>When you say that the snails may be at risk from Marisa, what exactly do you mean. I know Marisa are herbivores, do you mean they will crowd out the other herbivores?<

Marisa does not always know that it is a herbivore.  It will eat eggs and snails, though it does not seem very skilled as a predator.  It also poses a threat as a competitor and might carry parasites that could cause problems for native species.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa