Understanding the drivers of expert opinion when classifying species as extinct

The criteria as laid out by the International Union for the Conservation of Nature (IUCN) Red List are the gold standard by which the extinction risk of a species is assessed, and where appropriate biological extinctions are declared. However, unlike all other categories, the category of extinct lacks a quantitative framework for assigning this category. Given its subjective nature, here we explore attributes used by expert assessors working on a diversity of taxa when declaring a species as extinct. Using a choice experiment approach, we found that data availability, time from last sighting and population decline were critical attributes favored by assessors when inferring extinction. Although several of these attributes were significant in the decisions of assessors, this information provides a clear hierarchy of preference for certain attributes. This provides a basis for informing the development of specific criteria for more accurately assessing species extinctions.