Re-habitation and changes in the genetic diversity of aquatic insects in a newly restored shoal type habitat created as the result of a nature restoration project in the central basin of the Chikuma-gawa River

Abstract

The channels of almost all rivers in Japan have been fixed through the construction of artificial riverbanks to control flooding. In addition, to prevent flooding, maintenance works including the removal of gravel from the channels must be conducted regularly. As a result, the level of most riverbeds within river channels has been lowered, and riverbanks have become far steeper. These large changes to riverside environments have significantly altered the type of habitats available to plants, causing the level of vegetation growth on the riverside to increase. To improve such flood control methods, a new excavation project has commenced in the central area of the Chikuma-gawa River basin, under the auspices of the newly commissioned “Government Nature Restoration Project”. As part of this project, a large shallow environment approximately 1 km in length along the river’s course was newly created. We have attempted to evaluate the impact of this project and the subsequent environmental response, focusing on two dominant benthos, Stenopsyche marmorata and Isonychia japonica, particularly the dynamics of their genetic structure and diversity. Following the excavation of riverbanks and channels, the population density reached the same levels as at the control site, in a relatively short period of time. This is because the research site was limited to a small area within the large-scale river basin, with robust habitats located both upstream and downstream. The two target species in this study represent typical dominant species in the central basin of this river, and occur at high density. In other words, they could be transferred smoothly from the surrounding robust habitats, especially by the flow from upstream.