A Review of Condition Metrics Used in Biodiversity Offsetting.

Biodiversity offsets are commonly used to compensate for environmental impacts, but their effectiveness is often questioned. Estimations of expected losses and gains often rely on what we called condition metrics, which measure a site's quality or condition using certain ecological attributes. Condition metrics are central to most offset policies, but their attributes and calculations vary substantially. We reviewed the academic literature to draw a profile of existing condition metrics used in the offsetting context. We found 17 metrics that differed in how they included attributes from the three "dimensions of equivalence": biodiversity (present in 15 metrics), landscape (in 10 metrics) and ecosystem services (in 5 metrics). Most metrics included many ecological attributes and required fieldwork and GIS data to be calculated, but few used modeling and expert opinion. Generally, metrics aggregated the attributes into a single final value and were created in Global North countries. To favor more transparent and ecologically equivalent offset trades worldwide, we suggest condition metrics should include the three dimensions of equivalence in a disaggregated way, i.e. measurements done separately and analyzed in parallel. The use of modeling, expert opinion and GIS may facilitate the inclusion of the dimensions and reduce the need for intensive (and expensive) fieldwork. Testing synergies and trade-offs among attributes could indicate if metrics can be simplified without losing information. Finally, development of fit-for-purpose condition metrics is especially important in Global South countries, where few such metrics exist.