As with Quammen's The Song of the Dodo, this book is a magnificently researched mixture of travel writing and ecological observation. It is also a great read.
Quammen visits the habitats of four alpha predators (his own term to describe animals that sometimes kill and eat humans): Indian lions, Australian crocodiles, Russian brown bears and Siberian tigers. His main argument is that it's all well and good for the likes of us to say that such animals must be saved, but we don't have to live in danger of being eaten by them. The best hope for the survival of such species is to ensure that the local people who have to live in the presence of these species can profit from it. This doesn't sound like a particularly controversial argument, until you realise that one way of exploiting the species for profit is to allow them to be farmed and/or hunted in a sustainable manner.
Quammen also explains, but in less detail, how many predators can be seen as key species in their environments. Remove them, and the whole balance of the environment changes for the worse.
Unfortunately, Quammen is ultimately pessimistic as to the future prospects of his alpha predators.