How much of that biome is the result of a previous ecological disaster? The US is covered by those from what I've understood. Vast tracts of lands are arid because beavers were hunted to extinction for example. Protecting the accident of the previous 100 years doesn't sound so compelling.
The west is not arid due to beaver hunting. It’s been arid for thousands of years due to tectonic plate activity and a cold deep ocean that flows clockwise bringing colder water down from the north. The cold water and tall mountains produce arid inland conditions. This happened so long ago that the ecology evolved to the arid land.