Cacti (family Cactaceae) are the strange and beautiful plants best known for inhabiting deserts and being a pain in your butt, hand, leg, or whatever body part that happens to brush up against them. These traits are the result of a fascinating evolutionary story that played out in the American deserts, at least initially.
So, cacti don’t have leaves, right? Well, kind of, but those spiky bits are technically modified leaves. That is why they are, in botanical lingo, spines. (Sharp plant parts derived from the epidermis are called prickles and modified branches or stems are called thorns). At some point early in the evolution of cacti their leaves evolved to be smaller and smaller and eventually into hardened points that did not photosynthesize. Cacti also evolved green stems which took over the photosynthetic tasks.
So why would cacti evolve these two unusual features of green stems and non-photosynthetic stabby leaves? Well, these combination of traits are adaptations to dry and resource-poor environments. Big and flat leaves have lots of surface area which is typically a good thing from a plant’s perspective as it captures lots of sunlight for photosynthesis. But many plants have more sunlight than they can handle and more photosynthetic area means more water loss. All photosynthesis requires gas exchange, carbon dioxide in and oxygen and water vapor out. So for plants in water-limited and very sunny environments, like deserts, adaptations that limit photosynthetic area (and thus limit water-losing gas exchange) are likely to be advantageous and selected for.
So as adaptation to very dry desert environments cacti evolved photosynthetic stems which presumably lessen water loss during photosynthesis. Great. But there are more photosynthetic tricks in cacti that you can’t see. Early cacti evolved an additional solution to the photosynthesis-water-loss problem, a whole new physiological mechanisms for photosynthesis called CAM photosynthesis. There are a lot of differences between CAM photosynthesis and what most other plants use, but the most important part is that cacti, and other CAM plants, can leave their stomata (the little pores that allow gas exchange) closed during the heat of the day and instead do all their gas exchange at night when it is cooler and more humid. This little trick greatly reduced water loss and gives CAM photosynthesis plants an advantage in very dry environments.
So why the spines? Why not just lose the leaves all together? The short answer is that spines are a defense against herbivores. Herbivory, or the consumption of plant matter, can be really bad news for a plant: get a bunch of your photosynthetic tissue eaten, have to grow it back, not enough resources left to produce seeds and reproduce, less offspring, lower fitness. Plants that get less damage from herbivores might have higher fitness so evolution should favor plants with traits that reduce herbivory. This is particularly true in environments where resources are scarce, like deserts, where regrowing tissue lost to herbivory is very difficult (this is called the Resource Availability Hypothesis). Thus, plants in low resource environments, like cacti in deserts, invest very heavily in defenses, like big gnarly spines.
The >1500 cactus species all live in the Americas (well except one in Africa, probably dispersed there by birds) and mostly in arid environments. However, there are also many cacti that live in the tropical rainforests of Central and South America. At first this doesn’t make sense why a arid-adapted group would have a center of diversity in some of the world’s wettest habitats, until you look at the micro-habitats they live in, namely, up in the forest canopy. Rainforest cacti are almost all ephiphytes, meaning they grow on other plants, normally the branches of large trees. From plants’ perspective these environments are actually very dry, it is hot, there is no soil to hold water, and airflow from all directions desiccates. So the elegant adaptation that cacti evolved in the deserts gave them an advantage as tropical ephiphytes and when they arrived in these new habitats they thrived and diversified.
Around 30 million years cacti emerged and in the last 10 million years spread across North and South America. This is amazingly fast for the evolution and spread of a plant lineage across such a large geographic area with a huge range of ecological conditions. This incredible success was likely due to the evolution of the morphological, physiological, and defensive traits that make cacti so unique and fascinating. Understanding their evolution requires thinking about the environmental and ecological conditions that cacti live and thrive in, and also remembering that those spines are not just a pain in your butt but also a pain in the butt (or mouth!) of anything that wants to eat them.
(All photos from the Sonoran Desert in Arizona)