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The US government has announced that 23 species of American animals and plants are believed to have gone extinct.
The ivory-billed woodpecker, the San Marcos gambusia and the little Mariana fruit bat are among those species set to be recognised as having gone over the brink, the US Fish and Wildlife Service (USFW) have said.
Habitat destruction, collectors and pollution have been named as causes for their loss.
While the International Union for the Conservation of Nature (IUCN), the body which compiles the Red List of Endangered species, is yet to declare if many of these species are extinct, the outlook is bleak for the 23 named on the list.
'Just because you don't see something doesn't mean it's not there,' says Alex. 'I think we can probably say some are no longer there, and I would put the ivory-billed woodpecker in that category, but others depend on how well-searched those areas are.
'There may be some populations that persevere.'
Extinctions are a natural process that come about as environments change. On average, for every one million species, two of these species could be expected to go extinct in any given year.
Since 1900, however, rates of extinction have increased sharply as humanity's impact upon the world began to make itself known. A UN report in 2019 estimated that almost 2.5% of all amphibian species had gone extinct since 1500, in addition to around 2% of all birds and mammals.
This has seen animals such as the Saudi gazelle, Pinta giant tortoise and the northern gastric-brooding frog declared extinct by the IUCN.
The majority of animals featuring on the US list are from specific groups vulnerable to change, including eight species of freshwater mussels. These animals rely on pure, reliable water sources, and increased demand for water in the US and pollution are thought to be to blame for their loss.
Island birds also feature heavily on the list, with nine species from Hawaii and Guam in the Pacific Ocean.
'As they can fly, birds often get to islands and can diversify if left in isolation for a long time,' Alex says. 'But that also means that these species can be more susceptible to risk, such as when their habitat is threatened.
'Birds in general are also highly visible and there is a huge public interest in birds that you don't necessarily get for other vertebrates. This means we're more likely to notice when they're not there.'
While some species on the USFW list, such as the Moloka'i creeper, have been declared extinct by the IUCN in the past, others remain Critically Endangered.
'The IUCN takes a global look at species and their risk of extinction, considering species across their entire range, not just a country,' Alex says. 'The capercaille is a good example. It went extinct in the UK before being reintroduced, but the species still existed elsewhere.'
Occasionally, species that are regarded as extinct can make a comeback. Surveys can miss species if they are rare, live in hard-to-reach places or are easy to misidentify.
'Extinction is, in that sense, reversible,' Alex says. 'Species that haven't been seen in over 100 years, like the New Zealand storm petrel, can be found all this time later. For these rediscoveries, it has to exist, it has to be seen, and then it has to be identified. If any of those aspects aren't in place, then it will remain missing.'
As time goes by, and a species continues to be missing, the chance of discovering it again decreases. So while Phyllostegia glabra var. Lanaiensis (a Hawaiian plant related to mint last seen in 1914) is unlikely to be found again, those lost more recently may still be found.
Even when they do go extinct, these species are still valuable to scientists.
'Extinct species help us learn about the processes that led to extinction,' Alex says, 'as animals like birds don't go extinct on this timescale by themselves. We can only understand the extinctions of the future through the lens of the past.'
They also provide a powerful reminder to the entire world of how fragile nature can be.
'We've had hundreds of bird extinctions,' Alex says, 'but in Europe we don't really have that, not really since the great auk.
'We need to remember that as the world becomes more connected, we do have an impact on species elsewhere. Just because we don't see them, it doesn't mean they're not vulnerable.'
For the 23 species on the USFW list, a public consultation runs from 29 September until 29 November as they look for any information to decide whether or not to declare these species extinct.