Regional Riverscape Resonance: Assessing Congruency in Patterns and Determinants of Freshwater Fish Communities Across Subtropical Riverscapes of the Eastern Himalayas

ABSTRACT

Understanding differences between ecological communities is key to understanding community assembly. Analysing these dissimilarities, including using various indices, helps assess community uniqueness, understand the effects of disturbances and guide conservation efforts. This approach is particularly relevant for vulnerable freshwater fish communities in regions like the Eastern Himalayas, where factors like isolation and connectivity have shaped high diversity and endemism. This study focuses on the Teesta–Jaldhaka, Manas, Kameng and Subansiri River basins of the Eastern Himalayas. We aimed to test consistency among the fish community dissimilarity and uniqueness across the freshwater basins of the Eastern Himalayas. We also aimed to identify the key ecological and biotic drivers across the river basins. We employed two approaches: a dissimilarity‐based (Jaccard index) and a uniqueness‐based (local contribution to beta diversity) approach, using variation partitioning and regression‐based models. Presence–absence data were procured from in‐field surveys and peer‐reviewed literature, along with bioclimatic, topographic and human‐induced variables. Community dissimilarity and uniqueness are driven primarily by the presence of dams, precipitation of the wettest month and slope, contributing significantly to total variation. Mid‐elevation zones in the Teesta–Jaldhaka, Subansiri and Manas basins have the highest local contributions to beta diversity, indicating greater uniqueness, particularly in the Subansiri basin. The high level of unexplained variation in community dissimilarity and uniqueness underscores the complexity of the ecological system under study. Furthermore, variation in ecological patterns has largely been attributed to anthropogenic, climatic and landscape factors. Nevertheless, the predominant focus on local environmental variables must be balanced against the broader influences of existing dams, planned hydropower development and ongoing climate change.