Metrics based on habitat area and condition are poor proxies for invertebrate biodiversity
There is increasing demand for standardised, easy-to-use metrics to assess progress towards achieving biodiversity targets and the effectiveness of ecological compensation schemes. Biodiversity metrics based on combining habitat area and habitat condition scores are proliferating rapidly, but there is limited evidence on how they relate to ecological outcomes. Here, we test the relationship between the statutory biodiversity metric used for Biodiversity Net Gain (BNG) in England -- and as the basis for new biodiversity credit systems around the world -- and invertebrate richness, abundance, and community composition. We find that the combined area-condition BNG metric does not capture the value of arable farmland and grassland sites for invertebrate biodiversity: invertebrate communities were highly variable across sites that had the same type and condition under the BNG metric. We found no reliable relationship between scores under the metric and either invertebrate abundance or species richness, with the risk of the metric undervaluing sites of high invertebrate value. Our results highlight the need to incorporate factors beyond habitat type and condition into site evaluations, and to complement metric use with species-based surveys.