Communities associated with the Functional Process Zone scale: A case study of stream macroinvertebrates in endorheic drainages.

Rivers are being increasingly analyzed from a holistic scale focus, imposing the challenge to establish a clear sampling framework that integrates complex valley-to-reach hydrogeomorphic features. Here, we address this challenge by examining macroinvertebrate communities of different hydrogeomorphic patches, or Functional Process Zones (FPZs), established by the GIS-based model RESonate. We delineated FPZs across three endorheic drainages in the Great Basin, USA, using a self-emerging clustering method that classifies segments of rivers with similar hydrogeomorphic characteristics. We sampled macroinvertebrate communities across different FPZs. We examined the taxonomic and functional organizations of these communities, and we assessed the relative contributions of in-stream and watershed-scale environmental filters in structuring these communities. We found discreet macroinvertebrate communities associated with FPZs across drainages, where elevation prevailed on valley confinement in structuring these communities. Communities of upland FPZs exhibited a higher heterogeneity suggested by higher β-diversity and nested structure of communities, while lowland FPZs showed a higher pairwise abundance agreement across communities. Eltonian trait composition, primarily describing bionomic traits, showed a higher degree of niche differentiation in upland FPZs, thereby increasing the overall ecosystem function. Differences in variance partitioning among environmental filters acting at different spatial scales show a strong spatial structure in the response of communities in different FPZs. Overall, environmental filters had a stronger control of the communities' functional organization than the taxonomic composition. Our results support the paradigm of different FPZs having distinct communities that express different ecosystem properties. Findings of this study constitute a fruitful avenue for expanding community-based research using the FPZ template as a tool for riverine ecology. However, the unique nature of rivers in endorheic basins needs to be considered when applying our conclusions to other systems, as some findings (e.g., the higher community homogenization in lowland FPZs) might be specific to this rarely examined type of river systems.