Subject: Do shrubs poison other plants when overgrazed, and if so, how?

I found this comment on the "Scientific American" article "Plants Cannot Think 
And Remember" (http://www.scientificamerican.com/blog/post.cfm?
id=plants-cannot-think-and-remember-bu-2010-07-16). 

"I watched an interesting documentary a year or so ago about antelopes dying in 
some fenced in African reserves, but not others.  It turns out they were being 
poisoned by the shrubbery.  Once this particular type of plant had been 
overgrazed by the antelopes, it released a chemical compound into the 
atmosphere, signaling other plants to increase their production of toxins."

I was curious wheather the above claim in the documentary is true and, if so, 
by what mechanism do plants release this chemical compound when overgrazed, and 
how do they sense when they've been overgrazed? 

Re: Do shrubs poison other plants when overgrazed, and if so, how?