Efficiency of unlocking or locking existing protected areas for identifying complementary areas for biodiversity conservation.

It is well known that existing protected areas (PAs) should function as focal areas for expanding PA systems. The optimal complementary conservation areas are often identified by implementing two approaches in systematic conservation planning, i.e., unlocking or locking existing PAs. However, evidence-based studies are lacking for clarifying the efficiencies of these two planning approaches. With Sichuan in southwest China - part of a global biodiversity hotspot - as one case, this study first assessed the ecological representativeness of existing nature reserves (NRs). Using 32 natural vegetation types as the conservation features, we then implemented a systematic conservation planning process by running Marxan software with NR-unlocked and NR-locked scenarios. A human disturbance index was also included as a penalty function in Marxan for achieving cost-effective planning. We finally investigated the efficiencies of the unlocking and locking planning approaches by comparing the outcomes of the NR-unlocked and NR-locked scenarios. We found that existing NRs were geographically biased towards the western mountainous regions with high elevations and low human disturbance levels. For achieving the same quantitative conservation targets, the total area of the NR-locked priority conservation areas was 18.6% larger than that of the NR-unlocked areas, whereas the area of NR-locked complementary areas to existing NRs was 15.3% smaller than that of NR-unlocked ones. Moreover, the NR-locked priority conservation areas had higher ecological representativeness than NR-unlocked areas. The results suggest that if a completely new PA system is to be established without considering existing PAs, the unlocking approach could more efficiently achieve the full conservation targets at lower costs of land area and with better connected habitats. When existing PAs must be used as focal areas for expansion, the locking approach is more cost-effective for filling conservation gaps by requiring smaller amounts of complementary areas. Our analysis provides evidence-based support for expanding the current PA systems in a cost-effective manner.