Illegal trade and the spread of farms in arid areas are among threats that have put almost a third of cacti at risk of extinction, making the spiny plants among the most vulnerable plant or animal species in the world, according to scientists.
"We were surprised to find that such a high proportion of cactus species are threatened ... and by the diversity of threats," lead author Barbara Goettsch told Reuters of the findings by an international team of researchers.
The study said 31 percent of 1,478 types of cacti assessed were at risk of extinction - a higher rate than the 25 percent of mammals or 12 percent of birds that are rated as vulnerable to dying out because of human pressures.
Cacti range from 0.4 inch across to 62 feet high, and many are prized by collectors for brightly coloured flowers that bloom unpredictably. An international treaty bans trade in many rare cacti.
Inger Andersen, director general of the IUCN (International Union for Conservation of Nature), which oversees a Red List of threatened species, called the findings disturbing.
"They confirm that the scale of the illegal wildlife trade, including trade in plants, is much greater than we had previously thought," she said. Goettsch also works for the IUCN, based in Cambridge, England.
The study said that cattle ranching in arid lands, a spread of other farms and of roads and urban areas were also threatening cactus habitats in the Americas, from Chile and Uruguay to Mexico and the United States.
All species of cactus grow in the Americas except the mistletoe cactus found in Africa, Madagascar and Sri Lanka, the scientists wrote in the journal Nature Plants.
As a group of plants or animals, cactuses were the fifth most threatened behind cycads, amphibians, corals and conifers, the study said. Cycads are a type of fern-like plants.