Stochastic processes dominate community assembly at large spatial scales in Patagonian fjord ecosystems

Understanding how community assembly processes vary across spatial scales and environmental gradients is key to predicting species coexistence and informing conservation priorities. Beta-diversity represents the spatial variation in biodiversity and is intimately linked to processes ranging from fully niche-based to neutral assembly processes. Here we investigated beta-diversity patterns of rocky subtidal macrobenthic assemblages in 2 environmentally contrasting fjords of Chilean Patagonia, examining 3 spatial scales and dimensions within each fjord: vertical (subtidal depth; 1-21 m), fine horizontal (within fjord sections; 1-10 km), and broad horizontal (between fjord sections; 10-100 km). We applied generalized dissimilarity models (GDMs) to evaluate the contributions of 3 underlying assembly processes shaping ecological communities: density-independent responses to abiotic conditions (salinity and temperature differences), dispersal (geographical distances and depth), and stochasticity (expectations from null models), using Bray-Curtis dissimilarities as a measure of beta-diversity. Stochastic and dispersal processes dominated beta-diversity along horizontal gradients, particularly at larger spatial scales, whereas environmental filters consistently contributed to beta-diversity along depth gradients. The southern fjord presented greater community variation, consistent with its higher environmental heterogeneity. Stochastic processes dominated the northern fjord, suggesting stronger effects of habitat homogenization. Overall, these findings emphasize the scale-dependent nature of assembly processes in fjord ecosystems and underscore the importance of integrating multiple spatial dimensions into biodiversity assessments.