Missing species listing reveals undiagnosed extinction among reptiles, tropical, and island vertebrates

Rediscovery of a species feared to be extinct generates conservation optimism and positive media attention. The hope of evading extinction contrasts with the gloom of the biodiversity crisis, although the outlook for rediscovered animals is in fact grim, particularly if they are small and uncharismatic species as is increasingly the case (Fisher, 2011a). To be rediscovered a species must first be missing, but how do we know when a species is missing, possibly undetected or possibly extinct? Scientifically documented yearly distributions of sightings vary among species and depend on search effort (Fisher & Blomberg, 2011), location remoteness in relation to searchers (who are typically not local practitioners in biodiversity hotspots (Fisher, 2011b)), identification expertise available, species distinctiveness, and features that affect conspicuousness of the animal such as body size and microhabitat (Fisher & Blomberg, 2011). In 1994, the IUCN recognised that the appropriate time missing to detect extinction varies between taxa. Based on the work of the late Prof. Georgina Mace and colleagues, the IUCN Red List definitions changed to reflect evidence-based criteria (IUCN, 1994). Now Species Survival Commission Specialist Groups and their expert assessors judge if search effort has been adequate and appropriate for each species. Authors typically provide this rationale in the Red List species account. A component of ‘appropriate’ surveys is of course the number of years of effort, but time missing is not an explicit criterion. Martin et al. (2022) acknowledge this complexity but have taken a simple approach to define lost terrestrial vertebrates (mammals, birds, amphibians and reptiles) as species not recorded by science for at least fifty years, evaluated in the IUCN Red List of Threatened Species, and not designated as EX (extinct). They identify 562 lost species, more than 90% of which are classified as Critically Endangered or Data Deficient by the IUCN. More than 90% of these lost species are tropical, concentrated especially in countries with diverse habitats, islands and mountain-tops where distributions are restricted, and remote forests, such as in Indonesia, Mexico and Brazil. Nearly a third of lost terrestrial vertebrates are island endemics and a third mountain endemics. More than 60% of EX species are island endemics (Spatz et al., 2022). In mammals, missing island endemics are extremely unlikely to ever be rediscovered because their small ranges are searchable, and the particularly lethal driver of invasive predators is their main cause of extinction (Fisher & Blomberg, 2011, Clavero & Garcia-Berthou, 2005), which suggests that many of the ~170 lost island endemics are extinct, although they lack a ‘Possibly Extinct’ flag. These findings enhance our understanding of extinction and suggest ways to improve the value of information in Red List accounts by increasing the use of the ‘Possibly Extinct’ flag. Martin et al. (2022) suggest that the PE flag should be applied more widely when consistent with recently published guidelines. They also propose that a separate last sighting date field in Red List accounts should be routinely included with species classified below Critically Endangered. These measures would help to quantify the change in status of lost species in future. This study also contributes to our understanding of extinction because it provides an independent source of evidence of reliable correlates of species extinction and detectability. There is a striking agreement between the approach of Martin et al. (2022) and other work on the numbers and features of missing and extinct terrestrial vertebrates. For example, despite different methods and thresholds, the conclusions of Martin et al. (2022) are remarkably similar to those of Re: wild (2022) (non peer-reviewed research published online by NGO Re:wild in collaboration with the IUCN and others). Re:wild researchers contacted SSC Species Specialist Groups to collate a list of species judged to be missing by taxon experts based on their knowledge of survey effort, taxonomy, and other factors. The Re:wild database thus broadly uses IUCN criteria, and includes ‘NE (not evaluated)’ species and ‘EX’ species judged to be lost by experts. Re:wild (2022) identified 1008 lost terrestrial vertebrates, and a third of the total lost species were NE. The taxon expert approach thus concluded a similar number of lost terrestrial vertebrate species to the 50-year rule approach applied by Martin et al. (2022), if we omit NE and EX species (excluded from Martin et al. definition of ‘lost’) from the Re:wild list. Martin et al. (2022) emphasised the worrying disparity between the proportion of lost and extinct reptiles, and show