Cacti are a diverse family of flowering plants, Cactaceae, famous for their adaptations to arid environments, yet they inhabit a surprising range of ecosystems. These plants are defined by the presence of an areole, a specialized structure on the stem from which spines, flowers, and branches emerge, differentiating them from other succulent plants. The approximately 1,750 known species of cacti exhibit a vast array of shapes and sizes. While widely recognized as desert dwellers, their native distribution spans a remarkable geographic area across the Western Hemisphere.
Exclusive Geographic Origin
The cactus family is native only to the Americas, spanning from Patagonia in the south to parts of western Canada in the north. This exclusive distribution suggests the family evolved after the major continental landmasses separated. Molecular studies suggest the first cacti appeared approximately 30 to 35 million years ago, likely in southern South America. The subsequent continental drift and the formation of the Isthmus of Panama allowed for migration and diversification into North America. The lone exception to this New World rule is Rhipsalis baccifera, an epiphytic species whose populations in Africa and Sri Lanka are thought to have been dispersed naturally across the Atlantic, perhaps by migratory birds.
The Unexpected Habitats of Cacti
The common image of a cactus enduring a hot, sandy desert tells part of the story, as these plants thrive in a wide spectrum of environments. Certain lineages have adapted to conditions far removed from the desert floor, including the shaded, humid canopies of tropical rainforests. These “jungle cacti,” such as the Christmas cactus (Schlumbergera) and Rhipsalis, are epiphytes, meaning they grow non-parasitically on other plants, typically trees, for support and access to light. Their stems are often flattened and lack the prominent spines of their desert relatives, reflecting an adaptation to environments where water conservation is less of a struggle.
Cacti also occupy high-altitude regions in the Andes Mountains. These mountain-dwelling cacti must cope with extreme temperature fluctuations and intense ultraviolet radiation, a challenge different from the heat of a lowland desert. Other species flourish in coastal scrublands and dry forests, demonstrating the family’s ability to adapt to varying degrees of aridity and soil composition. The ancestral cactus was likely a scrubby tree from a seasonally dry, warm climate, suggesting the family’s adaptation to extreme aridity evolved later.
Major North and South American Cacti Ecosystems
The greatest diversity of cacti is concentrated in specific ecosystems across the Americas, with Mexico holding the largest number of species. The Sonoran Desert, spanning Arizona and Mexico, is the classic home of the iconic Saguaro (Carnegiea gigantea). This region is also home to the Organ Pipe Cactus (Stenocereus thurberi), which requires warmer temperatures and is restricted to the southern part of the desert.
Further east, the Chihuahuan Desert, which extends across Texas, New Mexico, and a large portion of Mexico, hosts a different set of specialized species. This desert lacks the towering columnar cacti of the Sonoran, instead featuring smaller, ground-hugging species like the Peyote (Lophophora williamsii) and the Living Rock Cactus (Ariocarpus fissuratus). In South America, the Atacama Desert in Chile, one of the driest places on Earth, harbors unique cacti that survive almost entirely on coastal fog.
Cacti Outside the Americas (Introduced Species)
While the Americas are the native home of the cactus family, many species now grow in populations across the globe due to human introduction. Cacti were transported for ornamental purposes, for use as living fences, or for their edible fruits. The Prickly Pear species, primarily from the genus Opuntia, are the most widely introduced and have successfully naturalized in many regions.
Invasion Hotspots
These introduced populations have established themselves in areas with climates similar to parts of their native range, leading to invasion hotspots in places like Australia, South Africa, and the Mediterranean Basin. In Australia, for example, the Prickly Pear became a devastating invasive species, rapidly covering vast tracts of land before it was brought under control by the introduction of a cochineal insect.
Global Distribution
This global distribution of naturalized cacti completes the picture of where these resilient plants can now be found growing outside their original evolutionary boundaries.