Decoupled responses of protist composition and food-web structure to environmental and spatial drivers in mountain wetlands

Understanding how environmental filters and spatial structure reorganize community composition and trophic structure simultaneously is essential for predicting ecosystem responses, yet this relationship remains poorly understood in microbial food webs. Here, we evaluated the relationship between food-web composition and structure along environmental gradients and spatial scales. We reconstructed protist-based food webs from 18S rRNA metabarcoding data using trait-based trophic inference across 219 mountain wetland communities distributed across five Iberian mountain ranges. We show that compositional turnover and food-web structural dissimilarity were positively but nonlinearly related, with limited food-web reorganization except at high compositional turnover. This suggests that community composition and inferred food-web structure represent distinct, non-interchangeable dimensions of protist biodiversity. Protist community composition responded to broad-scale climatic gradients, fine-scale environmental conditions and spatial structure, with pH emerging as the main driver. In contrast, food-web structure responded only to fine-scale conditions, with water table depth and vegetation structure defining contrasting network configurations. Shallower water tables were associated with larger, more internally structured food webs, while greater vascular plant canopy cover was associated with lower protist basal representation, consistent with shading by vegetation. The observed decoupling between protist community composition and food-web structure is consistent with functional redundancy in protist communities, whereby taxonomically distinct taxa can occupy similar trophic roles without substantially altering food-web structure. Our results suggest that pH-driven turnover may occur among protists occupying comparable trophic roles, whereas changes in water table depth or vegetation structure may promote replacement among more functionally distinct taxa, leading to greater food-web reorganization. The association of spatial structure with community composition, but not with food-web structure, suggests that geographically distant communities may differ markedly in protist community composition while maintaining similar trophic structure when they experience similar fine-scale environmental conditions. Climate-driven changes may therefore indirectly reshape microbial food webs by altering local environmental conditions.