I disagree slightly, To add, I would say that there are natural indications that the island is dangerous in the first chapter. There is a great deal of foliage which hides things and the forest is described as being dark. Additionally, Piggy seems to be more leery of the island than Ralph who perceives the island as a paradise and freedom. Piggy can't swim so essentially he is surrounded by water and this is also dangerous. Finally, there are tangles of creeper vines all about and when the boys explore and climb the mountain, they find lose granite rocks which are also potentially dangerous and foreshadow the end of the book.