by diurnal insects such as bees and flies visited frequently. Whereas small native bees and hoverflies were the most frequent flower-visitors in Muko-jima Island, the alien honeybee was the one in Chichi-jima and Ani-jima Islands. Native bees were not observed at all in Chichi-jima Island, and were observed in Ani-jima Island, but at much lower frequencies than the honeybee. During night, the flowers of S. cleyerifolium were visited by nocturnal moths and geckos of Lepidodactylus lugubris in all of the three islands. On the other hand, the flowers of P. obovata open in the evening, and secrete nectar during the night. Its flowers were observed to be visited by nocturnal flies, oedemerid beetles and moths in all of the investigated islands. In addition, diurnal flies, hoverflies were also observed to visit the flowers at high frequencies in Muko-jima Island. The honeybee was also observed to visit in Chichi-jima and Ani-jima Islands, but at low frequencies. For both plant species, the species composition and frequency of daytime flower-visitors differed much more greatly among the three islands than those during the night. It might be because nocturnal insects are not under the predation pressure by diurnal green anoles. Therefore, plant species that depend their pollinations on diurnal insects should be more strongly disturbed by green anoles. Moreover, the number of diurnal flower-visiting insects in Muko-jima Island was more than 10 times that in Chichi-jima and Ani-jima Island. Considering the fact that honeybees frequently visited the flowers of S. cleyerifolium in Ani-jima, they might compete with and exclude native pollinators there. It is necessary to investigate the effect of honeybee on plant-pollinator ecosystems in Ani-jima Island. In this study, we could show that flower visitor observation using interval photography is quite effective to monitor the plant-pollinator systems especially in uninhabited islands or at night. We could also show that nocturnal flower visitors might contribute much more significantly to the pollination systems than formerly expected. We consider that we should perform detailed observation of pollinators of various native plant species in the Ogasawara Islands in order to promote the conservation of their wild organisms.

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