Debris flow weakens the ecological role of river microhabitat heterogeneity in mountainous regions

Abstract

Mountain rivers provide habitat or refuges and create migration corridors for diverse aquatic and riparian organisms. River microhabitat heterogeneity (RMH), which plays a key role in ecological restoration, is sensitive to external disturbances in mountain rivers. However, the effects of RMH, induced by hydro-geomorphological processes, on local macroinvertebrates have not been quantitatively studied. To explore the ecological significance of RMH, we proposed a new RMH index (RMHI) for quantitative evaluation of RMH and selected five debris flow-dominated mountain rivers (DMR) and five equilibrium sediment transport mountain rivers (EMR) as contrasting examples based on the richness of sediment supply. We found that RMH supported macroinvertebrate α-diversity and functional richness in both DMR and EMR, but debris flow weakened the ecological role of RMH in DMR. The proposed RMHI should be ≥ 8.0 to maintain the ecological health of mountain rivers. Besides, the macroinvertebrate communities were mainly driven by species turnover in EMR, while species turnover and nestedness were balanced in DMR. And macroinvertebrate community shift from an R-strategy to a K-strategy due to deposition and erosion. According to the research results, we put forward the following suggestions. At the watershed scale, ecological conservation of mountain rivers requires a regional approach focusing on multiple sites in EMR. Priority should be given to protect the river with high macroinvertebrate species richness and close attention needs to be paid to multiple sites of DMR. At the river scale, we can protect the biodiversity of mountain rivers by maintain RMHI above its threshold.