Desert plant defense mechanisms

[Sincere]

Hi,

First time poster, long time reader checking in. I was recently watching an episode of Survivor Man, in which he was in a dry desert environment. I noticed that all the plants he used for food, had alot of defenses that seemed to deter grazers. These were mostly thorns, barbs, and little seeds that would embed in a grazers tounge.

The weird thing was the lack of grazing animals in the environment. So the question is, why do desert plants have so many defenses for so few natural enemies; where as grass and wheat are defenseless and live in an environment with many more threats?