Extinct, obscure or imaginary: The lizard species with the smallest ranges

Within the same lineage, some species have continental-wide distributions, whereas others are restricted to a single locality. Although ranges can be very labile, range size is thought to be the product of ecologically relevant traits such as body size, population density, and dispersal ability. Crucially, from a conservation perspective, range size is known to influence extinction risk. Species with small ranges have fewer individuals and lower genetic variation than wide-ranging relatives, often leading to elevated extinction probabilities. The International Union for Conservation of Nature (IUCN) Red List assessments reflect the importance of range size in evaluating extinction risk. One of the five criteria the IUCN uses to evaluate threat, criterion B, uses estimates of range size to designate extinction probabilities. Species with ranges smaller than 20,000 km2 can qualify as vulnerable under criterion B, while those with ranges not exceeding 5,000 km2 can qualify as endangered. The threshold is lowered to 100 km2 for the highest level of threat—critically endangered. A comprehensive dataset of all the world's lizard species was assembled and analyzed to identify species with the smallest ranges. These species are known only from their type localities and are often elusive, poorly known, and little studied. Many face severe extinction risk, but current knowledge is inadequate to properly assess this for all of them. The study found that one in seven lizards (927 of the 6,568 currently recognized species) are known only from their type localities, including 213 species known only from a single specimen. Geckos are especially prone to having tiny ranges, and skinks dominate lists of such species not seen for over 50 years, as well as of species known only from their holotype. Two-thirds of these species have no IUCN assessments, and at least 20 are extinct. The study recommends that such species become the focus of taxonomic, ecological, and survey efforts. The findings suggest that range size is not strongly associated with biological attributes such as body size or geographic attributes such as latitude, but rather with the year a species was described.