Optimizing Protected Area Expansion Across Protection Gradients: A Dose–Effect Framework Using Cohen’s d to Maximize Conservation Gains

Global biodiversity loss poses critical challenges to achieving the UN Sustainable Development Goals, particularly SDG 15 (Life on Land) and the 30 × 30 target. However, translating broad sustainability targets into regionally optimized conservation strategies remains challenging, as current planning approaches typically treat coverage targets as fixed constraints rather than optimizing ecological outcomes. Here, we propose a dose–effect framework that integrates spatial prioritization with standardized effect-size analysis to identify protection coverage that maximizes gains in ecosystem multifunctionality, supporting sustainable ecosystem management. Using the Loess Plateau of China as a case study, we constructed a composite ecological function index from five ecosystem services, identified conservation priorities using Zonation with existing protected areas locked in as the highest-priority foundation, and quantified the dose–effect relationship between incremental coverage and standardized multifunctionality gains. Results revealed a nonlinear, threshold-dependent curve, with an optimal threshold at 30.79% coverage where the standardized effect reached its maximum. Beyond this threshold, marginal gains declined as the study incorporated lower-quality habitats. This study demonstrates that explicit dose–effect assessment provides a replicable, sustainability-oriented protocol for optimizing protected area expansion, enabling decision-makers to balance conservation effectiveness with sustainable land-use planning across diverse contexts.