Disruption of local plamt-pollinator ecosystems in the Ogasawara Islands by alien species
Oceanic islands, which have never been connected to the large continents, generally have poor flora and fauna, but have unique ecosystems composed of limited numbers of species. The Ogasawara Islands are oceanic islands that locate 1000 km south of Tokyo, and have unique plant-pollinator system. The main pollinators are solitary bees, flies and moths, whereas social bees such as bumblebees, which are the most efficient pollinators in the continents, are absent there. However, native insects have been depleted especially in inhabited Chichi-jima and Haha-jima Islnads of the Ogasawara Islands due to predation pressure of North American green anole, Anolis carolinensis since 1980s. Moreover, honey bee, Apis mellifera was introduced to Chichi-jima Island for bee-keeping in 1880s, and is the main flower visitor in the island now. Thus, plant-pollinator ecosystem in the Ogasawara Islands might be disrupted by these alien species of animals. However, such ecosystems in the Ogasawara Islands have not been well documented, especially due to lack of quantitative observations of flower visitors through day and night times. In this study, I quantitatively investigated the flower visitors of the Ogasawara native plants using interval photographing function of recent digital cameras. I observed flowers of Syzygium cleyerifolium (endemic plant species to the Islands) and Planchonella obovata (cosmopolitan plant species), which abundantly grow together in the similar habitats. The study sites in this study were set in the inhabited Chichi-jima Island as well as in uninhabited Ani-jima and Muko-jima Islands. As the results, more than 50 types of flower visiting animals to the flowers of two plant species were recorded and identified. Species composition of flower visitors and frequencies of each visitor greatly differed among the three islands. To the flowers of S. cleyerifolium, which open early in the morning, diurnal insects such as bees and flies were frequently visiting. Whereas small native bees and hoverflies were the most frequent flower-visitors in Muko-jima Island, 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 in much lower frequencies than the alien bee. At night, the flowers of S. cleyerifolium were visited by nocturnal moths and gecko 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 night. Its flowers were observed to be visited by nocturnal flies, oedemeridae 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. Alien honey bee was also observed to visit in the Chichi-jima and Ani-jima Island, but in low frequencies. For both of the plant species, the fauna of daytime flower-visitors differed much more greatly among the three islands than those during night. It might be because nocturnal insects are not under the predation pressure by diurnal green anole. Therefore, plant species that depend their pollinations on diurnal insects should be more strongly disturbed by green anole. Moreover, the number of diurnal flower-visiting insects in Muko-jima Island was more than 10 times that in Chichi-jima and Ani-jima Islnad. Considering the fact that honeybees frequently visited the flowers of S. cleyerifolium in Ani-jima, they might compete and exclude native pollinators there. It is necessary to investigate the effects of alien honey bees on plant-pollinator ecosystems in Ani-jima Island. In this study, I could show that flower visitor observation using interval photographing is quite effective to clarify plant-pollinator ecosystems especially in the uninhabited islands or at night. I could also show that nocturnal flower visitors might contribute much more significantly to the pollination systems than formerly expected. I consider that we should perform detailed observation of pollinators of various native plant species in the Ogasawara Island in order to promote conservation of their wild organisms.
首都大学東京, 2016-03-25, 修士（理学）