Chickens are the best snail eradicators, my chickens eat the shells as well, which gives them added calcium. They love lawn beetle grubs and slater bugs as well. I let them dig over my vege garden before I plant new seeds or seedlings, and the get rid of all the nematodes as well.
Ducks are also good snail and slug eaters, but they can be messy, and duck poop is really unpleasant.