An illegal trade in live plants and seeds as well as unsustainable harvesting are the main threats to cacti, putting a third of the globe's prickly species at risk of extinction, a study finds.

Conservationists have expressed worry over the fate of the world's cacti, a vital component of arid ecosystems that provides a much-needed source of food and water for many animals.

"The results of this assessment come as a shock to us," says Barbara Goettsch, co-chairwoman of the Cactus and Succulent Plant Specialist Group of the International Union for Conservation of Nature.

"We did not expect cacti to be so highly threatened and for illegal trade to be such an important driver of their decline," says Goettsch, lead author of a study appearing in the journal Nature Plants.

Eighty-six percent of threatened cactus species used in horticulture are taken from wild populations, with illegal trade in those species occurring at the national and international level, the researchers say.

Cacti illegally taken from the wild and exported to Asia and Europe can sell for as much as $1,000 per plant.

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

Many species of cacti are attractive to collectors because of their diverse forms and often-attractive flowers, and other species are heavily utilized for foods and medicines.

Because many cactus species grow in very isolated ranges, they are particularly susceptible to any disruption, Goettsch explains.

"They tend to occur in very localized places, so the distribution range is generally quite small," she says. "They are also very slow-growing species so this makes them particularly vulnerable to disturbance."

The researchers cite the example of Echinopsis pampana, a once-abundant species endemic to the desert region of Peru.

Illegal collection for the ornamental plant trade has occurred at such a high rate that 50 percent of the plants have disappeared from their native habitat since 2000, they say, and the species is now listed as endangered.

"This study highlights the need for better and more sustainable management of cactus populations within range countries. With the current human population growth, these plants cannot sustain such high levels of collection and habitat loss," Goettsch says.