Extinct in the wild

A species that is extinct in the wild (EW) is one that has been categorized by the International Union for Conservation of Nature as only consisting of living members kept in captivity or as a naturalized population outside its historic range. Classification requires exhaustive surveys conducted within the species' known habitat with consideration given to seasonality, time of day, and life cycle. Once a species is classified as EW, the only way for it to be downgraded is through reintroduction.

Not all EW species are rare. An example is the Brugmansia genus, where all seven species are widely cultivated, but none are found in the wild. Ultimately, the purpose of preserving biodiversity is to maintain ecological function to prevent ecological extinction.

Examples of species and subspecies that are extinct in the wild include (in alphabetical order):

Reintroduction is the deliberate release of individuals into the wild, from captivity or from other areas where the species survives. However, it may be difficult to reintroduce EW species into the wild, even if their natural habitats were restored, because survival techniques, which are often passed from parents to offspring during parenting, may have been lost. Reintroduction efforts, also referred to as translocation, are complex and a common source of complication is how animals behave upon release. Climate suitability has been shown to influence reintroduction outcomes as well. Though many efforts translocate populations to historic ranges, climate change may be causing those previously inhabited areas to no longer be suitable for the species.

The Przewalski's horse was downgraded from EW to Endangered in 2011 after decades of reintroduction efforts. In China, they are still classified as EW since they are given supplemental feed over the winter to aid survival. Of the 2500 living, about 1360 are in the wild, and all 2500 are descended from 12 wild-caught ancestors, causing an inbreeding depression that contributes to factors, such as shorter lifespans and high mortality, that impede conservation.

Northern white rhinos have been extinct in the wild since 2007, and only two females remain in captivity. The San Diego Zoo Global is planning to save the species by using living cells from 12 rhinos that have been cryopreserved, turning them into stem cell lines, using in vitro fertilization to create embryos, and then having Southern white rhinos serve as surrogates. Currently, there have been no successful embryo transfers in rhinos. It is estimated to take at least 40 years for the target of 25–40 northern white rhinos to be reached.

Some people critique efforts to save species with such small populations due to the possibility of inbreeding as it can reduce the population growth rate. Small effective population sizes are another critique. Effective population size is a measurement of the loss of genetic diversity. Multiple populations have been found to have an effective population size below conservation goals. Additionally, monitoring effective population size and using it to aid estimations of the success of conservation efforts has been shown to provide a better overview of determining population trends when compared to population size.

The IUCN developed a system of classifying species recovery efforts in 2012 entitled the Green Status. The species recovery score is a 0%–100% scale, with 0% being the species is extinct or extinct in the wild and 100% being fully recovered. In addition, the Green Status also classifies previous and future conservation impacts with the Green Scores of Conservation Dependency, Conservation Gain, Conservation Legacy, and Recovery Potential.