LOS ANGELES — Africa's Western black rhino has officially been declared extinct and other subspecies of rhinoceros could follow, according to the latest review by a leading conservation organization.

The International Union for Conservation of Nature listed the Northern white rhino in central Africa as "possibly extinct in the wild" and the Javan rhino as "probably extinct" in Vietnam.

The organization blamed a lack of political support for conservation efforts in many rhino habitats, international organized-crime groups targeting the animal, increasing illegal demand for rhino horns and commercial poaching.

"In the case of both 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, chairman of conservation group's Species Survival Commission, said Thursday. "These measures must be strengthened now, specifically managing habitats in order to improve breeding performance, preventing other rhinos from fading into extinction."

The last Javan rhino in Vietnam is believed to have been killed by poachers in 2010, reducing the species to a tiny, declining population in Java.

The rhinos were among more than 61,900 animal and plant species reviewed for the conservation group's most recent Red List of Threatened Species. One-fourth of the mammals on the Red List were found to be at risk of extinction. But the organization said there have also been conservation successes.

Fewer than 100 Southern white rhinos survived at the end of the 19th century, but the population in the wild is believed to be more than 20,000.

Numerous other species are threatened, including many plants.

The Chinese water fir, which used to be widespread throughout China and Vietnam, was listed as "critically endangered," due primarily to expanding intensive agriculture.

The conservation group also listed five out of eight tuna species as "threatened" or "near threatened," and added 26 recently discovered amphibians to the Red List, including the blessed poison frog.

"This update offers both good and bad news on the status of many species around the world," said Jane Smart, director of the group's Global Species Program. "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."