Species Conservation Assessments in Oceanic Islands: the Consequences of Precautionary Versus Evidentiary Attitudes

The application of IUCN Red List criteria to oceanic islands often produces uniform species assignments to high-threat categories, but in some cases this may result from uncertainties in the data and overly precautionary attitudes to risk. We illustrate this problem using the endemic vascular flora of the Cape Verde archipelago, and show that changing risk tolerance along the precautionary-evidentiary gradient greatly affects conservation assessments. Most taxa qualified for threat categories due to small areas of occupancy, but while precautionary assessments classified 88.9% of 81 evaluated taxa as Critically Endangered, evidentiary assessments assigned taxa to a wider range of categories. Taxa with very small ranges and restricted to one–three islands were consistently considered Critically Endangered. Our results suggest that conservation assessments under uncertainty may benefit from a more evidentiary attitude, which seem to provide higher discrimination ability among taxa, thereby enhancing the contribution of Red List assessments to prioritize conservation action.