Thirty-one percent of cacti, some 500 species, face extinction due to human encroachment, according to the first global assessment of the prickly plants, published Monday.

The finding places the cactus among the most threatened plant groups on Earth, ahead of mammals and birds and just behind corals, according to the IUCN (International Union for the Conservation of Nature), an inter-governmental conservation organization.

"The results of this assessment come as a shock to us," lead researcher Barbara Goettsch, co-chair of the IUCN's Cactus and Succulent Plant Specialist Group, said in a statement.

"We did not expect cacti to be so highly threatened."

The IUCN Red List is widely recognised as the gold standard for measuring extinction risk for animals and plants.

Cacti -- native to the Americas, but introduced over centuries to Africa, Australia and Europe -- are crucial links in the food chains of many animals, including humans.

They are an essential sources of sustenance and water for deer, woodrats, rabbits, coyotes, lizards and tortoises which, in return, help spread cacti seeds.

Cactus flowers supply nectar to hummingbirds and bats, along with bees, moths and other insects that help pollinate the largely desert-dwelling plants.

In many parts of Central and South America, cacti are used by people for food and medicine.

Mexico's prickly pear cactus stem, for example, is highly nutritious, while the root of another species listed as near threatened -- Ariocarpus kotschoubeyanus -- is commonly used as an anti-inflammatory.

- 'Far-reaching consequences' -  

Depending on the species and the region, different forces have driven the decline in cacti, found the study, published in Nature Plants.

The top threat to cacti is expanding agriculture, especially in northern Mexico and the southern part of South America.

Species native to coastal areas are being decimated by residential and commercial development, while in southern Brazil conversion of land for eucalyptus plantations is harming at least 27 species, some of them already on the endangered list.  

"Their loss could have far-reaching consequences for the biodiversity and ecology of arid lands and for local communities dependent on wild-harvested fruits and stems," Goettsch said.

Researchers were also surprised to find that illegal trade in highly-prized plants is also a key factor in their disappearance.

"The scale of the illegal wildlife trade -- including trade in plants -- is much greater than we previously thought," said Inger Andersen, IUCN director general.

Some 86 percent of threatened cacti used in horticulture are taken from wild populations rather than cultivated, other studies have shown. Europe and Asia are the biggest markets for this illicit trade.

Mining is also a problem, as illustrated by the critically endangered -- the highest level of threat before extinction -- Arrojadoa marylaniae, which only grows on a single type of white quartz rock.

Even the expansion of fish farming in huge man-made basins has encroached on cacti habitat as land is cleared for aquaculture, especially in northwestern Mexico.

Cacti range in size from one centimetre (half-an-inch) in diameter, to some 19 metres (62 feet) high.

The iconic Saguaro cactus -- de rigeur in Hollywood Westerns -- grows nearly as tall, and lives for up to 200 years.
Source: AFP