There is not enough room for existing species to split into new ones. So as existing species go extinct, from even the most natural of causes, none will be replaced. Earth itself is an island with no immigration. Terminology has changed over the years. Whether any reserve in the world is actually large enough is not now known.
Nothing is perfect.
One or two sections of the book are extraneous. Occasional coarse language jars by its contrast with the rest. The author can be coy, as when he bids us to close our eyes while he uses an equation, or when he tells us to relax because he is not going to burden us with pages and pages of details about a list he has just given. But on the whole it's a book that's going to be read for many years to come, even though it's too big to carry around as Quammen has toted Wallace's The Malay Archipelago for so many years. The book ends back in Indonesia.