I have been surprised a few times in the concrete jungle of Taipei to find these rather large suckers crawling all over hedges at night (during the day they burry themselves in the earth). They are really big when compared to the tiny land snails we have back home in Canada so it makes them doubly striking to find in the middle of the city. I have accidentally stepped on a few of them crossing the sidewalks of Taipei and the feeling of their cracking shells is not unlike stepping on Xmas bulbs.
[Bushman has a good account of coming across one of these giant suckers]
At the time I just assumed that someone's meal had escaped and had managed to find another of its own to reproduce. I was wrong on both counts.
First, what, you say, are African snails doing here in Taiwan? Well they were probably accidentally introduced through trade. The problem is:
"They are a highly invasive species, and colonies can be formed from a single [...] individual. The species has established itself in Temperate Climates also, and in many places, release into the wild is illegal."
"The Giant East African Snail is a simultaneous hermaphrodite."
It is this factor that makes it a pest. All it takes is one stray snail and the right environment for these snails to flourish into colonies.
Even so, Taiwanese have probably responded to the invasion by serving them on the dinner plate. Other infested countries have, though, tried control methods and haven't been so succesful:
"Suggested preventative measures must include strict quarantine measures to prevent introduction and further spread. Many methods have been tried to eradicate the Giant East African Snail. Generally, none of them have been effective except where implemented at the first sign of infestation. Methods include hand collecting, use of molluscicides, flame-throwers [!!!], and the failed attempts at biological control discussed below. In some regions, an effort has been made to promote use of the Giant East African Snail as a food resource, collecting the snails for food being seen as a method of controlling them. However, promoting a pest in this way is a controversial measure, as it may encourage the further deliberate spread of the snails."
So, although not dangerous, the snails are certainly pesky foreign guests whose numbers may or may not be welcome in Taiwan.