The prickly pear cactus introduced into Australia in the early 1920s became invasive because

When exotic species are introduced into a new geographical area, they become invasive because of the absence of its natural predators in the invaded land. The prickly pear cactus introduced into Australia in the early 1920s caused havoc by spreading rapidly into millions of hectares of rangeland. Finally, the invasive cactus was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced into Australia.