Plant Symbiosis
Much of the desert is used to graze cattle, sheep and/or goats. Plants that are good to eat will soon be eradicated because the grazing animals are very clever about finding and eating them. Sheep, for example, are considered dumb, but they are very adept at finding food plants.
In a desert area that has been grazed for a while the only high nutrient plants left will be those that are growing in the shelter of some other plant(s) with thorns and spines which prevent the grazers from reaching them.
