What is the central idea (thesis) of Desert Solitaire by Edward Abbey?

Desert Solitaire by Edward Abbey is not really a book with a singular thesis, but a series of meditations about the Great Basin desert and his experiences as a ranger in Arches National Park outside Moab, Utah. Over the course of the book, Abbey reflects extensively on wilderness and displays several consistent attitudes about it, but does so in an exploratory manner rather than by linear argument.

His first major point is that the wilderness...

Desert Solitaire by Edward Abbey is not really a book with a singular thesis, but a series of meditations about the Great Basin desert and his experiences as a ranger in Arches National Park outside Moab, Utah. Over the course of the book, Abbey reflects extensively on wilderness and displays several consistent attitudes about it, but does so in an exploratory manner rather than by linear argument.

His first major point is that the wilderness is the spiritual home of people in North America. Notably, he believes that if we destroy our wilderness areas or stop visiting and preserving them, we destroy something at the center of our own spirit, saying:

A civilization which destroys what little remains of the wild, the spare, the original, is cutting itself off from its origins and betraying the principle of civilization itself.

He believes that wilderness and the flora and fauna inhabiting it have intrinsic worth to the human spirit. 

One of the most well-known and characteristic sections of the book is his description of a trip down Colorado River and his meditations on the Glen Canyon dam, and the vast destruction it wrought on the river ecosystem and the irreplaceable remains of the Anasazi, who created stunning cliff dwellings before Europeans settled the region. He sees the dam as showing the government complicit with the forces of urbanization and corporate greed, stealing a magnificent heritage of natural beauty that should belong to everyone and destroying it to profit a limited number of people, something he considers unambiguously evil. 

In his evocation of the beauty of the high desert, Abbey argues that we ourselves are part of nature, and that to destroy our environment or other species within it, we ultimately destroy ourselves, both in terms of our spirit and eventually our survival, as our ranching and dam building degrades our environment.