The island rule

Last November I learned that Homo floresiensis was a distinct species, and that possibly an earlier species migrated to the island of Flores and shrunk in size. But why do animals either shrink or grow once they inhabit a lonely island? This answer lies in what biologists call the island rule. In essence, small animals become larger because they don't have as many natural predators, and large ones dwarf because they don't have as many resources as they once did on the mainland. Less food on the island would mean that the smaller young would have a better chance of surviving. That is probably what happed to the Flores hobbit.