2009 Volume 70 Issue 3 Pages 255-260
The spread of alien organisms has drastically increased in the past few years, and alien plants have directly damaged our living space and caused ecosystem imbalances. The Ministry of Environment of Japan has enforced the Invasive Alien Species Act since 2006, by which specific alien plants are identified, and is focusing its efforts towards the breeding prevention and their eventual extermination. However, the environmental factors that nourish the luxuriant growth of alien aquatic plants remain largely unknown and the attendant risks are poorly understood. In this paper, I proposed how we might cope with the problems caused by alien aquatic plants based on a survey of their distribution and the relevant environmental factors of aquatic plants in the water of the southern Okayama area. This survey revealed that the habitats of alien species were characterized by water that was heavily enriched, and that they were distributed mostly in irrigation and drainage canals with artificial banks near urban districts. This suggested that their preferred habitats were places where disturbances occurred. Further more, even though Trapa sp. is native to Japan, they also showed invasive tendencies and habitat preferences similar to those of alien species. Therefore, when we are faced with alien aquatic plants, it is necessary to accurately determine the risk and to define the direction of risk management, instead of making blind assumptions that the alien species are harmful. Moreover, alien species are sometimes introduced repeatedly, as exemplified by Pistia stratiotes. Thus, we must keep in mind the concept of a time-axis.
