Beyond compliance: Strengthening mitigation hierarchy implementation in environmental impact assessment practice

Development projects continue to drive biodiversity loss, especially in densely populated and heavily modified areas. Environmental Impact Assessment (EIA), guided by the mitigation hierarchy, is meant to limit these impacts and pursue No Net Loss of biodiversity. However, persistent shortcomings – such as neglect of avoidance, narrow biodiversity metrics, and weak treatment of cumulative and residual impacts - are known to undermine its effectiveness. Here, we developed a novel comprehensive set of biodiversity-inclusive exemplary practices to assess both theoretical and procedural implementation of the mitigation hierarchy. These practices address the full mitigation sequence, including avoidance, minimisation, restoration and compensation, but also the persistence of biodiversity loss despite mitigation. Applied to 20 major infrastructure EIAs in Flanders (Belgium), a region under intense anthropogenic pressure, overall performance averaged 0.46 on a 0–1 scale (range: 0.29–0.55). We found frequent procedural formal compliance but limited use of avoidance and inadequate remediation, often failing on ecological equivalence, coherence, or monitoring. Semantic ambiguities and conflated mitigation types obscured proper sequencing of actions, potentially blurring the line between preventive and remedial steps. To address this, we propose a set of substitutability principles that prioritise avoidance, account for ecological complexity, and recognize the limits to biodiversity substitution. We argue that advancing EIA practice requires not only integration with biodiversity policy and spatial planning to enable upstream decision-making within ecological limits, but also appropriate instruments and governance arrangements to support this integration. This requires enforceable remediation standards, stronger ecological follow-through, and clearer normative guidance.