Island of the Lost by Joan Druett

True story: in 1864, a few months apart, two different groups are shipwrecked on opposite ends of the same Aukland Island, 285 miles south of New Zealand in the subantarctic. One group, blessed with cooperative, ingenious folks, leads an almost Robinson Crusoe-like existence for over a year. They figure out how to make soap, tan leather, construct a smithy, and ultimately make a boat (forging hundreds of hand-cast nails!) that they sail hundred of miles to find help. In the other group, anarchy and social division reign. Most of the survivors die, and the remaining three are ultimately rescued by a plague ship. All the castaways survived by eating a plant called stilbocarpa; it prevented scurvy and bleached their teeth, so that when they were rescued, they all had uncannily gleaming grins.