The title of this book is not a person, or a thing, but a place, and a very specific place at that: the Island of the Blue Dolphins. This title tells us that we are concerned not just with the tale of one girl (the book, after all, wasn't called Karana or even Karana's Island), but the story of the whole island, and everything on it. This includes Karana, of course, but also the blue dolphins, Rontu, the otters, the waves, the abalones, the sea elephants, and well, pretty much the whole shebang.
In this way, the title reflects the worldview that Karana will come to adopt: that the island doesn't belong to her (if we want to get technical about it, the title implies that the island really belongs to the blue dolphins), but to all the living creatures on it and near it. She's just one part of the island's rich and thriving ecosystem. Compare this attitude to the discussion between Captain Orlov and Chief Chowig about who owns the island in the first chapter of the novel. We think Karana's got it right.