Looking for information on ecological changes in the western US over the past 200 years

I'm trying to find sources that give me a 'picture' of how the western US
looked before european settlement. If anyone knows of a good book, or
article, which explors this then I'd love to hear about it. And I'm looking
for both wildlife and habitat changes.

It can be regional, ie. sonoran destert, great basin, forests, or even
habitat, ie. how grasslands have changed, what the pacific nw forests were
like 200 years ago. Or wildlife, ie. numbers of beavers, elk ,etc.

I have found some sources so far but nothing that had good information. The
best I could hope for is someone who studied the issue and says something
like, "we estimate there were x hectares of sagebrush in x area before
european settlement and now this has grown/reduced to x heactares".

thanks for any help.