<div> Conventional environmental governance frameworks largely treat nature either as a passive resource base or as an externality correctable through market pricing mechanisms. Such approaches rely on implicit assumptions of marginal damage, proportional system response, and reversibility—assumptions increasingly contradicted by empirical evidence of non-linear ecological stress accumulation, delayed collapse, and irreversible threshold breaches. </div> <div> <br> </div> <div> This paper introduces the Adaptive Environmental Capacity Theory (AECT), a systems-based environmental governance framework that conceptualizes the environment as a finite, heterogeneous, and regenerative capacity system constrained by biological regeneration rates and critical thresholds. Rather than proposing a single optimization equation, AECT formalizes a Capacity–Load–Regeneration–Threshold (CLRT) structure and an adaptive decision rule that governs economic and institutional activity within ecological safety limits. </div> <div> <br> </div> <div> AECT reframes sustainability from an efficiency and pricing problem into a continuous capacity management challenge, emphasizing precautionary buffers, threshold respect, and regeneration prioritization. The framework explicitly rejects marginal optimization beyond ecological safety boundaries and argues that certain losses cannot be compensated through financial mechanisms once thresholds are crossed. </div> <div> <br> </div> <div> By integrating ecological limits directly into governance logic rather than post-damage correction tools, AECT offers a policy-relevant, institution-compatible foundation for managing environmental stability across production, service, and urban systems. The theory is presented as a conceptual governance framework that invites future empirical calibration and subsystem-specific operationalization. </div>