Weeds don't need cows to spread
Thank you for the thoughtful coverage dealing with weeds across the West, and especially the discussion of the cheatgrass/fire cycle problem. A quick point of clarification, though. Your articles seemed to return to a theme of grazing as a central cause. In my experience, grazing may not be necessary for land to experience replacement of native shrubs and grasses by cheatgrass and its close relative red chess.
A good example is the Nevada Test Site, where grazing has not been permitted by federal order for over 50 years. During this time, red chess and cheatgrass have invaded and begun to replace the native shrub cover. They do that by germinating months earlier and at seedling densities exceeding 10,000 per square meter: native species just cannot compete and their recruitment becomes a rare event.
So, although livestock and fire may have assisted the invasion and dominance of cheatgrass and red chess in many locations, I suspect that these and other weeds (examples include white top and Russian knapweed) may not need any type of disturbance for them to establish and replace native species within suitable climatic zones. This is a truly scary situation, potentially affecting tens of millions of acres across the west.
Santa Fe, New Mexico