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The image of a cactus is one of arid, remote, and unperturbed living. In this day and age, that is no longer the case, and cacti find themselves under siege from humankind.

A new global assessment from the International Union for Conservation of Nature (IUCN) published in the journal Nature Plants has determined that almost one-third of the world’s cacti face the threat of extinction—making them more vulnerable than mammals and birds when considered as a taxonomic group.

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The report found that human activity, in the form of illegal trade, unsustainable harvesting, and developmental and agricultural encroachment, are putting increasing pressure on cacti. According to the IUCN, more than half the world's nearly 1,500 cactus species are used by people.

Barbara Goettsch, lead author of the study and Co-Chair of IUCN’s Cactus and Succulent Plant Specialist Group, told me that the diversity of the threats affecting cacti came a surprise to her.

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"There are many factors affecting cacti throughout their range: shrimp farms, eucalyptus plantations, small holder agriculture, mining, and unscrupulous collection, which is in many cases done illegally."

Goettsch said that people can help by leaving cacti where they are and simply appreciating them in their natural habitats.

"Many people collect cacti from the wild when they go on walks or picnics because they are very attractive and relatively easy to transport," she said. "When buying live plants, make sure they are plants that come from propagation and not from wild populations, and the same applies to furniture and other crafts made of cactus wood."

According to the assessment, 86% of the threatened cacti used in horticulture are taken from wild populations, with European and Asian collectors being the biggest contributors to the illegal trade.

Dr. Eric Ribbens, a botany professor at Western Illinois University, told me that the report's findings don't surprise him, mainly because cacti grow slowly, often in small populations, and are "quite vulnerable to harvesting."

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"Cactus thievery can be a very major problem," he said. "A world with fewer cacti is a duller world. We are losing species at an unprecedented rate, and the cacti problem is just one example."

Ribbens said it's important not to buy a cactus unless you are certain it wasn't collected from the wild.

Cacti were originally only found in the Americas, however global trade and travel has lead to their proliferation in other parts of the world. As BBC reports, "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."

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For example, since arriving in Australia in the 19th century, prickly pears cacti have spread throughout the Outback, often at the expense of native flora.

The IUCN now considers cacti to be the fifth most threatened group of species worldwide, after cycads (ancient plants that look like palms or ferns), amphibians, corals, and conifers. Mammals are sixth and birds are seventh.

Cacti often offer other species a key form of food and water in the arid ecosystems where they are typically found. According to the IUCN, these cacti-reliant species include deer, woodrats, rabbits, coyotes, turkeys, quails, lizards, and tortoises. Cactus flowers also provide nectar to hummingbirds, bats, bees, moths, and other insects. Goettsch said that without cacti around, many of these species will "struggle to survive in arid lands."

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"Cacti provide, vital water, nutrients and places to nest for many species in these harsh environments," she said.

With climate change predicted to make many hot and dry environments even hotter and drier, the outlook is even grimmer for cacti and those relying on them.

"We are already seeing the effects of climate change on those cactus species that are found in areas that are already very dry, such as the deserts in northern Chile," said Goettsch. "Even though these plants are adapted to dry climate they have tolerance thresholds too and climate change can affect many of them."