Significance of small stream restoration and river network on freshwater fish diversity.

Abstract

The river network is one of the key functions for maintaining riverine biodiversity. However, the location of most restoration projects was determined by only the site scaleʼs biodiversity status. Additionally, although the effect of large river restoration projects was often well assessed, most small stream restorations have been conducted without monitoring. In this study, to provide information for restoration prioritization, we clarified the influence of stream connectivity and stream restoration on the α and β diversity of freshwater fish based on the samples obtained from 21 small streams belonging to 1st to 3rd-order streams in Tokachi river, central Hokkaido, Japan. Here, we hypothesized that (a) small stream restoration could increase α diversity particularly at high connectivity stream, and (b) β diversity would increase at a lower connectivity stream but will be not changed by restoration. Results revealed that high α diversity was supported by restoration and proximity to the mainstem. Additionally, α diversity was higher at restoration sites (characterized by various stream widths and max water depth) than at non-restoration sites. In contrast, although β diversity was negatively influenced by dIIC, we could not find any relationship between restoration status (restored or not). Our results suggested that restoration sites should be selected based on the interaction of conservation target (α diversity or β diversity) and spatial position within stream network. For instance, small stream restoration nearby mainstem is a priority to conserve α diversity of freshwater fish. Additionally, given our results that stream restoration increased α diversity regardless of habitat connectivity and had no effect on β diversity, restoration at low-connectivity streams (i.e. high β diversity stream) may contribute to both α and β diversity conservation.