Iriomote island: ecology of a subtropical island in Japan
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This article aims to introduce the reader to the charm of Iriomote island. Iriomote island is in the subtropical region of Japan. I have been visiting the island since 2015, for four consecutive years. I have also been taking graduate students with me to provide them with field experience with mountains, rivers, and the sea, all together in one place. The students belong to the Kyoto University Leading Graduate Program in Primatology and Wildlife Science (PWS), a 5-year PhD program (http://www.wildlife-science.org/index-en.html).
Primatologists have been carrying out research on Yakushima since the 1950s because of its monkeys (Maruhashi 1980). Many publications have resulted from this work, which continues to the present day (e.g., Pelé et al. 2017). In contrast, they paid very little attention to the other Ryukyu islands. Nonetheless, it might be interesting to explore various aspects of other large islands, such as Iriomote, that, unlike Yakushima, have no monkeys in their ecosystem.
Not only are monkeys absent from Iriomote island but it has no rats or mice either, except for the human-introduced black rats (Rattus rattus). There is only one carnivore on the island—the Iriomote cat. This cat was first discovered in 1965. At that time it was reported as a new species (Felis iriomotensis). However, recent progress in DNA analysis revealed that the cat is in fact a subspecies of the Bengalese cat; consequently, its scientific name was changed to Prionailurus bengalensis iriomotensis.
The third part of the field course was snorkeling. When I was young I was a mountaineer, and had only limited experience of the sea. I had only snorkeled twice in my life before this Iriomote practice course, once in Baja California in Mexico and once in Hawaii, both times utilizing the opportunity while visiting these places thanks to international conferences. The sea surrounding Iriomote island was as beautiful as those that I had experienced elsewhere. We enjoyed exploring the coral reef that harbored various kinds of tropical fishes and sea snakes.
Just then, I was reminded of an experience in the Himalayas. I was climbing a mountain named Muztagh Ata (7509 m) at the northern edge of the Tibetan Plateau. I was exploring the climbing route with my partner. Suddenly, I fell into a huge crevasse. It was a so-called hidden crevasse that was covered by snow at the top—I had not noticed it. I stepped on it and broke the snow surface. I fell vertically, almost 15 m, and was stopped by the rope tying me to my partner. He successfully stopped my fall but he was not able to do anything else. I hung there, at the end of the rope. I had to climb up the rope by myself. The crevasse was huge. When I looked down, I could see only black space and no bottom. When I looked sideways, I recognized that the crevasse was about 2 m in width. Both sides were ice walls that continued far into the distance. I was alone, hanging in space, surrounded by blue ice. That scene now came back to me, as I floated in the Iriomote sea. By the way, it took me almost 2 h to climb out of the crevasse, up the vertical rope to reach the surface.
The Iriomote field course provided a night safari too. I have experienced night safaris in tropical rain forests in Borneo: Danum valley, Maliau Basin, and Imbak Canyon. There, I saw sambar deer, flying squirrels, tarsier, civets, chevrotains, and so forth. On Iriomote island, night safaris let you observe the subtle way of life of many tiny creatures, such as bats, lizards, crabs, coconut crabs, frogs, etc.
I was particularly impressed by the fireflies (Lychnuris atripennis) this year. It was already November. On the four main islands of Japan, we see fireflies in the summer but not in November. There are about 2000 species of fireflies in the world; about 50 of these occur in Japan. Japanese people love to see fireflies flashing in the bushes along stream banks. The fireflies in Iriomote this season were flashing on the road. We were mystified—why the road? We carefully examined the light sources and realized that it was firefly larvae, not the adult form, that were flashing. Their underbellies were lighting up! This was my first time seeing firefly larvae flashing.
There are four World Natural Heritage (WNH) sites designated by UNESCO in Japan: Shiretoko peninsula, Shirakami mountains, Ogasawara islands, and Yakushima island. Iriomote island is now a candidate to become the fifth. The Japanese government has nominated Iriomote, Amami, Tokunoshima, and a part of Okinawa as WNH sites. Those islands and Yakushima are included in the 1000-km span of the Ryukyu islands. My aim is to continue my visits to Iriomote and the neighboring islands across different seasons to learn more about the unique features of their ecosystems.
I am grateful to Dr. Mutsumi Nishida, the vice president of the University of the Ryukyus, and his staff. They gave us the opportunity to visit Iriomote island and provided the home base for carrying out the field course for the students. Thanks are due to my colleagues, Drs. Shiro Kohshima and Takakazu Yumoto, who visited Iriomote island with me and taught me about various aspects of its unique fauna and flora. I also appreciate the assistance from Profs. Reiko Takizawa and Seiko Fukushima who managed the Iriomote field course. Financial support for preparing this manuscript came from MEXT-JSPS grants #24000001, #16H06283; the Japan Society for the Promotion of Science (JSPS) Core-to-core Program CCSN, and the Leading Graduate Program of Primatology and Wildlife Science (U04) to the author. Special thanks are due to Dr. Shin Watanabe for guiding me around Iriomote island, and kindly showing me how to operate the drone in 2015—my first experience of using a drone. Finally, I thank Dr. Dora Biro of Oxford University for editing the English text.