Potential impacts of flooding events and stream modification on an endangered endemic plant, Schoenoplectus gemmifer (Cyperaceae)

Abstract

AbstractRiver management often conflicts with the conservation of species in river ecosystems. In Japan, almost all river systems have been covered by concrete walls. Such river improvement works caused critical damage to river ecosystems. Here we report the ongoing extinction process of a rare aquatic plant by several consecutive heavy rains. The aquatic plant, Schoenoplectus gemmifer C. Sato, T. Maeda & Uchino, is an extremely rare endemic species that is strictly associated with springwater. The species is only found in 23 locations in Japan, including only two major habitats: Hamamatsu and Oita. We monitored the population fluctuations of S. gemmifer at three river systems in Hamamatsu. In the largest habitat, Higashikanda River, the population size of the species decreased to nearly 1/10th in 2004, due to several severe floods. Spatial and temporal records exhibit four stages of damaging process. The stepwise damages were found to be caused by a rapid flow of water accelerated by the river improvement work (made in 1985). The reproduction and growth by seeds and gemmae did not evidently cover the losses by flood washed out. In the other two rivers, one was extinct and the other is now at the risk of extinction. The modified river structures may be responsible for the near‐extinction of S. gemmifer in Hamamatsu area. We propose two policies for the conservation of this species: (1) the artificial cultivation of gemmae and seedlings and (2) the modification of river structure to decrease the number of washed‐out plants. In particular, it is important to decrease the water velocity at floods by some methods.