According to the article in the Texas Monthly regarding the Texas Parks and Wildlife have been in the process of eliminating animals that would be predators to the Big Horn Sheep that they are relocating to the park.
They are killing donkeys, elk, mountain lion and other animals that would be a threat. Their reason to reintroduce the Big Horn Sheep is for trophy hunters.
This is the link to the article: http://www.texasmonthly.com/2012-04-01/feature3.php