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Cacti are one of the world鈥檚 most beguiling and easily recognised plant families, but how and why the spiky family evolved into its many diverse shapes and sizes has been a mystery. Now, researchers are untangling the ancient history of the Cactaceae, revealing some surprising forces that shaped these plants, and prompting concern for their future.
Fossils can鈥檛 tell us much about the origins of cacti because there aren鈥檛 many. The Guinness World Records lists the oldest cactus fossils (prickly pear spines and seeds) as being , but the cactus family is thought to be than this. The problem is that things that grow in dry environments are much less likely to become fossilised, and it means that researchers seeking to understand the origins and evolution of cacti need to focus on computational methods instead, looking at today鈥檚 species and their DNA and making backwards predictions about what happened and when.
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We know that a lot did happen. The Cactaceae family today contains somewhere between 1000 and 2000 species, and while many of these look broadly similar 鈥� thick and fleshy stems with prickly spines 鈥� they are a diverse group. They range in size from just 12 millimetres to a towering 19 metres, and include some lesser-known non-succulent species, as well as species that don鈥檛 live in deserts but instead in wet tropical or colder places.
Even though cacti reproduce relatively slowly 鈥� which usually causes organisms to evolve more sluggishly 鈥� the Cactaceae stand out among plants as having some of the fastest rates at which new species evolve. Circumstantial evidence has provided some hints about the factors behind this: larger species, and those that have evolved to use birds, bats and moths as pollinators (instead of bees and ants), show some of the highest rates of diversification.
And the places with the largest number of cactus species tend to be arid and semi-arid regions. But, as Jamie Thompson at the University of Reading in the UK and his colleagues write , ancient aridification 鈥� an increase in dryness seen in several American regions 鈥� cannot fully explain cactus evolution. Some big periods of diversification seem to have happened more recently than the transition to a drier climate, for example, and new species seem to have emerged more quickly in places that aren鈥檛 as dry.
So Thompson and colleagues turned to machine learning for help. They built a family tree of more than a thousand cactus species and used various computer analyses to model and predict the role that 39 factors may have played in spurring cactus diversification. Their work, although preliminary, produced some interesting and surprising results.
They identified three factors that appear to have been major drivers of cactus speciation 鈥� none of which were the well-worn hypotheses of aridity and a change in pollinators. Instead, the researchers found the biggest factor was the difference in daytime and nighttime temperatures, with more species forming when these differ by a relatively middling amount, around 10掳C. Cacti are highly adapted to their local temperatures 鈥� they are famously skilled at conserving water, mostly carrying spines instead of leaves to reduce evaporation, and also employing a special kind of trick for photosynthesis, known as CAM, which enables them to make sugar at night and curtails the loss of water during the day. But this tactic is less effective at high temperatures, and at the other end of the scale, the succulence of most cacti makes them vulnerable to freezing. This leaves them highly specialised for life where water is scarce but temperatures never veer too far towards either extreme.
Their Goldilocks tendencies also apply to soil sandiness 鈥� the study found that cacti more quickly evolve new species in medium-sandy soils. Cactus roots are highly specialised for finding water in sandy soils that rarely stay wet for long. Many cacti have wide-ranging nets of shallow roots, so that when it rains, they can rapidly draw up water from a wide area, and some have very deep roots that dig down to find hidden moisture. They are so specialised to live in these soils that, if there is too little sand, it鈥檚 possible that cacti might be edged out by other types of plant. But if soils are extremely sandy, even cacti could struggle to get what they need.
After temperature variation and soil sandiness, plant size was a good predictor of speciation rates, but in a funny pattern 鈥� middle-sized cacti were found to diversify the slowest, with both smaller and larger cacti showing faster speciation rates. The modelling also suggested that cacti do best in areas that have more annual variation in temperatures than is seen in the tropics.
Although all these drivers are just hypotheses that need further testing, the ones related to daily and seasonal variations in temperature add to growing evidence that cacti aren鈥檛 as resilient to relentlessly hot temperatures as we might assume. This is prompting concern that cacti, around a third of which are largely due to habitat loss and unsustainable collecting, are also not well equipped to survive the extremes of climate change.
We are already seeing this. The struggled with intense heat in the summers of 2020 and 2023. Excessive warmth weakens the plants, causing some to go squishy and collapse or to be more susceptible to diseases. Old, established saguaros seem to have the water reserves to withstand this, but younger plants do not, leading to concerns that the next generation of saguaros 鈥� a long-lived, slow-maturing species that 鈥� will struggle to survive.
Another cactus species has become a sad emblem for climate change: the Key Largo tree cactus, which earlier this year is thought to have become the first species to go locally extinct in the US . The species lives on in some parts of the Caribbean, but it serves as a warning that some of nature鈥檚 toughest-looking species are not as resilient as we thought.
