Giant African snails (非洲大蝸牛, Achatina fulica) can be found pretty much everywhere in Taiwan. These are not indigenous to Taiwan at all. There is a long history of how they got here.
Record shows that it was Prof 下條 久馬一 (Shimojyo Kumaitsu) of Imperial Taihoku (Taipei) University who first introduced these giant snails in 1932 from Singapore; they were to be bred as a protein source.
Prof Shimojyo actually had impressive credentials in infectious diseases: he graduated from Imperial Tokyo University, then became a professor at Kanazawa University specializing in typhus research before moving to Taiwan. He also headed Taiwan Tropical Research Institute and was pivotal in the formulation of leprosy managing policy. Unfortunately, snails were not his forte. The idea of snails as a food source did not work out so well at all. It might have been because of the complicated processes required to remove the slime (for the curiously minded, a good post on how to prepare the snails can be found here). The biggest problem is, however, that the African Giant Snails are carriers of 廣東住血線蟲 (Angiostrongylus cantonensis), a parasite that can cause eosinophilic meningitis. The infection rate of the parasites is a very high 50%. Even though they can be destroyed by heat at >60C, the usual quickie stir-frying is, however, ineffective. It is therefore best not to use the snails as food.
As a result, the abandoned African snails were free to roam and procreate since, consuming everything green in their paths.
You'd think this is a lesson learned. Well, not really. In 1980, another snail, the 福壽螺 (Pomacea
canaliculata), was introduced from Argentina. Again, abandoned as a protein source, these pests now invade rice fields all over Taiwan causing tens of millions of dollars in crop loss, and efforts in eradicating them have not been successful thus far.