Definition of middle class flood disturbance for increasing biodiversity at gravel bar

摘要

Frequency and magnitude of middle class disturbance is related to the increment of biodiversity. However, 'middle class disturbance' is not clearly defined in river habitat because of its complexity. Two indices, BOI and WOI, are defined for expressing breaking or washout situation of trees respectively and one index, WOI_d, is defined for expressing the removal of annual grass. The condition at which WOI_d is larger than one (annual grass is washed out), and WOI is less than one (tree is not washed out), is correlated with the biodiversity index (B_i: defined in this study) at three gravel-bed bar and islands in the Arakawa River, Japan. However, the trend is changed with the BOI. When BOI is larger than one (tree trunk is broken), the biodiversity is increased, but when the BOI is less than one, it is decreased. The three indices have possibilities to express middle class disturbance that is closely related to the biodiversity at the habitat.