The popularity of succulents leads many to wonder how many species exist globally. Getting a firm answer is complex because “succulent” does not refer to a single, related group recognized in formal biological classification. Instead, the word describes specialized physical features that evolved independently in many different plant lineages. This functional description, rather than shared ancestry, is why a definitive species count remains elusive and debated among botanists.
The Defining Characteristics of Succulents
A succulent is defined by its physiological capacity to store significant amounts of water in specialized, fleshy tissues. This adaptation, known as succulence, is a survival strategy allowing these plants to thrive in arid environments where water access is sporadic. Storage occurs in the leaves (which become thick and engorged), the stems (which often expand into columns or spheres), or sometimes the roots.
These water-storing tissues are composed of specialized parenchyma cells. Beyond storage, succulents employ structural and metabolic adaptations to conserve moisture. Many species utilize Crassulacean Acid Metabolism (CAM) photosynthesis, opening their stomata only at night to minimize water loss during the day. Their outer surfaces also feature a thick, waxy cuticle and a reduced surface area to limit evaporation.
Estimating the Global Species Count
Since “succulent” is based on a functional trait rather than a formal taxonomic category, providing a precise species count is challenging. Current reliable estimates suggest there are around 12,380 succulent plant species worldwide, but this figure is often used as a reference point within a broader range. The total count can fluctuate significantly depending on how strictly the defining characteristics are applied.
The primary difficulty lies in establishing a clear boundary between a true succulent and a semi-succulent or a mesophyte. For instance, some definitions include geophytes (plants with underground storage organs like bulbs), while others exclude them, leading to substantial variation in the final tally. Furthermore, the number is always changing due to ongoing botanical exploration where new species are still being discovered and formally described.
Taxonomic revisions also contribute to the variability in the count through the processes of “lumping” and “splitting.” Lumping occurs when botanists decide that previously separate species are variations of a single species, reducing the count. Conversely, splitting happens when one species is divided into two or more distinct species, increasing the count. These reclassifications are common as scientists refine their understanding of plant relationships.
The enormous diversity found across approximately sixty different plant families that contain succulent members also complicates any attempt at a definitive census. Due to these factors, the most realistic answer is a dynamic range, generally falling between 10,000 and 15,000 species. This range accounts for the various scientific interpretations and the continuous nature of botanical discovery and classification.
The Primary Taxonomic Homes of Succulent Species
The trait of succulence is considered polyphyletic, meaning it evolved independently in many different plant families as a convergent response to environmental pressure. Despite this wide distribution, a few major plant families contain the vast majority of the world’s succulent species. Understanding these families reveals the primary taxonomic homes for these thousands of species.
The Cactaceae family (Cacti) is perhaps the most famous, with almost all members being succulents that primarily store water in their stems. Another major group is the Aizoaceae (ice plants or stone plants), which is virtually entirely composed of leaf succulents, many native to Southern Africa. The Crassulaceae family, including genera like Crassula, Echeveria, and Sedum, is also a dominant home for succulents, typically storing water in their leaves.
These three families account for a large proportion of the total species count. Other families also host significant numbers of species where succulence has evolved, such as the Euphorbiaceae (Spurge family) and the Asphodelaceae (including Aloe and Haworthia). The independent evolution of succulence across so many families highlights the success of this strategy in various harsh climates.