Quantifying the IUCN Red List: Using historical assessments to calculate future extinction risk

The IUCN Red List of Threatened Species assigns species to discrete categories based on a common set of criteria to capture relative extinction risk. Some quantitative analyses require a continuous probability of extinction not provided by discrete categories. Furthermore, the criteria may not translate to extinction risk in a universal way across species, because some species attributes are not incorporated into the criteria. Here we calculate extinction risk using Markov Models parameterised with historical IUCN Red List assessment data, and we provide quantitative extinction risk for use in downstream analyses. We find that taxonomy, body size, and habitat specialism interact with extinction risk in ways not fully captured by the existing categories. For example, an Endangered habitat specialist appears equally at risk compared to a Critically Endangered habitat generalist. We hope our findings will support more accurate quantitative biodiversity analyses of species extinctions and declines using Red List data.