A feared mass extinction of wildlife also endangers billions of humans who depend on them for food and livelihood, according to a new assessment of species loss issued Tuesday at the Rio+20 conference.

Experts presented a grim tableau of the planet's biodiversity as world leaders were to arrive for a three-day summit on Earth's environmental problems and enduring poverty.

Out of 63,837 species assessed, 19,817 run the risk of following the dodo, they said.

At threat are 41 percent of amphibian species, 33 percent of reef-building corals, 25 percent of mammals, 20 percent of plants and 13 percent of birds, the update of the prestigious "Red List" said.

Many are essential for humans, providing food and work and a gene pool for better crops and new medicines, it said.

The findings are "a clarion call to world leaders gathering in Rio to secure the web of life on this planet," said Julia Marton-Lefevre, head of the International Union for Conservation of Nature (IUCN), which compiles the benchmark.

"Eighty percent of our calorie intake comes from 12 plant species," said Professor Stephen Hopper, head of Britain's Royal Botanic Gardens at Kew, London.

"If we care about the food we eat, and the medicines we use, we must act to conserve our medicinal plants and our crop wild relatives."

Around 100 heads of state and government are expected to attend the summit-level part of the UN Conference on Sustainable Development, starting in Rio de Janeiro on Wednesday.

Widely respected, the Red List looks in detail at a small fraction of the world's known species in order to get a benchmark of biodiversity health.

UN members pledged under the Millennium Development Goals to brake the rate of loss in species by 2010, but fell badly short of the mark.

After this failure, they set a "strategic plan for biodiversity" under which they vowed to prevent the extinction of "most known species."

The Red List assigns surveyed species to one of eight categories.

Out of 63,837 species it assessed for its update, 27,937 were in the "of least concern" category or were "near threatened," and 255 were considered at "lower risk."

Another 3,947 were critically endangered, 5,766 were endangered and 10,104 were vulnerable, making a total of 19,817 species at threat.

Sixty-three species have become extinct in the wild and 801 have become completely wiped out. The remaining 10,497 species in the survey lacked data to make a judgment.

An American river mollusk called the ovate club shell is among four species that have sadly joined the list of the extinct, according to the Red List experts.

The good news, though, is that two species, including an amphibian called the hula painted frog (Discoglossus nigriventer), which is only found in Israel's Lake Hula marshes, have been rediscovered.

Species loss is often the result of habitat destruction. But invasive species and, increasingly, the suspected impact of climate change, are also factors.

The report placed the spotlight on reckless exploitation of oceans, lakes and rivers.

"In some parts of the world up to 90 percent of coastal populations obtain much of their food and earn their primary income through fishing; yet over-fishing has reduced some commercial fish stocks by over 90 percent," the IUCN warned.

It said that 55 percent of the world's coral reefs, on which 275 million people depend for food, coastal protection and livelihood, are overfished.

In Africa, 27 percent of freshwater fish are now listed as threatened, while in Asia, the important commercial species known as the Mekong herring (Tenualosa thibaudeaui) is vulnerable.

Insects, bats and birds that pollinate crops provide an "ecosystem service" to humans worth more than $200 billion per year.

But 16 percent of Europe's endemic butterflies and, worldwide, 18 percent of of bats are at threat.

The Red List is updated each year or more. The 2012 count of threatened species is unchanged from 201, except for birds, where the proportion is 0.5 percent higher, the IUCN told AFP.