Several species of rhino have been poached into extinction or to the point of no return, according to an update of the Red List of Threatened Species, the gold standard for animal and plant conservation.

All told, a quarter of all mammal species assessed are at risk of extinction, the International Union for the Conservation of Nature (IUCN), which compiles the list, said on Thursday.

About a third of the 61,900 species now catalogued by the IUCN are classified as "vulnerable," "endangered," "critically endangered," or extinct, with some groups, such as amphibians and reptiles, in particularly rapid decline.

Rhinoceros have been hit especially hard in recent years. Their fearsome horns -- prized for dagger handles in the Middle East and traditional medicine in east Asia -- can fetch hundreds of thousands of dollars on the black market.

The new assessment shows that a subspecies of the western black rhino (Diceros bicornis longipes) native to western Africa is now extinct, joining a long list of creatures -- from the Tasmanian tiger to the Arabian gazelle -- that no longer stride the planet.

Central Africa's northern white rhino (Ceratotherium simum cottoni) is listed as "possibly extinct in the wild", while the Javan rhino (Rhinoceros sondaicus) is making a last stand after the remaining specimen of its Vietnamese counterpart was killed by poachers last year.

"Human beings are stewards of the earth and we are responsible for protecting the species that share our environment," Simon Stuart, head of the IUCN Species Survival Commission, said in a statement.

"In the case of both the western black and the northern white rhinos the situation could have had very different results if suggested conservation measures had been implemented."

There were a few slivers of good news showing that species can be prevented from slipping into oblivion.

The southern white rhino subspecies (Ceratotherium simum simum) is back from the brink, its numbers up from 100 at the end of the 19th century to some 20,000 today.

Central Asia's Przewalski's horse (Equus ferus), meanwhile, has moved from a status of critically endangered to endangered.

"We have the knowledge that conservation works if executed in a timely manner," said Jane Smart, the Global Species Programme director.

The general trend, however, is an acceleration in extinction across a wide spectrum of fauna and flora. Indeed, many scientists say Earth is on the edge of a so-called great extinction event, only the sixth in half-a-billion years.

Some groups are especially vulnerable. In Madagascar, home to a dazzlingly rich diversity of life, an alarming 40 percent of reptiles are threatened.

Plant species are disappearing too. Such was the fate of the Chinese water fir (Glyptostrobus pensilis), once common in China but now apparently extinct in the wild due to habitat loss.

The new classification also recognises new species, including 26 recently discovered amphibians such as the blessed poison frog (Ranitomeya benedicta) and the summers' poison frog (Ranitomeya summersi).

Both are threatened by habitat loss and harvesting for the international pet trade.

"The world is full of marvelous species that are rapidly moving towards becoming things of myth and legend," said the IUCN's Jean-Christophe Vie.