Almost one-third of cactus species are under threat as a result of over-harvesting and illegal trade in the plants, a global study has concluded.
Conservationists voiced concern, saying the level of threat to cacti was much greater than previously thought.
The plants are a vital component of arid ecosystems, providing a source of food and water for many animals.
The results of the assessment by the International Union for Conservation of Nature appear in Nature Plants journal.
According to the study, 31 percent of the world's 1,480 cactus species were under pressure from human activity, such as illegal trading, agriculture and aquaculture as well as land-use change.
"The results of this assessment come as a shock to us," said lead author Barbara Goettsch, co-chairwoman of the IUCN's Cactus and Succulent Plant Specialist Group.
"We did not expect cacti to be so highly threatened and for illegal trade to be such an important driver of their decline."
The assessment reported that the illegal trade of live plants and seeds for the horticultural industry and private collections, as well as their unsustainable harvesting, affected 47% of threatened species.
These plants which have evolved to cope with the harsh conditions found in arid landscapes are native to North and South America, with exception of one species that is native in southern Africa and South Asia.
Although cacti are a familiar sight in other regions, such as Europe and Australia, these plants have been introduced to these landscapes either intentionally or accidentally.
Many species are highly sought after by collectors for their attractive flowers, and half of the species are used as a source of food or medicine.
Thriving in landscapes where very little vegetation can survive in the intense heat or drought conditions, the cactus group plays a pivotal role in sustaining arid ecosystems.
Among the species that use cacti as sources of food and water are deer, coyotes, lizards and tortoises. The animals, in return, help distribute the plants' seeds.
However, Dr Goettsch said that cacti species were very isolated.
"They tend to occur in very localised places, so the distribution range is generally quite small," she explained.
"They are also very slow-growing species so this makes them particularly vulnerable to disturbance."
Dr Goettsch said that extending protected area networks would "definitely benefit the species, because we did find that a lot of the threatened species do not occur within protected areas".
She added that the national-level enforcement of international agreements, such as the Convention on International Trade in Endangered Species of Wild Fauna and Flora (Cites), would help curb the illegal trade in cacti.
"The whole family of cacti is included in Cites, which means that you can trade the species but you need to have permits. This is what needs to be enforced in some of the countries where the species occur," she said.
"The other thing that would really help these plants would be to raise awareness of the importance of harvesting sustainably, because in many cases the plants are not destined for international markets. They are just traded in local markets so many local communities need to be aware of how they should harvest them or if they should harvest them at all."