Abstract/Summary

1. Biodiversity net gain is a policy focus worldwide, acknowledging ongoing losses of biodiversity to development, and a commitment to offsetting any residual impacts on biodiversity elsewhere. At least 37 countries have mandatory offsetting policies, and a further 64 countries enable voluntary offsets. Offsets rely on credible and evidence-based methods to quantify biodiversity losses and gains. 2. Following the introduction of the United Kingdom's Environment Act in November 2021, all new developments requiring planning permission in England must demonstrate a biodiversity net gain of at least 10% biodiversity net gain from 2024, calculated using a statutory biodiversity metric framework. The metric uses habitat as a proxy for biodiversity, scoring habitats' intrinsic distinctiveness and current condition. 3. We carried out a study of the metric's performance across England in terms of outcomes for biodiversity. We used generalized linear mixed models to regress baseline biodiversity units against five long-established single-attribute proxies for biodiversity (species richness, individual abundance, number of threatened species, mean species range and mean species range/population change). Data were gathered for species belonging to three commonly used indicator taxa (vascular plants, butterflies and birds) from 24 sites, including all terrestrial broad habitats except urban. 4. In baseline assessments, metric-derived biodiversity units correlated with most plant biodiversity variables, but not with any of the bird or butterfly biodiversity variables used in this study. Plant species recorded in habitats with higher baseline biodiversity units had slightly more restricted ranges (slope −16.22 ± 1.52, p < 0.001) on average and had shown stronger past declines (slope −0.02 ± 0.00, p < 0.001) than those in habitats with lower baseline biodiversity units. Each additional baseline biodiversity unit was associated with a 1% increase in plant species richness (p < 0.01). 5. Synthesis and applications: Using the statutory biodiversity metric to define 10% biodiversity net gain without additional species-focused conservation management is likely to translate into small gains for plant biodiversity, and negligible gains for birds and butterflies. We make specific recommendations to improve the metric's efficacy in achieving desirable biodiversity outcomes. Our results provide a valuable case study for other countries interested in developing metrics to support biodiversity net gain policies.