The Cactaceae family represents a highly specialized group of plants that have successfully adapted to some of the planet’s driest environments. These organisms exhibit remarkable physiological and structural modifications, allowing them to thrive where other plants cannot. Determining the precise number of distinct species is complex, as the count continually shifts due to ongoing scientific discoveries and changes in how botanists define a species. This variability highlights the dynamic nature of taxonomy.
The Current Estimate of Cactus Species
The number of recognized cactus species is generally cited to be in the range of 1,400 to over 2,000 species across approximately 130 genera. For instance, the International Union for Conservation of Nature (IUCN) recognizes approximately 1,480 species, while other sources suggest counts closer to 1,750 or more. This wide variability stems from the continuous process of scientific review and the difficulty in classifying organisms that exhibit high morphological variation.
The primary reason for the fluctuating species count involves the debate between “lumpers” and “splitters” in taxonomy. “Splitters” define new species based on small differences in morphology or genetics, while “lumpers” group organisms with minor variations into a single, broader species. The application of new genetic and molecular data has led to significant reclassification, often supporting the consolidation of previously split species or the recognition of entirely new ones.
Organizations like the International Cactaceae Systematics Group (ICSG) work to establish a consensus, but the process is slow. Genetic analysis often reveals that a widespread species is actually a complex of several distinct populations, or conversely, that species previously thought to be separate should be merged. The count is also affected by the discovery of new species in remote regions, particularly in South America. The most reliable figures are those recently published in major botanical monographs that incorporate the latest molecular phylogenetic studies.
Defining Features of Cacti
All members of the Cactaceae family share unique characteristics that separate them from other succulent plants, the most distinctive of which is the areole. The areole is a specialized, cushion-like structure representing a modified, condensed side branch. This structure is the exclusive source from which all spines, hairs, flowers, and new vegetative shoots emerge.
The spines growing from the areole are highly modified leaves, serving functions like defense, shading, and collecting condensed moisture. In many species, especially the Opuntia group, the areoles also produce tiny, barbed bristles called glochids, which detach easily. The body of most cacti consists of a fleshy, succulent stem that has taken over the role of photosynthesis.
Another defining biological feature is Crassulacean Acid Metabolism (CAM photosynthesis), an adaptation to arid environments. Cacti open their stomata only at night to absorb carbon dioxide, avoiding excessive water loss through transpiration during the day. The carbon dioxide is stored as an organic acid until daylight, when it is used for photosynthesis while the stomata remain closed, significantly conserving water.
Major Classification Groups
The large number of cactus species is organized into a few major subfamilies that reflect their evolutionary history and distinct morphology. Phylogenetic studies generally recognize four main subfamilies, with the vast majority of species falling into the largest group.
Pereskioideae
This subfamily contains the most primitive members of the family, which still retain broad, non-succulent leaves and woody stems. These species look more like typical shrubs or trees and provide a window into the ancestral form of the cactus. Some members utilize the standard C3 photosynthetic pathway instead of CAM metabolism.
Opuntioideae
The Opuntioideae subfamily is the second largest group and includes the familiar prickly pears and chollas. A key defining feature is the presence of glochids, the tiny, barbed bristles found in the areoles, in addition to the larger spines. Many species in this group have stems divided into distinct, flattened pads or cylindrical segments.
Cactoideae
The largest and most diverse group is the Cactoideae subfamily, accounting for about 85% of all known species. This group includes the classic globular, barrel, and towering columnar cacti. These species represent the most highly evolved forms, typically lacking visible leaves and exhibiting the most extreme stem succulence.
Maihuenioideae
The fourth and smallest subfamily is Maihuenioideae, represented by only two species found exclusively in the high Andes of Argentina and Chile. These low-growing, mat-forming plants are unique because they possess persistent leaves and do not utilize the CAM photosynthetic pathway.
Geographical Distribution and Biodiversity Hotspots
Cacti are almost entirely native to the Americas, ranging from Patagonia in the south up to parts of Canada in the north. The single exception is Rhipsalis baccifera, an epiphytic species also found naturally in parts of Africa, Madagascar, and Sri Lanka. This unique distribution is thought to be the result of seed dispersal by migratory birds.
The greatest concentration of species richness is Mexico, often called the epicenter of cactus diversity. Mexico is home to approximately 586 species, with a significant number of those being endemic. This high concentration is due to varied topography, complex geological history, and wide diurnal temperature ranges, which have driven rapid evolutionary diversification.
Other major biodiversity hotspots include the arid and semi-arid regions of South America, specifically the Andean slopes and the dry diagonal. The high species count in these regions is heavily influenced by endemism, meaning species are restricted to a very small, specific geographic area. Many cactus species have extremely limited ranges, contributing significantly to the overall diversity of the family.