Life history, niche, and neutrality: A tripartite balance in a dam-impacted mega-river metacommunity

Understanding the mechanisms of community assembly is a central theme in ecology, yet these processes remain critically underexplored in mega-river ecosystems, which are increasingly reshaped by anthropogenic disturbances. The Pearl River, China's second-largest river by discharge, combines extensive natural heterogeneity with one of the world's most intensive dam networks, providing a unique natural laboratory to disentangle how natural and human-induced drivers jointly shape biodiversity. Here, we employed an integrative framework—combining metacommunity structure analysis, deterministic analytical approaches, and neutral community modeling—to provide a holistic view of the processes structuring macroinvertebrate communities. Our results revealed a Quasi-Clementsian metacommunity structure, an ecological signature characterized by strong environmental filtering (high coherence, clumped boundaries) but suppressed species turnover. Hierarchical partitioning identified local environmental conditions as the strongest direct drivers of community composition. Structural equation modeling further revealed that catchment-scale factors exerted powerful, albeit indirect, control by shaping these local habitats. Crucially, spatial models explicitly incorporating dams (AEM dam ) vastly outperformed natural connectivity models (AEM natural ), unequivocally identifying anthropogenic fragmentation as a critical modulator of spatial patterns. Furthermore, neutral community models indicated a nearly balanced interplay of niche and neutral processes, a balance subtly mediated by dispersal traits: niche selection was marginally more important for highly mobile aerial dispersers (AET), while stochastic processes like dispersal limitation and ecological drift were more influential for constrained aquatic dispersers (AQT). Collectively, these findings demonstrate that community assembly in this heavily exploited mega-river is not governed by a single ecological paradigm but by a complex interplay of deterministic and stochastic forces, fundamentally reshaped by human activities. Our study advances the mechanistic understanding of biodiversity dynamics in working riverscapes and underscores the urgent need for conservation strategies that simultaneously address watershed-level land use and restore ecological connectivity.