Legacy effects of cropland use trajectories on stream environments and insects across contrasting change contexts

Agricultural land-use change can leave persistent legacies in freshwater ecosystems, yet the consequences of contrasting cropland-use trajectories, particularly expansion and contraction, remain poorly understood. In this study, we examined how multi-decadal cropland-use trajectories were associated with contemporary stream environmental conditions and aquatic insect communities at selected stream sites within Zhejiang Province, China. Using snapshot measurements of stream habitat, water quality, and aquatic insect assemblages, we related historical cropland-use trajectory attributes at the sub-catchment scale to current environmental conditions and to taxonomic and functional community composition under expansion and contraction contexts. Historical cropland-use trajectory attributes were significantly associated with most contemporary environmental variables and with both taxonomic and functional community composition. Although community-level comparisons provided no conclusive evidence that the strength of historical effects differed between expansion and contraction contexts, separate Threshold Indicator Taxa Analysis (TITAN) revealed distinct threshold-response patterns among individual taxa and functional groups within each context. Under cropland contraction, several sensitive taxa showed threshold responses along recovery-related trajectory attributes, whereas under cropland expansion, some disturbance-tolerant taxa increased along disturbance-enhancing attributes. These results indicate that cropland-use legacies are associated with multiple environmental and biological responses, while analyses of indicator taxa and functional groups provide complementary information to community-level assessments. Our study highlights the importance of considering long-term land-use trajectories, particularly the direction of change, and integrating taxonomic and functional responses when assessing the long-term consequences of agricultural change and informing stream restoration and land-use management.