When plants become invasive, they can create environmental havoc. Several of the cacti and succulents are considered invasive in various areas of the globe.
The Hottentot fig was originally native to South Africa. Extremely adaptable, it has now naturalized in various regions of the world. These include some parts of Europe (Portugal, Spain, France, Belgium, and Italy). This is also invasive in coastal areas of Great Britain, Ireland, and California.
Originally introduced as a landscape plant, it has now naturalized and started to crowd out native plants. Now, it covers vast areas of coastal regions where it grows on cliffs or rocky and sandy terrain. The plant does further damage by increasing the salt content of the soil.
A related species, the red Hottentot fig has naturalized in the Mediterranean region. This was originally native to South Africa.
Some of the prickly pears are also known for their invasive nature. These have now naturalized in dry valleys of Switzerland, western China, Bhutan, Africa, and the Canary Islands as well as vast areas of Australia.
One species of prickly pear was originally introduced to the Canary Islands as food for the cochineal insects. Then, it escaped from cultivation.
According to experts, a single prickly pear plant was originally introduced to Australia in 1839. Eventually the plants covered more than 25 million hectares. Much of the land was abandoned as a result.
To bring the plant under control, Australian scientists brought in the pyralid moth from Argentina. As the larvae feed on the prickly pear pads, this exposes the plants to a bacterial problem, which lead to the death of the plants within a couple years. Sadly, the moth itself has become a problem since it was introduced to the Caribbean. From there it spread to Florida and now threatens native prickly pears in the U.S.
The Barbary fig has also naturalized in Portugal and the Mediterranean region, while the prostrate cactus has become invasive in southern Europe.
Of course, there is also the question of purslane (Portulaca oleracea). This was reportedly first introduced to North America as a vegetable crop. However, it escaped and became a widespread weed. This has also naturalized in other areas of the world, such as the Philippines.
In one of his gardening encyclopedias, garden writer Donald Wyman wrote that purslane was one of the worst weeds in the U.S. However, the purple loosestrife and the thistle are far worse problems. At least, purslane can be pulled and eaten as a vegetable or used as animal feed.