Scirpus hattorianus, a perennial sedge, was initially described as a Japanese species. Although this species has disappeared from Japan, recent works have treated it as conspecific with a common sedge in eastern North America. Herbarium studies and a search of the literature on Japanese S. hattorianus and the taxonomically confused naturalized species, S. georgianus, were performed to identify the origin of the former. Herbarium studies showed that Japanese S. hattorianus had been collected from only one, two, three, or four sites in a small area of Fukushima Prefecture between 1925 and 1939. The literature search revealed that a hydroelectric power plant had been built in the vicinity of the sites from which S. hattorianus had been collected, and that the materials for construction of the power plant had been purchased from a company located within the native distribution range of S. hattorianus in North America. This suggests the possibility that materials from North America were transported to the collection sites of S. hattorianus in Japan at that time, and that the plants may have been introduced accidentally with these imported materials. Accordingly, at present, it is deemed more appropriate to treat the Japanese S. hattorianus as a transiently naturalized sedge from North America rather than a native and extinct sedge.
Reproductive Interference is a negative interspecific interaction during reproductive processes, which may act a critical role in displacement of native species by its alien congeners. To understand the alien RI effect on distributional relationships of Japanese Taraxacum, we focused on a native species in Izu Peninsula, putatively T. platycalpum, and investigated the coexistence with the alien congener T. officinale and its vulnerability to the alien RI. Our field survey revealed strong negative correlations in the local density between the native and the alien, and reductions in seed-set of the native only when relative abundance of the alien was 80% or more. These results suggest considerable negative effects of the alien RI on the native in Izu only if the native is the small minority at a spatial scale within which RI works, but otherwise the native should be impervious to the alien.