Most invasive cactus

The most invasive cactus is the Indian fig (Opuntia ficus-indica), a species of prickly pear originally from Mexico. A 2014 study published in the journal AoB PLANTS found that this cactus had become naturalized in 22 countries outside of its native range.

Able to thrive in both arid and semi-arid conditions and with a lack of natural predators outside of its native range, O. ficus-indica has now established itself in many temperate areas around the globe including much of Africa, Australia, southern Europe, Yemen, and certain islands in the Pacific, such as Hawaii, USA, and the Indian Ocean, such as Mauritius.

The same study found that Australia was home to more invasive species of cactus than any other country, with 39. It was followed by South Africa (35) and Spain (24).

For the most part, cacti seem to be originally introduced to non-native countries for ornamental/horticultural purposes, with a small number of cases where they have brought in as a food crop or to use for demarcating property borders (e.g., hedging).