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Volume 70, Issue 1
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Original article
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Tetsuya TSURUTA, Tsubasa TADA, Nobuyoshi KOTERA, Izumi AKAGAWA, Kei&pr ...2009 Volume 70 Issue 1 Pages 1-11
Published: 2009
Released on J-STAGE: May 20, 2010
JOURNAL FREE ACCESSThe purpose of this study is to elucidate the factors that influence the community structure of benthos in conventional rice fields. First, we investigated the benthos faunas of paddy fields in Shiojiri and in those with water sources from Shirinashi River and Goka-Mae Pond in Goka around the Chikuma River, Ueda, Nagano Prefecture. The numbers of taxonomic categories of carnivorous aquatic insects and ratio of individual numbers for carnivorous aquatic insects within the entire benthos in the paddy fields of Goka were higher than those of Shiojiri. On the other hand, the paddy fields in Shiojiri were significantly more abundant in numbers of Oligochaeta and Chironomidae than those in Goka. Therefore, it is most likely that the abundance of carnivorous aquatic insects influences the abundance of Oligochaeta and Chironomidae. Second, to evaluate the impact of herbicides on benthos in Goka paddy fields, numbers of individual Oligochaeta and Chironomidae were compared between paddy fields on which two different herbicides were used. Numbers of Oligochaeta in the paddy fields on which imazosulfuron·oxaziclomefone·clomeprop·daimuron was applied were significantly lower than those in the paddy fields treated with dimethametryn·pyrazosulfuron-ethyl·pyriftalid·pretilachlor. This result may be attributed to direct and indirect effects due to the difference in toxicity between the two herbicides.View full abstractDownload PDF (540K)
FEATURES:Endemic freshwater biota and environment of inland water in Ogasawara Island
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Keiichiro YOSHIDA, Yoshihiro IIJIMA2009 Volume 70 Issue 1 Pages 13-20
Published: 2009
Released on J-STAGE: May 20, 2010
JOURNAL FREE ACCESSWe examined the hydroclimatic conditions during the last decade on Chichi-jima in the Ogasawara (Bonin) Islands using climatic data (1998-2007) recorded at the Chichi-jima observatory. Climatological wetness indexes (precipitation/ potential evaporation) were approximately 1.0 from 1998 to 2001, indicating closely balanced dry and wet conditions, while wetness indexes were lower than 1.0 in 2002-04 and 2006-07, and dry conditions were predominant during those years. In particular, the early northwards shift of the Asian monsoon and the intensive development of the North Pacific anticyclone (the Bonin high) in 2004 caused severe drought conditions over the Ogasawara Islands. The lower values (< 0.3) of the wetness indexes during April and May of 2004 suggested that such severe dry conditions affected the island ecosystems through the depletion of soil moisture and outflow. These results suggest that the drying trend during the late 20th century continued after 2001, and that the severe drought conditions, such as occurred in 2004, could be risk factors affecting plant and animal survival. The long-term change in the hydroclimatic conditions therefore implies that this drying trend will trigger the abrupt changes in ecosystems on the Ogasawara (Bonin) Islands.View full abstractDownload PDF (558K) -
Ryuhei UENO, Kiyoshi SATAKE2009 Volume 70 Issue 1 Pages 21-29
Published: 2009
Released on J-STAGE: May 20, 2010
JOURNAL FREE ACCESSAt irregular intervals in 1998-2000 and 2006-2007, chironomid fauna was studied in Chichijima and Hahajima in the Ogasawara Islands, of subtropical Japan. A total of 17 species were recorded, among which Dicrotendipes yaeyamanus, Glyptotendipes tokunagai and Tanytarsus magnihamatus were new to those islands. Compterosmittia sp., that lacked an anal point seemed to be a new species. In spite of the fact that aquatic environments in the Ogasawara Islands are typically lotic, our samples were totally lacking in Orthocladius, Rheocricotopus, Paratrichocladius and Polypedilum that often dominate in lotic environments on the Japanese mainland. The adults of seven species were successfully reared from pupae or larvae in the laboratory, thus confirming their habitats. The habitats of abundant endemic chironomids found in lotic environments that are relatively stable and numerous in the Ogasawara Islands suggest that they are in no urgent need of conservation measures. Nevertheless, since precipitation levels are on the decline in the islands, the entire aquatic environment might eventually be reduced to the point where aquatic life could be critically at risk.View full abstractDownload PDF (511K) -
Tetsuro SASAKI, Kiyoshi SATAKE, Kotaro TSUCHIYA2009 Volume 70 Issue 1 Pages 31-38
Published: 2009
Released on J-STAGE: May 20, 2010
JOURNAL FREE ACCESSStenomelania boninensis is a thiarid snail endemic to the Ogasawara (Bonin) Islands. Although an alien thiarid snail, Melanoides tuberculata, which presumably has a potential to replace native snails, was recorded from Chichijima in 2005, nothing is known concerning the relationships between these two species. Field surveys on the geographical distribution of the both species were conducted in 39 stream systems on eight islands of the Ogasawara Islands, including Mukojima, Chichijima and Hahajima. As a result, whereas S. boninensis was collected from 33 streams of five islands, M. tuberculata was collected from only five streams in Chichijima. In addition, whereas while S. boninensis was found at both natural and concrete bank sites in freshwater zones, M. tuberculata was found observed at concrete bank sites in both freshwater and tidal zones. The effects of a stream bank construction on both M. tuberculata and S. boninensis were also assessed. Quantitative samplings of the snails were conducted at six stations along the Yatsuse River in Chichijima during a period from immediately before to nine months after the construction was completed. Before the construction, S. boninensis and M. tuberculata coexisted in the reaches of the construction site. During the construction period, both snail species were eliminated by the diversion of all water from the stream. Whereas M. tuberculata recovered at the construction site three months after the construction, S. boninensis failed to recover even after nine months. Moreover, M. tuberculata expanded its distribution range to the upper reaches after the construction. In conclusion, a disturbance like a stream bank construction possibly induces the replacement of native species by aliens.View full abstractDownload PDF (533K)
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