The 40 schemes represent international and national-level approaches, and tend to be either area-based (Table 2, Additional file 1; two-thirds of the schemes; e.g., wetlands of international importance [Ramsar]) or taxa-based (Table 3, Additional file 2; 20%; e.g., IUCN red-listed species). A subset (15%) is a combination: they identify valuable areas based on their importance to particular taxa (reported in Table 2, Additional file 1; e.g., centers of plant diversity). About 10% of the schemes are essentially compilations of designations made by others (reported in Table 3, Additional file 2 and Table 2, Additional file 1, depending on their focus; e.g., world database on protected areas).
The schemes share overlap in the criteria for inclusion. For example, a review of the eight best-known, scientifically-based schemes (identified by † in Table 2, Additional file 1; ) found that a few criteria were shared by most of the schemes, while other criteria arose only in a few (or none) of the schemes, in order of occurrence: intrinsic value of nature/wildlife (8 of 8 schemes); functionality (6); efficiency (6); international recognition and cooperation (4); representation (3); sustainable development (2); engaging local stakeholders (1); and utilitarian or sustainable use of wildlife (0). Endemism emerged as the most-often cited scientific criterion. Intactness was second.
Most of the earth's surface (79%) appears in at least one of the nine leading terrestrial-based schemes (identified by * in Table 2, Additional file 1), but it seems unlikely that business is going to restrict itself to the remaining 21%. Thus, companies need information at a spatial scale comparable to the impacts of their individual operations, i.e., at the site-level. Unfortunately, many schemes are defined over such large areas – hotspots, for example, cover entire countries – that few if any companies will chose a policy to stay out of hotspots altogether. This may explain in part why "a number of authors have pointed out that global conservation prioritization has had little success in informing actual conservation implementation" .
Coarse resolution also obscures variation in conservation value within a given priority area, e.g., within a hotspot, certain habitat types (such as wetlands or breeding sites) will have greater conservation value than others. Position within a landscape, such as proximity to wildlife corridors, creates further variability. However, the database (Table 3, Additional file 2 and Table 2, Additional file 1) reveals that only a few schemes operate at a resolution that can provide guidance at the site level. But even among schemes that identify site-level priorities, almost none (10%) rank values on a continuous scale. The vast majority of the schemes are dichotomous: a few ecosystems are identified as a priority while most other areas are left off the list.
Likewise, if a scheme does not assess threat then it is impossible to determine if an offset would provide any new benefits. If a site is not threatened then conservation actions are unnecessary (at least to prevent loss in the short term) and an offset would not provide an additional conservation gain. Unfortunately, the majority (80%) of schemes that focus on individual sites avoid the issue of current threat levels. Those that deal with threat are generally the schemes focused on endangered species.
It is not surprising, nor is it meant to be a criticism, that existing schemes are not more suitable for offsets – the schemes were developed with other objectives in mind. (Most are associated with particular conservation organizations and play a large role in their planning and fund raising.) Nonetheless, existing schemes can inform business of the major conservation issues in the regions where they operate. Moreover, where schemes do offer site-level priorities, such as important plant and bird areas (Table 2, Additional file 1), companies can use the schemes to identify no-go areas of such importance that no further impact is acceptable.
Where priorities have not yet been identified at the appropriate spatial scale and resolution, companies should collaborate with stakeholders to conduct a regional prioritization exercise. For example, a company could engage in an exercise akin to The Nature Conservancy's (TNC) Ecoregional Planning Framework or World Wildlife Fund for Nature's (WWF) Ecoregion Conservation Process  to meet the combined requirements of prioritizing sites based on representation, threat and viability. For a species-level approach, priorities could be derived from a process like that of Conservation International's (CI) Key Biodiversity Areas or Wildlife Conservation Society's (WCS) Range-Wide Priority Setting.
A substantial literature exists for the selection of representative reserve systems [13–18], including improved algorithms that explicitly incorporate threat levels (e.g., [19, 20]), which should aid the ordering of conservation priorities for biodiversity offsets. Irreplaceability scores may serve as a relative measure of conservation priority, thus assisting in determining equivalence, and areas identified by high values of irreplaceability (e.g., 95%–100%) should be considered no-go areas [21, 22]. The relative vulnerability of sites can then be used to further inform prioritization, albeit as a static assessment . Despite the potential bias inherent in such a process, stakeholder workshops can also be an effective means of initiating a prioritization exercise .
The Convention on Biological Diversity calls for all signatory nations to prepare National Biodiversity Strategy and Action Plans, and in some cases these documents may serve as a comprehensive conservation 'needs assessment' for the nation (i.e., if they include identification of essential and priority targets for conservation as well as the interventions needed to sustain them).