Cactus - Distribution

Distribution

Cacti inhabit diverse regions, from coastal plains to high mountain areas. With one exception, they are native to the Americas, where their range extends from Patagonia to British Columbia and Alberta in western Canada. A number of centers of diversity exist. For cacti adapted to drought, the three main centers are Mexico and the southwestern United States; the southwestern Andes, where they are found in Peru, Bolivia, Chile and Argentina; and eastern Brazil, away from the Amazon Basin. Tree-living epiphytic and climbing cacti necessarily have different centers of diversity, as they require moister environments. They are mainly found in the coastal mountains and Atlantic forests of southeastern Brazil; in Bolivia, which is the center of diversity for the subfamily Rhipsalideae; and in forested regions of Central America, where the climbing Hylocereeae are most diverse.

Rhipsalis baccifera is the exception; it is native to both the Americas and the Old World, where it is found in tropical Africa, Madagascar, and Sri Lanka. One theory is it was spread by being carried as seeds in the digestive tracts of migratory birds; the seeds of Rhipsalis are adapted for bird distribution. Old World populations are polyploid, and regarded as distinct subspecies, supporting the idea that the spread was not recent. The alternative theory is the species initially crossed the Atlantic on European ships trading between South America and Africa, after which birds may have spread it more widely.

Many other species have become naturalized outside the Americas after having been introduced by people, especially in Australia, Hawaii, and the Mediterranean region. In Australia, species of Opuntia, particularly Opuntia stricta, were introduced in the 19th century for use as natural agricultural fences and in an attempt to establish a cochineal industry. They rapidly became a major weed problem, but are now controlled by biological agents, particularly the moth Cactoblastis cactorum.