I cannot imagine that planting exotics would "protect" native vegetation.  Having studied deer and elk over many years in New Mexico, and observed that "ice cream" plants are always nuked- it doesn't matter what they are growing with.  The presence of non-natives had absolutely no effect that I could observe, except that they take up space that palatable plants would inhabit and the non palatable ones put even more pressure on the smaller amount of remaining native palatable vegetation.  Non-natives also seem to support populations of exotic invasive invertebrate plant pests.  I certainly probably would not recommend planting them in this kind of situation.- Marcia H. Wolfe