River network position and cascade dam effects on macroinvertebrate communities: implications for hierarchical river conservation

The interplay between the natural gradient of river networks and the fragmentation effects of cascade dams in shaping macroinvertebrate diversity and community assembly remains unclear. This study focused on the middle Jialing River in the upper Yangtze basin, a highly developed area. We quantified taxonomic and functional α- and β-diversity across the river network. Generalized additive models revealed a clear spatial gradient. Dam impacts amplified in distal secondary tributaries and attenuated in proximal mainstem reaches. The strongest dam-related effects occurred in secondary tributaries, where high local damming intensity co-occurred with low background connectivity. Random forests and structural equation modeling showed that dam distance consistently affected diversity. This effect was strong on taxonomic β-diversity in secondary tributaries, with a direct path coefficient of 0.53. Functional β-diversity was highly responsive to dam distance and exhibited a pronounced nestedness pattern in secondary tributaries. Variation partitioning revealed that community assembly mechanisms shifted along the river network. Dispersal limitation dominated at the basin scale. Environmental filtering drove the mainstream and primary tributaries. Secondary tributaries were jointly driven by dam distance, environmental filtering, and spatial constraints. These findings extend the network position hypothesis to regulated rivers by re-parameterizing its dispersal-directionality assumption. We recommend designating secondary tributaries as priority conservation areas, primary tributaries as connectivity management zones, implementing multi-site protection along the mainstream, and incorporating functional β-diversity into routine monitoring as a sensitive indicator for ecological assessment. This study provides new insights into the coupled effects of cascade dams and natural river network structure, offering a scientific basis for hierarchical river management in the upper Yangtze River.