Condition Assessment Variability in England’s Biodiversity Net Gain: Implications for Nature Credit Markets

The demand for standardized metrics to measure biodiversity outcomes is becoming an economic and political priority for countries around the world to tackle the biodiversity crisis. England’s Statutory Biodiversity Metric is a pioneering model being adapted by other countries, particularly as the European Union recently published its Nature Credits Roadmap. This Roadmap aims to increase private finance for nature restoration and to establish markets for biodiversity credits. England’s mandatory Biodiversity Net Gain (BNG) requires a minimum 10% uplift in units to deliver positive gains in biodiversity. However, its calculation is dependent on subjective habitat condition assessments done in the field by competent assessors through a series of habitat specific criteria. We used simulations to understand the influence of uncertainty in these assessments on calculated biodiversity units, showing where assessment variability can generate the required 10% uplift in biodiversity units, even when the true ecological state of the habitat remains the same. We validated these simulations through an online test targeting 155 professional ecologists using England’s Biodiversity Metric, followed by semi-structured interviews with a subset of 21 participants. Our analysis reveals that ecologists were more likely to correctly assess habitats in poor condition than those in good condition, showing there is a significant bias introduced by inherent variability among assessors. While we are aware that an online test is not the same as carrying out an assessment in the field, our findings suggest that there is a high demand for training to reduce inter-observer variability and for improved guidance, to ensure the metric calculation shows  genuine measures of nature recovery. Our findings can be an important lesson to the EU and other nations when adapting England’s metric. Detailed guidance, high-quality training , and the integration of new technologies and responsible AI use need to be considered when designing metrics. England’s statutory biodiversity metric was ten years in the making and is the 7th in a series of test metrics. It is crucial that metrics go through rigorous testing before becoming mandatory to ensure transparent methodologies that capture real change in biodiversity, and to achieve trust and integrity in nature credit markets.