Inconsistencies and Underperformance in US Biodiversity Offsets Policy

Biodiversity offsets aim to compensate for the ecological impacts of development in protected species’ habitat. Despite the increasing prominence of offsets in conservation governance worldwide, their effectiveness at producing positive species-level outcomes is rarely explored empirically. Here, we evaluate US Species Conservation Banks in California, one of the longest-running biodiversity offset programs. We used 30 years of on-site species monitoring data across 36 sites to understand the consistency, scientific adequacy, and ecological outcomes of conservation banks for four focal species.

Overall, we find that most banks are not required to demonstrate the presence or breeding success of target species to sell credits for them. We also find that banks differ significantly in quality, measured by consistency of focal species presence, breeding, and achievement of habitat goals. In our sample, 23% of banks fail to document any presence of the target species on-site and 56% of banks fail to demonstrate successful breeding. Detection rates varied by up to two orders of magnitude across banks within species, with no evidence of improving outcomes over time. Banks’ ecological success varied by species: for two of our four species, most banks produced consistently low or zero detections of their target species despite substantial accumulated survey effort, while for the other two species, banks consistently provided occupied habitat. We found that conservation banks have highly inconsistent monitoring and management programs that do not align with best scientific practices, inhibiting analyses of program success. Notably, we find that adaptive management is often invoked to shift monitoring practices, resulting in inconsistencies both between and within banks. Species monitoring data for impact sites was not available, precluding an assessment of ecological equivalence between impacted and protected habitats. Despite these data limitations, when combined with evidence on the low additionality of banks in California, our results indicate that this long-running biodiversity offset program provides marginal and highly inconsistent biodiversity benefits over several decades. As governments increasingly adopt offsets as a central mechanism to resolve conflicts between development and biodiversity, our analysis highlights the uncertainty of positive outcomes for species even under mature regulatory frameworks.