Interactions among land use, climate, leaf litter traits, and consumers shape large-scale patterns of decomposition in streams of SW Europe

Habitat degradation is one of the primary drivers of biodiversity loss and ecosystem alteration. In streams, leaf litter decomposition plays a key role in energy and nutrient cycling, which is influenced by leaf litter quality, microbial and invertebrate consumers, and environmental conditions. Land use changes alter stream habitats and can modify decomposition rates and dynamics, but large-scale assessments of these anthropogenic impacts remain limited. We explored the consequences of major land use changes (tree plantations, agricultural land, and urban settlements) on stream ecosystems by comparing sites in human-impacted catchments with reference unimpacted sites (i.e., surrounded by native vegetation) across several regions of southwestern Europe encompassing a wide climatic gradient. We quantified decomposition rates of multiple leaf litter types to evaluate the interaction between leaf litter traits and environmental variation, exploring changes in associated invertebrate assemblages. Overall, land use changes reduced total and microbial decomposition rates, with agriculture exerting the strongest effects, and caused marked shifts in invertebrate assemblages including replacement of sensitive by tolerant species and reduced detritivore richness, mainly in urban streams. Climatic variability seemed to modulate these responses, with seasonal climates showing higher vulnerability to impacts. Lastly, leaf litter types differed in their sensitivity to detect degradation, although no consistent trait-based patterns were observed. Our results show that land use changes alter stream ecosystems across climatic gradients, highlighting the urgent need to integrate environmental and biological drivers across spatial scales in watershed management and to provide actionable baselines to optimize riparian restoration and buffer the effects of ongoing climate change.