Although mature native grasses can get water from lower soil regions, seedlings cannot get their roots deep enough into soil to access water before drought sets in, and thus, die of thirst. Without this ability to reproduce, native grasses inevitably decline, and so over time, cheatgrass becomes more and more common until eventually it dominates. Cheatgrass also encourages fires. Because it dries up earlier in the year and burns easily, where cheatgrass is abundant wildfires occur earlier and more often, which damage or kill native grasses and make it impossible for sagebrush to grow back after fire. A decrease in sagebrush also means decreased numbers of native wildlife species because many shrub-steppe animals depend on this shrub for food, hiding, cover, or nesting.
Science is constructed of facts as a house is of stones.
But a collection of facts is no more a science than a heap of stones is a house. Take a long walk or hike. You'll find cheatgrass almost everywhere you go in the shrub-steppe, including those sites listed below. Compare areas that have a lot of cheatgrass with ones that have little or no cheatgrass. Park in the small parking area and head east through native grasslands.