IUCN Red List criteria fail to recognise most threatened and extinct species

Many species have benefited from management actions associated with listing as threatened on the IUCN Red List, resulting in fewer extinctions relative to business as usual. Red List criteria used to categorise threat status have, however, been designed with primary focus on higher vertebrates, and consequently produce inconsistent and deficient threat assessments for other taxa. Equally valid decisions on population trend shape, generation length, period of assessment, and habitat quality can result in threat status determinations ranging from Least Concern to Critically Endangered. Moreover, inconspicuous species exhibiting catastrophic population decline to below detection limits can ultimately be regarded as Data Deficient, thus categorised as neither extinct nor threatened. Under-estimation of extinction frequency biases our understanding of global environmental change and the urgent need for societal action. Improved extinction risk assessments can be achieved with changes and simplification of Red List criteria, and detailed case examples that guide progress through decision-points.