“Biodiversity offsetting” is an increasingly prevalent conservation practice worldwide to compensate for the negative environmental impacts of anthropogenic development. The minimum goal of offsetting is to achieve No Net Loss (NNL) of biodiversity values. NNL means that biodiversity losses to development will be equivalent to gains via ecological restoration and management. In practice, offsetting suffers from a lack of ‘proof of concept’ due to insufficient monitoring of offsets and from failures of regulatory oversight. Furthermore, little work has been done to critically test the ecological efficacy of offsetting practices. We sought to evaluate the efficacy of offsetting practices in Aotearoa New Zealand using two tools developed there in 2008, to calculate requirements for freshwater offsetting. The Stream Ecological Valuation (SEV) and Ecological Compensation Ratio (ECR) methodologies have been applied in Auckland, Wellington and elsewhere, and provide a baseline for empirical assessment of offset success.
We took a three pronged approach to assess the state of freshwater offsetting in Aotearoa. We first carried out an extensive desktop and physical review of resource consent documentation for developments applying SEV offsets in Auckland and Wellington. From this, we assessed the state and trends in freshwater offset regulation. We then carried out field assessments of offset sites with sufficient baseline data for restoration analysis. We evaluated progress towards NNL by analysis of stream functional and structural response to restoration, and investigated environmental factors driving these responses. Finally, we carried out eco-regulatory analyses to identify the regulatory drivers behind stream loss due to offsetting. Our results indicate that due to regulatory obstacles, it was not possible to accurately measure restorative success. Furthermore, offsetting is enabling inequitable loss of headwater streams. We highlight systematic issues leading to offset failure, with recommendations for best practice to alleviate biodiversity loss moving forward from regulatory and ecological perspectives.