The hardy cactus, which likes heat and aridity and is adapted to rough terrain, may not seem like a possible victim of climate change. However, even these prickly survivors could be close to reaching their limits as the planet gets hotter and drier over the next few decades, according to research published Thursday. The study estimates that, by mid-century, global warming could put 60% of cactus species at greater risk of extinction.
This prediction does not take into account the illegal logging, habitat destruction and other man-made threats that already make cacti one of the most endangered groups of organisms in the world.
Most cactus species “are somehow adapted to the climates and environments they live in,” said Michiel Pillet, a doctoral student in ecology and evolutionary biology at the University of Arizona who led the new study, published in the journal Nature Plants. “Even a small change can be too much for them to adapt to on shorter timescales.”
For those who think of cacti as masters of all-weather hardiness or as interesting, easy-to-maintain houseplants, the sheer variety within the cactus family can come as a shock.
For starters, not all cacti inhabit the desert. Some live in tropical forests or in cold climates at high altitudes. Some store little water in their stems, relying on rainwater and dew. Some occupy highly specific environments: limestone cliffs in Mexico, pink granite hills in Brazil, a 1.5 square kilometer sandy patch in Peru. In the Amazon, the moonflower cactus revolves around a tree trunk, high above the ground, so it stays above the waterline when the forest floods and water can disperse its seeds.
In part, it’s this defined taste for specific environments that makes certain cacti vulnerable not only to climate change, but threats of all kinds.
“If it’s found in just a very small area, and someone comes and plops to plant anything, the entire population will disappear,” said Barbara Goettsch, another author of the new study and chair of the International Union’s Succulent and Cactus Expert Group. for Nature Conservation.
The study looks at 408 cactus species, or about a quarter of all known cactus species, and how their geographic range could change in three different ways by global warming this century. To the researchers’ surprise, their results didn’t vary much between the different pathways to climate change, Pillet said: even if the planet warms only modestly, many types of cacti could experience declines in the amount of territory where the climate is hospitable to them.
Overall, 60% of cactus species are expected to experience declines of any magnitude, the study found, and 14% could experience sharp declines. Only one species, the xique-xique, in Brazil, tends to have a substantially greater range.
According to the study, the places where the greatest number of species could become threatened are generally those with the greatest species diversity today, including Florida, central Mexico and large swaths of Brazil. Tree-dwelling cacti seem to do especially poorly, perhaps because their lives are so intertwined with those of other plants.
The outlook doesn’t look so bleak for the American Southwest, home to the iconic saguaro, Pillet said. But scientists still don’t know enough about certain rarer cacti to predict how they might react to harsher climates, he said. This means that the study’s projections may not paint a complete picture for some parts of the world.
Cacti, by nature, do not reveal their secrets easily. Scientists who examine other plants’ sensitivity to environmental changes can observe, for example, the size and thickness of their leaves. “Most cacti don’t have leaves, so what would you measure?” Pillet said.
The study’s predictions also don’t account for extreme events like droughts and wildfires, Pillet said. In the Sonoran Desert (southwest US and Mexico), rapid infestations of “buffelgrass”, a drought-resistant grass native to Africa, Asia and the Middle East, have made the landscape highly flammable. Forest fires have killed thousands of saguaros in recent years.
“It’s a popular image of cacti,” said David Williams, a professor of botany at the University of Wyoming who was not involved in the new research. “Oh, we don’t have to worry about cacti. See, they have thorns, they grow in this terrible environment.” But cacti, like most plants, exist in delicate balance with the ecosystems around them, he said. “There are a lot of these tipping points, boundaries and interactions that are very fragile and reactive to changes in the environment, land use and climate change.”
The new study is “fundamental,” Williams said, in showing how widely these changes can affect cactus communities.
About a decade ago, when Goettsch was preparing a comprehensive global assessment of threats to cacti, only a few scientific studies had looked at the potential impacts of climate change specifically on cacti, she said.
But other experts on these plants kept telling her during her field visits, “You know, we’re back now and a lot of plants are dead. There’s no real reason, so we think it might be climate change.” Evidence has only accumulated since then, she said.
Brazil is a high point for cactus diversity. As the semi-arid lands of the country’s northeast experience warmer temperatures, more intense droughts and desertification, this plant wealth is at risk, said Arnóbio de Mendonça, a climate and biodiversity researcher at the National Institute for Space Research in Brazil, who did not work in the new study.
“Either species adapt or go extinct,” he said. “As adaptation is a slow process and current climate change is occurring rapidly, it is likely that many species will be lost.”
Translated by Luiz Roberto M. Gonçalves
