The humble cactus is an icon of resiliency. Stuck in a desert with little rain and abundant searing sun, these plants don’t just survive — they thrive and in turn provide habitat for owls and other birds, and flowers for bats and other pollinators.
Yet cacti are in serious trouble, due to habitat loss and the poaching of species for private collections. Now we can add worsening droughts to those woes, according to a new study in the journal Nature. Researchers modeled not just a future of relentlessly rising temperatures, but the attendant dry spells, finding that the majority of nearly 1,000 cactus species will lose habitat as the climate radically changes. More than a quarter of those species could lose at least 80 percent of that habitat by 2100.
“The public perception of cacti tends to be of these organisms that are extremely drought-adapted and are just resilient, can handle anything that you can throw at them,” said Michiel Pillet, a conservation ecologist at the University of South Florida and lead author of the paper. “In reality, cacti are absolutely amazing — and they’re my favorite plant group, clearly — but they’re not that different from other plants.”
That means cacti need water. In the deserts of the southwestern United States, for example, they rely on seasonal monsoon rains which tend to make July or August the wettest month in a given year. Because this is still a desert, cacti have evolved a range of adaptations to survive, like a waxy skin that helps them retain that bounty of rainwater. “Just like other plants, they evolved under conditions that they’re used to,” Pillet said. “When you take them outside of those conditions, they might not be able to handle that.”
Think of these cacti like sick hospital patients. Someone might be admitted for heat-related illness, as their body struggles to cool itself. At the same time, though, they might be dehydrated. Cacti are dealing with the same stresses, as global temperatures rise and dry spells intensify. “When we consider drought conditions, then it turns out that they’re much, much sicker than we previously thought,” Pillet said. “These results really do change the outlook of cacti, and most likely plants more broadly under climate change.”
Instead of simply modeling these threats going forward, the researchers turned to the citizen science platform iNaturalist, where people upload photos of plants and animals. This provided 650,000 observations of cacti in the wild, helping paint a clearer picture of how species might be faring.
Really, this isn’t a simple matter of cacti withering and dying. Sure, if relentless droughts take hold, individuals can’t get water. But the stress could be temporally complex, too. “Now we can move towards questions of: What life stages are most affected?” Pillet said. “Is it the adults or is it the seedlings? But also, which processes? Is it reduced growth or is it increased mortality? Or are plants failing to flower?”
Put another way: Cactus species could be suffering from sublethal effects of drought — the lack of water may not kill them immediately, but still interfere with their reproduction. And without procreation, populations shrink. In the Sonoran Desert, for example, the iconic, towering saguaro is reproducing less frequently during prolonged droughts, because they’re not always flowering.
Part of the problem is that while desert ecosystems may look tough and hardened overall, they’re in fact quite fragile. “They don’t recover quickly once they are disturbed, or once they are degraded, just because there’s so little water and such higher temperatures,” said Marcos Robles, the Nature Conservancy’s lead scientist in Arizona, who wasn’t involved in the research. “It takes multiple decades to recover, and in some cases, they don’t recover.”
Now drought is compounding with other stressors, like habitat loss and poaching, to drive cactus species to the brink. On the opposite end of the spectrum, too much water from rising seas has led to the local extinction of the Key Largo tree cactus in Florida. Higher nighttime temperatures are a problem, too, since that’s when saguaro and other cacti respire to avoid water loss. “If the nighttime temperatures are very high, they can’t do it,” said Rod Haenni, president of the nonprofit Cactus and Succulent Society of America, who wasn’t involved in the paper. “Then if you water on top of that, the plants just rot.”
Still, some cactus species may persist by picking up and leaving. Well, not individual plants, but populations could creep into areas with climates that more closely match what the species adapted to all those millennia ago. A similar thing is happening in the Arctic, actually, where some plants are marching farther north in “search” of increasingly agreeable weather and more water.
In the end, the purpose of this sort of research is to better inform where conservation professionals might concentrate their efforts. They can’t stop droughts, of course, but they can evacuate enough cacti to establish populations in botanical gardens, where they can be cared for as the climate grows more unrecognizable to even the hardiest desert vegetation. Species can even be reintroduced to the wild in some cases, so long as drought and high temperatures don’t remain a threat to a population.
Governments, too, need to protect the lands where cacti are threatened, Haenni said. This new research, he added, could make that happen. “Hopefully, it’ll wake people up,” Haenni said. “But the problem is a lot of the governments involved are indifferent to this. There’s a lot of people in the countries that are not indifferent, but they’re not running the governments.”