At one time, the plains of the Snake River all across Idaho and into Eastern Oregon were populated with herds of Bison.  By the early 1840's they had disappeared due to a series of particularly severe winters and overhunting.  Normally, it would take many decades with good conditions for the herds to wander back onto the range and rebuild their numbers - even without hunting.  They never got the chance.  By the 1870's the vast numberless herds of the Great Plains had been pretty much killed off due to the demand for leather, Buffalo Tongues, the relentless pressure from buffalo hunters and the westward migration by the pioneers;  fewer animals each year and the survivors stopped breeding.  Each animal killed did not get replaced.  It was only a matter of time.  An estimated 40,000,000 buffalo at the time of the Revolution  were reduced to a dozen and a half in Yellowstone Park.  Thankfully, there were survivors across the border in Canada that were later used to help rebuild the numbers enough for survival in this country.