Drivers of ecological network quality change and multi-scenario simulation and optimization in fragile alpine regions: a case study of Lhasa

This study investigates the spatiotemporal evolution of ecological network quality in Lhasa, a fragile alpine region, under the combined pressures of urban expansion and climate change. It aims to clarify the driving mechanisms of network change and support future network optimization and regional ecological security. An Identification–Evaluation–Attribution–Simulation (IEAS) framework was developed for this purpose. The framework integrates MSPA, InVEST, and MaxEnt to identify ecological networks from structural, functional, and species perspectives; evaluates network quality through a multidimensional system covering scale, structure, and function; applies OPGD to identify dominant drivers and their critical thresholds; and uses the SD-intPLUS model to simulate ecological network responses under different SSP–RCP scenarios. The results show that: (1) From 1980 to 2020, the ecological network in Lhasa shifted from early degradation and contraction to subsequent recovery in ecological source extent and area-weighted functional connectivity. Ecological source coverage expanded westward, and the identified dispersal paths became shorter; however, several newly emerged western source clusters remained disconnected from the pre-existing network, indicating that topological integrity did not recover synchronously. (2) Changes in network quality were jointly driven by natural constraints and human disturbance. Increased grazing intensity was the main factor associated with network degradation, whereas the mid-altitude zone (4793–5050 m) represented a sensitive threshold for network improvement and can serve as a key zone for targeted restoration. (3) Ecological network quality followed different trajectories across future scenarios. Under the SSP1–2.6 scenario, targeted restoration improved overall network quality and promoted coordinated gains in scale, structure, and function. By contrast, SSP2–4.5, SSP3–7.0, and SSP5–8.5 showed varying losses in source extent and functional connectivity, although their topological responses differed among scenarios. Under SSP5–8.5, ecological source area decreased by 46.1%, ECA decreased by 60.2%, and NC increased to 4. (4) Linear fragmentation by transport infrastructure and continued climate change were identified as common threats across all scenarios, supporting management strategies based on targeted restoration and control at barrier points. The IEAS framework provides a new perspective for understanding the evolution of ecological networks and their driving mechanisms in fragile alpine regions. The findings offer theoretical support and spatial guidance for ecological network planning and quality improvement in Lhasa.