2021 年 76 巻 p. 50-58
Spartina alterniflora, a halophytic plant native to the east coast of North America and the Gulf of Mexico, grows in river estuaries and tidal flats, forming large colonies. Given that the invasion of this species often has substantial detrimental effects on local ecosystems, including the survival of benthic animals and native plants, it needs to be appropriately managed. Since 2009, S. alterniflora has been spreading into the Ohno River, which flows into in the inner region of the Yatsushiro Sea. As the Ohno River environment has obstructed access to dredging machines and thus restricted the removal of S. alterniflora, colonies of this plant have rapidly proliferated. To characterize its spreading patterns in this river, we quantified chronological changes in the area covered by S. alterniflora colonies based on a comparison of aerial photographs. The colonies were found to be distributed primarily in the intertidal zone of the Ohno River. Although the number and area of S. alterniflora colonies have increased markedly along the river estuary up to 2 km upstream, these colonies are rarely found in the upper reaches of the tidal zone, which could be attributable to the fact that large stands of Phragmites australis reed cover extensive parts of the habitat in the upper part of the intertidal zone. Nevertheless, there remain areas in the intertidal zone that are potentially suitable for further growth of S. alterniflora (i.e., bare fields), indicating that it will continue to expand its distribution along the Ohno River in the future. Continuous monitoring of the Ohno River and neighboring rivers will facilitate the early detection of this spread and thereby minimize the costs of controlling S. alterniflora growth.