Biodiversity Conservation

When is a species extinct?

It may sound like a silly question but the world is a large and complex place. To this day, there are credible sightings of species thought to have been extinct for decades. So what determines is a species is, in fact, extinct?

From the Extinction entry on  

How Do Scientists Decide a Species Has Gone Extinct?

Writing in The Scientist, Andy Carstens begins by detailing recent sightings of the near-mythical ivory-billed woodpecker, which was thought to have gone extinct in the 1930s. As a result of credible recent sightings by, among others, Mark A. Michaels of the National Aviary, US Fish and Wildlife Service (USFWS) were convinced this past January to offer the bird a 6-month stay of execution. A ruling of “extinct” would have meant the removal of protections required under the US Endangered Species Act, such as preserving habitat and taking other steps to try to increase population size.

An ivory-billed woodpecker (Campephilus principalis) in 1932. Arthur A. Allen and The Macaulay Library at the Cornell Lab of Ornithology.

The ongoing case highlights some of the challenges researchers face in determining whether a species has actually gone extinct. It’s “difficult to prove the absence of something,” says H. Resit Akçakaya, an ecologist at Stony Brook University, and so a lack of verifiable sightings is not necessarily evidence of extinction. According to guidelines issued by the International Union for Conservation of Nature (IUCN), an organization that tracks species’ conservation statuses on the basis of surveys, modeling, and expert opinion, “A taxon is Extinct when there is no reasonable doubt that the last individual has died.” But researchers typically don’t know when or if that last death has occurred, Akçakaya adds.

The outline of a bird at the center of this photo, taken in Louisiana in 2021, is one of the pieces of evidence for the existence of the ivory-billed woodpecker cited in a preprint coauthored by ornithologist Mark Michaels.

There are perils on both sides of this equation. Continuing to classify an actually extinct species as endangered can lead to underestimating extinction rates, and obscuring the bigger conservation picture, as well as misdirecting financial resources away from protecting vulnerable species to searching for ones that no longer exist. On the other hand, declaring something extinct when it really isn’t can inflict further harm on a struggling species. Additional issues can arise if a species that’s been declared extinct is later found. Discovery of such a “Lazarus species” can cause the public to lose faith in scientists, according to Akçakaya, and may increase poaching demand in some cases.

To aid consequential extinction decisions, IUCN has developed a methodology to help scientists make the best use of available data, says Akçakaya, who also chairs the organization’s Standards and Petitions Committee as a volunteer. One approach uses so-called exhaustive surveys conducted throughout the species’ historic range during times and seasons when it’s expected to be present. The second estimates extinction probability based on the extent and severity of threats that a species faces. The methodology has value, according to Stuart Butchart, an ornithologist at BirdLife International who was one of the first scientists to test it. For the ivory-billed woodpecker, Butchart’s analysis estimated a 75% probability of extinction using the threat-based method, a result primarily due to habitat loss. From surveys and recorded sightings, the odds of its extinction were lower—around 20%.

Kelsey Neam, a conservationist with the nonprofit Re:wild, has tested IUCN’s framework on amphibians, although she hasn’t yet used it to recommend the extinction status of a species, in part due to the dearth of information. Lack of data is the biggest challenge for extinction declarations. Whatever the level of available data, decisions ultimately come down to the verdict of a jury of experts. As an assessment facilitator for IUCN’s Amphibian Specialist Group, Neam leads working groups of experts from particular regions in reviewing species’ status. “Sometimes it’s unanimous,” she says. “Everyone goes, ‘Of course, this is totally extinct.’ Other times, there’s a lot of debate.” Her job as an expert in using the IUCN criteria is to remain unbiased. “I often do feel like I’m the head juror,” she says. “It’s a lot of pressure.”

Tasmanian tiger extinction dated to late 1990s

Long considered a poster child for 20th Century species’ extinction, it turns out the Tasmanian tiger may have endured almost until the 21st Century!

Photograph is of the last captive Thylacine, taken on 19th December 1933 at the Hobart Zoo by zoologist David Fleay (image courtesy David Fleay trustees).

An international group of researchers led by the University of Tasmania has taken a fresh look into the disappearance, and conceivable reappearance, of the Tasmanian tiger thylacine. The last thylacine confirmed killed in the wild was in 1930, and the last specimen in captivity died at a Tasmanian zoo in 1936. Since then, sightings have regularly persisted across Tasmania, though no captured creatures or images have been offered to prove its survival.

With the possibility that the creature had persisted well past its addition and eventual removal from the endangered species list with an official designation of “extinct,” the researchers wanted to model the most likely last refuges of the iconic predator. In the paper, “Resolving when (and where) the Thylacine went extinct,” researchers modeled 1,237 reported sightings from 1910 to the present day.

For the study, published in Science of The Total Environment, researchers pulled from every available source: records from government archives, published reports, museum collections, newspaper articles, contemporary correspondence, private collections or other miscellaneous citations and testimony. The team even poured over microfilm records to compile their sighting database.

This resulted in median extinction dates of 1999 and 2008, with the most likely (overlapping) termination date by the late 1990s—a highly controversial result unless you are a Tasmanian tiger enthusiast hoping they may still be out there. However, when restricting data to physical specimens, the models indicated extinction by 1941. Looking at the data as a whole, the annual number of reports in the six decades spanning 1940 to1999 was relatively constant but fell substantially from 2000 to the present. This suggests the possibility of a small group of thylacine beating the odds of extinction by retreating to more remote areas, vanishing just a few years before smartphone cameras could have captured conclusive evidence.

Endangered vulture returns after being extinct for 36 years

Cinereous and Griffon Vultures feeding in the wild. Credit: Hristo Peshev, Fund for Wild Flora and Fauna

The Cinereous Vulture (Aegypius monachus)—also known as Black Vulture, Monk Vulture or Eurasian Black Vulture—is the largest bird of prey in Europe. Globally classified as Near Threatened, its populations in southern Europe, once abundant, have been experiencing a dramatic decline since the late 1800s. So dramatic, in fact, that by the mid-1900s, these birds had already been nowhere to be seen throughout most of their distributional range across the Old Continent. In Bulgaria, the species has been considered locally extinct since 1985. Thanks to the re-introduction initiative that was started in 2015 by three Bulgarian non-governmental organizations: the leading and oldest environmental protection NGO in Bulgaria: Green Balkans, the Fund for Wild Flora and Fauna and the Birds of Prey Protection Society, the species is now back in the country.

The re-introduction of the Cinereous Vulture is the latest in a series of conservation projects focused on birds of prey in Bulgaria. An article published in Biodiversity Data Journal details the process.

These animals went extinct in the wild. Scientists brought them back

Writing for CNN back in 2021, Rebecca Cairns details sixteen animals that were extirpated in the wild, then brought back from the brink of extinction in captivity and reintroduced to their former habitat. A rather gorgeous pictorial accompanied the article, and included such iconic species as: the Eurasian lynx, the Tasmanian devil, and the Steppe bison.

Hunted for its meat, hide and horns, the Arabian oryx disappeared from the wild in the 1970s but has since been reintroduced in Israel, Oman, Saudi Arabia, Jordan and the United Arab Emirates.

Personally, I would make a lousy adjudicator for the IUCN. I would always be inclined to believe that a species is still alive. The world becomes so much poorer when we lose even one!

From Getty Images.

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