Story highlights Subspecies of black rhino in Africa extinct according to leading conservation network

One quarter of mammals on IUCN Red List are at risk of extinction

Plants, including a tree used to produce a chemotherapy drug now on endangered list

Five out of eight tuna species are either "threatened" or "near threatened"

Africa's western black rhino is now officially extinct according the latest review of animals and plants by the world's largest conservation network.

The subspecies of the black rhino -- which is classified as "critically endangered" by the International Union for Conservation of Nature's (IUCN) Red List of Threatened Species -- was last seen in western Africa in 2006.

The IUCN warns that other rhinos could follow saying Africa's northern white rhino is "teetering on the brink of extinction" while Asia's Javan rhino is "making its last stand" due to continued poaching and lack of conservation.

"In the case of the western black rhino and the northern white rhino the situation could have had very different results if the suggested conservation measures had been implemented," Simon Stuart, chair of the IUCN species survival commission said in a statement.

"These measures must be strengthened now, specifically managing habitats in order to improve performance, preventing other rhinos from fading into extinction," Stuart added.

The IUCN points to conservation efforts which have paid off for the southern white rhino subspecies which have seen populations rise from less than 100 at the end of the 19th century to an estimated wild population of 20,000 today.

Another success can be seen with the Przewalski's Horse which was listed as "extinct in the wild" in 1996 but now, thanks to a captive breeding program, has an estimated population of 300.

The latest update to the IUCN Red List of Threatened Species reviews more than 60,000 species, concluding that 25% of mammals on the list are at risk of extinction.

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Many plants are also under threat, say the IUCN.

Populations of Chinese fir, a conifer which was once widespread throughout China and Vietnam, is being threatened by the expansion of intensive agriculture according to the IUCN.

A type of yew tree ( taxus contorta ) found in Asia which is used to produce Taxol (a chemotherapy drug) has been reclassified from "vulnerable" to "endangered" on the IUCN Red List, as has the Coco de Mer -- a palm tree found in the Seychelles islands -- which is at risk from fires and illegal harvesting of its kernels.

Recent studies of 79 tropical plants in the Indian Ocean archipelago revealed that more than three quarters of them were at risk of extinction.

In the oceans, the IUCN reports that five out of eight tuna species are now "threatened" or "near threatened," while 26 recently-discovered amphibians have been added to the Red List including the "blessed poison frog" (classified as vulnerable) while the "summers' poison frog" is endangered.

"This update offers both good and bad news on the status of many species around the world," Jane Smart, director of IUCN's global species program said in a statement.

"We have the knowledge that conservation works if executed in a timely manner, yet, without strong political will in combination with targeted efforts and resources, the wonders of nature and the services it provides can be lost forever."