Towards the Full Operationalization of the Dark Diversity Concept in Environmental Impact Assessments: A Call to Revise International EIA Standards

Environmental Impact Assessments and related safeguard standards are key tools for mitigating the ecological damage of development projects; however, they predominantly evaluate what is observed during baseline surveys. This approach, which is inherently constrained by imperfect detectability, seasonality, and uneven sampling effort, leads to a chronic underestimation of an ecosystem's true biotic potential, with direct consequences for the mitigation hierarchy (avoidance, minimisation, restoration and offsets) and for strategies that aim to meet the "No Net Loss" of biodiversity target. This paper proposes the systematic integration of "dark diversity", i.e., the set of species ecologically suited to a site but currently absent, into the EIA framework. Assessing a habitat's ecological "completeness" (observed diversity relative to dark diversity) reveals its true level of degradation and restoration potential, helping to distinguish systems that are simply complex to detect from those that are impoverished or constrained by dispersal and establishment barriers. This approach provides a more robust and precautionary baseline, allowing for the design of mitigation and compensation measures based on an ecosystem's full potential, not just one part. We outline pragmatic pathways to operationalize dark diversity in EIA practice, combining existing occurrence data, co-occurrence approaches, and emerging tools (e.g., eDNA and niche modelling) to guide targeted supplementary surveys, impact evaluation, and measurable restoration objectives.